350/th350 question

[Irish_Alley]

just curious but why a 750 cfm carb?

[86gmc23]

WOT accelerator pump jet squirts nice solid stream for around 2sec.  I sprayed some carb cleaner over carb while I snapped throttle and it revved up without stalling. Jet is a 31, do you think that means I should set up the jet size?

Bd I didn't get a chance to check that since I just saw it but will get into it more tomorrow. Float levels are just below sights. Distributor is vacuum advance.

[rich weyand]

WOT accelerator pump jet squirts nice solid stream for around 2sec.

In both primaries?

I sprayed some carb cleaner over carb while I snapped throttle and it revved up without stalling.

So you are definitely going way lean for the fire to go out.  I wouldn't think it is the primary or secondary jets, though, since fuel delivery from them hasn't begun yet at the time it "instantly dies".  The accelerator pump settings should dominate the mixture for about the first second after WOT application.

Here's the best on-line reference I've found. 
http://www.stockcarracing.com/techarticles/scrp_0601_holley_accelerator_pump_adjustment/

Sounds like you need to increase the pump volume adjustment and go to a bigger accelerator pump jet.  That will put in more gas, solving the lean condition, while keeping the pump shot down in the 1.5-2 second range.  What diameter jet is in there now, do you know?  Remember that flow goes as the area of the jet, so a 10% increase in diameter is a 21% increase in flow rate.  Adjustment in small steps is advised.

[86gmc23]

31 jet I was referring to is the accelerator pump jet, I will try to get some larger ones tomorrow and try it out. 
The 750 carb came on the motor, I have a 650 but it needs a rebuild.

[rich weyand]

31 jet I was referring to is the accelerator pump jet, I will try to get some larger ones tomorrow and try it out. 
The 750 carb came on the motor, I have a 650 but it needs a rebuild.

Understood.  I was just ruling out other variables.

What's in it now, a 28?  Going from a 28 to a 31 is a 23% increase in cross-section, which is about a 23% increase in flow rate (actually, a little more, since the edges have to be discounted a bit -- the whole diameter does not flow).  If you kick up the volume about the same, you will get 23% more fuel into the transition.  The A/FR has to be above 17:1 to be lean enough for the fire to go out.  Adding 23% more fuel to a 17:1 A/FR should take it to 13.8:1 A/FR, which would be just about perfect on acceleration.  Of course, you might be higher than 17:1, but you can't be much higher or it wouldn't be so responsive to the carb cleaner spray.  So 31 sounds like a good thing to try.  You should see a lot of improvement.  If that doesn't quite do it, there's still a little balking, you could try just a touch larger, or try polishing out the 31s a touch.

[86gmc23]

31 is the one thats in it now.
Im going to have to order them, how big of increase is recommended. Think ill start with .035

*Just got couple small drill bits, .035 & .038 ill try and if works ill order one in correct size

[rich weyand]

.035 is 27-1/2% bigger cross-section than .031.  That ought to be plenty.

[86gmc23]

If I drill out the .031 to .035 and it works, I should still buy a new .035 rather than using drilled one right?

[rich weyand]

If I drill out the .031 to .035 and it works, I should still buy a new .035 rather than using drilled one right?

If it works, why bother?  They are conical shaped for better flow, but drilling it out won't change that.  All they do for the different diameter orifices when they make them is drill them out.

[86gmc23]

Sounds good to me, should know how it works later today.

Thanks again for your help Rich

[rich weyand]

De nada.  Hope you get it sorted.  I think we are on the right track, though.  The carb cleaner proved the issue.

[86gmc23]

Having same issues after opening it up to .035 and at .038. Not quite sure where to go from here. Should accelerator pump lever be a lot stiffer with fuel in it then one on the bench? Also from the primary vent tube I'm getting lots of fumes and some fuel spatter spraying out, not sure if that's from me snapping throttle or not.

Is it possible for a vacuum leak to cause this? I can hear a hiss but when I cover carb I can't hear it anymore and motor is steady pulling 20" of vacuum at idle.

*closed choke blade 3/4 of the way and snapped throttle and it took off fine. Is this more proof that it's to lean

[rich weyand]

*closed choke blade 3/4 of the way and snapped throttle and it took off fine. Is this more proof that it's to lean

Yes.

Did you also increase the accelerator pump volume adjustment, or did you just go to the bigger orifice?  Is your squirt still 1.5-2 seconds long?  Did the behavior change, or stay exactly the same?  Like, does it take a bit longer to stumble now, maybe a half-second, or does it still die instantly?

[bd]

Having same issues after opening it up to .035 and at .038.... 

*closed choke blade 3/4 of the way and snapped throttle and it took off fine. Is this more proof that it's to lean

Did you let the engine run to operating temp or is it cold?

[rich weyand]

Having same issues after opening it up to .035 and at .038.... 

*closed choke blade 3/4 of the way and snapped throttle and it took off fine. Is this more proof that it's to lean

Did you let the engine run to operating temp or is it cold?

Excellent question.  I missed that one.