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Discussion Starter · #1 ·
This morning on my way to work my SL350 started running very poorly at mid RPMs. It idles fine, and runs great (like it always has) above 5000 RPM. Under 5k (and above 2-3k or so) the engine sputters and chokes, and sometimes only runs on 1 cylinder. With the bike in neutral, just revving the engine, there is no indication the engine is running poorly. It is only noticeable under load.

Prior to today, it always ran great, and I never saw this problem. The carbs, which are Mikuni VM28s from a Yamaha R5c were completely disassembled in the fall for rejetting. After rejetting I put several hundred miles on it last year. They were drained for winter storage, and this spring, I have put another several hundred miles on the bike, with no signs of trouble until today.

I checked the obvious things that I thought could be the problem, that I could easily fix before I leave work today. The battery measures 12.3V with the ignition off, and with the engine idling the voltage is stable at 12.3V. Revving the engine to 4-5k in neutral raises the voltage to 12.6V. I think this is a healthy charging system for a 350 engine. I checked the electrical connections to the coils and the leads to the spark plugs, they all looked fine.

I also filled up with gas on my way in today, thinking that the gas might have gone bad/got water in it. The engine honestly ran worse after I refilled the tank.

My current thought is that the timing slipped, but I am not sure. My previous old bike had the electronic ignition upgrade, so this was never an issue. But I have points on this bike. Any other ideas? It doesn't seem like it is a fueling/carbs issue since everything worked fine when I last rode it on Friday. I am going to check the points/timing when I get home, but looking for other potential causes/fixes I can do before I leave work. Thanks!
 

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12.3V is bad. Your battery is on its way out.
I'd replace it.

A good battery should read 12.6V or more at rest.
 

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Discussion Starter · #3 ·
12.3V is bad. Your battery is on its way out.
I'd replace it.

A good battery should read 12.6V or more at rest.
Likely cause of running poorly though? we have a battery charger at work, just threw the battery on... hopefully it will build up enough charge to run all the way home.
 

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Absolutely, a bad battery will cause the bike to run like crap.
 

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^^^but if it turns out not to be the battery, check the points and timing. A narrow points gap from rubbing block wear will cause retarded timing, making the bike run poorly at lower rpm and could cause stalling at low rpm and/or idle as well. Points need to be checked every few thousand miles and the felt lubricated lightly to reduce wear on the rubbing blocks to keep the friction and wear to a minimum, which directly affects timing
 
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