Running on two cylinders (it's a four cylinder)

My GPz750 has once again broken down and I had to leave it in order to get home. I'm pretty sure it's only running on two cylinders because it sounds like chain saw instead of it's normal glorious purring. I thought it was the coils because, during a rain storm, the bike kept lurching and cutting out randomly. It did this again, once on my way to get gas, then another time on my way home. So, I replaced the coils. I took it out for a ride (it sounded great, very full and strong) and then it started cutting out and lurching again.

So right now it's in a different neighborhood and I have no idea how to get it home. But, until then, any ideas on what's going on?

Here's another tidbit. On my latest test ride, every time the engine turned over it made a slight *pfft* sound. When I tried starting it after it died, the engine would turn over but then make that same *pfft* and not start.

Could it be a vacuum leak making that noise? If so, there are only like 4 vacuum lines and I HAVE gone over then, unable to pinpoint anything.

Thank you for the help

When you get back to the bike if you can pinpoint which 2 cylinders are not firing you may be able to isolate the problem to a specific coil or part of the ignition system. Ed

How would I determine which cylinders are firing?

Assuming you can get it running, you can spritz a little water on the header pipes to see which ones get hot. If you cannot get it running you can check for spark at the plugs to see if 2 of them have spark and 2 do not. Ed

Wow, that is so simple. Thank you good sir.

If it happened to be a vacuum leak, where could I find new hoses? Is there a kit perhaps?

I don't think there is a kit, but most of the hoses are fairly straight so maybe generic hoses will work. Otherwise you may want to consider blocking off the clean air system. Ed

The hoses are metric in size. A German car dealership might carry them, otherwise you can use SAE (American) sizes with a zip tie to tighten them up. 7 or 7.5 mm comes to mind, although I might be off slightly.

i tried the water test and cylinder 3 is not running quite as hot as the others. It does fire, but not as hot. My carburetors are in need of a rebuild, perhaps #3 is a major culprit?

I did solve the "pfft" noise, there was a detached vacuum hose. It did not solve the poor running condition though.

I did determine that as the engine warms up, that's when it starts to die.