Combustion chamber volume

[Simon ELC]

Having just built up a 255cc TS1 motor 64x71 I set about re-jetting the 30mm VHSH.

Pilot 44
Slide 50
Needle K72 (oddball) was K24
Atomiser DQ268
Main 130 (adjustable power jet)

Augusto 7000 retarding to 15 degrees at WOT

Ron Moss head re-profiled for Wiseco IT 250 piston

Franspeed Super Tourer

Although I'm getting a good plug colour across the range and the motor fells right, My Westach CHT is reading higher than I would like at 175 degrees C under load. Before upgrades it was about 150.

I haven't worked out the proper head comp ratio, corrected or otherwise, but the head volume is 24cc.

My question is can you still get a good plug colour but run hot due to head comp ratio being too high?

Forgot to mention. Barrel has been extended 5mm to bring timings to sedate levels. Transfers 125 Exhaust 185. No packers No gaskets.

[Warkton Tornado No.1]

Using your figures you can calculate the ‘uncorrected’ compression ratio, which is about 11.6:1.

You may not like using the uncorrected figures, but that ratio looks high to me, for pump fuel @ any rate.

[Simon ELC]

Thanks WT1,
I guess I need to take a couple or so more CCs out of the head and work out the corrected comp figure. What's a safe figure?

[Warkton Tornado No.1]

I guess I need to take a couple or so more CCs out of the head and work out the corrected comp figure. What's a safe figure?

I am of the persuasion that believes in using the 'uncorrected compression ratio’ or geometric compression ratio as advocated by A Graham Bell amongst others.
I was hoping to find some notes of specific CR for an engine of similar capacity to advise you an actual figure, but haven’t found them….yet.

However, if you were to use the geometric figure, I reason that 10:1 should be safe with your retarding ignition & an expansion chamber & a good, tight squish so that heat can be successfully absorbed by the cylinder head.

[Simon ELC]

I took the combustion chamber out to 28cc which gave a trapped volume of 31cc. This gives a non corrected head comp ratio of 9.2:1. 5.6:1 corrected. So pretty safe I hope.

Lovely sunny afternoon I thought I'd go for a test ride. But No. Wouldn't start. No spark. The scoot's been on the bench a week and the LT coil has packed up. AARGH!

Put another stator in just to start it but couldn't be bothered to re-strobe so it'll have to wait another week.

[Warkton Tornado No.1]

S’funny that so many points ignitions get ditched for the sake of reliability.

I say it tongue in cheek & I’m sorry if that sounds cynical. It’s not meant to be sarcastic as an electronic ignition does lend itself to the MASSIVE benefit of being able to vary the advance through the engine’s range.

What actually prompted me to respond was neither your ignition failure nor the CR which sounds a lot safer BTW. Although retarding much more than 17° BTDC surprises me a little.....

I envy your 64 mm crankshaft stroke! Damn you! My Rapido is only Ø 71 mm x 61 mm but is very ‘plonkable’ & long stroke is the way to go! You haven’t mentioned your squish. I know, it’s sometimes best kept secret as so many have much too big a gap which stops the head doing as good a job as it should.

The other feature you deserve an accolade on may seem whimsical to other Forum users, but

No packers No gaskets.

means less to go wrong. Top Man! The gaskets more so than the packers which can be useful for transfer port matching.

To my point:

I just re-read your carburettor settings & would say they are a little different to what I would have anticipated, but I know comparatively little about reed valve motors.

So forgive my ignorance, but I would expect the slide to be 40 to 45 which might mean you could drop down from the DQ268 to DQ 266.

What is the adjustable power jet, please? Just my intrigue…..

[Simon ELC]

The crank is an LTH 64 with a 115 rod that I bought on a whim. Although it looked very well made I had my doubts about pin strength. So took it to John Howe at Jahspeed who gave it tap with the medium hammer which knocked it way out of true. I decided to get John to re-pin it with one of his 'special' pins and also weld it. Belt and Braces.
This is going in my rally bike and I want it to be a reliable fast tourer.

The squish is 1.3mm. I got the barrel extended 5mm at the top and got barrel and head machined to do away with gaskets. They can't fail if they are not there.

Your take on the jetting is interesting. I had planned on going the other way with the slide making it leaner. At present to get the mid range safe I seem to be a little rich at 1/4 throttle. Are you suggesting a leaner atomiser with richer slide and bigger main + powerjet could be the way to go? I intent to get it on the dyno when I have time and when I think I've got the jetting somewhere near. I've got loads of needles and all the atomisers from 264 to 269 but the slides are £55 a pop so don't have any variants yet.

The powerjet is an adjustable one I got off a fella from Greece who goes by the name of Two Stroke Terror. He used to sell them on ebay a few years back. I think he has a website of the same name.

[Warkton Tornado No.1]

That squish of 1.3 mm is fine.

Lambrettas seem to need a richer slide than perhaps might have been thought by comparison with karts, for instance.

The slides are extremely expensive but shopping around can sometimes be productive, especially on the Continent. However, a 40 slide can always be modified to convert it to 45 or more. A common enough practice amongst race tuners is to file the edge…..

I tried checking my memory/sanity(!) by trawling the Lambretta Forum carburetion sites & it’s amazing how many neglect to detail the slide they’ve used! Anyway, most veer toward 40 or 45 with very, very few 50’s.

With my own road, hybrid set-up, it is in particular something I tried to work around having had 50’s to spare, but eventually had to concede a slide more like a 40 was the way to go. In my case, I had to modify a 50 which was not easy.

The needles & atomisers I tried are all wear free & brand new & I came down to a 264, having anticipated settling upon a 266 or even 268 all of which I bought specifically.

That may surprise you, but even when a Group Four Rapido in 2012/2013 with an efficient expansion chamber raced against mostly Mugellos on flat out tracks such as Snetterton & Darley Moor achieved a third, the atomiser was still only a 265.

As for your crank, you were right to check it & get it stripped. It's a pity you got John to weld it as well, because it would have been interesting to know if the interference fit alone (of John's oversize pins) would have been enough.

[Mike Davis]

I took the combustion chamber out to 28cc which gave a trapped volume of 31cc. This gives a non corrected head comp ratio of 9.2:1. 5.6:1 corrected. So pretty safe I hope.

Lovely sunny afternoon I thought I'd go for a test ride. But No. Wouldn't start. No spark. The scoot's been on the bench a week and the LT coil has packed up. AARGH!

Put another stator in just to start it but couldn't be bothered to re-strobe so it'll have to wait another week.

5.6:1 corrected is super safe, youll have no problems there, you could run 18 deg static at that no problems, Ive found up to 6.4:1 is still plenty safe and have plenty of engines out there running around that with thousands of miles on them, so yours will be no problem.