Compression Loss

[B-Boyblue]

Hi all,

I have had my motor rebuilt last year and had mahle 1641 barrels and pistons fitted and twin 34 weber ICT carbs. It runs fine but has always leaked oil the cause of which has now been identified as an over pressurised crankcase despite it having a bugpack breather kit and vented rocker covers. it had a billet oil filler neck on but that was changed to a standard filler and the filter changed on the breather and that made a big improvement to the oil leak. I was told to maybe fit a tower breather on such as the CB performance one. Before doing this, it was recommended that a compression test and leak off test should be carried out. We found compression at between 98 and 102 PSI on all four cylinders but when we did the leak off test, there was very little leakage and the pressure was holding.

Does anyone have any idea as to what could be causing the lack of compression as I'd expect to see better values than that having had a rebuild and done less than a couple of thousand miles.

Any advice would be appreciated.

Cheers all

 

[privatejobs]

i had lots of oil leaks on my rebuilt engine due to using 20/50 oil mineral and it smoked a lot i then tried a 5/30 semi syn and it pissed out almost evry where and smoked like a deisel have now put 15/40 mineral oil in it no leaks no smoke could not beleive oil would do that

 

[Happystamps]

Crankcases become overpressurised because of piston ring wear. They're all slightly pressurised, but too much will blow oil out. Fitting "performance" breathers doesn't help at all- they're often more restrictive than standard (meaning crank pulley leaks) and you don't need them.

Fit a standard breather/filler assembly (You already have) and get rid of the breather filter
Run a breather hose into one of the carb filters using a decent sized fitting on the air filter base.
Put stock rocker covers back on.

That should get rid of the oil leaks.

Normally I'd say that you could get your low compression readings from poorly seated valves or worn rings. Do your compression test, then put a few spoons of oil down each spark plug hole and try it again. You'll either see the readings improve and you'll have to sort new rings and possibly a cylinder hone, or you won't see them improve and you'll need to check your valves out. They can often be sorted by grinding them back into the valve seats, which should get rid of leaks (unless you've got cracks in 'em)

TBH it sounds like your rings are a bit shit.