Back Firing

I have been working on this bike for a couple months now, and I am about ready to give up. This bike is a 1981 KZ750 H2. I recently sent in my Keihin to get worked on, and found out they had a problem. Many problems to be exact. To make a long story short, I ended up with some 26 mm Mikuni's instead. Now I believe these are good carbs, but I still can't get my motorcycle to run right. I get a ton of backfiring, but I can't seem to locate the problem. It also doesn't like to start. I have reset the cam timing, measure the valve clearances, and made sure the intake boots are correct. Anyone have any ideas on what I could have missed?

Backfiring is usually an indication of an air leak or leanness. Have you checked the nipples on the carb boots (if applicable) to make sure that there aren't any leaks? Also, a weak ignition can often be a source...are each of your cylinders firing? If you remove each spark plug lead, is the spark big, blue, and fat or is orange? You might also want to check the float levels in the carbs and make sure that they are synched. Also, check to make sure that your problem isn't in your tank...if fuel can't get to your carbs, then the bike will run like crap. These are just a few starting points. I'm sure that a few others will chime in. Just check the simple things first and then go with the more involved.

Jeff

How about the timming? If your bike has points they also could need work. Also have you put new plugs in? How's the spark look when you ground the plug agains the motor?

Rember there are 3 parts a motor running.
1) Fuel (it has to get through the carbs and into the motor.)
2) Air (Fuel alone wont burn (for long) air must be moving through the carbs)
3) Spark (A nice powerfull spark that is consistant)


If you have checked the electrical and still cant get it right you might want to check compression.

Well I can take some of those off the list. I think the float levels are ok, because I just got them rebuilt by someone that really knows what they are doing in this area. I was worried going from a 34 inch intake, to a 26 but I was told they would work great. They do seem to be getting gas. I thought the same thing about the gas issue, but there is more than enough getting into the bowls. I have also made sure that I am getting spark out of every plug as well. As a matter of fact I just cleaned and regapped them then other day. 2 of them got really black, but I was told that this can be normal with a new set of carbs.

Neotrion wrote:

2 of them got really black, but I was told that this can be normal with a new set of carbs.

That sure doesn't sound "normal" to me...

I can recheck the timing tomorrow to make sure something didn't slip when I was resetting the cams. I was very carefull to make sure I left the T right on the mark when I got the cams adjusted correctly. I did notice that when the T was on the mark, the oval part of the metal was past the sensor on the pick up coil by about 3 degrees or so. Should the oval be right on it when the crank case and the mark are at T?

are you talking about back fire in the carbs or exhaust? also how did you set the timeing, and one last thought, sure you dont have any issues with the plug wires, coils , etc.

dont think I was clear, I was thinking of ignition timing

Well it does sound like it is backfiring both ways at times. If I had to pin it down, I would say it is back firing out of the exhaust. It occassionally sounds like it is in the carbs too, but I could be wrong there. I have the electronic ignition and no points, so I didn't mess with the timing other than what I did with the cams. I followed the procedure in the manuel to the letter, so I doubt it is off.

I just pulled the plugs out again today. I had just cleaned them yesterday, and found that 2 and 3 are foweled with black sute worse than before. 1 and 4 where a little bit on the white side, but looked pretty normal. Does anyone have any ideas?

this is going to be a dumb question, are you sure you dont have spark plug / coils crossed,

hit enter too fast, kind of sounds like its wired wrong, ( spark plug wires ) or the coils, sometimes the colors dont match up on the electronic ignition , mine didnt on my 650,