Hesitation/Backfire @ WOT with new engine....
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Melting Slicks
Joined: Jan 2003
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From: Shreveport LA
LA Events Coordinator
Hesitation/Backfire @ WOT with new engine....
I have a backfire/hesitation at WOT with my new engine..it starts right up & idles great. Everything that's related to the running of the engine is new...MSD HEI ignition, Taylor Wires, New AC R45TS spark plugs (gapped at .045). Timing: initial 12* at 2700rpm=36*...so I focused on the carb.
Holley SA 670. Checked the float level: OK. Checked the accelerator cam/pump relationship: OK. Fuel pump pressure: 6lbs and steady. Powervalve: OK. I read about the lean situation with SA's so I increased the jets: Primary from 65 to 68 & secondaries to from 68 to 76 (as per Lars). Saw a dramatic improvement...BUT still a slight hesitation/backfire at WOT...not as bad....but still there! It is VERY noticeable when the engine is cold....only slight when the engine is warmed up...
Holley SA 670. Checked the float level: OK. Checked the accelerator cam/pump relationship: OK. Fuel pump pressure: 6lbs and steady. Powervalve: OK. I read about the lean situation with SA's so I increased the jets: Primary from 65 to 68 & secondaries to from 68 to 76 (as per Lars). Saw a dramatic improvement...BUT still a slight hesitation/backfire at WOT...not as bad....but still there! It is VERY noticeable when the engine is cold....only slight when the engine is warmed up...
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,713
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From: Shreveport LA
LA Events Coordinator
I don't see how...it's a new engine (GM Performance crate)...just got it installed about 2 months ago. But I tried your suggestion...and she run fine...no popping...only under load is when she pops.
Thread Starter
Melting Slicks
Joined: Jan 2003
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From: Shreveport LA
LA Events Coordinator
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From: Dayton, Ohio
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Thread Starter
Melting Slicks
Joined: Jan 2003
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From: Shreveport LA
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From: central FL, near the beach.
Tech Contributor
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From: At my Bar drinking and wrenching in Lafayette Colorado
Race Director
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From: central FL, near the beach.
still sounds like a miss to me. MUST check the plugs!
.
with your upjet, the rear is 48% richer! if it was 15:1 it is now ~10:1.
front is ~10% richer. (check my math)
so average is likely rich enough!
Rookie mistake: WOT on a cold engine, can spin a bearing.
.
with your upjet, the rear is 48% richer! if it was 15:1 it is now ~10:1.
front is ~10% richer. (check my math)
so average is likely rich enough!
Rookie mistake: WOT on a cold engine, can spin a bearing.
Last edited by Matt Gruber; Sep 10, 2007 at 10:58 AM.
Thread Starter
Melting Slicks
Joined: Jan 2003
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From: Shreveport LA
LA Events Coordinator
Idle mixture screws won't affect WOT. To richen WOT you need to change jetting. Also, the power valve won't affect WOT - it only affects transition to WOT: at WOT, any power valve of any rating will be open. The PV only affects the kick-in point of the power enrichment as you transition to WOT.
So what do I increase? The primaries only? to what? The secondaries only? to what? or both?
Tech Contributor
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From: At my Bar drinking and wrenching in Lafayette Colorado
Keep an 8-jet size split between primary and secondary on a carb without a secondary power valve.
Changing the front from 65 to 68 is a 9.44% increase in metering area - that's correct. Then, when the power valve opens on the primary side, it is equivalent to an 8-size jet increase, so that makes the primary side flow equal to a pair of #76 jets at WOT. To keep balanced air/fuel mixture at all 4 corners of the carb, you then need to run 76's on the secondary side. If you bump primary jetting up 2 sizes (to #70), you also need to bump the secondary up 2 sizes to keep the 8-size difference between primary and secondary side. If you go too rich, the worst that will happen is that you'll soot the plugs at WOT - jetting has little to no effect on idle and cruise since fuel is metered through the IFR at idle and cruise (carb runs on the transition slot fuel at cruise). So bump that jetting up and give the car a few hard runs through 1st and second gear at WOT. Pull the plugs and see how they look after the WOT runs - if the plugs are clean, you're going the right direction. Don't be afraid to make those jet changes and see how the car responds. That's what tuning is all about...
