Please Help Me By Helping My Friend!

I know this isn't XJ related, but I've been using this forum for years now and have complete faith in the knowledge base!

My friend has a late 70's Honda CB 350(?) twin. He's been working away on it for a while now and finally got the carbs jetted properly (he thinks). He's taken it out on the highway for the first time twice now and had some issues. The first time he found that it was running fine, but if he went as fast as 70mph for any length of time he would lose power and he'd have to pull over. Soon after it would start running fine and off he'd go. As long as he didn't go too fast it was fine, but any time he pushed it a bit it would eventually die again. I recommended he rebuild his petcock (I had a very similar problem a couple of years ago and this was the problem - bowls not filling fast enough).

He rebuilt his petcock and took it out again yesterday. It started to do the exact same thing. He stopped and fiddled with the mixture and rode on. At some point the bike started running extremely poorly and he basically ended up limping it home. It idles fine and runs decently at city speeds - this problem only seems to manifest at high speeds.

Pulling one spark plug he found this disaster (see photo). This spark plug was new a week ago (before the 2 highway rides). What the heck is happening? Can anyone point him/us in a direction to start? I've told him no matter what he should check the compression - I'm concerned about what the missing spark plug fragments have done to his rings etc.

We're in a bit of a panic as we are supposed to be going on a week-long trip to Deals Gap Tennessee in a week and a half, but now this! (this is why the subject line is "Help Me By Helping My Friend" - I don't want to cancel/postpone the trip!).

I apologize for the non-XJ related question, but would greatly appreciate any help or advice!

Jimmytime

 

Hi Jimmy you should post this in Other Motorcycles section.

Is that the right plug? from the look of the treads it appears to be too long? I hope it wasn't hitting valves or the piston...

 

To me it looks like either the spark plugs is to long for this engine or the cylinder head is in a very bad shape - check if is the right type. (Nasty picture though)
The problem with lack of power after a certain time could be a problem with the fuel supply - check if there is enough flow of gas out of the fuel line - what about the ventilation hole in the the fuel cap, is it clogged?
Good luck.

 

Oops! I didn't realize that there is an "Other Motorcycles" section.

Sorry!

Thanks for the advice too!

 

Umm... if the top of the piston looks like that too you may have a problem...

I would first do a compression test. If everything looks OK, I would install a set of brand new plugs and go for a SHORT "warmer-upper" ride and then have a look at the plugs.

 

Is there any way if that one cylinder was running lean (he suspects the intake manifold may have a crack) that it could cause this kind of damage?

 

Lean usually results in white plugs these have a lot of "color". It certainly looks like it got very hot. I'd be looking at the top of the cylinder through the hole too. What plug is it he has in there?

 

Yes. Bill is right you really need to get a look at the top of that piston too.

 

Unfortunately we don't live in the same city, so I won't be able to get a look at it myself - but I will pass all this info along to my friend. Seems like a wrong-sized spark plug is the lead suspect.

 

Ya you should have maybe 1-2 threads that show colour from the chamber, not the 7 or so you show there.

 

Funny thing is I did some looking and CB350 uses NGK B8ES which is standard 3/4 inch reach and not a shorty and the one in the pic does lokk too long based on the thread coloration. I read they make a long reach B8ES too but that does not make sense the E designates 3/4 inch reach.

Curios to know the answer here.

 

How is the spark plug from the other cylinder? Something is realy wrong here, never seen anything like it.

 

Apparently the other cylinder is running really well - better than it ever has in the past! That's what's strange. I can say with 100% certainty that he wouldn't install 2 different sized plugs!