Compression loss when hot.

[flatdog]

One obvious thing is, are you adjusting the correct valves (won't be the first time I've adjusted the exhaust valves to inlet gaps.)

Failing that, spend a little money at your local dealer /bike shop, at least they can rule out most issues and hopefully find out what is wrong. To me it does not seem too far off being ok.

 

[Brendan]

One obvious thing is, are you adjusting the correct valves (won't be the first time I've adjusted the exhaust valves to inlet gaps.:

To me it does not seem too far off being ok.

I am absolutely positive that the valve clearances are as per BMW spec.

I'm not taking this to a dealer or bike shop. The collective knowledge here is far superior to that available at any individual bike shop or dealer.

There's a challenge here and I'll not be beaten or else the bike dies.

 

[John Armstrong]

Are you checking the compressions with an airline gauge or one that is like a tyre pressure gauge? If you're using the 2nd then using the 1st type will tell you much more as you can then move the piston around and also listen in the intake / exhaust / sump to see where it's leaking. My 2p's worth from doing compression tests on Lycoming engines.

 

[Brendan]

Are you checking the compressions with an airline gauge or one that is like a tyre pressure gauge? If you're using the 2nd then using the 1st type will tell you much more as you can then move the piston around and also listen in the intake / exhaust / sump to see where it's leaking. My 2p's worth from doing compression tests on Lycoming engines.

I've done the compression test using a screw in type max reading gauge.

I've also made up a hose and connector to allow me compress the cylinder with air. Using this method I can push the piston back when set before TDC or forward after TDC?
Holding the kick start at TDC I can put more pressure into the cylinder than an average human can hold back. All the above done with the engine hot.

I'm now waiting on new head gaskets to reassemble the engine.

 

[John Armstrong]

And I guess when you're pushing air in you can't hear anything leaking?

Or, seen any evidence of leakage when splashing soapy water over the heads / barrels etc?.

Just had a thought, not something stupid like lack of oil pressure / flow, so when the engine gets hot the lack of oil means the gaps are too big? Pure wild guess and maybe totally wrong.

 

[Beemerboff]

Try checking the valves by feel with a hot, running, engine - that is the way almost every auto professional checks them.

There should be just the slightest free play.

And the two pools of oil on the floor will confirm that oil is reaching the rockers---.

You said that the previous rings were broken - this can indicate excessive side clearance in the ring groove - were the grooves checked, trued up and oversized rings or spacers fitted or did the new rings go back in the same grooves?

 

[Brendan]

Thanks for the reply

Try checking the valves by feel with a hot, running, engine - that is the way almost every auto professional checks them.
There should be just the slightest free play.

Sorry don't understand what I'm checking for, is it free play between the valve stem and guide. If so I've already heated the head to 200 deg c. If not can you elaborate.


And the two pools of oil on the floor will confirm that oil is reaching the rockers---.
Oil is reaching the rockers for sure


You said that the previous rings were broken - this can indicate excessive side clearance in the ring groove - were the grooves checked, trued up and oversized rings or spacers fitted or did the new rings go back in the same grooves?
Rings went into the grooves that looked to be in good condition. The old broken rings which were on 1 piston only came out in 2 pieces and 3 pieces. No bits missings and all pieces were quite large. Didn't feel that any remedial work was required to the pistons

 

[roamer]

compression loss when hot

Hi Brendan, your cold compressions are low, i am assuming the throttle is wide open , does it get higher with more kicks,
when you do the cylinder leak test with the air line, only put a little pressure in as you are listening for where the air is escaping from especially the crankcase breather and valve ports,
if you still have it apart , slip a barrel off and drop the top ring in the bore about 15 / 2o mm from the top and and check for daylight between ring and bore ,and most important end gap against bmw spec, move the ring to the bottom of the bore and compare,
you are trusting others regarding the cylinder bores, thats where i would be looking,
pm me for my phone number [if thats allowed] i will be happy to go through it with you,,,,,,,roamer

 

[stever1]

OK. I know its a bit out of the box thinking, but could you be looking in totally the wrong direction ?. The symptoms you describe could simply be fuel starvation. Try changing any fuel filter, and clean/change any fuel lines.
If you think about the age of the bike, if the fuel system is original the fuel lines etc could be degrading.
Good luck,

