Low Compression after long weekends of no use

Hello, I have what I consider to be a strange issue with my ’88 xr250r  which is confuising me.

When the bike has been un-used for a very long weekend or longer, sometime, when trying to start it has zero compression.

After kicking for 20 to 30 times, compression slowly returns until it runs(badly) then after a couple of minutes or riding it’s back to normal with good compression. I use the bike everyday for commuting, it uses lots of oil through leaks and burning. These will be fixed one day. Promise.

 

Possible causes for intermittent low compression.

1. De-comp sticking on. The bike has no de-comp cables, all removed by previous owner. Only control I have over decomp is by twisting the end of the cam sticking out of the head. This was in its normal position when I lost compression.

 

2. Rusty valves. Can valves rust this quickly?

 

3. Sticking valves, I would have thought kicking the bike over would instantly fix this.

 

4. Knackered rings. Could an oily bore be disguising knackered rings so only when the bore is dry do I loose compression?

 

5. Knackered head-gasket, again, can daily use disguise a knackered head gasket. Can a head gasket be so bad that you lose all compression?

 

Any advise on the above would be much appreciated. As I use the bike everyday I don’t want to strip the engine down until I have a fairly good idea what needs fixing.

First thing I'd think is bad rings. 

 

Bad rings = low compression.  But, if you drop some oil down into the chamber, it makes a better seal and compression goes up.  I'm thinking since the oil ring may be bad, it's letting a bit of oil up into the chamber by kicking it, then making a better seal.

 

Although a ring job may not be much to do, I have a feeling a cylinder hone may be a good idea too.  I think I've heard that mentioned on here a few times.  So that could increase your time and cost.

 

I'm sure more experienced minds will weigh in.

The only way to determine what is needed, is to take it apart, inspect, and measure the clearances.  Honing a worn out bore with new rings will continue to burn oil.

 

I'm guessing you have a lot of oily carbon build-up in the combustion chamber.    

Thanks for the input, seems that everyone is leaning towards the rings then. The bike is 26 years old so god knows what condition that bore is in.

Noted regarding honing a warn bore. As I can't afford for my bike to be in bits whilst I work out what size rings I need and whether I need a re-bore etc I think I'll have a look around for a second hand barrel, piston and head and refurb the lot. Could go down the big bore route. .

Whilst I'm at it, new valve seals etc.

And then sell my old stuff assuming it has any life left in it for someone else to do the same thing.

 

Anyone in the South London area got any old parts that they want to get rid of for a fair price?

You could easily pinpoint the problem by doing a cylinder leak down test.

A new engine that was properly broken in and warmed up would have up-to 3% leakage max.

A test that shows 10% leakage or more would not be running as well as it could be with less than 10%.

The test would also allow you to pinpoint the leakage problem.

Intake or exhaust valve(s) piston rings or head gasket.

IMO: you have a carbon build up problem minimum, and more than likely a worn piston, rings and cylinder bore.

You might try using a powerful de-carbon detergent in you fuel tank like "Seafoam" to remove all carbon from your piston, rings and valves.

And then do another leak down test again after using a whole bottle with 3-4oz cleaner per tankful just to find out if the results will be less than 10%.

But plan on a fresh bore, piston, rings, valve job, oil change and properly servicing your air filter minimum if you plan on keeping the bike.

Hello All, Just some feedback to the problem above (bike randomly loosing all compression during start-up).

I re-built the top end, hone and new rings. Cylinder diameter was checked and all in spec so no need for a re-bore.

The rings were very clean, free to compress so seemed in good condition.

Exhaust valves had carbon build up but the seats were good. Valve stems all clean. Seals good.

Cause of the oil burning was a loose valve guide.

 

After the re-build, smoking has stopped, compression is still good, however, it still randomly looses all compression when starting in the morning. Not always. Just sometimes.

Bike has no kick start compression cables attached or hand level cable. Removed by previous owner.

When compression is lost, I always check that the decompression cam on the rocker cover is in the right location. It always is. However, when kicking whilst I'm twisting the cam to double check it's not the cause, the valve doesn't appear to be applying any pressure to it. You can normally feel the valve pushing back on the cam.

 

I don't understand how this is possible????  :banghead:

When ever we store our bikes,always put on TDC.This helps from getting rust on valve seats,also helps valve springs.We sometimes have same problem,even need to pull start bike.When your bike is 26years old,rebuild it right,ride another 26years.Even if valves look good,after 5/30 years,they can become work hardened.Valve stem snap,you know what happens then.Also if the 88 250 (I cant remember) has Dc on cam,we throw it away.One time had rust on valve stem,No Comp.Pulled bike to start,No more problem,and that was in Calif nice and dry.In England very wet,easy to get rust.

I have had this happen on my 600 periodically.  I have seen the same thing happen to Honda cars too.  Honda even put out a service bulletin on the problem once.  What happens is the valve stems gum up for various reasons, oil quality, fuel quality, etc.  When the engine cools the deposits thicken and sometimes when you go to restart it cold the valve opens and stays that way.  The Honda car bulletin stated the "fix" was to introduce a upper cylinder cleaner into the intake as you cranked the engine to free up the valves. you could hear the cylinders pick up compression one by one as you did that.  On the cars that had this problem it was usually after a long trip where they had been running several hundred miles.  I figured they got warm and melted the deposits on the valves and stuck them the next morning.

 

On my 600 it has happened twice.  One time I just kicked it several times and it finally had enough compression to run.  The other time I was conveniently close to a hill and coasted it down and started it.

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