according to the dyno test I read on the MX400B, it makes peak power at 8000 rpm and it drops off after that.

I will type out the result on 1 of the tests starting at 2000 rpm. it makes peak torque at 7000 and peak hp at 8000 rpm. from test rides I had done so far, the dyno numbers reflect the power produced very well. the Hooker did not hurt the power as bad as I was expecting.

there is a mild hit at 6000 rpm but begins to go crazy by 7000 and it hits 8000 rpm very quick. I would love to get this on a motorcycle dyno and do a baseline run once it is broken in.

at 2000 rpm 5.33 hp and 14.02 ft lbs torque
at 2500 rpm 8.27 hp and 17.36 ft lbs torque

at 3000 rpm 9.48 hp and 19.46 ft lbs torque
at 3500 rpm 12.25 hp and 18.38 ft lbs torque

at 4000 rpm 14.82 HP and 19.4 ft lbs torque
at 4500 rpm 18.52 HP and 21.60 ft lbs torque

at 5000 rpm 22 hp and 23 ft lbs torque
at 5500 rpm 23.91hp and 22.80 ft lbs torque

at 6000 29.13 hp and 25.50 ft lbs torque
at 6500 34.38 hp and 27.78 ft lbs torque.

at 7000 37.85 hp and 28.40 ft lbs torque
at 7500 38.19 hp and 26.75 ft lbs torque

at 8000 38.76 hp and 25.44 ft lbs torque
at 8500 31.51 hp and 19.47 ft lbs torque.

as it gets past the peak power numbers at 8 grand it begins to fall away pretty quick. this is from a issue of Cycle in a 1975 test. I seriously doubt once past 8000 rpm in top gear it would not do much after. I think it could run 100 mph for sure and this is not bad at all considering a stock DT400 is all done by 85 or so mph.

this test was done on the Webco dyno. I will post the dyno results of a stock DT400 from a Cycle 11-74 test of a 1975 DT400B. this is interesting reading for the power hungry.

for a interesting comparison I will post the Dyno results from a 1975 DT400B. this test too was done on the Webco dyno.

at 2000 6.01 hp and 15.81 ft lbs torque.
at 2500 rpm 7.92 hp and 16.65 ft lbs torque

at 3000 rpm 11.02 hp and 19.31 ft lbs torque
at 3500 rpm 13.22 hp and 19.84 ft lbs torque

at 4000 rpm 15.54 hp and 20.41 ft lbs torque
at 4500 rpm 18.95 hp and 22.12 ft lbs torque

at 5000 rpm 22.64 hp and 23.78 ft lbs torque
at 5500 rpm 23.10 hp and 22.07 ft lbs torque

at 6000 rpm 20.29 hp and 17.76 ft lbs torque
at 6500 rpm 18.09 hp and 14.61 ft lbs torque.

that is it on this test.

anyone that has ridden a stock 1975 or 1976 DT400 will tell you it is futile to spin past 5500. the dyno results reflect this very well!

Those torque figures at any given rpm are impressive. So this could be a great woods bike or open class. In other words by the figures it can act like a dt400 mountain goat at low rpm but be in the next canyon on the straights when winding out. If this is correct it is a one off motor indeed. I have heard many other bikes of that era would faint in tight technical mountain climbing. This is what as you suggested is what I want for my bike.

yeah, it has great torque. if you rode it and kept the revs low, you would think it is a stock DT400. this combo came out really good!

the only Achilles heel on this bike is fuel economy.

looks like I will be putting on a bigger tank sometime. a Clarke or something of that sort ought to work pretty good in the 3 gallon size. that might give me a 50 mile or more range I would guess.

Yah, the fuel mileage can become a problem. I had a 68 Goat with a 389 tri-power, modded heads, Top end Norris cam shaft, 411 buick posi rear end, turbo 400 custom built and valve body, and switch pitch torque converter- smoked the tires in 3rd gear- from simi to Malibu and back used 3/4 tank- just 60 miles or so. But gas was so cheap back then.

The big culprit with fuel mileage is that 38mm carb.....I ran one on my "where's the beef ?" RT360 project and I emptied the gas tank in under 30 miles. You can run a 34mm or 36mm and it will really improve your mileage with surprisingly little performance loss !

Cool project....glad you finally got it together

Keep the updates coming

thanks Mies! been thinking about a 34 mm carb! this is a possibility and it would work with the factory air filter.

if I could get it up to 30 + MPG and loose no power or revs doing it, this would be great!

I think for the 38 mm carb, I will try a UFO and Racer Clam's trick divider plate mod. the great thing is the divider plate makes the 38mm carb act like a smaller carb at low throttle and the UFO will really clean up the throttle response and because of the change in air flow, it would require a smaller pilot jet.

and of course when all is said and done a sleeper pipe! hard to hide a 38mm carb and a Webco head but other than that you would be hard pressed to tell it has been modified at a casual glance.

This has been an interesting read. I have a nice MX400 mill sitting on the foor in the shop and a titled DT400 roller that is missing the motor. Seems like a match made in . . . Purgatory? My plan was to mate them up, add a DT side cover to allow for an oil pump, and use my thinly dissguized through frame chamber capped with a stock end piece to keep it quiet.

I had the original MX stator modded by DEET to include lighting coils, but the Powerdynamo is a real possibility.

I just need to get that thing out of the boxes and into production. Probably going to go with a more DT inclined Carb to keep it usable.

Really enjoying the thread as I have a lot of the same challenges to overcome.

I say go for it!! you won't regret it!

the transmission ratios on a MX400B are a bit different from the DT400 but it would not matter much as a MX400B engine would very happily pull either one of them! I really like this engine! all the same I would love to have a 1975 MX400B whole motorcycle to go with my modified DT400.

the MX400B and DT400 has the same 1:1 ratio in 4th but the other gears are a little different. the MX400B has a closer ratio transmission but it could still be considered a wide ratio transmission.

the excellent torque output of the MX400B would pull either transmission quite well.

desert racers would gear a MX400B up for high speed work and they performed well at that. it was also a Hare and Hound scrambles weapon and they were also very good at Motocross.

the only real flaw of a MX400B was the front forks. upgrade that and it would be a great one for Evo class.

on a 34 mm carb, I would get a 34mm carb from a 1977-78 DT400 and rejet it. the lean jetting of a untouched late model DT400 is a no no but I bet you can go to JetsRus and get what is needed to make it work. I would not go smaller than a 34mm carb.

you can also get a 1975-76 DT400 clutch cover with a oil pump and retain the oil injection.

then go for the mods that Racer Clam does being the UFO install and the divider plate. there is so many possibilities and great potential with this setup.

also get a sleeper pipe from Meis as well!

all set up and I think it could match or beat the fuel economy of a stock 1975-76 DT400 with much more power than with a stock DT400.

if you do this Makotosun, please do a build thread on it and consider me

Edit: the crankshaft on the MX400B does not have the slot for the oil pump drive. the crank would need to be modified or use a 1975-76 DT400 crankshaft.

another option is get the RH side crankshaft half for a 75-76 DT400 and get the crankshaft rebuilt with the DT400 half on it. $peed and $port does have the DT400 crankshaft half at about 245.00.

This thread is great!