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Discussion Starter #1
friend brought it over and its having issues, it will fall on its face when you get int the throttle, below 3k rpm it runs but above that it stumbles and no power.
so far I have torn down the carb and cleaned it including the primary jet ( screwed all the way back in and then 2 turns back out) when I first started working on it it had to be choked to start now it fires up on the first try without the choke.
everything inside the carb was spotless with the exception of the pilot screw I cleaned that, also on the needle jet there were 2 "spacers" on it between the eclip and bellows. every thing I saw shows only one spacer (hard plastic O ring) so I removed one of them. needle jet is clean and shows no real signs of wear neither does the float needle.
fuel pump holds vacuum (5 inches of vacuum) for a while.
new spark plug installed.
one other issue the wheel speed sensor wire was cut in two so I spliced that back together and the speedo registers MPH again.
good voltage on battery both running and with the machine off.
intake tube between carb and head is good ( Just went thru this on my 700 ho) intake tube between carb and air box does have cracks in it, I sealed them with some electrical tape for trouble shooting purposes.
compression is with in spec @60 psi. (book shows 50-90 psi)

next thing I am going to look into is the sticking choke issue they have although it doesn't seem like its sticking but I am going to double check it.

3 pin connector on back of the speedo checks out as does the 12v+ and switched 12v on the speedo.

now here is a question, the book says to trouble shoot the speedo that you unplug the unit and start up the machine and drive it, if the issue(similar to the reverse limiter but in forward gears) goes away that it is the speedo that is bad and to replace it, but this machine will not start with the cluster unplugged??

so any ideas?
 

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How does it run off the ground on jack stands in gear?? Think any think is bound up in the suspension/drive-line/brake systems???
 

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Discussion Starter #4
seemed to run just fine up on jack stands, now for the kicker..
I tightened up the choke cable to where it's supposed to hold in whatever position you set it at, and for shiggles I took it out for a spin with out the air box on it, and the damn thing ran like a scalded dog... put the box back on and it boggs down, took just the top off the air box and it still boggs but not as bad as when the lid is on. now I'm thinking it's the needle jet either the jet needs to be leaned out or a new jet..I am going to set it up one notch and see how it does. its in the factory #3 position right now,
I tested out the fuel level in the bowl with a clear hose and it appears to hold the correct level when running so I dont think it's in the float.
 

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Discussion Starter #6
it ran good with the air box off and with just the lid removed,
I think I found the issue, both the needle jet and float needle are worn.
I changed the needle jet settings to lean it out and it ran better but still bogged down at a higher RPM, I pulled the carb off again and looked at the float needle and it is indeed shot and so is the inlet where the needle goes.
I'm going to order a new needle and seat along with a new needle jet assembly.
will update when I get it back together..
 

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Discussion Starter #7
back to square one I guess. installed a new float needle and seat along with a new main jet, nothing changed. It will run like a scalded dog with the air box off, and it will slightly bog with just the lid off and filter off. I have had the carb off numerous times, there is nothing clogged, it is not flooding out (float bowl is not overflowing)
so when the airbox is off it runs just fine, with it on and you slowly increase the throttle it gets to @1/4+ throttle and it starts to bog if you increase throttle it gets worse.
if you hammer the throttle from idle it will do the same [email protected] the same throttle position.
I pulled the pilot screw and cleaned it, put it back in and screwed it down till it stopped( not forcing it just lightly until it stopped) then turned it back out 2 turns, then I proceeded to try different settings on it @1/4 turn at a time and nothing changed.
I have even tried running the main jet one notch leaner and one notch richer from the factory setting in the middle.

so should I start looking into the valve clearance and timing?
 

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No - it's not valve clearance or timing - it's a fuel/air issue

Now screwing with the fuel screw will not help - it's not a chain saw with a slow screw and a fast screw - the carb has just two external adjustments one for idle speed and one for the fuel/to air mixture at idle - the fuel screw will not help with performance above about 1/8 throttle.

Once idle is set, then you are starting to deal with mid-range - the BST carb has two controls for air and one for fuel - the butterfly valve is the primary air control and the slide is the secondary air control and the mid-range fuel control - when the butterfly valve is opened by the throttle cable, vacuum drops - when the vacuum drops, the vacuum controlled slid lowers in the venturi decreasing air volume and raising air velocity - the velocity of the air passing over the needle jet draws fuel into the air stream (like air passing over a chimney draws smoke out) - the taper of the needle controls the amount of fuel being drawn through the jet - the slide will 'bounce' at low speed with the 'pulse' of vacuum during the intake cycle - as engine speed increases, the slide will not fall as far as it rises and eventually becomes fully open.

Now if the fuel is not metered correctly with the air, engine performance will suffer - too much fuel and the engine will bog and run like the choke is on and slowly closing the throttle will allow the mixture to correct and the engine may speed up some, but there will be a point where the engine simply cannot accelerate past. Likewise, if there is not enough fuel, the engine will accelerate to a point where it simply cannot go any faster and continued opening of the throttle results in a progressively leaner mixture resulting in a loss of speed and ultimately the engine dies.

What you have described is a classic example of being overly rich - any restriction to air flow lowers the point where the engine loses power (bogs).

Based on this information, I believe the main jet in the carb is too big and the engine is simply being flooded with fuel when the air flow is restricted.

I do not see anywhere in this thread where you have disclosed the size of the main jet - all your references have been to the idle circuit and mid range - if you want it to run full throttle, install a new 155 main jet just to assure that the currently installed main jet has not been enlarged by drilling or improper cleaning. Using a cutting torch tip cleaner to clean a jet will enlarge it and screw up it's calibration. If it runs the same with a new 155 main jet, try a 145 and go progressively smaller until it loses performance, then go back to the size of jet it runs best with.
 
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