I have tired reading though this section but there is no clear information, most posts are 2+ years old, mostly about blown up Kawi pistons.

Having just finished my "stock" R5K restoration I want to move on to something more custom starting with an identical R5K I scavenged for spares. Got enough to build it back up but engine needs service:
crank will be shipped off to Andy for repairs any day now but both barrels have cracks on the exhaust bridges, I expect by the time they are welded, plated and adding cost of new pistons, rings, gaskets, seals... total expense will be approaching the cost of 300 kit.

Any downsides to doing 300 conversion at this point?

This is for a street bike but not for daily use, just to have fun on weekends as I use LC4 Austrian Tractor for daily commute

Hi there ,
my honest advice , heres a long story !
The 300 kit will give you a beter ride , it has much more midrange and pulls beter than your 250 ever will .
To make your 250 perform like a 300 - you will spend more than a 300 kit .
Get your self a exhaust gas temperature gauge to sort out your jetting .
I have raced hard with the cast pistons and the wiseco pistons and both have performed well .
Forget about comparing hp on the dyno and feel the difference when you ride the bike .
You are welcome to test ride my bike anytime to come here - no questions asked .

Ask plenty questions and we will get you sorted out .

cheers , Neal

I strongly disagree with Neal, but not on all points.

I agree with the way the 300 rides. It does have bags of mid-range, but to make one rev (and therefore feel like a buzzy 250, which I personally think is the point of owning a buzzy 250), they lose that mid-range. Consequently, I don't see the point! They still won't make the big power you would normally expect from a big-bore.

We have yet to run any 300 that can out-perform the best 250, or even match it, and it's not for the want of trying. The best 300 we have run was actually tuned by TYGA, complete with VHM heads, PJ38s etc., and could be considered for all intents and purposes, an "F3" 300. It still barely scraped by 70hp on the same dyno that the better 250s regularly make mid~high 60's on, and the best makes 75hp on. It does however, as mentioned above, make a chunk more mid-range than a 250, as you would hope (and expect) it to.

I do think Matt is getting ever closer to nailing the unquestionable reliability issues, and this may also release horsepower at the top. I have a reputation as a 300 hater (particularly by Neal), but quite the contrary! I actually pressured Matt 12 years ago to complete the project, and encouraged StephenRC45 for a long time (and through 7~8 barrels, and even more seizures!) to try and nail some of the problems. Personally, while I will never choose one over a good 250, I do want the kit to be reliable and successful, if for no other reason that the sheer amount of time and effort Matt has put into it!

In my opinion, a 300 should be taken for what we've shown countless times it is. It's a lazy 250! They make on average about 65~66hp, and a chunk of mid-range. They turn a 250 into a lazy 4-stroke. You could well be faster on-track with one, because you have more margin for error with more mid-range... a missed or wrong gear is not so critical, but the bike won't ultimately be any faster with 66-300-hp than it will with 66-250-hp!

There certainly IS a substitute for cc's... it's called a well tuned and set up 250! It will be more reliable too!

The benefits? You get new barrels with Honda plating.

Remember too, the 300 kit will not work with stock exhausts. You will need to add race pipes to the cost of any build. That's approximately £1200 + £500 (for new pipes). Add another couple hundred Pounds/Dollars for a twin EGT setup as Neal is recommending, and you have just spent best part of £2000!
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Thanks guys, it's refreshing to get an honest opinion on things that is not polluted by "mine is bigger than yours" and other personal insults, especially from two opposing sides of this.

Couple points I would want to make that make me lean towards 300:

•I already have stock 250 and planning to build "Tuned" 250 some day to compete in P89 class (Production pre-'89),
this will be a an independent project, not expecting to complete this any time soon as I need to clear up my storage of all the old junk, hopefully sell some of it so I can free up some funds

•this bike will need pipes either way as all I have at the moment is MC21 frame MC28 ProArm and R6K engine (also 21 subframe, Radiator, some bodywork, yokes... etc)

•always likes 4 stroke twins, love the low end power, also like having couple completely different bikes I can throw a leg over on any given day depending how I feel.

