LCGB Forums

Ok......what happened here?
BGM/RT 195, no reed conversion,squish 1.4mm,timed static at 17 degrees (strobed to make sure),Dellorto PHBL 25mm, Ram air remote filter,100 main jet,50 pilot,AQ264 atomizer,D22 needle 2nd clip down,300 float needle jet, 42mm series 2 clubman exhaust,fast flow tap,B9 NGK spark plug,Super unleaded fuel using synthetic Castrol oil at 3%. Running a bit rich as you will notice but its been no prolem for over 2 years. Stripped to replace the rings,and this is what i found
IMG-20220510-WA0009 by Kevin Payne, on Flickr

Air leak ?

Despite your suggestion of a rich mix, no doubt attributed to the oily appearance - which could be due to overdoing the oil - I'd say that the crater may be due to a lean top end.

Unless I've missed it, you don't mention the slide, but presuming 40-45, main jet 100 seems small. I'd have thought 110, 105 minimum.

A 100 main jet is very weak for this setup.

110 main on my 25 phbl for the RT

All input appreciated.....
Initially (when first built) i followed the instructions to the letter. The suggested main jet for the 25mm running open is 100!
I thought that running with a filter and using a 100 main would be a bit rich,but would be fine to bed all the petrol/oil lubricated parts in.
As it ran so well and there were no problems,i left it!
After two years of use which has included a few high speed trips down the M4 i was obviously surprised at the state of the piston. I would have thought it would have gone bang way before!!
Anyway,starting from fresh,ive ordered a jetting kit plus a few various atomizers,and ill have another go!

What made you investigate matters if the engine was running fine? Was it just time for a top-end de-coke?

Do you think the damage to the crown might be an old (but not fatal) injury and then the set-up has made the crown oily and black?

Two years running without any real maintenance (just the basic plug clean,filter in carb,oil check) then reading about how the nickasil plating is a bit harsh on the rings initially,i thought it would be a good idea to replace them before this years rallies. The oil/petrol mix has always been done using the little jug to measure out 1 and a half jugs to 5 litres of fuel,which by my reckoning is 3%.
Over the years i have had scooters,this is the first time i have encountered this.
I have had seized pistons,holed pistons,broken piston rings,had circlips come out,shattered small end bearings etc.... but this one i found really odd as i cant see why its happened!
As suggested i am going to fit a new piston,then re jet the carb using a bigger main,but as i stated earlier, a main jet of 100 is suggested in the instruction manual that came with the kit!

kevspeed wrote:...a main jet of 100 is suggested in the instruction manual that came with the kit!

Personally I wouldn't trust the estimated jetting figures quoted in the RT195/225 manual. On page 33 they suggest a 'standard' atomizer for the TM24.

The 'standard' TM24 atomizer is a 454 series Q-0 and IMHO it is way too lean for any piston-ported Lambretta set up (I ended up with Q4 on my RT195) and that makes me question the validity of the other recommended set ups. It's a shame as most of the other info in the instruction booklet is great.

Reading your article with interest I am running the same kit, also run 100main 266 atomiser, d22 needle 2 nd clip through opened air box, I think my engine runs rich, I have jus repositioned needle from 3 rd clip, runs much bettter, but was going to change the needle to a 29 to see if I can improve further, interesting thread.my Atomiser is richer than yours, I am also on standard N4 plug. You may be to rich top end affecting cooling. I have some friends at 95 main.