Back pressure

Hi All

I previously posted my question about my 850mk2 a blowing head gaskets ! and apart from the good advice I was given before re the front two studs in the cyl head being a weak spot around the pushrod tunnels , I still have a nagging doubt with regards to back pressure or oil in the bores ?? I can see no obvious leakage from the head gasket but I am still burning off a bit too much oil/fouling plugs .
I have the bike stripped now and I am about to pull the barrells off and check out the situation with the piston rings / the bores are good with no signs of wear or scoring.

Cheers Ken.

Ken,
I had the same on my 850, after many miles I took the head off and the bores looked OK. Took the barrels off....every compression ring was broken!

Mick

Provided it is oil & not a carburation or ignition problem, oil can come from the following,

Knackered or loose intake guides.
Knackered or missing intake guide seals.
Piston rings stuck in piston.
Piston ring gap right?
Rings not run in properly.
Valves seating?

It would be worth emerying the bore with 120 or 180 grit to get rid of the shine, then running on 10-40 oil for a while to run the rings in. Don't ride it too gently running rings in. I have seen rings not bedding in with straight 50 oil & forever smoking.
I now always lap the head onto the barrel during rebuilds with grinding paste, makes a perfect match. Run my bike on 10-40 oil all the time, but that is my preference.

Or give up & sell it to me cheap :lol:

Ken Graham said:

I still have a nagging doubt with regards to back pressure or oil in the bores ?? I can see no obvious leakage from the head gasket but I am still burning off a bit too much oil/fouling plugs .
Cheers Ken.

Click to expand...

Could you explain what you mean by backpressure? How much oil are you burning?

when pulling your barrells off look at the pistons for signs of blow by (black markings ) Doing so you can eliminate the different causes as mentioned by Flo
i strongly advise AGAINST deglazing . what you will get is more oil consumption etc
whatever you do please post your results on this subject so we can get rid of that idea of deglazing once and for all

Cheers guys , off to the garage just now . will update later possibly Saturday due to work and let you all know what I find when the barrells come off.
hopefully that will show me what is going on.

Many thanks Ken.

Result ! some time ago when having the top surface of the barrells skimmed the engineering shop cracked the skirt around the cutaway for the conrod , this area was then cut out to eliminate the crack from growing ! it would appear to have picked up a small piece of schrapnel that has lighty scored the lower area barrell allowing oil to pass the already not run in rings which is then being burned off in the top end .
I do have a pair of unfinnished barrells which will now be honed and retain the standard bore.
I will also split the bottom end now and have a look at the big ends ect to try and acertain any further damage - hopefully none.
Thanks for all the advice guys , parctice makes perfect.
Cheers Ken.