Engine is Happy at a Certain RPM Range

[ThickCobra]

I hope everyone is enjoying the gorgeous Wisconsin weather. For me, it's a great time to put some miles on the car, which I've done for the last couple of days. But, now I have a question for you all with stroker engines. I find that with my engine, tko 600 and IRS that it is happiest in an RPM Range while under a load of 1800 - 2200 rpm. Anything below this in any gear, the car noticeably lurches, begging for a lower gear. Needless to say, driving thru my neighborhood requires a good amount of clutch feathering.

Is this normal and expected?

[Railroad]

That 1800-2200 rpm cruise range is great, not wanting to run below that is not good.
It could be the engine, cam combination, or just the carb adjustment.
How does the engine idle, what rpm, choppy, or clean and steady?
Is this condition new or consistent since day one?
Lastly, what gear and speed are you encountering this? You cannot lug down a manual trans in too high of a gear, especially with a carb.
Your carb might be not adjusted ideally, but it could be, just down shift and keep on trucking.
More information is necessary.

[Jeff Kleiner]

Is this normal and expected?

That is quite dependent on the camshaft used. If it is profiled more for higher RPM power it generally won't be happy on the low end. Do you have specs?

Jeff

[ThickCobra]

The engine idles smooth at 1,000. That may be high, but any lower and it won't run.

The condition has been consistent from day one, I'm just getting around to posing the question.

Speed doesn't matter. It will lug in any gear between 1000 and 1800 rpm. However, it is most notable driving in and out of my neighborhood at low speeds. At approx 20 mph, i either keep the rpms high in 1st or shift to 2nd and deal with the lugging.

I have read reached out to Mike Forte for cam and lifter specs. And, I have a Quick Fuel Hot Rod 780 carb with a vacuum secondary which I had tuned by a local hotrod mechanic. With power to it, the car really moves...with my annoying condition of course.

[Railroad]

Any chance you have a vacuum gauge? If you can put a vacuum gauge on the intake manifold vacuum at idle, it will reveal more about your cam. If the cam specs are your limit, that is what you will have to live with, short of major surgery. A smaller carb might provide some relief, but will compromise other areas.
If you consider the issue too severe, start getting informed about fuel injection. It is more coin, but a world of difference.

[RRussellTx]

I'm also running the Forte 427w with the TKO600 and the 3.31 rear end. The main difference is that I have the Holley Terminator EFI.

Here are the details on my heads and camshaft:
Heads are Edelbrock 60255. They have 2.020 intake and 1.600 exhaust valves.
The camshaft is an Edelbrock #2281 which is 235/[email protected]" .573/.582 112 LSA. Installed on a 108 ICL.(4*advanced)
http://www.edelbrock.com/automotive/...2281&submit=go


I don't know if it is "normal and expected" but I definitely have the same lugging issues driving in and out of the neighborhood.
It is definitely frustrating trying to cruise anywhere below about 25 mph.

Mine does idle pretty well at 800 RPMs though - it's not exactly smooth but I love the sound.

[Clover]

I would say this is a combination of a fairly big cam that pushes the power band high in the RPM range and the idle being set high at around 1000 RPM. 1800 - 2200 rpm is about 800 to 1200 about rpms above idle. You are probably more accustomed to engines that idle at 500-600 rpm. Comparably, your 800 to 1200 rpm above idle is 1300 - 1700 rpm on a modern car's engine. You don't often up shift to be that low in the rpm range if the engine makes its power up high. I think you are just going to need to drive the engine around at a little higher RPM then you are accustomed to.

[ThickCobra]

I can't imagine I have a very aggressive cam as I requested a conservative hp of only 450. I know some guys with the same 427 DART engine have up upwards of 600hp. And that I would suspect is a very aggressive cam. I'll will wait to see what info Mike forwards.

And, for what it's worth, a while back Mike suggested 800 rpm at idle should be about right. But, im not there.

