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Author Topic: 250 Single - Performance Upgrades  (Read 1125 times)

Offline paddyshepherd

  • Posts: 57
Re: 250 Single - Performance Upgrades
« Reply #10 on: Apr 14, 2018, 11:41:31 »
Cheers Gents.

You're right Teazer - it is immaculate inside. I guess I'll just do oil seals and gaskets for now then and consider more power internally at a later date. I might give megacycle a look though as the cam does have a little bit of pitting on it. In that case, I'll just be looking at carbs and exhausts for now.

The stock carb is a Keihin PD70 - 33mm carb with a straight outlet (i.e no venturi). How do I go about deciding what would be a worthy upgrade?

Offline crazypj

  • Posts: 12991
  • Split personality, I fake being smart
Re: 250 Single - Performance Upgrades
« Reply #11 on: Apr 14, 2018, 16:55:45 »
Flatslide off a modern MX 250? (probably a bit big?)
Should be easy to find as maintainance was low hours on early models (RMZ 250 was only 30 hrs on crank, CRF dropped valves pretty regular, etc)
 Maybe one out of a 600/750 inline 4 set, pre-2005~6? (before they all went fuel injected)
'you can take my word for it or argue until you find out I'm right'
Best thing I ever overheard
"yep, PJ's my boss, he taught me everything I know, just didn't teach me everything he knows"
Brian Morgan, 1982

CB360's,  http://www.dotheton.com/forum/index.php?topic=11736.0
XS650,  http://www.dotheton.com/forum/index.php?topic=11922.0

Offline ex119x

  • Posts: 46
Re: 250 Single - Performance Upgrades
« Reply #12 on: Apr 17, 2018, 03:34:00 »
For a carb, I'd either look at a VM32 Mikuni, or if you wanted to stay with a CV, then the stock 35mm off an FT500 Ascot would work. The stock carb off an XL500s is a 32mm slide needle Keihin. The 250 motor and the 500 motor are almost interchangeable. I think I heard the 500 is a little wider in the rear for the motor mounts and won't quite bolt into the 250 frame without some slight mods. None of the RFVC single parts interchange with the earlier XL or RS motors and none of those parts interchange with the first generation XL250 or 350 motors. 3 pretty distinct families of engines. I have looked at it, but not measured anything out, but I wonder if a 500 crank and top end would work with 250 cases?

Offline crazypj

  • Posts: 12991
  • Split personality, I fake being smart
Re: 250 Single - Performance Upgrades
« Reply #13 on: Apr 18, 2018, 00:40:42 »
VM32 would be OK if you could get it with even close to correct parts. By the time you change just about everything except body and float, it's cheaper and easier to use something more modern. Using CV you can use a much larger carb but not have to learn to ride all over again, using a mechanical slide carb means you need to be a lot more careful not opening throttle too far too fast at lowish rpm, slide and jets need to be a lot more carefully matched to engine
'you can take my word for it or argue until you find out I'm right'
Best thing I ever overheard
"yep, PJ's my boss, he taught me everything I know, just didn't teach me everything he knows"
Brian Morgan, 1982

CB360's,  http://www.dotheton.com/forum/index.php?topic=11736.0
XS650,  http://www.dotheton.com/forum/index.php?topic=11922.0

Offline paddyshepherd

  • Posts: 57
Re: 250 Single - Performance Upgrades
« Reply #14 on: Apr 18, 2018, 15:47:08 »
For a carb, I'd either look at a VM32 Mikuni, or if you wanted to stay with a CV, then the stock 35mm off an FT500 Ascot would work. The stock carb off an XL500s is a 32mm slide needle Keihin. The 250 motor and the 500 motor are almost interchangeable. I think I heard the 500 is a little wider in the rear for the motor mounts and won't quite bolt into the 250 frame without some slight mods. None of the RFVC single parts interchange with the earlier XL or RS motors and none of those parts interchange with the first generation XL250 or 350 motors. 3 pretty distinct families of engines. I have looked at it, but not measured anything out, but I wonder if a 500 crank and top end would work with 250 cases?

The 500 swap is very easy on the CB250RS, slight modification to top engine mount and frame apertures need opening slightly to fit larger bolts. The hard part over here is finding a complete motor in the first place. I did wonder myself if the top end of the 500 would fit on the 250 cases - the bolt pattern looks correct but I guess it depends on oilways and clearences etc.

