|850T Hotrod Dyno Runs by George Dockray
I finally got around to a dyno-tuning session with my glorified 850T and thought I would pass long the results to you. The basic engine is a 1975 850T that's had the following engine changes made:
949cc 1000SP barrel - piston assemblies.
Mistral 850 LM1 mufflers and larger-dia. headers w/ cross-over, T-3 'H' pipe
LM1 flywheel (9 lb. as opposed to the 14 lb. original)
dual-spark ignition. w/ Dyna III & Dyna coils (timing set to 28 deg. full advance)
36mm Dellorto PHF carbs from a Cali III / 'Euro +" jetting (from Agostini)
K&N 'pod' filters
Inlet opened to accommodate 36mm manifolds
The “Euro+” jetting was as follows:
pilot jet- 50
atomizer - 268
needle - K-18 (clip in 3rd groove below the top)
slide - 60-3
main jet - 130
Other than that, there are no other mods that relate to power. It's running stock valve gear and cam. I have not measured the compression ratio to see how the 850T heads did with the 949cc barrels.
I tried the carbs just the way they arrived and although it was rideable, there were definitely flat spots and a lag at higher RPM and a reluctance to rev. The first dyno run showed that the fuel - air ratio was lean just off idle, a bit rich at mid-range and lean again in the main jet range. After a few runs with different pilot jets, main jets and needle positions, we ended up with the following chart. FA ratio looks in just the right zone above 3500 RPM with nice smooth torque & power curves all the way to redline. The weak mixture that you see at low RPM is a result of my not knowing at the time that to increase the output from the accelerator pumps, the screws need to be turned out (I was turning them in). My allotted dyno time ended before we could address this lean spot, so I trimmed that portion of the settings later by feel - about 1/2 turn out (richer) from the stock setting. In the end I picked up about 6 HP at the rear wheel. Here's what we ended up with:
pilot jet- 60
atomizer - 268
needle - K-18 (clip in the top (leanest) groove
slide - 60-3
main jet - 135
I'm amazed at the difference in the ride, it pulls with authority right to red-line through 3rd. I have not had the opportunity to wind it out in 4th or 5th, the dyno showed it revving to red-line in 5th with a top speed of 124 MPH & it feels like it will.
The flatness of the torque curve is impressive, although I don't know if this is a result of the dual exhaust crossovers, the dual spark or both. Looking at some other dyno charts that are posted, I'm pretty impressed that a small valve engine with the soft stock cam could do this well.
I did a bunch of engine work last winter that included a B10 cam with LM4 valve springs, "black diamond" valves w/ K-lined guides and another visit to the dyno. These were the only changes that would effect power. Otherwise a RAM clutch was installed, the cases modified for a proper oil filter, crank assy. balanced to 52% and a windage plate. Here's the result.
I did a bit more tweaking on the accelerator pumps (less squirt) and needle positions by road test tweak & try. Started with a K9 needle, but found it too rich with 268 needle jets so went back to K18s until I get some 265 NJs to try with the K9s back at the dyno.
The 2.7 HP increase with no difference in torque from the stock cam is a bit deceptive by seat-of-the-pants. It feels more responsive and revs more freely with the B10. Still a nice wide power band and easy to loaf along when necessary.