[CraigS]

First step is going to be to get a wide band O2 sensor setup so you can see what the mixture is. I know my SS650AN was super lean at low throttle openings and steady speeds. Mine was OK down to about 1500-1600 so similar to yours but not quite as bad. I learned a lot about carb idle circuits getting it straightened out. One trick the QF tech guy told me was get my 4 idle screws perfectly set and drive it. Then back all of them out 1/2 turn and drive again. Back them all out another 1/2 turn and drive again. The idle itself will not be right but what you are looking for is whether this extra richness helps the problem area. The idle circuit is active to much higher speeds than I realized, up to about 2000. I did this test process because I already knew the engine was going real lean at those speeds.

[Sanford]

Swap from single plane to dual plane, stronger signal to carb? Give up some top end but gain driveability.

[myjones]

One more thing to remember on a stroker. Because of the faster piston speed they need less spark advance, sometimes as much as 6*
If you have a timing curve that's not set up for a stroker it could be part of the problem and hurting power output at the top end.

If you dyno the car at some point that top end number will prove out.
Dale

[Railroad]

CraigS's advise is the easiest, no cost and comes from first hand experience. Without buying the O2 reader, I would start backing out the idle adjusters. You can easily get back to your original tune.
I look forward to your response to trying his advise.

[Jim1855]

In a previous life/cars I ran a 427w, 9.7:1 compression, 780HP carb, cam w 242 duration .610" lift, 108LSA, 34-36 degrees of advance, AFR 225 heads, Vic Jr manifold, 3.27 gears, TKO600. This may or may not be similar to your configuration but it identifies my starting point. It would idle at 900 but 1,000 was better for the motor. Would drive at 1,800+ RPM but didn't like much under that. With the same motor and Holley 670 Street Avenger and a RPM Dual Plane manifold it would run well in the lower <1,500 and didn't suffer significantly at the upper end. It was also well tuned having worked with two different tuners and their dynos.

ThickCobra, not sure what you expect. Some tuning might help, a smaller carb would help; it's a charge velocity/vacuum thing. But as has been mentioned, EFI would help the most. It's a Cobra w/ a carb; how civilized do you expect it to be?

I offer this following site / calculator. The developer is a friend, PhD Engineer and Cobra owner. The calculations are derived from the Holley and many other calculators. I've spent a lot of time working through and discussing results with the developer. The calculations are more geared to drivability than upper RPM HP which was his focus. http://www.secondstrike.com/Technical/CarbCalc.asp

Not sure this info / comments will help but you are not alone or unique. New Corvettes are civilized, Cobras aren't supposed to be.

Jim

[RR20AC]

Not sure what is different about my Forte 427 Holley EFI motor, but it runs amazingly well at lower rpms. Just took out a friend yesterday and demonstrated how my car will go down a 40 mile per hour road in 5 gear at 1200 rpms. You can actually hear the two side pipes firing off independently. Has a lot of low end torque. https://youtu.be/zj_RVi1txsI

[ThickCobra]

[QUOTE=RR20AC;327035]Not sure what is different about my Forte 427 Holley EFI motor, but it runs amazingly well at lower rpms. Just took out a friend yesterday and demonstrated how my car will go down a 40 mile per hour road in 5 gear at 1200 rpms. You can actually hear the two side pipes firing off independently. Has a lot of low end torque. [url]https://youtu.be/zj_RVi1txsI[/ur


Unbelievably cool. Mine would have been jerking all around those turns. Now I'm motivated to fix my problem and not live with it as I had no idea it could sound so smooth and drive so well.

I am going to start with the basics and work my way to more complex. I'm realistic that I probably won't solve my issue in a few weeks.

Thanks, Everyone!

[BigUgly]

ThickCobra,

I feel your pain. I have the same engine combination as you and have the same problem, it is not a joy to drive the car. In addition to the jerking and bucking mine runs so rich the plugs and side pipes look like I'm burning soft coal. I have not taken the carb apart to see what jets are inside and because I never received the card that Quick Fuel supposedly puts in the box with the carb identifying the carb specs I do not know what all the specs are for my carb.

The air bleeds that I can get to through the top of the carb are, the four inner ones are 31, and the four outer ones are 70.

I guess I will take the carb apart and see what is inside.

