Topic: 90psi after rebuild

[notaleclively]

I have a mostly 1972 CB750. The motor is a 72. So we will go with 72. With the help of my father-in-law we rebuilt everything on the motor that can be done without separating the case.

New pistons, rings, wrist pins, valves, gaskets, seals, and all the other top-end bits. Head and cylinders were bead blasted. The head was machined by a good local shop after bead blasting. Nice fresh crosshatch on the walls. Valve timing is dead on. Bearing and cam tolerances checked out. Some small headaches with the cam cap studs' and old soft aluminum, but nothing some careful helicoiling couldn't fix.

While it was sitting on a stand we did a compression test and got 90psi across the board. Nice and even. But low. That test was with no carbs, exhaust, or valve cover. Valve tolerances were checked many times. Compression was tested many times. Valves were in spec every time, 90psi every time. Adding a drop of oil to the cylinder could get you to over 100psi. We tried multiple gauges. 90psi every time.

After a little fussing, it's now in the frame and running. I am still working on tuning the carbs to get it to run properly. #1 is way too lean, and #3 is not sucking fuel on idle. They all hit at 2k RPM or so. I put a vacuum gauge on the carb on #1 and it read very low. But it's still eager to start and run #1. So the vacuum appears to be strong enough to get enough fuel to run and rev.

I saw multiple posts that said a low compression reading on a fresh rebuild could become higher after some period of break-in. I have doubts that 90psi can turn into the factory spec of 160psi.

Has anyone ever seen this on a new rebuild?

[newday777]

Did you bore oversize or did you use stock size pistons and rings?

[notaleclively]

Everything inside the engine is stock spec.

[Don R]

 Some compression testers read low on small cylinders. It sounds like your pilot jets may need to be cleaned, they are tiny and easily plugged, especially on a bike that was sitting a while. 

[BomberMann650]

Improper cam timing can drop compression numbers too.

[notaleclively]

Yeah, carbs need some sorting for sure. I rebuilt them while they were off the bike. I didn't sonic clean the bodies, but I did replace the jets. I don't have a solid track record of getting that right the first time. I think they are pretty close though.

We tested compression with my harbor freight cheapo gauge with big multi-hose adapters and an old snap-on with a very short intake hose. Both read 90psi on all 4. The consistency gives me confidence there isn't a big scary assembly error somewhere. But the low value still worries me. The low vacuum value on #1 also seems to indicate a mechanical timing issue.

I had a bunch of 750 engine parts around when I built this thing. Is there a chance I have the wrong cam in there? Are there cams with the same lift and different durations or timings? We checked the height of the lobes and the bearings. I did not check on the duration or timing of the lobes. Could I have an F-model cam in here or something wacky? Did someone put a different crank in this thing at some point? Are there different cranks that could cause this?

[Stev-o]

...but I did replace the jets.

Hope you used genuine Keihin jets or you will have trouble tuning. 

There is a good chance your compression tester is not accurate and we always say that if the numbers are within 10% that it should be good

[Tim2005]

What pistons did you install? Also, was the cylinder head used on the motor beforehand/ is it definitely the correct head?

[notaleclively]

I did use authentic jets. I have made that mistake before as well. I have made a fair amount of the classic errors. But I know I am bound to make plenty more.

I keep hearing that I should be fine. 90psi seems really low. I just wanted to make sure I didn't make some classic mistake that everyone knew about but me.

Bomberman, if the valve timing was off enough to affect compression would it be even across all 4? Or would affect 1/4 and 2/3 differently?

[notaleclively]

Tim,

I used this kit.
https://4into1.com/engine-rebuild-kit-w-pistons-honda-cb750-1969-1976/

The head and block were together when I pulled them from a bike that had been in a barn for 20+ years. History before that is unknown. There was some corrosion inside the cylinders. They moved freely. The cylinder walls cleaned up really easily. But we determined the pistons were too pitted and replaced them. The valves were replaced with OEM and seated by a trusted professional shop. Springs are original to that head as far as I know. 

[bryanj]

If you honed rust out of the bore then used std pistons and rings it is 50/50 that the piston clearance and ring gaps are too large, also you wont get full compression before 500 ish miles.
MOST IMPORTANT is are you certain you removed ALL the blasting media from the oilways, many people have trashed cams, towers and followers after that mistake

[TwoTired]

Most auto motive compression testers add lots of volume to the compression chamber of small engines.  This lowers the compression ratio unless the check valve is located right at the spark plug hole.
  I did the calculations some years ago in a post here somewhere.  But from memory, the hose of an automotive tester added 10 - 20 cc to the combustion chamber. This necessarily lowers the compression ratio, making it impossible get Honda book numbers.  Also it is easy to alter the compression ratio by adding a volume of oil, a teaspoon is about 10cc which is not far off from the total volume of the combustion chamber at TDC.

  One of the Honda shop manuals shows the gauge Honda used to make their published numbers for compression.  It had the dial gauge atop a very long and very skinny solid tube that may have added  1 or 2cc to the combustion chamber.  This would be typical for a small engine compression tester that is not easily found in auto supply stores.

[newday777]

Everything inside the engine is stock spec.

Did you measure the ring end gaps in 3 different depths of the cylinders?
What is the ring end gap measurement you got with the rings?

[notaleclively]

Bryan - The rust was very minimal. A cylinder hone was able to take it off while putting on a fresh corsshatch.

Lloyd - I hear this a lot and I don't question it. BUt 90psi seems too low even with that difference.

Stu - I do not recall what the values were. But I do recall that we were satisfied with the rings out of the box.

[Tim2005]

I can't see it's the gauge hose making this much difference, on mine the hose is 10-12" long and it still measures the little cylinders on my 400f as 160psi. 

Next idea, could you have fitted the compression rings the wrong way up?

[BomberMann650]

The low compression would be consistent - these bikes are too old to have variable valve timing 😆

The way cam timing effects compression is advance opening too much BTDC and closing too soon BBDC - blowing the intake charge up the pipe, and closing before the stroke bottoms out.  So on the ignition cycle, the volume to be compressed is less than ideal.

Retarding the valve timing has the same effect, but will reach a point where exhaust valve touches the piston during the overlap.  Dont ask me how i found that out.

[notaleclively]

Bomberman - Makes sense to me. I believe the valve timing to be good. The horizontal marks on the camshaft were a few degrees off of the line made by the cam bearing. It was ever so slight, and moving the timing one tooth in either direction was clearly worse. There were witness marks on the cam gear we use to ensure it was seated correctly. The cam chain tensioner looks to be working well. The adjustment screw is nowhere close to bottoming out. No wear was seen on the chain or the cam gear.

Sorry I know it sounds like I am playing "Stump the Chumps" here. But I promise I am not. It was suggested I do a "Run-in" compression test. So I am going to try that after I clock off today. Just to see what the values are. I hear they might be lower?

What compression values have you folks seen on your newly rebuilt cb750 motors? What tools do you use for compression testing on these motors?