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Technical Backfire/Spitting through carb - troubleshoot?

Discussion in 'The Hokey Ass Message Board' started by NewToMeT, Apr 29, 2019.

  1. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 5,216

    2OLD2FAST
    Member
    from illinois

    Last time I chased a problem like this (under load backfire) , it was a bent exhaust valve (compression was down about 12# on the offending cylinder ....... Found it doing a leakdown test .....in your case , it could be a tight valve ......
     
  2. TRENDZ
    Joined: Oct 16, 2018
    Posts: 386

    TRENDZ

    What do the spark plugs look like?
     
  3. 429DB034-0661-452C-BB8B-1756F781A5DA.png Hook up a vacuum gauge
    And see what it does.
    Could be chasing an 1 of a dozen issues that have nothing to do with “distributor” timing .
     
    MantulaMan likes this.
  4. sevenhills1952
    Joined: Mar 14, 2018
    Posts: 956

    sevenhills1952

    What you may consider doing is find a local guy (mechanic or engine builder) to look at it. There's a fellow a few miles from here, he's 70, machine shop, dyno at home, he's been a Chevy drag racer and engine builder well over 40 years.
    He could tune it blindfolded, he wouldn't mind you watching and asking questions.
    Someone like that's your best bet.

    Sent from my Bell Candlestick
     
  5. NewToMeT
    Joined: Dec 6, 2018
    Posts: 80

    NewToMeT
    Member

    Gotcha. Opening up the covers to adjust the valves is on the list, but hadn't thought of it in relation to this problem.
     
  6. NewToMeT
    Joined: Dec 6, 2018
    Posts: 80

    NewToMeT
    Member

    If I could find a guy like that it would be great. There is a car show in my town this weekend, so was planning on attending.
     
  7. Ok then,,, it’s the 4 Holley 94 carbs that’s been rebuilt then? Am I understanding that correctly now?

    Are we to be sure the carbs have been done correctly and functioning properly? All equally for use as a set?
     
  8. carbking
    Joined: Dec 20, 2008
    Posts: 3,715

    carbking
    Member

    Virtually always, backfire through a carburetor is not the carburetor's fault; but I am happy some still think it is, as I can sell more carburetor rebuilding kits ;)

    Virtually always, the issue will either be a defective valve (burned, bent, out of time) or ignition (carbon tracks in distributor, crossed wires, defective wires allowing cross-fire).

    You mentioned one cylinder is 110 whereas the others were about 130. Not good, but, is the one that is 110 one of the ones fed by the carburetor with the backfire?

    31Vicki mentioned this in his first post: a carburetor backfire normally occurs when a cylinder is firing, and the intake valve does not perfectly seal.

    Think about it. If the intake valve seals, HOW does the backfire get out of the cylinder to the carburetor???

    Jon.
     
    Last edited: Apr 30, 2019
    31Vicky with a hemi likes this.
  9. NewToMeT
    Joined: Dec 6, 2018
    Posts: 80

    NewToMeT
    Member

    The carb experiencing the backfire is over cylinder 5/7. Cylinder 3 is the offending low one.

    Makes sense about your points RE: ignition or mechanical. Seems the internet thinks Lean condition is also a cause. So in your experience that is not usually a cause?
     
  10. NewToMeT
    Joined: Dec 6, 2018
    Posts: 80

    NewToMeT
    Member

    Correct, just the carbs were rebuilt. I did the work. Checked as best I could, leaks at the PV, accelerator pump action, flow though all the circuits, Float action.
     
  11. squirrel
    Joined: Sep 23, 2004
    Posts: 55,933

    squirrel
    Member

    The large plenum in that intake also might be affecting things...lean mixture can be a cause, but that can be due to factors other than a carb that is not working properly. Think mixture distribution, and total carb size for the engine/rpm, and how much air velocity through the venturis it takes to get the fuel moving.
     
    Fordors likes this.
  12. Rich S.
    Joined: Jul 22, 2016
    Posts: 296

    Rich S.

