Technical Q and A for classic motorcycle maintenance and repair.
If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with "Keith's Garage" as your subject.
Rough running KZ750
Q: Last summer, my 1982 Kawasaki KZ750H with 15,500 miles on it started running a little rough. One morning, I went to the garage and found gas on the floor around my bike. The vacuum operated fuel shutoff had failed. I purchased a repair kit for the shutoff, changed the oil and filter (due to gas filling the crankcase), removed the carbs to replace the needles and seat, replaced the spark plugs, and ran a compression test, with all cylinders showing 125psi. Up until this, the engine had run fine. After replacing these parts the engine would run well under acceleration, but cruising it would miss and feel like it was just going to die.
I removed the carbs way too many times, making sure all the passages were clean and all the adjustments were right. I can now rebuild one of these carbs in my sleep! Nothing seemed to help, so I went to the electrics. I checked the timing and the timing advance unit — it was rusted and would not advance, so I repaired it. I then substituted known good coils from a friend’s bike, with no change. I then synchronized the carbs, and after attempting this several times I noticed no change.
Going over all that I had done, I realized that probably when I took the carbs off the first time the rubber carb insulators had lost their seal at the head. I replaced them and now the engine fires up very quickly. It hasn’t started this easy since my problems first began. I also replaced the gasoline in case it might contain ethanol, but it still had the cruise problem. I removed the air filter and cover, and now the cruise problem is almost gone, I can barely feel the hesitation. I am going to install separate air cleaners and see what happens. According to my repair manual (which I have memorized), this means that I have a rich mixture condition. Why do I now have a rich mixture when all I did was clean the carbs? - Matt/Wichita, Kan.
A: Matt, I think you hit the nail on the head when you realized the carb insulators weren’t sealing to the heads anymore. That was my first thought when you said the bike would miss at cruising speed (light throttle). You don’t say, but did you buy new insulators or used ones? I ask because old cracked rubber insulators can cause a lean condition throughout the power band. A quick test is to spray some WD-40 on the insulators as the machine is running. Any change in idle speed will indicate an air leak. You say the cruising problem is almost eliminated by removing the air cleaner. Ensure the air cleaner is clean and not clogged, and that all the lines to the air injection system are intact, or if removed, properly plugged. Check the rubber diaphragms in the carburetors for cracks, and make sure the float heights are correct; they could have changed when you replaced the needle and seats. I’m glad you fixed the advance unit, but I don’t think your problem is electrical. I think once you confirm all your base carburetor settings are right, and that you don’t have any air leaks, you’ll find you’ve solved your problem.
Kawasaki carb parts
Q: I just purchased a 1982 Kawasaki KZ1100A with 30,000 miles that needs all four Mikuni BS34SS carburetors rebuilt. Do you know any company that has NOS parts or one that specializes in obsolete parts like the rubber diaphragms? My Kawi has the full Vetter touring package, with frame-mounted front fairing, removable saddlebags/luggage, and the large back fender luggage trunk with passenger backrest. I’m a religious off-the-rack reader of every issue you put out! - Glenn/Bridger, Mont.
A: It looks like you can get all the parts you need to rebuild your carbs, but make sure those diaphragms need replacing before you shell out the $$$ to replace them. To the best of our knowledge they’re only available NOS, and they come complete with the piston. Bike Bandit shows them at $173.17 each, while Kawasaki Parts House shows them for $154.14. All the other carb parts seem to be affordable. MC