Engine Mods to Sherpa T

[rootsman2]

I am interested in doing some engine mods to one or both of my Sherpa T's. I read that YRGO runs a 280 cc 250 bored out from a 250. My buddy has a factory 370 engine made for one of the heavier American riders from back in the day. Can anyone shed any productive light or opinion on this subject? Thanks!

[sherpa325]

I think the 280 conversions use a gasgas piston. Most people I know are using electronic ignitions,newer carburetors [Keihin, OKO, Dellorto, Mikuni] and some are reducing flywheel weights, which can be done by using a 250 magneto on the 325's and likewise smaller flywheels on the primary side. The problem with the lighter flywheels is that obviously the motor revs better, but it puts extra pressure on the already average brakes as you lose engine braking. Would love to see some photos of the 370 as I would assume it would be a copy of Martin Lampkins bike if not one of his. I think the factory riders tried a 348 long stroke engine[64mm] in 1976 or 1977, not sure of exact year, but they all went back to the 325 except Martin Lampkin who apparently ended up with the larger bore and larger stroke 370. I am not 100% sure on the factory riders engine sizes so if anyone can add more that would be great.

Cheers Greg

[pschrauber]

The later 199b engine with 340cc is much slower in any reaction compared.to 199a model in standard set up. The 326cc feels snappy against the 340 in my opinion.

I had mounted the lighter flywheel of the 250cc to the 340cc and that was for me a bit too much power you might try yourself.

Improvements I liked:

- carn swap fo 28mm Dell'Orto,

- rear silencer with less low reduction of the exhaust vapors.

- clutch improvements,

- no changes to the ignition, I can't see the super improvement beside less maintaince.

[oldjohn]

I think the Sherpa engines are very good for their intended purpose in stock form; I really don't think there's a lot to be gained and I can't remember ever wishing my 199 had more power. Apart from a decent carburetor they don't need much modification - I think your time is best spent on sharpening the tune-up.

I experimented with different flywheel weights and found that it's a trade-off between responsiveness and traction. I swapped the big drive-side double weight for a smaller Pursang weight and found it improved response without impacting on traction too much. But when I fitted a lighter ignition side flywheel as well it became very much harder to maintain grip and the bike became exhausting to ride as it took so much more effort just to keep moving on loose, steep climbs. It seems that reducing the weight on one side only is OK but if you remove weight from both sides it destroys the bikes ability to find traction on loose ground - something they're very good at in stock form.

Unless you're using the bike for something other than what it was originally designed for I don't think the stock engine needs much help.

[rootsman2]

I have a 26mm OKO carb and electronic ignition on both by Model 49 and 92 "350". It was a big improvement on both bikes but mostly they start nice adn crisp on the first kick. The 250 is much quicker but it would be nice if I could get more power and retain the snap of the 250. I'm pretty sure this is what Yuro is after and why he went to a 280...I also put a later model Alpina front wheel on the 250 and that improved front braking (alot)

Edited August 15, 2017 by rootsman2

[nigel dabster]

Its a twinshock built in the 70's if you want to modify thats fine but why?

[metisse]

Sadly,   because modern classic  sections need better brakes, better handling,  better reliability,  certainly if you don't have the know how or time to tinker in the shed several nights a week.