missing, stumbling

[bvergeer]

It is a 1970 390 with a manual tranny. I have replaced all the ignition components and it still stumbles horribly between shift and when decelerating. It seems to idle fine but I also notice a light miss at even 15-1800 rpm. It does seem to be fine under load.

Still has the original intake and 2V carb.

Any suggestions?

Thanks,
BOb

[Leadfoot]

well sitting miles away does not help much but..........

I would look close at the point gap first. then follow it out right to the spark plugs. and hope its just a loose plug wire or something real simple.

if thats all good I would check for a vacuum leak. buy simply cutting some or the air off at the carbs air horn/ or intake hole with your hand if it picks up rpm and runs smoother you got a leak 10:1. likely at the distributors vacuum advance, or carb. ie. the acell pump.

but none of this helps look at the spark plugs one more time whats the color and how thick is the spark, because with what you are saying it runs bad at ish high vacuum conditions, it normally means weak spark at high cylinder pressures. so you may want to check the gap on the plugs likely they are open to far.

also do a balance test, buy stabbing the wire boot at the distributor with a test light. what you are looking for is a even drop in rpm. over all the cylinders . and ya some think it will screw you wires but in the real world
thats minimal unless you stab the live hell right out of it.

any way good luck and happy hunting.

[Dragon]

Shoot the original carb. The shafts are worn it can be rebuilt but will still weird problems.

The clue is the original carb.

[smclaren]

Carby issue, Mate !

[bvergeer]

Points, condenser, plugs, cap, rotor, wires and coil all less than 200 miles on them. Autolite plugs gaped at .035 and points at .017.

It didn't change the issue, it was there before all the ignition stuff was changed and it is there now.

I have a "hardly been used" Barry Grant Road Demon Jr. 625 on the shelf. Guess I'll have to break down and get an intake for it.

Thanks all,
Bob

[bugser]

Yeah, it stumbles between shifts and decel...which to me is the same thing.

[bvergeer]

I replaced the carb (not with the BG) and nothing changed. I sprayed carb cleaner around the carb and spacer gaskets. Didn't hear any difference.

My next guess is a leak around the intake somewhere.

Any other suggestions?

Thanks,
Bob

[Dragon]

I have a stumble I built into my truck. Might give you clues. I set my Holley's Accel pump to not squirt when I am just barely moving the throttle. So a shifts and idle take offs it stumbles just a hair. The increase in mileage 1 mpg. The change in the carb makes for some odd sensations during acceleration. Make sure the Mechanical advance and the vacuum advance returns. In a manual that will cause similar problems Snapping the throttle shut during a shift or decel will make the engine go rich for a few cylinders.

[willowbilly3]

Did you do a compression test? Put a vacuum guage on it? Is the distributor shaft/bushings sloppy? Timing chain stretch? You can't tune an engine that isn't mechanically healthy.
My guess is the distributor (not dizzy) shaft bushings are shot, in which case you cannot maintain the point gap. Also the rubbing block on points goes away real fast. they don't make many of them anymore so the qc sucks. I always can tell when my points are going shut, it stumbles and makes a little backfire on deceleration.

[bvergeer]

Getting Closer.

This turned out to be a 2 part problem. Compression test showed reasonable results ranging from 125 to 140. Willowbilly's guess got part of it, replacing the distributor cleared up the stumble. I just went for a rebuilt single point OEM replacement from the local auto parts store for $40. I'll add a Pertronix module to it some day if I get sick of messing with the points.

I noticed a crack in the drivers side manifold that is probably the cause of the popping between shifts. I found a set of used manifolds that I will get surfaced and that should take care of it.

Thanks all,
Bob