Top Speed?

[CRXSi90]

http://www.bikez.com/motorcycles/kaw...e_175_1981.php

So I went there and it says the top speed for this bike should be 70.2 mph. I took mine on my first long-ish ride of 90-140 miles. (didn't check the odometer when I left!) This was participating in the Slimy Crud Run. I hit a lot of country roads where the speed limit was 55 mph. The bike cruised just fine at that speed. If I pulled the throttle to the stop on a long stretch, I could hit maybe 58. When I got back, I swapped out the standard 14-tooth sprocket for a 15-tooth. A couple days later, I took it out for another run, and was able to hit 62 or so.

So, I'm thinking, WTF?!? These runs were on flat ground, fullish 2-gallon gas tank, my ~150 lb self sitting more-or-less erect, and not drafting off of other vehicles. I'm thinking that if I laid down on the tank, went down hill, and drafted a semi, I could probably hit mid-70's. But is that the way this is supposed to work? Or should I be taking a closer look at my bike and see if there's trouble in the carb/ignition?

[fredintoon]

http://www.bikez.com/motorcycles/kaw...e_175_1981.php
So I went there and it says the top speed for this bike should be 70.2 mph. If I pulled the throttle to the stop on a long stretch, I could hit maybe 58. When I got back, I swapped out the standard 14-tooth sprocket for a 15-tooth. A couple days later, I took it out for another run, and was able to hit 62 or so.
So, I'm thinking, WTF?!? - - -

Hi Kevin,
that road test top speed was on a shiny new bike ridden by a 75lb guy laying flat on the tank.
Now it's 31 years later, what do you expect?
New hiway tires (not knobbies), new chain and sprockets, an engine rebuild, lose 25lbs and buy a set of skinny racing leathers and you might get another 5mph.

[DGXSER]

If your bike still has great compression, the power is still there. However, to Fred's point, you have not spent hours tuning the bike on a Dyno to squeak every ounce of power from it, then tried every sprocket combination to find the perfect setup for your weight and aerodynamics, and you probably do weigh a bit more than the dude testing that thing.

IMHO, if you can get a 175 cc bike up close to 60 MPH, you're doing good. I ain't seen 150 since high school, so I think it would be a pipe dream for me.

[CRXSi90]

LOL, thanks guys. I remember my mom telling me that back in the 70's, she used to do freeways on her Honda CB200 ( ), so I was just wondering what I could squeak out of this thing. But it's not too surprising that the manufacturer's ratings are a bit optimistic. I'll just stick to the back roads and trails where this thing was meant to play.

[jetmechmarty]

I have it on good authority that these price point motorcycles were of inconsistent quality. Some came from the factory tuned spot-on and screamed, while another you could coax plenty of power out of with some fine tuning. These inexpensive motorcycles often left plenty of room for improvement.


Do you think Yamaha built it this nice?

[CRXSi90]

Yep, that makes a lot of sense. As with any manufacturing process, there is likely to be some variation between units. And it makes sense that this range would be larger for lower price point units. Also, the ones that the power/speed tests were performed on were likely to be the prototypes or pilot production units that were assembled and evaluated with more care and scrutiny.

Anyway, it probably wouldn't hurt for me to give it a shot doing some tuning on this thing. I'm not sure that the idle circuit is working quite right since it's a real dog trying to get moving from a stop. It's much worse if it's not warmed up or it is cold outside. But revved up to 3-4k rpm (??? - no tach), it usually does just fine. I was hoping my Iridium spark plug would cure all that, but no cigar.

[650mark]

top speed

your story reminds me of my first "top speed run" on my new honda s90 in 1967. had about 100 miles on it, rode 3 miles over to a long flat stretch of road and opened her up! laid flat on the tank, throttle pinned, and just as she hit 60mph she went dead. i coasted to a stop scared to death and sure that i had blown it up. walked the 2 miles home, told my dad, and he drove me back to the bike. i just knew he was gonna kill me. he looked the bike over, muttering things like "blown piston, bent valves, camshaft destroyed" and i just knew he was gonna kill me! he looked me right in the eyes and asked me if "i had learned my lesson" then he started to laugh like crazy and said "it's out of gas,dummy". he put it on reserve and started it up. he was still laughing when we got home and he told that story to anybody who would listen. i sure miss him more than you can imagine

[CRXSi90]

That's a fun story. It's good to remember that we were all young once, and that's true from both sides. It was probably so long between filling the gas on the S90 that you forgot about it. I haven't done a good test on the KE175, but I'm guessing I'm in the 45mpg range. That happens to be about the same as my mom's 883 Sportster.

