Valve clearance/compression

[jaycee]

Hey everyone, I just bought a 79 CB750.  I hope to start a build thread soon but had a quick question that I was hoping someone could answer.  What is a good cold compression range for these?  I read the manual and it stated to do it warm but the bike won't idle so I did it cold.  My compression with the carbs removed where 110, 30, 90, 40.  I dropped some oil in the cylinders and got 110, 35, 100, 50.  This leads me to believe that he low compression is valve related.  The information I received with the bike indicates that the intake valves are set between .0025-.0035 inches (which seems acceptable).  However the exhaust clearance has several listed between 0 and .0015 inches (which seams really low as I thought they should be in the .003-.005 range).  Now would these low clearances be causing my low compression (ie valve not fully seated) in which case a re-shim might fix it?  If so I'll try to re-shim before I remove the head.  Or is it more likely that something else is wrong (bent/burnt valve) and the head needs to come off anyway?  Thanks for any guidance.

[TOOLS1]

You might want to start with downloading the Honda factory service manual from our manuals thread on the home page here if you don't have it. If you have a Clymer, or Haynes manual throw them away.
Low valve clearances can cause the valve to not seat, and the cylinder to not have compression. I would start with setting the valves. If you still have low compression, it is time for a rebuild.
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[jaycee]

Thanks for the reply tools1.  That's kinda what I was thinking.  I already ordered the shim tool so when it gets here I can start.  I've seen a lot of shims for sale online but what is there availability locally (dealer) do they still keep them in stock?  If not maybe I'll just order a kit to have some on hand.  I've been wanting another bike since I sold my last one about seven years ago.  Now that I have one, there is nice weather and I can't ride its really getting to me.

Oh and I have been using a Clymer manual as it came with the bike, but I'll download the Honda FSM.

[TOOLS1]

Most dealers still have shims, and will take yours on trade for the ones you need. Just like buying a car.
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[nilsthenomad]

I believe the early 90's toyota's had 25mm shims, which is what the bike took. I was able to get some at my local auto store when I told them the measurement and not that it was for a Honda motorcycle. They mentioned something about them being for a 90's camry.

[shinyribs]

nilsthenomad wrote

I believe the early 90's toyota's had 25mm shims, which is what the bike took. I was able to get some at my local auto store when I told them the measurement and not that it was for a Honda motorcycle. They mentioned something about them being for a 90's camry.

Thats REALLY good info. Thanks for sharing.


We really need to start a sticky and put info like this in there as it comes about. So many gems of knowledge buried on this forum.

[nilsthenomad]

Did a little digging, the 1988-92 toyota corolla took 25mm shims. If your local part store can't find em for your bike, this might be the way to ask.

[jaycee]

Thanks for the info Nil, figures I have 5 toyota heads and none of them are the correct ones.  Such is life.  I am still waiting on my valve shim tool, which I hope will be here tomorrow so this info may come in handy this weekend.