650 backfire???

Hello,
I am new to this forum but have been looking for something like this for over a year! That is when I bought my 1980 xj650 from my uncle. I has always had a backfire but I really need to fix it. I was doing some reading on here and thought I would adjust the air mixture screws. It had never been done before so I had to drill out the covers. One (BIG) problem. I drilled too far on the far left carb and ruined the screw. I cannot back it out or screw it in!!! I turned them all the way down and then backed them off 1/2 a turn. The bike runs fine but the left hand side exhaust pipe is where the backfire comes from. I am wondering if it is not that one carb that is not adjusted properly. Am I on the right track here? I have put new plugs in it, new air filter and pulled the carbs off and sprayed a whole can of carb cleaner throughout it. (No, I did not touch anything when I was inside of them.) Thanks for the help guys! I know I am in the right spot! I use forums for my other vehicles and I always find the right help! :wink:

 

Basic setting for the pilot mixture screws is all the way in and 2.5 to 3 turns out. Correct way to do it is with a colortune plug. The #4 screw is probably closer to being right than any of the others. They are not "air" mixture screws. The air is determined by the air filter, butterflies, air jets and the venturi size. This screw controls the amount of fuel that can be pulled into the air stream.
The backfire is most likely caused by a rich condition on either #3 or 4 cylinder. This could be the result of a bad needle valve causing the carb to flood slightly. You should have replaced the float valve seats and needles when you went inside the carbs. I would also make sure the choke is completely off. The choke works by adding extra fuel to the mix. Most auto chokes close off the amount of air. So the choke circuit must be off to set the mixture on the carbs. Hope this helps and feel free to keep asking for help! That's what we are here for.

 

where is a good place to buy the valve seats, needles, ect? I would like to replace that stuff but am a little intimidated. Any suggestions?

 

I like dealing with these fine folks:
http://www.motorcyclecarbs.com/CARBURET ... _P5657.cfm
Probably not the cheapest, but they are one of the best.

I was anxious about doing the carbs on my bike too; then look what I did:
http://www.zillaphoto.com/MaximX/2-rebu ... Carbs.html
http://www.zillaphoto.com/MaximX/2-rebu ... Carbs.html
In the end "twernt nuttin' to it." Well, it takes some getting used to, but your confidence working on the carbs will grow quickly. Your first tip is to leave the carbs mounted to the rack if at all possible, as shown in my links above.

We're here to help you, the same as I was helped when I had problems.

Z

 

What about the jets and needles? I saw a stage 1 and 3 carb kit on ebay. Would this be what I need? I tore into the carbs last night and the main jets are 'stuck'. I cannot turn them with a screw driver!! They were almost stripping out! (if that is a term :? ) I have put off driving for a while until I fix this (obviously!) Thanks!

 

This looks like a good time to say that our bikes were extremely fine-tuned in their original design. Unless major modifications have been made to the bike, like a performance exhaust, there's probably no need or benefit to installing a performance carb kit. It will just make it that much harder to tune.

It's very important to use really good fitting screw drivers with the relatively soft metal parts that we have to deal with, like brass and aluminum. If you just can't get the jet out with a screw driver, I would suggest an appropriately sized screw extractor, and replacement with another normal sized jet. If it's not too late, and you haven't munged the jet passage itself, you may consider just leaving it alone and cleaning as good as possible. Cleaning jets & passages does not include using drill bits and such. I was taught to only use carb cleaner and air. Others sometimes use very fine nylon or wire strands.

Z

 

no yu dont need no stage kit. as stated earlier yu have a rich condition in one of yure idle circuits. sounds like yu may want to remove yure carbs and take em to a dealer and have em soaked and cleaned. only costs about 50 bucks......maybe all the stuck parts will come loose.

 

alright, so I cleaned everything out and put it all back together again. Started it up and no backfire! I could have swore the problem was fixed. I drove it around for about 5 minutes and then it started again :x I know everyone knows what a backfire sounds like but sometimes a video does help. I want to exhaust (no pun intended) all of my options before buying anything. I am waiting for the video to upload so I will add it in a few minutes.
http://s166.photobucket.com/albums/u109 ... 010100.flv

 

Does anyone have any ideas or more suggestions for me??? Thanks!!

 

Kcoop, I'd start with a very thorough disassembly of your carbs, blowing each and every hole, port and crevice with compressed air and carb cleaner. Then re-assemble, making sure the entire thing is spotless along the way. Bench sync the carbs, then put them back on the bike and sync them with a manometer. A final colortune, and they should be running like a clock.

Total time to do said cleaning, maybe 2 hours tops, with a little know how, and that should get it running smooth across the block.

 

Just got done cleaning carbs extremely well, synced the carbs and still have the backfire on the left side (1 or 2). We are stumped :? When you spray the 3 and 4 intake boots (after carbs) with starting fluid, the idle revs up. They are visibly cracked but 1 and 2 do not leak. Could the fact that the 3 and 4 are leaking be causing the 1 and 2 to overcompensate and backfire?

 

Regardless boots on 3 and 4 must be be fixed before you go any further. They will upset your attempts to tune and trouble shoot (and might be the cause of your problem).

Quick fix, glue and bike innertube.

General backfire guide:

When does the Popping happen?

If you are slowing down and therefore backing off on the throttle does it pop then?

The popping could be a lean backfire.

Lean is BAD!

If you get popping on acceleration check the exaust gaskets for leaks.

Popping on idle (and hesitation or surging at even throttle) check plugs and coils.

What do the plugs on 1 and 2 look like?

 

The popping is when it is slowing down. The plugs look great! I am assuming lean because of the 3 and 4 boots that are shot. Would 3 and 4 running lean cause 1 or 2 to backfire???

 

Might be worth checking that the 1 and 2 slides aren't hanging up.