high compression engine - can i run on petrol?

[nozila]

Hi all,

I recently purchased a race engine that from what I know, built for flat track/motocross. This engine has a compression of around 13:1 or slightly higher, high compression pistons, crank got a cut and rephased to 270 degree and the carbs are re-jetted to runs on methanol. The previous owner stated that I could detune it by fitting normal 750 pistons and rejetting the carbs to run on petrol.

My question is, can I just re-jet the carbs to be tuned for petrol without opening up the engine? And can I run E85 on its current setup?

Thanks,

Noz

 

[TwoManyXS1Bs]

Not really, on regular pump gas anyway. But, there's some avenues to pursue. Member kopcicle has posted a bit about alcohol mods, worthwhile to read thru some of his threads.

http://www.xs650.com/threads/the-red-bike.18430/

If the cam has very long durations, that'll bleed-off a lot of the compression, until you hit higher rpms, then it may be tolerable.

High octane racing fuel may be available.

If the camchain is stretched enuff, you could install a thicker headgasket. Try for 10:1 or less.

This kinda stuff is beyond typical mechaniking, best to study-up on alcohol mods and dust off the slide rule...

 

[angus67]

Search thru xsjohns threads on lowering compression. He took almost a full point off a stock comp with a streched cam chain and double head gasket.

 

[nozila]

thanks guys, I'll look into your suggestion. I thought I could kick start it but the PO told me he needed to put it on a roller to get the engine running yikes!!

 

[Signal]

Please post details of the carbs and methanol jetting when you rejet for petrol.
I bought a road going XS 650 with a 750 kit that measured 11.5:1 and it was a pain on pump petrol. I lowered the compression to around 10.5:1 by fitting two head gaskets and a thicker copper base gasket and it became a thing of joy. I had an adjustable cam sprocket so fixing the timing was easy.
If you are going to pull it apart measure the compression ratio before you start, knowing that and the thickness of the gaskets will take some of the guesswork out of the work you need to do.
Please let us know how you get on.

 

[weaselbeak]

The E85 might possibly work, but I wouldn't risk it. My son runs pulling tractors with high ratios on alcohol and those ratios give an inordinate amount of blowby, fouling the oil easily.

 

[Tomterrific]

You will need both the lower compression pistons and reject the carb much leaner. You could try E85 but the ratio of alcohol to gasoline is not fixed at 85% so the octane may not be high enough for your compression.

Tom

 

[nozila]

thanks guys. still deciding on what to do with it - sell it off or keep it