misfire... carb issue??

[Kyle]

i got a fresh rebulit 400 in my truck. everything is new and machined. new valvetrain. everything. got the idle mixture set with a vac guage to max vac at idle. carb is a holley 670 street avenger with vac secondaries, truck idles just fine and will run just fine throughout the rpms with just light throttle. but when i stab the throttle to kick the trans down or get up and go real quick and it stumbles and misfires and wants to stall out and sometimes backfires into the carb.... i can let off the throttle and feather back into it and it will run fine again. it only does it when i stab the throtttle. timing is set to 4 degrees also... am i having an issue with the carb? is it not getting enough? do i need to raise the float levels maybe? is it the valvetrain, do they need to be adjusted now that theyve had all had time to properly seat even tho they dont tick or anything? do i need a lighter or heavier secondary spring maybe? cant figure it out!!  

[rangerwheelinmike]

Accelerator pump?? Might not be giving it enough gas

[Kyle]

Accelerator pump?? Might not be giving it enough gas

checked it. it shoots instantly and shoots a shitload of gas

[Ranger Dave]

if its  backfired,  that blowes the (Accelerator pump)   holly  know for that.

just like these guy stated above.


change  carb, and ill be that goes away. 
or  put new Accelerator pump in that one.

[rangerwheelinmike]

try adjusting the acc pump if its cuting out it needs more gas

[Orangecrush]

check your timing, is the cam after market? cam card will indicate how muchn timing it may need,or fuel delivery issue,

[old school truck dude]

Try 8 degrees on the timing. The power valve is what gets screwed up if it backfires very much. Round thing that screws in inside the bowl. Quick and easy check to see if its blown: Put your finger over the bowl vent tubes and block them off. Motor SHOULD slow down sometimes even die. If it just keeps on runnin the power valve is blown or possibly a bad internal gasket. There are many different kinds that all look pretty much the same so dont just grab any old one and try to use it, get the right one.  

[Kyle]

check your timing, is the cam after market? cam card will indicate how muchn timing it may need,or fuel delivery issue,

its a comp cam. part # 12-422-8. card dont say nothin bout where the timing needs to be for ignition anywheres

[old school truck dude]

This problem really sounds like timing. Set it by ear. With motor idling in nuetral, turn dizzy counter-clockwise till max rpm is reached. Now turn dizzy clockwise slowly till idle begins to drop. STOP. Have buddy put truck in drive and hold the brake. Feather the throttle and see if it pings or bogs and backfires. If it bogs and backfires, turn a little bit counter clockwise. If it pings and spark knocks, turn it a little clockwise. Play with it for a bit till it seems just right, then drive it and make slight adjustments accordingly. Who needs a light? 

[Kyle]

checked if the power valve seal was still good by putting my fingers over the vent tubes. it didnt do it at first but after a minute it started boggin down and startin to stall out. checked the valvetrain one cylinder one cylinder at a time in the firing order and everything seems to be good and not too tight. messed with the advance a lil, bumped it up to 6 degrees and it still did it then droped it down to 2 and it still was backfiring...

although i did find it weird that when i loosened the dist hold down bolt and fired it back up int was running rough even tho when i shut it off and didnt change anything. could i have the wrong clamp or it on the wrong side of the dizzy, or does it even make a difference?

[Orangecrush]

did it say to install 4 degrees advanced??  some are like that.  just a thought..

[Kyle]

did it say to install 4 degrees advanced??  some are like that.  just a thought..

no part i bought gave me any advance timing to set it to. i set it to 4 degrees cuz thats what the stock motors call for so i at least had a starting point to work from

[old school truck dude]

4-6 isnt enough, I dont think.. Try at LEAST 8 if not more. I dont know what the elevation is around here, but try at least 8. Is it possible you have a vacuum leak? If its a motor that has a big cam, I usually get rid of the PCV valve and use breather elements only in its place. Ive found that with some motors they will not develop enough vacuum to operate a standard PCV valve properly and that ends up bein a huge vacuum leak. You CAN buy an aftermarket PCV valve that is designed to operate correctly on lower vacuum if you feel you must have one (for WHAT I say, dont care about emissions!), they come in different types depending on how much vacuum the builder/cam grinder or whoever figures it will develop. Once you change anything from the factory setting, you have to do your math, and you also have to expect to dink around a bit and play with it to see what works best. Put the light back in the drawer n see if you can time it by ear using above ear method and get the huff outta the carb. Ask BigFrank, timing by ear works good sling lotsa mud witht he Rat Phucker. I dont see where you said what kind of ignition system your usin.. Is it HEI??

