Topic: low compression, poor idle

[pooshine]

Hi, I'm on my 4th 550. this one I inherited from my ex girlfriend and it is definitely a "project".



The bike is not firing on #3 cylinder at idle, though it runs fine at speed.  I checked the compression with my cheap-o compression tester. It' not accurate but it did show about 90 pounds on 1, 2, and 4 but less than 60 on 3. Oil poured in the cylinder made no difference. My question is this: Do you guys in all your infinite wisdom and experience think that the lack of full compression could be the cause of that cylinder not firing at idle.  If that seems possible or likely then I guess that I've got some work to do. If that lack of compression doesn't seem like enough to keep it from firing (which is what I suspect)  then I'll look into other possible causes first and get around to looking at the valves later.

[Terry in Australia]

Wow Poo-ey, that is really low comp mate, even for the "good" cylinders, I had better comp than that in my CB750F2, and it had a gouge 1mm deep where the wrist pin had worked it's way up and down the bore due to a missing circlip!

The fact that it runs at all is a testimony to Honda's build quality, but if the "oil down the cylinder" trick bore no fruit, you've probably got a stuffed exhaust valve mate, and that's why it's not firing in that cylinder. Looks like it's time for a top end rebuild................... Cheers, Terry.

[750goes]

Before you rip the top end apart, make sure some other things get checked as well.

Just a quick thought,, check the valve adjustments on #3, make sure they are spot on, then do another quick compression test and let us know the result/s...

Cam chain adjustment also.

The #3 not running at idle problem could be a clogged slow jet, this would still permit the bike to run fine at higher revs.

But as Terry said those compression figures are pretty poor.

[TwoTired]

I think I'd get a second opinion from a known good compression tester, before ripping it apart.

I don't know your experience level, so forgive the insulting questions.

Was the engine hot during your comp test?
Choke open?  On all carbs?
Throttle open? On all carbs?
Kick start or electric start?
How long since the engine was regularly run?  Or, how long was it sitting before you ran it?

Do check valve clearances and watch the valve excursion while rotating the crank.  Do they move the same distance as the other?  Are they "sticking"?

I concur, a plugged slow jet in one carb will make a cold head pipe at low RPM.

Also, a spark plug condition comparrison across the cylinders, may confirm a problem on #3.

Cheers,

[pooshine]

Thanks for the insult Two Tired, I deserve it.  I used the kicker with the engine hot and choke open but forgot to open the throttle- dumb ass. No wonder that tester was giving such low numbers.

 I pulled the idle jet out and it was clear. The valve clearances and cam chain should be good as I adjusted those 1 year and 100 miles ago. The bike had sat for many years before I bought it. I took the carbs apart and cleaned them as well as I could, rode around a little bit and then it sat for another year. Recently, I put some of that yamaha carb cleaner through it and have been riding it 30 miles a day to work and back. The #3 not firing at idle has been consistent since I first got it running.   I'll redo the comp test by saturday and share the results.   Thanks everyone for your help.

[Vatch]

Don't mean to thread hi-jack, but I'm looking to pick up a compression gauge.  What to look for, what to avoid?

[Terry in Australia]

Don't mean to thread hi-jack, but I'm looking to pick up a compression gauge.  What to look for, what to avoid?

Buy a decent "brand name" item, I'm in Oz, but I hear that Sears sells a good one, (Craftsman?) the ones with rigid tubes are great for cars, but difficult with multi-cylinder bikes, particularly our Honda's.

Trouble is, the flexible hose items tend to leak around the swaged hose clamps when twisted into extreme angles, and often leak between the O rings that seal the attachments as well. Just make sure you spend your money wisely and buy a good one. Cheers, Terry.