Topic: 350 twin - plug fouling problem

[petersan]

I've got a 71 350 twin that I've been trying to get going -- almost everything is looking good, except the #2 plug fouls and then stops working. 

I'm comfortable diagnosing problems on my other bikes, but don't have too much experience with this one, so am looking to everyone else for suggestions.

Should say that the carbs were completely gone through and cleaned up with new carb kits.  As far as I can tell the two carbs are balanced (though this is not quite the science that it is on the 4cyl engines). Also, running the plugs at recommended gap.

I get about 30 miles (city driving) on the bike with a clean plug, before it gets fouled and stops firing. cleaning it up or popping a new one in corrects the problem for another 30 miles or so.

Even when the #2 cyl is firing - it seems to do some intermittent and minor backfiring. 

So - I'm wondering if this is an ignition problem or carb problem.

Thanks all,
Paul

[florence]

when you say the plug fouls, what is getting onto the plug, oil, fuel, soot?

Are the points and condensors in good order and have you checked the ignition timing?

Is the carburettor flooding?

When you put in a clean plug, will the bike run on just no.2 cylinder? (take lead off no.1 temporarily)

[petersan]

looks to be running too rich - fouling is sooty rather than wet or oily.

set points and condensers once, but wouldn't hurt to do again, along with timing. (should be able to get to that this weekend)

carb flooding -- do you mean the float not closing the valve correctly in the carb bowl, ie. incorrect float height adjustment?  When I went through the carbs, I set float height to spec according to manual

haven't tried just running the bike on #2 with a fresh plug - I'll try that this weekend too.

But I know it runs on #1 by itself :-)

[petersan]

And thanks for the help!

[florence]

If it is sooty then it is running too rich but if it is causing this problem then it is very rich.

If it is flooding, which I suspect, then the float height is wrong or the float valve is not closing properly.  If you pull off the air filter rubber and run the engine you may find lots of petrol sloshing around in the mouth of the carb.  It has happened to mine, caused by debris from the bottom of the petrol tank.  I cured that with in-line filters.

Without looking at your bike it is hard to tell but maybe the carb slide is sticking open or the diapragm on top of it is perished/damaged, or the spring is missing?.  Push the slide up with your finger and make sure it falls steadily.  Use the other side as a comparison.

[petersan]

Well, compared to the fours, the carbs on this bike are a joy to remove and disassemble, I'll take a look for fuel in the carb body and recheck the float height.  I do have inline filters installed.

It's been a few weeks, but last I checked, the carb slides moved freely on both.

thanks again.

[lordmoonpie]

By #2 if you mean the right hand cylinder as you're sat on the bike than also be aware that the generation of motors using this design from Mr Honda always run slightly rich on the right pot. Don't ask me why it must be some funny design issue somewhere and maybe a porting wizard like Mike Rieck will tell us all.

But - on my 350 racer I ended up having slightly different needle height settings in each carb to adjust. Float height and engine was totally sound - it just ran that way!

[petersan]

yes - do mean the RH cyl, and funny you mention it, I've talked with two other people local who have dealt with these bikes and they both mentioned intermittent problems with that side.

unfortunately, the needles in the carb kit I got do not have clips like my 550 and 750 and are not adjustable (that I can tell)

if I have good weather, than I'll look at this tomorrow.