1973 Honda CB175 synchronization issue / 360* parallel twin

Question

Hello All. I picked up a 1973 CB175 a few weeks ago. I'm having a little issue with getting the bike to run right. The issue: About 1-5 miles into riding the left cylinder cuts out then the whole bike dies out. After waiting 1 minute it will start up and run good then do the same thing. I was told from the seller that the carbs needed synchronized but the guy selling it really didn't know anything about the bike. He didn't even ride and had only bought the bike to sell it to make a couple dollars.

Now as far as the troubleshooting, here it goes:

New Points and timing set

Carburetors have been rebuilt

Spark plugs replaced w/ gaps set

Battery fully charged

Air cleaners checked

Verified fuel was getting to the carbs

Fuel not leaking out of the carbs

Throttle slides seem to be matched up

Compression checked: 165psi on the right side, 155 on the left side

Choke seems to operate the way it should

After short runs, the left plug shows rich, and the right one is showing excellent. After temporarily removing the left air filter, the plug then had a white tip showing somewhat lean.

I'm really trying to get it to run on one cylinder at a time so I can get both cylinders running the same setting the idle/mixture. I removed the plug wire and put a spare in the wire and grounded to the engine on one side, but doing this on either side the bike wouldn't run. I know there is an issue with the idle because I thought it was running nice when it was idling, until I discovered that the left exhaust wasn't warm at all, as the right was, and after putting a board up to the right exhaust, I could hear that the left wasn't even firing except a couple putts here and there. I suppose all the air I'm feeling from the left exhaust is just the compression from the cylinder. After checking the spark plug on the failing side, the plug was showing an over rich condition. To compensate, I tried taking off the air cleaner because the I've been unable to successfully set the idle/mixture correct on it with the right side running. This made the cylinder fire up, heat up and run, but trying to ride eventually ended up with the same result. Also when I checked the plug after the addition of all the extra air, the tip was white indicating over lean. The right side is running excellent as far as I can tell.

Does anyone know a better way to run a parallel twin on one cylinder to set the idle/mixture?

Anybody have any suggestions? Anyone have any experience with some old Honda 360 degree parallel twins? Thank you very much. Your answers will all be appreciated.

Answer 1

To check the ignition, swap the plug wires. Since your bike fires both plugs every revolution you can do this. If the problem moves to the other cylinder then you have a bad coil or spark plug boot. Also try swapping the plugs. The coils on old motorcycles are notorius for failing. They were poor 25 years ago and dont get any better with age. Coils love to fail when they get hot. This can really give you head achs when you are trying to trouble shot the problem because they cool off then start working again.

BikeBandit.com has parts break downs on almost ever thing made look under OEM parts. I wouldn't recomend paying the money they want but the pictures a great help.

Answer 2

First off, the carbs not being syncronized wouldn't cause one side to cut out after running a bit. At idle, it could cause one side to appear to not be firing though, but the engine should run on one cylinder regardless of which side it is.  

First off, it sounds like you need new air filters.  If you can't find any for that bike, consider washing the old ones out in gasoline, especially if they have any oil on them.  Also know that running an engine without air filters won't hurt anything unless you're running it in very dusty conditions such as a dirt or gravel road.  Also know that a white spark plug isn't necessarily a sign of an overly lean condition, but a white plug with little blisters is.

Are you getting spark on both sides?  The ignition coil/s are pretty old and weren't nothing to brag about in the first place.  Upgrading to good aftermarket coils is a good idea whether that's the problem or not.  My old CL360 had crappy coils and at only 2 years old couldn't fire a spark plug with any more than 7,000 miles on it.  A good healthy spark can ignite a crappy air-fuel mix but a crappy spark can't ignite a perfect air-fuel mix.

Let me know what happens.