My current line up is the recently finished R5K, RG500, Tuono and KTM LC4, that's what's on the road and ready to ride, another half dozen projects like VF1000R (x2) RD400 (x2 and hopefully both sold) Yamaha AT1 125 cc Enduro, Hawk GT (Bros in UK) it was my old race bike with Ducati inverted front end, 700 big bore, cams, Ohlins etc.. that one been shelved for 10+ years since I married and quit racing, some day it will be back on the street. Add 10 or so scooters and I got my plate more than full, everything always needs work and money so got to pick the projects wisely and sell some of the toys that I no longer use like my GS150 and P200 and RD's and VFR's

Nah Andy your'e cool anyway !
I would like to make some more comments , While Andy and Steve are very good with NSR'S . Building a motor to the book is what they have proven . Great mechanics but maybe when it comes to the design and working out of things 300 related need more info .
When i became interested in the 300 i compared the 300 to RS barrels , the 300 came out tops . There are many spare barrels and heads for sale compared to RS parts and my conversion was cheaper than the RS route .
When we did our first attempt at the 300 it was with the help of a clever man , many people ask him for advice and use his software when building race motors .
My first dyno run as a comparison reflected what i felt when riding the bike . It was stronger in the midrange and peaked at a similer rpm to my 250 but with a bit more hp . You can tune the bike to perform how you would like .
If you want more on top with a kick in the band it can be done .
Forget about hp figures - look at the graph .
We have learn't more now compared to when the guys first started with the 300 . A process that could have been speeded up IMO .
You can't use the F3 to compare against as it is out of reach for most people but a 300 is within grasp of a few more .
A SP 250 can't match a R 300 . The only problem with the 300 is that it has a bad rep from seizing - something that is very closely linked to the owner and "bad" preperation IMO .
I have seized - but it was hole behind the powervalve shaft pushing pressure into the system and blowing water pipes ( 2nd hand barrels ) .
For me , the nsr300 will allways win when compared to a nsr250 !
I am open to discussion too .

http://i561.photobucket.com/albums/ss56/deon42_bucket/NSR250vsNSR300.jpg

this was our first attempt at the 300 . If you look at the shape of the graph you will notice how it follows the 250 curve but is just stronger all the way .

Umm- i hope this link works .

Neal wrote:
The only problem with the 300 is that it has a bad rep from seizing - something that is very closely linked to the owner and "bad" preperation IMO .

If you saw any of Steve's bikes you wouldn't dare say that, and I think there are dozens on this forum that will back me up on that. Over 7 dead barrels, and a dozen or more pistons are no reflection on his preparation, and the comment is way off the mark.

His F3 has been thrashed by all and sundry on track days, and is now run buy a total 2-stroke newbie, and it has never failed. It makes more peak power, and similar torque. You can't argue with facts.

Many F3 parts are indeed difficult and/or expensive to come by, but proving tuning methods cost more in labour than they do in bolt-ons. £1700 (including new pipes) gets you (on average) a 66hp 300. That makes our 67~70hp 250's look cheap!
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Hang on Andy , I am not saying Steve or you are bad mechanics , I paid you guys a compliment in the paragraph . It would benefit all 300 owners if you guys posted up your hidden details of porting , squish , squish band , port maps , timing etc .
Let's judge the guys that have posted about seizing their 300's in this forum .
Let's assume that the 300's that you guys have tested are not correct that's why they only make x hp .
Then why does my bike produce a desired curve on the dyno and reflects an improvement compared to my 250 that was tuned by the same guy and went well for a 250 .

*Andy , you are misquoting there *

If you apply the same rules of tuning to a 300 as you do to a 250 then a 300 should be beter than a 250 IMO . At least that is what our comparison shows .

Neal wrote:
If you apply the same rules of tuning to a 300 as you do to a 250 then a 300 should be beter than a 250 IMO . At least that is what our comparison shows .

But it doesn't. The "F3" 300 with VHM's and PK38's that TYGA tuned and was run up on Steve's dyno showed that. It makes 72hp... within 0.5hp of it's previous run at an independant facilty a few months prior.

Interesting note:

A very good NSR250, without all the HRC goodies, will make 69~70hp. (Bill Limbs made 69hp with stock pipes)

Steve's NSR500Vs make 137~140hp.

By those figures, a 300 should make around 83~84hp. It doesn't, and none have come even anywhere close... not even within 10hp "close".

Something is flawed with the design. The 300 kit does not compliment the 250 bottom-end, the pipes, the ignition, or possibly a combination of all three. This is why we now accept it for what it is, not for what it was hoped it promised 10 years ago. You can't fight the physics of it.

