Compressions

oldchuffer

CB750 New member
Messages
4
Reaction score
0
Points
1
Location
Isle of Anglesey UK
I've tested the compressions on my CB750KZ as its been difficult to start and I get 125-100-105-130. Putting a squirt of oil in the bores didnt make much difference but that maybe cos its been stored for a few months and i did put some oil in before storage. I did put new rings in when i rebuilt it 2 years ago (its not been on the road since) but i didnt glaze bust the bores. Any ideas why the compressions are on the low side? Should i check the valve clearances first?
 

amc49

CB750 Guru
Messages
3,476
Reaction score
25
Points
36
Location
Fort Worth TX.
You have to remove the cylinder glaze or the rings ruin, they cannot seat in correctly.

The throttles need to be fully open while testing and the engine will read better if starter turns it fast and steady. Use car battery and can even be one in car but do NOT! start the car up. Always suspect the gauge used as they are commonly crap. If not using one that positively screws in then you are wasting time doing it.

The valve clearances should have already been checked FIRST, they lend toward low compression on the twin cam engines.
 

dirtdigger

Moderator
Staff member
Messages
3,319
Reaction score
48
Points
48
Location
Black Hawk SD
Like said rings will never seat without de glazing the cylinders. You will never realize proper compression and will more than likely burn oil. Check valves but be ready to pull the engine and do a correct re ring job with new rings and cylinders honed providing they are round and straight.
 

Chrisraven

CB750 Enthusiast
Messages
32
Reaction score
0
Points
6
Location
Abbotsford, BC, Canada
Hi amc49: Just wanted to ask you about the compression in my 1980 750C. I have 150 on number one and 145 on the other three. Is this an ok pressure, I was reading in the manuals and they were saying the pressure should be 175 plus or minus 15. What is your thoughts on this? Should I tear down the head and cylinder and redo/rebore the cylinders? The bike has 38000 Kms on it, but has been sitting in a heated and unheated shop over the past 32 years or so.
Thanks
Chris
 

Chrisraven

CB750 Enthusiast
Messages
32
Reaction score
0
Points
6
Location
Abbotsford, BC, Canada
Do you think I should do a leak down test to verify everything inside the cylinder? Just trying to find out what might be causing the roughness on the running of the engine. While it seems to idle pretty good at 1000rpm, there seems to be a roughness when I crack the throttle. I have had 750's before but it was quite a while ago and my memory of the sound and response it probably not what it used to be.
I'm thinking it may just be in the carbs due to the years of sitting bare and dry. It has gotten better since I have had gas in the carbs for the past week or so. Each time I start it it seems to settle down a bit more. If I hold the throttle at 1000 rpm it idles fine, if I let go of the throttle it will drop to 900 and after 30 seconds or so will die.
 

amc49

CB750 Guru
Messages
3,476
Reaction score
25
Points
36
Location
Fort Worth TX.
Carbs need to be synced to idle smoothly and pull off idle correctly.

A leakdown test is a waste of time, they are only for drag racing up to 10% leak, with compression you have you are likely past that.
 

amc49

CB750 Guru
Messages
3,476
Reaction score
25
Points
36
Location
Fort Worth TX.
If you have rechecked all valves to around .005" or so then the best thing to do is take it out and run it to get engine to realize it is back running again and then ride it hard for a bit. Put some solid upper rpm revs on it a good couple times like racing it.

Somebody I know used to call it the Amish speed test. It works the valves hard to get them to beat through any carbon holding them up and older engines can actually come back off that running better. Or sometimes the carbon breakup can let a valve or two settle in deeper and they then need to again be reset. It can be common to have to do that at first then the engine will settle in at valves to be a good long runner. That shows most when the bike ran a long time with nobody setting valves when supposed to.

The valves are commonly any compression leak issue on these because nobody ever does them because of the difficulty. The rings if engine never stored outside to get heavy humidity and no pods used will last forever compared to valves.
 

Chrisraven

CB750 Enthusiast
Messages
32
Reaction score
0
Points
6
Location
Abbotsford, BC, Canada
Hi there amc49: Thanks for the input, just managed to get the carbs off and the number one slide is stuck solid so that is where the problem lies I believe. Will know more after I have done the carbs. Did a teardown last night removed the seat, tank, exhaust and some goofy after market back rest, made by Yamaha...hum that should not be on a Honda....lol I will do a leek down test now that the exhaust and intake are off the bike. Should be easy to see what, and if there is a problem. I will be setting the valves properly so at least I know that will be done.
Ordered all the spares that I need to complete the resto so the next couple of months will be spend getting things back to showroom condition. Looking forward to May, should be ready for the street then.
 

amc49

CB750 Guru
Messages
3,476
Reaction score
25
Points
36
Location
Fort Worth TX.
If the slide is stuck at the top big ID you may be looking for a new carb. I'd soak top and slide stuck together in solvent in case it just might be varnish stuck. If it is due to condensation, parts may be bad, you CANNOT sand the top big OD of slide or inside the top cover to clean them up, they are matched parts. You can sand the lower smaller OD part of slide where it fits in the lower carb body.
 

Chrisraven

CB750 Enthusiast
Messages
32
Reaction score
0
Points
6
Location
Abbotsford, BC, Canada
I reconsidered doing the carbs myself and have decided to send them off to Mike at The Carb Project. Talked with him on the phone and he said that he could do that without a problem. So they are going to be on their way via Fedex today. I thought better safe than sorry on such a critical part of the air system. Thanks
 

Chrisraven

CB750 Enthusiast
Messages
32
Reaction score
0
Points
6
Location
Abbotsford, BC, Canada
Yes that's what I figured. I talked to him (Mike) in person. Seems like a nice guy and says he believes in service. Says it will be about a month or so and I should have them back. No rush, I have lots of other things to get cleaned up. Spent some time last night on the old chrome and my polisher. Looks like new chrome now. Really happy that the exhaust on both sides is like new. Any idea how I can tell if it is the original exhausts? Marks? Numbers, names or letters. These seem really to perfect to be original but it was stored in a heated shop for most of it's life so it is possible. These ones have "SANKE 2069" on the mufflers just behind where the header pipe attaches to the muffler. If they are not the original mufflers how do I find what the original looked like. These look like all the pictures that I have seen so far for a 81 model year.
 

Indydave

CB750 Enthusiast
Messages
33
Reaction score
0
Points
6
Location
Indianapolis
You have to remove the cylinder glaze or the rings ruin, they cannot seat in correctly.

The throttles need to be fully open while testing and the engine will read better if starter turns it fast and steady. Use car battery and can even be one in car but do NOT! start the car up. Always suspect the gauge used as they are commonly crap. If not using one that positively screws in then you are wasting time doing it.

The valve clearances should have already been checked FIRST, they lend toward low compression on the twin cam engines.
Why not start the car? I have tried just using the car battery and there isn't enough voltage unless the car is running.
 
Top