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Discussion Starter · #1 ·
91 GMC Sonoma
Unfortunately, I don't have a whole bunch of info on engine or cam specs. Supposedly it is a 93 nitrous build has H beam rods and forged pistons. Compression ration is unknown and cam specs are unknown.

400 SBC
Brawler 750 cfm race carb (no choke mechanical secondaries)
6AL and 8570 distributor w/mechanical advance. Msd blaster 2 coil. Set to 18* idle and 34-36 ish on the advance.
14-16hg in park/neutral around 9-11 in drive.

Currently running stock jetting other than I have changed IFR from 33 to 28.
74/84 mains. 4.5pv. I'll be changing the IFR to 31 as the 28 are affecting cruise too much causing low load low speed to go into 15's and some surging. Part throttle is slightly rich as well. 12-13 on the afr under medium load.

The problem I am having is that the truck seems to be really strong and right on at wot (12.8-13.2) until it gets up to 190 and above. Once the engine is run hard a couple times, it seems to go really rich at wot, off the scale on the wideband and if I let off it wants to stall. I assume it's flooding at some point, dumping raw fuel in. Plugs also indicate the same.

I assume I should start with reducing mains but I am a EFI minded person so I want to get input from the carb gurus.

TIA!
 

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I would start at that Power Valve - 4.5 is a bit low for an engine that has 9 in gear. Plus, it might be blown out. If it truly is flooding after a couple of runs, are your fuel pressure and float adjustments stable? You didn't mention what you are using for a FP and Regulator.
 

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If your carb is going so rich at wide open throttle besides your power valve, the power valve channel restrictors also will play into how rich your wide open throttle will be in relation to your jetting. When the power valve opens up its about a six to ten jet size difference on average from what most of my tuning books say.

You might want to see what your power valve channel size restrictors are and if they are changeable. I don't know how much size of a change you need to make a difference in the tune. I know on the ifr's you go up or down two sizes at a time to make a change and on air bleeds you go up or down four size change to make a difference in things to see a difference. I have never had to really mess with changing the power valve channel restrictor size before as I don't hardly ever go wide open throttle or enough to open up my power valve to often except when I pass somebody on the highway.

I know a 750 would most likely have a .050 to .060 size range restrictor I am guessing but can't say for certain. If it is that rich I would maybe try to do nothing else but first change out your power valve and see how much of a difference it makes and if not much of a change then I would look into going smaller on the restrictor size of the power valve by .004 smaller and see what difference it makes.

It might take a combo of the power valve restrictors and maybe the secondary jetting down a few sizes to help with the wide open throttle.
 

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Discussion Starter · #4 ·
I am using a Holley FPR. I believe it is this one. Holley 12-843 Holley HP Billet Fuel Pressure Regulators | Summit Racing

Holley tells me the PVFR is 55.
It had a 3.5 PV in it and I changed it to 4.5 based on the vacuum readings I got. I am going to order a 6.5 PV and also drop down the mains a couple steps to see what that does because I have the main jets here.

What exactly do the mains control if not wot?
 

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Discussion Starter · #5 ·
I would start at that Power Valve - 4.5 is a bit low for an engine that has 9 in gear. Plus, it might be blown out. If it truly is flooding after a couple of runs, are your fuel pressure and float adjustments stable? You didn't mention what you are using for a FP and Regulator.
Fuel pressure seems stable. I have a gauge and keep it around 6psi at idle.
The docs on the carb say to adjust the level to the middle of the sight glass. I have them set just below the arrows on the glass. I have also replaced needle and seat to be sure.
 

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Are you sure on a WOT run, fuel is not coming out the fuel vents?
many put a rubber line connecting both fuel bowl vents, and then cut a slice out of the top of the half circle shaped rubber hose. The slice is just deep enough to allow air in the vent, about an 3/4" inch long,



This keep fuel from coming out the vents and getting dropped/sucked into the carb under hard throttle and/or braking.
 

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Discussion Starter · #7 ·
Are you sure on a WOT run, fuel is not coming out the fuel vents?
many put a rubber line connecting both fuel bowl vents, and then cut a slice out of the top of the half circle shaped rubber hose. The slice is just deep enough to allow air in the vent, about an 3/4" inch long,



This keep fuel from coming out the vents and getting dropped/sucked into the carb under hard throttle and/or braking.
That is something I can't confirm but sound like a real possibility. What would cause that to happen? Fuel shouldn't come out of the vents unless something is wrong, correct?
 

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That is something I can't confirm but sound like a real possibility. What would cause that to happen? Fuel shouldn't come out of the vents unless something is wrong, correct?
go into your kitchen, get a bowl, put water in it, and now while holding the bowl, move quick, what does the water do, it goes up and out. as it hits the bowl side. use tupperware container if you need to see it happen with straight up walls. Same thing happens. Fuel in a carb bowl is no different. HOLLEY based carbs have a fuel bowl at both ends, so when at WOT the fuel is trying to get out the front fuel bowl (bowl closer to front of car), when you brake, it tries to get out the rear bowl vent. REMEMBER when these were designed, tires with traction on the street, were not a thing, nor was brakes that stop a car on a dime. Road course Racers found to keep the cars from falling on face or stalling , they add the rubber vent line. As the fuel out the vent is un metered fuel.
 

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Discussion Starter · #9 ·
go into your kitchen, get a bowl, put water in it, and now while holding the bowl, move quick, what does the water do, it goes up and out. as it hits the bowl side. use tupperware container if you need to see it happen with straight up walls. Same thing happens. Fuel in a carb bowl is no different. HOLLEY based carbs have a fuel bowl at both ends, so when at WOT the fuel is trying to get out the front fuel bowl (bowl closer to front of car), when you brake, it tries to get out the rear bowl vent. REMEMBER when these were designed, tires with traction on the street, were not a thing, nor was brakes that stop a car on a dime. Road course Racers found to keep the cars from falling on face or stalling , they add the rubber vent line. As the fuel out the vent is un metered fuel.
I see. So what prevents the same fuel from coming out of the slit at the top of the hose? Is it that it doesn't make it up that high?
 

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Discussion Starter · #12 ·
Update. I tried the hose over the vents and it didn't do much. It actually caused me to have a hard start issue after putting gas in the truck. I stopped to get gas and when I tried to start back up she didn't want to start. I had to remove the hose and she cleared up. Thanks for the suggestion.

I'll be changing jets tomorrow and I may even lower the float level on the secondary. I'll follow up with the outcome.
 

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Discussion Starter · #13 ·
Update:
Changed IFR from 28 to a 31
Changed primary mains from 76 to 70
Changed secondary mains from 82 to 80.
Test drive indicated lean into high 15. 2 to 16 and even saw 17 for a second. Obviously drove it home under light throttle and it was around 12-13 under light load. She was hovering around 12.5-13.8 wot. I increases from a 70 on the primary to a 74 and it cleaned up nicely. I am still seeing slightly leaner than ideal 14.8 under moderate throttle though pulling hard and clean on the primaries. When getting in to the secondaries she is in the high 12 low 13 range. It's a bit cool outside, so I think I'll drive it a little more before I make another change but it is nearly perfect.

Thanks for the help!
 
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