Holley/Demon Carb stumble

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Cudafever

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Its a 850 double pumper carb running on a 12.8:1 compression HYd roller cam 408.
4200 stall and 4.56 gears out back.

Now this stumble started show it ugly head the first of this year. And Every Holley book and YOUtube i have watched say if it stumble at the first of the full throttle stomp. YOU NEED BIGGER SQUIRTERS.
I started out with what can in the carb, which is 31's, then 33 and seamed to fix the problem......
next race had the same stumble problem so i when to a 35 and stumble when a way............
Last race had same problem and when to a 37. problem solved and ran good again. Vary confused but fix the problem so well ok.................

So today i install my 90/10 shock up front and put my GoPro back under the car to see how the 90/10 work.
Guess what!!! Yup the stumble is BACK.
Now one more tidbit before you watch the video. I made sure there was 0 play between the squirter arm and lever. secondary need a slight adj and figured it was going to run even better. But NOOOO

With all of this i don't' think the Squirter is the problem, but what is?????

You can watch the hole video but the "drive and stumble" is about 1.30 into the video
If i roll the throttle in it works just fine. but if i stab it, it stumbles........

 
Does the carb have the plastic cams that change the duration and timing of the pump shot?
 
Look for erratic fuel bowl level and or fuel pressure. Bigger nozzles masked the problem short term. Your motor may need more than a 31 for best 60' but you need to check the fuel system.
 
mderoy340 said:
Look for erratic fuel bowl level and or fuel pressure. Bigger nozzles masked the problem short term. Your motor may need more than a 31 for best 60' but you need to check the fuel system.
Click to expand...
stock squirter was 31, i'm all the way up to 37's front and rear.
 
This is what's in my mind.
every time i when up 2 squirter size, the problem when a way for the day.........Next time......2 more sizes and good for the day.....exc.
Now with that said, whatever the problem is, it requires a heat sook before it fixes it self. i just don't think it the squirter change that fixed my problem.

The problem doesn't go away after it gets warm.......it goes away after i have done several stomp down the track.

If it was fuel pressure i would have ran out of fuel down the track. the fuel bowls are in the middle of the sight glass every time i look at it. fuel pressure is at 9 psi all the time. Does it fall off going down the track????? Maybe but if that was the problem it would once again fall on its face down the track not at the hit.



Here is a question mark in my mind. the factory squirter (Demon carb) is a different shape squirter then the standard holley squirters. here is a picture of them one upside down and the other right side up. To the left and down is the original Demon squirter the pair the right are the squirter that i have been installing (33,35,37)

Demon Squirters.PNG
 
Here is why i don't think adding any more squirter is going to help anything.
Last year i had the exact same combo. EXCEPT i didn't have my 4200 stall. It was only a 2800 stall.
2800 stall SHOULD have required a bigger squirter then the 4200 stall.....Right?
2800 stall was happy with a 31 squirter and the 4200 stall needs 37's.??????
also when i first install the 4200 stall converter, there was no stumble, no need to add more squirter.

My mind goes to the power valve. But if it was blown, it would run rich at idle, or not idle at all. Idle quality has NOT changed.
 
I had the same issues a while back... I had a 950 ProForm that uses the same venture size as the 850. I juggled shooter's, shooters with tubes, cams, and even a 50cc in the rear.
I finally went with a 750 HP I modified. I'm happy now, and I'd rather take a ball peen hammer to the forehead, than do that again !!
our motors are different.. mines a 360, yours a 408, so not a direct comparo, but nevertheless.

How much Ignition lead/total timing are you using? Open spacer, or 4 hole? Cam in the 240s @ .050 ?
 
'63GT said:
I had the same issues a while back... I had a 950 ProForm that uses the same venture size as the 850. I juggled shooter's, shooters with tubes, cams, and even a 50cc in the rear.
I finally went with a 750 HP I modified. I'm happy now, and I'd rather take a ball peen hammer to the forehead, than do that again !!
our motors are different.. mines a 360, yours a 408, so not a direct comparo, but nevertheless.

How much Ignition lead/total timing are you using? Open spacer, or 4 hole? Cam in the 240s @ .050 ?
Click to expand...

I agree. you need to check timing first.
then consider temperature. It will stumble if the engine is cold.
Go back to the stock squirter for now. consider rebuilding the carb. best to know what you have.
Also I had 2 new power valves be defective.took a while to sort.
So, if you have the timing set, engine warmed up,carb gone through, then consider trying different carb tricks.
 
I was running a mighty demon 825 for awhile and frankly got “lost” in tuning it. I took a step back and went back to the stock setting and started over. Like stated above I would go back to the original squirter. It is probably just slamming raw fuel onto the bottom of the intake plenum and puddling. Getting it warm helps it evaporate, so the squirter could not have “fixed” it for a day. Have you tried to delay the pump shot a bit? Looser converter lets it gain rpm easier so it doesn’t need the pump shot to cover the transition until it has enough air velocity to pull fuel.
 
An afr meter would tell you what it needed. I love mine, honestly it was ok to get you close, but you really need to do the old trial and error thing to get the last bit out of the combo. What I really like is if the car flakes out/stumbles during the run, i look at the meter and it helps point you to possible causes. If it shows lean, fuel. If it shows rich ignition.
 
Cudafever said:
Its a 850 double pumper carb running on a 12.8:1 compression HYd roller cam 408.
4200 stall and 4.56 gears out back.

