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MG Midget and Sprite Technical - compression test - is it time to worry?

Hi guys

I have been a bit concerned about my inability to get a smooth idle on my new engine. Being ever the pessemist I just did a compression test (a dry test with a engine cold) and got the following result:
No1 200psi
No2 175psi
No3 200psi
No4 200psi

Dropping some oil into the bores raised the cmpression equally across all bores by around 25psi.

What am I to make of this? On the face of it this is worrying BUT is it too early to worry given that the engine has only done around 500 miles since new and needs to be given more time to fully bed in. Or do I have a problem? If I have a problem is it rings or valve(s) given the equal impact of oiling the cylinders?

Chris
Chris H (1970 Midget 1275)

Hi Chris
Good to see you Sunday. good show.
Is this the engine P.May rebuilt? if so I would be interested what he has to say, I hope you read the sign on the way out of his work shop that clearly sates there is No warranty on his engines! or his diffs apparently... nuff said.
Rob Newt

If it all went up equaly then the piston/bores are no problem.
Might be a slight problem with a valve/seat.

What spec is the engine and at what rpm are you trying to make it idle

I would not worry and enjoy the slughtly unequal idle.

Onno Könemann

Hi Rob

Good to see you too for a chat. Yes - the Peter May 'No warranty is what is worrying me!

Onno - I was trying to set the idle to around 1000 rpm. The engine is 1330 bore, MG Metro head, Peter May 270+ camshaft, HIF44, LCB etc. I did wonder if the lightened steel flywheel might make for a poor idle but the compression test got me a bit worried, perhaps unncecessarily you think.
Chris H (1970 Midget 1275)

With that setup i would not worry about a slight uneven idle at 1000rpm.
To make the most of it you could remove the head and regrind the valves.
That should see the compression test all even.
But you will not notice it during driving.

Just put away the compression meter and do not use it until something is realy wrong.

If it aint wrong don't go looking for problems
Onno Könemann

I doubt if that compression variation would result in an uneven tickover. And if the compression increase with added oil is the same then it doesn't seem to be a rings problem. With only 500 miles on it I would first do a re-torque of the head and then make sure the valves are spot on for adjustment. (16 thou min. but check the cam specs. for correct setting.
Then do a re-test, making sure you do this WOT.

It could need a valve lapping, but I would try the above steps first.
Guy

Guy

I am going to check the basics like valve clearances etc. Interestingly I initially set them at 15 thou but Peter May told me later to reset them at 12 even though he knows the cam he provided me with. I will open the clearances up a bit (it can't hurt) and see what happens).
Chris H (1970 Midget 1275)

I assumed all things like valve clearances would be spot on before you go and do things like compression measuring.

Onno Könemann

I think an engine has to be of merchantable quality and fit for purpose and merely saying no warranty would not inpress a Judge at arbitration. Even a Race engine has to 'do what it says on the tin'. we warrant our race engines for a couple of races as long as it has been set up on a reputable rolling road. warranties are difficult, we tend to do goodwill rather than a specific warranty.....eg race engine mid season....customer pays for parts we stand labour!

I agree with Onno, just drive and enjoy. 200 psi region will not lend itself to smooth idle, you need low cr on an A series for smooth running, but for power you need the higher CRs.Big single carb trying to feed 4 cyls on a siamese head not good either. Do a Morse test equivalent and see which plug drops the idle speed most....maybe it would surprise you to see it might not be the lower cranking pressure one. Also a repeat of the plug chop test shows differing results you will find. What you do is remove the plug leads one at a time and note the rpm drop (need good tachometer) repeat with each lead in tuen, note rpm drops and compare results, repeat at least three times.


Peter
P Burgess

Thanks Peter - that sounds like sound advice.

Chris
Chris H (1970 Midget 1275)

Hi Peter. With apologies to Chris for slightly hijacking his thread, when I came to your RR with some of the guys from the NW region of MASC in June(Friday before midget50) you reckoned that my 1133cc mk3 Sprite,(the green one)was set up pretty well. You managed to squeeze an extra few BHP out of it by tweaking the ign timing to get 50BHP at the wheels. You may remember that I have a 731 cam, skimmed head(195psi compression across all four cyls) and an HIF44 carb with BDL needle. From what I read above, are you saying that I would get more power by reverting to my twin 1.1/4inch carbs?

Bernie.
b higginson

WOW.... (.12 valve clearance)....that dont sound right, are you sure that there isnt a miscommunication between peter and yourself. Id drop a dime and have a quick chat.

Id think .12 is great on a pure stock 1275 but im running .17 using keith culvers artical in minispares website for figuring valve lash, and my engine is no whhere as radical as yours.

Id fear that you could have a burnt valve face/seat in #2 cly at the vary least id open it to .15 just as a wide open guess.

Just remember.... This is your engine, you paid for it, that makes you more then intitled to run any and all test you desire and fear any little hick up.... Theres no argument im the same way, i maaybe a nanny to my engine that im vary proud of, but i want to catch any issue no mattwr how small before it becomes a bigger issue

Its easier to kill 20 bees in your shop in april then to deal with a 5000 member swarm in aug...lol

So feel free to test away and question everything till you feel safe in peace of mind

Prop
Prop

.12 and .17 now that could be an issue....LoL

Sorry about that, please add in the appropriate zero

Prop
Prop

And as usual Prop comes in to spread panic.
Onno Könemann

Hi Bernie

Usually the large single carb gives less grunt below say 3000 rpm then out classes the twin 1 1/4" carbs and keeps revving.

Prop....don't you run bigger clearances cos you have roller rockers?


Peter
P Burgess

Hahaha... Yeah but im so good at it.

Prop

Somebodys been eating there weaties for breakfast

Good catch Peter....

I thought about that after my posting, i am running high lift rollers but i was fearing to correct myself would only contribute to the mass hystiria of the moment...hahaha

Prop
Prop

Prop, you should give your engine what it's screaming for; a good spanking and a hard shake down, let it SCREAM!!!

Soft doctors make stinky wounds you know!
Alex G Matla

Hey Prop - you got me worried about those bees now! In any case my car runs on Bee Pee!
Chris H (1970 Midget 1275)

Peter. Thanks for that. I didn't fancy going back to twins. Anyway, I want to sell them.

Prop.
I like the Bee analogy, but don't go killing them, they're in enough decline as it is.;)

Bernie.
b higginson

This thread was discussed between 06/09/2011 and 08/09/2011

MG Midget and Sprite Technical index

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