Compression test completed - time to rebuild motor?

Pedey · Post by **Pedey** » Sun Feb 17, 2013 3:35 pm

Hi everyone,

I'm the first to admit I don't know all that much about the old L28, so any advice would be most welcome.

I just did a compression test on the L28 and got the following results:

1: 172
2: 173
3: 172
4: 171
5: 130
6: 172

Now, obviously there is an issue with cylinder 5. We put a few drops of oil down there but it made no difference, indicating that it may not be the piston ring? The valve may not be seating properly, and I understand they are adjustable and can be shimmed? What would you guys do next? Do I need to rebuild the motor? Should I just buy another motor that's already built? Do you have one you can sell me for cheap?

Also, I'm trying to get even a very rough idea about what kind of compression ratio the motor is running. Supposedly it had a high comp rebuild before Morgan (who sold it to us) got hold of it, but he said he had his doubts. Do those numbers give anyone a rough idea? I was hoping for a 9:1 ratio or somewhere around there.

Again, any help or discussion would be most welcome.

Thanks!
Pedey

nustad · Post by **nustad** » Sun Feb 17, 2013 4:26 pm

Has the car sat for a long time and had very little use?

Have you been for a decent long run in it recently?

My engine has receipts for a fresh rebuild but I have oil control problems that seem to get better with increasing use......

Ps my car is now at GT Refinishers getting rust work and maybe a new air dam depending on how much the rust works comes to.....

djz · Post by **djz** » Sun Feb 17, 2013 4:29 pm

A leak down test would probably be the next thing I'd do, it will help narrow down where the problem is but if a valve isn't seating there's not much you can do without pulling the head.

nzeder · Post by **nzeder** » Sun Feb 17, 2013 7:34 pm

I with DJ on this a leak down test is the next step.

us2 · Post by **us2** » Sun Feb 17, 2013 8:07 pm

Before doing too many kms. you should check valve clearances. If you have a tight tappet, it could lead to a burnt valve.
Take off the valve cover, and check for a tight tappet on that cylinder. Valve/tappet clearance is not adjusted with shims. It's a matter of a couple of nuts to turn. You need a feeler gauge and some open end spanners. 14mm and 17 mm from memory,
and a socket to turn the engine over by the cam bolt. Clockwise, otherwise you will loosen it.
Adjusting valves is not difficult on these engines. GOOGLE

If the valves are OK, then do some miles on the engine, and don't panic. 130 is still compression, and i would wager it still hauls ass.

Pedey · Post by **Pedey** » Mon Feb 18, 2013 6:09 pm

Thanks for the replies, an engine builder I spoke to today suggested pretty much the same thing, so I guess I'll be whipping off the rocker cover soon!

ZMAD · Post by **ZMAD** » Mon Feb 18, 2013 6:23 pm

Get a new gasket first and don't run engine with cam cover off.

us2 · Post by **us2** » Mon Feb 18, 2013 6:39 pm

ZMAD wrote:Get a new gasket first and don't run engine with cam cover off.

Correct. And definately not with your favourite white shirt on.

That does NOTHING for marital bliss.

Ask me how i know this.

ZMAD · Post by **ZMAD** » Mon Feb 18, 2013 10:47 pm

Ha ha!

Pedey · Post by **Pedey** » Mon Feb 25, 2013 11:19 am

So I pulled off the rocker cover, checked everything. Everything appears fine, also checked the valve clearances, they are spot on. My next thought is a burnt valve, so I need to pull off the head. Anyone that's done it a few times before have and tips for me?

Also, I decided to do this, as the motor has to come out anyway so everything can be painted:

As always, any input is appreciated

djz · Post by **djz** » Mon Feb 25, 2013 11:45 am

Make yourself a chain wedge and don't forget about the two M6 bolts at the front of the head. Get another person to help lift it off if you are leaving the manifolds on, if you drop it on the dowels you'll regret it.

Andy · Post by **Andy** » Mon Feb 25, 2013 12:03 pm

I have no advice that doesn't involve a lot of time or a lot of money, but flowing the head and fitting better valves would be my suggestion, a visit to Gary at The Cylinder Head Shop on the North Shore will give it the necessary go.

Other than that its a quick repair, check the other valve seats and guides and fit a decent head gasket. Head on and off and readjusted is a 3 hour job with manifolds etc. If you find substantial wear or other issues think of spending this lot of repair money on a modified head rather than fixing now and modding in 12 months time.

us2 · Post by **us2** » Mon Feb 25, 2013 1:13 pm

Pedey wrote: As always, any input is appreciated

First do some reading.
Then get the engine at TDC on firing stroke. Take note of where the chain is in relation to the marks on the sprocket. If there is no mark on the chain, put a dot of white paint or your wifes fingernail polish if she is not home.

Orange colour is OK. The rotor should be pointing towards the left headlight roughly.
And as DJZ suggested, wedge the timing chain, If you don't, the chain falls off the bottom sprocket, and you loose the timing.

Then unbolt the cam sprocket.
Then you can unbolt the head. You would have noticed you need a allen key socket for this. Size????

Don't turn the engine over with no front cam sprocket. You could bend valves. For that matter, don't turn the engine at all untill it is reassembled as you will disturb the wedge holding the chain.

May the force be with you.