Technical WOT stumble-quick question

I have been trying a few things to get a stumble to go away and came upon this. By plugging the vac advance port and running the dizzy mech only it got better. Not perfect but better. Does this mean the vac canister is bad? It seems to hold vacuum (****ing on hose) but I don't have a pump to test. Its a pertronix flame thrower II with a B22 canister non adjustable. 350 motor, torker intake, comp mag 280 cam. Stumble is only at WOT, I tried diff. squirters and sec opennings. Timing is 14 initial and light springs with 20 limiters so 34 total coming in 1500-2000 Thanks in advance

 

I know it sounds like a dumb question but I'm starting to think the dist. is bad not just the can or at least it may have the wrong can. I need some sleep.

 

Are you sure you're not starving for fuel?
Pic up another distributor and test with it. I bought one with points and cap for $10 at a swap meet and keep it on the shelf just for eliminating possibilities.

 

^ It came with .31 and I tried .35, stumble stayed but it seemed a little better so I went to .37 and it got worse. It was just mentioned to me to try a diff. pump cam and a little more initial timing. I will try this tomorrow and go from there. I do have an extra dizzy but it requires a diff. coil that I do not have. That will be my next move. If the can is OK it might be a pinched wire shorting when the vac opens or moves.??

 

Pete

 

^ I pretty much went through all that with not much effect. I am going to try a different pump cam though. My question/concern is why did the stumble all but vanish when I plugged the vac. advance?

 

Maybe the numbers for your total advance are not what you actually have in real time? Is you vacuum canister adjustable? I went through this same thing and finally got the timing good and then found I also needed to get a little more fuel in.

 

The can is not adjustable but it does seem to hold vac. gotta get me a vacuum tester or pump. I am going to play with the timing some more today so I guess I should be happy that its getting better. Thanks

 

Ok, read it all and have a question or 3. Is the canister going to manifold or ported vacuum? While we all colloquially refer to that part as "vacuum advance" it's really not designed for that. It's a load compensation device designed to reduce timing for rapid throttle operation. Manifold vac drops, timing is retarded a few degrees, as vac returns so does the degree of advance. If you're not on manifold then your basic curve needs some rethinking. You've proven that by disconnecting it. Your engine "likes" the lead (advance) you have w/out being changed by the vacuum reaction, which ever way you have it connected. How's it run with less advance? And does your canister advance or retard? You can answer all of this by simply looking at the direction it pulls the plate. I always go for the basics because they never let you down. Good luck, and I hope this helps.

 

Could be that your advance plate is not returning to home position as fast as your foot hits the floorboard, especially if the stumble is short lived and then recovers.

WOT here doesn't have a relation to Rpm. Is it WOT right off idle, is it wot after climbing the converter then Wot, is it WOT from 60 mph?

 

An easy way to test the can....Push the arm or rod in the same as it would be if vacuum was applied, then put the tip of your finger over the vacuum port and release the rod. If it stays in, the can is good, if it comes out the can is bad.

It will probably move out slightly but not it's full travel if good. When you remove your fingertip it will complete it's travel out.

 

Drunk Man Thanks, did not even think of that.

 

Highlander, it is hooked to the manifold. I'm guessing that the vac advance is advancing -motor speeds up when you hook up tube to port after setting initial timing set. I am at work now so can not check direction. I just realized something I did not mention before. When I plugged the vac. port I did not have to compensate the idle. It ran good right where it was.

 

The WOT stumble happens at all attempts. Stop light in first, 50/60 mph and power braking to get rpm's up.
It recovers when foot comes off pedal slightly. Gradual increase in throtle and motor runs good. Standing next to car in neutral and slapping open and throtle responce is great. Thats what bugged me at first, it seemed it only happened when car was under full load ( changed from 3.40 to 3.80 gears). All the **** I have fooled with has not had much effect untill I tried it without the vac advance hooked up. When I get off work I am Going to change the pump cam and see if it will except a little more initial. I am going to keep vac advance unhooked untill I find out why. I did not have any time left to dial in motor after plugging vac. I will also try the suggestions made here. Thanks, I will let ya'll know.

