Question for carburetor experts - Cold Starting

[Peder_y2k]

Working on my low mileage (2100) '87 Yamaha Virago xv535 with the air cooled v-twin engine. When warmed up, it runs perfectly, idles smoothly at 1100rpm, has quick throttle response, and has no misfire or exhaust pop, and delvers 55-60mpg routinely.

Here's the issue: When cold starting (45*F), and the fuel pump has filled the carbs, I set the enrichment lever at full 'on' and the engine starts immediately, then runs for 2 seconds and begins to stumble to the point I must readjust the lever and can't find a point where the engine will keep running without both tweaking the throttle and fiddling with the enrichment lever to 'help' it along to where it has warmed up enough to stay running. Experimented with setting the lever 1/2 'on' but then it won't start untill readjusted to full on. Warm restarts no problem.

 

The carbs and fuel system are spotless clean inside, carb fuel level is correct, carbs synch'd, enrichment lever and linkages in excellent condition. SEAFOAM in fresh fuel.

 

I thoroughly understand these CV carbs and know more than most people about carbs, but on this one I am stumped.

 

-Pete, in Tacoma WA USA

[csdexter]

I used to have a '97 XV535 (wonderful bike, BTW, they don't make them like that nowadays) and it was more or less the same: starting from cold was a two hand job. I should also mention it was getting worse as the weather improved.

 

Ok, rephrasing:

- cold engine, cold outside (like winter cold): starts on the button with choke to full on

- cold engine, cool to warm outside (like a spring night): starts on second attempt with 1/8 ~ 1/4 throttle and choke fiddling

- cold engine, warm to hot outside (like a summer day): starts only with 1/4 throttle and no choke

 

I didn't mind, got used to it and thought it was part of its "character". One day, I took it in for a full service and when I came back to pick it up, the guy there (awesome mechanic) looked at me and went "You messed with the mixture screws, didn't ya?". I said, no, I don't even know where they are on this bike! (back then I actually didn't).

So he showed me and told me they were set to way richer than factory setting, probably because the bike has been restricted before. He also explained me he re-set them to factory and synchronized the carbs.

 

Of course, after that, the bike started on the button every time, except very cold winter nights when it would need full choke that you could move to 1/2 after the first 20 seconds or so and close completely after the first 2 minutes.

 

I don't know if that's your issue, but that fixed it for me.

[Guest tx2sturgis]

It might be that the idle mixture screws are set too rich. I dont know if the screws are capped on those carbs.

 

Or maybe the enrichner jet needs to come down a size or two. Since it probably has no 'fast idle' setting, its not that unusual to have to 'help' it idle using the throttle a bit when its cold. If setting the lever at 1/2 works then that is probably the answer to the problem.

 

Just guessing but its a place to start.

 

 

 

 

[Peder_y2k]

With this mornings cold start event, I first turned the pilot screws 'out' 1/4 turn and the start process was worse than before. I stopped trying after a few seconds and readjusted the screws 'in' 3/4 turn with the engine still cold and it started immediately and continued running at high idle without fiddling with it. I'd say the cold start mix was rich. The engine warmed up nicely and I could move the enrichment lever to 1/2 in about 30 seconds running, then completely 'off' at one minute. After some warming up, the throttle still has quick response off-idle, and the engine idles relatively smoothly at 1100RPM......... just not as smooth as before. But then it is winter temps here in the 40's and I wasn't out riding, just parked in the driveway.

 

I think I have the solution.

Thanks all.............

-Pete, in Tacoma WA USA

[Marcarl]

With this mornings cold start event, I first turned the pilot screws 'out' 1/4 turn and the start process was worse than before. I stopped trying after a few seconds and readjusted the screws 'in' 3/4 turn with the engine still cold and it started immediately and continued running at high idle without fiddling with it. I'd say the cold start mix was rich. The engine warmed up nicely and I could move the enrichment lever to 1/2 in about 30 seconds running, then completely 'off' at one minute. After some warming up, the throttle still has quick response off-idle, and the engine idles relatively smoothly at 1100RPM......... just not as smooth as before. But then it is winter temps here in the 40's and I wasn't out riding, just parked in the driveway.

 

I think I have the solution.

Thanks all.............

-Pete, in Tacoma WA USA

You can probably do better with the idle if you can adjust the mixture properly for each cylinder. They will all be set a bit different, there is no such thing as them being all the same, they are individuals, just like us,,,, but it seems you're on the right track.

[Guest tx2sturgis]

I'd say the cold start mix was rich. The engine warmed up nicely and I could move the enrichment lever to 1/2 in about 30 seconds running, then completely 'off' at one minute.

I think I have the solution.

Thanks all.............

 

You're welcome. Have fun!