327 idles great, backfires with throttle

[1986c10]

Hi, I picked up a mint condition 1979 C10 project that has traded hands alot and was never completed. The engine was rebuilt in 1998 and never started, its a .040" over 327, small cam if not stock, intake, headers, holley 600 double pumper etc. I bought it not knowing if the engine was junk or not. I changed the oil, rebuilt the carb because the floats were stuck, distributor was somehow bad, replaced that. Fired the truck up and broke it in for about 15-20 minutes. I set the timing at 14*, adjusted the mixture screws on all 4 corners of the carb and it now starts and idles great. Problem is, when you smack the throttle it backfires or just hesitates, so bad that i couldnt drive it up the slope in my driveway without it stalling. I sprayed carb cleaner on everything, found no vac leaks, i even blocked off the brake booster and pcv with no change. Vacuum from memory was kind of low, like 12" i think. I bumped up the carb squirters from 21 and 28 to 28 and 31 with no change, ive advanced the distributor more with no change. I'm kind of getting frustrated with the thing. Anyone have any thoughts? I'm debating on slapping a friends carb on it just for a sanity check.

[garyd1961]

Sparkplug wires crossed, Cracked or bad distributor cap, Bad cam wiped lobe, timing not set right, bad carb. Just a few things that may cause your problem. Oh I forgot bad coil.

[YoungPup1977]

That first sentence that states mint condition maybe challenged a bit I am guessing....sure hope you didn't pay a mint condition price for that 79 c10, I am with Gary, check the things he mentioned and go from there. Maybe perform a compression test just to see where you are....working behind others not knowing what they have done is always a great challenge... Enjoy and have fun !!

Oh yea just wanted to add....any time you swap out parts is not a bad thing....just know the working condition of the part you are swapping to.

[Dead Parrot]

Vacuum on a near stock cam should be closer to 18~20.

Don't trust the timing mark. Even if the balancer was new when engine rebuilt, it is now 21 years old. Rubber might have gone bad and let the ring with the mark shift a bit.

Also possible the builder put the timing tab in the wrong spot.

Spark plugs gapped correctly?

Check the rubber suction lines from the tank to frame and frame to fuel pump.

[rennat_2006]

Did you spray all around the intake manifold with carb cleaner or similar checking for a vacuum leak? Ive had best luck with the non flamable stuff because it will drop the idle and you smell it in the exhaust.

Dealt with this on my nova when it had the 327 in it. Idled fine and had a stumble while revving in park, try to drive it though or even touch the throttle while it was in gear and it would sound like elmer fudd was under the hood.

[cashforclunkers]

HEI or points distributor?

[1986c10]

I would say its mint condition, no rust, no bodywork, no dents, scratches etc, got it for $4500. It has about $20k into it since it was first worked on. Pretty much everything on it is new, mechanical and electrical.




Thanks for the suggestions so far, I will start with a compression check, verify valve lash and lift, I'll toss some new plugs in if all seems well. With a lot of throttle it will backfire with a flame just like if the timing were too retarded. I may stick my timing light on it with the vac advance hooked up and see if maybe the timing is retarding too much meaning too loose of dizzy springs??

I did spray around the base gasket on the carb, all vacuum connection ports etc.

The distributor is an MSD streetfire hei

[cashforclunkers]

Very nice truck. I would try changing the module or coil. I feel like you might have weak spark.

[geezer#99]

Lean and/or retarded makes backfires.
Lean from a vacuum leak or cold intake.
Retarded timing of course.
What intake?
Have you confirmed timing tab/balancer are accurate?

[YoungPup1977]

That’s awesome !!! Good luck with it

[mrtoni]

I’ve never looked at 2wd trucks but I love yours. Prob the nicest styling I’ve seen

[garyd1961]

When I was a lot younger I took a 455 Pontiac out of a Grandville and put it in a Grand Prix. It did the exact same thing, would idle fine but when you give it the gas it would spit back through the carb. I checked everything I could think of even taking the distributor cap off and looking at it under a magnifying glass. Finally I just sold the car at a loss only later to find out the new distributor cap was cracked.

