GS 650/550 carb question

[Gateway]

I recently rebuilt the carbs on my GS 550 and had no problems at all. Before the filter or gas tank was in place I hooked up a weed eater gas tank let the carbs fill and fired it right up. Ran ok so I put it back together, warmed it up good and went about fine tuning.. all is well in 550 land. This weekend I did the carburetors on my 650 and did the same thing and it would only run with the choke on. I thought what the heck? Took 'em back off & looked at them and found nothing wrong.... put'em back on... same thing. A buddy of mine said " pu the air filter back on " well, I did not think it would make any difference but I tried it. Fired right up and ran ok, but a slight dead spot. Put the top on the air box and it runs like a champ.So, should I be thinking all is ok? Or ...not??

Thanks!!

Brian

[Steve]

Yep, these carbs are sensitive to airflow. Not just how much, but what it looks like. The airbox has what amounts to velocity stacks inside that smoothe the airflow through the carb. Not having the airbox in place leaves rather turbulent air, which wreaks havoc with trying to add the proper amount of gas.

If you want to try the carbs without the airbox, fold a shop rag in half and clamp it over the carb intakes. You only have to clamp over the outer carbs, just make sure it is taut over the inner ones. That seems to work decently enough to even take a ride around the block, but is nowhere good enough to do any jetting checks.

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[Gateway]

Thanks!! I guess coming from restoring vintage MX bikes there is a definite learning curve. You could fire up a YZ 250 with no filter at all.

[gs11ezrydr]

Yep all should be well with air box installed.I believe they are called CV carbs for constant velocity and that is what you have with the air box installed.

[Steve]

Yep all should be well with air box installed.I believe they are called CV carbs for constant velocity and that is what you have with the air box installed.

Actually, you have rather constant velocity over the jets whether you have a stock airbox, pods, velocity stacks or ... nothing.

The difference is in the quality or smoothness of the airflow.

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