Will have a look at the carb suggestions by both you and CrazyPJ.

Not got a lot of time at the moment due to university commitments (exam time!) but fitting it in as and when I can.

Offline crazypj

  • Posts: 12991
  • Split personality, I fake being smart
Re: 250 Single - Performance Upgrades
« Reply #15 on: Apr 18, 2018, 18:05:31 »
There must be a few 500 motors around, one was fitted to the CJ250 stolen from me a few years ago (you are in Britain?)
'you can take my word for it or argue until you find out I'm right'
Best thing I ever overheard
"yep, PJ's my boss, he taught me everything I know, just didn't teach me everything he knows"
Brian Morgan, 1982

CB360's,  http://www.dotheton.com/forum/index.php?topic=11736.0
XS650,  http://www.dotheton.com/forum/index.php?topic=11922.0

Offline ex119x

  • Posts: 46
Re: 250 Single - Performance Upgrades
« Reply #16 on: Apr 19, 2018, 01:58:03 »
The FT500 Ascot, the XL500S and the XR500 all have the same basic motor. Th RFVC stuff is next generation and all different.

Offline paddyshepherd

  • Posts: 57
Re: 250 Single - Performance Upgrades
« Reply #17 on: Apr 20, 2018, 12:49:10 »
Not a single engine from any of those bikes coming up on eBay at the moment - since not many of the parts are available its better money to split the engines and sell each part separately.

Best I can find at the moment is an XR500 casing set with crank, gearbox and cylinder barrel. Would have to piece it up from there.

Offline crazypj

  • Posts: 12991
  • Split personality, I fake being smart
Re: 250 Single - Performance Upgrades
« Reply #18 on: Apr 22, 2018, 11:18:45 »
When CJ was stolen/recovered around 2010 (not worth returning to Britain for at the time, I only paid 30.00 in 1986)
I started looking around for a FT/XL/XR 500 motor. There were none available in USA at reasonable price. I take a look every so often, they are not common on eBay USA so will be far less common in UK. I don't think Americans realise the UK market has less 'restrictions' so a much greater diversity, there is 'a bike for everyone' of some capacity, manufacturer, etc but only so many people to buy them.
In 1980 I bought a 197? Yamaha DT400MX. Turns out, only 90 were ever imported into Britain. The market for 'large' trail bikes has always been limited, the conditions favour lighter bikes (much easier to haul out of a bog  ;D )
« Last Edit: Apr 22, 2018, 11:22:17 by crazypj »
'you can take my word for it or argue until you find out I'm right'
Best thing I ever overheard
"yep, PJ's my boss, he taught me everything I know, just didn't teach me everything he knows"
Brian Morgan, 1982

CB360's,  http://www.dotheton.com/forum/index.php?topic=11736.0
XS650,  http://www.dotheton.com/forum/index.php?topic=11922.0

Offline paddyshepherd

  • Posts: 57
Re: 250 Single - Performance Upgrades
« Reply #19 on: Jun 01, 2018, 07:48:36 »
So... I've been kinda sat on this for a while pondering over my options, I haven't really touched the bike itself much recently.

I think for now I'll stick with the 250 unless a good deal comes up for a 500 motor or complete bike.

I'll probably fit a high comp piston (I've found a 10.5:1 in standard bore, stock is 9.3:1) for now until the engine actually needs a rebore.
I'll get the cam reground with a slightly more aggressive grind (more of what we'd class as "fast road" set up)
The exhaust will be completely custom, either stainless or Ti. Will probably run open "shotgun" pipes out one side with an internal baffle.

It will never be a flying machine, but I think with some good tyres and a bit of a diet, the above should help it out somewhat.

I've found a good deal on a Keihin CR33 Carb to import from Japan. Comes with adapters and whatnot to fit the 250RS as they were much more popular over there. Also comes prejetted. Will still need a bit of a play around, but should be a good base point in my mind. I have the option of the standard kit or the Bito R&D Tuned version - I'm currently trying to find out the difference, but as all the info is in Japanese, it's proving a little difficult. I've e-mailed Bito to ask them and currently awaiting a reply.

What do we think? Depending on well how the carb works I may fit a quick action throttle too.

I recently rode a stock RS and it did bother me how unresponsive the stock carb felt.