 

[Brendan]

OK. I know its a bit out of the box thinking, but could you be looking in totally the wrong direction ?. The symptoms you describe could simply be fuel starvation. Try changing any fuel filter, and clean/change any fuel lines.
If you think about the age of the bike, if the fuel system is original the fuel lines etc could be degrading.
Good luck,

Hi, we fitted 2 brand new bing carbs, fuel tap and hoses about 18 months ago. The bike has done about 200 miles since then. First time it stopped on the side of the road we assumed electrical or fuel, we've done them to death at this stage. Above still would not account for low compression.

 

[JayC]

Brendan have you done a wet and dry test or just the dry? A wet test should point to your compression loss , whether it be rings or valves.
I am aware you have pointed out all the work carried out but that doesn't mean it was done correctly despite the years of experience of the workshop.

I would still be checking the bore sizes/ovality just for peace of mind. I agree that this shouldn't cause a loss of compression when hot over cold but something is causing this and all avenues need to be exhausted.
It's daft enough that your compression loss is happening A about F and so all the 'no it couldn't be thats' need to be eliminated.

Did the workshop provide you with their findings on the bore measurements so you could check against the permissable tolerance range? Was the honing their advice after carrying out checks or your request?

Edit: Ignore my comments re 'wet test'! I have just learnt it is inadvisable on airheads!

 

[g.s.john]

Did you have the valve seats replaced when you did that top end rebuild?
If one is not seated correctly that could be the problem.
And before you ask it took me ages to find that little bastard.

 

[thornley]

How much was honed out? What was the bore diameter at finish. Ring sizes fitted?. If the honing was heavy the rings might be under size at temperature?. Valve seats fixed or could they lift? Valve guides good solid fit?

Just trying to throw in a few questions!

 

[Brendan]

I would like to start by thanking everyone for their input.

Ok, so after listening to all the comments and suggestions, here is my plan.

Remove the barrels and check size and ovality. Piston is sized as 67.94 std.
Depending on result of barrel tests either leave alone or machine to next size and fit new pistons and rings

Reassemble and do a leak down test on the cylinders after barrels have been resolved.


Will have to wait for a few days until gaskets come in before I can do any further assembly.

I plan on burning the fuc*er if it's not sorted out this time.

 

[Brendan]

How much was honed out? What was the bore diameter at finish. Ring sizes fitted?. If the honing was heavy the rings might be under size at temperature?. Valve seats fixed or could they lift? Valve guides good solid fit?

Just trying to throw in a few questions!

Funny you should mention amount of honing as it was muh less than I would have expected.
Rings fitted were BMW parts having given the supplier piston size as stamped on the crown.
The valve seats are integral as far as I can see.
Valve guides were press fitted and internal machined after to suit new valves.
I left petrol in the recess of the head and there was no leakage into the port area.

 

[thornley]

Funny you should mention amount of honing as it was muh less than I would have expected.
Rings fitted were BMW parts having given the supplier piston size as stamped on the crown.
The valve seats are integral as far as I can see.
Valve guides were press fitted and internal machined after to suit new valves.
I left petrol in the recess of the head and there was no leakage into the port area.

If the bores were oversize to start with, I assume these are liners and not plated, honing 0.1mm plus could take them oversize. Without a known bore size, I can only guess. Stay with it you will have a Eureka!! moment

 

[roamer]

compression loss when hot

hi Brendan.
you have been thorough and precise with everything you have done
this excludes checking the bores and pistons
maybe you would be as well checking them as the cold compression is low
I am not the only one thinking this may be your problem and it is very relevant
broken rings are a symptom of piston and bore trouble
good luck,,,,, roamer

 

[Smug]

Are the piston rings fitted the right way up? Are the piston ring grooves in the piston worn?

Take the top rings out of the piston and try them down the bore. Measure the ring gap with the rings about 10mm from the top of the barrel and again with the rings further down the barrel.

If the barrels were honed take a look at the barrel surface to see how the rings have marked the honed surface. Is it polished all the way round and all the way down the bore?