 

Hmmmm. Thanks Vicky, you got me thinking...

 

"It recovers when foot comes off pedal slightly"

Coming off slightly is restricting the air and matching the Air to fuel. The idea is the match the fuel to the air.

Accelerator pump is running out of squirt or the squirt needs to be prolonged. Prolonging the squirt lessens the fuel thru the pump though, because you need a smaller squiirter to do it. Or a larger pump if there's an option. If you're living off of the squirt there's just not enough fuel.

***uming its a holly.
Going up on the power valve gets fuel quicker but its still limited by the PVRC. Drilling the PVRC and going down in jet size get more fuel under low vac heavy load conditions

 

It's the least I coud do for all the thinking you get me doing

 

Awesome! At least its working for SOMEONE. I have been dealing with the same thing on my truck,I have more thoughts on whats going on here, but I gotta take my kid to the pool. When I get back, I'll bring you into the pm, and we will discuss further.

 

Hi. I am no means a great mechanic but have you actually put a vac*** guage on the secondary side of the canister or to the port that feeds directly to distributor advance? Remember that a canister will hold more volume but will NOT change the inches of vac*** it holds. If you have poor inches of vac*** a cannister will not make it better.

Good luck, Jimmie

 

No problem.

 

Kinda sounds like the vacuum canister has a vacuum leak, that would explain why it runs better when plugged.
As far as the stumble goes, I'd check the antisiphon slug under the squirter. If somebody has left it out at high RPM dribbles of fuel will come out the squirter and leave you with intermittant miss.
Those numbers you said you tried 31, 35 & 37 - are they squirter sizes? You need a special squirter screw that is hollow when you get above a .31 to allow the extra fuel to flow. The existing screw cannot allow any more fuel past it - you can put a .50 squirter in there with minimal effect.

 

So when full vacuum goes into the pod it's advancing the lead even more. This would mean you're at 14 + whatever the travel is from the pod lever. Just pulling a number outta my ***, let's ***ume the total vac advance is 15 + the lead of 14. 29 degrees at idle seems like a lot. Even if it was only 10, 24 seems high too. Used to be that when using manifold vacuum and the pod as the load compensation device initial settings were low. As counter-intuitive as it might seem setting the lead at 4, add vac and it goes up 10 (?), now your lead is 14 again. You stomp the long skinny pedal and fuel need due to low speed is instantly compensated by timing, vac goes up, as does timing, "WWOOOMMBAAA!!!!" She revs good off idle and advances automatically. As revs go up the centrifugal advance joins the party and it gets all it needs. 14 degrees doesn't seem like a lot, but when you're trying cover up a "hole" created by higher lead (piston lower) the fuel requirement can muller up the rest of the tune. I hope this makes sense.

 

Highlander, PM sent.

 

Yes, squirters. I think I am OK without the hollow screw, I checked into it and it really comes into play at .40 and above. Canister seems to hold but I could be wrong.

 

That's one thing I have is good vacuum at idle (18) considering the cam its great.

 

accelerator pump is running out of squirt or the squirt needs to be prolonged. Prolonging the squirt lessens the fuel thru the pump though, because you need a smaller squiirter to do it. Or a larger pump if there's an option. If you're living off of the squirt there's just not enough fuel.


Would a pump cam change prolong the shot? With the same squirter

 

Back to the top for wraymen.

 

Yes it can. The cams also sometimes have 2 sets of holes for just that reason, shooter duration. I answered your PM and hope it helps.

 

Sent some PM's out. I was over thinking and created more of a problem than I had. Decided to go back to starting point, found a dizzy and a matching coil(mine had diff. resistance). Stumble is gone runs better and cooler. Never would have thought dizzy was acting up, damn near new (was in the car when bought) pertronix with igniterII. Canister held vacuum, could be plate, pinched wire etc. One good thing, I learned a lot of good info on set ups and how they react off each other. I also realized to stop and walk away for a while, get a good starting point and change one thing at a time. Thanks again
Just thought about it; I am not knocking Pertronix, I have used them in one form or another on most of my cars and this is the first problem I ever had.