[Eveready1100]

Mate, the first thing I would look at is the exhaust. It's a 30 year old 2 stroke and its had a lot of time to build up oily deposits in it and choke the motor at revs. It's been a while but I recall removing the baffle and using a baking soda solution to soften the stuff in the pipe then heating it with a torch then rapping it to shake the oily crap out of it.
Someone who's had more recent 2 stroke experience (later than 20 years ago) might be able to give a more updated version of decoking a 2 stroke exhaust pipe

[CRXSi90]

Thanks. Before I knew what I was doing (wait, that still applies...), I tried to do that once before. I think I took the baffle out and poked a propane torch down it's throat and kept on it for a few minutes. I could hear it crackling and such, but I probably didn't do it long enough. That was probably 8 years ago. I do remember that I fired it up without the silencer and rode it around a little bit. That was a riot!

I did some reading and it looks like various approaches include the torch, caustic soda, solvents, and detergents.
http://www.classicmotorcycle.co.uk/a...troke-exhausts

I went for my first longish ride on this bike the other weekend. For the first 6 miles or so, the smoke was just pouring out of the exhaust. It ran clean for the remainder of the trip. I figure it was either carrying some oil up from the bottom end (either stuff that got stuck on the walls, or leaky transmission seal), or it was burning the soot out of the exhaust. Anyway, the exhaust is certainly a reasonable place to start! Thanks!

[fredintoon]

- - - I went for my first longish ride on this bike the other weekend. For the first 6 miles or so, the smoke was just pouring out of the exhaust. It ran clean for the remainder of the trip. I figure it was either carrying some oil up from the bottom end (either stuff that got stuck on the walls, or leaky transmission seal), or it was burning the soot out of the exhaust. Anyway, the exhaust is certainly a reasonable place to start! Thanks!

Hi Kevin,
it didn't occur to me the bike was a 2-stroke so yeah, the exhaust might well be plugged up.
Old Britbike 2-strokes had mufflers that could be taken apart to clean them but not so with modern bikes.
What I did to my son's 2-stroke was to remove the muffler, shove my oxy-acetylene torch flame in until smoke came out the other end then turn the acetylene off and the oxygen up full.
Shoulda seen the plume of nasty brown smoke that came out.
Alas that I had to end the experiment early, before the neighbors called the fire brigade.

[CRXSi90]

Not bad! I just pulled the silencer out of the muffler and burned as much of the oil out as I could with my propane torch. I'm sure the people @ my apartment building appreciated that! Then I banged the silencer against the ground to knock the ash loose. *ting*ting*ting* Then I looked inside the muffler housing, and there was some crud blocking the bottom holes, so I took a wire and fished it around in there to break the crud loose. I couldn't exactly flip the bike and shake the stuff out, so I fired up the bike without the silencer and gave it a few good revs. I'm sure the people around here loved that too! Finally, I buttoned it all back up, and took it for a ride around the block. It stumbled a bit, but once it got warmed up, it had a lot more low end torque than before. I'll have to go take it for a high speed run and see if it did anything to the top end!

[CRXSi90]

No cigar. I went out for a ride, but couldn't make it past 60-62 on the flats. I tried easing the choke on a little to see if a little more fuel would help it, but that definitely was not the case. As far as breathing, we took care of the exhaust. The intake has a foam filter. I can replace the foam, but I wouldn't think that would have a remarkable difference. Maybe it's something to do with 32 year old reed valves? I guess I could dive in and give them a look.

[mharrington]

Does carbon and oil buildup on the inside of the total exhaust systems facor in to diminished potential as well? My daughters stock 1978 DT125 does 59 with me on it. I am 248lbs. I recently and briefly owned a 1975 DT250 that did 64mph with me on it.

[fredintoon]

Does carbon and oil buildup on the inside of the total exhaust systems factor in to diminished potential as well? My daughters stock 1978 DT125 does 59 with me on it. I am 248lbs. I recently and briefly owned a 1975 DT250 that did 64mph with me on it.

Hi Mike,
well it must do, eh? But if the DT125 is only 5mph slower than a bike with twice the engine capacity there can't be too much wrong with it?