125 to 175 conversion

[kclewis]

Hi Guys, I have just converted my my 125 LIS from 125 to 175cc with a barrel and piston and head kit, sourced from Ebay. The seller had honed out the kit to give the correct clearance 3 thou from memory, but having fitted it the compression is way too high, very hard to kick over, not tight but very high compression. It did come with a complete gasket set and taped to the outside was an indian GP175cc head gasket that is 2mm thick, I used the standard 0.5mm gasket, should I use the thicker gasket and what is causing this, could it be a small capacity head? I have just taken it for its first mot, it ran ok but did suffer from a heat seizure on the way, I may have been going too fast in 3rd as I could not select 4th, (now sorted) I can't ride it on the road yet as I am awaiting the registration as it was an italian import. Any help would be great, Thanks Keith

[holty]

hi kieth,
did you check the squish when you had the engine assembled, or even a dry build measuring the squish with the gaskets you have, 1.5 to 1.2 would be a good figure to aim for, bigger will still run ok but make less power, did you do a leakdown test after assembly, an air leak is a common cause of siezures , or just under jetted maybe?

[kclewis]

Hi Holty, thanks for the reply, I did try and measure the squish but did not seem to have much luck, as from memory the shape of the combustion chamber made it difficult, or so I thought at the time. I will revisit that and see what it is.
I did do a leak down test and found the exhaust flange leaking that was all.
Under jetting could be an issue, will check that but after a short run to MOT and back plug looked black with made me think it was a bit rich, again I can't do much about this till I can ride it on the road.
I did contact the supplier of the conversion kit today and he asked what compression I was getting, saying it should be around 90 psi, I have a compression tester coming tomorrow, so will try that before I strip it down.
He also mentioned the squish, so this is definitely something I need to sort.
I have not owned a Lambretta since the early 70,s so can't recall how hard they are to kick over but this one I can stand on the kickstart as it has so much compression.
Cheers Keith

[Scooterdude]

I've just completed a rebuild on a 175 conversion engine, one I suspect is an eBay special as the standard of the casting is very poor and showing a great deal of roughness inside the ports etc.
I set it up with a squish of 1.48mm using the standard base gasket but without a head gasket, the problem was I had the engine without the scoot so was unable to test the compression until I put it back in, well it felt excessive to say the least! But started and ran very smoothly, we may have to have a rethink if it proves problematic and fit a 2mm head gasket if needs be.
I think the problem lies with the poorly machined head (a reprofiled 150 head) but poorly finished and not at all the quality of the one I had from Harry Barlow for mine.

[kclewis]

Hi, I have (hopefully) attached some links to the cylinder head from when I rebuilt it, also 1 with the 2mm gasket temporarily in place. What do you think, does it look like yours?

DSC05186.jpg

DSC05190.jpg

Keith

[Scooterdude]

Yes it looks very similar to be fair and with the correct step in the head but the low volume combustion chamber, I think the use of the 2mm head gasket is "papering over the cracks" as it were for the incorrectly machined head.

When you went back to your supplier what did they say your squish should be ?

[kclewis]

Hi, they said "I would only use the gaskets to ensure you have a squish of minimum 1.5mm " What do you reckon, get a better head? It does start well and runs ok, but a friend who races 2 stroke bikes says with today's petrol and a very high compression the piston would not last long. Keith

[Scooterdude]

you really need to accurately measure the volume of the head you have, and find out what the volume of a 175 conversion head should be and go from there really.

[kclewis]

Yes that sounds the right way to go, thanks.
Keith

[Adam_Winstone]

Measuring the volume of the combustion chamber is the right way to go about this, especially as there is a narrow squish band running all the way around the top of the chamber and behind the plug hole, which suggests that this head started life as a smaller cc. If a 125 or 150 casting machined out to 175 then the combustion chamber should also have been increased in volume to suit, which explains why your compression is too high as the chamber (bowl) does not seem to have been reworked.

Put a plug in it (not meaning to sound rude!) and then stand the upturned head on top of a mug so that you can angle it so that the bowl is more or less flat/level. Then use a syringe to fill the bowl level, measuring the cc (ml) or water that it takes to fill it. NB: You're not measuring bowl + squish area, just the bowl. Once you have this measurement then we can start to talk about calculating the geometric compression ratio.

NB: This is not very accurate but it is perfectly OK to give you an idea of what the current geometric ratio might be. Ballpark figures, rather than rocket science!

Adam

[kclewis]

Great thanks Adam, will take it apart over the weekend all being well and I will certainly measure the capacity, and hopefully you can help me work out the ratio. It is really way too hard to kick it over at the moment.
From memory the head did have a 175 marking on the casting...
Cheers Keith.

[Adam_Winstone]

Yep, happy to help.

Adam

[kclewis]

Just done a compression test on the motor and it came out at 8 Bar or 116 PSI, I expected around 90psi?
Keith

[Scooterdude]

Just done a compression test on the motor and it came out at 8 Bar or 116 PSI, I expected around 90psi?
Keith

Thats not a lot. My 175 from Harry Barlow did 145psi. however i suspect your gauge may be out going by the small combustion chamber in your picture.

[kclewis]

OK, it was a top of the range pressure tester ( Cheap!) I will get the gauge checked tomorrow and replaced if no good and try again. Did you leave yours running with that pressure? Keith.

[Scooterdude]

OK, it was a top of the range pressure tester ( Cheap!) I will get the gauge checked tomorrow and replaced if no good and try again. Did you leave yours running with that pressure? Keith.

Yes as it dident seem overly excessive and when i managed to sieze it up i replaced the piston and honed the bore and was getting around 125psi.

[kclewis]

The supplier says the head was done by Harry Barlow of pro porting, so once I get the pressure gauge back and test again I will strip the top end and see what I can find.

[Adam_Winstone]

^... Harry may well have produced this for a specific stage of tune. What people need to consider is that the cylinder head and compression MUST be considered in respect of the tuning work done to the barrel as, whilst 'geometric' compression ratio will be the same regardless of what 175cc motor it goes on, the 'corrected' compression ratio varies massively depending on what the exhaust port duration is (basically because compression cannot start until the piston shuts off the gas from escaping out of the exhaust port).

This is an issue that has come up time and time again, with there being plenty of threads/info on scooter forums and on-line in general.

In light of this, a head for a Stg IV tuned barrel will typically run a much higher geometric compression ratio than a standard or mild tune 175, however, the corrected ratio will be similar. The problem comes when you fit the wrong head to the wrong tuning spec 175 cylinder, resulting in a tuned barrel that has very low corrected comp ratio and is completely gutless OR a standard/mild tune barrel that has WAY TOO HIGH compression. Lots of people run into having way too high compression when dealers regularly sell tuned kits with high comp heads, then send out the same high comp head with a standard/mild tune kit, resulting in it being hard to start, seizing and/or holing pistons.

You MUST match the right compression head to the right tune barrel, you cannot simply think of them in terms of cylinder capacity.

Adam

[Scooterdude]

Interesting points there Adam. Are there specific combustion chamber volumes for specific stages of tunes? I guess there must be in that case.

[Adam_Winstone]

^... 100% correct.