Author Topic: Backfire  (Read 4039 times)

Offline bd

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Re: Backfire
« Reply #15 on: October 19, 2020, 08:25:27 PM »
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.

Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline bigben5054

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Re: Backfire
« Reply #16 on: October 19, 2020, 09:03:00 PM »
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. 

Offline bd

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Re: Backfire
« Reply #17 on: October 20, 2020, 08:08:00 AM »
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.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline bigben5054

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Re: Backfire
« Reply #18 on: October 20, 2020, 08:55:32 AM »
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.

Offline Mike81K10

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Re: Backfire
« Reply #19 on: October 20, 2020, 01:51:18 PM »
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).
« Last Edit: October 20, 2020, 06:00:47 PM by Mike81K10 »
"Tell me and I forget. Teach me and I remember. Involve me and I learn." -Benjamin Franklin

Offline bigben5054

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Re: Backfire
« Reply #20 on: October 20, 2020, 02:46:37 PM »
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. 

Offline Mike81K10

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Re: Backfire
« Reply #21 on: October 20, 2020, 06:06:55 PM »
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.
"Tell me and I forget. Teach me and I remember. Involve me and I learn." -Benjamin Franklin

Offline TexasRed

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Re: Backfire
« Reply #22 on: October 21, 2020, 12:58:28 AM »
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.
« Last Edit: October 21, 2020, 01:25:45 AM by TexasRed »

Offline bigben5054

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Re: Backfire
« Reply #23 on: October 21, 2020, 06:30:15 AM »
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.

Offline TexasRed

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Re: Backfire
« Reply #24 on: October 21, 2020, 11:44:22 PM »
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.

Offline bd

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Re: Backfire
« Reply #25 on: October 22, 2020, 11:12:02 AM »
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...




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.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Mr Diesel

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Re: Backfire
« Reply #26 on: October 22, 2020, 12:07:34 PM »
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.
« Last Edit: October 22, 2020, 12:09:20 PM by Mr Diesel »
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Offline bd

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Re: Backfire
« Reply #27 on: October 22, 2020, 12:17:14 PM »
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.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline bigben5054

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Re: Backfire
« Reply #28 on: October 22, 2020, 12:25:09 PM »
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.

Offline JohnnyPopper

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Re: Backfire
« Reply #29 on: October 22, 2020, 03:02:02 PM »
Siphon or electric pump.
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