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1984 XR250RE head change

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Hi Guys

I bet you dont get many questions from the Isle of Man. I hope you understand the terminology.

Bit of a long story,friends twin carb xr going to the dump so decided to save it, replaced conrod,big end, little end, gearbox bearings etc. Ran ok but carbs all over the place, changed jets needle heights all over the place but always had a big bog even when trying to pull off from stationary, would start from cold and run fine until warmed up then bog would appear, very hard to start when hot. In the meantime replaced all rockers and sub-rockers and added a 1mm o/s Wiseco as bore was a bit worn as well, still the same, gave up !!!

Now the problem - I have put on a single carb head, looked and measured the same just combustion chamber slightly different, used a carb off an XL250, still RFVC motor and exhaust headers. Will it go no chance, i got it going with easy-start so mechanically ok but will not go any other way.

Has anyone else put single carb head on twin carb motor ? Help!!!!!

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There is more different in the later model single carb ('86 on) heads than you think. The exhaust valves are actually smaller and yes the combustion chamber is a different shape because they run a completely different piston. To bolt on a single carb head would be a gamble as you could have valves hitting the piston or anything, its not designed to work on that model.I recommend re-installing the twin-carb head.

It IS possible to get the XR250RE carb jetting right, it is a right pain in the butt but I have done it!

Here are the specs I ran in the carb.

Primary Carb

-#105 main jet (108 standard)

-#45 pilot jet (standard)

-XR250 RF slide, needle and needle jet, needle 2nd clip position from top (standard)

-float level 19mm (18mm standard)

-airscrew 1 1/4 to 1 1/2 turns out

Secondary carb

-#102 main jet (105 standard)

-XR250RF slide, needle and needle jet, needle 2nd clip position from top (standard)

-float level 19mm (18mm standard)

Secondary carb opening clearance- standard 3.5-4.0mm

You may need to slightly adjust these settings if your running a high-comp piston- say 1 size bigger on main jets but testing would tell, good luck!

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Thanks for the jetting info I have had lots of plays with the primary carb but the only parts I can get in the UK are the main jets, its really tough taking the twin carbs on and off to just change a jet isnt it. I think the carb slide and body are worn beyond futher use but I may have to try them again. I would still like the single carb option to work and cannot see why it wont, I would guess its got slightly less compression than std head, values dont hit the piston as it did run once, thanks again I will have to get back to it.

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I am running an '84 XR250R with a K&N filter, snorkle removed, header ground and wrapped. I also have an XRsonly exhaust and I am running 123 main, 120 secondary jets. A 45 main which I might take to 47 and needles are stock position. I spoke with XRsonly and they recommended 120 for both, but I was running slightly lean at the top end(white plug).

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