HEI Ignition timing and low RPM bucking (manual trans)

[novadude]

Sounds to me like the cam timing has to much overlap for that RPM range you are trying to make in run.

I agree, couple that with the 'springyness' of suspension bushes and engine mounts and the damper springs in the clutch plate and you have a perfect receipe for a bucker!
Vac adv on imtake vac wont help but is not the be all, as the engine loads up the vac drops and so does the vac advance timing- engine drops a little tq and slows down unloading itself from the driveline so vac picks up and so does the timing making some tq and rpm which loads it all up again! Self feeding cycle of bucking. That said if it bucks with the vac NOT connected to manifold vac you're back to what Walter suggested.
Drive with more rpm or slide the clutch when crawling in 1st, cars like that suck to drive.

Interesting you mention this. I was able to fix a bit of "fussy" behavior at low ~1200-1500 rpm by getting away from manifold vacuum and running ported in my manual trans car.

[GRTfast]

Throttle linkage is a deal breaker if it is too quick just off idle.

The lever above the pivot at the foot, is it behind or in front of the firewall?

Where does the rod connect to the carb lever relative to a vertical line above the center of the throttle shaft, forward, straight above or ??

The slowest motion will be obtained with the connection at the carb straight vertical above the throttle shaft.

The opposite end at the firewall, if the lever is straight horizontal from the pivot the motion of the pull rod is nearly vertical for the first several degrees of motion and will give the slowest possible carburetor shaft rotation.

Obviously, a 100% horizontal lever is impractical or impossible, but the general idea is the most gentle off idle range will be obtained with the pull rod connected to the levers closer to horizontal from the pivot at the firewall and more nearly vertical on the carb lever.

Study the throttle linkage on '55 and up Chevy V8, the lever on the firewall end arcs over the engine and almost lays on the intake manifold at idle, so the pull rod pivot is nearly 45 deg. forward of the pedal pivot at the firewall.

Another possibility is if the engine moves relative to the pedal attached to the body. If the engine mounts or the frame flexes the engine motion can pull the throttle.

Thanks, I understand perfectly, and feel stupid for not thinking of the linkage geometry before it was suggested here. That is probably a big part of the issue, I plan to modify it ASAP.

[GRTfast]

Sounds to me like the cam timing has to much overlap for that RPM range you are trying to make in run.

I agree, couple that with the 'springyness' of suspension bushes and engine mounts and the damper springs in the clutch plate and you have a perfect receipe for a bucker!
Vac adv on imtake vac wont help but is not the be all, as the engine loads up the vac drops and so does the vac advance timing- engine drops a little tq and slows down unloading itself from the driveline so vac picks up and so does the timing making some tq and rpm which loads it all up again! Self feeding cycle of bucking. That said if it bucks with the vac NOT connected to manifold vac you're back to what Walter suggested.
Drive with more rpm or slide the clutch when crawling in 1st, cars like that suck to drive.

Interesting you mention this. I was able to fix a bit of "fussy" behavior at low ~1200-1500 rpm by getting away from manifold vacuum and running ported in my manual trans car.

I plan on giving that a try. Can’t hurt anything.

[superpursuit]

Sounds to me like the cam timing has to much overlap for that RPM range you are trying to make it run.

I agree with Walter as well. Your tall rear tyres are also making your final drive ratio very tall which is not helping. With your "cam has a lot of overlap" I doubt you are anywhere near 'on cam'. A lower 1st gear or diff ratio will help.

[cjperformance]

Sounds to me like the cam timing has to much overlap for that RPM range you are trying to make in run.

I agree, couple that with the 'springyness' of suspension bushes and engine mounts and the damper springs in the clutch plate and you have a perfect receipe for a bucker!
Vac adv on imtake vac wont help but is not the be all, as the engine loads up the vac drops and so does the vac advance timing- engine drops a little tq and slows down unloading itself from the driveline so vac picks up and so does the timing making some tq and rpm which loads it all up again! Self feeding cycle of bucking. That said if it bucks with the vac NOT connected to manifold vac you're back to what Walter suggested.
Drive with more rpm or slide the clutch when crawling in 1st, cars like that suck to drive.

Interesting you mention this. I was able to fix a bit of "fussy" behavior at low ~1200-1500 rpm by getting away from manifold vacuum and running ported in my manual trans car.

Exactly! Cammy autos with not enough convertor can suffer a rolling/unsteady idle with excessive rpm drop when put into gear with manifold vac on vac advance. Ported vac advance is pretty foolproof.

[GRTfast]

Sounds to me like the cam timing has to much overlap for that RPM range you are trying to make it run.

I agree with Walter as well. Your tall rear tyres are also making your final drive ratio very tall which is not helping. With your "cam has a lot of overlap" I doubt you are anywhere near 'on cam'. A lower 1st gear or diff ratio will help.

