500XC - Fuel starvation

[subarumy98]

HI All

I need help please. We verified on a dyno last night that the Husky is simply running out of fuel at high revs.
If you rev slowly, then you can get too 8000rpm.
Gun it, and you get to 6500rpm max, then run lean.
If you hover at 6000rpm, then you can slowly reach 8000rpm.

Have checked the following:

• tank breather clear
• Fuel tap clear
• Fuel hose clear
• Fuel filter clear
• carb overflows clear
• jets clear
• main jet clear
• increased the main from 340 >> 390 = drain even quicker
• float levels set at 18mm as per manual

What amn I missing ?
All suggestions welcome.

Many thanks


Trevor


[/list]

[cyclegod]

What size is the carbie? (40mm Mikuni is stock size?)

What size is the fuel filter? (I had a smallish filter on my 250, a bigger hi-flo filter fixed that bikes fuel starvation problem)

What size is the fuel line?  (Too small here will restrict flow)

[fatboycrash]

Gudday Trevor!
I have a 1983 500xc and had the same problem at WFO... BRAAAAAAAPPPPPblughhhhhhhhh Shut throttle and wait, open then BRAAAAAAAAAAAAPPPPblughh....   I got rid of MOST of mine by ignoring the 18mm float level and adjusting "by seat of pants" until it mostly went away. From memory the float needle tang was almost level with the float supports. Hope that helps. As an aside, the 1982 two fiddy I just bought will hesitate for half a second in the same way, if you've let the fuel level get too low in the tank and you hit it WFO after a rough patch of track. But that might be the bloody Keihin carb that someone has fitted. Works well otherwise but I am hard on the heels of fleabay for a TM38....
FatBoy

[subarumy98]

@CG:

40mm brick with a hole in it Mikuni
7mm inlet on a 4" auto inline filter
8mm fuel line

@FBC:
I will check tonight, but potentially I am missig the thick washer on the bowl drain plug. Maybe the main is now too close to the surface of the plug.

Will advise later...


What I forgot to mention in this whole process, was the 12" flame out the tailpipe every now and again at about 7000rpm as the operator tried different techniques. I shat myself. Was expecting to catch a piston coming out there the next time.


T

[Lozza]

Fuel tank breather??? Set the float height to just where it's not pi$$ing out the overflow.

[GD66]

 Bud, if you're going back to first things first : fuel tap and line have to be bigger diameter than mainjet : then to float level : then check float isn't leaking : then check for air leaks etc :..... keep us posted....

[subarumy98]

I think I have it sorted.

I doubel checked all the hoses, its actually 8mm top to bottom, not 6mm.

Raised the fuel level WAY up. Every now and again I get a drop or 2 out the overflow.
drilled out the gas tank breather for good measure
main is now a 390 too.
fitted the 1mm washer by the bowl drain plug

NOW we are in business.
Blubbery at the bottom.
Doesn't cut at the top - YEAH BABY !!

[fatboycrash]

Glad to hear you got it sorted!!! That Fivehunge should straighten ya arms and pucker ya sphincter while giving you a silly grin.....

[subarumy98]

Roaaarrrrr brab brab bupbup its back.

But I think I have solved it too.
I am trying a 430 main jet to rule out a lean mixture.
I have changed to a straight through new 8mm fuel pipe.
Removed the large auto cartridge fuel filter,
Replaced the fuel tank breather with a stiffer 6mm item - the clear plastic one seemed tobe collapsing under vacuum.


So, right now its doesn't cut at the top, but also doesn't rev out cleanly - too rich.
I will try a 400 tomorrow evening.


But this is what's puzzling me the most:

there's a Husky 390 up here that is running a 410 main, and another with a 430 main. I would have thought the 500 would have required the same or larger main jets - not smaller ??

Comments please..

[GD66]

 Not relevant, Sub. Even identical machines can use different jetting to each other. As far as the other, smaller engines go, bore x stroke, porting, exhaust dimensions, and the like mean no similarities need apply. But to hell with the others, let's get YOURS on the pipe !

[subarumy98]

SHHEEEIIIITTTTTTTTTTTTTTTTTTT !!!


Ok, let me explain:

After some further research, I discovered the listed main jet is a 340.
I popped in a 360 to start with. This is a different bike.
The top end burbling is gone.
The fuel starvation seems to be gone too. (A little tricky wringing out a desert bike in the housing complex here )

I reckon the 2 biggest contributing factors have been the cartridge auto fuel filter and the fuel tank vent tube.

We have an invitational race on Tuesday night 22nd - I cannot wait...

[subarumy98]

Confirmed:

All issues sorted.