To Rebuild or not to rebuild?

[mmiller]

All spring on my rough idling, popping, grumbling 1977 CB750 I have (in this order):

1. Rebuilt the carbs (they badly needed it--new boots and top dust covers)
-followed the Hack A Week video - thanks Dino
2. Tested the float level
3. Synchronized the carbs
4. Cut plug wires back 3/8''
5. Cleaned up the spark plugs (All firing blue)
-tried new ones in every cyclinder (no change)
6. Gapped and polished the points
7. Put the motor to ATDC and did a "Cam chain adjustment"
8. Synchronized the carbs again - - idling fine at 1300 to 1400

So finally, I ordered a Compression Test Kit -- manual says, "must be above 150psi or within 10% of each other"

8. Compression Test:
Dry 1-110psi 2-105psi 3-100psi 4-100psi
Wet 1-140psi 2-135psi 3-135psi 4-140psi

My DAAAAD says ride it like I stole it...I think I've witnessed him being wrong too often...I just want my baby to run like it should. It has for the past four years!?

I want to know what you all think, should I rebuild it?

 

[amc49]

Compression says it all, I for one tune out wet results, they don't run 'wet' in the real world do they? 100 psi if true is a dead cylinder where I come from.

If the desire has flagged enough you have to ask others about it you already have your answer.

 

[mmiller]

Thanks AMC, I've got a 350 scrambler project to finish up this summer so I might just ride it a little, close to town this summer and then do the rebuild next summer.

Thanks for the advice.

 

[dirtdigger]

Provided your compression tester is reading right and you are doing the test right then you motor is wore out. The wet test would lead toward rings but too much oil can falsly bump the compression no matter what is leaking....rings or valves. Beings they are all very close but low either the gauge is reading low, you are doing the test wrong, or the rings and cylinder are worn...most of the time valve leakage doesnt cause an even low compression accross all cylinders, normally just one or two cylinders. Start saving your money...to do a good rebuild the right way you are going to spend some $$$. If you need suggestions or have questions on the motor rebuild dont be affraid to post on here.

 

[amc49]

X2. Always suspect the gauge used if all the numbers are low but pretty even. The odds are against that, usually one or more is way out from others. Gauges technically have a compression ratio similar to an engine, the length of hose affects that and why some long hose car testers used on bikes with smaller chambers don't read as high. Shorter the hose the better on a small motor.

You can't take too many tests as compression testing has errors all over it even at best. One test only and you can often be messing up there. I use a premium quality tester and even then you do multiple runs for backups to your initial one.

The DOHC guys have fits with this, being nobody ever shims the valves and then the valves recede to tighten up clearance and then leak. They see you SOHC guys with good running motors that are much older than theirs and automatically assume theirs will run as good or better and just not so if the bike was never valve adjusted. It makes for lots of problems.

Why on earth people pour scads of cash into projects that have such leaky motors they will never run right is beyond me, a compression test should be the very first thing done with ANY new bike with an unknown running condition. Most put the old finger to a plug hole and whirl it and call it good and most of that ilk have not a clue. The same goes for cars. On the car sites I frequent there have been uncountable folks that spend thousands on engines that should have never had a cent spent on them.