Low top speed

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denesdr
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Low top speed

Post by denesdr » 17 Jul 2017, 12:44

Hi!
I bought my 1993 Mk2 two years ago. The engine had good compression by feeling and measuring, but very hard to start cold and lean symptoms at cruising speed and not too great top speed around 130km/h. I tried to adjust the float level, jet needle, jet change and air screw with no luck. I decided to try a PWK33 carb. After jetting the PWK ( 45 pilot, leaner straight section needle, 128 main) the cold start problem is gone and starts 1-2 kick cold or hot. The cruising lean symptoms is gone too, but the top speed is same as before.
The engine has now completely rebuilt from top to bottom all seals, new bore, bearings etc. I hoped the best. But the top speed is horrible same as before around 130km/h with 17 inch wheels and 15-39 sprockets, this is near the same but slightly longer ratio as the original with 18 inch wheel and 14-42. The bike pulls great and has good power at lower gears but reach top speed in 4 gear and no more acceleration in 5-6 gear. Brakes, chain, sprockets, wheel bearings are all in good condition, power valve works and i am not too heavy weight with my 68kg. Ahh i almost forgot the bike has an aftermarket silencer which i cleaned and repacked, the front pipe is original but i removed some big dents and the inner sound dampening layer and carefully welded the pipe but again, no luck.
Does anyone have any idea what's going on here?

Thanks!
Denes

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Re: Low top speed

Post by gregorious77 » 19 Jul 2017, 07:55

Could be a number of things really. Get it on a Dyno for starters to see if the jetting is all good. If it was lean at the top end revs I am sure it would have seized by now if you are WOT trying to go faster. It is my understanding a std CRM will do around 100mph (160kph). Maybe its rich at the top? could be a weak spark? could be your expansion chamber is tuned for low end torque? could be you powervalve not being in the correct position or not opening far enough? could be worn rings? Reeds?

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knackeredMk1
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Re: Low top speed

Post by knackeredMk1 » 19 Jul 2017, 23:22

Two things -
Check your powervalve is working properly - Video in FAQs
Junk your expansion chamber - It will be so full of crud by now. Get a new aftermarket one which will give you slightly more power than the original had and a huge weight saving.

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denesdr
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Re: Low top speed

Post by denesdr » 03 Aug 2017, 06:19

Thanks for the suggestions! The power valve is working properly, the expansion chamber is in good conditions that's for sure, but maybe i try an aftermarket pipe later.
But i checked the cylinder compression few days ago and it's topped around 140 Psi. Fuel on, full throttle, cold or hot i read same values. This is normal? I think it's not so much compression for a 800 Km top end (rebored, new piston kit, new seals and gaskets). Reed valve is good too.

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Re: Low top speed

Post by gregorious77 » 03 Aug 2017, 08:29

Compression should be around 170psi for a fresh engine. I put new rings in mine when it was showing 125psi but it still had good power.
You say you removed the inner part of the expansion chamber? that means you have changed its volume and shape so effectively changed the characteristics of the engine. The expansion chamber design is crucial for power delivery. Some pipes give good low end power with top end sacrifice and vice versa. Maybe you have created a low end power chamber.
Other thoughts are you engine may have had the port timing changed in the past to give low end power?
First and simplest thing to try would be an expansion chamber.
http://www.dragonfly75.com/motorbike/ECtheory.html
http://www.dragonfly75.com/motorbike/portdurations.html

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Re: Low top speed

Post by knackeredMk1 » 03 Aug 2017, 09:46

I still think that you have an exhaust problem.

When I use PWK33s on my CRMs it improved throttle response massively compared to other modified CRMs. It maybe gave some extra power but I couldn't tell for all the modifications I made at the same time.

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denesdr
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Re: Low top speed

Post by denesdr » 07 May 2018, 22:56

Ok, i've talked about the top speed of a CRM250 at several forums, but nowhere can I find any certainty that an unmodified Mk2 can reach 160km/h (or roughly 100mph). Most users reported 130-140km/h as an absolute maximum, one owner said 170km/h but with the original speedodrive combined with a 17" front wheel so this is an indication error nothing more.
The funny thing is that for e.g. a CRM125 in good condition can reach this performance easily, ok not so grunty down low but it has half the capacity of a 250.
So my question is that an aftermarket pipe (DEP for example) can give wings for this old horse to performe like as it should be?
Does anyone have such real experience of this kind with an Mk2?
Again, my bike is in good shape from A to Z, including the exp. chamber, carb etc., thats for sure.

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Re: Low top speed

Post by knackeredMk1 » 08 May 2018, 10:49

An aftermarket exhaust system will give a slight top end power increase but will free up a little torque lower down compared to a low mileage original system.

The point being the comparison is with a relatively new system. If you change the front pipe and the silencer then you should get a noticeably bigger gain over what is I assume, a still original system.

From my experience it is actually the silencer that is more often the cause of reduced top end performance rather than the expansion chamber but a lot depends on the condition and previous maintenance!


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