Summary: Bike was running normally, it tipped itself over while idling on kickstand and stopped running, now won't start, troubleshooting has revealed I have low compression, but I am confused since I don't think dumping over would create low compression

1987 CBR 600F w/ 56,000 KM
Purchased in 2010 and have not done any motor maintenance
Cleaned carbs last year, but they didn't really need it
Rotella synthetic

Problem: Motor cranks easily, but will not fire, not even a burble.

Trouble shooting the Holy Trinity:
1) Spark
- Pulled all plugs and tested each plug and wire, all are sparking
2) Air/Fuel Mix
- Air Filter looks OK, Fuel filter is new, Fuel pump is pumping, float bowls are being filled
3) Compression (at ~3000 feet)
- PROBLEM: Compression test is showing 65-70 PSI on all 4 cylinders
- Poured a bit of oil down into cylinder and tested again -> compression jumped up to 120 PSI, so I think the rings are the main culprit, not the valves

Why I am perplexed:
-Bike was running fine (for an oldie) before it tipped, so I assume the problem was caused by the impact or laying down
- First thought was Flooding, but I have drained all the bowls and pulled the plugs
- Second thought was a kill/ignition switch, but since the plugs are sparking I figure these are OK

Thoughts
- Common advice seems to be that with compression that low, it needs a rebuild and therefore troubleshooting is a waste of time
- BUT, I don't feel like tipping over would suddenly drop compression in all cylinders
- If it was running previously even with low compression, something else must have changed when it tipped
- I am tempted to pull the carbs again, but wonder if I am barking up the wrong tree based on the low compression
- I might try putting oil in each cylinder, then putting plugs in, and trying to fire it with the hope that compression will be artificially high for a few strokes

Questions
1) After cranking for about ten seconds, I pulled a plug. I couldn't smell or see fuel on it. Is there a way to test that fuel is actually getting into the cylinder?
2) Should I try starter fluid?
3) Could some switch be broken that I am not considering?
4) Should I really just stop putting energy into troubleshooting this since the compression is low?

TIA

 

Compression certainly seems low, but readings will vary greatly between testers and their different setups, so it's hard to get a bona-fide result from them, and it's especially difficult on small-displacement engines. And like you said, tipping over shouldn't cause low compression suddenly. Oil in the cylinder will always create higher compression, so that's not a definitive factor either.

Give a shot of carb cleaner/ether down the breather tube and see if it goes. Carb cleaner isn't quite as explosive as ether and the engine will still run on it. If it fires you aren't getting any fuel into the motor.

As far as switches go if that was the problem you would have no spark. Any switch designed to kill the engine will kill spark.

Sounds like you aren't getting any fuel to me. The easy test is a shot of carb cleaner or ether. That's what I would try first. The plugs would be wet if you truly had low compression or no spark.

 

Tipping it over could wash the rings with gas, leading to low compression, but I'd think that squirting oil in should fix the problem.

Sounds like a kill switch for tipping (I suspect that all bikes are legally required to have one) but as MrG pointed out, if you have spark, that should mean the ignition is working fine.

I wish I had more ideas for you but I wouldn't junk it. There is no way that a perfectly good motorcycle becomes junk just from tipping over.

How about something clogging the fuel intake from the tank. Maybe tipping it over caused some garbage to cover the inlet to the fuel line.


Good luck.

 

I also think the cylinders were probably flooded with fuel from the tipover leading to the poor compression readings. I wouldn't give much thought to those low numbers at this point.. they are probably false readings if the bike started and ran fine prior to the incident.

Since you have spark to all 4 cylinders the ignition system is working properly and there is no damage to anything in that circuit. I would drain the carb bowls and then crank the engine for 10-15 sec. and verify the fuel pump is still working and supplying fuel to the carbs. If there is fuel getting into the carb bowls then everything is in proper working order in the ignition & fuel systems. Also make sure the engine oil is not contaminated with fuel, or if in doubt do an oil & filter change before running the engine again.

Assuming it was due to fuel washing down the cylinder walls, it can be difficult to fire the engine back up the first time until good compression is restored. It takes a minimum of around 100psi compression before a cylinder will be able to fire the air/fuel mixture. Pull all 4 plugs and crank the engine over to clear the cylinders of any remaining fuel or oil. Adding a small amount of oil to each cylinder will help to boost compression and fire easier. Then either try bump starting the engine down a hill or connect a jumper battery to the bike and crank it for awhile for 10-15 sec. at a time with a break in between each attempt. It may take 5-10 tries before it wants to fire, but you should see some sort of result eventually.

 

Thanks for the responses everyone, I got the bike started yesterday.

For what it is worth, This problem occurred a while ago, and I had already done almost all of the things suggested above. That is why I was thinking it might be time to scrap/rebuild.

Short story:
Changed oil and plugs. Hooked up to car battery. Got a burble/sputter and was encouraed to keep cranking. With throttle held open it finally caught. Doesn't idle well, is running on 3 cylinders, looks like plug wire is wrecked.


Longer story:

I spent quite a while checking things, but I only made 3 actual changes
1) New oil & filter
2) New NGK copper spark plugs
3) Hooked up to car battery for lots of good cranking

After changing the oil, I pulled all the plugs and checked spark again.

I realized that I could not see spark on cylinder number 3.

Perhaps I imagined it earlier when I thought I had spark on all 4. The spark on the other three is very thin and hard to see.

Did compression test again. Still reading around 65 psi (at 3000 feet with a cheap gauge) and it bumps above 100 with a bit of oil poured into the cylinder.

Put in all new spark plugs.

Confirmed that fuel was flowing.

Cranked it on regular battery. Had some slight burble, and a bit of white smoke.

Hooked up to bar battery and cranked for a while. Was encouraged by burbles and sputters so I continued cranking and then opened the throttle.

Motor caught and revved very high (looks like bar/grip was bent a bit from the tipover and was holding throttle open) causing a lot of white smoke.

After 15 minutes the smoke went away and idle became more normal, but was not holding idling well with throttle closed.

Pulled the wire off cylinder 3 plug and it runs exactly the same. That wire has damage to the outer sheath. It looks like it was pinched when the bike tipped over.


So, I plan to replace the plug wire(s) and go from there.


I think the major difference was hooking up a big battery, and being able to crank hard&long.

Although I expect improvement with spark on all 4 cylinders, it still remains that the compression was quite low and this motor is likely still close to its end.

That said, I was able to ride it like it is. So even on 3 cylinders, these bikes can still run with low compression readings.