importance of WOT during compression test?

[Lucky 1]

You cannot just take used valves and lap them!!!  You cannot lap your way to good valves in worn out guides.

Some new valves specify no lapping.
You cut the seats and take the valves out of the package and install them THATS it!

[seestheday]

My compression numbers are pre-anything. I stopped riding, then did the readings.

I checked my valve clearances about 2,000 kms ago and they were all bang on.

Sent from my BlackBerry device on the Rogers Wireless Network

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From: "Lucky 1 [via Honda CB750'S]" <[hidden email]>

Date: Mon, 10 Dec 2012 17:12:23 -0800 (PST)

To: seestheday<[hidden email]>

Subject: Re: importance of WOT during compression test?

Are these numbers AFTER the valve adjustment?

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[easyrider12]

I'm affraid to go near my valve cover now and/or post about it
signed- Anonymous
ha, JK glad you went first

[seestheday]

Honestly if I need to spend $2K on this project I'm cool with that.  I just don't like buying tools that I'll only use once if they're not necessary.  One from space issue, and two because I'm frugal.

Spending that money will be worth it to learn.  I'll probably ham fist a few things on the way, but I accept that.

[seestheday]

Also, numbers with the carbs off:
135
140
140
150

No change dry vs. Wet

What are your thoughts on these numbers?  Are they bad enough to require me to pull the head?  Now I'm second guessing myself.

Is there another way of fix that head gasket leak?  I really don't like a dirty oily engine, and it would be worth it to me to do a fair amount of work to fix a bunch of oil leaks.

[shinyribs]

Those are pretty good numbers IMO considering new was 175. <--- I believe that's what you said new was???

If you have to pull the head to fix the leak and notice alot of wear on the cylinder walls that would be one thing,but you don't have bad numbers. I checked to compression on my silver bike the other day and it was only reading 125. Way less than yours and I think it runs great. It's runs better than alot of people say it should.

And if the valves aren't split or burnt you can just lap them for almost free. HF sells some kits real cheap.Like $20 IIRC.

[shinyribs]

Oh,and you don't need thousands of dollars worth of tools.

If you find you need new valves ( highly doubtful with how much you've been riding with no valve related issues) it's much cheaper to pay a shop to it than set up to do it yourself. If/when you go in to the head you'll probably put in new valve seals while your there. Wiggle the valves in the guides to check for wear. If you feel any wiggle at all you might need guides,but not valves. The valve stems are much harder than the guides and will be the first to go. Check to see if they rotate freely in the guides and check for sticking while spinning them and pulling them up and down. There may be some gummed on oil vapor residue that might make for the feeling of sticking so be sure to clean the stems as good as you can before checking them to prevent false alarms. If there is no wiggle the guides are good. If there is no sticking the valves are nice and straight.

Whatever the leak down will show will tell you if the valves are sealing or not. If you want yo do a visual inspection for peace of mind color the seats with a Sharpie. Spin the valve and see what pattern develops.If you find a spot where the ink doesn't wear off you found a leak. You dont have to push real hard when you spin the valve either. Just enough pressure to keep it in contact with the seat.Or at least you can see how much of the valve face is seating. You may find that you are leak free,but very close to one if very little of the valve face is meeting the seat. Also keep in mind that the more of the face of the valve that contacts the seat,the better it can cool. Valve heads are only able to be cooled for the split second the valve face actually touch the seat. Whether its an air or liquid cooled engine. It's a miracle they last at all. They have a hard life!

So, just to check your head out and possibly reseat your existing valves you're looking a $20-ish lapping kit and a Sharpie marker which it like $2.

If you do decide the rings need replacing than all you need is feeler gauges,a hone ( bottle brush type is the easiest to use) and a way to file the end gaps. Actual piston ring filers are nice for this and can had fairly cheap. I think I paid $30 for mine and have built dozens of V8's with the same stone. But the same job can be done with a nail file honestly. Not the metal ones,the stiff foamy type. Even if you do buy an actual ring filer you have to go back and deburr what you file with it with a nail file. A ring filer will give you straighter edges than you can produce by hand but it's a catch-22 since not all bores are the same diameter and no one stone could ever produce perfectly parallel edges like they claim they do. There is always a certain degree of angle inherent to using a ring filer,unless you buy a $1,000 plus motorized unit will adjustable guides. But in all honestly even the 'big gun' engine builders I know don't use them cuz the math involved in setting up the stops is uber complex and they cant afford to spend dozens of hours setting the machine up time and time again. You may be able to buy drop in rings that dont have to be file-fitted and void all of this other than the hone. I really don know if those are available or not.

If you do indeed need to re-piston the motor whatever machine shop that does your bores will do all the measuring on that. Even if you did do all you own measuring any shop worth it's salt would never trust your numbers in the first place. So no need for all the dial calipers and bore measuring equipment.


This is how we would check out stuff on the race cars. But it should relate to any four stroke engine IMO

[seestheday]

Thanks for the encouraging words Shiney.  I was planning on farming out any work that required any expensive tools or a high level of skill anyways.  Was planning on checking here before doing any of that.  Good to know that a lot can be done for a reasonable price.

Oh, factory spec for my DOHC is 170 +/- 14 psi, so the cylinder that is at 150 is within 6 psi of factory spec, and the worst cylinder (135 psi) is 21 psi from factory spec.

[seestheday]

Haha, I've learned that I need to take Lucky's comments with a grain of salt.  He's a very smart guy and obviously knows a lot, but can get crusty, especially if you're a newb like me that is inclined to ask a lot of dumb questions.

[seestheday]

Why do you need the steel/formica table?  I assumed it was to check to make sure there was no warping of the head which is why I suggested glass.  Would a flat piece of steel that could be set on top of a table work?

Edit: Nevermind - you said that a glass table top would work.  I misread it as "would NOT work".  I don't have a glass tabletop, but I do have a decent sized piece of glass I can put on top of my wood table that I can use to check to make sure the head is flat.

[RodneyCB750K1979]

When it comes to head rebuilding, I prefer taking it to a motorcycle machine shop. Scope: hot dip cleaning, disassemble valves, inspect seats and seat angles, machine valve seats, lap valves, replace bad valves or all valves, check springs, replace guides, replace seals, reassemble valves, extract any broken exhaust studs, tap exhaust stud holes, check deck for straightness and flatness.  I never tried this work, but I feel I lack the equipment and skill to do this work.  I would need to consider the cost of tools, how often I would use these tools, and consequences of screwing up the head.