Carb Cough

[fin_676]

As tourbox said that perhaps a larger main jet may help
As all the jets flow fuel all the time sometimes changing the less obvious jet will assist in a good tune
The main jet is partially blocked by the slide needle but not fully shut off as the slide moves up it draws the needle out of the main jet as the needle is tapered the further up the slide goes the greater quantity of fuel is drawn through the main jet
By increasing the size of the main jet you may add too much fuel at higher engine speeds so it is a wee bit of a balancing act to get good fuel ratio at all engine speeds
If it proves that 1 main jet is too small and the next size up is too large incremental steps can be achieved by raising the needle further on its taper by adding brass washers under the needle seat up to a max of 4 washer

Brian

 

[tourbox]

Very good explanation of the needle movement fin 676,Brian.
tourbox

 

[fin_676]

Thanks tourbox i have been trying to fully understand carbs for many years and when the rest of the world moves to fi i start to understand just a wee bit of what makes them work

Brian

 

[FLDL02]

Looked again for leaks with WD 40 sprayed generously around seals with no change in engine. Diaphragm looks intact. Will a washer or two richen the mixture or just add fuel in the idle range like the enricher knob?

The Pilot is 48 and the main 195. There were no washers on the needle. I have inserted one #4 washer to shim the needle. Idle mix screw was out 1 and 5/8 turns. I will tune and test ride Monday.

 

[fin_676]

The jets that are fitted do seem to be on the large side
Having a washer under the the needle seat raises the needle throughout the rev range so there is a wee bit more fuel throughout the range and will give you approx a 196 jet size

Brian

 

[Bodeen]

I agree with Brian, that jetting seems awfully large. Unless your bike is modified quite a bit. Whats really surprising to me is that you get 45mpg.

 

[Rockdrummer]

I have sprayed around the intake seals with no changes observed. Must I use a fluid sprayed through a tube touching the entire circumference of the seal to be sure or is a flammable vapor sprayed in the intake seal vicinity enough to reveal a leak?

What does it mean that the cough did not happen even once in the southwest desert? A leak goes away there... or the lean mixture is fine there?

Have you ruled out the ignition module?

 

[FLDL02]

I've got a 48 PJ and a 195 main jet. Shimmed now with one #4 washer, Mix screw at 2 and 1/8 out, idle at 1000 RPM. Just rode the other day to test and tune. Problem is the weather was hot and dry around 30C. The bike runs fine in that weather like it did in the desert. I i need a cool wet day/night to test again. It did not cough at all with the current tuning. When I get the right atmospheric conditions I will test ride again to see the result. The washer is the only thing I changed. I will post the results when I have them.

 

[flst89]

If I understand your original post correctly, you say this problem is new.

So we know that in its current configuration, this carburetor ran fine before the problem occurred.

That rules out improper jetting.

The problem is most likely lean mixture, caused by an obstruction in the low speed circuit, an intake leak, bad accel pump or damaged diaphragm.

Clean the carb and replace the intake seals, and also replace the vacuum hoses to the voes and petcock.

 

[HDDon]

When you have the carb apart make sure you replace the accel. pump diaphragm.