Compression test

[Triffid]

I've just bought a cheapo ebay compression tester (which I suspect is inaccurate). What range of readings should I expect from a standard Lambretta engine? Cheers!

[CHRIS in MARGATE]

I open my throttle wide and kick until needle goes no further. I get 105 psi. Engine is 9000 miles old without problems. Same reading as 9 months ago. That's my reference point.

[johnnyXS]

I suppose if you have a screw in type compression tester you could also run alongside it in second gear to turn the engine over instead of kicking.
I'd be in cardiac arrest if I had to kick my scoot over in todays heat
On a 4x stoke I usually aim for 8-10 revolutions but I've never tried to measure the compression on a scoot yet.
I might give it a try it tomorrow .

[coaster]

Just be wary of assuming any particular reading is good. I took my Avanti for a Dyno session last year at Lowestoft Lambretta Works as it was running very rough and down on power although started and ticked over ok. Sash's resistive dyno confirmed it had a problem and set about checking everything including the compression which was reading 125lbs. Unable to clear the problem it was obvious the top end had to come off and found that the plating had started to come off near the top of the bore, the rings were badly worn and a lot of piston was stuck to the barrel below the exhaust port. There must have been enough 'seal' to give a half decent reading but not having a healthy reading on that engine to compare it to, it could have been misleading

[CHRIS in MARGATE]

Exactly
If you have a compression tester it is only a guide; not a bad guide at that but nice to have a reference point to start from and refer back to.
Nice when you get a non runner with little or no compression and you get a reading of about 35psi though. That's definitely when to get the top off.
I have used mine on lots of other scooters and recorded readings on sound engines without problems of between 100 -125 psi

[johnnyXS]

Just be wary of assuming any particular reading is good. I took my Avanti for a Dyno session last year at Lowestoft Lambretta Works as it was running very rough and down on power although started and ticked over ok. Sash's resistive dyno confirmed it had a problem and set about checking everything including the compression which was reading 125lbs. Unable to clear the problem it was obvious the top end had to come off and found that the plating had started to come off near the top of the bore, the rings were badly worn and a lot of piston was stuck to the barrel below the exhaust port. There must have been enough 'seal' to give a half decent reading but not having a healthy reading on that engine to compare it to, it could have been misleading

Ouch ! thats a timely warning .
I was flying along on my old banger yesterday touching 50mph which I rarely do when it suddenly occurred to me what the consequences might be if the piston suddenly failed or the chain gave way and the engine or gearbox jammed.
I slowed down a bit after that

it might be a useful thread if some of us add their compression readings and their engine specs, mileage etc so that we can build up a bit of reference information .

[CHRIS in MARGATE]

Here we go. Stage 4/5 ex TASS Racing ported iron barrel at 70.8 = 228 ccs.
Compression ratio measured at 8.5:1.
Bore and piston 9000 miles old.
Compression = 105psi

[coaster]

.................it might be a useful thread if some of us add their compression readings and their engine specs, mileage etc so that we can build up a bit of reference information .

Good idea, I'll measure my repaired Avanti next weekend if I get a chance.

[Triffid]

Thanks for all the feedback. My Jet 200 and li150 read around 75psi whereas my SX reads around 55 - slightly higher after putting some 2T oil down the bore to seal it. I doubt the gauge is accurate but the SX is relatively low so that might indicate a lack of compression which may explain the poor starting/running.

[davla]

I've just bought a cheapo ebay compression tester (which I suspect is inaccurate). What range of readings should I expect from a standard Lambretta engine? Cheers!

I was thinking of buying a compression tester but not sure which one to buy. Out of interest what reading do you get?

[johnnyXS]

I've just bought a cheapo ebay compression tester (which I suspect is inaccurate). What range of readings should I expect from a standard Lambretta engine? Cheers!

I was thinking of buying a compression tester but not sure which one to buy. Out of interest what reading do you get?

This is ok for a cheapy.
If you look through ebay you'll see that most of them are identical with different labels on the box .
Get a screw in one not a push and hold type so that it leaves you both hands free and gives a good seal .
Also make sure that it contains 18mm as well as 14mm
http://www.ebay.co.uk/itm/Petrol-Engine-Compression-Tester-14-18mm-INSTRUCTIONS-psi-bar-kg-cm2-kPa-/282177513702?hash=item41b31728e6:g:xkgAAOSwT6pV4qqA

[Triffid]

I found loads on ebay - most probably out of the same Chinese factory and built down to a price. I couldn't justify Snap-On and it's only to give some indication of compression issues to do with bad starting. As the Jet starts well with it's reading of 75+psi and the Sx doesn't with its 55 psi then I think the SX has too low a compression.

[johnnyXS]

I found loads on ebay - most probably out of the same Chinese factory and built down to a price. I couldn't justify Snap-On and it's only to give some indication of compression issues to do with bad starting. As the Jet starts well with it's reading of 75+psi and the Sx doesn't with its 55 psi then I think the SX has too low a compression.

Have you done a squish test on the SX ?

Theres a how-to in Sticky I think. Theres a brilliant article about squish and poor compression in part 1 ' Bang for Bucks ' series Scootering back in May 2016 . Just a tiny reduction in squish gave a significant increase in power .
Just need a length of 2mm solder to put into the sparkplug hole .

[Chadley]

Italian GP150, standardish engine rebuilt 2500 miles ago. 117 psi engine cold.

Measured using a gauge that looks to be the same as the one from the earlier comment:

http://www.ebay.co.uk/itm/Petrol-Engine-Compression-Tester-14-18mm-INSTRUCTIONS-psi-bar-kg-cm2-kPa-/282177513702?hash=item41b31728e6:g:xkgAAOSwT6pV4qqA

[johnnyXS]

nice one Chadley

better get on and do mine this morning

[Triffid]

I haven't done a squish test so that's something to explore. However, I suspect the piston/bore clearance is out of spec so when I find some time I shall take off the barrel and piston, check the clearence and check the squish at the same time. Thanks for all the feedback, folks!

[EddieStone]

A compression test is good for telling if your piston/rings are worn, but the value is pretty arbitrary.

Test the engine cold, then pour a little oil in the plug hole and test it the same way again. If the reading is significantly higher the second time your engine is worn. If it stays pretty much the same the it's in good shape. The value isn't as important as how much it changes between dry and oiled.

My 2p worth.

[Andie]

Bought myself one of those gauges, thanks for the Heads up.

S3 with TV 175 iron barrel and Meteor Piston. 110 psi from cold.
Engine rebuilt 1500 miles ago

[johnnyXS]

1964 LiS150 Special
cold reading 125-130psi

I suspect that this is very good compression for a series 3 standard tune engine of dubious pedigree.
Although the scooter was an an appalling state of neglect when it was imported from Venice it had clearly been in recent use pulling some sort of trailer . I wouldn't be surprised if the engine hadn't been rebuilt in the not too distant past . I shan't know for sure until I get to rebuild the engine this Winter

[johnnyXS]

I found loads on ebay - most probably out of the same Chinese factory and built down to a price. I couldn't justify Snap-On and it's only to give some indication of compression issues to do with bad starting. As the Jet starts well with it's reading of 75+psi and the Sx doesn't with its 55 psi then I think the SX has too low a compression.

They are very low compression readings Triffid
As they are low on both your scoots it might be worth checking the compression tester on your car as a crude calibration check.I would expect most petrol engined cars to have at least 160psi
Either you have a significant compression leak on both scooter engines or you have a faulty compression gauge I should have thought