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Old 06-12-2006, 08:48 PM
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carb bogging from idle when quickly accelerated

sorry if this has been posted before but i did a search and found nothing. Anyhow here is my problem. I have a SBC 350 with aluminum heads, performer rpm intake, MSD ignition (pro billet), Speed demon 750 CFM carb with vacuum secondarys. THis was dynoed at 407hp and 431TQ. Anyhow My problem is when i put my barry grant on it and then try to tune it i cant seem to get some problems to go away. FIRST i just did the Idle mixture according to: http://chevyhiperformance.com/howto/45638/ I am sitting at around 10 pounds of vacuum?? (not sure of the measurement) at 900RPM. The truck seems to running fine, no pings, no smoke, Not getting hot like before i did this. BUT when i punch the gas it bogs almost to where it kills itself. The timing is set to 15 degrees and i believe total is 35. (cant remember its been awhile) anyhow i was searching online to try to figure this out with no avail. I seen something saying i should get a larger accel diaphragm, but then i read other posts with similar problems and they say since it is a 750CFM on a 350 change the secondary spring?? to make it open slower. all the help you can give will be greatly appreciated. Oh yeah i know the best way is to get a wideband and tune it but i am sitting at already over 14k in this damn thing and i am buying a new house and if i ask for 1 more penny i believe my wife will murder me.

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Old 06-13-2006, 07:28 AM
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Quote:
I seen something saying i should get a larger accel diaphragm, but then i read other posts with similar problems and they say since it is a 750CFM on a 350 change the secondary spring??
If you are positive the MSD is working correctly, the next step will be finding the correct secondary spring. If the secondarys open too soon, the engine will fall all over itself. HOLLEY makes a spring kit which gives you all of the varying tensions.

There is also an adjustable secondary housing available but I do not know for ceratin whether it will fit a Barry Grant.
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Old 06-13-2006, 12:22 PM
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What are your cam specs? For 407 hp, 10" vac seems a little low.....
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Old 06-13-2006, 08:18 PM
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cam is xe274h

that vacuum is at idle. when i punch it, it goes down to 2 then jumps to 20's. I ended up putting #68 jets in there.. the off the trottle response is good now, it doesnt bog, UNLESS you punch it and hold it. then it sputters, backfires a few times and goes.. PLEASE HELP!! I AM PULLING MY HAIR OUT ON THIS THING!!!!!! As for secondary spring, it is the stock non colored which reads it opens at 1800 and is fully open at like 3800. any help would be GREAT!!
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Old 06-13-2006, 10:46 PM
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I've got a 383 with 226/228 @ .50 and it hasn't much vacuum. I have yet to tune the timing and carb though.
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Old 06-13-2006, 10:51 PM
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Had the same problem with my 950cfm Holley 3-bbl on my 354 hemi until I adjusted the squirter cam properly. It should be adjusted so there is ZERO play with the throttle closed. ANY slight movement of the throttle should start the squirters flowing.

If that is adjusted properly, lock the secondaries closed (replace the diaphragm spring w/ a piece of wood or something that won't compress). That will turn it into a 2bbl carb. Should still take off like a banshee on the low end - you don't need the secondaries 'til the top end. If it takes off strong like that it means the spring is too weak and you need to tune it with a stronger one.
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Old 06-13-2006, 11:00 PM
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Quote:
Originally Posted by willys36@aol.com
Had the same problem with my 950cfm Holley 3-bbl on my 354 hemi until I adjusted the squirter cam properly. It should be adjusted so there is ZERO play with the throttle closed. ANY slight movement of the throttle should start the squirters flowing.

If that is adjusted properly, lock the secondaries closed (replace the diaphragm spring w/ a piece of wood or something that won't compress). That will turn it into a 2bbl carb. Should still take off like a banshee on the low end - you don't need the secondaries 'til the top end. If it takes off strong like that it means the spring is too weak and you need to tune it with a stronger one.
as for the squirter cam, there isnt any play in it. i also upgraded the squirter from a 31 to a 35. I read a few carb books and i was seeing that when the engine is under fast hard acceleration the engine vacuum drops, then the power valve opens when it reaches its vacuum. THe power valve assists the engine to provide the fuel when vacuum drops. mine is a 6.5 my idle vacuum is 10. SO my truck must hit 6.5 before the power valve opens to spit more fuel. i am thinking if i go up a size or two so that my engine drops only like 2 pounds of vacuum before the valve opens then it will get fuel faster when i punch it. any ideas if i am in the correct area?
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Old 06-14-2006, 12:04 AM
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ok.. got the power valve changed.. runs good down low. the acceleration is good, but now when you romp on it and hold it accelerates but it pops alot and keeps going.. secondary's need changed now?
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Old 06-14-2006, 08:34 AM
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willy's great points.

