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Reputable East Coast XR250 Engine Builder

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Its a shame you guys so far away in So Cal I have done over 10 Xr300s. Another option if you wanted Copy cat pro built 300s. If your lower end good shape. You can hand bore cases like we did,no case split. I know all parts Only would need carb spec again from Stevethe. You could ship head to So Cal as far as boring and sleeve Ship to So Cal also I would need ask Mike first. He has done all our Machine shop work over 30 years.Machine shop work super important as most will over bore and  press new sleeve incorrect. On 8 300s we have the Xrs only welded big fin heads. On all 10 of them Xr4 oil coolers. Mikes own 300 only oil cooler no big fin head. I think his is for sale also because of our age we are now on built 230fs. We also have a Stroked Xr250 305ccs. We also have our 1 off custom grind web cam. Its not for go fast only to reduce off idle snap slight more on top.Also more easy on valve train all of us have had zero problems or overheating.Stevethe was the master 1st build on our motors. As you all know only weakness on late xr250s intake valves.You must never use OEM valves. To build a copy cat Xr300 does cost $$$$ not one thing expensive. It does add up all the parts.

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4 hours ago, bajatrailrider said:

Its a shame you guys so far away in So Cal I have done over 10 Xr300s. Another option if you wanted Copy cat pro built 300s. If your lower end good shape. You can hand bore cases like we did,no case split. I know all parts Only would need carb spec again from Stevethe. You could ship head to So Cal as far as boring and sleeve Ship to So Cal also I would need ask Mike first. He has done all our Machine shop work over 30 years.Machine shop work super important as most will over bore and  press new sleeve incorrect. On 8 300s we have the Xrs only welded big fin heads. On all 10 of them Xr4 oil coolers. Mikes own 300 only oil cooler no big fin head. I think his is for sale also because of our age we are now on built 230fs. We also have a Stroked Xr250 305ccs. We also have our 1 off custom grind web cam. Its not for go fast only to reduce off idle snap slight more on top.Also more easy on valve train all of us have had zero problems or overheating.Stevethe was the master 1st build on our motors. As you all know only weakness on late xr250s intake valves.You must never use OEM valves. To build a copy cat Xr300 does cost $$$$ not one thing expensive. It does add up all the parts.

Have you ever tried Keihin PWK on the XR250/280/300?  Since Keihin PWK28 is working so well on CRF230/240/250 I can only assume we would get same great results with XR250/280/300 as well.  I wonder how a big Keihin PWK would be on a CRF270 stroker?

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17 hours ago, VortecCPI said:

Have you ever tried Keihin PWK on the XR250/280/300?  Since Keihin PWK28 is working so well on CRF230/240/250 I can only assume we would get same great results with XR250/280/300 as well.  I wonder how a big Keihin PWK would be on a CRF270 stroker?

On Xr300 only used 35 pumper and flat slide 34mm some bored to 35mm. On Crf230/270cc Mike is building his will use 28mm PWK carb. You already know the hold up waiting for 6mm stroker crank. My 230 is only a 254cc with Pwk carb I dont think we would want to use any other carb as they run so good. Will try to get picture of big fin Cyl head Xr300.

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3 hours ago, Off-Road Adventures said:

How would a stock XR400 carb work on an XR250 that has been punched out to 300?

Only a guess but likely a bit big unless the engine spent the majority of its time right at redline.

I used a very old formula to check sizes for 300cc @ 10,000 RPM:

  • Min (Torque):    30 mm
  • Mid (Mild/Stock):    33 mm
  • Max (Race/Wild):    36 mm

For CRF230 with 270 cc stroker @ 10,000 RPM:

  • Min (Torque):    29 mm
  • Mid (Mild/Stock):    31 mm
  • Max (Race/Wild):    34 mm

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Here is why I would not use Xr4 carb Xr250. By removing the stock carb and changing to 36mm pumper,and BSR 42mm carb. It made more power then the 440 big bore. Means buddy had 440 stock stock carb,mine stock 400 with carb. No contest roll on drag 10 times. On our stock Xr250s with just 34 flat slide very little improvement.

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2 hours ago, bajatrailrider said:

Here is why I would not use Xr4 carb Xr250. By removing the stock carb and changing to 36mm pumper,and BSR 42mm carb. It made more power then the 440 big bore. Means buddy had 440 stock stock carb,mine stock 400 with carb. No contest roll on drag 10 times. On our stock Xr250s with just 34 flat slide very little improvement.

We tried XRs Only 32mm Mikuni TM kit on XR250 and while mid-to-top was much better low-to-mid suffered miserably.  Carb size MUST be matched to the engine operating range you spend the majority of your time using.  Since my buddy was almost always riding slow short tight stuff the bigger carb just could not be made to work out.

Bigger is NOT always better...

Edited by VortecCPI

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1 minute ago, VortecCPI said:

We tried XRs Only 32mm Mikuni TM kit on XR250 and while mid-to-top was much better low-to-mid suffered miserably.  Carb size MUST be matched to the engine operating range you spend the majority of your time using.  Bigger is NOT always better...

The Jetting we use on the 34 flat slide completely different then Xrs only and Sudco. On stock late Xr250 no loss of power down low but little gain. 

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52 minutes ago, bajatrailrider said:

The Jetting we use on the 34 flat slide completely different then Xrs only and Sudco. On stock late Xr250 no loss of power down low but little gain. 

Ah...  Very interesting!

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