WOT Stumble with Pinging

[KJMason]

I'm trouble shooting a stumble on my newly rebuilt Qjet (rebuilt with Cliff's kit) and, of course, there are lots of threads on this but I seem to be having a symptom that nobody else mentions, detonation.  When I floor it it'll immediately stumble badly and then I'll get detonation in the stumble.  The stumble isn't momentary, it'll continue until I back off on the throttle a bit at which time the stumble will clear and the pinging will stop.  If I ease the throttle in to full WOT the engine accelerates well with no detonation. 

I've replaced the air valve spring and cam in the rebuild and tightened the spring to 3/4 turn (Cliff's recipe calls for 1/2 to 3/4 turn).  The air valve shows works freely with no signs of any sticking.  Vacuum pull off is pretty slow, about 2 to 3 seconds, and according to the recipe I should be able to shorten that time up but I'm not going to do that until I solve the stumble.  Vacuum advance is plugged for now and full cent advance is set at 26Deg (same as it was before the rebuild). 

I'm going to go ahead and tighten the spring some more but I'm curious if the pinging is a sign that this isn't related as much to the air valve opening speed but perhaps to a secondary fuel circuit issue, either POE, rod size (they're 44) or hangar (J) creating a very lean condition.  BUT, I did not have this problem before the rebuild and didn't change any of that except plugging the tiny holes adjacent to the POE tubes per Cliff's recommendation.  I also increased the size of the fuel inlet (.135) and put in a new needle and float which is set to 1/4 inch.  There were a few other modifications to the idle circuit but, as I said nothing else to the secondary.  Any ideas?

[Cliff Ruggles]

What is the carburetor number?

If things work fine going into it slowly, then you have a transition issue. 

Things that control transition are the pull-off, spring tension, POE system, secondary metering rod height and primary accl pump shot duration......Cliff

[KJMason]

Cliff, carb is 7040562.

I have the original rods (DA/0.044) and hangar (J), these did not change during the rebuild and I didn't change the rod height.  Secondary POE well restriction was increased from .032 to .040, Sec POE restriction stayed at .061 which was already slightly large.  Sec tube restriction is .044.  Fuel inlet seat is increased from 0.120 to 0.135, I installed one of your accelerator pumps and increased the accelerator pump discharges to .0295.  The air valve spring is still at 3/4 turn as I haven't had a chance to work on it.

[Cliff Ruggles]

Nothing mentioned would cause a lean condition or stumble, aside from the secondary airdoor tension too loose, or pull-off too quick....Cliff

[KJMason]

I've been sidelined for a while and just got back to working on this....unsuccessfully as it turns out.  Vacuum pull-off is 3 seconds and I've tried increasing the airdoor tension to one full turn (originally set to 3/4 turn) and also reducing it as far as 1/4 turn.  Car still stumbles badly, if "stumble" is the right term.  Stumble sounds like a momentary problem but if I go past 2/3 throttle it'll bog down and stay bogged down until I let up on the throttle.  As for troubleshooting this, rather than just randomly changing things I'm trying to understand more about what's actually going on in the carburetor and why. Is there a difference in how the engine responds to too much airdoor tension and/or too long a pull-off vs too little tension and/or too short a pull-off?  Generally, all I've seen described is that it'll "hesitate, stumble, or bog" if the adjustments are wrong but not much of a lead on what specifically happens with "too much" or "too little" and what direction I need to go to fix it.  I'd think that if the tension were too high or pull-off too long the car would just have slow the throttle response, maybe hesitate, but then take off as the door opened.  On the other hand, if too little tension or too short a pull-off then the air door would dump a bunch of air in and leaning the mixture causing a bad bog or stumble but even here I'd think that it would clear itself as the air/fuel mixture stabilized.  Mine doesn't seem to clear at all short of reducing throttle.  Right now with the big bog it seems to me the last thing I'd want to do is reduce the pull-off time but I'm not sure what else to try.  Sorry about the long post but I'd like to understand more about what's really happening here.

[429bbf]

lets think about whats going on. you say if you only go 2/3 throttle it works fine . what happens at 2/3 throttle thats right when the rod starts opening the bottom barrels. it seems to me your going lean, ether not enough fuel coming from the poe or maybe we need to lift the needles on the secondary a little higher or faster. i would have to have this in front of me to figure out .it sounds like its not getting enough fuel before the top flaps start to open and raise the needle . it acts like a vacuum leak between the base plate and the fuel bowl in the back. not allowing the fuel to be pulled in the right amount. i had a carb that had an extra hole in the fuel bowl going to the base plate . exiting in the rear venturies . massive vacuum  leak when the secondaries were engaged.acted a lot like your talking about .that old of carb shouldn't have that.i tapped and plugged it and fixed the problem. hth

[von]

Another possibility is a worn plastic cam that lifts the secondary rod hanger when the secondary air valve (flaps) open. If the hanger doesn't raise enough the rods don't get lifted to the smallest (richer) diameter in the jet opening.

[KJMason]

lets think about whats going on. you say if you only go 2/3 throttle it works fine . what happens at 2/3 throttle thats right when the rod starts opening the bottom barrels. it seems to me your going lean, ether not enough fuel coming from the poe or maybe we need to lift the needles on the secondary a little higher or faster. i would have to have this in front of me to figure out .it sounds like its not getting enough fuel before the top flaps start to open and raise the needle . it acts like a vacuum leak between the base plate and the fuel bowl in the back. not allowing the fuel to be pulled in the right amount. i had a carb that had an extra hole in the fuel bowl going to the base plate . exiting in the rear venturies . massive vacuum  leak when the secondaries were engaged.acted a lot like your talking about .that old of carb shouldn't have that.i tapped and plugged it and fixed the problem. hth

I completely rebuilt this with one of Cliff's kits so I'm pretty sure there aren't any extra holes that fit your description.  I tapped and plugged the typical fuel leak sources in the primary and secondary fuel wells.  Now that I think about it, I wonder if I might have set the plugs too deep in the secondary wells.  When I was rebuilding mine I read where someone had inadvertently blocked the wells so I made sure they were clear (even ground the set screws that form the plugs down to make sure they didn't go too deep)  but I suppose I could have gone a little too deep causing a restriction.

Von, I installed a brand new cam during the rebuild but is it possible to install the cam the wrong way?  IIRC, the large lobe on the cam points toward the front of the carb and the rods seem to lift correctly when the air door is opened but maybe I got the cam backward?

I guess I should just pull the carb again and disassemble/reassemble it to make sure I didn't do something stupid.

[429bbf]

just a thought . are your secondary tubes pushed to far into the housing? im sure you checked this on rebuild but sometimes i guy gets in a hurry and misses a few small details .fwiw

[KJMason]

just a thought . are your secondary tubes pushed to far into the housing? im sure you checked this on rebuild but sometimes i guy gets in a hurry and misses a few small details .fwiw

Actually, the tubes were originally pushed in all the way to the pickup tubes so I pulled and trimmed them at an angle per Cliff's book so there's no obstruction there.  I opened the holes into the secondary wells a tiny bit and plugged the very small holes adjacent to the POE tubes in the air horn.  Again, these mods were all detailed in Cliff's book so I don't think they'd cause the problem.  Still stumped here.

[Cliff Ruggles]

The pull-off may need to be a little quicker, as you are seeing no change in setting the air door tension.

I'd also raise the secondary metering rods a bit, as mentioned earlier, to fatten things up as the secondaries first come in.....Cliff