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Hot Rods Does this look like an exhaust leak that could cause a backfire?

Discussion in 'The Hokey Ass Message Board' started by HardcoreZ28, Jul 13, 2022.

  1. HardcoreZ28
    Joined: Feb 24, 2010
    Posts: 446

    HardcoreZ28
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    I did that many years ago on my Camaro and almost started an engine fire so I've never tried it again hahah. I did relash the valves on that side after warming it up with no change though. And compression and leakdown tests were good.
     
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  2. NoelC
    Joined: Mar 21, 2018
    Posts: 668

    NoelC
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    https://www.chevrolet.com/content/d...engines/02-pdf/sp-350-357-deluxe-19367083.pdf

    - Set spark timing at 32º before top dead center (BTDC) at 4000 RPM with the vacuum advance line to the distributor disconnected and plugged. This setting will produce 32º of total advance at wide open throttle (WOT). The HEI vacuum advance canister should remain disconnected. This engine is designed to operate using only the internal centrifugal advance to achieve the correct timing curve.

    I'm going by memory, but this doesn't sound like what you did Hardcore.

    To be clear on who's paying attention, a tooth off @cam sprocket, is not going to yield the same result as a tooth off stabbing in a distributor.
    In some cases, it'll yield a more spectacular result.

    But isn't that two separate yet related events? The cam sprocket is opening and closing the valves in relation to piston up and down. The distributor gear and cam gear engagement are they not related to spark in the cylinder as the piston is approaching TDC in the up motion.

    But yea, I've seen some stuff run and wondered how. Do that enough times, you have something to base an opinion on. All I'm willing to agree with is that it isn't a distributor issue. Both delivered a spark. The when may still be in play.

    I am looking forward to carb talk.
     

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  3. Budget36
    Joined: Nov 29, 2014
    Posts: 13,174

    Budget36
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    I think @Atwater Mike was referring to cam timing. The posted reply of @HardcoreZ28 suggests the fella is looking at that, or wants to.
    Other thing is since it seems what he dug up that this engine “may” have issues, I’d be more inclined to think an improperly ground group of cams if that’s what pops up.
     
  4. HardcoreZ28
    Joined: Feb 24, 2010
    Posts: 446

    HardcoreZ28
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    NoelC those are GMs instructions.....however it seems they give the same generic timing settings for several different engines despite different cams, heads, etc. The instructions also list 2 different spark plugs in 2 different spots lol. Seems as though GM gives very conservative timing numbers including the no vac advance to prevent as many warranty claims as possible.

    There is a shop in the mid west here that sells these crate engines already broken in on the dyno. Spoke to someone there yesterday and he said he knows they set them with more than the recommended timing to make them run properly but the actual dyno guy is off until Monday. He said to definitely call back next week and pick his brain on what they're seeing for vacuum, timing, etc. It appears they are setting them up on the dyno and getting about 25-35 more than the advertised horsepower in street applications.

    As for cam timing that would be opening a whole other can of worms that I'd have to research. I'm assuming I'd have to install a timing tape on the balancer possibly and also get a degree wheel.
     
  5. HardcoreZ28
    Joined: Feb 24, 2010
    Posts: 446

    HardcoreZ28
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    Not saying it couldn't happen but the other guy I'm talking to got his engine 4 years ago while mine was about 10 months ago. But they could possibly be from the same runs.
     
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  6. How many teeth are on a SBC distributor gear ?

    Tommy
     
  7. HardcoreZ28
    Joined: Feb 24, 2010
    Posts: 446

    HardcoreZ28
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    13 I believe....but I've put the distributor issue to rest. No such thing as a tooth off as long as you can rotate the housing to the position you need without hitting the firewall or engine. And I just put in a brand new MSD unit with no change.
     
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  8. HardcoreZ28
    Joined: Feb 24, 2010
    Posts: 446

    HardcoreZ28
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    I've been talking to the guy on the other forum with the same engine and symptoms. His backfire isn't nearly as bad as mine....happens once in a blue moon. I haven't confirmed but I think he has a full exhaust.