    Swap the carb with another one and see if the problem follows the carb


    Sent from my iPhone using The H.A.M.B. mobile app
     
    carbking, Fordors and jaw22w like this.
  13. DOCTOR SATAN
    Joined: Mar 21, 2014
    Posts: 710

    DOCTOR SATAN
    Member
    from okc

    Have you synced the carbs.....my son kept having a hiccup in his z and it
    Ended up being a spec of dirt floating around in there getting in the way of everything...
     
  14. TRENDZ
    Joined: Oct 16, 2018
    Posts: 386

    TRENDZ

    It surprises me that someone that builds carbs would think a lean condition wouldn’t make a pop through the intake.

    You can take a perfectly tuned engine in 100% perfect mechanical condition, and if you purposely render the accelerator pump inoperative you will get an instant lean pop through that perfect engine.
    Lean pops are common with otherwise perfectly fine engines.
     
  15. carbking
    Joined: Dec 20, 2008
    Posts: 3,715

    carbking
    Member

    Please note "virtually always" in my comment; not completely always. I did not state that a lean condition will not cause a backfire.

    As far as your accelerator pump analogy is concerned: Extremely common during fuel rationing of WWII to disable the accelerator pump to save fuel; plus accelerator pumps were not used on carburetors during the first 40 years of the automobile.

    One of the biggest issues in keeping happy carburetor customers is to listen to the customer for his/her symptom(s), and NOT rebuild a perfectly good carburetor ;) Waste of the customer's money.

    During the 1950's and 1960's literally millions of good accelerator pumps were replaced with good accelerator pumps. The reason: points would burn, or the contacter arm would wear, changing dwell. The changed dwell would in turn change the timing, which would cause a hesitation upon acceleration. The mechanic would do an electrical tune-up, which solved the problem, and also replace the accelerator pump at the customer's request. The hesitation went away (because of the electrical tune-up) and the customer was happy (and poorer than he/she needed to be).

    Jon.
     
    Last edited: Apr 30, 2019
    scott27 likes this.
  16. squirrel
    Joined: Sep 23, 2004
    Posts: 55,933

    squirrel
    Member

    Sure would be interesting to find out if the timing is really retarded as far as we think it might be, and if changing it helps the problem....if not, we'll move on to the next easiest thing to check.
     
    427 sleeper likes this.
  17. Unknown engine....pops through carb on acceleration....Sounds like a flat cam to me. Easy to find with valve covers off.
     
  18. Rich S.
    Joined: Jul 22, 2016
    Posts: 296

    Rich S.

    I agree with squirrel. Get your timing squared away 1st and if the problem doesn’t go away then maybe try a carb swap like I said.


    Sent from my iPhone using The H.A.M.B. mobile app
     
  19. carbking
    Joined: Dec 20, 2008
    Posts: 3,715

    carbking
    Member

    Rich mentioned swapping two of the carburetors, and seeing if the backfire follows the swap.

    This is an easy test to determine if you have a carburetor problem or something else.

    One test is worth 1000 opinions.

    Jon.
     
  20. 2X REMOVE AND VACUUM CHECK THE POWER VALVE ON THE ONE CARB.
    IF YOUR ENGINE BACKFIRED, IT BLEW OUT THIS OR OTHER POWER VALVES.
    I WOULD SUGGEST RUNNING 4 CARBS YOU DO NOT NEED THE POWER VALVES.
    MAKE SURE YOUR MECHANICAL ADVANCE IS WORKING PROPERLY (TAKE IT APART)
    ARE U USING REC 90 FUEL? NO ALCOHOL., EASIER TO READ THE PLUGS.
    MIXTURE CHECK: NEW PLUGS WOT FOR 1/8 MILE, SHUT DOWN, NEUTRAL OR CLUTCH, CHECK PLUG COLOR. CHECK HEAT RANGE OR REJET AS NEEDED.
     
