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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: bigben5054 on October 10, 2020, 11:24:43 am
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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.
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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?
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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?
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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.
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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.
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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.
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Just a hunch, but I would make sure all the ignition is right first. Sounds like an intermittent miss-fire to me.
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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.
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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. ...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...
It sounds to me like it is over-fueling and fouling out the spark plugs still. When you state that you see "gas squirting on each side," are you refering to fuel squirting from the accelerator pump nozzles as the throttle opens or fuel delivery from the main nozzles? The carburetor might have a perforated power valve diaphragm or a warped metering block.
What brand and part number spark plugs are installed? Did you replace them or are you still monkeying around with the "old" ones that were sooty? Describe where you have the initial timing set and the total advance produced and at what RPM.
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I see the gas coming from the accelerator pump nozzles. I’m not sure where I should be looking for the main nozzles. Below the butterflies I assume. Spark plugs are AC Delco R44LTS. Still monkeying with the same set...I’ll venture out to the parts store tomorrow. I was just hesitant to possibly ruin another set of $30 plugs if there is an underlying problem still. Timing is set at 12 initial. There’s 22 mechanical in the distributor, so 34 total. I’m not sure at what RPM it hits that.
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Is it the same carburetor that was used before the engine overhaul/replacement? Did the original engine manifest similar symptoms? Main nozzles reside above the throttle plates and below the choke plate, protruding into the center of the throttle bores just below the lip (restriction) of the venturies. Often, they appear as annular rings centered in the throttle bores and are connected to the rim of each bore by a bridge. To observe main nozzle discharge, with the choke plate fully open, you may need to momentarily throttle the carburetor very rapidly to full throttle while peering down the bores. Limit engine RPM to ~3,500 and keep your face off to one side in case of a flame poping out of the carburetor throats.
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Thanks for the pic; that is helpful. I will check again, but from what I was seeing, it looked like about the same amount of fuel dripping onto the throttle plates on each side. Same carb, but I rebuilt the carb the same time I rebuilt the engine. I have always wondered about how much this carb might have been monkeyed with. It is a Holley Street Avenger 670, electric choke vacuum secondaries. For one, I looked up on Holley about what size jets should have been in the carb and they said 65 and 68. It had 64 and 68 in it, so I bought the correct sizes. But there are a couple other oddities - the primary metering block does not have a timed vacuum port (every 670 I have looked at does have one) and it does not have idle mixture screws on the secondary block (I've seen it both ways). I have always thought the truck ran rich based on 1) horrible fumes in garage and 2) black, watery discharge from both sides exhaust.
I was trying to dial in the fine tune. I started with the idle mixture screws out 1.5 turns each. Hooked up a vacuum gauge and adjusted them to try and achieve highest vacuum. I ended up turning both side in a great deal (I think they ended up being about .5 turns out each side). At that point, I thought I sensed an intermittent misfire and a hesitation off idle. Tat led me to checking into the accelerator pump nozzle. Instead of risking dropping something down the intake, I pulled the carb. Removed the pump and I cant remember the size but it wasnt anything crazy. Put it back in and replaced the carb. No changed. Double checked timing. Then I backed out the idle crews a bit more - to maybe 1 turn each and it seemed to cure the mis and helped with the hesitation. But that is when the backfiring started. I've tried backing them out more, I've tried turning them back in. No change. It is definitely a new problem. I broke the cam in without issue and I have driven it around the neighborhood several times, including several climbs up my steep driveway without issue. Now I can't climb the 2" lip into the garage without the thing bogging down and starting to backfire. For what its worth, the folks over in the Holley forum do not think its a carb problem since my sooty plugs were 1, 7, 2 and 4 as opposed to 1, 7, 4 and 6. They suggest I take a harder look at ignition and to also run compression and leak down tests.
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bd knows his stuff! Have a question though! Is there a chance your timing is off or perhaps your timing chain slipped. Could you have a blockage (comes and goes) in your fuel lines that may be moving like debris. I had a rust problem with my fuel system and there were rust particles floating in my fuel system (I have changed out my whole fuel system during my build in progress).
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I suppose anything is possible. But it's a brand new double roller from Comp Cams. New balancer as well, and I made sure when assembling that the dots were correct and mark on balancer lined up with 0 on tab at TDC. Your fuel question raises a point that had constantly been nagging in the back of my mind. The gas is pretty old. I've had the truck a little over two years, so it is at least that old. I know I should drain it and put in fresh gas - 91 octane given my compression is probably up there with flat tops and Vortecs. But I didn't. I put in a new Fram fuel filter and if I'm not mistaken, each of the fuel inlets (x2) on the carb have a filter as well. Just figured between 3 filters they would catch anything significant while I run the tank dry.
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My old fuel sending unit picture will give you an idea of what my fuel tank and lines looked like. I had off and on fuel flow problems with chunks of rust moving around in my fuel system.
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What distributor did you get? Did you provide your own coil? I don't think the timing would cause that much of an issue when the engine is not in gear. EDIT: I see you said MSD, but they seem to have several. If it's the one with the regular 4 pin module, you may find a parts store that can check the module on their tester. If it tests bad, I would get a new coil (bone stock no super duper internal hei coil, they are junk).
The jets being 64 instead of 65 isn't that big of a deal. Holley runs everything fairly rich. If you follow some of the non-holley "holley" tuning guides, a lot of times your power valve number goes up substantially and the jets go down somewhat (at least in the front). Plus, some 64s can flow as a 65 or a 63. Manufacturing "tolerance".
