Stumble from low voltage to MSD?

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mellow_utah

mellow_utah

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Hi all, I have a 73 scamp with a 360 from a van, airgap intake, quickfuel carb, headman headers, msd 6al. I have had a stumble issue at low rpm. I had a quickfuell 600 and now a 750, messed with both, new dizzy, messed with timing (I have even taken the vacuum advance off and increased my total timing to compensate) , no matter what I have tried the stumble is still there. The stumble goes away around 1200rpm, having messed multiple times with fuel and timing to no avail I have been brainstorming other possible issues. The other day I drove the car for the first time at night and at an idle my lights are extreamely dim until my rpm rises, I think I have a standard alternator. My question is, could the low output of my alternator at idle have an effect on the MSD box's ability do deliver enough spark causing my stumble? Has anybody had a similar issue?
 
i think bad grounds have caused people problems with msd's in the past.

i would start by making sure that the ground straps to the wiper motor on the firewall, the head to firewall, battery to head, etc all are cleaned and in good contact etc. make sure msd has good ground as well.

with the car off and a meter you can check the ohms between different spots on the msd unit , head, wiper motor bolts, battery neg etc.

p.s. the headlight harness also has it's own grounding point, altenator gets its ground through the attaching bolts if they are rusted clean them up too
 
I dont think so the 6Al is hooked up directly to the battery at 12V. Have you ever done the MSD spark test without the car running? It's quick and easy to do.

I am running a 6AL and thought it was having issues but it turned out to be my amp guage. So I bypassed it.

With car idling what is the voltage at your battery?
 
Thats a good idea on the grounds, the stumble is there day or night, the lights were just an indication to me that the output from the alternator might be low and if so the voltage to the MSD would be low. I dont even know if low voltage from the MSD would cause the stumble but it was a though I had because changing fuel/timing doesnt seem to effect the stumble at all. It also doesnt have the issue when I am in park or neutral and rev it up just in gear. Maybe it becausethe rear end is still the 7.25 " and has like 273 gears ?
 
crj1968 said:
vacuum leak I think
Click to expand...

Doesn't sound electrical to me either.
If it was from low voltage the lights being on would make it worse I think.

Not sure where the 7 1/4 question came from though.
Maybe it was meant for a different thread? :)
 
Oh I thought maybe the ratio would bog it down lol, shooting in the dark lol. I thought maybe vacuum leak but I sprayed carb cleaner around the intake and carb and the rpms never ramped up. I could try it again though
 
Well,.. I gotta say it,,..

Have you visually looked at the way the accelerator pump-shot come out when you open the throttle..???

It should be a instant, strong, continuous squirt of fuell... any hesitation, interuption,, or splutter, in that squirt,, is likely your stumble..

hope it helps
 
yes, the accelerator pump is squirting emeadiatly and steadily through the whole stroke. The car does have power brakes which are very touch in the morning lol. I have a large vacuum purt at the rear of the carb which i have a t-fitting going to the brakes and the pcv
 
You know, speaking of shots in the dark it occured to me that it could be possible that during that low RPM/high vacuum point it could be getting over advanced.

This would be easy to diagnose by pulling the timing back a few degrees and see if the stumble changes, or goes away. (or disconnect the vac advance for a test drive and see if that makes it go away)
Either one should work for testing.

Just another shot in the dark.
 
good thought, i did try that though, currently I dont even have the vacuum advance attached, i detached it and advanced the timing (as it is now) and it still does it. Before I did that though, I did try and back the timing off and advance it both with the original dizzy and a new one, same deal.
 
Maybe a slack timing chain? or off a tooth? It really has me baffled, no matter what adjustment I make it stays the same, ive tryed 3 carbs, two dizzys, all kinds of air/fuel adjustments, timing adjustments, the stumble is still there.
 
Plug the big port and see if it goes away. You won't have PB so be ready. Also, this eliminates the PCV so you can determine if that may be the issue.

If it's a stock 360, set initial timing in the 12-15 area. Check total timing without vacuum, should be in the 34-38 range. If total is higher than 38, the distributor likely needs to be recurved. Also see at which RPM it begins to advance off idle.

Most of the time the issues you have are either timing or carb related. Verify timing is doing what it should first. Then go to the carb
 
ok bear with me, so hook up the light and make sure at idle its 12-15 then give it throttle while watching the timing mark to see when it starts advancing and where it tops out? Is it common to have to re-curve a stock dizzy?
 
I'm going to say a dredded, nasty, mean and evil thing here, but worn or leaking valves can cause this.

Seems you have eliminated pretty much everything else, right.
Comp test that puppy so we can eliminate that to.

Recurving isn't supposed to be a cure for a miss usually, but for better performance.
Besides, changing your timing as you did should have answered that, because it would have smoothed out at some point in the changes.
 
mellow_utah said:
ok bear with me, so hook up the light and make sure at idle its 12-15 then give it throttle while watching the timing mark to see when it starts advancing and where it tops out?
Click to expand...

Yep, if it's stock, the 12-15 range is good. Having the baseline set properly allows the carb to meter fuel as it should in proper ratios. If that is out of whack at idle, you get problems that stack on top of each other as engine rpm increases and different circuits in your carb are accessed.

mellow_utah said:
Is it common to have to re-curve a stock dizzy?
Click to expand...

Ummm, yes... the stock distributors have curves in them to deal with smog/emissions testing requirement. Absolutely pitiful if a good running engine is desired.

Advancing the crap out of it because you disconnected vacuum advance is not a good way of doing things.
 
crackedback said:
Plug the big port and see if it goes away. You won't have PB so be ready. Also, this eliminates the PCV so you can determine if that may be the issue.
Click to expand...

That's a good call also.
I recently had a PCV so bad it made the car smoke.
 
yeah plug off all vacuum stuff you can find and see if it helps. Like crackedback says though; be prepared with the braking !
 
My wife say's she wants me to go do something "usefull" now. :)

Later
 
I have not done a compression check only because I had 22-25in in vacuum at idle (if memory serves) and have no burning oil, but it could be worth buying a comp tester and trying it. So, factory dizzys are bad news, I was told that you could run one without vacuum because the vacuum was added for emmissions/drivability, not true? Any recomendations on aftermarket dizzys? MSD cause I have an MSD ignition box?
 
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