Stumble under light load: any ignition experts out there?

[Blown4.6]

Well, I'll repost this one. Maybe someone happens to have had the exact same experience and can help. I'm tiring of throwing $$$$$$$$$ at every little problem only to have it not exactly fixed and then throw more $$$ at it. I've got about $60,000 in this one and it's not even perfect yet.

Okay, when pulling from a stop, under light to moderate acceleration, everything is fine from 600-1300 rpm. Then, at 1400 rpm, it stumbles hard. Then, it comes back to life and all is fine from there on. It stumbles so predictably and suddenly, I am almost certain there's just a spark cutoff. If the engine is cold, it will often stall completely. If I accelerate very gradually, it doesn't happen. If I ROMP on it, all is fine with no stumble. This is not the same "stumble" you get when you jump on the throttle and it bogs for a second because it's become too lean for a slplit second. This is a different thing. I'm pretty sure it's spark.

This also will happen on the freeway cruising at say 75 mph. It seems like I just get back in that "stumble zone" where it just wants to die. If I accelerate, it clears up. It also seems to be worse the longer I drive on the freeway .

Now, based on this, it seems like there is just a dead spot which comes on at a specific load and/or maybe vacuum. Here are some of my suspicions:

* Vacuum advance screwing up: unplugged and capped the tube but the problem remains the exact same. That's not it.
* Coil: The coil is an el cheapo from Pep Boys. The original coild cracked just a few weeks ago and I replaced it with a $15 OEM model. The ignition is Pertronix electronic. Could driving with the old cracked coil have damaged the pertronix ignition? Could the new coil not be working with pertronix electronic?? (I don't know if the electronic ignition requires a coil upgrade??)
* Pertronix Ignition: See above. Could it just have a dead spot where no spark is being thrown at all?? It seems to be load or vacuum specific problem and happens even with the vacuum hose disconnected and capped.

Details on this engine:
> 224/230 @.050 cam with 110 LSA
> Deamon 525 cfm carb with vac secondaries and electric choke
> Guages: all new electronic (I only include this b/c I've heard of the electric tach sometimes creating a problem / short in the ignition system)
> Dual plane manifold with 2" spacer
> No vacuum leaks
> Just spent like $500 on a tune from a pro tuner (old hotrod guy here in LA). I don't think he's leaned it out too much. Also, the plugs are nice tan color.

Guys, I'm stumped.

 

[Great68]

Are you running the ignitor off the resistor wire?

 

[12sec67]

Details on this engine:
> 224/230 @.050 cam with 110 LSA
> Deamon 525 cfm carb with vac secondaries and electric choke
> Guages: all new electronic (I only include this b/c I've heard of the electric tach sometimes creating a problem / short in the ignition system)
> Dual plane manifold with 2" spacer
> No vacuum leaks
> Just spent like $500 on a tune from a pro tuner (old hotrod guy here in LA). I don't think he's leaned it out too much. Also, the plugs are nice tan color.

Guys, I'm stumped.

first... you can check the setting on the pertronix. it has to be a certin measurement away from the "black thing" on the distributer. i had a something almost like a stumble and i put the pertronix at a little closer adjustment and it fixed it. the pertronix also didn't last very much longer than a year after that which meeans it could of been going bad.


second...don't think its too lean,if it is lean it will backfire out thru the carb. i think it is too rich. there isn't supposed to be any bog when you romp on it either. he may have tuned it for WOM wide open throttle and not for cruising around.


third.....have you recently added the 2" spacer after the tune? if so it could be making more vacuum then it was previously causing the carb to pull more or less fuel thru the carb.

hope this helps...

Rich

 

[nds03]

my demon would stumble until i put bigger squiters on the accelorater pumps.

 

[ozstang65]

Don't know about the Demon carbs, but i've experienced similar problems with Webbers/Dellortos. The problem could be in the jetting between the idle and the power circuits.

If you can borrow another carby for it, try it on - process of elimination...

Good luck

Edit:- I think if it were spark related it would happen no matter how hard or soft you were driving it.

 

[Blown4.6]

thanks for the help guys. The car's out getting the tranny (c4) rebuilt for the 3rd time in the last 500 miles. When it gets back, my tuner said to bring it to him and he'll fix the problem (why didn't he fix it the last time I handed him $2000? for the new cam??). He thinks it's too lean. The thing is, I think the jets are stock and the demon is supposed to be tuned right out of the box. Also, the plugs are even tan and really clean. Well, I'm no tuner; so, it could be.

I didn't mention that it does sometimes spit back through the carb. My corvette used to do that terribly when it was tuned too lean (it also would hang the idle at 2000 rpm and other classic stuff that this car isn't doing).

Good point about the spark. If the spark is completely cutting off, I can't think of why/how it would be happening only at a certain load. ?? Why I didn't just buy a 400 hp crate 351w I have no idea. It would have cost me much less! and it would run...........

 

[ForceFed70]

Can you get it to happen while the tranny is in neutral or park? If so, take it to a shop and spend the $60 to have them scope it...

 

[Blown4.6]

Can you get it to happen while the tranny is in neutral or park? If so, take it to a shop and spend the $60 to have them scope it...

Nope, it won't do it in neutral. Only pulling away from a dead stop (fairly gradually and stedilly) or cruising on the freeway at 75. Both situations are probably creating about the same: throttle angle, vacuum and load.

Goofy.

 

[thehueypilot]

Could be your power valve is not sized correctly. Power valves are usually the source of light throttle stumble. To size them correctly you need to moniter your vacuum at cruise and idle and size it about two inches (steps) below the worst case of both of these. Example would be if your cruise vacuum going up a steep hill drops to 10 inches and idle sometimes drops to 12 inches I would size the power valve to 8.5 inches. For some reason the carb. companies size their power valves too low which will cause this condition.

 

[Blown4.6]

Could be your power valve is not sized correctly. Power valves are usually the source of light throttle stumble. To size them correctly you need to moniter your vacuum at cruise and idle and size it about two inches (steps) below the worst case of both of these. Example would be if your cruise vacuum going up a steep hill drops to 10 inches and idle sometimes drops to 12 inches I would size the power valve to 8.5 inches. For some reason the carb. companies size their power valves too low which will cause this condition.

Man, that might just be it! I don't see why my tuner allowed me to pay for a "tune" and give my car back to me like this??

That's a good possibility. Thanks