Diagnosing a carb problem

[pdx_r100s]

I just stripped down my carbs and cleaned them in an ultrasonic cleaner, replaced rubber, mounted them, synched them, but now having a problem.

The bike is running nicely at idle and off idle, but as soon as I ride a block or so I get a very loud backfire, and the bike dies. I can’t get it to start again until it cools off for 15 mins, then it seems ok until I ride a block and it does the same thing.

Before doing this carb overhaul, I installed a new electronic ignition. I had the mechanic set the timing and adjust valves. I rode for a few weeks like that without issues other than that the carbs seemed a bit out of tune (he suggested I do a good overhaul next). So I’m thinking that because this problem appeared after I had the carbs apart, and the bike was running decently after he did his work, it is related to the carbs and not ignition/valves.

Do my symptoms sound like a rich condition or a lean condition? I was going to go back in and double check float heights, but if anyone has other suggestions, please tell me. The air screws are about where they should be (3/4 turn out—spec is is 1/2 turn for this year/40mm Bing).

Thanks

[robert]

Sounds electrical to me.

[gspd]

I get a very loud backfire, and the bike dies. I can’t get it to start again until it cools off for 15 mins, then it seems ok until I ride a block and it does the same thing.

Sounds like textbook ignition failure.
If it was carb related both cylinders wouldn't die simultaneously.

The air screws are about where they should be (3/4 turn out—spec is is 1/2 turn for this year/40mm Bing).

These are fuel screws, not air screws, Pretty sure they should be more like 1.5 to 2 turns out, but I don't think that's your immediate problem.

[pdx_r100s]

If it was ignition failure, why would it run fine for a time? Also, the ignition is brand new and worked fine for the last few weeks until I took the carbs off, and then this problem immediately appeared. I’m just having trouble understanding why we’d suspect ignition right away when that wasn’t touched.

[barryh]

If it was ignition failure, why would it run fine for a time? Also, the ignition is brand new and worked fine for the last few weeks until I took the carbs off, and then this problem immediately appeared. I’m just having trouble understanding why we’d suspect ignition right away when that wasn’t touched.

A backfire in the exhaust is unburn't fuel in the exhaust being ignited. That typically happens when the ignition system intermittently fails to ignite the mixture in the cylinder so that it passes into the exhaust system and is then the next time the ignition does work you get a back fire.

Why after the carb rebuild and not before ? A possibility is that if the ignition is marginal it may work fine with the correct fuel mixture but will fail to reliably ignite a mixture that is excessively too lean or too rich.

[Rob Frankham]

Some years ago, I had a problem with a bike that would run perfectly for some time then, for no apparent reason the engine would stop. Sometimes, it would restart again before coming to rest and often that would be accompanied by a backfire. Sometimes it would restart immediately, sometimes it would take some time before it did. Took me ages to work out the cause... to cut a long story short, the problem was a broken conductor in the wire between the battery and the ignition switch (the insulation wasn't broken). running a parallel positive feed to the ignition switch solved the problem permanently.

I'm not saying that this is necessarily your problem but the symptoms are quite similar so in your shoes, I would be looking at something that is causing the ignition to fail. Bad connections, bad wires, kill switch, igntiton switch... all of them might have a similar effect.

Don't rule out the ignition system itself. It is a known phenomenon that many things, especially electronic ciruits, are more liable to fail in the first few weeks than at any other time. I would certainly not rule it out as a source for the issue.

Apart from anything else, many ignition systems have overheat protection which causes the ICM to switch off it it gets too hot. You don't say which system you had fitted... Should it be fitted with a heat sink? in an airflow? not close to hot components? was this done?

Why is everyone assuming it is ignition... well that's because it's what the symptoms suggest.

Going back to carburettors. Is there any possibility that, when you rebuilt the carbs, you swapped the enrichener (choke) components from side to side or replaced then upside down. This might cause the engine to fail run OK when it's cold but fail when it is hot... although I would expect that to lead to progressively worse running as the temperature increases not a sudden cut out.

Rob

[gspd]

Not sure what happens when both chokes are reversed, never tried it.

Backfiring 101, just for fun..
Next time you rent a bike, try switching the kill switch off for a few seconds and then back on while riding.
It's a blast! (literally)

[pdx_r100s]

Thanks. This is all very helpful—I’m learning a lot.

The ignition is an enduralast unit—it has an optical pickup. The mechanic set up the timing after he set the valves, and I had a few hundred miles on it without issues before I tore down the carbs.

I also replaced the coils when it was installed.

One thing that may be significant—when I first remounted the carbs after the overhaul, the left one overflowed with fuel when I turned on the petcock. I just assumed it was a stuck float needle and gave it a whack. I want to at least ensure that the carbs are set up correctly before I go exploring the ignition situation.

[gspd]

When it's in its won't restart mode, pull a spark plug out, rest it on the cylinder fins and crank the motor.
Do you have spark?
I'll bet you don't.

[gspd]

"I also replaced the coils"
Just a thought...
Which coils did you use?
About 50 years ago an airhead with brand new Bosch coils came to me running very poorly, cutting out, etc. (I forget the exact symptoms). Turns out the owner had installed two identical looking 12v coils instead of the required two 6v coils (wired in series).