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MG Midget and Sprite Technical - What am I doing wrong?

Hi all

Hope someonce can help. I've just replaced the head gasket on my 1500 midget as I was losing water with no puddles under the car and the cooling system was being pressurised. All four compression readings have now increased by about 1/3. The car starts and idles fine at about 800 rpm.

The problem appears as I take my foot off the pedal after revving it. I get a spitting and poping from the exhaust. So far I've done the following:

Plugs cleaned and gapped
Points cleaned and gapped
Rotor arm clean
Distributor cap clean
Plug leads clean
Dwell set to 54 degrees
Dynamic timing set to 10 degrees BTDC
Balanced carbs

Can't remember what the result should be if you raise the carb piston lifting pin. Should the engine falter then pick up or falter and ultimatley stall? Mine stalls immediatley.

Another symptom maybe she seems reluctant to rev much over 4000 rpm and slow to move from 3000 to 4000 whilst driving.

I think the answer's really basic but I've been tinkering for so long, I can't think straight!

Help please, what have I missed/done wrong?

Andy
Andy Kenning

Popping on over-run could be weak mixture, e.g. an air-leak into the manifold. But this wouldn't explain the reluctance to rev - that might be sticky pistons? (Assuming your timing is correct...)

A
Anthony

when you lift the piston, if the mixture is correct then the revs will rise slightly and then drop a little.
I've found that it's all a little vague and find colour tune easier to use (maybe my carbs are tired?)

If the engine dies then it's way too lean, if it's leanish then you'd expect the revs to drop, but not for the engine to go out.

things to check:

Air leak on the inlet manifold.

All manifold hoses present and correct?

Dash pot oil still in carbs?

air filter fitted?

You could try richening the carbs, but if you didn't alter the head (straight off and on) then I doubt the tune would be miles out on re-assembly...unless a fault has be 're-assembled in'

I'm sure others can think of more things to check as well....but it does sound like carburation rather than ignition related

robnrrugby

oh yeah I forgot the traditional response





condenser!





please ignore the final comment of my previous post!
robnrrugby

I think it sounds like weak mixture too. Although a leak into the exhaust system at the manifold or in the pipework will cause the same affect. I have known people diliberately drilling a small hole half way down the exhaust to create this very "cool" affect. :-)

I think why many people find the "lifting" the piston technique a little problematic is that you do NOT push the pin all the way up. You have to "feel" the pin onto the piston, and then lift it a further 1mm or thereabouts. If the engine dies then the mixture is weak, if it speeds up then it is rich, if it speeds and then returns to normal it is fairly spot on. All that assumes the carbs are in balance.


Could be condenser ;-) (receives brown evelope from condenser suppliers)
Bob (robert) Midget Turbo

Do 1500 carbs have lifting pins?

Dave
Penwithian

Yes they do.

Alan
Alan Pritchett

No brown envelopes received here from Nick Swift, either, but Swiftune have just launched a 'competition condenser'. Hardly dirt cheap at £27, but what price knowing it ISN'T that little piece of ... trouble?

http://www.swiftune.com/Product/535/competition-condenser.aspx

TC
Tom Coulthard

Thanks guys

It was a weak mixture causing the problem. Well done all.

Now to sort out why I continually have to top up with water, about a pint every 20 miles or so. I've a suspicion that what I thought was a blown head gasket may have been a cracked head as well. I looked for any cracks whilst the head was off and couldn't see anything untoward. The crowns of pistons 1, 2 & 3 all had slight carbon deposits on them but number 4 was clean of any deposits. Is this a clue?

The missing coolant is not appearing as puddles under the car or as leaking type deposits under the bonnet so I guess it must be getting into the cylinders.

The next step will be to get rid of the contaminated oil, flush it through an d refill with fresh oil to see if that gets contaminated as well, unless anyone has any better suggestions.

Andy
Andy Kenning

glad to hear you've made progress on the mixture side of things..

Has it got an expansion tank?, are you sure that after the 1st pint has gone the coolant continues to disappear?

if it does then you're not overfilling it, so I'd say yes you have a head problem, start looking in and around pot 4. time for a coolant system pressure check?

robnrrugby

Yes, it does have an expansion tank.

I've filled the engine through the thermostat housing plug, started the car with the plug off and as it runs and warms up the air bubbles out of the hole where the plug should be. I remembered doing this with my MGBV8 last year and it seems to work on Midgets too!

I've taken the car for a 'spirited' drive and it's now been cooling down in the garage for about 1 1/2 hours. The engine is warm to the touch, the expansion tank is still at the correct level. I had a peep in the thermostat housing and the level has dropped by about 1/2". The temp needle got halfway between the first and midway (normal?) mark. Is this right?

Sorry to be so vague, but this is early in my Midget ownership. I've restored this one for my wife without having driven one before! I've only ever worked on B's before this.

Andy
Andy Kenning

Andy,
I am experiencing similar problems to you with the car wanting regular top-ups. I suspect a head gasket problem, but hadn't thought of the possibility of a cracked head. It is only recently that I switched to an MG Metro head so it could be faulty. Hope not!

After one heating / cooling cycle the expansion tank should remain about half full - if you don't top it up or open it whilst hot, ;-) it should then stay pretty constant at that level. Mine doesn't! Another clue is the expansion tank foaming if the cooling system is being pressurised with engine gasses.

A compression test my show up the fault. A cooling system pressure test might also if you can find someone who does such a thing. In your case one clean piston top could indicate it was getting rinsed off perhaps?
Guy Weller

Guy

Prior to replacing the head gasket, the cooling was pressurising to such an extent that opening the expansion tank resulted in a loud high pressure pop/whoosh noise! Having replaced the gasket this no longer happens. However I did examine the old gasket closely before binning it and couldn't see an obvious breakdown in its fabric. If I'm still losing coolant then it must be an airlock (still), a cracked head or cylinder wall. The pessimist in me knows it's not going to be an airlock!

I'll have a look tomorrow to see how much coolant is missing.

I was told today thay getting rid of airlocks in these 1500 engines is a real trial, can anyone else confirm this or is it just waffle?

Andy
Andy Kenning

Andy -

Do you know if the pressure cap on the expansion tank is good, i.e., rated at 15 lbs or so, and working properly? Seems like the system shouldn't be pressuring up that high if the cap is venting anything above 15psi or whatever.

Good luck!

-:G:-
Gryf Ketcherside

This thread was discussed between 04/06/2008 and 05/06/2008

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