And I'm afraid what we've learned over the last 10 years is not up for discussion on the forum. It makes us money, and makes us fast motors. It makes NSR250 that out perform NF5's and hang with NX5's! It's cost us a lot of time and money to get where we are. Thousands and thousands of Pounds, and hundreds, maybe thousands of hours.

A lot of information has been shared both ways with Matt, and Steve and I have both sat for hours with him over countless beers going over thoughts and ideas relating to 250's and 300's. Steve has even lent Matt NSR500V parts to look at now. However, you will note that TYGA do not pour their knowledge out on a forum, and nor does Frank Wrathall, Clive Padgett, Bob Farnham, or any other tuner.

Edited for grammar.
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RG500 which is essentially two parallel 250 twins, makes 100 hp on a good day so 250 with even only 60 ponies seems like a great result.
Got some wheel spin exiting the corners (wet) on my bone stock R5K which I did not expect at all, loads of torque and very usable power mid-rangs, would not want to trade that for peaky 70 hp with a big hole in the middle, if I want the excitement of "Wile E. Coyote strapped to a missile" I'll just tide my RG500, there is no 3D ignition or variable RC valve, it's either ON or OFF.

Appreciate al the response!

Gonna add up some numbers and make a decision at some point, got plenty of time to mull it over

Did the F3 300 have custom pipes made for it ?
I have shown a comparison of custom 300 pipes vs 250 pipes and the 300 performed beter with its own pipes designed for it .
Andy , we are not concerned about 250 secrets - I am asking about 300's . You have said that they don't perform so I doubt people bring their bikes to you guys for a detune . Then surely the recipe to ruin a 300 should be put up for all to see to warn other riders . It can't be top secret if it nothing special is shown by the results .
Only when you have a true complete 300 can you compare it to a 250 . Even the F3 250 ! We're talking motor , carbs , pipes and ignition tuned for a 300 vs the same for the 250 which the F3 is tuned for .

I can't explain why I am 2 seconds a lap quicker on my 300 compared to my 250 .

I can only sing praises for the nsr300 .

There's only so many ways I can say "a 300 tuned to the hilt (effectively F3, and beyond, spec) by Matt made 72hp"! Accept it. The owner was disappointed... Matt was disappointed! Both were convinced the first dyno read low. The motor also lunched itself the first track day it did, before it even ever made it to a dyno!

If anyone knows what they are doing with the 300, it's the guy who's spent the last 12 years of his life trying to develop it! The guy who has worked with some of the best in the business in Japan. HIS MOTOR made 72hp.

I'm party to a lot more information that you will ever be aware of, and entrusted not to reveal it. I will not be drawn into revealing any of it. It's confidential. The 300 is what it is. Don't believe your own hype.

250 tuning "secrets" and 300 tuning "secrets" are secrets full stop. Reliability issues and potential fixes need to be discussed with Matt and/or other 300 developers as we don't touch 300's any more. They simply cannot be guaranteed reliable! Even setting them up over-rich doesn't stop them letting go -- that isn't, and never has been the answer. And yes, the 72hp "F3" 300 has custom pipes.

Edited to add the last sentence. Forgot to reply to that question.
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Last edited by Andy on Fri Sep 14, 2012 6:36 am; edited 2 times in total

Dmitry wrote:
...would not want to trade that for peaky 70 hp with a big hole in the middle...

A 70hp NSR250 doesn't have a hole in the middle, nor does the F3 which is knocking on the door of 75hp. In fact, it's hugely tractable, far more so than a 60hp bike, far more so than an HRC RS250, and easier to ride than both too. It's using the last 5hp and 1000rpm that takes the doing!

In fact, we know people that haven't revved it much past 11,500rpm and come back and said they didn't need to as it just had so much drive. What they mean is they couldn't ride any more than 70hp!

Actually, even those who've ridden my 250 have all remarked, without fail, at just how much drive it has too. The NSR is unlike any other 250 2-stroke. To a lesser (3XV) or greater (RGV) degree, all of the others will lose more and more mid-range as you tune them.

The 300 will however, out-grunt just about any 250. The more stock each is though, the more noticeable the 300 is. The more you (equally) tune them, the less the difference. You do however reach a point of diminishing returns with the 300 far sooner than you do with the 250. We've proven it time and time again. The 300 hits a brick wall at a TRUE 70hp. Getting power after that is very hard work, as is getting RPM. If they rev, they don't make power and they lose the mid-range... if you keep the mid-range, they won't rev.
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