350 Popping backfiring fire

[moto57]

hey guys.

At this point ive done so much research I think my brain is going to explode, so Ive finally decided to ask some poeple that know.

Alright I'll start of with what im running.

-engine-
I have an 84 chevy 1500 with a 350 in it (atleast thats what im asumming) I was told it was bored 30 over wouldnt that make it a 355? I think my buddys just bragging up the motor he sold me.

-carb-
It has a Holley 4160 600cfm with stock jets which is 55 I believe. it has a #25 pump nozzle in it, and a vacuum secondary, which has a medium heavy spring in it, the brown one. the primary can is like an off white set in the #1 position.
The air/fuel mixture is at 1 1/2 turns out.

-distributor-
Im running a HEI vacuum advance distributor, and stock AC-Delco plugs.
I know that I have the Distributor on TDC and the plugs are in order ive check a 1000 times.

-issue-
All right down to the fun stuff.
Before I found out that the motor is suppose to be set at around 12 degrees adv. I was trying to tune the carb at TDC. and now all my plugs are soaked and black (im going to grab some tomorrow)

My question is while I was fiddling around with the carb with the motor retarded at TDC would that cause my plugs to get soaked and potentially foul out?

Is a #25 pump nozzle too big or too little possibly?

If this truly is a 355 should the timing be advanced even more?

Now that I have the motor at 12 degrees advanced it runs alot smoother but still falls on its face when you open it up quick, and still backfires through the carb and exhaust, is it safe to say plugs are roached or is the carb still set way to rich?

I also ordered some new nozzles im going to try in it too. #18 #28 and #31 nozzles.

I really appreciate the help guys.

 

[rich weyand]

When you snap the throttle open, the air speeds in faster than the fuel circuit can accelerate to keep up, the air/fuel ratio goes up over 17:1, and the fire goes out until the gas flow catches up. To compensate for this, carburetors have an accelerator pump. It squirts raw gas into the bores when the throttle opens. The accelerator pump needs to be adjusted to keep from getting that bog when you snap the throttle open. If you had an Edelbrock, I would walk you through it, but you'll have to wait until a Holley guy wanders through and he can talk you through it. But that's what's happening. Very typical of a bad or poorly adjusted accelerator pump.

 

[moto57]

Alright makes sense. I have adjusted the accelerator pump both ways. Im pretty sure I have it way to far down at one point. this makes me think that I dont have enough fuel coming in when the pump gets initiated, so possibly I do need a bigger pump nozzle.

I appreciate the reply man.

 

[74 Shortbed]

A 25 squirter is usually too small unless it's a real light vehicle, don't need to buy them though you can drill them one size at a time until the stumble goes away, the number is the size hole in thousands #25= .025. As far as duration of the shot you can adjust the cam on the side and try it, you can get different cams also if needed, at idle the arm should be at the top and the diaphragm not compressed but no slop it has to move at the slightest movement of the throttle, also check clearance at WOT should have .015 between the arm and diaphragm fully compressed, if not you could rupture the diaphragm, adjustments are easily made with the screw at the end of the arm.

 

[moto57]

I don't have anything that small to drill out the squirter or I would. I already bought those 3 from summit I'll just trying the 31 first and see what happens. I will let you guys know what I figure out.

Thanks short bed

 

[74 Shortbed]

Drill bits are cheap, try the 28 first the 31 may be too much but you won't really notice it and you'll just dump fuel for nothing, only takes a second to change it.

 

[moto57]

Alright that was %100 my issue. I can crack my engine WTO now with no hesitation.
I tried the #28 and the #31. it needed the #31 the other didnt give her enough. runs great drives very well, but if you hammer the throttle from a dead stop it kinda lacks on power like it feels like its there but its lacking fuel or something could that be the pump cam possibly? of maybe secondaries? Ive also cam across a few posts that say they use a #35 pump nozzle. could I go that route and possibly just turn the accelerator pump down a bit.

Thank you very much rich and step side, I really appreciate it!

 

[74 Shortbed]

This is why I said to drill it one size at time, too big will dump too much fuel and make it lazy, you go up in size until you can whack the throttle free engine without a stumble but it's crisp, now you try it under load it will stumble so again you go one size at a time until the stumble goes away, now when you get to that point and if it lays down for an instant and goes again then the cam needs to be adjusted or changed for a longer pump shot. Secondaries have nothing to do with the primaries, that's another circuit to deal with later.

 

[bucket]

The #31 and #35 seem to be the most common for street engines. I don't think I've ever run a Holley with anything smaller than a #31. Playing with a pump cam assortment can do a lot of good. Sounds like you might benefit from a profile that ramps up real quick and then tapers off.

 

[74 Shortbed]

The #31 and #35 seem to be the most common for street engines. I don't think I've ever run a Holley with anything smaller than a #31.

All depends on the application and every motor is different, ton of variables there, only way to know what works "right" is to work up to it gradually from the bottom, just because it runs don't mean it's right, that's why Holley gets a bad rap for bad mileage because they're not tuned right..

 

[bucket]

All depends on the application and every motor is different, ton of variables there, only way to know what works "right" is to work up to it gradually from the bottom, just because it runs don't mean it's right, that's why Holley gets a bad rap for bad mileage because they're not tuned right..

I understand that. All my Holley's have been able to get me respectable fuel mileage

 

[gmachinz]

A stock 600 Holley should have 64 jets in front-not 55-I don't believe Holley used odd number jetting anyway and I think 58 are the lowest they go anyway-at least that's the lowest in my jet assortment kit-106 being the highest. So put a pair of 64 jets in if not for a better baseline on the primary fuel delivery-and combined with your 31 discharge nozzle you should be good there. Make sure you have your accelerator pump adjusted correctly-I didn't see the cam specs but if you need more fuel volume, a swap to a 50cc fuel pump kit is easy to do too.

Now, unless you have a custom ground cam I'd say having to set the timing at 12 advanced instead of "0" I'm wondering if you are a tooth or two off on the gear.

 

[74 Shortbed]

I understand that. All my Holley's have been able to get me respectable fuel mileage

Awesome..

 

[74 Shortbed]

I don't believe Holley used odd number jetting anyway and I think 58 are the lowest they go

http://www.smileysracing.com/shopping/productDetails.aspx?i=98346&c=2054

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[gmachinz]

http://www.smileysracing.com/shopping/productDetails.aspx?i=98346&c=2054

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Well dadgum! I guess Holley does make a lot of smaller CFM carbs too....so smaller jets would be needed for them like the 450 CFM 2-barrels and such. My assortments don't go that small but then I have jets for alcohol applications too which are huge!