The International CBX Owners Association

Dave, you hit it on the head! Compression on 6, 5, 4, 3, 1, are all 100psi #2 is almost 0. Right at 10 psi. What happened here? How could this just all of a sudden happen? I'm looking at a major job here. I'm dumbfounded. Now what?

Leslie:
Whew! - now we're getting somewhere. Do not spend another dime until you check the valve clearances. With 40,000 miles - when can you verify that the valves were last checked, if ever? I don't want to assume you don't understand the workings of these DOHC engines, but you CANNOT ignore the maintenance of checking the valve clearances and camchain adjustments. I really meant it when I said that these valve clearances can and will reduce to zero over time and can cause expensive damage. You absolutely have to start at the basics first, which I assumed you did. The longer this went on, the more apparent it became that was not the case.

If you can rebuild carbs, you can check valve clearances, but you'll need a shop manual and a couple of tools. Are you willing and able to do that?

Don't obsess about your voltage readings at the spark units - I don't think there's anything wrong with them. The bike runs too well and the problem is only with one cylinder.

And whilst I'm typing I see your update - now we're really getting someplace. You likely have a too-tight exhaust valve, but possibly a burned one. You have to check the valve clearances and if you are lucky, you'll just find a tight one - that's holding a valve open and causing compression loss. If clearances are OK - then you'll have to go deeper. I mentioned before that it is not unheard of for these engines to burn a valve.

If you need a blow-by blow of the valve adjustment procedure just ask. Glad we are finally getting somewhere. This is the conversation we needed to have a month ago.

Dave

Dave, as ticked off as I am at the whole ordeal with the carbs, I'm elated to have determined the root cause of the problem. Thanks to your persistance, I finally gave in and checked as you indicated. Knowing however that my carbs are clean is satisfying. Right that excercise off my list. I've had to rebuild another 82 I had, years ago, but never gone through the carbs till now. I could do the job without a doubt. I have the manual and will get any special tools needed. Hopefully I'll start cracking wrenches tomorrow.

Dave, when you say "blow by blow" adjustment procedure, are you meaning the standard manual? or do you have something better?. I also remember seeing somewhere a valve shim referencew chart. Could have been in a past issue of the CBX club magazine. Wasn't there someone offering a shim exchange program?

I use the manual for the actual procedure - no real shortcuts there. But for removing the cover, reinstalling the gasket properly and reinstalling the cover there are some tips, all of which have been spoken of here many times. You also have to adjust the camchains while the cover is off - different from the manual's procedure.

I use the 2-piece shim tool, but the one-piece seems more available. Check all the valves as per the manual, but accept no clearance tighter than .003 inch. .004 or .005 is better. I'd be willing to bet that most of your's, especially the exhaust, are much tighter than that. And of course, you'll want to focus on # 2 especially. On any that have zero clearance - you won't be able to turn the bucket. You will likely have to loosen the cam to get them to turn.

As for shims, my local Honda dealer doen not work on these DOHC bikes any longer, nor does any dealer. So they have tons of shims in the shop that they do not use any longer. My dealer just gives me any I need, but you might try for at least an even swap, noting that they no longer use them anyway.

I'll send you a PM with my phone number - I think you better call me about all this.

Dave

Sounds good! Just let me know when it's more convenient to call. Thanks again!

I thought that bike sounded like a "5"!

P.S. The other cylinders are low; could be a gauge thing. Probably though, like Dave said, this bike hasn't had a valve adjustment in forever. You were holding the throttle wide open when you checked the compression, right?? If you were, put a little oil (a couple of ounces or so) in a couple of the good cylinders and recheck the compression. If the rings are tired, the readings will usually go higher.

Ron, the second time I checked them I opened the throttle all the way. Readings were in the 125 psi range. Engine cold.

Leslie,

Glad you now have a fault to deal with, Dave was a great help to me when I had my carb problems a couple of years back along with many others on the other site.

I would say that a compression check should be taken before and after a valve adjustment or at least once a year and noted in your maintenance schedule. It is a way of understanding the general condition of your bores, rings and valves and can help you understand the wear and tear on your engine as time goes by.

Hopefully, this will be a simple fix for you.

Regards

Goss

Here are the preliminary results so far.

Leslie -

Adjust them.

Does the mark on the left exhaust on #2 mean "less than .0015'"? That could be your problem with low compression.

Again, shoot for .005 on them all.

Steve

Yes, less than. Thought of leaving anything .003 and haigher alone, since they've already been seated. Whats your opinion?
After replacing the shim on #2 and doing a compression test, am I to expect an oil bath while cranking the engine with the kill switch on?

We know how opinions go.

Mine is for .005 across the board. I'm sure someone with more experience than me will pipe up soon.

Steve

SPLINTER wrote:and doing a compression test, am I to expect an oil bath while cranking the engine with the kill switch on?

I believe the correct way to do a compression test, is with the engine at operating temperature.
Anything else will be but an indication of what you are looking for, but WILL tell you if any of the no2 exhaust valves are burned (which I hope they aren't ).

Aris

Here's an illustrative anecdote that may help shine some light on this subject.

In a previous life I was head of technical support and failure analysis engineering for a well-known farm equipment manufacturer. Weary of thousands of queries as to why valve clearances were so important, I trapped a cold valve in a very accurate caliper. I then heated the valve stem GENTLY with a torch until it was warm. It grew in length by EXACTLY .006 in., which was, incidentally, our published specification. I documented all steps with a camera.

I published my graphic in a service bulletin and it went further toward helping folks understand the basic principles than any “explanation” I had to offer.

Simply put, a cold valve needs room to grow—and is going to expand in length even after it runs out of lash space. With nowhere else to grow at the top, it will simply not seal when at operating temp. Even a ten thousandth of an inch gap between the valve face and seat will result in a major loss of compression. The flow of super-heated ionized gases will also burn valves in a very short time.

Obviously, the valve guide is also expanding, but so is the camshaft—although to a lesser degree. All these factors being considered and accounted for, .005 is the ideal clearance. Set your valves when they're at room temperature and the rest, being elementary metallurgy, will take care of itself.

Hope this helps.