Honda Motorcycle, CB900C needs expert advice?

Question

The Patient: 1980 Honda CB 900 C, stock except for Jardine 4 into 2 headers, being restored after 3 years abandoned in garage. 30,000 miles total.

Condition and Treatment so far: Carbs replaced with overhauled unit by reliable shop, and synced to bike. (Original carbs were cleaned, but still popped as noted below; were never tested at high rpm.) Jets stock at 105, running in Houston Texas at sea level and 80 degrees F. Compression all excellent. Valve clearances all within specified tolerances. New plugs D9EA (recommended). Plug wires checked for 100% continuity. New battery: Alternator output 12 amps at 14.5volts, battery voltage at 12, coil voltage at 11.2, Pulsar gap to .016, static time checked Ok. Waiting on new timing light for dynamic timing. (Note: Pulsar was gapped with metalic gauge, although not magnetic.)

She runs fine except:

Problems: #1 When no accelerating (ie. loafing along, not much load on engine or backing down) get popping on right side Jardine, some times a lot of popping. Not full backfire, but a lot of popping.

#2- Main problem: On opening her up, whether off the line, or on the highway, at 7500 rpm, power just quits. Will not go above 8000. Can sneak up on 8000 at less than full open throttle, but then same problem and will not go above 8200. Pulling on some choke makes no difference. Ran at 6500 for 2 minutes -- lots of popping on right side-- shut down and pulled 1 and 4 plugs immediately. Very light beige color, with a touch of pink(?). Absolutely no oil or soot or black. Possibly too lean due to Jardine headers??? Would that cause these symptoms?

Diagnosis?

THANK YOU ALL VERY MUCH FOR YOUR ANSWERS. It will take me several weeks to apply them.

Answer 1

Mechanics not familiar with Honda CV carbs, don't clean or thoroughly check them.
The air cut-off valves and throttle slides aren't checked.
The air cut-off valve plunger gets stuck in their bores due to old gas gumming them up. A basic carb clean will not clean them. And the diaphragms get torn/deteriorated from carb cleaner fluid.
http://www.mrcycles.com/
# 11 Carburetor Com. Parts
The slides - with the carbs assembled, lift 2 slides and release them at the same time. They should drop to the bottom of the bore at the same time. Check all 4. If they don't -
Clogged slide lift holes.
Worn out springs.
Slide getting stuck in the carb top.
Dirty throttle slide.
Leaking gasket on the carb tops.

Answer 2

The EX500 is a *great* bike (this coming from someone who's been riding for over 25 years on a great many bikes both big and small). The throttle response is good and the power comes in with quite a kick when you give it lots of throttle and revs. However, if you treat the throttle sensibly, you're rewarded with a bike that's very easy to control. The handling is very sweet as well, once you've learnt to handle the bike properly, on all but the straightest of roads it'll take a very commited rider to leave you behind. Cornering (on good rubber) they're as good as bikes much larger / faster. Even on the straights, they'll run up to 125 mph if you've got the early model. Do test-sit on one though, if you've got long legs you may find the riding position a little cramped. The GS500 is also very good, but is less exciting when the throttle is wound fully open. The engine response is very linear which makes for a predictable (some may say boring) ride. Once you've learnt, you;ll be wanting to change this bike whereas you'd want to keep the EX. The Ninja 250 - good although it is - won't give you the buzz that either of the 500s will, and are actually more difficult to ride because of the need to keep the engine on the boil to make progress. A Buell? Cool looking bike, but not exactly trouble free. Carry a rucksack to collect all the bits that fall off when you're riding it and buy shares in Loctite.

Answer 3

You may have to readjust your main needle valves in your carbs since you opened up the exhaust with the headers.
You may have to rejet as suggested but you can tell that by looking at the plugs. Do they look powdery white? If not your not too lean. Those bikes were not designed to run lean. Popping was very common upon messing with the exhaust. I would be more concerned about what sounds like fuel starvation at high rpm. Like I said try the needle valves.

Answer 4

Now let me ask this. When you put that header on did you rejet to the headers? otherwise pull the valve cover off and see if you have a flat spot on a cam or do a leak down test to see if your drawing air either from an intake or exhaust port. or perhaps one of your carb boots have a leak or crack, and I well assume you synced the carbs?

Answer 5

sounds like it is running lean up top end, try going up a size or two on the main jet, and check for exhaust leaks sounds like it is sucking air into the pipes some where. im guessing you changed the air cleaners? best bet would be to run the bike on a dyno and see what the exhaust emissions are doing. good luck.

Answer 6

Cb900c Exhaust