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backfire though carb

Discussion in 'The Garage' started by mud390, Jul 26, 2003.

  1. mud390

    mud390 1/2 ton status

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    My 1984 K5 with a 350 is backfiring though the carb. I did a tune up today. New plugs, wire, cap, and rotor. The coil looked good, so I let it ride. It fired up a lot easier, but it still backfires through the carb, just a hair less. My next step is the rebuild the carb. What do y'all think? Am I going the right direction or way of course? Any thoughts, help, and input would be great! Thanks!

    Kris
     
  2. COCHEV

    COCHEV 1/2 ton status

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    did you check your timing? also, are you getting a bog a long with your backfire? if so, your accelerator pump may be bad. i just had this happen. what carb are you running?
     
  3. mud390

    mud390 1/2 ton status

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    Haven't checked the timing yet. Don't have a timing light. I tried to move the distributor thinking it may have jumped, but it was tight as could be. It doesn't bog down when it backfires, just pops. It only does it when I get to around half throtle or so. Maybe a little less.

    Kris
     
  4. COCHEV

    COCHEV 1/2 ton status

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    did you maybe cross a couple plug wires?
     
  5. mud390

    mud390 1/2 ton status

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    It was backfiring before I did the tune up. I swapped the wires 1 for 1 as to try to make a mistake. I checked it with the book and it all matches up. It started backfiring after riding on the highway at 65-70mph for about 15 minutes. I did the same thing two days prior for over and hour and a half total driving time and never had a hicup. Would a carb rebuild seem logical for this problem? Maybe just do it anyway for the insurance. Its a stock Q-jet. The motor still has all the emissions crap on it. I don't have inspections here, so I want to take it all off. Way to many vacumm line for me. Thanks!

    Kris
     
  6. COCHEV

    COCHEV 1/2 ton status

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    you might also have a burnt valve. you could take one plug off at a time and open it up to where it pops. when it doesn't pop, you've found your cylinder. but first, i'd go get youself a timing light and start there. hope this helps /forums/images/graemlins/thumb.gif
     
  7. mud390

    mud390 1/2 ton status

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    I might have to try that tomorrow. I'm gonna borrow a timing light this week...hopefully. Thanks for the help!

    Kris
     
  8. Twiz

    Twiz 1/2 ton status

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    From another post,

    [ QUOTE ]
    Backfireing through the carb is a sure sign of a lean mix.
    Sure, cam, spring or anyother odd-ball problems might be present or cause the condition. But with a "new" combination, such as this, the most likely cause is a lean mix.

    Understand that,
    There are 5 independent, allthough interrelated fuel metering systems within the carb.
    These are,
    1 - Cold/Start System. Which include the Choke, Idle system, Transition port and........(not all that interesting)
    2 - Idle System. Which include the Idle adjustment screws, Fuel inlet, Air inlet (both, past the butterflys and past the emulsification inlet ports) and the Transition Ports from Idle Sys. to Main Sys.
    3 - Main Metering System. Which include the Transition Ports, Main Metering Jets, Main Metering Rods or someother fuel en-richment system, Main Emulsification port.
    4- Seccondary Metering System. Which include the Seccondary Transition Ports, Seccondary Metering Jets, Seccondary Mettering rods or someother fuel enrichment systemt, emulsification port.
    5- Exceleration System, Which include the excellerator pump, port, lever, linkage and so-forth.



    [/ QUOTE ]
    My point in that post, was that each "system", allthough interrelated - operate primaraly within specific conditions. So, if your haveing problems in one specific area, such as: Cold-Start, Idle, Transition from Idle to Cruise, Cruise, Transition from Cruise to Wide Open Throttle, W.O.T., Or Hard excell.
    Knowing which system within the carb covers that area, might help in finding and correcting the problem.

    In this case, I'd say you might have a clogged Fuel Enrichment System. With Q-jets, the Main Metering Rods control that section. So, maybe the lil' rubber seals wiped out, or a ports clogged - Tough to say without driveing it and inspection the carb.

