97 F650 rejetting with stock jets...how could it have gone so wrong?

[Homer Susan]

f650 funduro 97 45000 miles

I bought the bike with 11000 miles on it a few years back. The previous owner had rejetted, cut holes in the air box and put a performance pipe on to match. I didn't think too much of this at the time but... the engine had been running so rich I started to call the bike little piggy. It was backfiring and behaving badly, black smoke etc.. so

I changed the pipe for stock and adjusted (thanks to my friends) the aftermarket jets. but still it was using too much fuel. So I thought why not go back to the original stock jets. I had them installed in January and things were going fine until, the bike sat for about a month while I waited on a new starter switch to be sent from the states. It had started to stall a bit before that, but not too bad. Now the plugs are fouled within a day... black as night, and the bike won't start without a new set put in. I have a good mechanic working on it here and he says the jets are too big and letting too much gas in. He wants to try smaller jets.

Have any of you run into this problem on the f650?

Does anyone have any information on aftermarket jets that are smaller than the stock. I found some info on f650. com website but they were for bigger jets.

also the airbox was modified..... should I go ahead and buy a new airbox and try that?

How about problems with running the engine too lean with little tiny jets? Should I be worried about that as well?

Thanks
Susan

[gsworkshop]

A lean mixture can be very dangerous for any motor. The temperatures go higher and you can easily burn valves etc.
Rather too rich than too lean but then again it should be perfect because a happy engine means a happy biker.
I have never worked on these models before but I will try and help as I understand a fair amount of how the jetting work.

[gsworkshop]

To Start I would recommend that you get the air-box back to original.
A carb will normally have 4 jets all operating different stages of acceleration.
1 - Idle Jet
2 - primary jet - operate when of idle to roughly about 4000rpm
3 - Needle jet + Needle - from 4000rpm to full throttle
4 - Main jet - This jet will only limit the fuel flow at full throttle and because one would only ever run your vehicle at full throttle all the time in racing conditions, this jet will have almost no influence on the carbs performance in normal riding conditions.

The idling jet will have little effect, but there would be no sense in changing this jet so I am sure it will be standard.
On the F650 Carb I do not see a primary jet so I would assume that the next stage will be the Nozzle needle and needle jet. Again there don't seem to be a needle jet but it seem as if the venturi is doing the jetting in this case.
I am sure the jetting problem will be between these two parts that will have to be replaced as a pair and because of the dual carb arrangement it will mean you need two sets.
Part number for the Nozzle Needle - 13 11 2 343 397
Part number for the Venturi - 13 11 2 343 395
Main jet - 13 11 2 343 396
Because I have never worked on this carb before I would recommend that you double check the info.
The idea is just that in most cases only the main jets are changed which hardly make a difference unless if your are constantly racing around at full throttle.
The mixture can also be regulated normally by adjusting the needle position up or down.

[gsworkshop]

Before you do anything make sure you reset the valve clearances!!!!!

[Homer Susan]

Thanks
GS workshop... you've made some good points that I will be sure to pass on to my mechanic..
He has a friend coming from Miami on the weekend and I am hoping that he is able to carry parts down when he comes. My plan of leaving at the end of next week doesn't sound too promising but.... just maybe it will work out.

Cheers
Susan

[Walkabout]

Susan,

Reading the sequence of posts in this thread, for interest sake, I am confused.
If I get the drift, you have set the jets back to "stock" in January and now you intend to replace them with another standard set (reference GSworkshop's part numbers) OR a set which is smaller (according to what your mechanic wants to do).

I agree with GSworkshop; check the part numbers against what you want to achieve - your initial description of the symptoms = a rich mixture, so that is your start point.

Good luck with the trouble shooting,

Dave