Changing the front from 65 to 68 is a 9.44% increase in metering area - that's correct. Then, when the power valve opens on the primary side, it is equivalent to an 8-size jet increase, so that makes the primary side flow equal to a pair of #76 jets at WOT. To keep balanced air/fuel mixture at all 4 corners of the carb, you then need to run 76's on the secondary side. If you bump primary jetting up 2 sizes (to #70), you also need to bump the secondary up 2 sizes to keep the 8-size difference between primary and secondary side. If you go too rich, the worst that will happen is that you'll soot the plugs at WOT - jetting has little to no effect on idle and cruise since fuel is metered through the IFR at idle and cruise (carb runs on the transition slot fuel at cruise). So bump that jetting up and give the car a few hard runs through 1st and second gear at WOT. Pull the plugs and see how they look after the WOT runs - if the plugs are clean, you're going the right direction. Don't be afraid to make those jet changes and see how the car responds. That's what tuning is all about...
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lars
i hopei'm wrong,
help me understand.
1. U open the rear to help idle?(instead of drilling hole in primary)
2. this works, but now it cruises on transition?
3. why not close rear tight. drill primary so more air flows in primary? then it will hit main sooner.
4. is this a bad idea? why?
thanks!
i hopei'm wrong,
help me understand.
1. U open the rear to help idle?(instead of drilling hole in primary)
2. this works, but now it cruises on transition?
3. why not close rear tight. drill primary so more air flows in primary? then it will hit main sooner.
4. is this a bad idea? why?
thanks!
Tech Contributor
Joined: Aug 1999
Posts: 14,380
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From: At my Bar drinking and wrenching in Lafayette Colorado
Matt -
Any Holley or BG carb has idle fuel discharge holes on the secondary side (even with 2-corner idle mixture screws). If you're discharging fuel, you need to flow some air to mix with it, otherwise the rear cylinders run rich at idle and light cruise. The way you do that is to crack the secondary idle screw. In addition to equalizing air/fuel mixture through all 4 corners of the carb, this allows you to close the primary throttle down to avoid exposing more than .020" of the transition slot. This setup produces best possible idle quality as well as best transition off idle and into cruise.
Yes, the Holley and BG carbs cruise on the transition circuit - not on the main metering circuit. A normal car needs less than 40 horsepower for steady-state cruise. 40 horsepower is produced with the throttle blades barely cracked open - they will only be cracked open far enough to pull fuel from the transition and idle circuits at this light throttle opening. Mixture is controlled only by the IFR and the low speed air bleeds. You won't get the carb into the main metering circuit at light cruise by drilling the throttle blades... there is no need for the carb to be on the main metering circuit when producing less than 40 hp.
Any Holley or BG carb has idle fuel discharge holes on the secondary side (even with 2-corner idle mixture screws). If you're discharging fuel, you need to flow some air to mix with it, otherwise the rear cylinders run rich at idle and light cruise. The way you do that is to crack the secondary idle screw. In addition to equalizing air/fuel mixture through all 4 corners of the carb, this allows you to close the primary throttle down to avoid exposing more than .020" of the transition slot. This setup produces best possible idle quality as well as best transition off idle and into cruise.
Yes, the Holley and BG carbs cruise on the transition circuit - not on the main metering circuit. A normal car needs less than 40 horsepower for steady-state cruise. 40 horsepower is produced with the throttle blades barely cracked open - they will only be cracked open far enough to pull fuel from the transition and idle circuits at this light throttle opening. Mixture is controlled only by the IFR and the low speed air bleeds. You won't get the carb into the main metering circuit at light cruise by drilling the throttle blades... there is no need for the carb to be on the main metering circuit when producing less than 40 hp.
3rd time today. I no longer assume what i have learned on the 4165 applies to SA's. Maybe they are good on a dump truck