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  1. #1
    FEP Supporter Broncojunkie's Avatar
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    Default Backfire...revisiting

    Hey guys. I have discussed this problem on here before, but I still haven't figured it out. In my pace car, I installed a fresh 331 stroker. Aluminum heads. Right around 11:1 compression ratio. Carbed. A local machinist, who has always done good work for me, did the machine work, balanced everything, and assembled the short block. I finished the top end and did everything else. When I first fired it up (a couple years ago) it had a backfire whenever you get in the throttle and let off of it. It would happen while driving or parked and revving in neutral. The car wouldn't hardly run without advancing the timing further than normal. About 30 degrees initial timing (distributor not locked out...this happens with oem and MSD distributors, as I've tried both). I've installed 2 different MSD 6AL boxes, a duraspark 2, and currently, a Pertronix Digital HP.

    Yes, plugs are wired properly according to the Comp Cams spec sheet. After exhausting all avenues, I decided to pull the timing cover today and check that the cam is clocked properly. It is. At one point, I thought I had it figured out. I fixed a couple header leaks and it seemed to not be as bad, but never went away. I've had 3 or 4 different carbs on it, including pulling them from other good-running cars. It currently has a new Quick Fuel Brawler 650 on it. I've adjusted the idle mix and rejetted to where it's running correctly. I even installed a wideband to confirm and fine-tune.

    I'm pretty much at my wits end here. I guess it's possible that I still have a small exhaust leak somewhere, but this isn't an annoying "pop" on deceleration. This is like a shotgun going off. A buddy suggested that the compression ration may be higher than what my machinist figured up, and that the 93 I've been running might not be enough. But it doesn't seem to be a pre-detonation problem. If my cr is too high, could it cause a bad backfire like this?

    Other things I've tried are a compression test, new/different plugs, plug wires, and several other things I'm probably forgetting. What am I overlooking here?
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  2. #2

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    First think I would check is for a slipped balancer. 30 degrees initial timing and my car wouldn't even turn over. Take out #1 plug and insert something long and skinny (that won't drop all the way in. You can either pull all the plugs and turn it over by hand (safest) or you can use a starter button. Don't look at the balancer yet, just turn it over until you feel or see the rod rise to its highest point, then look at the balancer. It should be darn close to -0-, due to the quick and dirty method, it might be within 5 degrees, if not, time to change balancers.

    Are you running HO pattern of 13726548 or standard SBF 15426378? Which cam exactly? Maybe you have one that was boxed or ground incorrectly? You could try the other pattern and see if it makes a difference. You will be able to tell right away.

    Check for vacuum leaks. Anything that will lean it out once the throttle plates close. Spray carb clean at all intake gasket connections and see if it changes in sound and/or smoothness. Intake to head, carb spacer to manifold, carb to spacer, heck, plug off the power brakes or any vacuum connection to the body of the car. You might be surprised what can leak.

    I would almost bet you will find you issue in one of these methods. Good luck, keep us informed. Compression will not cause a backfire, vacuum leaks will. Too much advance could do it also but usually that is back up through the carb. Start with checking the balancer and getting timing in the ballpark.

    Kenny

  3. #3
    FEP Supporter Broncojunkie's Avatar
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    Balancer is a new SFI solid unit. Not the bonded oem style. At TDC (yes, verified exactly the way you suggested) my timing mark is right on the "0". Cam is the H.O. firing order and I've tried both. It will barely run on the non-HO firing order. I have checked for vacuum leaks. There are very few vacuum connections on this car. All of the stock smog equipment has been removed and it just has the bare necessities. I have a vacuum gauge I use for tuning and checking everything. Vacuum is good. I've also tried the carb cleaner method. I couldn't find any leaks anywhere and I'm pretty confident there aren't any, but I will probably try swapping intakes just to see if that hellps. I've had 2 different intakes on it with no change.

