4 cycling 2 cycle

Joined
Aug 20, 2001
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#1
New to the forum so if this isn't the place just say so. I have a 1996 Beta Techno Trials 250, recently purchased and learning the trials thing. The bike runs excellent with the exception that it four cycles anytime that it isn't under a serious power demand. I think that the bike is bone stock ( according to the previous owner), the spark plug runs with a nice tan color and doesn't foul out. The engine has loads of compression and power. The MC1 that I am running at 50 to 1 has always worked for me with no difficulties. I did drop the needle 1 step with no results. My feeling is that the trials bike needs to hit right off of idle for instant power, mine is like it is loaded up but doesn't belch out the smoke. Maybe this is how it is supposed to be but I think it should hit sooner. Open for suggestions.
 
Joined
Dec 10, 2000
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#2
This irregular and erratic firing when under no load is inherrent to a two-stroke. it can be minimized with careful jetting, usually of the pilot circuit and, to a lesser extent, the needle, but you will never totally eliminate it. Start with the airscrew adjustment first.
 
Joined
Mar 15, 2001
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#3
I don't know if this helps you any, but I've had like 25 or 30 bikes, and out of all of them, the one that would four cycle the worst just happened to be the only trials bike that I ever owned (not the same brand as yours). It ran great otherwise. I'm wondering if it is aggravated by the large flywheel, low porting, and smallish carburetor. Could it just be the nature of the beast? I haven't ever been to a trials competition, so I can't say if this is normal...
 
Joined
Jan 25, 2001
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#4
Yeah my Beta definitely responded most to pilot jet mods as you are riding the bike from there all the time & it needs to clean out instantly.

That is after I threw away the junk carb that was on there before.

Fortunately the Italians decided to use a Mikuni on the later models like yours so its just a case of trying smaller jets until it hesitates & can’t be tuned out by airscrew any more then go back one. Mines running 22.5 but airscrew is fully in so really needs a 25, but don’t take this as gospel as mines somewhat older but I started with jetting specs from a late Beta & didn’t end up too far off.

PS think you should be down to ~ a 140 main