Now this stumble started show it ugly head the first of this year. And Every Holley book and YOUtube i have watched say if it stumble at the first of the full throttle stomp. YOU NEED BIGGER SQUIRTERS.
I started out with what can in the carb, which is 31's, then 33 and seamed to fix the problem......
next race had the same stumble problem so i when to a 35 and stumble when a way............
Last race had same problem and when to a 37. problem solved and ran good again. Vary confused but fix the problem so well ok.................

So today i install my 90/10 shock up front and put my GoPro back under the car to see how the 90/10 work.
Guess what!!! Yup the stumble is BACK.
Now one more tidbit before you watch the video. I made sure there was 0 play between the squirter arm and lever. secondary need a slight adj and figured it was going to run even better. But NOOOO

With all of this i don't' think the Squirter is the problem, but what is?????

You can watch the hole video but the "drive and stumble" is about 1.30 into the video
If i roll the throttle in it works just fine. but if i stab it, it stumbles........

Click to expand...

Your primary shot pump my be going to hell, hence the change in running characteristics.
What cc shot pump do you have? Have you checked the pump diaphragm?
I had to go to a 50cc shot pump on my BB stroker to lose the stumble.
Sounds like it's running out of fuel
 
First off, thanks you all for your response!!!!!!!

I dont think its timing or it would be idling lower......but its on my list to check.
Go back to 31's, which i truly believe that is all i need.
Then i'm going to pull the front bowl off and check the pump diaphragm and spray carb cleaner threw all the orifices.( my gut tells me this is where my problem is................)
While the bowl is off, pull power valve and test it.

I do have a o2 meter, that will record a run...............never been able to make that happen. My o2 sensor is only 4 inches before the end of my header, colector.(header extensions) It don't read acurite at idle so i can't just whack it at idle and see what the o2 says.
Didn't look at it during the test run and i only get a few runs out of it before it quits working(race "leaded" gas) and have to replace the o2 sensor(them suckers are expensive)
I may have fixed the record problem but i haven't had time to load it on my computer to see if it recorded......maybe I will get time to do that tonight.
It is probably just slamming raw fuel onto the bottom of the intake plenum and puddling.
I really like this idea, but i would think the eng would load up afterwards. I kept looking hard for any sign of black smoke in the video but i saw non.
 
also, you might want to try and lower the front fuel float down a little bit. My old car would do the same thing, small block, 4.56's, it would bog. lowered the front float and the problem went away!
 
X2 ^^^ float level should be at the bottom of the sight window, unless the bowls have the over size glass windows.
 
Less of a tendency for fuel to enter the vent on acceleration. Also proper fuel level in emulsion channel of metering block.
 
It's a demon carb, has three notch in the side of the glass center one is in the middle of the sight glass.
I have a 650 holley, agree pull plug out off bowl and bump the fender, fuel dribble out your good, Demon is not this way.
If this fuel level was the problem it would have been the problem 2 years ago when i rebuilt it.
 
I had a similar problem quite a few years back, only occasionally it would flood after the hicup. Made it impossible to race. I took the carb apart and my rear float was not moving as freely as I liked, so I replaced the float and added jet extensions at the same time. No issues ever since.

Edit: Just reread...my issue was not really the same, but I'll leave this here all the same.
 
So i took skrews advice and when back to the original 31 squirtes and and replaced the Pink pump cam for the blue one.
Words grate........NO,.... stumble is still there. check my timing just be sure, still at 38 degrees.

After all of these changes i made sure that there was no play or delay time on pump lever. Just touch the throttle and it starts to squirt.
The direction that the squirter sprays is different with the standard 31 "Demon Squirter" then the Holley ones i bought.
The demon/ original, squirter actually shoot the stream of fuel across the top of the venturis. The Holley squirters shoot it just below the venturis.

normally i would say that this shouldn't matter, But i sure would like to try a bigger Squirter in a Demon design.
I'm can't seam to find any.

Does anyone know where i can find a "DEMON Carb Squirter??????
 
Have you gone through the carb. Demons carbs are notorious for their lack of QC. They always have machine swarf left in them. I used to be the guy that went through them at the engine shop I worked for. Every single new Demon had crap in it, not to mention the build and assembly problems. Had one come through that didn't have emulsion holes drilled in the rear metering block. Have also seen missing jets, air bleeds not drilled through, power valve channels not drilled, and numerous other problems.
You might see if you can borrow a known "good" carb from another racer.
 
skrews said:
Have you gone through the carb. Demons carbs are notorious for their lack of QC. They always have machine swarf left in them. I used to be the guy that went through them at the engine shop I worked for. Every single new Demon had crap in it, not to mention the build and assembly problems. Had one come through that didn't have emulsion holes drilled in the rear metering block. Have also seen missing jets, air bleeds not drilled through, power valve channels not drilled, and numerous other problems.
You might see if you can borrow a known "good" carb from another racer.
Click to expand...

First off Thanks for the link!!!!
This is a used carb that i rebuild several years ago.
Yes i have heard about the problem with Demon carbs including broken drill bit being left in them and still ship it.

The only real problem with this carb(until now) is that it has ford linkage and i had to drill a hole in it to get the throttle linkage to work.
Well last night that kick me right in the butt!! the faster accelerating squirter ramp hit my installed chry pin for the linkage.
couldn't get above high idle. Had to put a smaller nut and grind the foot to make clearance.

I my have to re rebuild the carb. some crap may have got down one of the bleeds or some dumb thing like that.
I do have a good carb for it but it is only a 600 DP that worked just fine before the demon.

I have two week to figure this thing out before the next race. works been crazy and last night was the first time i have had to work on it.
Thanks again! I will be ordering them jets and i might as well order a rebuild kit for it as well.
 
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