    A rebuild, with proper cleaning should nail it. (Don't forget to expoxy those wells)
     
  9. mud390

    mud390 1/2 ton status

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    How hard is it rebuild a Q-jet? I have never rebuilt one before. Any good article that I might read to help me prepare? Thanks!

    Kris
     
  10. COCHEV

    COCHEV 1/2 ton status

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    not hard at all. get the book by doug roe-rochester carburators. even without it its not hard. get a nice clean spot on the work bench and go for it. /forums/images/graemlins/thumb.gif
     
  11. leadfoot067

    leadfoot067 1/2 ton status

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    ok i had a similar problem on my truck....would seem fine till i got goin down the road,crusing alon and lean on the throttle and it would backfire on me.... /forums/images/graemlins/confused.gif...spent a week figuring it out...it was the mechanical advance springs in the distributor..the weighst had locked up kinda and werent really moving enough /forums/images/graemlins/deal.gif...workes well now... /forums/images/graemlins/thumb.gif its just something quick and easy to check before rebuilding the carb...could also have a vacume leak somewhere /forums/images/graemlins/confused.gif
     
  12. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Back firing through carb is a problem with timing or valve not closing <font color="red">PERIOD! </font>

    It is got nothing to do with lean mix Burned holes in pistions and burned exhaust valves are what happens with lean mix.

    For a back fire to happen on the intake end there has to be an ignition source where there souldn't be. Remeber that the spark plug isn't supose to fire till the compression stroke. The valve should be long closed on compresion stroke.

    You have a timing issue more then likley or a stuck valve. Stop looking at the carb...it's not your problem. If your having problems starting then it's probably timing. The other possiblility is there is a problem with the distributor.

    Does yoour truck have ESC ignition? ESC uses a knock sensor and a computer to control timing and will not have a vacuum pot on the distributor.

    On the core suport should be a sticker that tells you how to properly set you timing. there will be something that has to be disconnected on either system.
     
  13. COCHEV

    COCHEV 1/2 ton status

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    [ QUOTE ]
    Back firing through carb is a problem with timing or valve not closing PERIOD!



    [/ QUOTE ]
    not true.

    read the roe book on page 48 in the section "off-idle operation" if you accelerator puump is not working right at tip in/off idle you will have a bog and a backfire. just went through this with my q jet. took it a part and fount the rubber plunger to be dryed and cracked. replaced it and the problem went away.
     
  14. COCHEV

    COCHEV 1/2 ton status

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    i believe what happens is if you don't have enough fuel to air (no shot from the accl. pump) you get a lean mix, plus your throttle is open so you get the backfire through the carb. maybe someone can pitch in here and fill in the holes on my engine theory.
     
  15. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    [ QUOTE ]
    [ QUOTE ]
    Back firing through carb is a problem with timing or valve not closing PERIOD!



    [/ QUOTE ]
    not true.

    read the roe book on page 48 in the section "off-idle operation" if you accelerator puump is not working right at tip in/off idle you will have a bog and a backfire. just went through this with my q jet. took it a part and fount the rubber plunger to be dryed and cracked. replaced it and the problem went away.

    [/ QUOTE ]

    I had the truck sitting for a while and the fuel bowl went dry. Pump rubber stuck to the wall and pulled off the shaft. Then it got jammed into the bottom of the bore. Drove it for half a year like that before I bothered to fix it. It never once back fired through the carb.
    Fell on it's face and stumbled everything I got on the gas hard till the mains got flowing. Sometimes hard to start....never backfired through the carb. Don't mistake a stumble and the resulting detonation for a back fire in the carb.

    Only ways I could see that pump causing that issue is at high RPM with max advance Get off the throttle and then get back into it full throttle. Other way would be poor valve sealing and the extra fuel dams against the valve seat causing enough delay not to get a full igniton before the thing got back on intake stroke. I basicly don't buy that without some other problem helping to begin with or 4000 RPM cruise and getting into the gas.

    He has a hard start problem...that says too much advance.
    If he has ESC it would get to a point where the ECM would give up and not be able to back off the timing enough to stop the detonation. It would let it go full advance to try to get itself back to square one, Backfire though the carb then go full retard the next stroke. It might even have the roter close enough to ignite two plugs at the same time and cause this.


