On the wifes 1991 Formula mx 462, the engine was rebuilt last season, compression is 160 in both. I rebuilt the fuel pump, new fuel lines, cleaned the carbs, checked the crank seals and the idle still varies alot. It pulls fine on mid to top end but it won't idle unless it's at 3,000 or more. If you feather the throttle it will stay running, but when you can it it falls flat and but when it recovers the mid and top end is fine. One odd thing is the fuel is not pumping into the carbs. You can see it through the clear lines, the fuel is up to the curve in the fuel lines but is not pumping all the way to the carbs, it just kind of falls in. I tried using 3/16 fuel line so i know it's a tight fit. The gas tank was cleaned and the in tank and inline filters were replaced. How much vaccum should the motor produce to the fuel pump? and How much pressure should the fuel pump put out? I don't know what is wrong.
thanks...........

I bet your fuel pump is screwed. If your engine has compression to run, more then likely it has the vacume to power the pump.

The fuel pump does not operate by vacum, buy by positive & negetive crankcase presures, check the pulse line from the crackcase to the pump.

Thats what i was talking about.

However since the sled runs after 3000rpm I would assume that the line is connected. (possible a crack in the line.)

But I would still say the pump is broken. Probably the diapram not working properly. Just take the pump apart and put it back together. Try fliping the diapram.

i would definatly check the pulse line for cracks,kinks etc. and if that is fine i would clean or replace the fuel pump they are cheap enough.i had an old yam ss440 that had the same probs u described. i pulled the fuel pump apart and cleaned it up in an ultrasonic cleaner and it worked fine.

I just cleaned and rebuilt the fuel pump!

I will bet you put a diafram in backwards or something on that order. Maybe you hooked the fuel line to the pulse line?

Doubt very much if there is a problem with the pump as it runs good off idle, where it uses the most fuel. If the fuel pump was bad it would be the other way around, no top end 'cause its starving for fuel. Sounds like a pilot jet problem. What are the plugs like after you run it WOT? Lean or normal? Air in the lines is not all that uncommon from the vibration of the machine and you will not see the fuel move! If you pull off the supply line to your primer you should be able to tell how your pump is doing as it is teed into your fuel line. Is the machine hard to start or just to keep running?

If it runs mid to full fine than you dont have a fuel delivery problem. Better get back in the carb and check the pilots, or the settings of the pilot air screws. Also check for air leaks causing a lean condition.

There is a fuel delivery problem, mudwieiser says his fuel lines are not full, so I would thing the fuel level in the float bowls would be low. For the pilot circuit to work it has to have fuel. Won't idle (no fuel) feather the thottle to get revs up and it starts to draw on the main jet wich is in the little fuel that is in the float bowl. I would check the fuel lines from the tank to the pump, a leak in one of those line will draw air in to the lines.

I already replaced the lines from the pump to the carbs, I used 3/16 line because 1/4 fit loose. The carbs have been cleaned and checked 2 times. The pilot screws are set at 1 1/2 turns as the book specifies. I rebuilt the pump, and i pulled the valves out 1 by 1 so I know they are in correctlly. The plugs are burning cardboard brown after a w.o.t. run. The sled always starts fine as it has the primer. It will stall out unless i have the idle set above 3,000 then its fine. :cussing: , SWRules

Was the rotary valve timed correctly when it was rebuilt?

Yes the rotor was timed correctly, it ran great until this season, I pulled it out of storage and it hasn't ran right since. I used stabil and fogging oil all that good stuff, I'm very picky with my toys. All the fuel lines are replaced and it's a nice tight fit. I'm going to pick up a new fuel pump today, I rebuilt it the other day but I have never liked using anything but Mikuni parts, these were Winderosa.

Carb float level? If you can try taking the line off the primer and see if that level drops. Or remove the pulse line from the engine, put on a longer one to suck and blow in it yourself to see if it pushes fuel. If you remove your fuel delivery line the fuel should move up the hose and stay there when you do this.

Mudwieser, you say you have replaced the the fuel lines from the pump to the carbs, how about the line from the fuel tank to the pump? incl the pick-up line in the tank?

The fuel problem was the pump, however the idle is still irratic. I can get it to idle right but when you hit the gas it goes way up and the only way to get it to down is to back the idle way down then readjust it. Does the engine have a bad seal? How can you test it without a leak down tester?