BigUgly

[ThickCobra]

In a previous life/cars I ran a 427w, 9.7:1 compression, 780HP carb, cam w 242 duration .610" lift, 108LSA, 34-36 degrees of advance, AFR 225 heads, Vic Jr manifold, 3.27 gears, TKO600. This may or may not be similar to your configuration but it identifies my starting point. It would idle at 900 but 1,000 was better for the motor. Would drive at 1,800+ RPM but didn't like much under that. With the same motor and Holley 670 Street Avenger and a RPM Dual Plane manifold it would run well in the lower <1,500 and didn't suffer significantly at the upper end. It was also well tuned having worked with two different tuners and their dynos.

ThickCobra, not sure what you expect. Some tuning might help, a smaller carb would help; it's a charge velocity/vacuum thing. But as has been mentioned, EFI would help the most. It's a Cobra w/ a carb; how civilized do you expect it to be?

I offer this following site / calculator. The developer is a friend, PhD Engineer and Cobra owner. The calculations are derived from the Holley and many other calculators. I've spent a lot of time working through and discussing results with the developer. The calculations are more geared to drivability than upper RPM HP which was his focus. http://www.secondstrike.com/Technical/CarbCalc.asp

Not sure this info / comments will help but you are not alone or unique. New Corvettes are civilized, Cobras aren't supposed to be.

Jim

Jim,

Inputting my specs into the second strike website indicates my 780 Hot Rod Quick Fuel carb is way too big for my engine set up. But my engine was built starting with a Dart 351/ 427 stroker and with the supplied carb. I apologize if I missed something, but the calc program does not ask for cam specs. Or, is that not relevant?

I did make progress today. I tried CraigS approach but it must have been well dialed in as I couldn't turn the idle screws more than a quarter inch each way without a significant drop in performance. So I returned them to their original positions. I then hooked up a vacuum tester to test the vacuum, the exhaust vacuum and the distributor advance and they were all within specs.

On to next steps.

[Jim1855]

ThickCobra,

Cam specs will matter as these will determine your general power range but...., This can really add to the complexity of the calculations. If you know the specs then you have a shot of determining the powerband and watch the results of different sizes in the carb calculator.

As I indicated the SecondStrike calculator is really designed for drivability and will be biased toward the smaller carb. I can attest to the validity as I've run a 670 vac (1st), 770 vac (2nd), 780HP (3rd), 940HP (4th) and back to the 780HP. Mixed in dual plane and single plane manifolds and three different sets of headers and four different muffler / sidepipe configurations all in the same 427 Windsor. The projections were good from my experience. The dual plane and 670 gave the best around town easy drive, it still ran well when into it but not like the big carb and headers.

About the QF780. I really don't think it is too big, actually about right but it will exhibit the low RPM drivability that you've indicated not liking and should easily run to the upper 6,000 range. Depending on your manifold and other specs a carb change should help. (Real life no guarantees) If there was an opportunity to try a 670-700 you might be pleasantly surprised.

But again as others have indicated EFI might be the best solution. The Holley Sniper is getting good reviews, FiTech seems to be good as well. There are others. I'd guess by the time you get an EFI system installed and tuned you'll be close to $2k, (system + parts + install + dyno tune).

Hope this helps.

Jim

[Mark Dougherty]

I have a question
What flywheel and clutch do you have

[ThickCobra]

I have a question
What flywheel and clutch do you have

Mark,

Looking forward to seeing you again when your back to Mike's in Windlake, WI.

To your questions, the clutch is a Ram 88794T. The flywheel is a Ram 1529, 24lb billet steel.

[bobl]

What you are experiencing is definitely not normal unless it has a really big cam. First red flag is that it won't idle below 1000 RPM. Could be initial timing, the timing curve or vacuum advance (if it has it) not connected correctly. Obviously carb tuning could be the cause. If it has a similar cam to what Russell listed it should easily idle down to 700-800 rpm and lug smoothly. Hopefully Mike Forte will chime in with specs on the cam.

[CraigS]

Thinking about a replacement carb or efi is way premature. 780 is fine for a 427 especially a vac secondary 780. Based on your 1000 rpm idle I think there is something wrong somewhere that is fixable;
- too much timing advance
- vacuum leak. Are the three black rubber caps in place or have a hose connected to them? look at the pic of the bottom of the carb
https://www.holley.com/products/fuel...arts/HR-780-VS
- idle speed adjustment of the primary and secondary throttle plates as well as the choke high speed idle screw. The secondary adjustment is usually a small screw adjusted w/ an allen on the side where the black choke assembly is. Usually not easy to get to.
- is there a pcv system and how is it set up?
Let's keep working on what you have.

