4 stroke 250 engine pre 85 which is the best air cooled

[jimg1au]

looking for a 4 stroke 250 engine to put in a flattrack frame for classic dirttrack.i have a single downtube frame for it to go in.engine must be air cooled.
thanks
jim

[sudman]

My pick would be the RFVC XR250 twin carb, they are by far the fastest 250 4 stroke in the Evolite's class here in UZD

[Hoony]

I agree with Sudman, i had two of those XR250's both the 1984 and 85' models. and they were indeed fast for a 250cc 4 stroke in standard trim.

The only problem these days would be finding a good engine as they suffered from cracked heads between the Valve seats.

What about a TT250 engine? they may have been DOHC 4V in the 1984 model, i know the 1981/2 models were only OHC 2 valve engines.

[sudman]

The TT250 4 valve goes ok & it was a twin carb as well but they are basicly the same engine as the tt350 so it is heavy for a 250.

[jimg1au]

are the xr250 twin exh ports i have a single downtube frame.also can they be coverted to single carb for methenol use
jim

[Hoony]

Hi Jim,  1984/85 are indeed twin exhaust ports (as well as twin inlet) i don't know if they can be converted to single inlet but anything is possible. i do know that 1986 XR250 onwards were single carb models, i don't know if the heads were different or the manofold was 2 into 1.  what carb are you thinking of running 34mm dellorto ?

[sudman]

Xr frame is a single down tube type & you could just make up a tube manifold to suit.

[firko]

Jim for my 2c worth.....I'd pick the '84 XR250 but why bother with methanol? I suppose it'd be an advantage if you were chasing an extra bit of oomph to get that little bit in front of the pack but in reality for a club day, once a year Nats racer why not just keep the twin carbs and run it on petrol? Lifes's complicated enough without going through the dramas of converting to a single carb setup and then going through the pain of jetting the bugger.

When you do go looking for an engine you'll find that Hoony's right, most of the heads will be knackered with cracking between the valve seats. The problem was caused by there not being enough metal in the casting and the engines running too hot due to too little finning. Ballards helped the cooling problem with their 'big fin' welded fin conversion, all of which were done by KK's own Chris Ellis who was the "aircraft trained 'heliarc welder" mentioned in the Ballards (XR's Only) catalogue of the day. Chris literally did hundreds of these fin conversions which also included welding some extra meat around the valve seats so when looking for an engine, if it's got welded fins, there's more chance it'll be OK.  

[sudman]

I raced one back in the day at National level & had no problem with the head as we used a oil cooler.

[Hoony]

here you go Jim, a few old photo's from the 1985 Model i had.

[Nathan S]

The 4-valve TT250s didn't come out until 1985, but the very similar twin cam XT250 motor was definitely available as a 1984 model.

Stock, the XR wins.
Modified, I think they'd end up even but the Yammie would be more reliable.

The fact that the same basic motor is still fitted to the TTR250, while Honda had several major revisions to end up with a motor that was not obviously better than the TTR donk, says something.

[TT5 Matt]

pritty sure the single carb manifold bolts on the twin carb head,the twin carbs were on a angle to clear the centre mount single shock where the single carb had the shock offset to the left to clear the larger carb which came out on the right hand side on a slight angle but check for fit before laying out the $$$ its been a while since i worked on those 250's

[LWC82PE]

My pick would be the RFVC XR250 twin carb, they are by far the fastest 250 4 stroke in the Evolite's class here in UZD

That would be the first model i wouldnt pick. They are notorious for cracking heads.

[suz125]

cbx250 is same as xr250 radial valve motor

[Slakewell]

The cracked heads can be fixed I'm doing one for mate right now there a barstard to thou and not paying mates rates it would be expensive.