Won't run in the mid-range. Out of ideas...

[GTCarlos]

Hi All, I have a new problem too. Not as dramatic, I hope, as Buellers cam, but perplexing. The story goes...

I rode my '98 Skorpion Tour to work the other day. It's about 20 miles on the freeway at speed, 4500 RPM +. Everything ran well until I got off the freeway. At the bottom of the off-ramp it died. As I was at a light at the head of a line of cars, I just pushed the Start button It started normally but wouldn't sty lit below about 2000 RPM. I limped the last blocks to work and trucked it home the next evening.

I'm still getting to know the bike. About 100 miles ago I fouled a plug so I leaned it out fairly severely. It ran great so I never re-checked it. Pulling the plug after my 'event' I found it looked extremely white & lean. Being an optimist I changed the plug, raised the needles, ensured both carbs are opening and put if back together. No change.

The bike starts normally with or without the choke but dies in a moment unless you catch the throttle up to 2K RPM +- or more. It runs normally there and *seems* to produce normal power. As I don't know the bike all that well I'm at a loss as to where to begin. My thoughts are a burnt valve, hung de-compression system or maybe even an ignition timing problem. My only clues are the sudden onset at speed and a very white spark plug.

So first, can I even check the compression (if I go get a 12mm gauge fitting)? Is it time to pull the head to have a look around or is there an easier approach? Your collective wisdom, or entertaining guesses highly appreciated.

Regards.

-Danny

[DAVID THOMPSON]

i can tell your in the usa home of crap in the gas
try www.pure-gas.org and try a tank full and work on sorting it out on the gas it was designed to run on

compression is hard to take on these as the cam is designed to reduce compression at slow start speed

so compression checking is a bit of a crap shoot

dave

and i am looking in my pdf files for one on the carbs if i find it i will forward it too you (i hope its find-able )

[dandywarhol]

When you say you "leaned it out quite severely", how did you do this? By lowering needles, changing jets or just by altering the pilot screw?
If it was the latter, then it will have little or no effect on running at 4000 rpm on medium + throttle.

[Chuck in Indiana]

When you say you "leaned it out quite severely", how did you do this? By lowering needles, changing jets or just by altering the pilot screw?
If it was the latter, then it will have little or no effect on running at 4000 rpm on medium + throttle.

That was my first thought, too. What did you do?

As David alluded to, I was in Florida on my Centauro when it suddenly decided to not idle coming off the interstate. Same scenario. Wouldn't run below 2K rpm. Decided <sigh> It's a Guzzi, so it'll get me home. By the time I was in Tennessee, it was running normally again. No doubt a load of bad gas.

[GTCarlos]

David, "Dandy" & Chuck,

Thanks for your comments. I leaned it only by dropping the needles to their extreme. I don't recall but I might have also gone down xx5 on the main jet of the small carb as I happen to have a spare floating around. IIRC it was a 135 to a 130 ? Anyway a small step if I swapped them at all. Then I ran it for perhaps 0-80 miles over several days with it performing nicely.

I also don't recall when I'd last bought gas but the problem developed on my way to work in the early a.m. so I know it had at least 30 miles on that tank if it's gas related. As the bike is down I haven't added or swapped in different fuel. I'll try that later in the week.

In a haze of optimism tonight I dismantled the carbs to make sure everything was clean and secure, which it was. I also gapped and fitted a new -8 spark plug. All that resulted was the gathering of a few more clues.

1. The bike was definitely more prone to catching and closer to running below 2K RPM +- with full choke on.
2. At minimal sustainable RPM, c. 2000 or so, it would "cough" every few seconds. That made it almost die and produced a "clank" sound in the head somewhere.
3. The "cough" was accompanied by a back-fire with a short blue flame from the (HoleShot) pipe. See, there are advantages to trying to fix your bike in the dark!

So now I'm wondering about the decompression mechinism. Could it fail rapidly like that? Would it produce those symptoms & make that noise? Would it alow the bike to start and then die a second later if you don't jocky the throttle? And , would a burnt valve allow for a flame to come from the pipe or is that a sign of something else?

Thanks again. I'm looking forward to your collective wisdom or collective guesses.

-Danny

[GTCarlos]

Yep, I'm definitely in the Land of Crap Gas, California. The BEST stuff you can get here is 91 avg. octane and 10% alcohol. And G-d only knows what in the "summer Blend" we're afflicted by at present.

The nearest station on the PureGas list is about 65 miles north. way too far for testing but I'll keep it in mind!

Thanks.

-Danny

[DAVID THOMPSON]

check at local boat dealers or boat docks

or a farm supply that sells fuel
pure gas .org may not list them

[breakwellmz]

Valve clearances?