[ThickCobra]

Thinking about a replacement carb or efi is way premature. 780 is fine for a 427 especially a vac secondary 780. Based on your 1000 rpm idle I think there is something wrong somewhere that is fixable;
- too much timing advance
- vacuum leak. Are the three black rubber caps in place or have a hose connected to them? look at the pic of the bottom of the carb
https://www.holley.com/products/fuel...arts/HR-780-VS
- idle speed adjustment of the primary and secondary throttle plates as well as the choke high speed idle screw. The secondary adjustment is usually a small screw adjusted w/ an allen on the side where the black choke assembly is. Usually not easy to get to.
- is there a pcv system and how is it set up?
Let's keep working on what you have.

I plan to work thru your checklist of items today. I prefer the process of elimination for solving issues and items I can handle, as carburators have always been foreign to me. Any where l can find an instruction manual for my particular carb as one did not come with it.

[Railroad]

Until you get something on the QF, here is something on the Holley, probably the same, other than the QF having removable air bleeds and easier to tune.
https://www.jegs.com/InstallationIns...10-0-82750.pdf

[mike223]

Any where l can find an instruction manual for my particular carb as one did not come with it.

Try this linked article, pay particular attention to the part about "transfer slots" - as Craig mentioned (quoted below) the position of the primary + secondary throttle plates is a good place to look - if you get too far out on the transfer slots you'll have endless trouble with the idle.

http://www.hotrod.com/articles/ctrp-...uretor-tuning/

- idle speed adjustment of the primary and secondary throttle plates as well as the choke high speed idle screw. The secondary adjustment is usually a small screw adjusted w/ an allen on the side where the black choke assembly is. Usually not easy to get to.

- is there a pcv system and how is it set up?

I could not run a standard PCV - tried an OEM PCV valve for 1966 427 corvette - too much airflow, too big a vacuum leak - would not idle, would not run right.

There are various PCV options, but for now - plug it and get it idling / running right + come back to that later - it is a complication / variable you do not need at this point.


For the record I am running 393w / 750 DP / solid roller cam / T56.

Idles ~ 800 - and it will buck on me @ 1200~1400 rpm.

[ThickCobra]

Craigs,

I missed responding to your pvc set up question. My pvc from the right front valve cover is connected to the rear of the carburetor at the power brake vacuum port. It's the largest port on the carb.

[Gumball]

Craigs,

I missed responding to your pvc set up question. My pvc from the right front valve cover is connected to the rear of the carburetor at the power brake vacuum port. It's the largest port on the carb.

Jay,

I think I may have mentioned this before - so disregard if you found that the current set-up is proper for you. But, when I was working bugs out of my car, one of the things I did was to send my carb out to be blueprinted. I'm running a 650 DP with mechanical secondaries and no choke on a 347. I originally had my PCV set up from the right (passenger) valve cover to the 3/8" port at the bottom rear of my carb. The carburetor shop said that was incorrect, as it was acting as a huge vacuum leak. I have since re-routed the PCV so that it vents into the air cleaner - still pulls a vacuum from the valve cover to assist in scavenging the crankcase, but that vacuum is now before, not at, the carb. The result is that with the blueprinting and the re-routing of the PCV system, the car is running stronger than ever and will pull from down low in all five gears with no bucking or stumbling.

[mike223]

I have since re-routed the PCV so that it vents into the air cleaner - still pulls a vacuum from the valve cover to assist in scavenging the crankcase, but that vacuum is now before, not at, the carb.

Me too - works much better.


I also thought it was generally accepted that a 2500# 400(+)hp car was not a particularly good application for a vacuum secondary carb...

and will pull from down low in all five gears with no bucking or stumbling.

Mine will "pull" from down low just fine - but it doesn't want to grunt along at closed throttle much below 1500rpm.

