hanktohendrix

Hello,

I have slight/medium bucking on my 64 Coupe with an L76, manual trans.

Bucking appears: Engine warm, low and part load, whole rpm-range and even in coast down (downhill).

Free of bucking: During warm up (choke active), idle, and full load in whole rpm-range.

It felt like wrong ign. timing.
Base timing was O.K. but I supposed a too early timing due to vacuum and/or centrifugal.
So I disconnected the vacuum to get later timing –> no improvement at all. So timing is not too bad.

I have to mention, that I had to replace the ignition unit inside the distributor (the Mellory failed and the new one is an “Ignitor”, or so)
But I think if that is the reason, than it should be there also (and especially) on full throttle, or??

Next I will do a drive with partly closed choke flap to make mixture a bit more rich.
Even if gets better than, what’s to do?
I doubt in main jets ‘cause I have bucking also in very low load and coast mode.
I also thought about “false air” but I have bucking also at higher revs so false air must be massiv...

I’m not quite sure what to do next.
Any tips, experience?

Thanks!
Markus

Matt Gruber

where is the base timing? it sounds too advanced.

Frankie the Fink

Matt beat me to it but 'trailer hitching' is typically too much timing or a carb problem.

If it's a carb issue then what you do depends on what type of carb you run. With a Holley light load/cruise surging can most certainly be caused by main jet size. (I just went through this with a '67 Chevelle).

Was it bucking before your ignition swap out ?

hanktohendrix

Base timing is 10°; vacuum off and 700rpm.
I also supposed in too much advance while driving.
That's why I did a run with disconnected vacuum line to get 'later' timing. But no effect.

I'm not quite sure if it was already before the ignition swap. I can drive the car since August and soon the ign. failed.

Right now I remember:
With the old ignition the coil was supplied directly from the ign.key without using the ballast resistor.
With the swap we used the resistor which seems correct to me.

Maybe that's the cause??

Markus

midyearvette

could be you answered your own question with the diagnosis of a strong engine with the choke working during warm up
after the choke opens then things lean out to normal settings
tells me you have a vacuum leak for starters or debris in some air passages if it is carb related...time to do all the usual checks for vacuum leaks first after timing is verified and check the base gasket, intake gaskets and carb mounting bolts along with manifold bolts or bad pcv hose etc......

Matt Gruber

the pertronix 1181 uses a resistor.

hanktohendrix

It's been a long time...but I have to write down the solution:
After checking or replacing all parts on carb and ignition I gave up :-(
Last year the ignitor petronix failed and I swaped to old school points, and after a while I noticed: the bucking is gone! :-)
I assume the ignitor changed the dwell angle.
From now on I trust in points :-)

Markus

kingwoodvette

Frankie the Fink

MikeM

You've only given it a half hour. Let me take a minute and look up that big four syllable word that was thrown out and see if we can make a little hay off that.

Tampa Jerry

If you are back to points, you should wire in for the ballast resister. Jerry

Frankie the Fink

Go ahead - we'll wait -- and then you'll always have that word in your vocabulary (oops sorry!).... daily speech..