Diagnosing carb problems

[Hishman]

Iv'e had alot of problems with my carbs since the day i bought my bike. It has never run off the choke. When i turn the choke off it dies. If i give it gas with the choke 1/3 out it backfires into the intake.

I've seen people say clean your carbs and then clean them etc. Well ive cleaned them about 6 times. Twice with carb cleaner once with works toilet bowl cleaner(more on that later) soked them overnight in vinigar then boiled them in it. I soked them in chem dip as well. These suckers a clean. I can squirt carb cleaner in the jets and have it spray out the three little holes and the idle needle. So i don't think that is the problem.

More about the works cleaner. I read somewhere that toilet cleaner worked. I soked my carbs in it for about 5 minutes. Bad idea, it ate away at them turned the metal a dark gray and now it appears to be corroding.

So hear is the problem i cant seem to figure out why it wont run off the choke at idle. What steps can i do to check other problems.

Im not sure if the carbs are ruined or not and i may just buy a set of new set

 

[willem]

If it's firing back into the intake there might be another problem. I know a guy here in the Netherlands who had an intake valve which was bent, so it never sealed and he had it backfiring into his carbs a lot as well.

But now your carbs may be ruined by your cleaning attempts, so you may have two problems instead of one.

So make sure your carbs are OK first. I don't expect the aluminium alloy to be progressively corroding away since it doesn't really do that (well, aluminium actually corrodes so fast that it seals itself in a corroded layer within a few hours) unless there's still toilet bowl cleaner on there eating away the metal. Clean the carbs in some vinegar or lemon juice again, that might get the dark gray off.

As for the valve being bent, the only way to check it for real is to take the top end apart and have a look.

 

[Yamaguy55]

Compression check will prove if valves are not seating. I'd check my valve clearance as well. If they are so tight they don't truly seat, it acts just like that. Popping/spitting in the intake can also be a sign of lean mixture.

However, carbs from the 70s-80s don't like ethanol. You clean them often or you can take a beating. It usually is the pilot circuits and those tiny passages seen at the top on the engine side of the carbs, right under the mixture screw, but inside the throttle bores. they like to clog, and there's your idle problem right there. (guess, but no doubt an accurate one). you can't leave ethanol gas in these things more than a week or so without grief unless you keep some sort of fuel stabilizer in there as well. Add the usual cold/hot temp swings of this time of year, and the age of the bikes, and there you have it.

Be sure the idle passages are clean and the valves are set properly, it may start to run fine.

 

[XSLeo]

I agree you probably have them clean. If it only runs with the enrichener on, it's not really a choke, then you need to at least adjust the idle mix screws. More likely needs to be rejetted.
Once warmed up if it won't idle without the enrichener up the pilot jets one size, adjust mix screws to match. If still no idle step up the pilots one more.
Leo

 

[padre]

Same problem on my 80 xs 400. Is there an air mixture screw capped off somewhere and if so where

 

[Yamaguy55]

Same problem on my 80 xs 400. Is there an air mixture screw capped off somewhere and if so where

Under the block off plugs, on the top front, right up against the rubber carb mounts. There is a screw with a needle-like end, a spring, a washer, and an O'ring. The needles are to allow the already-mixed fuel-air mixture a path around the almost closed throttle plates. There are several holes inside the throttle bore. They allow a transition from closed to open throttle. Between the jets and passages in the bowls, this is the primary area the carbs clog. Unlike the slightly larger BS38 carbs that are used on the XS650, these seem particularly prone to clogging. Running fuel system stabilizer continually seems almost mandatory with ethanol based fuel and these carbs. I have no idea why these are worse, or seem worse.