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MG TD TF 1500 - Compression test on rebuilt engine

I just finished a compression test on the engine. It's been hard to get a good idle, and I think I've still got electrical issues (it just seems to uneven to be firing on all cylinders) but I did get results.
Firstly, it was hard to take a compression test on #1 because it kept cutting out when the cylinder wasn't getting a spark. Again, an indication of uneven sparking across the cylinders?
118-118-118-110 was the result.
I was initially worried about #4 because the first reading I got was 90, but a second test gave a better result. (The first test the compression gauge may not have been screwed in far enough into the cylinder for a good test)
Overall, it seems a bit low. But checking the archives, it seems like it is within acceptable range.
On a totally rebuilt engine I'm wondering if this is really OK. Admittedly, no real head work was done beyond skimming the head for flatness and reseating the valves.
Given that it's still running way too rich and I've got to figure that issue out, I'm not sure if I should be worried... or not?
Geoffrey M Baker

? Geoff

test for compression is done as far as I am aware just using a starter motor, not running.

crank it on the starter a few turns with the carbs wide open.

To do a proper compression check remove all the plugs and then crank it over using your gauge in each plug at a time. This will give you a true reading per cylinder.
David Honness

And finally, after doing the compression test dry, squirt about a tablespoon of oil in each cylinder and rerun the compression test. Cheers - Dave
DW DuBois

Geoff - That's a new one - a compression test with the engine running! There's an old joke about a mechanic and his doctor...

Remove all 4 plugs, block carbs wide open, and measure each cylinder on roughly the same number of rotations, maybe 4 or 5. Then repeat, adding a squirt of oil to each cylinder, and compare readings.

Your figures are definitely low, but I don't remember what you did to your engine - the rings would not be seated if they are freshly installed. I aim for 175# freshly rebuilt, and 145-50# broken in, 1500 miles later. A weak engine WILL run under almost any poor condition, but will not have any power, smoothness or reliability.

Tom Lange
MGT Repair
t lange

Learn something new everyday... I didn't know that 😊
Geoffrey M Baker

Geoffrey :
Richard Cameron

Anyone who knows what pre setting should be used with a supercharger 0.5 bar?
YS Strom

OK, retest.
Dry 120 120 125 120
Wet 135 135 140 135
New rings were installed.
Geoffrey M Baker

Now that's more like it should be -- Good!
Richard Cameron

Yes, I'm happier now :)
Hopefully, when the rings seat properly, it will go up a bit more.. but any rate, it's pretty much what it was before the rebuild.
I should add that the rebuild simply replaced all bearings, cleaned out the water passages, skimmed the block and reset the valves. Same pistons and valves were used, as the only damage that had occurred was to the camshaft bearings. I was very lucky because the engine overheated and lucked up during a parade going at 2 mph! At a high speed, it would have blown the crank and the pistons...
At some point in the future, I'll redo the head with new larger valves etc... but for now, I just want to get her back and running again!
On a side note, I replaced the crankshaft and piston rod bearings, because they suffered from a different problem... from many years of few oil changes, the acid in the oil had eaten pits in some of the bearings...
Regular oil changes will solve that problem...

Geoffrey M Baker

If when doing a compression test I get an unusually low reading in one cylinder I repeat the test and often get a much better reading. I'm thinking maybe a flake of carbon falls of the plug as it is removed and lodges on the valve seat until blown out with extended cranking? Just a theory.
John Quilter

I took a while but I found that joke !!!
Steve Wincze

There are many varieties of this joke; this was the one I meant:

A mechanic had just finished removing the cylinder-head from an MG TC, when he spotted a well-known cardiologist in the shop, waiting to pick up his Porsche.

The cardiologist was no little surprised to hear the mechanic shout across the garage "Hey Doc, come over here and take a look at this!"

The cardiologist walked over to the mechanic working on the MG. The tired mechanic straightened up, pulled a red shop rag from his hip pocket, and wiping his greasy hands he asked,

"Now Doc, take look at this XPAG engine. I can open its heart, take the valves out, repair any damage, and then put them back in. When I finish, it works just like new.

"So how come I make a measly $34,275 a year, and you get the really big bucks, $1,695,759, when you and I are doing basically the same work?"

The cardiologist shook his head sadly, put his hand on the mechanic's shoulder, and quietly replied,

"Yes, but I do it with the engine running."

Tom Lange
MGT Repair
t lange

You do not pay for the mechanic for repairing it with a hammer! You pay him for knowing what to do with it!
Len Fanelli

This thread was discussed between 23/01/2016 and 31/01/2016

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