How can this happen?

[nastyjones]

My Mate has a 1975 Suzuki gt 250 two stroke twin that is exhibiting some very weird symptoms. When he bought it, it would only run on the left cylinder. The right cylinder refused to fire even though we have spark. We changed the points & condensers & still only ran on the left. We thought it must be a carb issue. I cleaned both carbs in my ultra sonic cleaner, no difference. Then we put another spare cleaned right hand carb & I'll be damned if it now only runs on the right. We changed it back to the original configuration & it went right back to only running on the left. We are baffled as to why changing carbs causes the bike to run this way.

Am I overlooking something obvious?

[elbert]

Are one of the outlets from the fuel tap blocked? Could be that you switched outlets from the fuel tap when you changed to the other set of carbs?

[nastyjones]

no, seems to flow equally from both taps.

[TLRam1]

Make sure you have a strong battery

[nastyjones]

battery is holding a fresh charge. It boggles the mind to think that changing the right carb affects the left cylinder.

[tz375]

Are both float bowls full of fuel and do you have a strong spark on both sides?

[nastyjones]

Lots of fuel, spark seems strong, timing is spot onl

[tz375]

I'd pop that "good" right carb back on and see if the problem switches sides again. If it does then the question is what changed between the two carb set ups.

In other words, is it really following the carb swap or coincidental. If it really follows the carb swap it shouldn't be too hard to work out what changed.

It's possible that say a slow jet was blocked on the original carb and partly blocked on the left so a new carb worked on the right but the left failed to fire now. Did it run "fine" at all revs/throttle positions with either set up or was it bad low down or high up?

[nastyjones]

It seems to follow the carb swap. We have changed both the right & left carbs several times & every time the opposite cylinder refuses to fire. In other words, if it's not running on the right side & I replace that carb, the right side now fires but the left has stopped firing.

To answer your question, it has never run "fine" at any rpm

You think I should clean them again?

[nastyjones]

Alright, this is new. My mate had the bike running on the right cylinder. On a whim, he pulled the plug wire off the right side while it ran. The right side quit of course & the left side immediately fired itself up & ran on that side.
So now we're thinking electrical. Definitely a WTF moment.

[tz375]

You know what they say - if it seems like a fuel problem it is probably electrical.

That sounds like the old low battery trick that afflicts some bikes. Try hooking up a car battery with jumper cables and see if that fixes the problem.

[jabcb]

To make the low battery trick more confusing, some of the old T-series bikes seem to run fine without a battery.
Here is one: http://www.youtube.com/watch?v=YCiE2l6pC6o

Worth noting however...
The T250/T350 & most years of GT250s don't have a voltage regulator. They use the battery as a voltage regulator. So it doesn't take much rpms for you to burn out the lights.

[titan performance]

Good point JAB......I have two 75 T500m's,....one will run without the battery, the other wont.
What's going on there?

[jabcb]

The electrical systems are pretty marginal. Not sure how the +/- variations added up to cause this.

The T-series use single phase alternators -- without a battery the rectified voltage cycles between 0V & the maximum voltage. But the ignition coils act as inductors so you don't get the simple sine wave curves you would expect. Alternator output when the points open is certainly one of the factors influencing the ability to run these bikes without batteries.

[ja-moo]

If you have a small battery charger, stick in when the engine is running.....