  21. MantulaMan
    Joined: Jun 19, 2018
    Posts: 40

    MantulaMan

    Bump

    Sent from my SM-G390F using Tapatalk
     
    VANDENPLAS likes this.
  22. sevenhills1952
    Joined: Mar 14, 2018
    Posts: 956

    sevenhills1952

    Here's a pretty good backfire explanation, everything one's have said here already. Lots of things to check.
    I remember the Chevy guy I know saying something about the 5-7 cylinders running lean.(Many years ago...I could certainly be wrong). I do remember him showing me using an IR temp gauge on headers those two running hotter (lean) and he would compensate with valve lash adjustment, those two cylinders.
    As I say, many years ago. If you could find the right expert to help you, you'd be way ahead. Finding right person is difficult.

    http://www.badasscars.com/index.cfm/page/ptype=product/product_id=95/prd95.htm

    Sent from my Bell Candlestick
     
  23. NewToMeT
    Joined: Dec 6, 2018
    Posts: 80

    NewToMeT
    Member

    Guys thank you all for the info. No followup testing yet as I pulled the carbs off to check the passages and the PVs. Now for the PV to see if it was blown I filled the bowl and saw no leaks. No issues there. Whats the vacuum test for these?

    Plan of attack: Get all the carbs back on after confirming function again
    Confirm total and initial advance (mechanical, vacuum plugged)
    Confirm no vacuum leaks
    Confirm manifold vacuum readings
    Report back :)
     
  24. MantulaMan
    Joined: Jun 19, 2018
    Posts: 40

    MantulaMan

    Vacuum at idle should be decent and the needle should be solid as a rock (no wobble) if the vacuum is low or the needle wobbles then you have a problem. There are plenty of tests you can do with a vacuum gauge but the first one is just to check the vacuum reading at idle.

    Sent from my SM-G390F using Tapatalk
     
  25. sdluck
    Joined: Sep 19, 2006
    Posts: 3,185

    sdluck
    Member

    Sync the carbs and check the throttle linkage that it is operating the carbs correctly.
     
  26. Rickybop
    Joined: May 23, 2008
    Posts: 9,605

    Rickybop
    ALLIANCE MEMBER

    Years ago...
    My engine was running crappy.
    Replaced all ignition parts, sparkplugs too.
    Still ran like crap. It was running rich.
    Rebuilt the carb.
    Still ran like crap.
    Getting frustrated to my wife. Like wtf?
    She said, "Did you check the sparkplugs?"
    I said, "Yes of course, I replaced them."
    But it made me think.
    Pulled the plugs. Black and sooty...they fouled that quick.
    New plugs...all betta.
    I'm still guessing a flat cam, but I hope not.
    Let us know.
    I'm always curious what the problem ends up being when a bunch of us take the time to diagnose. But a lot of guys never let us know how it works out. I don't know why.
     
  27. NewToMeT
    Joined: Dec 6, 2018
    Posts: 80

    NewToMeT
    Member

    So, bunch of things done and it is much better, though still had one tiny backfire when it was cold. Cant pinpoint exactly what helped because did several of the changes while the carb was apart so didn't test each factor.

    Changed plugs (they were pretty sooty)
    Checked carbs for PV and circuit flow. Looked good so back together with new gaskets
    Got tighter vacuum hoses (some were a bit loose)
    Rearranged a few spark plugs that were kinda close together

    Once started. Set total timing (no vacuum) to 35. This put initial timing at 15.
    Hooked up vacuum gauge. Engine was reading about 15 inches. Then adjusted idle mixtures, and was able to get this up to 20. Guessing I still have much more adjusting to do.

    Will report back more. Had it running well enough to get it down to a car show 3 miles away (came away with 1st place American modified :) Do note it was a British car show focus so my competition was pretty sparce and voted on by the attendees). But more testing to come this weekend.
     
    Rich S. and 427 sleeper like this.
  28. squirrel
    Joined: Sep 23, 2004
    Posts: 55,933

    squirrel
    Member

    The backfire when cold...do you have chokes on the carbs? if not, then this might be considered normal behavior.

    thanks for the report!
     

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