[nskaats]

Thinking about a replacement carb or efi is way premature. 780 is fine for a 427 especially a vac secondary 780. Based on your 1000 rpm idle I think there is something wrong somewhere that is fixable;
- too much timing advance
- vacuum leak. Are the three black rubber caps in place or have a hose connected to them? look at the pic of the bottom of the carb
https://www.holley.com/products/fuel...arts/HR-780-VS
- idle speed adjustment of the primary and secondary throttle plates as well as the choke high speed idle screw. The secondary adjustment is usually a small screw adjusted w/ an allen on the side where the black choke assembly is. Usually not easy to get to.
- is there a pcv system and how is it set up?
Let's keep working on what you have.

This is absolutely the correct way to start. Diagnose what you have and figure out what needs to be changed. The combination is most likely good, you just have to dial it in properly. Knowing how to use a vacuum gauge and adjusting idle fueling and spark will be the best place to start. Once you have the idle dialed in you can work from there. Your driveability issues definitely sound tuning related to me. I would guess a combination of both fuel and timing are quite a ways off.

Unfortunately taking your car to a "professional" tuner doesn't mean as much as people think it means these days. Everyone with access to YouTube thinks they can tune a car. Most of my customers are bringing me cars that were "professionally" tuned elsewhere. The majority of them that have a dyno strap the car on the rollers, do some wide open throttle pulls to get fuel kind of close, and throw in the same timing they they put in everything else. The ones without a dyno will usually just do some wide open throttle runs on the street and adjust by feel. A few of them know how to read plugs to make adjustments. Very few of them ever touch idle and part throttle, which are the most important areas for a street car. There's an order in which you do things and idle is the starting point.

[ThickCobra]

As a reminder, I have a Quick Fuel HR 780 cm vs (vacuum secondary). It rained most the day so all the adjustments a made will need to wait til tomorrow for test drive. I downloaded some instruction manuals and watched some you tube videos. Oh well, a rookie does what he can.

Chris, I was able to download my carb instructions and it specifically instructed to use the fitting in the back of my carb as the main vacuum port. Believe you me, I would have preferred that rerouting the pcv tube would have solved my issue. I March on.

I began by revisiting the vacuum setting. Final adjustments resulted in 3 of the 4 screws,( 2 on each side) set at 5/8 turn. The 4th was set at 1/2 turn and resulted in a steady vacuum of 14. I read somewhere that the range should be 15 -22. Am I out of spec? And if so, do I need to look for a vacuum leak?

Next, I revisited the timing. Mike Forte had suggested a while back 14° btdc at idle and 34° at 3500 rpm. Talk about a bear to locate and read with everything so tight in the engine bay. I was able to get it to 16 and 36. Should this be a problem.

[Railroad]

You should be able to back your base timing up 2* to attain Mike's recommendation. Just for laughs, when you are able to drive it, block off the big vacuum port you are using for PCV and drive it. PCV valves are specific to engine vacuum. My point, your PCV valve may be a vacuum leak, when it should not be. Mike is on top of this stuff and what he provided should be right, but it does not cost anything to check it. With the timing and idle mixture optimized, you are on your way to getting right.

[ThickCobra]

You should be able to back your base timing up 2* to attain Mike's recommendation. Just for laughs, when you are able to drive it, block off the big vacuum port you are using for PCV and drive it. PCV valves are specific to engine vacuum. My point, your PCV valve may be a vacuum leak, when it should not be. Mike is on top of this stuff and what he provided should be right, but it does not cost anything to check it. With the timing and idle mixture optimized, you are on your way to getting right.

Ok. I'll try it but just for laughs.

[CraigS]

"Next, I revisited the timing. Mike Forte had suggested a while back 14° btdc at idle and 34° at 3500 rpm. Talk about a bear to locate and read with everything so tight in the engine bay. I was able to get it to 16 and 36. Should this be a problem."
Not sure I understand this. You should be able to twist the distributor to get it to zero for instance. Does the dist run out of room? Sometimes the vac pot on the dist contacts something. If that is the case, it is kind of a pain especially if it's your first time doing this. Pop the cap off, set if off out of the way so you can pull the dist out of the block, turn the rotor by one tooth and re-install. Ck youtube for better instructions. 16 and 36 isn't horrible but it is definitely on the high side. The 16 could be why you can't get the idle any lower.

[mike223]

PCV valves are specific to engine vacuum. My point, your PCV valve may be a vacuum leak, when it should not be.

