Topic: Top end rebuild- was not expecting that

[Dime]

Should of known better. The bike has been sitting for a
number of years. Compression is in the crapper, this is after a valve adjustment.

1- 72 lbs
2- 71 lbs
3- 65 lbs
4-100 lbs

Oh well. This just turned into a long term project. It gives me an excuse to play with the engine all the same. Full head rebuild as well as oversize pistons from cruzinimage.

Not what I wanted but it is what it is. 1978 cb750k. It was actually running.like this except number 1 cylinder. 

[scottly]

What was the compression before you adjusted the valves?

[Dime]

What was the compression before you adjusted the valves?

Don't know never tested it.

[scottly]

Does the engine still run? Did you have the throttle fully open when you did the compression test?

[Dime]

Does the engine still run? Did you have the throttle fully open when you did the compression test?

The engine runs except on number 1. Number 1 has spark an fuel confirmed.

I forgot to test with throttle wide open. Will retest tonight

[Dime]

Retested and the numbers went up to 120 across the board. Still not satisfied with that so I am gonna pull the engine and re ring and hone at minimum.

[fizzlebottom]

The compression numbers individually are all but meaningless unless you're using the exact setup that Honda used when they originally tested. The goal here is that you have equal compression, or a range of +/- 20psi across the board.

The problem with most commercially available compression testers is that they essentially increase the combustion chamber volume through all of the rubber hose between the threaded end and the actual gauge. To get it as accurate as possible, there needs to be a check valve as close to the threaded end as possible in order to minimize the added volume of air.

So you might not be in for rings, but you may have a problematic valve. Could see how a tbsp of oil down the cylinders changes your compression numbers.

But on the other hand, if you pull the head to check your valves and potentially lap them, pulling the cylinders and pistons is very little extra work.

[newday777]

Might be stuck rings. Mix 50/50 acetone and Marvel Mystery Oil, set cylinders so they are half way, fill cylinders with mix and let soak a few days to a week. Suck out remaining mix, drain oil and put in fresh oil. Start it up outside and heat cycle it a couple times. Report back how the compression reading changes.

[Dime]

This project is long haul. I might not be doing a full on restoration but I want the engine to have a fresh top end. I will go through the head and see what the cylinders need. I would rather ride it later than sooner and know what I have to start with.

[scottly]

Retested and the numbers went up to 120 across the board. Still not satisfied with that so I am gonna pull the engine and re ring and hone at minimum.

I would just run the bike for a while, after fixing whatever is keeping the #1 cylinder from firing.

[PeWe]

Full throttle during comp test?
If not, it will be lower.

[Dime]

Full throttle during comp test?
If not, it will be lower.

Did it again with full throttle and it went up to.120's across the board.

[Mark1976]

Retested and the numbers went up to 120 across the board. Still not satisfied with that so I am gonna pull the engine and re ring and hone at minimum.

I would just run the bike for a while, after fixing whatever is keeping the #1 cylinder from firing.

   Best advice so far, it's been sitting a while, get it to fire on all 4 (you're going to have to figure it out eitherway) put a few hundred miles on it and retest. I think far too many individuals pull the topend off just because they can. It really looks like a great project, I've seen far worse. Just an opinion... your bike your project....

[seanbarney41]

+2...just run it.  Trying to put these together as good as Honda did is very expensive and difficult.

[RAFster122s]

A motor that has been sitting pretty much always is going to show lower compression unless the bike was properly put away and all the intake and exhaust ports are sealed so it won't rust while sitting. They rarely are, so...common for clutches and rings to be stuck or not sealing well and often clutches are gummed up or not releasing correctly. Often running them will free things up and compression comes up, seals sometimes reseal after they have rehydrated with oil or gas. Running them can show you after a bit what it needs and save you time and money. But, if a motor is locked up, different story.
Fwiw, my opinion.
Electrical issues not withstanding...they rarely improve and require a different std of care and work before resurrection of a long dormant bike.

[HondaMan]

120 PSI is NOT bad compression in these engines, particularly after sitting unused a long time.
While mine is modified to have higher compression than (K2) stock, it hits 135 when hot, but less when cold (haven't tested it in a while, but that's what is was in 2020 during Covid shut-in). The K7/8 should have similar numbers. These numbers, though, vary a LOT between the types pf compression gages used: those with long hoses on them read much, much lower than the ones Honda used, which had a 2.5" long (angled) aluminum pipe on the gage to reach the plug holes. Those gave numbers like the ones seen in their manuals, but those with longer pipes/hoses do not.

[rotortiller]

I get 150psi on a stock K7 engine kicking it over with the throttle open. You want a test gauge with the check valve in the spark plug end of the hose fitting otherwise you have altered the compression ratio substantially and should expect lower readings. Not so much hose length as it is test equipment design.

[scottly]

Dime, is this the bike from your introduction that only has 14,000 kms? That's less than 10,000 miles!!

[Don R]

 Fare too many of these bikes come apart and never get re-assembled. I always make an effort to ride it first to get to know the bike a little. Yes, I've still had to take them apart but at least I knew what needed attention.

[Mark1976]

Fare too many of these bikes come apart and never get re-assembled. I always make an effort to ride it first to get to know the bike a little. Yes, I've still had to take them apart but at least I knew what needed attention.

+1 to that...

[carnivorous chicken]

+2...just run it.  Trying to put these together as good as Honda did is very expensive and difficult.

+3

[HondaMan]

Fare too many of these bikes come apart and never get re-assembled. I always make an effort to ride it first to get to know the bike a little. Yes, I've still had to take them apart but at least I knew what needed attention.

I actually have 2 whole (not yet assembled) rebuilt 750 engines left here (now 7+ years) by others who took out their engine, shipped it here for rebuild, then bailed out (or something), never to be heard from again. I guess I'll assemble them and sell them this winter(?).

[robvangulik]

I get 150psi on a stock K7 engine kicking it over with the throttle open. You want a test gauge with the check valve in the spark plug end of the hose fitting otherwise you have altered the compression ratio substantially and should expect lower readings. Not so much hose length as it is test equipment design.

Sorry, I don't agree.
The place of the check valve doesn't matter, the moment that valve is pushed open the volume/pressure is the same for the whole compression tester system.

[Stev-o]

+2...just run it.  Trying to put these together as good as Honda did is very expensive and difficult.

+3

+4 Run it! 

[Medyo Bastos]

+2...just run it.  Trying to put these together as good as Honda did is very expensive and difficult.

+3

+4 Run it!

+5


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