Long time no type, Desmo conversion update
Posted: Fri Apr 19, 2019 4:25 am
Been quite a few years, I’m sure, so it’s time to update you on my desmo adventures. I built up a cafe’d 250 single with a head ported by Syd’s and a full race cam from MegaCycle. The head was a small port, opened up for a 30 mm carb, larger valves and a coil spring conversion. Beautifully done and the engine ran sweetly all the way to 11K -- where budgetary concerns got the better of valour. All was good for a 1000 happy, hard miles. Then the valve guides were worn, not toast, but worn none-the-less. I guess what else can you expect with a full race cam and high revs?
Chance, ebay and a light head all came together one night when I ended up with a desmo head for $800. Even though complete desmo heads were then trading for $2000, it wasn’t quite the deal you might think. The seller -- AKA “the butcher” -- had decided to “race-port” it himself. Must have been a Texan because he liked wide-open spaces. So wide he went through the intake port into the valve spring area! Naturally all the rockers were shot as well...
So I cleaned the head carefully and sealed the intake port back up, and narrowed the port to match Syd’s porting (and added some drop), using Devcon Alloy putty. The seller had also hacked up the exhaust port so -- in for a penny, in for a pound -- I filled that as well. New seats, guides, rocker pins and bushings, valves, and full race cam from V2 and I was back in business. Inclination and necessity (the repaired area) had me run the head springless -- desmo singles usually use 160 valve springs.
Well, it all ran a treat. Seemed just as quick as the springer head though perhaps a tad more “tappet-ty” when the oil was hot. Several years and 2000 miles go by and I figure it’s time for a look-see. Off comes the head and I find the guides are like new, no play whatsoever. Both valves need one size larger closers (converted to twin closers so I could use valve seals), and the intake, one larger opener. No wear on the cam or rockers. The Devcon held up in both ports, yes in the exhaust as well! Tickled pink is what you would have to call me, that’s for sure.
Only one other thing to report. I use Penrite Classic Heavy oil (40-70). The bike had sat all winter before I pulled the head. The oil was still all over the cam and rockers, impressive.
Well, that’s it for now.
Cheers,
John
Chance, ebay and a light head all came together one night when I ended up with a desmo head for $800. Even though complete desmo heads were then trading for $2000, it wasn’t quite the deal you might think. The seller -- AKA “the butcher” -- had decided to “race-port” it himself. Must have been a Texan because he liked wide-open spaces. So wide he went through the intake port into the valve spring area! Naturally all the rockers were shot as well...
So I cleaned the head carefully and sealed the intake port back up, and narrowed the port to match Syd’s porting (and added some drop), using Devcon Alloy putty. The seller had also hacked up the exhaust port so -- in for a penny, in for a pound -- I filled that as well. New seats, guides, rocker pins and bushings, valves, and full race cam from V2 and I was back in business. Inclination and necessity (the repaired area) had me run the head springless -- desmo singles usually use 160 valve springs.
Well, it all ran a treat. Seemed just as quick as the springer head though perhaps a tad more “tappet-ty” when the oil was hot. Several years and 2000 miles go by and I figure it’s time for a look-see. Off comes the head and I find the guides are like new, no play whatsoever. Both valves need one size larger closers (converted to twin closers so I could use valve seals), and the intake, one larger opener. No wear on the cam or rockers. The Devcon held up in both ports, yes in the exhaust as well! Tickled pink is what you would have to call me, that’s for sure.
Only one other thing to report. I use Penrite Classic Heavy oil (40-70). The bike had sat all winter before I pulled the head. The oil was still all over the cam and rockers, impressive.
Well, that’s it for now.
Cheers,
John