    Something I find odd is his install instructions supposedly call for MR43LTS plugs which are marine plugs. My instructions list R44LTS and R45LTS in 2 different spots. Does anyone know on Delco plugs which are the hotter and which are colder? Also I don't see how that could be causing a backfire so may not be of real concern to me anyway....right?
     
  9. Budget36
    Joined: Nov 29, 2014
    Posts: 13,174

    Budget36
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    Always thought the lower number was a cooler plug.
    BTW, do you know what the “L” in LTS is for? Long reach maybe? I ask because seems AC doesn’t make R44TS anymore (least that’s I could find).
     
  10. HardcoreZ28
    Joined: Feb 24, 2010
    Posts: 446

    HardcoreZ28
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    Yes it's for long which Vortec heads require. But I'm almost positive Delco plugs don't just go numerically for heat ranges. I'll have to research a bit tonight. But like I said I can't see how that'd cause a backfire in the exhaust anyway.
     
  11. Yeah I know HC ,
    It’s just that if anyone divides 360 by 13 it will show how many degrees that one tooth will move the rotor.
    Like you said,,,,you already have that out of the equation.

    It was just a rhetorical question,,,,,,trying to get past the dead horse in the room ,,,,lol .

    Tommy
     
  12. HardcoreZ28
    Joined: Feb 24, 2010
    Posts: 446

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    Lol got it. So frustrated with this backfire and been on nights for my one job so running on 4ish hours of sleep a day lately.
     
  13. NoelC
    Joined: Mar 21, 2018
    Posts: 668

    NoelC
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    July 13 was the first post. 2 and 1/2 months now.

    I'm going to suggest that while math isn't my strong suit, let say 28 degrees.
    And the point of that was what?
    I'm sure your good intentions were to add value to a solution, not piss me off by taking a jab at my offering HC is a tooth out.
    While you came close to the latter with you dead horse remark, the former not so much with your response.

    So let me return the favor.

    Why not take a moment to collect your thoughts and tell us, what's causing the backfire? And what does Hardcore have to do to fix it?

    Like that manifold thing... how would he tune that?

    That's what on the carbs? Tell us more. The floor is yours.
     
  14. HardcoreZ28
    Joined: Feb 24, 2010
    Posts: 446

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    Let's not start a pi$$ing match boys.....there's bigger issues here....like my sanity.
     
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  15. 427 sleeper
    Joined: Mar 8, 2017
    Posts: 2,872

    427 sleeper
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    F.W.I.W... I had a similar situation with popping out the exhaust on deceleration while tuning a dual quad set-up. Too lean of a mixture was the culprit. Added fuel and the popping went away.
     
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  16. HardcoreZ28
    Joined: Feb 24, 2010
    Posts: 446

    HardcoreZ28
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    I've gone up a few jet sizes from where I'm at now and saw no change in the backfire. I started with 62s and worked my way all the way up to 65s. With 65s my plugs started looking quite a bit more dirty. Worked back down to 63s in the primary carb from there. I've also adjusted the idle mixture screws out and in quite a bit to see if it helps with no change in the backfire.

    Monday I'm thinking of running a leakdown and compression test again just to double check my original. Are there any other tests I can do to check the valves aside from pulling the head and removing them all?
     
  17. 427 sleeper
    Joined: Mar 8, 2017
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    427 sleeper
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    Before tearing into the engine unnecessarily, see if you can get a hold of 2 more primary carbs with idle adjustment, and swap them into the end positions, sync them all and see if that helps. You might be better off with a 1:1 linkage, too.
     
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  18. HardcoreZ28
    Joined: Feb 24, 2010
    Posts: 446

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    If another compression and leakdown test look good I see no reason to tear into the engine anyway.
     
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  19. F-ONE
    Joined: Mar 27, 2008
    Posts: 3,271

    F-ONE
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    from Alabama

    I have consulted a Small Block Chevy guy. We had a brief talk about this issue today.
    [​IMG]

    [​IMG]
    His answer was great. He asked me, "Do remember going to the short tracks?" ...."Those cars pop like hell when they let off, don't they?"....

