Compression loss when hot.

[Brendan]

BMW R50US 1969.
I'm having a problem with the above bike and would appreciate if you could read my report and if you have the time possibly reply with some suggestions. At this stage I've gone grey and nearly bald as a result of this bike.

So here goes;

When cold the bike will start first or second kick, never needs more teasing when COLD.
When run for 3-4 miles power reduces. Bike will splutter and hesitate when revs are let go low. If you then stop the engine it will not restart until cooled.

What we've tried today;
Compression when cold 130psi on left and 120psi on right, both after 5 kicks.
Compression when hot 60psi on left and 70psi on right, both after 5 kicks.
No noticeable smoke from either exhaust pipe.
No noticeable smoke from crank breather pipe.
Heads removed again today to check for sticking valve stem, all seems fine.
Checked the valve clearances when hot as well and they are + 0.05mm above cold set.
No signs of blow back into inlet port from exhaust.
No oil leaks around head gasket.

Work carried out on the bike;
Barrels honed.
New piston rings fitted.
New valve guides and valves fitted. (proper job by machine shop, old timer)
New head gaskets.
Heads bolts torqued down as per spec.
Valve clearances set and checked several times.
Rocker end float below 0.05mm.
New mag coil and condenser.
New points, plugs and HT Leads.


Please help.

Any help would be greatly appreciated.

 

[(RIP) Mark H]

Dont know if the heads are still off, try inverting them and filling with petrol to see if valves are seated well ?

Broken compression rings on reassembly ?

Did it run fine for a time after the engine work, or has it been like this since rebuild?

 

[Brendan]

Thanks for your reply and questions, I've tried answering them below.

Dont know if the heads are still off, try inverting them and filling with petrol to see if valves are seated well ?

Butchered a spark plug to make a way of putting compressed air into the cylinder. Just before TDC on each cylinder I'm able to pressurise and hold a pressure in each cylinder when both cold and hot. Also new valves, and machine ground when heads were having new guides done.

Broken compression rings on reassembly ?

Would this not also show up when doing the compression test cold.
(also I'm kinda careful doing things and would be surprised if I managed to damage rings on both cylinders)

Did it run fine for a time after the engine work, or has it been like this since
rebuild?
No, the rebuild was done for several different faults including broken rings and sloppy guides.

 

[Brendan]

Just another bit of info to go on. Today I heated both heads to 200deg C and all valve stems move smoothly in the guides.

 

[(RIP) Mark H]

Did it run fine for a time after the engine work, or has it been like this since rebuild?

What I mean above is, when you completed the engine rebuild did it run fine for a while (Good starting hot & Cold) and then go on to develop these symptoms?

From what you say and what tests you have already done its difficault to imagine what could be wrong here

 

[Brendan]

What I mean above is, when you completed the engine rebuild did it run fine for a while (Good starting hot & Cold) and then go on to develop these symptoms?

From what you say and what tests you have already done its difficault to imagine what could be wrong here

No, it did not run ok since the work was done. The symptoms I described have been there since rebuild.

I was thinking of making up a blanking plate to allow me pressurise the domed area of the head and reheat and pressure test after activating the valves a few times and then check for leakage.

 

[Kenny]

Brendan,

Surely the compression readings should increase when the engine warms up !
Could the rings be the correct size? Tiz puzzling !

 

[bumpmuncher]

sounds like a valve clearance problem to me. are you using the correct clearances? I have never worked on an R50 so I am probably way off the mark

 

[Brendan]

Rings were OEM parts.
Valve clearences 0.15mm inlet cold. 0.20mm cold exhaust.

 

[JayC]

Has the motor had a rebore or just honed barrels?

 

[flatdog]

sounds like a valve clearance problem to me. are you using the correct clearances? I have never worked on an R50 so I am probably way off the mark

Me too, perhaps increase the tappet gap by a couple of thou and see if it improves. Your symtoms sound like the clearences are too small? noisey tappets are happy tappets etc.
or the timing is out.

 

[Brendan]

Has the motor had a rebore or just honed barrels?

Still on original bore. Barrels honed.

 

[Brendan]

Me too, perhaps increase the tappet gap by a couple of thou and see if it improves. Your symtoms sound like the clearences are too small? noisey tappets are happy tappets etc.
or the timing is out.

Can you explain your theory behind the valve clearances been too tight reducing the compression when hot. Remember the valve clearance has been measured at + 0.05mm above cold set when hot.

Regarding the timing been out I think not. It's been set and checked several times very carefully according to spec.

Sorry if I sound blunt and too the point but this bike has me driven demented.

 

[flatdog]

Can you explain your theory behind the valve clearances been too tight reducing the compression when hot. Remember the valve clearance has been measured at + 0.05mm above cold set when hot.

Regarding the timing been out I think not. It's been set and checked several times very carefully according to spec.

Sorry if I sound blunt and too the point but this bike has me driven demented.

No worries Brendan, tappet gap reduces when the engine gets up to temp, so if they are tight to start with (not saying they are, but it's worth adjusting a bit more) then as the metal parts expand the gap reduces and in some cases the valve will not be completely closed on compression.
too tight = when cold the engine runs fine then as it warms up the valves don't close to a compression seal etc.
too loose = valves operate later and reduce engine performance and are a little noisey.

I'll take it off you no problem, the bloody unreliable piece of junk................

 

[(RIP) Mark H]

If the valve clearances are tight they get tighter as the engine gets hot, I think the suggestion is you open the valve clearances by a few thou temporarily to see if it cures your problem ........... have your valve springs have been replaced in the rebuild ? if not are they tired, broken, wrong height, have they been measurered against spec.

 

[JayC]

Has the motor had a rebore or just honed barrels?

Still on original bore. Barrels honed.

So the obvious question then , have the bores been checked/cleared for any ovality? Not having a clue of the mileage, could it be that your barrels are oval?

 

[Brendan]

So the obvious question then , have the bores been checked/cleared for any ovality? Not having a clue of the mileage, could it be that your barrels are oval?

Let's assume they are oval, why have I good compression when cold?
The honing was also done at the machine shop, would have expected he checked for ovality.

 

[flatdog]

So the obvious question then , have the bores been checked/cleared for any ovality? Not having a clue of the mileage, could it be that your barrels are oval?

That probably won't be an issue here, the compression fades as the motor warms up. Your scenario would be present cold or hot.

Sorry, Brendan got there first. edit

 

[Brendan]

If the valve clearances are tight they get tighter as the engine gets hot, I think the suggestion is you open the valve clearances by a few thou temporarily to see if it cures your problem ........... have your valve springs have been replaced in the rebuild ? if not are they tired, broken, wrong height, have they been measurered against spec.

I'm getting greater clearances when hot. Springs have not been replaced or checked other than visual inspection.

 

[JayC]

Let's assume they are oval, why have I good compression when cold?
The honing was also done at the machine shop, would have expected he checked for ovality.

Sorry, my eyes or brain , both even , totally overlooked that detail!