vetteguy75

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...

73, Dark Blue 454

First thing I think of when I hear of back-firing is a vacuum leak. Might check everything,..hoses, carb gasket, etc. Sometimes you can hear a vacuum leak.

Good luck!

vetteguy75

It's not a real loud backfire (like BANG!!!)...more like a several muffled popping noise out the back..

Matt Gruber

small exhaust leak?

...Roger...

Do you have a cam lobe starting to go flat?Try a quick stab of throttle while sitting in neutral with your hand.See if it pops back.

vetteguy75

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.

vetteguy75

I thought about that too...but after the new engine, I had a complete exhaust system put in...2 1/2" pipes all the way back with new FlowMaster 40 series.

...Roger...

Probably not a lobe then that would be pretty quick to happen in 2 months I would think.Good glad its not a lobe.That would be a real PITA.

Matt Gruber

poor ignition?
gap plugs .035
if any are black, replace it

lars

If richening it up helped, richen it some more and see where it goes.

vetteguy75

Do I richen it up via the Air/Fuel Mixture screws or by increasing the Primary & Secondary jets?

Matt Gruber

see:
http://community.webtv.net/MATTGRU/pv
make the pv kick in early, huge difference!

lars

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.

Matt Gruber

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.

vetteguy75

So what do I increase? The primaries only? to what? The secondaries only? to what? or both?

lars

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...

Matt Gruber

VERY SAD NEWS
(cruising on the transition)
will add the SA 670 to my "CARBS TO AVOID" list.
Good luck making that turkey fly

Matt Gruber

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!

lars

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.

Matt Gruber

Thanks lars!
i revised my pv page yet again 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