[cadillac_al]

It sure sounds like crossed wires.

[BossHogg69]

Wires 3 & 6 being crossed, will still idle okay and sputter on acceleration.

I've also had a cracked distributor cap act similar to what you're describing.

I've also seen a vacuum secondary carb put together with the spring on the wrong side of the diaphragm that would idle perfect and not accelerate, and would backfire and bog, due to the secondaries slamming open when you hit the throttle and no fuel being pulled in when the secondaries opened. (Lean backfire). Even had a guy argue with me about it saying it was the timing, until I pulled the vacuum secondary can apart...

Of course, as already mentioned everything else being right and the timing not being where the engine likes will also cause a lot of issues.

Good luck and let us know what you find.

[bigdaddy4x4]

check the valve train rockers and push rods and adjust the valves .you could have one to tight that will cause a backfire.

[BossHogg69]

Originally Posted by bigdaddy4x4

check the valve train rockers and push rods and adjust the valves .you could have one to tight that will cause a backfire.

This is very true

[jbarron]

If the carb accelerator pump is bad it will cause a lean condition on throttle.

[1986c10]

Thanks for the compliments on the truck. I'd hope I can drive it soon!

Well, since I knew nothing of the previous work I figured i'd start from scratch. I pulled all the plugs, checked compression, 152-154psi on every cylinder, verifiied cam timing, re-lashed all valves, bought new plugs, gapped to .040", verified wire routing, inspected distributor cap with no issues, found an old breather port on the back of the block (old old sbc), I capped that off, set timing to 15* at idle and 34* at 3200rpm, adjusted my mixture screws for max vacuum. The thing idles great, still hesitates with quick throttle... I am having a friend pull off his holley and we're going to try it on this truck and hope for the best. I think it has to be the carb at this point.

[Dead Parrot]

Unless MSD specified otherwise for their dizzy, normal HEI gap is .045. But don't see a gap of .040 causing your issues.

Could be as simple as the motor just needs some down the road running to get better broke in.

[1986c10]

Update: Tried a friends Holley 600 and that fixed the issue. Kind of annoying that I rebuilt this carb and got it looking perfect inside and its still acting up. Maybe I'll tear it apart again but I know i'll probably not find anything wrong because I dont look at them everyday. Might just buy a new one and be done with it since its cruising season haha. Thanks everyone for the help, glad to hear it running healthy.

[kwmech]

What carb was on there that you had to "adjust all 4 screws on the corners of the carb"?

[1986c10]

It was a holley 600 double pumper like the one below but with an electric choke.

https://www.holley.com/products/fuel...SABEgKaMPD_BwE

When i first tried to crank it over it sprayed gas straight up and hit the hood. I tore it apart and found junk got stuck between the needle and the seat. I cleaned everything out, blew it out, rebuilt it with a holley kit, set float height, verified pump arm travel, got it running then set the mixture screws then idle speed. I then increased the squirter size with no change. Maybe it still needs a bigger shot but this is a stockish 327.

[A1971Blazer]

Originally Posted by 1986c10

It was a holley 600 double pumper like the one below but with an electric choke.

https://www.holley.com/products/fuel...SABEgKaMPD_BwE

When i first tried to crank it over it sprayed gas straight up and hit the hood. I tore it apart and found junk got stuck between the needle and the seat. I cleaned everything out, blew it out, rebuilt it with a holley kit, set float height, verified pump arm travel, got it running then set the mixture screws then idle speed. I then increased the squirter size with no change. Maybe it still needs a bigger shot but this is a stockish 327.

Did you check the power valve when you rebuilt the carb?

[mattfranklin]

There was a really nice episode on Roadkill Garage about tuning the Holley 4bbl. It's officially behind a paywall (I subscribe and love it), but there are complete copies out there. The episode has the title "CARB TUNING AND THE TATER TRUCK!" They systematically worked through all of the carb tuning parameters including power valve and jet selection, all that during a road trip.

[Troy Sr]

With the price you paid for this truck splurge for a Fuel Injection!!!