Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Dialing in your rebuilt Quadrajet carburetor => Topic started by: Overcamber on April 20, 2016, 08:17:48 AM
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finally had a chance to drive my truck with the new rebuilt carb. Having a couple issues. First off wide open throttle is crisp and clear. transition seems great!
My problem is low speed and city cruising speed. Very light throttle , approx. 1400-1800rpm, 35 to 50 mph the mufflers are loading up and backfiring. Even at 1/3 to 1/2 throttle.
Can you suggest something I can try to alleviate this problem?
thanks Gary
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just tried turning in the apt 1/2 + another 1/2 and it didn't improve(thought it would make it leaner). there is a slight grade and corner on my street that you have to take at 10mph and I can barely make it up without it sounding like a machinegun...I have louder type mufflers which amplifys the whole scenario
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just trying different things cause I'm off today. turned in the apt all the way to find its original setting was at 4 3/4 turns out from bottom.
just tried 1 1/2turns out and not much difference. its as though as soon as I squeeze the fuel its getting too much and begins to backfire??
let me know your thoughts/ this isn't my daily driver so I have some time to get it right.
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Man I give up ! Adjusted the mixture screws and everything seemed happy at 2 1/2 turns out . Go to take it for a ride and it shut off . No fuel at idle . Runs with the choke on but quits the minute I kick the choke off ?
Glad I'm working tomorrow cause I'm about to burn this truck ! Lol
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it seems like you have a couple of issues , first it acts like its flooding, then it sounds like your running out of fuel, vacuum leak, float sticking,filter, plugged , is the carb staying full of fuel . you say its not your daily driver ,is there crud in the tank and it ran good for a little while and now garbage has gotten into the needle and seat.did you build the carb?these may seem like dumb questions but you need to verify them one at a time before you go making a ton of adjustments. fwiw
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no , I just got the carb back from Cliff. I drive it a lot but I meant I have a second vehicle to use at this time.
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What is the carburetor number and application?
Backfiring is usually lean not rich. May be a vacuum leak if it's acting up an idle and low speed/light engine load......Cliff
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my carb number is 17086248
2315 JLH
its a 1986 gmc 1/2 shortbox 2wd 305 4bbl.
its isn't backfiring out the carb. its backfiring in the mufflers.
thx Cliff
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Sounds like a possible issue with the ignition.
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although I wouldn't rule that out ,I find it very coincidental or odd its that as the carb worked fine in that area before I sent it away?. just saying?
also it runs perfect at wide open throttle? would an ignition problem only affect the problem area I'm having?
thanks Gary
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The wires tend to break on the pickup coil in a HEI & can cause a similar deal. Not saying that's it but it might be worth looking at.
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Well I've had the truck since new and I know the pickup is original . 30 years old and 230000km so perhaps that's worth a look?
Curious what cliff has to add?
Really appreciate the help guys ! Keep it coming , it's great discussion .
Thank you
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this might sound stupid , but I've seen a cracked cap,wet cap or sticky weights in a distributor.act similar to this. fwiw
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Backfiring is almost always ignition, or the carb super lean at light part throttle, or a little of both. Since it fattens up things at heavy/full throttle you may not have any issues there, but if trying to burn a lean mixture ignition issues show up quickly.......Cliff
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Ok I'll pull the distributed and start there /. Hope I find the cure !
Thx
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been working long hours/ hoping to get to it wed/thurs this week
thanks guys
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So I checked my vacuum advance and it won't hold a stitch of vacuum so tommmorrow I will be trying to locate one of those . Today I picked up a new pole piece( ignition pickup) for in the distributor ( it tested ok but it won't last forever so now is the time to change it . ). Also picked up a new fuel pump at the GM dealer as well . I need to rule these things out as I proceed troubleshooting this issue .
Please add comments if you feel it will help me .
Thanks in advance .
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The vacuum advance is a big player here. It adds timing at light throttle cruise to help burn a lean mixture and provide improved engine efficiency. Without it performance will be poor and you will have running issues, poor fuel economy, etc.
Also check the mechanical advance to make sure it's working correctly........Cliff
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I'll be doing that today
Thanks again Cliff
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Well its been one hell of a bad/expensive day. Started doing more t/s today and noticed the BrandNew choke pulloff on my carb is pooched> wont hold a stitch of vacuum> not sure what happenes now?? Cliff? I tested my old one and its still good so I put it on.
anyhow, this is what I've changed today and still have the same results:
new choke pulloff
new cap and rotor
new pickup coil
new vacuum advance
new plug wires
new spark plugs
new msd HEI coil
replaced 8 ft of vacuum line so every connection has a tight fit
tested every vacuum device under the hood to rule out any vacuum leak
$$$$$
was able to set the timing as per my emission decal, suppose to be 6deg @550rpm in drive with vacuum advance disconnected and plugged... was able to do that but the truck is constantly missing and backfiring..i noticed with the timing light that it hits the timing mark for a couple beats but then the light flashes and the timing mark is gone??? did check the mechanical advance and it is in fact working.
next step is to check for garbage in the carb?..I don't know anymore? any ideas?
tthanks
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somthing to add since my earlier post today> I checked the spark at every cylinder while its running and its bright and strong for what its worth.
so in frustration I removed the carb and started a further inspection. First thing I noticed was that the primary metering rods hanger had 1 half of it really bent up..like about 1/8" higher than the other side> I thought for sure this was the overfueling problem.
then I noticed that one of the main well bleed tubes was plugged at the end with a very hard substance(looked like an epoxy) I was able to grind it out after a period of time with a needle and flush it with brake cleaner. All the other orifices blew out clean. I thought for sure that between the 2 things I found it would of cured my engine chugging and missing but its still the same... grrrrrrrrrrrrr!
also the wide slot between both the primary and secondary flaps on the top is sealed with some sort of epoxy??( I think its the slot where you would put a float check tool in but I may be mistaken? I can provide pics if needed?. is this normal? I know my carb I sent didn't have this?
thanks
thanks
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The best thing to do at this point would be to send it back and we'll test it on our test engine and see if it has the same symptoms.
Backfiring is seldom a carb issue. Most often it's an ignition issue, or worn cam lobe, stuck/sticking valve or broken valve spring.
Not sure how the power piston arm could be bent, we spend a lot of time making sure they are exactly even on the APT carburetors and verify that the carb works correctly on our test engine. If they are off some it shows up quickly.
We block the "slot" in the airhorn or use the factory divider plate. This keeps fuel from being sucked over into the secondaries when the flaps first open......Cliff
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Ok Cliff , going to check a few more things and will let you know what I decide .
Thanks
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Well after further troubleshooting I've found my problem. Cliff you were right! ( no suprise) . Went out to look for a 5,7 crossfire with my new wires I installed before I put it away for winter storage . I didn't have the crossfire problem but did notice I had 2 plug wires side by side on the wrong posts on my distributer cap. I gazed in disbelief and couldn't believe my eyes !! Embarrassing to admit . I had a carb problem last fall and changed the wires because they were old . It was at that time I introduced a 2nd problem besides the carb problem and never gave the firing order another thought assuming it was correct . I learned a lot through all this and corrected a lot of problems I had going on under the hood . The carb is now setup and runs perfectly . Just want to thank everyone for all their input( especially Cliff for the support through this whole ordeal )
Cliff you have a pm from me regarding the choke pull off
Cheers. Gary
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Send you another pm cliff
Thanks