    I was planning on pulling the heads next...just to check things out and see if anything looks off. I also have a spare set of reworked iron heads (351w D0oe with some port work). I could even slap those on if anything looks suspicious with the aluminum heads. The aluminum heads are pretty cheap, but checked out by the local shop. I checked valve clearance when doing the top end and everything was good. I have checked and re-checked the valve geomoetry and rocker arms (hydraulic lifters, but everything is good. I've done plenty of them before without issue).
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  4. #4
    Moderator wraithracing's Avatar
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    I would probably have checked and done all the stuff you have up to this point. Something definitely sounds off with the engine. I still don't understand the 30 degrees of initial timing. That seems way off and way too much in my mind. Any chance the camshaft is installed incorrectly? Advanced or retarded too much and that is causing the huge initial timing requirement. Just spit balling here, but I think I would be looking into that if it were my setup. Good Luck!
    ​Trey

    "I Don't build it hoping for your approval! I built it because it meets mine!"

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  5. #5
    FEP Supporter Broncojunkie's Avatar
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    Quote Originally Posted by wraithracing View Post
    I would probably have checked and done all the stuff you have up to this point. Something definitely sounds off with the engine. I still don't understand the 30 degrees of initial timing. That seems way off and way too much in my mind. Any chance the camshaft is installed incorrectly? Advanced or retarded too much and that is causing the huge initial timing requirement. Just spit balling here, but I think I would be looking into that if it were my setup. Good Luck!
    Cam appears to be installed correctly. But I'm wondering how I go about verifying that the cam was ground correctly? I mean, is it possible that there is something wrong with the cam itself? I ordered it through Comp and it was an off-the-shelf grind.

    I believe what I'll do next is pull the heads and check some things out. I'll pull the pushrods first and check them. There's a couple things that I'd like to look at. For one, the valve springs in these heads are close to the max for the amount of lift the cam has. I spoke to the manufacturer and they said it was fine as-is, because they leave a little room in the specs for max lift. I have a new set of springs that are better suited.

    Also, when I first put the motor together, a tech at Comp gave me some bad info pertaining to the distributor gear. I removed the steel gear and installed a cast iron gear, which promptly started showing wear right after startup. Called Comp back, and another tech told me it needed a steel gear, which I changed out. The cam gear looked fine and didn't look like it had any damage, but I can pull it and inspect it more closely.
    Last edited by Broncojunkie; 01-25-2020 at 12:16 PM.
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  6. #6
    FEP Power Member qikgts's Avatar
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    What powervalve do you have in the carb? Is this a vac secondary? Front and rear PV's??
    '85 GT

  7. #7

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    30 degrees initial timing is way to much. You should max out at around 33-35 degrees.

  8. #8
    FEP Supporter Broncojunkie's Avatar
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    Quote Originally Posted by qikgts View Post
    What powervalve do you have in the carb? Is this a vac secondary? Front and rear PV's??
    I'd have to pull it apart to say what the PV size is, but it has been changed with no difference. It's had multiple carbs on it and multiple jets and power valves. None of it made any difference. It's a mechanical secondary. I've never had a vacuum secondary carb on it. They were all mechanical Holley/Holley type double pumpers.
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  9. #9
    FEP Supporter Broncojunkie's Avatar
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    Default

    I might add that I currently have 4 foxes... so i can swap parts between them just for testing.
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  10. #10
    FEP Super Member xctasy's Avatar
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    Since the carb changes dont settle or argrivate a massive lean or rich conition, you have two things to do.

    First, establish cold cranking compression at the 50 to 100 rpm a good starter and battery make. If it is more than 185 psi, then right there is your problem. Modern custom cams make insane amounts of cold cranking compression with the plugs out. One specific build in 1988 with another Engineering Technician was the 351 4v Phase III replica engine ( a Boss 351 with a special Hydraulic 295 degree Heatseaker cam), it made almost 200 psi and we retimed it to bleed off cold cranking compression to 185. It ran on 97 research rating leaded gasoline, and sat in a 11:1 with 57 cc closed chambers, -5.5 forged TRWs and 41 thou gasket with 16 thou puston shortfall. The key thing my Engineers Owen Adrian and Roy MacDonald said was dropping the CCC, not the cc's and CR. Dynamic compression ratio, the exhaust closing event angle, defines DCR, but CCC as well.

