Topic: 1973 cb350 power problem...

[JWINC]

My brother has a 73 cb350 that was running beautifully until he had to ride it in the rain without a rear fender. I believe he might have also ridden in the rain without a front fender as well. It is his only transportation so it couldnt be avoided. Needless to say he had some electrical issues afterwards and has since replaced the entire electrical system, as much to eliminate future problems as resolving the present one.

Before messing with the electric his bike would lose all power above 4000rpms. After the latest electrical work, new coils, cables, caps, plugs it gets to 6000rpms before losing power. He spoke to someone today and is thinking about investigating the points next. If anyone has any suggestions for him it would be greatly appreciated.

[Ogri]

Check all harness to frame grounds. You should have clean, bare metal on frame and connector. Replace points and condensers. Dismantle and clean carburettors, replace air filter. Check valve clearances. Check vent hole in gas tank cap.

[JWINC]

When checking the points should there be a visible spark or does that mean the spark is jumping the gap early?

[Ogri]

A weak spark jumping between the faces is normal. When it's a bright white spark it means the condenser/s needs to be replaced.

[JWINC]

He found a tear in one of the carb diaphragms but hasn't had any luck finding any aftermarket diaphragms for sale online. Does anyone know of a good resource?

[Ogri]

A 'get you home' fix is to use a bicycle inner-tube repair patch on the diaphragm.

I'm assuming it's a CB350K twin, rather than the four; that I don't think ever had diaphragm carbs.

I'm looking at this parts diagram:

http://www.cmsnl.com/honda-cb350-super-sport-350-k4-us_model433/partslist/E19.html

 ...and it looks like the diaphragm and slide is one unit - part#7

 I've looked it up on the David silver site and they say there that the part is discontinued. 

[JWINC]

He found the jet set, part #19 here http://images.cmsnl.com/img/partslists/carburetor-set_bighu0018e6019_6632.gif , in the bottom of the bowl. It had fallen off at some point on its own. It isnt immediately apparent how it stays in place. Has anyone ever had to mess with this piece?

[flybox1]

its a press-in main jet, (probably needs a new gasket, #1 in drawing) and is held in place by the leaf spring.

[JWINC]

The O-rings on both jet retainers of both carbs were shot, one jet had fallen off and the other was barely held in place by glue from a previous owner :/ He bought new rings and made a new pilot jet plug (all materials bought at Advanced Auto) and the bike is back to 8500rpm without issue. Getting a nice tight seal from the O-rings was crucial in getting good vacuum in the jets. He has a little backfire problem but it appears the jets were the main issue. Thought I would share so people can file it in the back of their heads. Thanks for all the help.

[1974CB750rider]

I believe you can order complete ready to go Mikunie carbs for that bike. Maybe a bit pricey but would solve the problem plus give the bike a new attitude. I rebuilt a set on a CB450K earlier in the year but all I had to buy were gaskets and carb cleaner.