jackson 63

I have a 1963 SWC with a 340HP engine. AFB carb. The car starts and idles good. Acceleration is smooth and the car seems to have a lot of power. When I am at a steady RPM in any gear the car jerks. Its not cutting out I don't think, it just seems to jerk. Thanks

426 Hemi

Trailer Hitching from too much advance possibly? A quick test is to unplug the vacuum advance and take it out for a spin.

tbarb

That seems to be a very common problem with the AFB and the 340hp engine, I have read that so many people experience this problem. The high performance camshaft has valve overlap (both valves open at the same time) and with light throttle the high engine vacuum produced pulls exhaust back into the intake manifold diluting the A/F mixture.

My feeling is the fuel has changed a bit from what was available when these engines were engineered and what you are experiencing is a lean surge.

Try the timing to see if it helps but IMO your carburetor idle circuit is too lean, the idle circuit consists of the idle restriction and idle air bleeds. I don't know if the venturi clusters are even available with larger idle restrictions but you can experiment and modify the existing venturi clusters but you better do some homework first.

jbmcrae

Boyan

tbarb

After eliminating the timing and any other tuning issue I could think of I would probably reduce the second idle air bleed by .006 to see if that helps. That would keep the idle circuit active longer plus make the mixture richer. The second air bleed is the hole on top of the venturi cluster, not the brass tube that sticks up, that is the main circuit air bleed.

If the casting is thick enough you could drill/tap and use a 6-32 brass set screw with a smaller hole, at that point if it does not work you can restore the original size hole. You could also J&B weld the hole shut and re drill .006 smaller to try and go back if it does not work but only one thing at a time or you will never know what worked or didn't work.

SupremeDeluxe

This sounds more like a timing issue than a fuel issue.

I would disconnect and plug the vac line to the distributor, test drive and see if there is any change.

You should have the B28 can, I tried a B20 first and the engine does not like that.

Verify the limit busing is in place, and that your advance springs are sufficiently stiff that the advance is not coming in too soon or going too far.

I would be surprised if some combination of above doesn't resolve your issue.

jbmcrae

MikeM

Bad spark plugs will do the same thing.

SupremeDeluxe

A good point and so will bad wires or cap/rotor as well. My car had the original (or near-original) cap on it and combined with lame aftermarket wires it was doing similar.

Zoomin

jackson 63

Drove the car today with the vacuum advance unplugged and the car ran great. Plugged it back in and it started jerking. Not sure if the vacuum advance is bad or if I have the timing too far advanced to start with. I will check the timing and report back.
When I suck on the vacuum hose I can see that the vacuum advance is working. Is there any other things that can go wrong?
Can I drive the car with the vacuum advance unplugged? Does it damage anything?

MikeM

Won't hurt a thing.

Critter1

As others have mentioned, too much timing can result in exactly what your engine is doing.

It's been popular, lately, to add a little too much lead into the system. It's not better.

MikeM

Highly recommened as a matter of fact. It was just a matter of time.................................... ......

SupremeDeluxe

I guess you could, but you are treating the symptoms of a timing advance curve that is messed up, rather than fixing the actual problem

jackson 63

We have a short season here in Minnesota. Just want to drive the car while I get this sorted out. Not sure what a B-28 can is. I buy my parts from Paragon and they do not list a new vacuum advance with that number.

Boyan

Frankie the Fink

Don't.

You will give up a smoother idle, cooler running and improved gas mileage....you just need to remap your timing curve a bit... Not rocket science... Pulling back on the initial timing a couple of degrees could be all that is needed - if it doesn't help then you need to dig a bit deeper...

Donald #31176

B28 VAC is a vacuum control made by Standard Motor Parts. The P/N s are as follow




> > > NAPA VC 1810

> > > AC D1312C

> > > STANDARD VC 177


> > > NIEHOFF DR 305

> > > Borg Warner V329


Wells DV1810

Airtex 4V1053