backfiring then died

Hi, first post on here. Bought an 81 XS650 last year and have been reading for a while. Hoping someone can point me in the right direction with some engine trouble.

I bought the bike with pod filters and straight pipes installed and the word of the previous owner that it had been jetted for that setup. Other than that the engine is stock. It's always been a little hesitant to idle without full choke for the first minute of running, but runs well after it's warmed up.

After about 10 miles last night, the bike started backfiring like crazy. A minute later, only the left piston was firing. Not long after that, the bike stalled while coasting. I got it started again and made the last half mile to my destination on one cylinder firing, with the other firing sporadically. I let the bike cool down for an hour and it will turn over and fire on full choke but not idle. Looked at the plugs and they seem good...not burnt up or oil fouled.

I ordered two rebuild kits and plan on running them through the ultrasonic at work. Any input as to if I'm on the right or wrong track would be appreciated. Thanks!

Well, it's never a bad idea to give the carbs on a new-to-you 650 a going through. Most of them need it. In your case, I'd say they definitely do, if not for a possible dirty condition, then at least to check and verify what jetting changes have been made. You can't trust what a P.O. tells you. He may have made some changes but were they the right ones and did he change all that had to be? You'll want to check the main and pilot jet sizes. If the rejetting was done correctly, both should have been changed. By the way you need nearly a minute of choke on a cold start, the pilots may not have been changed. Many other bikes can get away with just a main jet change for minor mods. These 650s aren't like that. They usually require both the mains AND pilots be changed. Here's a chart I made up showing the stock jetting specs. Your '81 is an H model and would have the BS34 carbs .....



What you should hope to find are pilots one or two sizes larger (45 or 47.5) and mains two to maybe four sizes up (137.5, 140, 142.5). Something else you'll need to check is if the mix screws have been uncapped. They were plugged over from the factory but you need to have access to them .....



A word on the rebuild kits. They are aftermarket and the quality of some of the stuff isn't the greatest. Don't replace any of the stock parts unless absolutely necessary.

Thanks for the info. I got the rebuild kits from mikes xs. I was planning on ordering a few different size pilot and main jets to try once it's cleaned. Is there better place to get replacement parts from?

Unfortunately, any rebuild kits will be aftermarket. But, you usually don't need to replace much. The original parts hold up well. About the only thing that may need replacing due to age are some of the o-rings and maybe that rubber cap over the pilot jet. The MikesXS kits are pretty basic. They have the rubber plug for the pilot jet and the o-ring for the float needle seat but don't contain the o-rings for the mix screw and the float bowl drain screw. That's OK, you can source those separately if you need them. The mix screw takes a 1mm x 3mm size and the drain screw a 1.6mm x 4.1mm.

Most of us prefer using genuine Mikuni jets. If you have a local shop near by that caters to dirt bikes and/or snowmobiles, they should sell individual jets. Those guys are always fooling with their carbs. Genuine Mikuni jets will have their trademark "square in square" emblem stamped somewhere on them .....







Two types of pilots were used in the different 650 carbs. You must use the type your carb set was designed for. The two types look very similar and will swap, but won't flow correctly if put in the wrong carb. Your BS34s use the BS30/96 type pilot .....

I was gonna get jets from mikes, but I'll stop by the dealer tomorrow. I tore the carbs apart this morning...definitely not rejetted. Everything was covered in buildup, the pilots were almost so stripped at the head that they needed an ez-out. Tons of deposits below the main jets. One of those rubber caps was missing.

Scrubbed everything down in the parts washer with aircraft simple green and then have been running the same solution in the ultrasonic cleaner. I don't think they've ever had a good cleaning until now.

Check your battery first.. Symptoms you describe can also be a bad battery..

That missing rubber cap would definitely cause running problems. They are there to make the pilot jets draw their fuel supply from the side, from the main jet area, not out of the main bowl. The bike won't run right without them in place.

Tore everything apart and gave it a run through the ultrasonic cleaner. Installed the new gasket, float valve and new caps for the mix screws. Went up one size each on the pilot and main jets. Set the float levels the same at as close to 22mm as I could get.

The bike starts up again, but the right cylinder (the one that stopped firing when the bike decided to quit) still doesn't seem right...doesn't sound right at idle and backfires under throttle when the left cylinder doesn't. Not quite sure where to go from here. Everything is the same in both carbs as far as I can tell.

Sounds like that side is running a little lean...richen the mixture a tad, see if that helps.

There's nothing in my manual about adjusting the mixture. Would that be done by backing the screw out or by tightening it?

I would do a compression check and a valve adjustment, you need to asess your engines condition.

JMD603 said:

There's nothing in my manual about adjusting the mixture. Would that be done by backing the screw out or by tightening it?

Click to expand...

On BS38's the screw is accessible... on 34's, I think they have a little cover you have to remove (drill out) to get access to them. I don't think it's hard to do...but should be done very carefully. If you've already done that/have access...then generally turning left is more rich, turning right is more lean.

There's nothing in the manual about adjusting your mix screws because your carbs have E.P.A. restrictions on them. The mix screws were set at the factory and then capped over so you wouldn't mess with them. But, once you start changing things and making mods, you have to mess with them. They need to be re-set, usually to a richer setting.

Your carbs use a true mixture screw, one that regulates fuel flow. Turning it out increase the flow and makes it richer. Turning it in reduces the flow and makes things leaner. This mix screw effects the idle or slow speed circuit. It's not an adjustment for high speed running.

When you cleaned the carbs, did you do so with the mix screws and their components (spring, washer, o-ring) removed? They must be removed because they effectively block the passageway you're trying to clean.

Because these are E.P.A. mandated carbs, they are set up rather lean. The mix screws usually need to be run somewhere between 3 to 3.5 turns out. Your popping problems could be as simple as not having the screws open enough.

Sounds like you could have an ignition problem.. You say the bike ran well after it warmed up.. Then it started back-firing and quit.. Why all the carburator jetting and such?? At the very least you proved it needed cleaning.. If one cylinder is backfiring before and after sounds like more than just carbs.. The idle mixture screws are just for idle.. If the motor will run long enough spray W-D40 around the manifolds and see if you have some kind of vacuum leak.. Too much air can lean out a cylinder and cause a cylinder break-down..

Is your bike using a battery?? As I stated before check it.. I have had similar problems as you describe and it was the battery.. These bikes are sensitive to ignition voltage.. Good luck!

It was the battery. I had charged it and the charger was showing 12.2v and full charge. New battery and both cylinders fire.

The idle seems good now. At wide open throttle it stutters like it's starved of gas. I'll go up another size on the main jet this week.

Thanks for all the help.