[Kyle]

4-6 isnt enough, I dont think.. Try at LEAST 8 if not more. I dont know what the elevation is around here, but try at least 8. Is it possible you have a vacuum leak? If its a motor that has a big cam, I usually get rid of the PCV valve and use breather elements only in its place. Ive found that with some motors they will not develop enough vacuum to operate a standard PCV valve properly and that ends up bein a huge vacuum leak. You CAN buy an aftermarket PCV valve that is designed to operate correctly on lower vacuum if you feel you must have one (for WHAT I say, dont care about emissions!), they come in different types depending on how much vacuum the builder/cam grinder or whoever figures it will develop. Once you change anything from the factory setting, you have to do your math, and you also have to expect to dink around a bit and play with it to see what works best. Put the light back in the drawer n see if you can time it by ear using above ear method and get the huff outta the carb. Ask BigFrank, timing by ear works good sling lotsa mud witht he Rat Phucker. I dont see where you said what kind of ignition system your usin.. Is it HEI??

im runnin full msd. ready to run dist with vac advance, 6al box, blaster 2 coil, msd 8.5mm wires and accell shorty plugs

i dont have a vac leak that i know of. had the vac guage on it when we set the carb and it was smoot and it can run at any prm smooth. i dont have any pcv on it. breathers on both sides. for some reason when i was driving it to work when it got to the point where it wanted to sputter a lil bit, bein set at 2 degrees seemed to handle it beter than at 4, and when it was running cold and the choke was on it seemed to not do it at all or as bad

[betterbeaters]

4-6 isnt enough, I dont think.. Try at LEAST 8 if not more. I dont know what the elevation is around here, but try at least 8. Is it possible you have a vacuum leak? If its a motor that has a big cam, I usually get rid of the PCV valve and use breather elements only in its place. Ive found that with some motors they will not develop enough vacuum to operate a standard PCV valve properly and that ends up bein a huge vacuum leak. You CAN buy an aftermarket PCV valve that is designed to operate correctly on lower vacuum if you feel you must have one (for WHAT I say, dont care about emissions!), they come in different types depending on how much vacuum the builder/cam grinder or whoever figures it will develop. Once you change anything from the factory setting, you have to do your math, and you also have to expect to dink around a bit and play with it to see what works best. Put the light back in the drawer n see if you can time it by ear using above ear method and get the huff outta the carb. Ask BigFrank, timing by ear works good sling lotsa mud witht he Rat Phucker. I dont see where you said what kind of ignition system your usin.. Is it HEI??

x2 I think mine was at 12 or more and 36-38 total

[Squirell]

I was always taught 10-12 degrees timing at idle, and 32-36 degrees at 3000 rpm. 6 degrees isn't nearly enough. And NEVER set timing by ear....

[Kyle]

well found out drivin to work that 2 degrees isnt nearly enouh. got the torque convertoer lockup controller so i could lock my converter up again and when it does lockup, it chugs and chugs and almost stalls out.... def gonna try advancing the timing today! the only question is, how do i know if its at 32-36 degrees at 3k when theres no way to see the marker? should i be going by what my msd dizzy box shows for vac curves?

[Squirell]

Why can't you see the marker?

[Cannaert]

You running electric fuel pump or mechanical one?
I it is eletric one I had an issue with my Holley doing the same thing. I had  run a regulator on it drop the PSI down  from 8 to 6 and it stopped the issue.

[Kyle]

You running electric fuel pump or mechanical one?
I it is eletric one I had an issue with my Holley doing the same thing. I had  run a regulator on it drop the PSI down  from 8 to 6 and it stopped the issue.

yea its an electric one. the stock one that comes in the truck actually. they run bout 15 psi to a holley billet hp return style regulator. regulator should be set to 7 psi from the factory but havent had a guage to hook up to it yet to be sure, waitin on the line to get here from summit. the instructions for the carb said its made to run between 5-7psi

[/quote]

Why can't you see the marker?

well i cant see it for 32-36 degrees. its just a stock damper. i dont have degree marks all the way around it