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Discussion Starter · #1 ·
I'll get the year in a bit. 2wd 300 fourtrax. I tinkered with it all day yesterday and got nowhere. Things I've done:
Rebuilt carb.
New oil, oil filter, air filter.
New cdi.
New plug, wire, coil.
Compression is 95-100.
It will start right up cold, after 10 seconds at most will die without choke. Soon after have to give it gas to keep it running. Once fully warmed up, will run worse and worse until it dies. I thought it was possibly electrical or fuel delivery related, but I'm just mechanically inclined enough to get myself in trouble. Never messed with valves, and only rebuilt top end on a 2 strokes. But I have tore 4 strokes apart before, I am thinking rings, or possibly valves need adjusted. If someone could point me in the right direction thatd be great, not sure what else it could be. Bought the machine at an estate sale for cheap and it's in almost showroom quality cosmetically, and from what I could gather it had been sitting for a long, long time. -John
 

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Hi John
I am struggling with a similar issue but on a TRX 420. Mine runs fine for about 5 km then dies and wont start up at all till it cools right down.
Reading your post makes me think you may have fuel starvation. It could be that the fuel filter is blocked or there is severe dirt/rust in the tank/fuel line. I replaced my fuel pump, filter and cleaned out plenty dirt in my tank.The more things you do the sooner you find the problem.:)
I suspect my problem is now electrical so I will try coil, plug etc next.

Good luck!
 

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First off, the Compression seems low.. Did you have the throttle wide open when you performed the compression test?

Where did you source you replacement parts from, eBay or Amazon by any chance?

Could be an ignition issue, if you bought a Chinese cool the spark could well be breaking down when hot and under load.

Inlet (air) leaks, have you tested for this yet?
How about the diaphragm in the top of the carb, is it perished, does it have a pin hole visible?

Timing... Have you checked it is within spec?
 

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Discussion Starter · #5 ·
Thanks for the response Sam, I did not, never used the compression tester before, didn't know that you need to open the throttle to get an accurate reading. All replacement parts OEM, ordered through honda dealer, tried swapping multiple times old and new, same deal. The boots are rock hard, but they still seal perfectly, checked with ether while running. Diaphragm is good. Timing:I have no idea how to check that..
 

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Sounds like fuel starvation (petcock screens dirty) there’s one on the tank inside and one on the bottom if the petcock.
Also, perhaps didn’t clean carb well enough—did you use pipe cleaners swabs and dental brushes in all those ports? Some I know will require a teeny carb wire set.
How did you rebuild the carb—complete carb kit with seals? Proper Carb synchronization and pilot screw adjustment extremely important
Did you replace the choke valve? If worn enrichment circuit will stay open, fouling plug. Is your plug sooty?
 

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Thanks for the reply Goob, that's what I thought, but I'll have to set up a bottle of clean fuel with a filter and try it that way, will do that later today. I think I did, did it the way i always do and it seems to work, i use torch tip cleaners, which are basically the same as Honda's "jet cleaners", didn't change seals, but none leak, and the only o ring was on the pilot screw, seemed pliable and fit nice, let the carb soak in napa's carb dip for a good while, then sprayed it, and cleaned it pretty well. And counted the turns of the pilot screw and put it back to where it was, and tried turning the pilot screw a few ways to see if I could get it to run differently, no change. I did nothing to the choke, all I checked was when I engage the choke, that the black plastic with a needle retracts and lines up straight, which it does, what else is it supposed to do?is there a way to check if the choke is not working properly? The plug was wet, smelled like unspent fuel, which I found odd.
 

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Hey john you didn’t say what year quad so I’m gonna post a pic of my Keihin VE90E. Make sure your carb is OEM and that the carb model matches the year of the quad.

you will see all of my seals are worn, cracked and stiff. Your carb likely needs a rebuild kit, a primer plunger and a choke valve. $100 in parts.

In the pic you’ll see the pilot screw; stack the spring first, then washer, oring last. In this pic the washer is hard to see.

inspect the pilot jet hole to ensure the port is not clogged by the tip of an old pilot screw. Insert the screw until it lightly seats—at this point you should be able to see the protruding needle tip and catch your fingernail on it. Back off the screw two full turns; at this point you can still see the tip of the screw but you won’t catch your fingernail on it.

next the lock ring on main jet needle needs to be on the middle setting.

Synchronization is simple. Set the idle screw so it just touches the throttle link. Set the throttle cable adjustment (handlebar) at midway. Install the carb andfasten the cable down, taking most of the slack. Use the handlebar adjustment to take out remaining the slack. When you slightly crack the throttle the vacuum piston should respond. Now you can run in the idle screw.
 

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You should verify that the float valve is sealing properly. I mention this since you've noticed the plug was wet which indicates over-fueling. An easy test you can try is to connect a clear fuel line to the carb which can still be attached to the engine or on the bench in the normal, upright position. Fill the float bowl with gas through the clear line which can be viewed (almost to the top). Now all you need to do is watch the level of the gas - does it hold it's level or drop at all? Some carbs will flood into the engine, others will leak out the vent eventually. It can happen very slowly so be patient.
 

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You should verify that the float valve is sealing properly. I mention this since you've noticed the plug was wet which indicates over-fueling. An easy test you can try is to connect a clear fuel line to the carb which can still be attached to the engine or on the bench in the normal, upright position. Fill the float bowl with gas through the clear line which can be viewed (almost to the top). Now all you need to do is watch the level of the gas - does it hold it's level or drop at all? Some carbs will flood into the engine, others will leak out the vent eventually. It can happen very slowly so be patient.
If it was a pilot valve (choke valve) issue to would have fuel spilling from the overflow pipe. The fuel jets control fuel inlet.

The tiniest bit of dirt or dust can play absolute havoc with Honda carbs. They need to be surgically clean, literally.. Not forgetting to clean the fuel tank, petcock and install an inline filter while your at it.
 

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The best way to diagnose needle and seat sealing problems is to attach a clear tube to the overflow fitting on the bottom of the carb and bend it up perpendicular to the float bowl. Open the drain screw, and you can see the float level from outside the bowl, especially while the engine is running.
 

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The best way to diagnose needle and seat sealing problems is to attach a clear tube to the overflow fitting on the bottom of the carb and bend it up perpendicular to the float bowl. Open the drain screw, and you can see the float level from outside the bowl, especially while the engine is running.
In my experience it either leaks or it does not.. Can't say I've ever tried your theory mentioned above however, float level is irrelevant as it's none adjustable.
Replacement of the valve needle and float is the only option
 

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Yes, but I like to see it for myself before shelling out bucks for an OEM float or needle valve. On bikes with a fuel pump, over pressure can seep by an otherwise good needle and seat.
 

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300's don't have a fuel pump....
 
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