Backfire

[bigben5054]

I could use some help trying to fix a backfire issue.  Instead of writing a book listing every last detail that I think is important, I'll just post the basics and can provide additional details as necessary.  350sbc with Holley carb.  Backfires through exhaust under a load.  Will also backfire if I open throttle up to say 2500rpm and keep it there for a few seconds.  Idles fine, but rpms vary.  For example, it may hover at 1000, but then I punch the throttle once and it settles down to 500.  Rev it up a little bit and it will stick to 1000 again, etc. 

[TexasRed]

Sounds like you may have choke or linkage problems on the carb. Make sure nothing is holding that fast idle linkage and choke up. When you punch the throttle, it'll usually kick off fast idle and then when you do a tip-in, it goes back on. Well it shouldn't but that's what happens if the choke isn't open yet.

Electric choke? Is it getting power and is there a good ground to it?

[bigben5054]

Linkage all seems fine as far as I can tell.  Electric choke and has power.  Cold start and it starts right up and gradually increases rpm as top choke plate opens.  Once I punch throttle after 30-60 seconds it idles down.  After it engine cools off and I touch throttle the choke plate snaps shut.  The idle variation issue is sort of hard to describe.  Sometimes when it’s idling high I can just barely punch the throttle and it backs down.  Other times I can romp it several times and it won’t back down.  Pretty finicky.  Exhaust backfires sort of randomly, but always under load and at higher RPM.  And sometimes when I punch the throttle off idle.  I pulled plugs.  A couple were very black and sooty.  Rest looked normal.

[TexasRed]

I would also check to make sure the air cleaner and/or spacer isn't running into the choke linkage. I've had to make a mark on the components to make sure the choke linkage moves freely when it's all back together.

Which plugs were sooty? The backfire could be because the choke plate has snapped shut and it's too much fuel.

You could check to see what it looks like when it won't back down without touching the throttle.

[bd]

HEI or conventional ignition? 

Install new properly gapped spark plugs.  Look deep into the boots at both ends of each spark plug wire, inspecting for black soot accumulation or greenish discoloration on the metal terminals of the wires and for black carbon tracks along the interior surface of the boots.  Inspect the spark plug boots for perforations and tears.  Check the full length of each wire, looking for brownish gray ash deposits while you feel for local rigidity or other evidence for arcing.  Use an ohmmeter to measure the resistance of the wires.  Assuming the wires have conventional carbon-impregnated cores, none should measure over ~8k Ohms.  Remove the distributor cap and rotor.  Inspect the cap for cracks, carbon tracks and the condition of the center carbon button.  Check the rotor, flipping it over and peering deep into the center hole for the distributor shaft looking for ash deposits or any evidence of perforation.

In addition to proper choke operation, verify the carburetor float level.  With the engine off, the choke blocked fully open, and the throttle held open 2-3°, check the axial (side-to-side) play of the throttle shaft.

[bigben5054]

Ok, I will double check choke clearance again.  Will also check float level and throttle shaft.  I have HEI (brand new MSD), new AC Delco plugs that I gapped, and brand new Taylor wires.  I’ll look that stuff over as well but since it is all new it should be in good shape.  #1 and #6 were the sooty ones.

[TexasRed]

I'd also check the firing pattern on your spark plug wires. #6 and #1 are 180 degrees apart.

What spark plug gap? I wouldn't run more than .045.

[bigben5054]

They are gapped at 45.  They are all hooked up in the correct order.  I looked at all the wires carefully and they all still look brand new.  I checked resistance.  Pardon my ignorance of using a multimeter, but when set at 20k ohms, all wires were between 1.15 and 1.73.  When I pulled plugs earlier, I would actually say #1, 2, 7 and 4 were black and sooty.  I put everything back together, deliberately putting the black sooty plugs into cylinders that did not have black sooty plugs and vice versa (after thoroughly wire brushing all of them clean of course).  I ran it around for a bit.  No change in symptoms.  Pulled plugs and 1, 4 and 7 are again black and sooty.  So I think there is not a problem with the plugs themselves.

[bd]

Based on 1-7 and 2-4 running rich, perhaps the driver side fuel circuits are dumping excess fuel.  Are the idle mixture screws balanced the same number of turns out from side-to-side?  Look down the carburetor throat with a bright flashlight and throttle the engine, slowly at first and then more rapidly.  Do both main nozzles begin to deliver fuel at the same time and at the same quantity?  Use some aerosol carburetor cleaner and spray around the base of the carburetor to check for potential vacuum leaks that may affect fuel tip-in.  Verify that the carburetor mounting biolts are appropriately tight.

FYI - Wire brushing spark plugs is never a good idea because it can coat the ceramic insulator with metal from the wire bristles and promote misfires.

[bigben5054]

I never connected those dots! Yes, both idle mixture screws are 1.5 turns out.  I’ll get these plugs put back in and follow your advice in looking down the carb.  I’ll report back.  What is the best way to clean up these plugs?

[TexasRed]

It's hard to clean plugs. Harbor Freight has a sandblasting thing. I'd try getting them with carb cleaner spray and blow them off with compressed air to get as much soot of as possible. They should still fire and once you've found the problem should clear themselves. Although you may want to eventually replace them.

Be careful with the carb mounting studs/bolts. It's really easy to crack a base plate.

[bd]

The best way to clean spark plugs is with a spark plug sandblaster followed by dry compressed air, a thorough rinse with aerosol carburetor cleaner, and more dry compressed air.  Eye protection is essential.  If the plugs are oil fouled, pre-rinse with aerosol carburetor cleaner to keep the blasting media from clumping inside the sandblaster.  Unless you use the appliance for more than one event, it's cheaper to replace the spark plugs.

[bigben5054]

I looked down the carb and at idle, you cant see any gas.  As you pull back on the throttle, you can see gas squirting on each side.  Seems to be about the same amount.  I also sprayed carb cleaner all around the base and didn't notice anything.  Still have same issues.  Backfires out exhaust under load or when I just rev it up (not in gear) to about 2500-3000 RPMs.  It will rev fine there for a few seconds, but then start popping off backfires.  Tempted to just get a new carb at this point.   

[bedwards]

Just a hunch, but I would make sure all the ignition is right first. Sounds like an intermittent miss-fire to me.

[bigben5054]

The entire ignition, distributor, wires and plugs are all new.  I’ve triple checked firing order.  I’ve triple checked to make sure all connections are secure.  I don’t know what else to do there.  Some wires touch each other when they cross each other.  Is that a big problem?  I started a thread in a Holley-specific forum as well.  Wish I had access to a known good carb to stick on there and try.