Very Brief History
Some folks have shown an interest in the big engine TZR project that I have had going for years. I am not going back to the beginning to recap in detail. Just understand that I started with a weather worn / low mileage TZR250 model 2XT. It is very similar to the 2MA, 2ME and 1KT sold in various international markets. The 2XT was the last of the parallel twins that have the exhaust out the front (unlike the later 3MA). In stock trim its a great light-weight that is under powered - like nearly all race-rep 250's. I bought this Japanese spec bike with the full intention of developing it over time in to something that made similar HP to the real-deal TZ250's of the '90's.
Well finally this summer I managed to achieve very important goals: I reached the 70 HP range, the engine is streetable and is reliable. The engine nets out at about 389cc's. I have used a pair of Yamaha DT200 cylinders with a slightly shorter stroke and custom parts to make the cylinders fit. Much thanks goes out to Gary Davis - HorsePower by Gary Davis for his tireless efforts (welding and machining) and his patience for enduring the endless questions I have. Yamaha is not known for the robustness of their 2-stroke ignitions (ask anyone with an '84 RZ350!) and so when the Zeeltronic ignition became available for the TZR - I bought one promptly! The application for the TZR is a nice one in that it allows full adjustability of both ignition timing and power valve logic. It really only makes sense as for years we could jet/carburate, modify ports and build pipes in attempts to tune our way to HP nirvana, but it is only the last ten years or so when the shade tree tuner has had this critical part of the puzzle.
I looked at a lot of ignition curves before deciding on a baseline to start dyno testing. The curve for the start of dyno testing this fall, I used all spring and summer (while watching the plugs very closely!). I was jetted way too rich for most of the summer, but finally this fall I got braver and jetted down on the mains even as daily temps dropped. I bought a bore scope and have been watching the carbon char - spread across the pistons as I jetted down. I was cross checking plugs against the piston crowns.
The following two charts are of the baseline ignition timing and the first repeatable run on the SuperFlow Dyno. Sorry for the display quality of the xls files ....... The black trace is of a stock TZR250.
Another note: The power valve was set to start opening at 5500 and to be fully open by 8500.
On the street it feels all of a 70 HP engine. That of course is owing to the fact that the bike now tips the scales at 291 lbs. with 1/2 tank of fuel. All kind of 4-stroke in the engine's delivery. I suppose has to do with the shape of the (dirtbike) transfers. Right now its not the type of engine that demands you tap the gear shift to assure forward motion. In the spring I was trying to ride it like it was peaky - silly and not necessary with a torque "curve" as flat as Kansas. The engine is saddled with Jolly Moto pipes designed for the 250. When the engine was still a 250, the Jolly's really gutted the mid range (by as much as 8 HP) from 6500 to 8000. Only besting the stock engine's output after 10,000 - holding on 'til 11,000. Made the bike unpleasant to ride unless you were flogging it all the time. So 73 HP and change is where the dyno day started .........