    If the engine runs well.
    If the engine is in time.
    If the valves are adjusted properly and you have good compression.
    These things you have checked out. From what I have read, everything seems good except for this popping and the backfires on deacceleration.
    Now I was able to adjust out the deacceleration backfires/ popping on my 51 Coupe, but that car has a full exhaust system.
    With this type of exhaust, popping on deacceleration may just be an attribute of this system.
    The only way to fully cure it may be plumbing in an exhaust system and running it under the car out the back with mufflers.
    Think about Harley Davidsons. They backfire and pop on deacceleration and this is just a trait. As a matter of fact, when researching your problem; backfires, popping on deacceleration I kept getting results about Harleys.
    Your exhaust is not much different than a Harley.
    Maybe you can play around with baffles.
     
  20. HardcoreZ28
    Joined: Feb 24, 2010
    Posts: 446

    HardcoreZ28
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    It has baffles in it and they were even repacked by me. That being said I'm starting to think maybe nature of the beast as well. Either way I'm planning to add a full exhaust this winter.
     
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  21. HardcoreZ28
    Joined: Feb 24, 2010
    Posts: 446

    HardcoreZ28
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    I did speak to the gentleman from that other site on the phone today and he finally made some good progress with his running a 4bbl. He made a power valve change from I believe a 4 or 6 to a 10 and it really woke his engine up. His vacuum is still right in the 13-14 range with the same fluctuation as mine. He seems to be in a similar boat now where it idles "good", and has good mid range power. His top end sounds better than mine currently. His timing is at the GM recommended 12* and 32* total all in around 2,800. He also has a full exhaust but hears the occasional pop.

    That being said what can anyone tell me about power valves in the 2G? Maybe I'm the opposite issue and mine is open too soon or stuck and overloading the cylinders?
     
  22. NoelC
    Joined: Mar 21, 2018
    Posts: 668

    NoelC
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  23. HardcoreZ28
    Joined: Feb 24, 2010
    Posts: 446

    HardcoreZ28
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    I watched the carb video.....really curious if I have a power valve issue but I think I'm grasping for straws. The 2g relies on vacuum to pull up the plunger inside which allows the stem on the power valve to come up and close. Maybe mine is out of adjustment and leaving it slightly open or the low vacuum of this engine isn't able to overpower the spring on the plunger and raise it, allowing the power valve to close? Then when the throttle snaps shut the overly rich mixture makes it into the exhaust and backfires? I feel like if it was stuck open I'd be pig rich all around though right? Again....grasping for straws at this point.
     
  24. NoelC
    Joined: Mar 21, 2018
    Posts: 668

    NoelC
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    It's so darn quiet you can hear a pin drop.
    Cold case file or what?

    Well, where are all the carb experts?
    I sure as hell am not going to put up a case for it being a carb problem after beating a horse to death saying it was a distributor installation issue.

    Funny thing. Second post in. Doesn't mention the carb but asks about ignition timing. And he's from Canada. What are the odds?:D

    Anyhow, in life's book of complication, the operation of a carb is pretty simple. A steady hand and a container of gas proves it.
    Distributors, not so simple. o_O

    Well, if I can't offer carb support, I'm going to offer this instead. When it came time to stabbing in a distributor in my off-topic 2002 computer controlled 5.7, one tooth back out of 13 made the difference.

    But yea...what about that power valve thing? I dunno. But maybe this little boost will find you an answer.
     

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  25. 427 sleeper
    Joined: Mar 8, 2017
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    427 sleeper
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    I think we're all trying to let this thread cool off a little bit before someone calls the Humane Society for dead horse abuse. They get kinda' touchy about that shit... :rolleyes:o_O:oops::eek:
     
  26. NoelC
    Joined: Mar 21, 2018
    Posts: 668

    NoelC
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    When the Authorities come and ask why I beat that horse to death, I'm going to offer up a darn good reason. I was out of bullets, and it was lame and suffering.

    Camshaft lobe separation angle: what does it mean? | Moore Good Ink
    “If you changed the lobe separation of a street engine from 112 degrees to 106 and didn’t do anything else,” says Jon Kaase, “the engine would idle a lot rougher and generate worse exhaust emissions largely because of unburned fuel.”
    ---------------------------------------------------------------------------------------------------
    tom muzik on November 22, 2018 at 1:39 pm
    My problem is raw fuel going off like a bomb on deceleration.

