2001 M750 tops out at around 90mph...?

[justin-branam]

I've had my M750 for a few years now. It was my first bike. I never really gave much thought to it as the bike runs great and has treated me well, but it tops out at around 90 mph. When I bought it, the dealer had just rebuilt the carbs, replaced the belts, adjusted the valves, new air filter, and gave it a general cleanup for the lady selling it. Everything seems to run fine on it, but I can't help but think I might be lacking some acceleration as well. I'm not too concerned with going over 90mph, but a 30 mph loss on the top speed leaves me thinking I am losing power across the whole rpm band. I understand the bike is 14 years old and isn't going to be like the day it was new, but any thoughts on where I can start looking for this mysterious power or what could be causing the power loss?

Thanks!

[DarkMonster620]

what is the gearing?

[justin-branam]

In all honesty I am not sure. I will have to count the teeth. However, even if the gearing isn't stock, it does not seem to accelerate any quicker then a few other guys I have ridden with, with the same bike.

[koko64]

Ive not owned a M750, but my modified M900 with 84-86hp topped out at 125 on the dyno with dropped gearing. You would think your bike would hit 100-110 at a guess.
Probably needs a good tune. Is it stock? Are your buddies M750s stock?

[Duck-Stew]

My '99 M750 hit 117mph once but was usually good for ~115.

Fuel filter or pinched fuel line come to mind.  Also, if the mileage is high your carburetor emulsion tubes may be worn enough to cause the bike to run rich which would hinder top speed.

[Howie]

Mine would also hit from 115 but not 120 GPS, not the way optimistic speedo.  Open air box, exhaust and jetting were the only modifications.  Poor fuel mileage?  Needle (emusion) jets  You can look yourself.  Remove the air filter, lift the carb slides and take a look.  Mangification will help.  If they are oval, replace them.  They are also expensive enough they should show up on your bill.  How many miles did you put on the bike?  Needle jets rarely last more than 20,000 plus a few miles.  What maintenance have you done on the bike since you owned it?  Belts every 12K or 24 months, valve check, spark plugs, air filter and gas filter every 6K or i year.  Brake fluid every year too.

[bond0087]

Howie makes a good point that when you're discussing speed on one of these, you need to mention if it's the real speed or what the speedo says -- the first time I took my 2000 M750 on the interstate, I thought "Damn, people are absolutely hauling ass today! I'm doing 80 and I'm getting passed by semis!" Then I strapped a GPS to my handlebar mount, found an open road, and discovered that my speedo was more than 10mph off at interstate speeds, so 80 indicated was a little under 70.  So if it's 90 on your speedo and actually 80, that's a bigger problem. If it's actually 90, then something is also probably wrong, but if it's bone stock, that's not an enormous difference from the rated 115 top speed.

In addition to everyone else's good advice, I'll tack on that if the tubes going to the plastic side pods come disconnected from there and hang out in open air, it causes a big decrease in power at high speeds. Those tubes need to be at atmospheric pressure, but when they get into the air stream at 90mph, the pressure drops significantly (Bernoulli's principle). That messes up the carb (technical term there), and it lacks power at speed but does pretty well around town.

PS: The vacuum operated fuel pumps on those can get less efficient but work well enough most of the time. If nothing else appears wrong, an inefficient fuel pump could explain low power at extended periods of WOT.

[jerryz]

Veglia speedos are more often termed   ''  Vaguelia  ''  due to reading 10% over speed most of the time,,, my M750 hits 115mph on GPS but speedo says 125mph   and at indicated 80mph its only doing 72 on GPS

[Howie]

Those tubes need to be at atmospheric pressure, but when they get into the air stream at 90mph, the pressure drops significantly (Bernoulli's principle). That messes up the carb (technical term there), and it lacks power at speed but does pretty well around town.


Good point!

[justin-branam]

Hey everyone, thanks for all of the suggestions. As far as maintenance goes, I change the oil/filter every 4 or 5 months, and the brake fluid is changed in the spring each year. Other than that, I have not really done anything else. Granted I bought the bike with 13K miles on it, and it now has just under 16K miles. Needless to say, it doesn't get ridden a whole lot.

The 90mph is on my GPS, and the speedo on the bike shows right at 100mph. My bike is ttock other than Termi exhaust. Buddies bike didn't even have exhaust.

I did replace the fuel filter when I bought it, but I will double check the fuel lines. I will also check out the tubes and needle jets.

[the_Journeyman]

My '99 M750 hit 117mph once but was usually good for ~115.

This is similar to mine, and I'm +3 on the rear sprocket,  IIRC it's a 44-tooth and 41 was stock.

I did change out my emulsion tubes when I did my 30,000 mile service.  I picked up a few MPG and better throttle response.

JM

[koko64]

If the bike has been sitting around unridden, deposits in the carbs can affect performance. Pilot jets block up, main jets close up smaller, float needles and float valves can be compromised.

[ducpainter]

A slide not lifting can cause poor top end too.

Have you checked the plugs?

[koko64]

True, it wont top out well with half a carb.

[brad black]

gearing?  it should go faster than 90.  put it on a dyno and see what power it's making.  has it had the snorkles removed from the airbox lid?  put them back in if so.

i had a kid complaining his m400 was sluggish over 110 (km/h).  it was a 95, with the shorter primary drive gear ratio, but it had the 14/46 gearing that they all get when they are imported.  so i figured it was doing about 9,000 rpm at 110.  i convinced him to fit a 15 front, but then he complained that it wasn't as quick through the gears.

typical kid, had all the answers to this old bloke trying to rip him off.