my 2001 scrambler 500 BST40 is giving me headache again


I discovered that the previous owner replaced the oem fuel pump with a scooter fuel pump or something like that; the pressure was higher, but the flow what about the half. the engine would flood itself at idle, and would run out of fuel after a few second at WOT with float level adjusted as indicated in the manual.

As I had everything teared appart, I decided to install a foam air filter and a jet kit from HPD inc. It is supposed to me tuned for running with open lid

If I remove the lid, it is way too lean. It might be the freezing temperature, but usually, 4 stroke doesn't need that bad of rejetting for winter. with lid on, the midrange and top end seem OK at best. I doesn't feel any improvement, really

the low end is way too rich. the engine die at idle. HPD recommend 3 turns out on the pilot screw, but even as lean as 1 turn out, it won't idle properly.

So far, I replaced the valve needle, because with the wrong fuel pump, fuel was getting out by the overflow.

with HPD jet kit, I replaced main jet, needle jet, piston spring.


is there any removeable part that could have been changed by the previous owner that would cause that kind of problem?

 

First off I would get rid of the scooter fuel pump and install the correct one. Higher pressure and less flow won't even each other out. Those needle valves are tested at 5 PSI and at an idle, your machine is not needing less flow because at idle, it isn't using enough fuel, but the high pressure could be making it flood at idle.
At high rpm your machine needs more flow(which the scooter pump can't do according to what your saying) so the higher pressure is useless and the flow is probably causing you to run out of fuel.
Just my opinion.

 

Sorry for the lack of information, but I replaced the pump with an original one

 

maybe put the original jets and needle back , did the crankcase get full of gas when the needle and seat were bad?

 

putting the original main jet, needle jet and slide spring might be an option, but the slide now have oversized holes, so I couldn't really put everything back to stock.

In fact, the needle and seat had never been bad, at all. I started there because fuel was getting out by the overflow, but it was caused by the higher pressure of the "wrong fuel pump" Then engine never flooded itself, because without vacuum, the fuel level would drop considerably.


After reading a bit more on the BST40, trying to discover if there was anything I didn't touch yet that would affect mixture and that could have been changed by the previous owner, I give a try at the Pilot Air Jet.

I simply removed it in order to lean out the idle as much as possible, and see what would happen. It began to idle smoothly and quite a bit quicker than before. Before I removed the Pilot Air Jet, I was at 1/2 turn out on the pilot screw, and the idle was still too rich. with Pilot Air Jet removed, I gave it a try with 3 turns out on the pilot screw (as recommend with HPD jet kit) and I feel like it is slightly lean, and tonight, we were just around the freezing point.

The low end now lack a bit of responsiveness, but the idle is lean, so i'm not really worried. Everything will probably be fine when i'll replace the Pilot Air Jet with the right one


The one that was in my carburetor has 130 printed on it. The service manual I got indicate that the original one is 160. I'll order the original one tomorrow morning.

 

I ordered the pilot air jet, and drilled the wrong one to approximative diameter of the 160 just to give it a try; il idle much better with 2-turns out of the pilot screw

I took a look at the needle, and i'm pretty sure it is wrong, too.

The numbers printed on the needle aren't what the manuel says for sure.

Should I be reading J8-6H25-94 on it, or something else ?