Seca 750 stumble on take off

I've got an '83 XJ750 Seca that is running great except for one problem: when starting out, or when the RPMs drop down low (under ~2500) it has zero power and sputters and stumbles. Once you get moving and over about 3000 RPM it is smooth and strong.

I've cleaned the carbs, adjusted valves, sync'd, and colortuned. It idles perfect. No surging. Starts easily with full choke when cold, no choke when warm. Idle stays smooth even as it warms up.

Stock air box, stock filter, stock jets. The previous owner put a Vance & Hines 4 into 1 on it though.

I live at about 6200ft, and rarely ride below about 5000.

The problem is worse when its hot out and the engine is hot. Starting out in the morning on a cool day its almost unnoticeable. Coming home on a 90 degree day is a challenge to get moving after a stop light. This makes me think rich.

But it idles and starts so well and runs so good on the top end, I don't think it is a pilot screw setting or main jet issue. Definitely something in the low speed circuit.

If this was an "normal" carb, I'd probably drop the needle down one notch and see what happens. But with the non-adjustable needle and CV carbs, I'm not sure what to do next.

A few ounces of Sea Foam in the tank seems to help a little, but doesn't totally eliminate it.

Any suggestions?

 

could be that pilot jet. id take a look at my plugs and see if it is in fact rich or lean.
The pilot circuit controls the mixture at idle and up to about that 3000ish i think? (i could be wrong on the number there)

i had a very similar issue. some days it would be perfect and some days anything below 3000. it ended up being the orientation of my fuel filter. had an inline fuel filter laying horizontal and it kept a huge bubble in it. once i changed the filter position everything since has been fine.

good luck.

 

My first thought too. I'm wondering if the aftermarket pipe makes it a tad rich in that range.

Is the pilot jet an air jet or a fuel jet?

Plugs looked pretty good last I checked. They didn't seem rich or anything. I should probably pull them and take a look though since its been a couple thousand miles since I did the tune up. (the problem has always been there both before and after the tune up)

Interesting about the fuel filter. I don't have one at the moment, so that wouldn't be the case here though.

 

there is a pilot jet for fuel and a pilot air jet. so there are both.