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hey guys,

im having some pretty big problems with a carb that i had made for me. Its a 750hp carb and i have it on top of a mild 340 mopar engine.

My problem is getting the car to stay alive. Before the lowest i could get it to idle was 1200 (much too high for a street car...with the old carb it idled down by 700) so the guy i bought it from told me to close the secondaries all the way at idle, which has seemed to help the problem...However, now even when warm the car wants to die, especially after i rev it up, it just stumbles like crazy and shuts down unless i throttle it...When adjusting the 4 corner idle, unless i have the driver side primary screw turned in all the way, the stumbling gets worse...as of right now the transfer slot is not exposed, and i was told by the builder that in order to fix this problem i would have to drill bigger holes in the blades until the problem stops and i get correct idle...i dont know how well this solution would work and id rather not drill any holes

The timing is set at 15 initial, 36 total, with 13 inches of vacuum..The car is also backfiring quite a bit (thinking i might have bad valves, but im hoping its just a carb issue)...its got 10.5:1 pistons, but im running 89 octane and the cam specs out 230 duration at .50, 480 lift (single pattern)...There is no pinging, but im not sure if the backfire is a sign of the gas not being high enough octance...

Any help anyone can provide i would really appreciate...the car is running like a total bag of crap right now, and its pissing me off since this carb was supposed to be a set the idle and go kinda deal (though nothing really ever is in this hobby).

thanks!
Matthew
 

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Pull the front bowl off and blow carb or brake cleaner through the idle air bleeds, and through the passages in the metering block with the mixture screws out. The transition slots should show like little squares, or about .020. After reassembling turn all 4 mixture screw out 1 turn from all the way in. When adjusting them, turn them all in or out an equal amount, 1/4 turn at a time in until it starts running too lean/dropping idle speed, then you can fine tune from there. I would also try more initial timing (keeping total the same), it will improve your idle vacuum and idle quality as well as make it more responsive. It should clear your backfiring issues. Try 20 to start, if it likes it go a little more. How much converter stall do you have? I used to run a similar cam in our 400 SBC's, with a 4000 stall 10" converter it ran best with the timing locked.
 

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Sounds like dirt or debri in the neddle and seat in the primary bowl causing fuel to run over in the engine. Take it apart and clean it and make sure the floats are set low enough to keep it from overflowing. Then set it up as stated above and try again.
 

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Sounds like you got a real mess on your hands. When you said 'drill the holes bigger in the throttle blades', are there holes existing? If so, warning bells are going off. As far as i recall the only holley 750 with holes and 4 corner idle had annular boosters - does yours have annular boosters? You made no mention of them, the reason i am asking this is i am thinking that somebody has been inside the carb fiddling with it and sold it because of mechanical problem/mismatch of the inside goodies. I (as well as the guys that replied above) do a ton of carbs, i have heard hundreds of guys bring me a carb to sell and say 'this one ran perfect' - well, why is it in your hand? Believe me, nobody takes a perfect carb off thier car. Nobody. Once they get a carb sorted out and jetted you couldn't get it off that car with a stick of dynamite!
I'd double check the valve issue you are concerned about, no carb will act right if you have valve or ignition problems, and if that stuff is ok it is time to take the carb apart for a looksee. Good luck, oj
 

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Discussion Starter #5
Pull the front bowl off and blow carb or brake cleaner through the idle air bleeds, and through the passages in the metering block with the mixture screws out. The transition slots should show like little squares, or about .020. After reassembling turn all 4 mixture screw out 1 turn from all the way in. When adjusting them, turn them all in or out an equal amount, 1/4 turn at a time in until it starts running too lean/dropping idle speed, then you can fine tune from there. I would also try more initial timing (keeping total the same), it will improve your idle vacuum and idle quality as well as make it more responsive. It should clear your backfiring issues. Try 20 to start, if it likes it go a little more. How much converter stall do you have? I used to run a similar cam in our 400 SBC's, with a 4000 stall 10" converter it ran best with the timing locked.
As far as increasing the timing goes, is the 15 initial going to really hurt me? i have a stock style mopar performance electronic dizzy and i dont believe i can go any higher on the initial unless i modify it. The car has no converter, it is a manual transmission. I have a distributor that i could run the timing locked out on (i have the start retard option on my mallory 685 box) but id like to keep the vacuum advance for cruising, plus i have heard that starting a car with locked out timing puts a strain on the flywheel and starter, killing the teeth (not sure how true this is?)
 