It is concerning that there is no ported vacuum outlet on the metering block. Maybe the block is not original to the carburetor body. How old is the carburetor? What rebuild kit did you use? Are the floats set externally and not raised too high? I am not sure a picture of the carburetor would help, but it may.
What kind of vacuum were you getting when setting idle mixture? A rumpy cam to begin with?
If you get new plugs, I would get some cheap autolites to throw in there until you figure out what is causing this. You have the correct spark plug for the vortec heads, but I would drop the gap down to .045. Much easier on the ignition components.
Also second edit: Seeing as holley now owns MSD, I think the forum would be careful "throwing" it over the fence. I've been somewhat disappointed by Holley and MSD quality in recent years.
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The distributor is the Streetfire HEI “by MSD.” Looks like an entry level type unit, but I figured it was as good as or better than stock. Pretty sure it is a 4 pin, so I’ll see if our parts stores test them.
The date code on the carb is 0331, so either 2001 or 2011. I feel like 2011 is more likely, but I have no way of knowing for sure. I used the Holley renew kit they sell. I think the floats are set right. No fuel coming out the vents and when I broke the cam in and when I have driven it before there has been no issue.
I can’t recall the vacuum. I have a cheap gauge and all I can remember was it was within the green “ok” reading. Cam is only a couple steps up from stock. It is not lopey.
I do have gap set at .45. Good idea on the auto lite plugs. I’ll do that.
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I am not sure what we can add here. I have a feeling that you may be pulling that carburetor again if the module checks out. Make sure they run it a few times to get it hot like it would be in the vehicle. They may even have one of those inline spark testers, and you can check to see if the spark is nice on a particular cylinder. https://www.harborfreight.com/in-line-spark-checker-63590.html
The only thing I can think of is if there's a powervalve plug for the secondary side or the powervalve for the primary is slightly unthreaded, leaking or blown. It's just weird it's not affecting ALL the cylinders, but I suppose the same could be said for the ignition as well. Or there's an issue with the metering block to carburetor body and it's allowing a lot more fuel to different boosters.
I would assume it's still doing the weird thing with idle? It may be the linkage for the secondaries being allowed to "crack" open occasionally and that could cause the rpm to be a little higher.
It may be worth doing a compression test at least first to see if you have compression on those cylinders and if there is a problem, to do a leak down test after.
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Install a new set of spark plugs gapped to 0.045" and run it to a smog shop for a pretest dyno run with exhaust sampling and printout. The exhaust gas contents should be quite revealing.
To check HEI or similar enhanced ignition spark performance, I recommend an appliance tailored to that type of ignition...
(http://forum.73-87chevytrucks.com/smforum/index.php?action=dlattach;topic=37117.0;attach=51885;image)
One more item to verify is rotor indexing. If the distirbutor gear was installed on the shaft 180° out, the rotor will orient "between" the distributor cap posts when the ignition discharges resulting in spark scatter. Although the circumstance is highly unlikely, it is not difficult to verify. Remove the spark plugs and rotate the engine by hand until cylinder #1 is at TDCC. Lift the distributor cap and determine "exactly" where the rotor is pointing in relation to the cap posts (spark plug wire towers). With initial timing set at 8° BTDC, the rotor should point within a few degrees of the #1 distributor cap post. If the rotor is midway between posts, the gear is indexed 180° off and the distributor will need to be removed for an easy correction. Now, would be the time to install the new spark plugs.
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Your fuel question raises a point that had constantly been nagging in the back of my mind. The gas is pretty old. I've had the truck a little over two years, so it is at least that old. I know I should drain it and put in fresh gas - 91 octane given my compression is probably up there with flat tops and Vortecs. But I didn't. I put in a new Fram fuel filter and if I'm not mistaken, each of the fuel inlets (x2) on the carb have a filter as well.
Maybe I'm missing something from the other posts, but I would address this before anything else. Gas doesn't last much more than a year, depending on how its stored. If it's old enough it will literally degrade to the point it will barely burn at all. Address the known issues first and go from there. Gas is cheap right now and you are going to need it anyway.
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That's funny. I read that a couple of days ago and totally forgot that he said it. I agree with Mr Diesel. Drain out that old juice and add ~5 gallons of fresh-squeezed.
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As always, I appreciate everyone's helpful input. I have not had any time to tinker with it yet, but when I do I will report back - good or bad. bd, I dont think we have smog shops here in Iowa. At least I've never seen one. But I've never looked for one either. Also, what's the easiest way to drain a gas tank? It's about half full.
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Siphon or electric pump.
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bd, I dont think we have smog shops here in Iowa.
But for the cold winters, I might relocate. 8)
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I wanted to check back in and give an update. I changed the spark plugs and I bought a new Edelbrock carb. It runs much better. Throttle is very snappy and the backfire is gone. I have not checked the spark plugs, but its on the to-do list after I drive it a bit more. It still has the goofy idle thing going on but I have narrowed that down to a minor bind/catch in my throttle cable. I did not change out the gas. I put on a new see-through fuel filter and the gas looks perfectly clean. There's not much left in there anyways. I appreciate everyone's help and if the problem returns, I'll be back!
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If it's that glass style fuel filter, I'd be very careful with that. The mr. gasket one is the one I'm thinking of.
https://www.jalopyjournal.com/forum/threads/fuel-filter-warning.1012119/
edit to add: that link has some bad language, so click at own risk
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It's plastic, but thanks for the tip!