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Fuel problem or what

James Moore

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First post here hope it is in right place as I get familiar with sight. Have a 2003 550 mule,kaf-300-c7. Put on a new carburetor  as old one was in bad shape, jet was frozen in body, although it cranked and idled fine. The problem is: it will crank and idle fine BUT with a slight movement of throttle it wants to die and when more throttle is given it picks up and runs fine. The old carb. and new one does the same thing. The adjustment with the plastic stop does no good. At a loss here. Could it be the fuel pump, as I know nothing about these kind of pumps. any help will be appreciated.

 Thanks, Jim.


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Welcome Jim!

let's see if we can get you going!

First i would start by making sure air filter is clean and not obstructed also your fuel filter too, it should be about 1/2 way filled with fuel., and that your fuel pump is pulsing good. It might even be some water in the fuel.

the way the fuel pumps work is:

you have a pulse tube that works off of crankcase pressure, the reciprocating movement of the piston creates the pulse. and that drawing in and pushing of air pumps a diaphragm in the pump, and that diapraghm pumping pulls fuel up the pickup tube and into the line.

I know mine has always hesitated slightly when accelerating... some of these FE290 engines just do that.

it could even be a spark issue, as in too small of a plug gap or its carbon dirty.



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Pumps have no repair parts.

I think they are $50 from Kawasaki.

you can find chinese ones for $12 or so.

I recommend pulling the spark plug wire off so it don't start, taking the fuel line off the carburetor side of the pump, crank the engine over. It wont shoot out a steady stream at  low starting rpm but you will get an idea of how good it works.

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I guess what gets me is why it will idle fine and run at a higher RPM great if it is the pump. I keep it at our place in Bedias, Texas but run it enough so gas does not go bad plus I put a fuel stabilizer in it. Think I will go up there and stay, away from Montgomery county.

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  • 3 weeks later...

need more info on the stumble.

cold start up......runs ok at idle BUT

A.  stumble when the throttle is opened slowly OR

B.  stumble when the throttle is rapidly opened 

C.  stumble with both A and B.

engine warm start up.....the stumble problem goes away or greatly improves????

Now you need to determine if it is a LEAN stumble OR a RICH stumble......simply give a squirt of starter fluid at intake.

depending on where you introduce the fumes, there is a delay before the rich mix makes it into the cylinder. 

Does the motor run easier as in no longer struggling at idle.  Get a feel for how the starter fluid boost reacts THEN have someone

SLOWLY open the throttle and also try the FASTER throttle blip when the fluid boost comes on.

analysis of results......a lean stumble will improve with warmer engine temps.

a lean stumble will go away or improve with the boost of fuel introduced into the intake.

Side NOTE.  10%, 15%, 20%  alcohol gas will have these problems on carb engines set up of OFF ROAD (0%) fuel.....required a rejet tune.

ALSO.........some dealers are blending in WATER until phase separation.....then add more OH to get the water back into solution.  This trick is really bad for the scooter crowd. 

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This happens when the engine is warmed up. It idles fine, runs fine at high speeds, it happens when the throttle is opened slightly from an idle but when it is opened more than at idle it runs fine.

Up-date: I noticed it doing it the last time I ran it and it was worse so I raised the seat and saw the carb was flooding out, gas coming out of carb from somewhere. I got back on and ran it fast back home and it ran fine. May be the float was sticking or trash in it that time. I drained the carb bowl but saw nothing come out, ran it twice since and it ran fine. Don't know what else to do.

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that 'll do it.  trash may have just moved out from under the needle in seat....meaning the problem could come back again.

the suggestion about flooding the engine oil with raw gas needs to be checked.  gassy smell test and or over full on dipstick.  besides cutting the oil, you will have a rich fuel/air mix when the gas cooks off and is reingested by the crankcase vent PCV system.

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the small hose from the carb body goes into the bowl space-----atmospheric vent.  they will get bugs nests or dirt, water, and then reduces the fuel flow thru the FUEL JETS.  There are AIR JETS (sometimes well hidden beneath the diaphragm porting on CV carbs) that draw air (clean air box air) into a junction within the carb body cavities TO the MAIN and IDLE fuel jets extending (aspiration) into the bowl fuel.  these "air flows" are separate from the full bore and throttle plate stream of intake air.  the fuel is effectively drawn into the bore with an entrance at the main jet brass nozzle and a small hole(s) south of the throttle plate or slide (depends on carb type).  simple way to think about the vent......you have to let air in when sucking the soda out of a bottle.  same goes for the fuel tank has to be vented....be it a simple hole in cap of a full blown EVAP system.....air in for fuel drawn off.

some machines run this hose (usually bigger because air has flow resistance) to a frame cavity.....think mice, mud dobbers, backwater floating trash etc.  you can change the carb 50 times...still a dog in the pile.

you might want to raise the end to a protected area but still have the loop end DOWN to avoid water flowing in.

The bigger hose from the crankcase PCV feeds oil vapor and blow by fumes to be reburned by routing into the airbox....thus the oil mess on filter.

EVAP canister gas tank fumes are also gated back to be reburned......that is why overflow carb----gassy oil will also cook off and screw up the F/A mixture.

Might as well cover the 3rd fuel/air circuit on carbs that DO NOT have a choke butterfly......that gags extra fuel from main and idle jets.  these will have a second cable to carb or a linkage on multi carb setups.  the basic function is to open an air passage (calibrated drilled hole) by lifting a slide (6mm dia or so.) off a seat and also pull the extended pin out of the fuel jets passage.....have a flow of air sucking metered fuel into a larger hole directly in to the side of the bore throat.  mis routing of the choke cable can open the enrichment circuit (choke) when turning handlebars and then be fine on straights.......H way back bad for this....plug a day habit!!!!!!

That's the short of it.  chow  

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