i have something to ask. your demon is a 4 corner idle 750(really a 850) correct? and your using a dual plane intake. im curious ,how well does it start in the morning ? and what are your idle screw settings, and secondary plate position ?

are you running a carb spacer over the divided manifold ?

do you have any other 4 barrel carb to try ? i love my demon, its a cool carb. i finally had to replace with a street avenger. i just couldnt spend any more time on crazy tranistional, tip in problems, that happened under certain conditions on the street.

no i believe its boging from too much, ,or not properly mixing the air fuel.

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Old 06-21-2006, 12:47 PM
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Quote:
Originally Posted by spinn
willy's great points.

i have something to ask. your demon is a 4 corner idle 750(really a 850) correct? and your using a dual plane intake. im curious ,how well does it start in the morning ? and what are your idle screw settings, and secondary plate position ?

are you running a carb spacer over the divided manifold ?

do you have any other 4 barrel carb to try ? i love my demon, its a cool carb. i finally had to replace with a street avenger. i just couldnt spend any more time on crazy tranistional, tip in problems, that happened under certain conditions on the street.

no i believe its boging from too much, ,or not properly mixing the air fuel.
my demon is a 750 cfm vacuum secondary. I am using dual plane intake, as for starting in the morning it SUCKS!! I took the choke off cause it was flooding it. as for the set screws... no idea. i took it in to get tuned on monday. i dont have another one to try no. i was told that this carb isnt the correct one for my application. i was told it needs to have a mechanical cause the vacuum on my engine is low due to the cam, so when i gun it, it has a low spot for the vacuum to build to open the secondary. i have a small spacer its around 1/2 inch. its a nitrous plate. but now it is running better. 1 problem was the number 2 spark plug wire was burnt thru by the header. the power valve was replaced with a 8.5 then the main jets decreased from 76 to 73. the secondarys where increased to 88. they tuned it with AF ration and showed me it. it runs good but is lacking the power i thought it would make. the engine made 407hp and 431Tq on the engine dyno, but at the wheels i only ended up with 260HP and 280TQ. the guy said my carb and headers are what is limiting me. oh well. with nitrous ill be close to 400. but i would have liked to be at 330's N/A
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Old 06-21-2006, 03:14 PM
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A mechanical secondary carb will not solve your stalling problem. Nor should it really generate any more top end power. For most street apps, vacuum secondaries are where it's at. What those guys said about not having enough vacuum to open the secondaries is complete BS. When the engine needs the air, the secondaries will open, period. If they're not opening up all the way, use a lighter spring. Basically, run the lightest spring you can without bogging, and your secondaries will open up when needed if you have everything else dialed in.
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Old 06-21-2006, 04:30 PM
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bog

What is your rear gear, converter, cam specs, cc's ?
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Old 06-21-2006, 08:59 PM
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Don't know if this will help but a coustomer brought a verry similar problem to us, turned out to be a stock converter in the trans. he had the complete RPM package we just went with a 3500 stahl. ba-bing! problem solved.
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Old 06-21-2006, 09:01 PM
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kevan

re carb bogging from idle... the fact that it sputters and backfires probably indicates a lean condition i'm having the same problem with my Holley 750 proform body.. i was reading in hotrod magazine where they put 81s in the primary and 88s in the secondaries on a demon 750 double pumper[the engine was a 383 though] good luck
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Old 06-21-2006, 09:47 PM
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Quote:
Originally Posted by tweaked
What is your rear gear, converter, cam specs, cc's ?
cam is xe274h rear end gear is 3.73's converter is 2800 stall.. and CC's.. no idea what that is. as for them telling me it is effecting my engine. they showed me on the dyno graph where they punch it, it goes lean a bit (loses power) then gains. i can post it when i get back to my truck to get it out.
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