Motorcycle Repair: 72 cb350, petcock, cb350

Question
QUESTION: Bill,

My 350 was running very well after a carb cleaning and rebuild of one.  Then on a.long ride it began to slow and stall at lights.  It ran rough and contours to stall occasionally at lights.  Then it stalled while I was.riding, backfired, fired up again and has run well since but still starts with some difficulty.  There is very little rust in the tank but I'm thinking that's the issue.  Your thoughts?  Are there filters I can get to put in the gas line?

Thanks.

Rick

ANSWER: Rick, certainly do your best to clean out any floating bits from the fuel tank. Pull the petcock and clean the screen over the Reserve portion of the body and clean out all of the passageways through the whole petcock body.

Check the petcock screen for debris, as that is the first line of defense on the ON selection and second for the Reserve portion.

There are small filters you can install inline. Most m/c shops carry them. The biggest problem is that the petcock fittings are 6mm and most aftermarket lines/filters are 1/4" (.240" vs .250") and the lines don't stay on the petcock fittings and if you use the OEM hoses, the filters ends are bigger than they need to be for 6mm hose. I usually use OEM Honda 5.5mm fuel lines and trim the ends of the filters down with a file to reduce a little bit of the OD without making them a loose fit.

Before attacking the carbs again, first drain the bowls into a cup and see what comes out. That will give you a good idea of whether there is debris in the jets.. and you know there are three of them... idle, primary main and secondary mains. MAKE SURE that your carb jets are correct for the series marked on the carb body. Late model carbs are generally marked 722A, 726A, 728A. Each carb series has different calibration and the aftermarket kits often are not made for each series.

Back up and check the valve adjustements, check compression readings, clean and gap the points to about .014" and then set each side to open at the F and LF marks for right/left side firing.

These carbs are sensitive to ignition timing settings, so be sure that the mechanical spark advancer is functioning properly. Diaphragms are also an issue with these carbs now, but you can get repair kits to fix them, rather than buy a whole slide/diaphragm assembly from Honda, if you can find them now.

Bill Silver

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

QUESTION: Thanks Bill.  I'll work through each of these.  Two more simple questions: can I just pull off the covers over the air filters?  Feels like they will break.  I adjusted the gap on the points and set the timing but don't know what or where the mechanical spark advanced is.  What's the best manual to get to help?

Thanks!

Rick

Answer
Rick, the mechanical spark advancer is behind the points plate. You can also give it a quick check if you have some good pliers and just grab the outer edge of the points cam and turn it gently to see if it moves against the spring tension and then snaps back into position when you release it. Otherwise, remove the whole points plate and twist the cam with your fingers. If it has some slop in the weights/springs, you can pinch the ends of the springs with needlenose pliers and get a little more return tension on the weights. If it is sticking or not returning properly, mark the position of the points cam in relationship to the advancer base, then remove the whole unit from the end of the camshaft for cleaning, lubrication and reinstallation. Recheck your ignition timing when you put the plate back on.

Honda manuals are the most accurate and complete, but can be expensive. Clymer's books are okay, basically a rehash of the original info, but sometimes they leave out some critical info.

I have sold a CD full of 350 info to interested owners, which has a copy of a photocopy of the first generation shop manual. It also contains all the Honda parts/service bulletins, OEM photos of the bikes and a incomplete version of a restoration guide file that I began years ago and never finished.

In regards to the outer side covers, which cover the air filters, you have to be very careful in pulling them off of the rubber grommets, which have hardened over time. Spray some silicone spray into the grommets and then use slow steady motion to work the tabs loose from the grommets. If you get them off successfully, replace the grommets with new ones from Honda and use some silicone spray/grease to keep them pliable.

Bill Silver
www.vintagehonda.com