Bingo.

Ok. I'll try it but just for laughs.

Just for laughs - you might want to review the operation of PCV valves.

If for any reason (several possibilities) the PCV valve you're running is not closing to the idle / low flow orifice, you're never going to get it to idle right plumbed the way you have it.


That would be a much bigger variable than 2 degrees of ignition timing.


Here's a detailed report on (yet) another carbureted stroked SBF that could not be made to idle with an off the shelf PCV valve plumbed in the conventional manner, and all the very specific reasons why:

http://www.engineprofessional.com/ar...Q315_34-44.pdf


I'm not saying you need to buy a high performance PCV valve - but plumbing what you have to the biggest vacuum port on the carb may not work for you (it didn't for me).

[mike223]

P.S.

If you're running vacuum advance on the distributor, and if eliminating a PCV vacuum leak increased your vacuum and pulled in several more degrees of vacuum advance...

Then you'd have another variable to watch for.

[Gumball]

Jay,

Building on what mike223 is saying -

The PCV will typically be an open circuit, allowing you to pull vacuum through the crankcase when the engine is running.... sort of acting like a shop vac. But, if you were using that lower port on the back of the carb to pull a vacuum on a closed circuit, like a powerbooster, once it has a vacuum, it will not be pulling air (cfm) through it... it just reaches a negative pressure and stays there as the engine runs.

Hence, a PCV plumbed to that port will be like a massive vacuum leak.... think of it as the difference between a healthy power brake booster plumbed there as opposed to one with a torn diaphram that just keeps letting air through.

[NAZ]

It's so very hard to troubleshoot complex problems over a forum. Folks will provide you with lot's of ideas in an attempt to help. Some will be competing ideas and it's easy to get frustrated. I grew up with carbs and made a living tuning them but hands-on troubleshooting is easy compared to trying to help with tuning problems over an internet connection. The 1000 RPM idle for a modified engine with a big cam is pretty much on par but it sounds like you have a mild motor and when you say it won't idle below 1000 RPM that's a red flag for me. So I'm in the camp that you should be able to tune your way out of this issue without having to throw expensive parts at the problem. The biggest tip I can give you if you're going to tackle this yourself is get a good book on Holley carbs (your QFT carb is a Holley clone) and read it before doing anything else. I've got a few reference books but the one that covers the basics best is one I got back in 1970 or so titled Holley Carburetors by Mike Ulrich. Must be out of print now as a quick a search didn't show any hits but anything from Ulrich dealing with Holley basic functions would be a great start. There is so much confusion about carb tuning that if you are not intimately familiar with carbs a review of the basics will be enlightening. A basic understanding will go a long way in helping you troubleshoot your problem.

[mike223]

I'm not sure I would even call them competing ideas, more like additional variables to watch for - I hadn't even thought about the PCV until CraigS asked about it.

That rang a bell about a previous (potentially) related experience...

[ThickCobra]

Let's recap. My engine will not comfortably idle below 950 rpm s thoroughly warmed up and lurches thru the neighborhood if under 1800 rpm s. Remember, this is a new engine supplied with the distributor, quick fuel carb and mechanical fuel pump. I want it to run well as equiped, as I'm certain others are buying this same set up all day long. Either theirs are running smooth or they are living with issues. Maybe I should have purchased a Coyote.

Update: I confirmed with Mike Forte I have an e303 cam in my 351/427 stroker. He said it should idle at 800 rpm s. Not happening.

Where I am after tinkering today.

Timing is at 34° btdc at 3500 rpm.
Best vacuum at idle is 13 - 14 but not smooth. Is this the best I should expect with a mild cam?
As for the idle, do I trust the speedhut that ffr provided or my 30 year old sears tach/ dwell tester. My tester reads 800 rpm s while my car gauges read 950 rpms. Either way, it's still a rough idle.
I plugged the rear 3/8" carb port and no noticeable difference. Remember, just for laughs.

This is where I am today.

[bobl]

An E303 is extremely mild in a 427. I would expect it to idle very smoothly. Also, the vacuum seems pretty low for that small cam. Since the timing checks out I would highly suspect a vacuum leak somewhere. Any chance you could borrow another carb for a test?