    I bet when he puts the timing light on that booger the timing mark will be jumping around like rabbit on a hot skillit.
     
  16. Twiz

    Twiz 1/2 ton status

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    [ QUOTE ]
    Back firing through carb is a problem with timing or valve not closing <font color="red">PERIOD!</font>


    [/ QUOTE ]

    <font color="white"> Bullshit!
    Sounds to me like your takeing a direct hit on my reply, so be it, slick. Thats the internet.
    I've read more than one miss-informed reply from the poster of death - that was entierly incorrect, but gracefuly, or in the least tacktfully, declined to rebuke, at the time.
    </font>
    [ QUOTE ]
    For a back fire to happen on the intake end there has to be an ignition source where there souldn't be. Remeber that the spark plug isn't supose to fire till the compression stroke. The valve should be long closed on compresion stroke.

    [/ QUOTE ]
    The ignition source is the previouse combustion cycle. When both valves are open during valve-over-lap, the remaining, and still burning exhaust gasses ignite the incomeing air/fuel charge. <font color="red">PERIOD! </font>

    [ QUOTE ]
    It doesn't bog down when it backfires, just pops. It only does it when I get to around half throtle or so. Maybe a little less.


    [/ QUOTE ]
    Related to throttle position = possible carb problem.

    [ QUOTE ]
    It was backfiring before I did the tune up.......... It started backfiring after riding on the highway at 65-70mph for about 15 minutes. I did the same thing two days prior for over and hour and a half total driving time and never had a hicup. (sudden problem *?*)

    [/ QUOTE ]

    It is entirly possible for the advance weights to stick. - The seccond reply, something I under-read, sounds like Timeing could be a possible cause. Like they are sticking after freeway speeds, then when the engine returns to idle *Pop*.
    I'll grant, I under-read that. But my original reply still holds true, regardless.


    [ QUOTE ]
    Would a carb rebuild seem logical for this problem?

    [/ QUOTE ]

    Yes.




    Trying to diag something first hand is tough enough - Tring to diag something over the "net" is just plain foolish.
    For the readers and the repliers.
    <font color="white">Did you see the word "might" - grimm ? I add it to the majority of my replies, something you "might" consider doing. </font>

    You can't fix it untill you find it. Thats the way it works.
     
  17. mud390

    mud390 1/2 ton status

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    I was planning on rebuilding the carb anyway, just for cheep insurance. Now it seems even more logical. Its cheep enough either way. I think I'll pull the heads off next weekend and peek in there and check the valves too. I'll look at the bore while I'm at and give the motor a once over. Thanks for all the input!

    Kris
     
  18. diesel4me

    diesel4me 1 ton status Premium Member

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    It could be a million different things,I have had backfiring upon acceleration when my accelerator pump went south,but since your problem comes and goes its more likely ignition or valve related--perhaps the pick up coil in the distributor is failing intermittenly,or you could have weak or broken valve springs(intake valves)or a burnt or warped valve,or sticking valves.In the shop my friend runs he has had several vehicles come in with similar symptoms and they drove him nuts until he discovered they had those bosch platinum plugs with the pinpoint center electrodes--they looked perfect,but upon replacement with ac-delco oem plugs the cars ran 100%better..Not bashing bosch plugs,but i have seen this to be true in many cars.I would get rid of the quadrabog,in my 25 years of gm ownership I've only had 1 on my 69 gto that ran halfway decent.I put a carter afb on my 400 small block and could not beleive the power and tourqe improvement! too bad the quadrajet had gas washed the rings halfway out of it before I changed it,now it smokes a little. /forums/images/graemlins/angryfire.gifShould have done it ten years ago.You may have low coil output,or a crack in the coil that will allow spark to jump elsewhere instead of firing the plug intermittenly--this would happen more on damp or rainy days.Also rocker arm may be binding or cracked,check the pushrods while your in there too!.Good luck!. /forums/images/graemlins/grin.gif /forums/images/graemlins/truck.gif /forums/images/graemlins/usaflag.gif
     

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