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Sounds like you got a real mess on your hands. When you said 'drill the holes bigger in the throttle blades', are there holes existing? If so, warning bells are going off. As far as i recall the only holley 750 with holes and 4 corner idle had annular boosters - does yours have annular boosters? You made no mention of them, the reason i am asking this is i am thinking that somebody has been inside the carb fiddling with it and sold it because of mechanical problem/mismatch of the inside goodies. I (as well as the guys that replied above) do a ton of carbs, i have heard hundreds of guys bring me a carb to sell and say 'this one ran perfect' - well, why is it in your hand? Believe me, nobody takes a perfect carb off thier car. Nobody. Once they get a carb sorted out and jetted you couldn't get it off that car with a stick of dynamite!
I'd double check the valve issue you are concerned about, no carb will act right if you have valve or ignition problems, and if that stuff is ok it is time to take the carb apart for a looksee. Good luck, oj
its funny you say this about the blades, as my dad said the same thing, as well as another guy where i posted this question...now its got me pissed as im trying to get a hold of the builder to see why he did this and for other solutions...in my opinion drilling holes like he said is a band aid solution, but this one already has them! as far as the ignition goes, its all working fine...i dont think i have valve issues, but i know its a possibility if the car is backfiring.

im hoping he gets back to me in the next few days, and then i can go from there on all of the advice you guys have all given...thanks so far guys! any other suggestions are appreciated!
 

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its funny you say this about the blades, as my dad said the same thing, as well as another guy where i posted this question...now its got me pissed as im trying to get a hold of the builder to see why he did this and for other solutions...in my opinion drilling holes like he said is a band aid solution, but this one already has them! as far as the ignition goes, its all working fine...i dont think i have valve issues, but i know its a possibility if the car is backfiring.

im hoping he gets back to me in the next few days, and then i can go from there on all of the advice you guys have all given...thanks so far guys! any other suggestions are appreciated!

The reason that you drill holes in the blades is that on some larger motors with smaller carberators you would have to open the throttle blades so far to let the engine breath at idle that you would uncover the transfer slots and cause an off idle stumble when you just eased on the throttle durring daily driving. The holes simply let you regain the proper position on the blades for increased drivability. The newer Demon carbs solved this problem by installing a large air bleed in the base plate between the venturies so that air could get around the throttle blades.
 

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15˚ is not enough. I'm not familiar with your distributor, but you need to limit the amount of mechanical advance. 20 to 25 initial would be more appropriate, and total where you have it at 36 is probably close WITHOUT the vacuum advance. On GM distributors that is done with the combination of weights, bushings and light springs to get the curve in sooner. Reading new plugs after a run will tell you where the total timing needs to be. The vacuum advance will add more timing on top of that under cruising conditions for the street, and a little is OK but with a performance cam you shouldn't need too much there either.
 

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Discussion Starter #9
i talked to my carb guy and he thought there was a blockage in the circuit near the problematic idle screw...well i took the carb apart today, blew air through all the passages and put it back together. Set the 4 corner idle screws each at 1.5 turns out and fired the car up...started right up!!! and adjusted from there! the car is running strong now, and doesnt stumble off the revs either, but i do have a slight vacuum leak however, and the lower the rpm at idle the worse it gets. It seems to be coming from the throttle shafts as they did have some play to them when i had the carb apart, and when the car is running if i push up on them to snug them the whistling goes away...whats the fix for this? new baseplate?

as far as the timing goes, i know it could go up more, but in order to this i have to weld up the slots in the distributor to increase the mechanical advance...the car is running good right now, but i will eventually get around to doing this...it is a street/strip car and seems to running fine right now on 89 octane, so i dont really want to go much higher (94 is tops around here)
 

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BLP sells a bushing kit but is not the easiest of tasks to do, they also carry stock and billet throttle plates.
 

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www.blp.com

Anything you need for a Holley from factory parts to custom billet parts. One of the best belt drive pumps available. And a few other goodies.
 

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That is only a replacement shaft, most likely the baseplate is what is worn and what the bushing kit is for. Or a new baseplate.
 

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Discussion Starter #16
That is only a replacement shaft, most likely the baseplate is what is worn and what the bushing kit is for. Or a new baseplate.
yea, i saw the bushing kits and figured that the machine work involved might be a little much. May be just better off buying a whole new baseplate...I wanted to stay away from that first because it costs more, but also because i have heard stories of guys going out and buying billet baseplates for their carbs, and then never being able to get them to run correctly...im just paranoid that with my luck, i will be one of those guys
 

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Probably trying to use an 850 base plat on a 750 WITHOUT making the right mods. BLP makes their billet stuff in house, NOT China knockoff. If you want to step up to the 850 base, tell them it's for a 750 and they can modify it for you.
 

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Probably trying to use an 850 base plat on a 750 WITHOUT making the right mods. BLP makes their billet stuff in house, NOT China knockoff. If you want to step up to the 850 base, tell them it's for a 750 and they can modify it for you.
Your probably right. Im gonna look into this further and probably order a base pretty soon since i want the car running right when i put it on the road
 
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