Low compression 3rd cylinder

steve2286w · Dec 28, 2019

It’s been a bit smoky, low on power, and checked the compression today it’s about 95 on cylinder 3 and about 270 on the others , I know it sounds low but it starts well
Disappointing as I’ve just got it back together
I’m not sure if it’s gasket or rings the exhaust pipe is wet and black
There may be specks of oil in top of rad, strangely no pressure in rocker cover
It’s out of mot now so not really sure if it’s losing coolant
With 5/6 valves shut, I connected my co2 cylinder to no 3 and put about 10-15 psi through I could here the noise with ear to oil filler cap

as a further check am I safe putting a couple of squirts of oil in glow plug hole before testing compression again
I guess it would not start or fire on that cylinder with such low pressure
Anyone take a guess at what the problem could be

jamesmartin · Dec 28, 2019

whatever, heads got to come off

steve2286w · Dec 28, 2019

Thanks James, if it’s rings what could have caused them to fail so soon , they looked ok when refitted, only done about 20 mile
Will the problem be apparent with head off do you think

jamesmartin · Dec 28, 2019

you need to the gasket before checking anything else

steve2286w · Dec 29, 2019

@jamesmartin ,
Well all sorted, found the cause of the low compression of one cylinder and I doubt you’ve had this one before
a month ago I cut the smaller diameter off the end of the inlet manifold to let a bit more air in where the inlet hose goes
With the manifold in the vice I stuffed a face flannel in the hole to stop the filings going in the inlet chamber
So 4 weeks later after much head scratching and testing took off the inlet manifold off and stuffed into the No 3 inlet port was said rag

So it could not get a good compression as no air getting in to compress
Now No 3 pressure good and running ok again, need to get it MOT now before further road trials
Another lesson learnt ,, never too old

Flossie · Dec 29, 2019

steve2286w said:
@jamesmartin ,
Well all sorted, found the cause of the low compression of one cylinder and I doubt you’ve had this one before
a month ago I cut the smaller diameter off the end of the inlet manifold to let a bit more air in where the inlet hose goes
With the manifold in the vice I stuffed a face flannel in the hole to stop the filings going in the inlet chamber
So 4 weeks later after much head scratching and testing took off the inlet manifold off and stuffed into the No 3 inlet port was said rag
So it could not get a good compression as no air getting in to compress
Now No 3 pressure good and running ok again, need to get it MOT now before further road trials
Another lesson learnt ,, never too old

Lol!
I did similar many many yrs ago.
Thrapping a Yamaha RD 400 down the motorway and suddenly lost nearly all power, just enough left to limp off at the next exit.
Didn't have a clue what to do, no tools no breakdown cover.
Lifted the seat (don't know why) and there was my spare pair of dry socks sucked up the air cleaner inlet

.
Power restored

steve2286w · Dec 29, 2019

Thrapping a Yamaha RD 400 down the motorway
glad I’m not the only one
They were scary fast, I fell off the back of one as mate wheelied it down the road years ago, my bike at the time was. Yamaha AS3 125cc

ifloochies · Dec 29, 2019

steve2286w said:
@jamesmartin ,
Well all sorted, found the cause of the low compression of one cylinder and I doubt you’ve had this one before
a month ago I cut the smaller diameter off the end of the inlet manifold to let a bit more air in where the inlet hose goes
With the manifold in the vice I stuffed a face flannel in the hole to stop the filings going in the inlet chamber
So 4 weeks later after much head scratching and testing took off the inlet manifold off and stuffed into the No 3 inlet port was said rag
So it could not get a good compression as no air getting in to compress
Now No 3 pressure good and running ok again, need to get it MOT now before further road trials
Another lesson learnt ,, never too old

I would pay good money to have seen the look on your face the moment you found out what your "problem" was

....
....and here you were ready to pull the head off

steve2286w · Dec 29, 2019

If there was a smiley face for an “embarrassed, jaw to floor, staring zombie like plonker” that would be me it took a bit for it to sink in and it took a while to remember it was me who put it there and then the joy of not having to tinker about with heads off/ engine out anymore

lynall · Dec 30, 2019

Ive seen it happen to pro mechanics before, last one guy got all parts from stores and stuck them in the large air filter to use as a basket, then proceeded to to service engine/adjust valvee etc, but could not find rocker cover gasket (a large rubber job) so got another from stores, fitted it and roadtested all good, driver came in coupled up and managed about 5 miles before gasket that was left in air filter got sucked into the turbo and wrecked it, 2.5k to fix and the guy got the sack.
Shame as although he was a lazy fcuk he was a likeable bloke.

Best thing is to use a rubber latex/nitrile glove to cover pipes etc.

I have seen rebuilt gearboxes with rags left inside blocking oil pick up and wrecking the box.

steve2286w · Dec 30, 2019

Glad I fitted the Di lol

Low compression 3rd cylinder

steve2286w

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