Motorcycle Repair: 1980 honda 900 C, bikebandit com, intake leaks

Question
Hey Bill,
Ive been working on this 80 900c for over a month. I dont like to bother anyone with my problems but im desperate. Heres the lowdown. The bike pops and backfires like crazy. The bike idles great. With a quick throttle snap, it dont seem to stumble. The worst part is it isnt always consistant in backfiring under the same circumstances. Except when riding and holding a steady speed, sometimes on decel from full throttle. When riding its like im doing a drive by shooting. Everyone looks to see if im actually shooting a gun. Usually when warming it up sitting, it will seem to get better. As soon as i start to let out the clutch i can hear a popping from the pipe. If i ride with the choke pulled, it dont backfire or pop. The bike always idles perfectly. Ive replaced plugs, coils, wires. Cleaned carbs twice. Valves are all between .003 and .006. Synched carbs, checked charging system @ 14.7 v. Checked static timing, compression all between 135-145, swapped out spark units, checked pulsers resistance, checked exaust leaks, checked intake leaks ( intake and box with carb cleaner ). Its wearing my down cause it seems that 2+2 dont equal 4 anymore. The bike also runs great with full throttle without choke pulled. I know what im saying is confusing but it is what it is. Help a brother out if you can.

Answer
Bob/// Check this site for parts illustrations:
http://houseofmotorcycles.bikebandit.com/honda-motorcycle-cb900c-1980/o/m2011
Go to carburetor components, look at item #11 and the diaphragm assy shown there. That is an anti-backfire valve and the diaphragms do fail. You can find them in the aftermarket catalogs for less $$ than Honda, but they should still be available. You mention cleaning the carbs twice, but didn't mention the air cut (anti-backfire) valve diaphragm.

These carbs have an accelerator pump, which helps richen the mixture when you crack the throttle, which can mask lean conditions on acceleration. Be sure that all the vacuum slides are moving smoothly in the carb bores.

FYI Honda shows normal compression at 171 psi. Now, with CV carbs in the way of air flow, even with the throttle held wide open, it may still show lower in your case. Maybe get a leakdown test done to verify pressure losses. If 125-145 readings were shown with no carb bank in place, then the motor is getting worn down inside. Oil, being drawn down the valve stem seals or past worn rings will alter the fuel mixture in the cylinders, as well as add extra combustible mixture to the exhaust side, which can ignite and cause backfiring.

If the intake or the exhaust have been changed from OEM standard, then there is a tendency to lean the mixtures out, plus non-standard mufflers will also cause a lot of popping and banging, due to lack of baffling.

Don't let the idle speed get too low when hot, otherwise there will be a bigger tendency to backfire. Should be idling about 1,000-1,100 rpm.

Did you replace the spark plug caps? Should have about 5k ohms each.

Float levels are 15.5mm
#38 idle jets
#68 primary mains
#105 secondary mains

Did you check the intake manifolds for cracks and air leaks?

Is the petcock screen clean, inside the fuel tank?

Is there a vent passage in the fuel cap that might be partially clogged?

I don't see the vacuum operated fuel shutoff on these illustrations, but they did use that device on similar models. A vacuum leak into the diaphragm will cause fuel restrictions and lean out the engine, as well. If your engine isn't equipped that that, GREAT!

The fact that it runs better/good with the choke on, pretty much points to a lean mixture situation. I don't know if your carb clean was a complete teardown or not, but the emulsion tubes up inside the carb bodies are cross drilled to help the fuel emulsify and mix more thoroughly. Be sure that ALL internal parts have been inspected and cleaned completely. #16 and #17 on the microfiche are between the main jets and the needle jet (secondary main) and must be checked.

With the spark advancer running in oil, they seldom cause problems, but anything is possible, so check it to be sure that it is advancing and retarding properly. Note the air gap between the advancer lugs and the pulser coils. Should be pretty small gap... usually around .008 to .012 in many applications, but check your book for specs.

Hope that helps you consider some other possibilities.

Bill Silver