Yeah the gearing was a bit of a comprise. Trying to get a good highway cruising rpm with a 4 speed, and a cammy big block can be tricky. I’m just trying to make it as good as it can be with the current setup.

[GRTfast]

Thanks for the suggestions and conversation everyone. I have some stuff to try. I’ll report back.

[cjperformance]

Just another thing to note is that this bucking is amplified by entire driveling 'springyness'(scientific term!) Simply changing one or 2 things can improve the situation by changing the frequency of the flex through the driveline. This can be as simple as some stiffer/denser bushes in leaf spring eyes or upper link bars, or a leaf spring clamp or radius bar, or different density engine insulator mounts, different weight springs in the clutch driven disc etc. All of these only mask the problem but can make a happy difference to driving comfort.

[GRTfast]

Just another thing to note is that this bucking is amplified by entire driveling 'springyness'(scientific term!) Simply changing one or 2 things can improve the situation by changing the frequency of the flex through the driveline. This can be as simple as some stiffer/denser bushes in leaf spring eyes or upper link bars, or a leaf spring clamp or radius bar, or different density engine insulator mounts, different weight springs in the clutch driven disc etc. All of these only mask the problem but can make a happy difference to driving comfort.

Yeah, the entire driveline has some composite stiffness, and consequently some natural frequency. The car is pretty solid. All the suspension linkages are 3/4 steel on steel rod ends. Engine and tranny are mounted in some crazy stiff steel belted rubber pads I made from industrial conveyor belt (way stiffer than most rubber or urethane isolators). I think my issues are a combination of:

Gearing
Cam
Ignition Timing (maybe)
Non optimized throttle linkage geometry (opens carb to fast off idle with small pedal throw)

Gonna play with the linkage and the timing. See how it goes.

[Walter R. Malik]

Some people are all about Wide Open Throttle horsepower and preach tight lobe separations with a lot of overlap, totally forgetting, (or not caring), that normal STREET cars run most of the time at PART throttle openings and need quite a bit less overlap.

EVERYTHING is a compromise and getting the right one is the object.

[blown265]

I noticed a significant improvement in drivability when I opened the love separation and reduced the overlap on a cam designed for a supercharged application. Cam duration and lift remained similar ((240@50* / 560 lift SFT) but the LSA was widened to 114*.
The car went from being uncomfortable at low rpm in parking lots/ heavy traffic etc, to driving almost like a stocker. Ignition timing and AFRs played a part, but the largest change came from a reduction in overlap.
Regards
Paulie

[cjperformance]

Totally, that narrow lsa is fantastic at WOT and rpm with no street exhaust etc. But wider lsa and or less duration just makes things way more pleasant at low and part throttle and 90% of STREET driving.

[Geoff2]

This problem will be the same with ported or man vac adv. The vac adv method is not the problem. This is because the problem occurs in 1st & 2nd gear at 1500 rpm, when either man or ported VA would be activated [ throttles blades open enough to engage ported VA ].

The 11" of vacuum is the clue. Assuming no air leaks/correct tuning & 11" is truly representative, then the cam has a lot overlap. A lot of overlap is at some point at lower rpms, going to cause surging/bucking; nature of the beast. Not helping is the light flywheel.

There are some things you can do to help the problem, but you may never totally be rid of the surging.

Do NOT use ported vac adv. Waste of time. Use Man vac Adv [ MVA ]. Above idle, they act the same anyway & that is where your problem is. MVA has benefits for idle quality, vacuum & engine cooling.

Use an adjustable vac adv [ Crane is best ] & adjust the Allen key [ AK ] so that timing is steady at idle. Start with AK fully CW, adjust 2 turns CCW & re-check timing. Keep doing this [ 2 turns CCW ] until timing drops off, then go CW 3 turns, timing steady.

You need AT LEAST 30* of idle timing & 40* would probably be better. The surging is caused my misfire, the mixture is leaned out from the cam overlap & is burning poorly. Lean mixtures need more time to burn, hence more ign timing at low speeds.

Finally, you will probably have to add more off idle fuel. This means increasing the size of the Idle Down Channel, which controls the volume of fuel & air delivered to the transfer slots, which is the cct that is activated. Increasing IFR may help also.

More on MVA benefits, scroll down to post #6.

www.hotrodders.com/forum/vacuum-advance ... 47495.html

[RevTheory]

We had this issue on a turbo C5 Vette years ago after installing a Lingenfelter LS3 with a Comp hyd/roller that was something like 250/2?? on a 114. STS turbos got it sorted out with the tune.

I know that probably doesn't give you any direction but ours wasn't resolved with changing some other mechanical part; it was hiding in the in the fuel injection/ignition map.

That car would barely let you idle in 1st gear without wanting to buck and jump. You had to either feather the clutch or start accelerating. I wish I could tell you what they found in the f/i program that fixed it.

[In-Tech]

Lean and low timing will definitely aid with the bucking. A lot easier to accomplish with EFI, not impossible with carb and dizzy just more time consuming.