Holley/Demon Carb stumble

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stock 31 squirters back in, Blue Cam installed on primary and secondary. Acks about the same, this is with a warm heat sooked eng,
when you hear the eng stop, that is all i recorded, the 2nt one is the last several throttle stomps. the last segment is in slow motion.
Tell me what you think................

 
IMO I think you have a case of too big of carb. If that's an 850 Mighty Demon it has 1.56 venturi with a 1.75 thottle bore base plate. Essentially the biggest combination Barry Grant offered in a 4150 style carb from what I know. On the flow bench that thing will go 1000+ CFM. That carb is better suited on a 450+ cube motor. Its going to be a finicky temperamental bastard on a 408. You may also be lean as that carb should have 85/93 jets, this could be why you have a stumble.

Du8ljMt.jpg
 
1000cfm wow no this 408 is not needing that much cfm
The reason you my think it's lean is that i race at a 1/8 mile track that live at 6600 feet and air density is usualy beween 8500-9500 foot.
I richen it up when i go to lower elevations. 4400 feet is the lowest i have raced with this carb.
I have a good running car but its only a 600 DP i have remove the choke and ported(cleaned) the throat and installed the button on the butter fly for better air. and it worked real good. but the demon work just a little bit better.....until lately.

Do you see any thing wrong with the spray pattern????
I fill like there is a problem with the front pump.
When the throttle is whacked wide open it seam like it squirts less fuel then when i open it up a little bit slower. Thoughts??

Yes it its the 850 Might Demon
 
One more Note
it has a 4200 stall and 4:56 gears out back pulling a 3860 pound car.
 
skrews said:
IMO I think you have a case of too big of carb. If that's an 850 Mighty Demon it has 1.56 venturi with a 1.75 thottle bore base plate. Essentially the biggest combination Barry Grant offered in a 4150 style carb from what I know. On the flow bench that thing will go 1000+ CFM. That carb is better suited on a 450+ cube motor. Its going to be a finicky temperamental bastard on a 408. You may also be lean as that carb should have 85/93 jets, this could be why you have a stumble.

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Looks like it has too much emulsion as well. .031 is pretty big and there are only two plugged.
 
Are the check valve needles in place under the squirtter nozzles ? IMO the spray pattern doesn't look bad. Things I'd try jet it up to factory spec and re drive it. Spray carb / brake cleaner around look for vacuum leaks. Try other carb, see if it runs good.
 
skrews said:
Are the check valve needles in place under the squirtter nozzles ? IMO the spray pattern doesn't look bad. Things I'd try jet it up to factory spec and re drive it. Spray carb / brake cleaner around look for vacuum leaks. Try other carb, see if it runs good.
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I'm going to pull the front bowl tomorrow to check just that and spay to hole system clean.
Im picturing the red rubber check valve is a little warped or something and leaks back into the bowl under hard stab
 
Emulsion area? help me to understand?
Im guessing Emulsion is air and fuel mixing??????
 
Its the holes in the metering block that intersect the main well channel. If they put too many or too large of emulsion orifices the fuel curve through the rpm range can be wonky. Personally I like Holley's old school calibration 2 E holes .028 in size. You have to have a wide band to see if your AFR is doing weird stuff to get a handle on trying to tune them. Best left alone.
 
skrews said:
Its the holes in the metering block that intersect the main well channel. If they put too many or too large of emulsion orifices the fuel curve through the rpm range can be wonky. Personally I like Holley's old school calibration 2 E holes .028 in size. You have to have a wide band to see if your AFR is doing weird stuff to get a handle on trying to tune them. Best left alone.
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Especially with someone that understand just enough about it to ruin a Carb!:D
Thanks for the explanation! I thought you were talking about the Squirters.:BangHead::)
 
It a little non profit deal in town, called Hypoxia Dragway(Evanston Wy)

Cuda launch.jpg

This is me trying to bet a dragster.
 
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factory jetting was 85/93(Asume sea level tune.
Do you think i'm running that lean?
 
Possibly. 10 jets leaner that box spec is a lot even at your altitude IMO. If fattening it up gets rid of the hack then your on to something.
 
skrews said:
Are the check valve needles in place under the squirtter nozzles ? IMO the spray pattern doesn't look bad. Things I'd try jet it up to factory spec and re drive it. Spray carb / brake cleaner around look for vacuum leaks. Try other carb, see if it runs good.
Click to expand...
Sorry i misread that earlier, yes the check valves under the squirters are in place.
 
Had some honey does today but got out there and pulled the bowls/metering blocks and attacked then with carb spray. blew a tiny black piece of crap out of the pump housing on the front bowl.
Didn't jet it as i wanted to see what would happen. In neutral it is better, out on the road..... stumble is still there but improved a little.(could have been just because it was heat soaked. If i don't get called into work tomorrow, i'm going to jet it up and see what happens.
 
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