Jump to content

topspeed as indicator for full horsepower...


Tom125mx
This post is 4089 days old and we'd rather you create a new post instead of adding to this one. You can't reply in this post.

Recommended Posts

hi all,

I have to admit, that I was a little bit disappointed about the topspeed of my 78 dt125mx.

--> not more than 90km/h (55mph) .. maybe the speedometer in inaccurate...

my kawasaki ke 125 (1978) appears more powerful and is faster as well..

... how fast should it go?

... any obviouse modifications to improve power?

... does the 175 barrel/piston/barrelhead fit?

if yes: what else need to be changed in order to have the full dt175 power?

thx in advance

tom

Link to comment
Share on other sites

Do you want an ad-free experience? Join today and help support the Yamaha Owners Club.
  • Moderator

morning Tom, first check the exhaust header, you may find a sleeve in there that you can take the dremel cutting disc to.

Yes the 175 barrel and head will fit straight on.

Also some of the early bikes had a 22mm carb but it would go better with the 24mm version fitted

Link to comment
Share on other sites

Topspeed as indicator for full horsepower. My kawasaki ke 125 (1978) appears more powerful and is faster as well.

Top horsepower for any given bike kicks in at a certain number of revs, depending on your bike model.

Yours reaches its maximum of 14HP at 6500rpm.

Your statistical top speed is given as 68mph (110kph), meaning you'd have to rev it pretty hard to get much faster than you already are.

The '78 Kawasaki, by comparison, is rated at only 10HP at 6300rpm, topping out at 58mph (94kph).

Bear in mind though that as these things get used, they do drop in performance.

Also, it depends how you rev it and govern the throttle.

If you're going along and whack the throttle open, you will be stressing the engine too much and it will actually lag in performance. If you gradually increase the throttle, building your speed up you'll find you eventually get a higher top speed... eventually.

A trick I used on my 125 Dragstar - Gear 1 was for moving off. Gears 2-3 were for playing in traffic. Gear 4 was for high speed moves (50-65mph, overtaking, etc) and getting up toward top speed. Once at around 65mph, I'd then shift to gear 5 and just cruise along, but slowly increase throttle over about 30 seconds to a minute. I'd then end up with a top speed of about 74mph.

Link to comment
Share on other sites

  • Moderator

my 175 will do 70 if I cane it but it's more comfortable at 65...I wont do it for long stretches though it seems a bit risky and I dont trust modern ethanol fuels!...I have one more tooth on the front sprocket to raise the gearing slightly

Link to comment
Share on other sites

  • Moderator

Same here. She will do 70 but 7krpm is my normal max.

I think the 2 compare on top speed. The extra cc counts for plugging the muddy lanes not tarmac tearing.

Link to comment
Share on other sites

One other factor " How heavy are you ,,, :eusa_think: 60mph will be tops, and remember Twinshock barrels are 4 stud . and MX 5stud... :yeah:

Link to comment
Share on other sites

One other factor " How heavy are you ,,, :eusa_think: 60mph will be tops, and remember Twinshock barrels are 4 stud . and MX 5stud... :yeah:

In other words if your a fat bastard that's why your bike is slow :jossun:

  • Like 1
Link to comment
Share on other sites

75kg... Not that fat ;-)

And it's the topspeed of the dt that is less than the 10hp Kawa ke 125.

Adjusted the ignition Timing yesterday. Plus I took away the airfilter. This added another 5 km/h in topspeed

Still far away from where i'd like to be ;-)

More Tipps?

The realistic tip that bring's the 100km/h+ will get a Free Pint!!! :)

T.

Link to comment
Share on other sites

  • Moderator

some of the early 125cc bikes had a 22mm carb venturi and the later ones a 24mm vanturi which is better, what is the number stamped into your carb Tom ?

Also mr Yamaha had a thing about fitting smaller diameter inserts into the exhaust header pipe, what is the number stamped into your exhaust and have you checked the header, I suggested this earlier but you didnt respond ?

It's not advisable to run without an air filter on a long term basis

Link to comment
Share on other sites

  • Moderator

Not sure about the carb, you will have to measure it using some inside calipers

but the exhaust has a restrictor tube, I know this because I just bought one myself, I use a dremel speedclick arbor and the smallest cutting disc they supply for it...even then you may have to wear it down a bit in order to gain access

looks like the pint is mine then

This is the sort of thing you'll find, you can see the end of the tube in this pic although the 2A8 has a longer tube so you'll need a torch

100_1307.jpg

Link to comment
Share on other sites

:) this Sounds promising!!

Would the ( unrestricted) exhaust of a dt 175 fit?

Any idea where to get an exhaust? .. I'like to keep the original one...

Are there any Tuning Exhausts for that Model available on the market?

Cheers

T.

  • Haha 1
Link to comment
Share on other sites

  • Moderator

I've tried various exhausts

aftermarket exhausts were made by several manufacturers over the years, of course being a 35 year old model they were mostly withdrawn from production

personally I have found that the performance of the original pipe is hard to beat, I say remove the restrictor in yours

Link to comment
Share on other sites

ok. cool..

since I want to keep the original pipe unmodified; i'm looking for a second exhaust.

any idea where to get a original part from? either used or new?

AND: would the exhaust from the 175mx fit to the 125cc barrel?

tom

Link to comment
Share on other sites

  • Moderator

looking for a second exhaust.

Best of. There are pattern ones by Big One but they are in the later front expansion chamber type. Pick a bodypart for an oe system in good nick.

Link to comment
Share on other sites

  • Who's Online (See full list)

    • There are no registered users currently online
  • Recently Browsing

    • No registered users viewing this page.
×
×
  • Create New...