Fuel Thread

[jumpjg]

Lately I have been having trouble getting my bike to idle right. The bike is a standard compression Mk III Interstate with 17k on the clock. The carbs were treated to the AMR Mikuni hard chromed brass slides trick (2006) and I'm running a Boyer. Before it was idling nicely at 1000 rpm with the air screws set a tad on the rich side to eliminate the flat spot just as the throttles are lifted off their seats (~1800 rpm no load), but suddenly a couple weeks ago it started to act up. I messed around with it countless times, adjusting the speed & air screws on arriving home from work (14 mile commute) & synchronizing with a manometer to finally achieve something close but not quite as smooth as it was before. Yesterday morning on my commute I had to flip over to reserve, so after work I topped off with 5.2 US gallons of 89 octane of a brand I normally don't use. Now the idle is better & the smoothness is back. Reflecting back, I'm thinking my idle troubles started about the same time as my last fill-up, and I'm thinking that I may change fuel brands. Certainly the previous fill-up could have had a slug of funk or maybe it was a coincidence, but I thought I'd ask the knowledge base here for their opinion; do different brand fuels make your bike run differently? In my case the questionable fuel was AMOCO, & yesterday's fill-up was Shell.

Thanks for your time,

Joe in St Louis

 

[JonC]

I have experianced the same issue on other bikes. Once on a trip far from home my Ducati ST2 started to run like crap wouldn't idle and was surging at low speeds. I had my wife on the back and we were a thousand miles from my tool kit, not that I knew anything about fuel injection anyway. I realized that I filled up the previous day just before finding a motel for the night, ran the tank out, filled up with a different brand and the bike started to run great again. It may have more to due with when the station got it's last fill up, that tends to disturb all the water and silt in the bottom of the tanks. If you are unlucky enough to get the fuel then, I think the result is a poor running machine.

 

[rons70commando]

My 1970 has the fiberglass tank so I have been using ethanol free fuel for a long time with good results. But when my source dried up I searched out a shop that sells race fuel; non ethanol and high octane. I bought a few gallons and I could barely believe how much improvement there was from pump gas. My idle jets no longer clog up, the bike idles better and I dont have to drain the tank after each ride now. My old Sportster runs better on the stuff also.

Just my 2 cents.

 

[concours]

I run AVGAS at home, cans in the garage. That said, I just completed a 1500 mile five state ride that saw me buying pump gas which included, among others, "REC90" and various other unknowns. Nary a hiccup. Ethenol stores poorly, but seems to run OK in most applications.

 

[rvich]

I buy 92 octane as I can tell a diference (I am sure of it, I think) when I buy the 87. With that said, I also have a 1987 Johnson 2-cylinder outboard that doesn't have an electric start either! I buy octane boost for that beast and not only does it start easier but I can tell by getting the boat up on step with a load in it that it really does run better. I can't say from brand to brand but higher octane in the old motors really doesn't hurt.

Russ

 

[Hobot]

If water and crud get in carb it can remain in bowl bottom so good to drain that before next top off. I don't think the octane really matters unless engine configuration and loading causes detonation other wise. In fact its better to run as low octane as tolerated for the faster burn efficiency. Of course if engine set up needs hi octane to retain its full spark advance combrustion pressure then yes better mileage on higher octane. I cut my motor sport teeth on the small outboards in the leaded gas daze, so I'd think maybe the ole motor is coking up Compression ratio or the timing is a bit advanced for lower octane to start as easy or rev up on full load. Definitely ethanol in gasoline lowers mileage and power unless timing and compression bumped up to get some use of its detonation suppression. Low octane fuel short of detonation does not cause rough runing, but for sure dirty fuel does of any octane.

 

[DogT]

Avgas, nothing better. Smells good too and is the right colour. May be problematical for your riding style though.

Try it, you'll love it, it certainly won't hurt anything. Last time I got it was about $5.70/gal. but the bike loves it. Doesn't clog the carbs either.

Dave
69S