Motorcycle Repair: cb400a, honda shop, jet sizes

Question
QUESTION: Hi
I have a 1978 Honda cb400a. I recently had the carbs rebuild buy a pro and rejetted for air filter pods and a 2 into 1 exhaust. I replaced the ignition coil, caps, spark plugs, and battery. My problem is it doesn't go over 30mph. I. also can't turn the throttle more than 1/4 turn without sputtering, popping and backfiring which also happens as I approach 25mph and can't get to 30mph. I'm wondering if it's the CDI or the Stator/Excitor coil. I think I can test the stator but not sure about the CDI. What should I do? Can it be anything else.
I hope I gave enpugh information.

ANSWER: Carlos, I worked at a Honda shop in 1980 and we saw a lot of the 400T series machines. A Honda rep once told me that all Stators would have to be replaced on the CB400A models. eventually. It might be that the CDI module isn't giving you a good spark advance, though. Pull the flywheel cover off and check the timing with the engine running, using a dynamic timing light.

If the spark timing is "stuck" at F marks, then most likely the module has failed. Generally, if the pulse generator coil fails, you get nothing out of the system. If the bike starts and idles properly, then It would follow that it probably isn't getting spark advance, as it should.

It might be helpful to find out what the rejetting sizes were in the carbs. Usually, you can just bump them up about 1 or 2 sizes to compensate for the changes in air intake and exhaust system.

Pull the plugs and get a reading on them... black, white, oily, dry, cracked insulator.

Bill Silver

---------- FOLLOW-UP ----------

QUESTION: Hi Bill

The jet sizes re 78 and 125. My carb doesnt have an accel pump. could that affect this problem? Im getting a test light tomorrow and will check this out. Should I test while in gear also? If advance is off then it's the CDI?

Carlos

ANSWER: Stock sizes are 75/125 so fine for stock exhaust/intake. Probably need to go up to 78/80 primary mains and 130/135 secondary mains.

http://www.cmsnl.com/honda-cb400a-hawk-hondamatic-us_model1009/partslist/E++19.h     Doesn't show any accelerator pumps on this model. Probably would help it, though.

Pull the choke out slowly as you reach the 25mph speed and see if it picks up power. If so, then you are way too lean. I am assuming that your carb mechanic checked the air-cut valve for any tears/pinholes and it is seated properly.

The true tests for carburation are under load, generally.

Make sure that the gas cap vent hole is clear and the petcock is all clean from screen to outlet.

The CDI module controls spark advance. It is supposed to advance at specific rpm levels.

Bill Silver




---------- FOLLOW-UP ----------

QUESTION: Ok got a test light and the "fn" marks is off at least an inch.
Will test it in gear tomorrow. I think the jets are goo because
my Clymer says 70/110 for the stock jets. Is the mark being off
relavent? Can I adjust or service it?

Thanks again
Carlos

Answer
Carlos, I have to retract an earlier statement, as I was looking at the wrong carb chart for the 1978-A model calibrations. Stock jets are 72/105 according to my Honda tune-up booklet. Based on your 78/125 settings, I think it may be too rich on the top end main jets, instead.  70/110 jet sets you mentioned in Clymer's book are for the 1979 model. They changed the calibration of the carbs several times.

You didn't say if the mark was off towards the T mark or away from it on the opposite side of the F mark. Timing may automatically advance when the engine is idling on an automatic.

The Honda book shows that TN timing is 7.5 degrees before T mark and F mark is 15 degrees. Advance begins at 1700-2100 rpm and finishes at 4500-5350 rpm with 45 degrees total. Put the bike on the center stand and check the timing in L or D and compare that to the above specs.

Bill Silver