14 replies to this topic

[steve1978]

hi, 

 

I’ve bought a compression testing kit from temu, only cheap but why not.

 

before I go crazy am I right in thinking that the testing process would be as follow…

 

1 disconnect all ht leads (making sure they are numbered!)

2 remove spark plug from first cylinder

3 insert finger tight my compression test

4 crank engine for 5 ish seconds and record result

5 repeat for all cylinders.

6 reconnect ht leads and plugs

 

what am I expecting results wise for the compression of the 998 engine?

[smurfomatic]

You will need to remove all the spark plugs, rather than one at a time. For accurate results, go down and back again - e.g. plug 1 to 4, then back 4 to 1.

 

Make sure you disconnect the coil too.

[Stevie W]

In addition to the above advice, keep the throttle wide open when cranking the engine. Testing with the throttle shut can give you an artificially low reading.

 

Cheers, Steve.

[Avtovaz]

as said above, wide open throttle so the air can be forced [no, not sucked!] into the cylinder, all other plugs out. If you suspect rings, put a small amount of oil down the bore and you should see the "pressure" increase. 

[Designer]

Thank you very helpful as I will be doing a compression test soon.

I take it it would be best to disconnect the fuel supply before the mechanical fuel pump.

Edited by Designer, 20 May 2024 - 01:18 PM.

[Cooperman]

Remove all the plugs first, then do each cylinder in turn making sure the throttle is fully open as you crank the engine. Crank for a minimum of 5 seconds for each reading.

[Designer]

Hi,

 

One question you say the throttle fully open. Can you do a compression test and get accurate results without the carb on the engine?

 

Paddy

[MiniMadRacer]

Make sure your battery is fully charged also, and if its a "cheap" compression kit, I woudlnt get too hung up on the absolute readings, them all being in the same ball park is far more important.

 

There were various 998 cc engines fitted to minis, with varying heads and pistons giving varying compression ratios, but I would expect with an accurate guage to see readings around 160 PSI.

 

In my opinion anything lower than 110 PSI maybe a concern.

 

I always checked compressions with the engine fully warmed up, so the oil is up to temp, and the pistons etc are working at the temperature, and the expansion they would be working at in a normal engine.

 

Cold compression testing can be quite misleading.

[Ethel]

No need to disconnect the fuel - the pump has to be able to pump more fuel than you'll need 90% of the time, so the carb is able to control the flow.

 

There are lots of variables, but it's more about consistency in order to get comparable results across all cylinders. The engine would rev faster once it started, so the throttle isn't that critical a restriction. Though it's very easy to chock the throttle linkage & it could help stop cylinders sucking & blowing in to each other.

 

The biggest variable will be the starter motor & battery performance tiring as the test goes on. Hence the advice to take all the plugs out & go down the line & back again to get a fairer average for each cylinder. Don't crank it for longer than is needed to get a steady reading & allow a minute or two between each test.

 

I assume even a Temu gauge has a hold button to catch the highest reading. Release it before moving on to the next cylinder

[Designer]

Got the refurbed Carb in, filled her up with coolant.

Then let her run till normal temp reached and then did a compression test.

Made sure the throttle was fully open.

Used a Gunson Compression Tester Hi Gauge.

 

Cylinder     1        2         3        4

1st run    172     185     185     191

2nd run   175     185     189     191  interesting that 4 is that much higher.

 

It would appear I have a problem with No1 which I think is valve related although Tappet clearances have been checked many times but I still get a tapping noise from the area of cylinder 1.

I hope that 2 3 and 4 are with in tolerances for a standard 1988 998 with an HS4 carb that has done 70,862 miles..

 

Paddy

Edited by Designer, 25 May 2024 - 04:38 PM.

[Cooperman]

That's pretty good for a high-mileage engine.

It may need the valves re-lapping, which is a simple job.

[Designer]

Thank you Cooperman for the reassurance that nothing is too drastic.

I am thinking come the winter I will lift the engine out, replace all the valves check the followers and replace those if need be.

 

Paddy

[Cooperman]

You will possibly only need exhaust valves.
Fit new guides, have the valve seats re-cut, lap the valves in and ideally fit new valve springs and stem seals.

[68+86auto]

Have a play with a leakdown tester. There are cheap ones available. If there's significant valve leakage you can detect it out the intake or exhaust.

[Designer]

Thank you 68+86auto we will look into that but we will enjoy Moop on the move over the summer and go into things a bit deeper come the winter.

I am sure this is been like this for a while as it is the first time I have done a compression test on her.

 

Paddy