ITS ALIVE!!! but it has a stumble, help!

[The4ork]

i took it out this weekend for some extensive carb tuning. with permission from the airport manager i was allowed to run up/down the taxi-ways at a local seldomly used airport.

initial timing is at 12* total at 32* no vac port
road demon 625


im having a stumble hesitation issue. when i am cruising at low -mid rpm and i increase the throttle to change speed say from 40 to 50... my wideband gauge spikes to 17:1 the engine boggs and hesitates, for a second or two and then it kicks back in and its normal again.

it also does this when i take off from a start it has a little bog then goes...

here is what ive tried so far....

with 65 jets in the primary side i tried upping from a 30cc accel pump to a 50cc pump - worse throttle response and problem still the same. so i went back to a 30cc

then i tried plugging the power valve and maybe the power valve seems to be the problem. still just as bad everything else runs roughly the same on the same size 65 jets

the other thing im noticing is it only seems to happen around 10-15 vacum, at all other vacums the engine runs fine. so maybe its a vacum thing?

im normally cruising or idling with about 20" vac, and at wot its around 5" but the 10-15" range the thing is just dry as a bone lean!

the other thing that i cannot figure out is arent you suposed to jet up 7-10 sizes when plugging the power valve.... but i had the same exact AFR as before the power valve. and im running the standard 6,5 power valve.

sigh... it runs like a raped ape at WOT tho



i had 2 diff guys at 2 diff hot rod shops tell me not to go less than a 65 jet on this setup even though its still rich 11:1 AFR at cruise, and 10:1 at wot "your just putting a bandaid on another problem" and both told me to plug the power valve and jet up

and yes, before you ask, my floats are at the correct level!

[torredcuda]

Have you tried different power valves?Go to a 5.5 or a 7.5 and see how it responds.

[brads70]

What about a smaller squiter and/or a less agressive squirter cam?

[The4ork]

It's no that the squirted is making it too fat, the problem is it spikes lean, but with the bigger accel pump it spikes rich then lean. It's a transitional period between the two cruise throttle positions that needs the attention(lean)

[torredcuda]

That should be the power valve area.

[74CudaDave]

That should be the power valve area.

 

Is the power valve new? Could it be blown out due to a backfire? What's your vacuum at idle?

[The4ork]

the power valve is new, and i just checked it, i have the carb off right now to double check my transfer slots were square and ill put it back on when i figure out what to do next....

its a 6.5 new power valve and my vac at idle is well... when i have it idling high and smooth (about 1000rpm) its about 20" when its idling low and a little rough (cam) it has about 18"

so my idle vac is between 18" and 20" does that mean i need like a 8.5 or 9.5 power valve? only sizes i can find locally are 6.5 only or a block off

[torredcuda]

Off the Holley site-
QUESTION How do I tell what size power valve I need?

ANSWER To properly size a power valve, take a vacuum reading at idle and if it is above 12" for a standard transmission a 6.5" will be safe to use. For automatic transmissions take a vacuum reading in gear at idle and if the vacuum is below 12" divide that in half for proper size. Example 9" of vacuum in gear at idle will require a 4.5" power valve.

-but every engine is different and it is only a starting point.Try going up to a 7.5 and see what happens.

[Chryco Psycho]

SO it spikes waay lean when you jump on the throttle , the pump is not the problem , the squirter is where the problem is , there will be a number on the squirter probably 25 or 28 , try going to a 32 or 35 squirter . The pump is delivering enough fuel with both  the the 30 & 50 cc pump , you just need a bigger squirter to get the squirt in faster to stop it going lean .

[The4ork]

SO it spikes waay lean when you jump on the throttle , the pump is not the problem , the squirter is where the problem is , there will be a number on the squirter probably 25 or 28 , try going to a 32 or 35 squirter . The pump is delivering enough fuel with both  the the 30 & 50 cc pump , you just need a bigger squirter to get the squirt in faster to stop it going lean .

It has 31 squirters in it, I found a 37 off an older holly in the garage I'm gonna try tomorrow!

Also I bought a cam kit and a secondary spring kit for use later on if needed. It's nice to have it running nicely

[The4ork]

well i put the 37 squirters in it which seemed to get rid of 90% of the stumble, and i bought a cam kit for the accell pump so ill play around with those and hopefully get the other 10%

i took the carb out and decided to take it apart and give everything a once over, i left the secondary bowl and metering block in tact, and took the primary side off. i gave it a wipe down with some carb cleaner and a lint free rag and didnt find anything out of the ordinary, and i also noticed that my transition slots made perfect squares, which was nice to see since the car was idling nearly perfect
it has always been a fat carb tho, all through the ranges

i put the carb back on with new bowl and metering block gaskets and went down 3 jet sizes to a 65 since it was always fat on gas (12:1 at part/cruise/wot)

now here is the odd thing. i started it back up and its really rich at idle, and now i screw the screws all the way in and the engine doesnt die, and im not seeing any fuel coming out of the boosters either, and i hadnt touched the fast idle screw, and my floats werent touched but to be sure i lowered them both a tad, and then some more, well below normal, and tried lowering to fuel pressure to 4-5psi and its still rich

i decided to check the timing, and its good at 12*

i shut the car off and gave the air bleeds a spray of carb cleaner, that did nothing. so i took the metering block out again and checked the power valve, its seated correctly and not blown and everything looks normal and the float is moving freely.


sigh... im giving up for the day... im just so frustrated!

[joelson6]

never mentioned what RPM the engine is idling at. if it's too high, you'll be out of the idle circuit and the A/F screws won't do anything.

[The4ork]

850

[brads70]

Opening up the holes in the butterfies might help.

[The4ork]

i thought about drilling the holes bigger in the butterflies, but im afraid if i go any larger my idle will be uncontrollably too high. as it sits now, both the primaries and secondaries are closed to the point i have "squared" transition slots showing.

and if i open the holes ill have more air, which will bring the idle up, plus if it leans it will bring the idle up. i have no way to lower the idle at that point.

so as a recap, i have squared throttle plates (transitions) with 1/8" holes drilled in each plate, im 10-11:1 at idle, and if i lean out (which i cant right now without some sort of major change to the carb) my idle will raise, if its 1000 or below i will not complain but if it raises higher than that i think im SOL

oh and the car is pretty much set to factory minus the drill holes right now. but the mixture screws do nothing. i have no idea where the engine is getting its fuel from if the mixture screws are closed shutting off the idle circuit and nothing is coming out of the boosters, and yesterday i even tried a power valve block-off to rule out the possibility of it being the culprit