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Bike: 1975 Honda CB550 Four, 12000 miles

Problem: So I buy this bike and I try her out just to see what she'll do. Charge the battery, Clean the plugs and presto. Sounds great, throttle response faster than any bike I've ever owned. After the bike runs in my driveway (mainly idling) for 8-10 minutes, it starts running really rough, starts putting out white smoke, and dies on me. Take out the plugs and they're dry fouled.

So despite the hidden thoughts of rings/head gasket I start hoping and check to see if someone put too much oil in it. Sure enough, I think I lucked out. I take out about 1.5 cups of oil and try it again. Same exact thing happens. After the bike runs in my driveway (mainly idling) for 8-10 minutes, it starts running really rough, starts putting out white smoke, and dies on me. The plugs are re-fouled.

It has done this every time, even after I made the mixture leaner and took out the oil.

What would cause the bike to run fine and then slowly get rough, put out white smoke, and die?

Only thing I can think of is that after it's been running for a while it leans out too much. Too rich would be black smoke, and ordinarily I'd say check your head gasket for a coolant leak, but that's an air-cooled bike. So my guess is you're running too lean.

Old bikes and similar problems... try idling her with the gas cap open.

will it foul the plugs if it's running too lean? i know it does if it's running rich.

no luck with the gas cap trick :-/

I'm thinking you need to rebuild and clean your carbs, paying special attention to your float levels.

Some times a bad coil will give you some symptoms like that. Worth checking out anyway.

Yeah JohnJ I was just trying to avoid that on yet another bike haha.
The other thing is, I'm hesitant to tear them open because it runs do darn good for the first 8 minutes. It has better throttle response than my friends 07 gsxr.

I tested out the coils already but I have another set I'll try anyhow. I did kinda rig the coil wires though (the pins in the boots were making poor contact so I cut back the insulation and pushed them in farther) so I'll go and order a new set of wires, I should anyhow, and see if that makes a difference. That'll take a couple days though.

Thanks so far guys. But aside from a carb cleaning that I guess I'm going to have to do and see if that does it I'm still stumped.

Crisp, almost too crisp, throttle response (no way it should be crisper than a new GSXR) and dry fouling after a few minutes sure sound like it's running lean. Any way to measure the temp of the header pipes, like in infra-red temp gun? They would show hot at idle if they are lean.

It may be just the idle or low speed carb circuits that are bunged up. How does it run when ridden?

Sounds like a carb cleaning session coming your way.

If you are tired of cleaning carbs, you may want to avoid buying older bikes that have been sitting.....

hahah yeahh Buzzz you're right. but the old bikes are the most fun haha.

The bike has been running reallllly hottt. (wayyy hotter than my 81 cb750)

So yeah. In the next few days I'll clean the carbs out and I'm guessing that'll do it.

Thanks.

just wanted to let you guys know what happened:

i cleaned the carbs, but they were pretty darn clean inside already. This didn't fix the problem, so I went and bought a new set of plugs. I'm guessing that the plugs weren't A-1, so they were slowly fouling and then not firing at all eventually. Replacing them did it. It's running great now and I have a guy coming to look at buying it tomorrow.

Thanks again guys