Fuel type

verticaltwin · Posted: June 22, 2005, 9:39 am Post subject: Fuel type

Being kind of new to the xs650's I'm just wondering what you all do for fuel. Are these bikes made to run unleaded or should I be using a lead substitute or some other top end lubricant? Are the valve seats hard enough to just run pump gas?

VT

mo650chopper · Posted: June 22, 2005, 10:31 am Post subject: Re: Fuel type

I run regular 87 octane unleaded without any problems.

flavaz33 · Posted: June 22, 2005, 9:35 pm Post subject: Re: Fuel type

After doing all my odds, it was running kinda crappy, some more tweaks and trying 89 instead of 87 its running pretty good. so ive decided to stick with the 89.

robertpokerplayer · Posted: June 24, 2005, 2:51 pm Post subject: Re: Fuel type

I run whatever is cheap, 87, the only problem I have sometimes is if the bike sits for a little while the gas goes "stale," a little water remover tends to take care of that.

79xs650 · Posted: April 24, 2006, 10:17 pm Post subject: Re: Fuel type

I have better throttle response with the 93 octane...and it idles smoother

grizld1 · Posted: April 25, 2006, 10:19 pm Post subject: Re: Fuel type

Valves and seats hold up well to unleaded fuels. I run the highest octane pump gas I can get; 93 octane when it's available. That's the closest you can buy to the "regular" of 30-odd years ago that these engines were designed to run on.

Tomterrific · Posted: April 26, 2006, 7:04 pm Post subject: Re: Fuel type

Please use high octane. That means 93 around here in Ohio. These bikes may have been made to run on regular leaded but I'm old enough to remember leaded regular was 94 octane depending on the brand. I read where 650 pistons are holed every summer from detonation. Give your poor engine some room to keep from blowing up and use premium gas.

I won't chance a holed piston for 20 cents.

Tom Graham

grizld1 · Posted: April 27, 2006, 8:02 am Post subject: Re: Fuel type

I've autopsied quite a few dead XS650 engines over the years. Every time I've found a stocker with a hole in a piston, it could be attributed to one of two causes--relieved exhaust and/or intake breathing with stock jetting, or a worn advance governor dialed in to the idle mark and throwing wide on full advance. Tom's quite right--octane rating has to do with additives which slow spark travel in higher compression environments, and higher octane fuel will give a bit of a safety margin. This has as much to do with the engineering of the combustion chamber as anything else. For example, my 2003 SV650 Suzuki operates at around 11:1 and runs premium pump gas. A Harley (or XS650) engine with compression bumped that high will require race gas or octane booster added to the fuel.

BTW, that margin of safety is not a substitute for proper jetting, ignition inspection, and reading of plugs for signs of detonation.

Phred · Posted: April 29, 2006, 2:51 am Post subject: Re: Fuel type

Another thing to consider is static vs. dynamic compression. Depending on your cam (or cams), even though your pistons may be 11:1 your *actual* compression may be much less, such as 8:1. This is because the valves are often held open during part of the compression stroke. Compression obviously will not build when the valves are not closed. At high rpm the compression can actually increase (slightly) due to high intake velocity creating a slight "ram" effect.

If too much compression is causing detonation, and better fuel, timing and jetting corrections don't fix it, a low-buck fix is to re-time the cams to allow a bit more valve overlap. Of course, only minor changes should be attempted, and clearances must be checked carefully, but this is much cheaper than new pistons. This is often done when the heads have been cut, and new heads are either unavailable or replacement cost is too high.

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