Clutch adjustment needed?

[Steve D.]

Car runs fine at idle and when revved when stopped. All is well when accelerating (smooth, pulls well, no missing) in all gears. But at constant RPM in any gear, it is very "jerky". The experience is similar to the jerking that occurs when going to slow too slow in a higher gear. Since it doesn't seem to be the engine missing, I'm thinking it must be the clutch adjustment. Comments?

Steve

[Joe L.]

Re: Clutch adjustment needed?

Car runs fine at idle and when revved when stopped. All is well when accelerating (smooth, pulls well, no missing) in all gears. But at constant RPM in any gear, it is very "jerky". The experience is similar to the jerking that occurs when going to slow too slow in a higher gear. Since it doesn't seem to be the engine missing, I'm thinking it must be the clutch adjustment. Comments?

Steve

Steve------


That's possibly the cause. Adjust the clutch as per instructions in the factory service manual and you should be able to eliminate this possibility.

Tip: you may very well not be able to achieve the factory-specified free play in the pedal and at the same time achieving complete clutch disengagement at full pedal deflection. What's important is that you have "some" pedal free play.

[Joe C.]

Re: Clutch adjustment needed?

If the clutch was slipping that badly I would think that you'd smell it, and a slipping clutch rarely exhibits symptoms as you describe.

Lacking any further description of the problem, I'd say that it is a misfire, either due to a lean condition (cruise operation is when the mixture is at its leanest), or a marginal plug/wire/coil/points/cap. etc. It's harder to fire a lean mixture and this is when "marginal" conditions usually show up.

Prior to 1965, clutch free play was specified at 3/4" at the pedal, for 1965 that was changed to 1 1/4". These specs are not good for "fast action" setting, which requires less free play.

[Timothy B.]

Re: Clutch adjustment needed?

I agree with Joe about the lean condition. What is the timing and hp of the engine, what carburetor and believe it or not what PCV valve.

Just as a test, plug the pcv at the carburetor inlet, leave the block side open to atmosphere so it will breath and see if the problem stops. This will eliminate one air leak and should make the mixture richer and will give some direction as to the problem being a lean condition.

[John M.]

Re: Clutch adjustment needed?

Had exactly the same problem with a 327-300 with massaged heads.
Idle and accelerated nicely but surged a little at cruise. It was running too lean.
I changed the jets two numbers up, I think, and it was the cure.
JBM 30025

[Domenic T.]

Re: Clutch adjustment needed?

Steve,
I had that problem on a car and fought it to no end, It was a 300/327.

I checked every thing then found that I was running at the vacume it took to operate the vacume advance can on the distributor.

The timing was moving and it made the car jerk which again changed the vacume and it started the next jerk.

I disconnected and plugged the vacume line to the can and found the problem.


DOM

[Duke W.]

Re: Clutch adjustment needed?

Disconnnecting the VAC is not a very elegant way to correct a lean surge problem, which appears to be the case here.

IIRC John increased his jet size by one over OE, which eliminated the lean surge.

In addition to the massaged heads is has the Special 300 HP camshaft, and the two modifications combined to lean out the cruise mixture enough to cause lean surge.

See the Fall 2008 Corvette Restorer for more info on the subject.

It would help if the OP would state the exact model year, engine and transmission option - also whether the configuration is OE or modified in any way.

Duke

[Steve D.]

Re: Clutch adjustment needed?

Mechanical details as requested: 63_327/340_LT-1 cam_Edelbrock carburetor_slightly massaged heads_B-28 can_no PCV valve_Keisler 5 speed transmission

I pulled the vac line at the can and plugged the hose. The described symptoms went away (Thanks for the suggestion Domenic). So now I know it is a lean cruise issue. I recently had the distributor out and may not have gotten it tightened properly. Will recheck the timing to make sure it's not way out and will report back.

PS: I just renewed my NCRS membership, and recouped my dues with this one post.

[Timothy B.]

Re: Clutch adjustment needed?

Steve,

After you recheck dwell and timing warm the car good and reset correct idle speed 800rpm +- and readjust idle emulsion screws. Is this a stock GM camshaft?

