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MG Midget and Sprite Technical - 1098 10CC Engine Low Compression

Hi,

I rebuilt a 10CC engine (bottom end only) for my frogeye about two years ago, since then it has only done about 1000 miles. I have subsequently had a headgasket failure between 1 and 2 which I attribute to using a 12G202 morris minor cylinder head designed for an engine with dished pistons.

I have acquired a 12G295 head which I have had skimmed, new guides and unleaded converted. I bought 4 new inlet valves and reused the exhaust valves from the 12G202 (25.4mm).

I have refitted the head and the car runs and drive ok but I have a strange noise which sounds a bit like a leaking exhaust but seems to be coming from the rear carb.

The car had the original (to the 10CC engine) exhaust manifold which had some damage near one of the flanges at the front of the engine. I therefore fitted a second hand LCB to the car but the noise continues.

The car idles fine (around 900rpm) and drives well, it only makes the noise on acceleration, when crusing or on the overrun its fine.

I checked all the plugs were tight and the timing and mixture seem fine (mixture set on an MOT test machine to around 4.5% CO).

As a last resort I checked the tappet clearances (all fine and set a 12thou). I have also tried lifting the valves against the clearance and none of them are sticking down.

I have carried out a compression test on cylinders 3&4 only, 3 is 170PSI where as 4 is only 90PSI.

Before I remove the head I will check the compression on 1&2.

Does anyone have any ideas before I start to remove the head again? Thanks for any help you can offer.

I did wonder whether to keep driving it and see if the noise goes away but I suspect that's only wishful thinking and likely to lead to damage.
Andrew Smith

Im not sure what your question is? Are you worried you blew a head gasket? (A quick compression test will tell you condition)If you are worried about the noise it makes under acceleration my guess would be a poorly sealed exhaust manifold or gasket. Do you have the stock set up on? Just trying to help but thats my guess. I usually crack out the big file and run it across the machined end on the contact pounts of the exhaust manifold to make sure they are straight and clean. he one I have is fine not coarse and I take my time to make it straight and clean.

Theres my 2cents.

let us know how you are progressing.

Goodluck!


Steven Devine

Im not sure what your question is? Are you worried you blew a head gasket? (A quick compression test will tell you condition)If you are worried about the noise it makes under acceleration my guess would be a poorly sealed exhaust manifold or gasket. Do you have the stock set up on? Just trying to help but thats my guess. I usually crack out the big file and run it across the machined end on the contact pounts of the exhaust manifold to make sure they are straight and clean. he one I have is fine not coarse and I take my time to make it straight and clean.

Theres my 2cents.

let us know how you are progressing.

Goodluck!


Steven Devine

Wow...80 psi differance from 3 & 4

My guess is a broken piston ring or a broken piston

I think a leak down test and a vacume gauge is going to tell you alot more

I wouldnt be driving it till you know why why your loosing so much compression

Prop
Prop and the Blackhole Midget

I really dont know much about 1098s ..but I think you can access the lifters thur a cover....any chance a lifter is broken

I wonder if the exhaust valve in #4 is seating properly

Can you feel any exhauste leaks moving your hand around the area of the noise...

not touching the hot engine obviously ...lol

Prop
Prop and the Blackhole Midget

Did the machine shop recut the valve seats after the new guides were fitted - and reface the old exhaust valves to ensure a seal?
F Pollock

Wow
I need to work on my reading comprehension.

Props got it in my opinion.

(#4 valve doesnt sound like its seated properly from your description)

Nice call PROP!
Steven Devine

If you did not have a pinking problems with the 202 head then it will not have caused the first HG failure.

So if any problems have risen since the head switch you should switch back and see if the trouble continues

The 202 is a litle bit more reliable then the 295 which tends to crack between the valves
Onno K

Can you do the compression test again, but this time squirt a spoonful of engine oil into cylinder 4?

If compression goes up, then you've got ring problems. If it stays the same, you've got valve seating (or head gasket) issues.
Growler

Thanks will put some oil down no4 and do another compression test.

The engine did pink badly with the 202 no matter what I did with the timing.

The machine shop recut the seats but I then ground in the exhaust valves.

If it is a valve problem, I sticking valve, as its new is it likely to get better or worse as I drive it?
Andrew Smith

It's possible you have a foul with the exhaust valve to block. Did you do any clearance checks? With skimmed heads and certainly with a hotter cam you have to pocket small bore blocks - EEK.
F Pollock

It certainly sounds like a valve not seating properly and blowing back through the carb.
Dave O'Neill2

To everyone who has responded thankyou, you've all been really helpful.

To everyone who said is sounds like a leaking valve then you were spot on. I pulled the head and found that the inlet valve on no 4 isn't seating properly. I'm going to try grinding it in again and see if that cures it.

I tested it by pouring petrol down the ports and found that only this valve leaked.

I'm going to desperately try and rebuild it tomorrow as i'm off work because I want to go to my wedding using wedding cars sevenoaks in it on Saturday! Maybe wedding cars Sevenoaks would be a better idea.

Regards

Andrew Smith
Andrew Smith

Hey Andrew,

Just a tip...When you grind on that exhaust valve, and the outcome does NOT look like this photo....then your doing something wrong

Just saying...hahaha

Good luck with it, im glad it wasnt anything more serious

Prop

http://www.poipassion.com/images/performers/performer_ian_hanness_02_coalition.jpg

Prop and the Blackhole Midget

Just to say that I have now reground the valve in and refitted the head and all is well, there was a small burr on the valve causing it not to seat properly.

Very strange as it wasn't there when it was fitted originally.
Andrew Smith

This thread was discussed between 17/07/2013 and 18/07/2013

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