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MG Midget and Sprite Technical - Compression check

Before i refit the carbs thought i would do a compression check. Bought gauge of e bay but it came with no instructions assembled as i think is correct in order as picture. But it dose not read anything if i remove the schrader valve at the bottom it reads but dose not retain the reading jumps all over,gauge has got 2 valves one on the bottom of the tube in the face of the adapter the other just below the gauge.I must be doing something wrong any ideas?

mark 1500 nearly on the road

Try screwing in the part to the far left in your photo into the spark plug hole...

no wrench needed it, just snug tight so no gases escape out the threads

Next screw the hose fitting into the 1st section

Make sure there are spark plugs in the other 3 cly holes (unattached to dissy) and spin the engine over until the gauge stops reading

There ((( should be )) a little jiggle pin at the bottom of the gauge on the hose area, push It and release the pressure

Did any of that make scence or work?

Prop

Prop and the Blackhole Midget

A release valve on the gauge face...hmmm

Ive not seen that before.... are you sure its a compression gauge and not a desial / petrol fuel pressure gauge /// leak down tester of some type

Try googling the product and manufacture... they may have an on-line pdf file instructions for how it works and diagnostics / fyi section

More photos please

Prop
Prop and the Blackhole Midget

found this on another forum must be my problem

I got one of those before Christmas and it did not seem to work either. After having a closer look I noticed that it had a one way valve fitted in the end of the pipe of the gauge. The purpose of this is to hold the pressure in the pipe when you crank the engine and then you can read it off the gauge after you have stopped cranking.

This valve is just a standard looking Schrader valve and it appeared that it would take quite a bit of pressure to make it open, try pressing on the pin. So I used another slightly different design of Schrader valve which I could alter by cutting the spring down a bit.

Now the engine compression easily opens this valve and just as importantly the pressure stays in when you stop cranking the engine.

Dont ask me why it was like that, but the kit was only a tenner from Ebay .

See the attached photos.

mark 1500 nearly on the road

And pic 2

mark 1500 nearly on the road

Congrats mark

Welcome to china, can you speak ""I ayah gotsa you mony na cow eye""

Hahaha

On my gauge the schader valve /jiggle pin is where the hose goes into the gauge its self...

So did the compression test out okay ?

Prop
Prop and the Blackhole Midget

Just a last thought,

Any chance the hose is installed into the gauge backwards

Prop
Prop and the Blackhole Midget

No it is connected correctly just el cheapo gauge, Like my old man use to say buy quality tools and the quality will remain long after the price is forgotten.
mark 1500 nearly on the road

Good one.. I like that and so true, unfortantly, I find it hard to pass up a cheap price esp at harbor fraight.

But I doulbt many of us can, so dont beat your self up....even if it works just 10 times ....thats still a deal

10 bucks is still good money no matter whos last dead leadership is on the front


At least its working... and besides how often are you really going to be using it, a couple times a year at best.... yepp its a deal, as long as it reads the correct pressure with in a few psi...its a winner

Prop
Prop and the Blackhole Midget

Prop,

I've never heard of doing a compression check with spark plugs installed in the other cylinders. Did you hear that from a reliable source?

Charley
C R Huff

Mark,
Are you sure that you need all of the components in your first photo? It looks to me that you might screw the right hand item (end with the valve in it) direct into the left most item (plug hole adaptor) Looking at the other 2 bits I wonder whether they are extra pieces to be used if you want to add in an extra length of flexible hose? And it also looks to me that if it were assembled in that order, the end of the hose connector (item 2) might foul the end of the valve and prevent it sealing so you would get no reading on the dial.

And, I always do compression tests with the other plugs all out so as to get a consistent engine spin. Prop, are you perhaps thinking of the process for a leak down test?
Guy W

In fact Mark, have you checked the thread on the end with the valve in it? It looks like that might screw direct into the standard plug hole, and that all of the other parts are just bits of the kit supplied to be used in different circumstances.
Guy W

You got me charlie

Good catch.... I was thinking Siamese head from the 1275....not the 1500

Prop
Prop and the Blackhole Midget

Getting refund on tester going to buy a good one.
mark 1500 nearly on the road

What happened mark.... is it not proving accurate?

Prop
Prop and the Blackhole Midget

no it wont hold the reading tried 3 different valves no joy
mark 1500 nearly on the road

Well that sucks....sadly, it will cost way more then a tennor in time alone, just to get a refund

Good luck

Prop
Prop and the Blackhole Midget

Prop,

I wouldn't think the 1275 head makes any difference since the valves are closed when it makes compression. You think it is proper to do the compression test on a 1275 with only one plug out at a time? Anyone else heard this?

Charley
C R Huff

Got back off holidays and new compression tester arrived, checked engine and 160 psi across all four so everything seems ok.So will fit refurb carbs and try to balance them.
mark 1500 nearly on the road

Hey charlie,

I may well be wrong on leaving the other 3 spark plugs in to test compression....I just assumed with a simese head you would loose compression in the next cly if the plug wasnt in...but I cant imagine any damage occuring.

Congrats mark...

i hope this gauge is better built, that last one was something else :-) 160 psi is not bad, still lots of life left in the old girl.

Prop
Prop and the Blackhole Midget

"....simese head you would loose compression in the next cly if the plug wasnt in..."

<< shakes head in disbelief >>
Steve Clark

Hey...

Its a prop thing

So if I understand correctly, if the intake valve on cly 1 was leaking and the intake opened in cly 2 which has no spark plug

There would be no false reading or inaccurate pressure ???

Hmmm ... I guess that woukd be correct because your testing at wide open throttle so the pressure would escape thur the in / ex ports of 1 & 2



I guess I just never gave it much thought...what is the down side of leaving the 3 spark plugs in place when doing a compression check...or is there a better benifit to be accomplished by pulling all 4 spark plugs


Well at least I know to change the oil filter after the new oil has been added....hahaha


Prop
Prop and the Blackhole Midget

Prop,

You pull all plugs so the engine can spin more freely. Also, if there is a compression difference between cylinders, the plugs would slow the engine down more when they were in a high compression cylinders than when in a low compression cylinders. So, the test may not be uniform.

As to leaking compression out of a valve on the cylinder you are testing, if it leaks it doesn't matter where it goes. It only matters that it leaks and so is not there to bump the guage.

Charley
C R Huff

Good points charley,

I wonder if that contributed to my high +200 psi compression testing

Prop
Prop and the Blackhole Midget

This thread was discussed between 11/08/2013 and 14/09/2013

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