    This occurs on my recently built 496 BBC stroker. It has big port raised-exhaust port heads and squared, 0.005 in the hole with Wiesco +0.020 dome pistons. The static compression ratio seems to be around 11:1. It produces 180 psi when closing all cylinders. It has 7 inches of vacuum with a Lunati hydraulic roller cam that lifts 0.629-629 with duration of 250 in, 255 ex., 110 centerline and 112 LSA.

    Originally, on startup I ran gasoline 93 and a Barry Grant 850cfm with no problem but the carb was noticeably small for this engine. So, I decided to go E-85 with a new 950 Holley E-85 carb, which is when trouble struck. Assuming it was a jetting issue, I spent much time with Holley “techs”. But after tuning this carb from a-z there was no change and I’ve come to the conclusion it is not a carb issue. The car pulls hard to 7000rpm but whether coming off full throttle or just granny shifting, I get this back-blast in the headers. Do you think it could be scavenging or cam timing or wrong cam? I’ve replaced the wires, 6AL box, and re-degreed the cam. I’d be grateful for your help. Thank you, Tom Muzik

    Reply
    • [​IMG]
      Steve Solo on September 29, 2020 at 5:07 am
      Tom, did you resolve your raw fuel igniting on deceleration troubles? I’m curious as to the size of your header primaries? I have built numerous engines of your spec. I run 2.125″ internal diameter headers. I also run 950cfm HP Holley street and 1050cfm Dominator track. In your case, I suspect your headers may cause excessive backpressure. I think your cam spec is okay but I’d also recommend re-checking your timing marks and verifying their zero-at-TDC accuracy. Mismarked balancers are not unheard of. Cheers!
    I confess to having done a bold highlight on buddies' recommendation. Shameful but I did.
    Moving on, it's easy to armchair much harder to wrench.
    Sounds like you need that degree wheel and tape after all if you want to get to the bottom of things.

    I didn't think however it was or could be an exhaust issue. But I'm trying to wrap my head around it and back firing just the same. It would, would it?
    Have to be running rich. Or just have unburnt fuel?
    Would it be rich because the needles were out of adjustment or to large?
    Would the fuel be unburnt because it was flooding or was it not ignited soon enough?

    I was wondering if the problem is the cam?
    Not that there is anything wrong with the cam, just you are attaching 3x2 to the top of it when in actual modern day, it was a throttle body/EFI engine by design cam.

    I'm sure my last remark is going to bring some energy to the discussion, maybe?
    But thinking out loud, if you're going to say it's a carb problem, you have to default into what the cam is doing. Hot rodding 101. The steep ramp is what's making it hard to adjust fuel requirements and the drop causing a lean occurrence? I could almost believe that. Sounds plausible, maybe.
    Would that be a carb problem or a cam issue?

    How camshaft grinds go awry: | Moore Good Ink

    Well worth the read.
     
  27. HardcoreZ28
    Joined: Feb 24, 2010
    Posts: 446

    HardcoreZ28
    Member

    I spoke to a tech this morning who does the dyno break-in and set ups for these crate engines from one of GM's distributors. He seems to feel that I've systematically checked just about everything. Vacuum seems consistent with what they see and my updated timing as well.

    2 things he noted....the lake headers may not be producing enough vacuum to scavenge the cylinders thus igniting it inside the header. And he also said to check my float level. Is it possible if the bowl is too full the engine is scavenging some fuel out of the carb when the throttle snaps shut? I don't have fuel out of the overflow of the carb.
     
  28. Gotgas
    Joined: Jul 22, 2004
    Posts: 7,175

    Gotgas
    Member
    from DFW USA

    Here is an example of one by our own 57JoeFoMoPar. Looks like a great idea!

    [​IMG]
     
  29. NoelC
    Joined: Mar 21, 2018
    Posts: 668

    NoelC
    Member

    First rule of customer service, the customer is always right.

    [​IMG]
     
  30. HardcoreZ28
    Joined: Feb 24, 2010
    Posts: 446

    HardcoreZ28
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    Except I'm not a customer. I'm a guy half way across the country that he took a half hour out of his busy morning to talk to.
     

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