Random pops from carbs

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Robert Thiemrodt
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Random pops from carbs

#1 Post by Robert Thiemrodt »

Recently completed a tune up of my super 90. Runs very strong. Picked up an extra 6 mph on top end. Out of the blue I get random pops/belches/spits from my solex split shaft carbs. Definitely not a backfire through the pipes. Any idea for a fix, or, is this a nature of the beast?
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David Jones
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Re: Random pops from carbs

#2 Post by David Jones »

Running lean. Which carb or is it both, I presume at idle? Try a 1/4 turn out on the mixture screws or the mixture screw only on the offending throat. There are other possible causes like dirt in idle jets or warped base causing an air leak but as you just did a tune up the likely cause is mixture screws in too far.
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Doug McDonnell
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Re: Random pops from carbs

#3 Post by Doug McDonnell »

Can also happen when you need distributor rebuilt.
1965 356C 2000 BMW 740i Sport 1967 Honda CL77 There is never enough time to do it right, but always enough time to do it over.

Robert Thiemrodt
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Re: Random pops from carbs

#4 Post by Robert Thiemrodt »

Spit/pop is not occuring at idle. Very randomly under acceleration. I can be cruising along at 50 m.p.h. steady, and it will spit. Very seldomly, sometimes not at all. Should I just check all four mixture screws? Thanks all
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John Clarke
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Re: Random pops from carbs

#5 Post by John Clarke »

Hi Robert
As Doug mentioned this could be a distributor problem, likely electrical with something momentarily losing contact or breaking down. Check the insulators on the contact Points. Also check how secure the leads are from the dizzy to the plugs. Could be a bad plug, sometimes showing by one of the exhaust headers running cooler than the others.
Are you running the stock dizzy? Replace the Capacitor and Coil and see if you still have the problem, before ripping the carbs apart !
Regards Jay
 

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Re: Random pops from carbs

#6 Post by Mark Schumacher »

Not sure who said it first: “90% of carb problems are ignition”. I have found this to be true over the years

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Re: Random pops from carbs

#7 Post by Robert Thiemrodt »

I will check all four exhaust points with thermostat gun, to see if one is running a little cooler. I'll check leads on distributor. All plugs and wires are new.
Not sure if Dist is stock, a little new to this business. Started doing it, only after the tune up. Other than the spits, the tune up gave me some extra hp's.
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Re: Random pops from carbs

#8 Post by tracy pellett »

My solexs have a cough between 3000 and 4000 rpm. I get worried when it doesn’t cough. Of course petal to the metal takes care of it .
tracy pellett

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Re: Random pops from carbs

#9 Post by Tim Berardelli »

Robert,

Also check for cracked plastic spacers under the carbs. Very common issue. At idle, spray carb cleaner around the base of the carbs-if the idle goes up you probably have found your problem. If you find a cracked spacer you must replace in pairs, unless you have some OEM originals in your "spider box"-current replacements are thicker and can't be combined with originals on the same intake manifold.

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Re: Random pops from carbs

#10 Post by Robert Thiemrodt »

Tim, cracked plastic spacers? Not sure where/what that is. I'm still fairly new. Exhaust temps are correct, so nothing running cold. I notice it most often when lifting off throttle between 3000-4000 rpm. Yes Tracy with pedal to the boards, no issue what so ever, just pure speed!!!

Can't get it to do anything in the garage to determine which carb it is.
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John Clarke
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Re: Random pops from carbs

#11 Post by John Clarke »

Good advice from Tim
Robert,
I know you said definitely not a backfire through the pipes, so I ignored an exhaust leak, but the clue to the problem could be related to lifting off. And when you immediately continue, you need a squirt of gas from the accelerator pumps. If not you could get a spit back. By removing the air filters on the Solexes ( you have got stock carbs?) operate the throttle arm and check that you can see fuel squirting out of the accelerator pump discharge tubes down into the throats of the carb. You don't need the engine running to see this, and it avoids a spit back in the face! These are very fine tubes and fuel sediment can build up in the pump diaphrams, blocking the outlet tubes. You may need a flashlight to see.
A simple check to eliminate this one.
Regards Jay
 

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Re: Random pops from carbs

#12 Post by Robert Thiemrodt »

John,

I know I'm good on that front. During tune up I used a vial David told me to purchase. I am getting equal squirts to all throats. One carb was getting too much fuel, so I adjusted the accelerating pump on that carb. Definitely not a back fire. Also checked timing, and that is spot on.

What about the plastic spacers Tim mentioned. Does anybody have a pic of that. Thank you!
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John Brooks
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Re: Random pops from carbs

#13 Post by John Brooks »

Black or brown spacers between the carb and the manafold.
Try .1 more cc of gas in the accelerator pump discharge. Lean will sometimes cause a pre ignition pop. Rich makes black smoke puffs. Rich is cooler than lean,
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Re: Random pops from carbs

#14 Post by Doug McDonnell »

1965 356C 2000 BMW 740i Sport 1967 Honda CL77 There is never enough time to do it right, but always enough time to do it over.

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Re: Random pops from carbs

#15 Post by Robert Thiemrodt »

Is 35 ml correct volume for 2 squirts of petro to catch in vial?
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