    If that is in ball park, then youve got the same effect as a blown intake or exhaust gasket leak, which could be casting porosity in alloy headed engine. This requires a kerosene pressure test, and a dye penetrate and magnaflux check. You can find that kind of leak and maybe easily repair it.

  11. #11

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    I would keep it simple. You have already had multiple carburetors on it and it still does the same thing. That should rule out a carburetor issue. Fix your timing and see what happens

  12. #12
    FEP Supporter Broncojunkie's Avatar
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    Xctasy, I will need to do some research on that. As usual, your post is a couple pay grades above me lol! Nothing about this engine was supposed to have issues like you're describing, but you never know where we could have miscalculated.
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  13. #13
    FEP Supporter Broncojunkie's Avatar
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    Quote Originally Posted by badnightrain View Post
    I would keep it simple. You have already had multiple carburetors on it and it still does the same thing. That should rule out a carburetor issue. Fix your timing and see what happens
    The problem with the timing is that I haven't been able to figure it out. After confirming everything is lined up where it's supposed to be, I figured there very well could be an issue with the timing set/ cam timing. I didn't set it (engine builder did) and I'm sure i checked it when finishing the top end, but I thought for sure I would find the timing set not lined up properly. I was surprised (and pretty disappointed) to find it set correctly.

    While I'm in the "tearing it back down" stage, I'm going to start checking some of the valvetrain parts tonight. I mentioned earlier that I had some better valve springs, so I will yank the heads anyway. And yes, I know it can be done without pulling them, but I want to check everything out anyway.

    I'm sure I'll get to the bottom of it, eventually. I sure do appreciate all the suggestions! If anything else comes to mind, no matter how unlikely it sounds, feel free to chime in. I'm open to any and all suggestions, at this point!
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  14. #14

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    we are not going to the moon here. keep it simple what ever the issue, it is something not to complicated. either way 30 degrees initial is not good. start there. I had a cheap timing light which made me nuts chasing issues. try a different light before you move anything.
    Last edited by badnightrain; 01-25-2020 at 05:51 PM.

  15. #15
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    Does it backfire through the carb or the exhaust? Have you run a compression test? Possible stuck/sticking valve? The timing thing is strange. I was expecting to read that you found the cam out of phase when you pulled the timing cover.
    '89 XR-7 5 Speed
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  16. #16
    FEP Senior Member liv2roc's Avatar
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    My question is, is it a roller cam or flat tapped? if a flat one maybe you wiped a cam lobe and lifter. if its a roller could be a bad lifter. when I changed my cam in my original engine I wiped 2 lobes and it started backfiring then valve started ticking.
    Last edited by liv2roc; 01-25-2020 at 10:18 PM.

  17. #17
    FEP Supporter Broncojunkie's Avatar
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    Quote Originally Posted by badnightrain View Post
    we are not going to the moon here. keep it simple what ever the issue, it is something not to complicated. either way 30 degrees initial is not good. start there. I had a cheap timing light which made me nuts chasing issues. try a different light before you move anything.
    Yep. I've been here before too. I have 2 timing lights and I borrowed one from my neighbor. Of the 3 lights, 2 have the timing advance knob. All are reading the same.

    I will also add that the same neighbor is a 40+ year retired (as of last week) mechanic and hot-rodder. His 71 chevelle was featured in 2018 or 2019 snap-on calendar. Great guy and knows engines. He came over and helped me with the timing and verified that I was doing it correctly.
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  18. #18
    FEP Supporter Broncojunkie's Avatar
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    Quote Originally Posted by KevinVarnes View Post
    Does it backfire through the carb or the exhaust? Have you run a compression test? Possible stuck/sticking valve? The timing thing is strange. I was expecting to read that you found the cam out of phase when you pulled the timing cover.
    It is a roller cam, roller lifters, and has roller rockers.... all new with this engine. I don't recall exact compression numbers, but they were consistent when we did the compression test. No noticeable lifter noise and all plugs appear the same. I'm not ruling out anything, at this point, though.