Remove the carburetor and double check how much transfer slot exposure is below the throttle blades. You want small squares or even a little less, this will allow correct adjustment of the emulsion screws so the vacuum pulls hard on the curb idle ports. If at that point the idle needs to be increased, a pcv valve may help allow more air for the engine to idle higher and then readjust emulsion screws again. You should have some sort of PCV ventilation on the engine even if it's a fixed oriface hole.

Which cfm Edelbrock carburetor are you running, and can you post a picture of the set up. How much timing are you running in the engine?

[Duke W.]

Re: Clutch adjustment needed?

There are often problems when you install a "generic" carburetor on a specific engine configuration. Massaged heads and a high overlap cam often require a richer cruise mixture than OE heads and a low overlap cam because of exhaust gas dilution, which reduces ignitibility of a lean mixture.

When AFB useage was limited to the 300 HP engine in 1964, it was leaned out somewhat compared to previous models that were used on both 300 and 340 HP engines because the low overlap 300 HP engine didn't need as rich a cruise mixture as the higher overlap 340 HP engine.

Look at the primary jets and metering rods. Edelbrock (and maybe still Federal Mogul) offer alternative jets and rods, but not a big a range as was available in the sixties and seventies.

Look at what's available to richen the mixture incrementally. Do the math on flow area and increase it 3-5 percent to start. If that doesnt' cure the lean surge, increase it another 2-3 percent. This can be very simple if you can richen the cruise mixture with only a change of metering rods.

Hook up the vacuum advance unless you want to suffer hot running and a ten percent loss of around town fuel economy. The proper solution for your problem is to get the carburetor properly tuned for your engine configuration.

If you are running the original '63 distributor, there are also benefits - stronger low end torque and better cruise fuel economy - by quickening the centrifugal advance to just under the detonation limit.

Assuming the carburetor has a provision for a PCV valve as the original AFB did I highly recommend that you implement the '63 design PCV system, then retune the carburetor.

Duke

[Steve D.]

Re: Clutch adjustment needed?

I disconnected the vac can hose for diagnostic purposes only, and will reconnect then work on the timing and craburetor tune.
Thanks alot.

[Joe C.]

Re: Clutch adjustment needed?

I disconnected the vac can hose for diagnostic purposes only, and will reconnect then work on the timing and craburetor tune.
Thanks alot.

Steve,

You have gotten good advice so far from all.

Please........do NOT go into the carburetor yet until you have FIRST checked distributor operation. In my first post I said that the mixture was the fault only if the ignition system had been verified first as being properly set up and in good order. Be sure that there is no carbon buildup within the distributor cap. Check your points dwell and your spark plug gap should be NO MORE THAN .035", with .032" being preferred.

Surge at cruise can also be due to too much spark advance, in addition to being caused by a lean condition. The fact that elimination of the vacuum advance stopped the surging points in that direction. What you must do is this, AFTER YOU HAVE DONE THE ABOVE:

1. Disconnect and plug the vac advance and verify that your total WOT advance (initial plus centrifugal) is no more than 38 degrees, with 36-38 degrees being acceptable. Verify that it is "all in" by about 2500 RPM (see note).

2. Reconnect the vacuum advance and verify that it adds no more than 17 degrees crankshaft timing to the total, such that cruise spark advance is NO MORE THAN 38 + 17, or 55 degrees.

Note: See the attached for baseline specifications. Minor adjustments may be made around them while observing the two criteria that are spelled out above. It is IMPOSSIBLE to recommend the specific timing of your centrifugal advance onset and RPM at which it maximizes unless static compression ratio of your engine is known. Because you are using an LT1 cam, I recommend that you start by using the centrifugal advance "map" of the L76 (all in @ 2350), and delay "all-in" time incrementally by adjusting springs if the engine detonates. I also assume that you burn PON 93 fuel.

[Steve D.]

Re: Clutch adjustment needed?

Tim

The cam is Federal Mogul, supposed to be ground to LT-1 specs. The carburetor is 600 CFM. No points; Petronix unit installed. I'll snap a picture today.

[Steve D.]

Re: Clutch adjustment needed?

Joe

Thanks for the comments. I'll work through the list and report back. One other item: the points have been replaced with a Petronix unit.

[Steve D.]

Re: Clutch adjustment needed?

Tim

Photo attached (I hope).