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Post by an171 on Apr 3, 2012 21:39:18 GMT -5
So I pretty much got my car all done for this year except my motor just isn't running right. The motor is an 89 318 w/ an edelbrock aluminum intake. I am running a holley 4412 carb and points dist. Timing is at 8* btdc. It runs like a raped ape sitting there idling, and revs up flawlessly. The problem is when I put it in gear and hammer on it it takes off perfectly. If I let off the gas and hammer on the brakes it dies. The float level on the carb is set so the fuel is just below the site screw. I am at witts end trying to figure this out so any and all opinions would be greatly appreciated. Thanks
Andy
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Post by redneckracing12 on Apr 3, 2012 22:22:16 GMT -5
maybe try turning up your idle a little bit
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Post by carson29 on Apr 3, 2012 22:37:56 GMT -5
I had the same thing happen to me last year. Has your carb been rebuilt lately?
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Post by nbr1derbydriver on Apr 3, 2012 22:39:37 GMT -5
are you running the brake booster ?plug the vacuum line and try it ? ive had bad booster do that ?
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Post by an171 on Apr 4, 2012 6:02:00 GMT -5
carb was rebuilt last year, and I was having the same problems then, I just limped it around. This year it is going to a show I can't limp it around. No brake booster. All vacume lines are plugged. Idle is plenty high. Thanks for the suggestions!
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Post by Johnny L on Apr 4, 2012 7:25:50 GMT -5
I had a problem with a holley 4bbl that would act the same way when I got hard on the brakes. Even though I thought the float level was set right I re-adjusted it all the way down, then all the way up, and finally back to the middle of the sight glasses and it took care of that problem. I don't know exactly what the issue with the float was but adjusting it from all the way down to all the way up and back to the middle again seemed to recalibrate the float position....or I just got lucky lol. Worst case scenario, try to get another carb on there and if the problem still exists then it has to be timing or a vacuum issue...
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Post by Comet Cyclone on Apr 4, 2012 7:58:09 GMT -5
idle air fuel mix adjustment screws could be off.Tune them by adjusting the motors vaccum.
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Post by dm440c on Apr 4, 2012 10:50:24 GMT -5
idle air fuel mix adjustment screws could be off.Tune them by adjusting the motors vaccum. ^^^^ based on what you described, this is what I would go to next. If you could get a sniffer on the exhaust to read air/fuel ratio I'd bet you're seeing a massive lean out under the conditions you described. Carbs have several different internal circuits for managing air and fuel flow for changing conditions. One challenging condition is what you described: the engine is spun up and under power then suddenly the throttle slams shut... there is a column of air moving through the carb and drawing fuel with it that suddenly is blocked off, yet the engine is still spinning and attempting to keep that column flowing. Carbs have special circuits designed to deal with this but it sounds like your carb is losing the air signal and starving out momentarily for fuel. Try adjusting the air bleeds at idle and see if you can improve on it. Another cause for this behavior is a vacuum leak (same idea- air demand is being filled by air that is not passing through the carb venturi which disturbs the fuel flow). Another good suggestion already given above is to try a different carb... it doesn't solve the problem but it can give you a clue about what components to zero in on. Still, I doubt your problem has anything to do with the ignition, it sounds like fuel starvation to me. |
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Post by an171 on Apr 4, 2012 11:28:25 GMT -5
Ok, that seems pretty complicated, I just want this thing to run! I'll add this, right before it dies, when jamming on the brakes, a bunch of black smoke comes out of the exhaust, which makes me think it is flooding out. To try to fix that we changed jets, it said they were 73s in it, but they may have been drilled. We ended up putting 69 in it, but didn't change anything. Also when I open up the sight screw for the float, the idle raises about 100 rpm, don't know if it's suppose to do that, just trying to provide as much info as possible. Once again all input is greatly appreciated!
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Post by dave386 on Apr 4, 2012 12:16:21 GMT -5
Ok, that seems pretty complicated, I just want this thing to run! I'll add this, right before it dies, when jamming on the brakes, a bunch of black smoke comes out of the exhaust, which makes me think it is flooding out. To try to fix that we changed jets, it said they were 73s in it, but they may have been drilled. We ended up putting 69 in it, but didn't change anything. Also when I open up the sight screw for the float, the idle raises about 100 rpm, don't know if it's suppose to do that, just trying to provide as much info as possible. Once again all input is greatly appreciated! Do you have the bowl vent plugged or something? The idle shouldnt raise from opening the sight plug. |
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Post by ZTR09 on Apr 4, 2012 14:24:19 GMT -5
rev it 6g and stick a clean rag over the carb still it almost about to die and pull it off do it 2 more times then see what the out come is had a sbc with a holly last year do the exact same
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Post by an171 on Apr 4, 2012 17:14:19 GMT -5
I think we got it figured out, thank you for all the advice. We tried a different carb and it was better, so we looked into what the differences between the 2 were and the different carb has a plastic piece going to the bowl vent. The carb in question did not. So we came to the conclusion that when jamming on the brakes, the fuel was sloshing into the bowl vent and causing it to die. Sounds good to me anyways.
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Post by Johnny L on Apr 5, 2012 7:25:02 GMT -5
Nice. Looking forward to seeing you guys run next week, hopefully you get a better match draw this year! : )
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Post by ynotracing on Apr 5, 2012 19:51:11 GMT -5
Nice. Looking forward to seeing you guys run next week, hopefully you get a better match draw this year! : ) x2 |
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