Newby here. I have an '85 Fourtrax that just won't take throttle. It idles perfectly and revs to a point if I feather the throttle. Seems to run better (not perfect) at 1/2 choke. History: Bought as a no start unit, found the fused harness disconnected under headlight. Fixed. Ran crappy so I cleaned carb (3X) and it is no better. Compression is 110 PSI with throttle wide open. Set valve clearance to .008 per manual still no better. Fuel flowing freely from petcock. Timing mark is lined up with the notch at TDC. When throttle is punched it feels like blowback from carb. This was purchased new by the farmer I got it from. Always garaged, plastics good, no cracks, just used to haul feed. It is really in excellent shape for a 30+ year old ride so I hate to have it useless. I know the compression should be 170 area but would the lower number cause my issue? I did squirt some oil in the cylinder to try to determine if the compression loss was valves or rings. Compression was same wet and dry so I assume valves are the culprit. Tried to provide as much info as I could, I hope it is enough for a guru to help me! Thanks for any help.
Help with '85 Fourtrax 250 dies on throttle
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think of it this way, not only does it take compression to fire the piston ?, but if the rings are not tight in the cylinder ?, then it won't '' draw '' fuel into the cylinder on the down stroke ( compression stroke ), in other words..its not sucking fuel in good enough because the rings are too worn.
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I agree 110 is too low , I would have to pull the manual on the 250 but think it is the same as the 300 185-205 psi , I find around 100 psi is where it gets to the point of running poor and then not at all ----- but also find that when the timing chain gets excessively loose , the compression goes down as the piston is not achieving top dead center before the valve closes , you can do a wet compression test , squirt engine oil into the cylinder thru the spark plug hole and retest compression , if the compression comes up then the rings are bad , if not then the problem is the valves or timing ------------ a head gasket or a bent connecting rod will also lower compression , if it were ever flowed with water while running then a bent rod is possible
Yeah, rotate the crankshaft (in the normal direction of rotation) to TDC on the compression stroke and stop. If you go too far rotate the crank two more revolutions until you stop on TDC on the compression stroke. Pull the cam chain tensioner off from the cylinder (the tensioner will lengthen automatically when removed) and hold it back in against the chain guide... squarely, as if you going to bolt it back on. Note the gap distance (fraction of an inch) between the jug and the tensioner base and report back. Then take the plug out of the end of the tensioner and using a narrow screwdriver crank the tensioner plunger back in and hold it there until you get it bolted back onto the jug. Then release your screwdriver to allow the tensioner to adjust. Rotate the motor by hand at least two complete revolutions in the normal direction so the tensioner can fully adjust before starting the motor.
Sorry for the delay on reporting but the problem is solved! I cleaned the carb 4 times with a spray can of carb cleaner, blew it out with the compressor and no joy. Tried the lemon juice cleaning method, blew it out with air and more carb cleaner, no joy. On advice of a snowmobile racer and with the help of his ultrasonic cleaner and a 3 hour bath in carb cleaner and blowing it out with air we have the old girl running like a champ! With the 110 compression I am sure she isn't going to win races but for working food plots and making trips to my deer stands and hopefully dragging out a 12 point it will work great. I appreciate all the input. For people who think their carb is "clean enough" it isn't! I have no idea what was blocked but it was the carb.
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