QuestionI have 1981 Honda CM 400C. I recently purchased this from a young kid and the bike seems to be running, but not great.
The problem is only relatable when cold/start up, but once warm, it runs fine, and has excellent power and excelleration... As I can ride all day, but the next day, the same problems until warm again. or at least until today....
I just recently learned the choke cable was missing, and have to manually hold it open. I cleaned the carbs out, partially. it ran great and started right up cold and sounded great. the next day, same thing, however.
now, I started the bike today after riding all weekend, but the weather dipped into the 40's the day prior, so it is running rough now.
I started it up today (low 70's temp outside)and it immediately backfired at first crank, and died immediately. Then, attempting to restart, It spat black OIL??? out of the tail pipe and there was a small bit of smoke coming out of the spark plug?? which it hadn't done prior. (or not that I have noticed anyway)
after manually holing the choke open all the way, it started, would run for 1-2 minutes and then die once I released the choke slightly. If I released the choke to less than half, the engine and carb would violently buck and jerk, forcing gas to come out of the carb from somewhere, which was causing a leak from some unidentifiable area in the carb near the carb housing that holds the carb. this is puzzling since it ran fine after warm up on Sunday. so, I attempted to warm the bike up but after five minnutes of failure, bucking and backfire I figured I should ask before causing serious damage to my honda, with all the sudden and different problems I am now facing since sunday. (I have started every day since purchase, this being the first time I have noticed any of these problems.
I am new to bikes/small engines, and don't understand some of the directions in my Clymers Manuel. could I have input the jets incorrectly or not correctly positioned the pilot screw/ or throttle adjustment screw? I believe it was runnng decent when I first purchased, and got it to even better running condition after just a small bit of carb cleaning...but have had to adjust the throttle adjestment screw, not being in the same position as first purchased, but was running then also
NOTE: can not remove the top portion of the carbs as screws are VERY tight, and fear I will strip them, so the top area with the springs and diaphrams haven't been cleaned.
I fear with all the problems listed, I was dooped by a young kid at my excitement for my wife wanting to get me a bike for my bday, buying the first 1500 running bike that came our way... Thinking the starting problem was just gummed carbs. so you can imagine my wofe is upset now that I emptied the bank account for a problematic motorcycle..
please offer any assitance you can please. I would be very thankful if you could assist.
AnswerBrian..... refer this illustration of the carb parts that may be involved.
Yes, you can reverse the primary and secondary main jets and cause a rich low end condition, coupled with severe leaning out as the throttles are opened. Primary jets are #72 (a 75 there won't hurt) as illustrated at #31. Secondary main jets are #118 as shown as #32.
You really need to get a replacement choke cable. Motion Pro makes reproductions for about $15 each. Check your dealer or Ebay auctions.
Note that there is an air-cut valve, which is an anti-backfire device, but when the diaphragm fails then it leans out the engine on that side and causes backfiring and bad idling. #14
Note, also, that there is an accelerator pump diaphragm fitted to these carburetors, which helps the engine make the transition from idling to mid-range. #7
"Smoke" from around the spark plug area may indicate a loose spark plug or arcing due to bad spark plug cap resistor. The caps should ohm out at about 5k ohms, disconnected from the spark plug wire. Fouled spark plugs will cause backfiring/misfiring.
Air leaks at the intake tract... intake manifolds, diaphragms, vacuum lines, etc.
For those tight screws on the top of the carburetors, you can do a couple of things to loosen them. Get the correct size screwdriver tip, insert it into the screw head firmly and tap on the driver tip with a small hammer. Hold the screwdriver tip bit with a 6" crescent wrench or a long 8mm (usually) wrench and use it for more leverage as you try to turn the screw out. If those methods fail to get results, get the smallest Vise-Grip pliers you can find with nice sharp jaws and grip the head of the screw with the pliers and rotate them out.
Probably wouldn't hurt to check valve adjustments and adjust the balance shaft chain on the right side of the clutch cover.
Bill Silver
