compression testing advice?

Checking out my non-running CB400F. Initial compression test was 4 or 5 to 1 and fairly uniform for all cylinders. Should be 9.5 to one per the Haynes manual. Added a squirt of oil to each cylinder with no change. Redid all the valve gaps to make sure they were fully closing again with no change. It ran last year although it was WAY down on power and just plain goofy. Had me chasing timing thinking that was the problem. New points and verified all the marks and good spark....still no go so that's when I started looking into compression. Any thing left to check before I bite the bullet? Thanks in advance.

 

How are you checking the compression RATIO? That won't change with oil in the cylinder or not. Compression test PSI is based on more than just the ratio, cam timing/overlap has a big influence on the test numbers as well, ignoring other issues like valve sealing/ring sealing, etc. If the compression test is low, I'd do a leak down next, see where you're looking seal.

 

60 psig (+ or-) Multiple strokes bump up the pressure until it goes no farther, ending up at 60 psig. The cylinder isn't capable of raising the pressure any higher due to leakage some where. What really has me baffled is why it's uniform across all 4 cylinders. I think my next try is to go to known good engine and verify the gauge is recording properly. I would hope to see around 140 to 160 psig on a tight cylinder. I don't have a leak down rig.

 

Leak down testers are very easy to fabricate or I’m sure Harbor Fright has some at very reasonable cost. They’re a good diagnostic tool.

 

Does it start on it's own ? I've found when engines get down around 100 pounds of compression they start really hard or not at all. Are you sure your tester is accurate ?

 

It ran last year but was way down in power and couldn't rev above 4k...that's why I thought it was timing. Won't start now. I will have to check my test gauge, it used to work last I used it (3 years ago?). It's a cheapo so it very well could be the culprit.

 

Worn out. But worn out evenly.

 

Choke off and carbs wide open, right? Maybe even take off air cleaners.

Tuning over the engine fairly fast is also needed.

 

Compression under 100 psi won't fire. Best test from there is leak down. is it in the exhaust, carb or crank case my guess is rings...
https://www.summitracing.com/parts/...tNjZDXFmR7_-zcBDtw53-q0HI1YnOaQhoCoAgQAvD_BwE

Thats looks just like my matco i paid more than 3 time that price for...

 

To the OP,when you said checked the points and ignition timing that has nothing to do with the compression the engine makes.If the timing chain jumped,that
will affect the valve timing which will have everything to do with engine compression.Don't know if this a common problem on this engine or not,but would explain low compression on all cylinders.Leak down test would help confirm this as if it jumped time you may have bent some valves.Good luck.

 

Checked the pressure gauge, it was good. Ran the test with all plugs out, this would eliminate the need for pulling carbs, etc except for the tested cylinder. The whole ignition timing thing was due to a previous guess as to the problem...the bike did run last year though really poorly. Thanks for the tip on the leak-down tester, guess that's next on the to do list.

 

You're doing it wrong.
Operator error.

 

Does that bike have CV carbs? You may need to manually hold the slides up if you're not going to pull the carb rack.

 

My excuse is it's been a long time.....After holding the throttle open while doing the tests it went up to 90psi...still very low but I'm surprised it wouldn't at least try to fire.

 

90psi is still very low to get good combustion. Leak down test will help you determine if you need to do a full tear down or just a head rebuild.

 

UPDATE: My compression tester (chinesium crap) was fubar. I had an old tester that I modified and got 125 to 130 psi on the 4 cylinders. The leak down test was all 4 less than 10 psi which should be good. Still no fire. Rechecked valve gaps and all within spec. swapped the 1/4 plugs to the 2/3 plugs, no bueno. What am I missing? Scrap the new points/condensers for another new set? Take it to a shop and tell them good luck????

 

Spark,compressed air,fuel.

 

Have you tried any starting fluid, sprayed into the airbox or the carb throats if the airbox is currently removed?
Is it firing at all?
Could you have mixed up the 1/4 lead with the 2/3 lead and firing at the wrong time?

 

You didn't mention you checked for spark at the plugs? If no spark then, you can check dwell while cranking, it's more accurate than using a feeler gauge, condenser bad? Point contacts bad/dirty? I'm not one for throwing parts at something, but it doesn't get much cheaper than that.

Pull the fuel line off, got flow? Do the carb bowls have a little drain screw? Check that they are full of gas. Pull off the petcock and check the screen.

Fuel/Air/Timed spark is all you need so check them all.

 

Mechanically, the hard parts are probably okay; the soft parts, probably not.
Rotten, vulcanized, cracked intake boots can equal too lean to start/run and no appreciable power if it does. If it does start and you run it, don't. You'll spend a lot more money than new intake boots if it burns up. On a bike 45+ years old, cracked intake boots are a good bet.
You could seal the boots with a high temp sealer, but surely someone has a kit.

https://4into1.com/insulator-set-honda-cb350f-cb400f/#product-reviews