    Edit: forgot to add that it is backfiring through the exhaust. I believe that is technically an "afterfire". Pardon my hillbilly terms lol! I haven't experienced any backfires through the intake, that I recall.
    Last edited by Broncojunkie; 01-25-2020 at 11:32 PM.
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  19. #19
    FEP Supporter Broncojunkie's Avatar
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    Quote Originally Posted by liv2roc View Post
    My question is, is it a roller cam or flat tapped? if a flat one maybe you wiped a cam lobe and lifter. if its a roller could be a bad lifter. when I changed my cam in my original engine I wiped 2 lobes and it started backfiring then valve started ticking.
    Looks like you and KevinVarnes are thinking alike! Well noted! I will be checking lifters and cam for any damage. Even though it's a roller cam, I realize lifter rollers/bearings can fail and wipe out a lobe.

    I was able to dig a little deeper tonight. All pushrods are good. None bent. Rockers fine, as well. Didn't get home till late tonight, but I'm still on boat hours, so I'm still in the garage. I'll probably have another update shortly.
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  20. #20
    FEP Supporter Broncojunkie's Avatar
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    Ok, guys. I didn't get far enough to pull the heads, but I'm still planning on it. Maybe tomorrow. after pulling the intake off, I think I may have found my problem. I'm not 100% on this one, but here's my theory. The intake doesn't seem like it's fitting very well. The imprint on the intake gasket is a lot deeper toward the bottom than it is on the top. My theory is that it didn't show up as a leak because I checked it at idle speed. At higher rpm, enough air is slipping by the upper part of the intake (where it seals against the heads) to cause my problem. I'm going to go ahead and order me a good intake tonight. I should have it Tuesday. Sunday and Monday, I'll work on valve springs and get everything put back together, if I can. I have to go back to work for a few days Wednesday, so there's a good chance I won't be able to test anything until I get back. I'll be out of town for about a week.
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  21. #21
    FEP Supporter Broncojunkie's Avatar
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    After some reading and inspecting parts, I don't know if the new intake will seal any better. I'm ordering some thicker gaskets (FelPro 1262R5) and go from there. Iirc, I had this problem with my original intake as well. It was an Edelbrock performer. My current intake is a Jegs (I think) knockoff of the vic jr.
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  22. #22

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    Part of the problem with V engines is the angles. This is why modern V8s like Coyote use horizontal ports with a pull down manifold and vertical bolts. Old Ford V8s like this, when you deck the heads, block, change the relationship to the intake flange. It could also be as simple as not overtorquing the intake bolts wedging the intake down into the V, which would produce the pattern you are describing, heavy to the bottom, less at the top.

  23. #23

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    Do you run the cork gaskets on the ends? If so, use a bead of ultra black rtv instead. You can gain a little depth that way.

  24. #24
    FEP Supporter Broncojunkie's Avatar
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    Quote Originally Posted by 4eyedblind View Post
    Part of the problem with V engines is the angles. This is why modern V8s like Coyote use horizontal ports with a pull down manifold and vertical bolts. Old Ford V8s like this, when you deck the heads, block, change the relationship to the intake flange. It could also be as simple as not overtorquing the intake bolts wedging the intake down into the V, which would produce the pattern you are describing, heavy to the bottom, less at the top.
    I've had trouble getting them to seal before, but not to this extent. Probably just a tolerance-stacking issue. Intake, heads, deck, gaskets. From what I understand, these thicker gaskets should fix it. I think they'll be here tomorrow. My goal is to get the heads off, valve springs swapped, and heads back on tonight, but more realistically, it'll probably be buttoned up tomorrow night. I'm planning on finally getting some drive time this Spring/Summer.
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  25. #25
    FEP Supporter Broncojunkie's Avatar
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    Quote Originally Posted by mudgepondexpress View Post
    Do you run the cork gaskets on the ends? If so, use a bead of ultra black rtv instead. You can gain a little depth that way.
    I've been using straight rtv (Ultra Black or "The Right Stuff") for the past several years. Definitely the way to go!
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

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