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Mike

Carb help

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Mike
Hey gang, I have 2 snowmachines and I am looking for carb advice( I know they aren't tractors, but they share the garage with the tractors) Both machines run perfect at any speed above idle, they just won't idle and will die without a slight amount of choke. They have fresh premium fuel, new fuel filters and plugs. My assumption is either too much air or not enough fuel. They are Polaris machines. I am going to try running some seafoam thru them next. Thanks for any and all advice. Mike

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rokon2813
What style Carbs ?? Chances are if they are old Tillotsen diaphram carbs, there is a pinhole in one diaphram. These things tend to run best if rebuilt every year.

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HubbardRA
Idle jets are very small in these carbs. They tend to plug up very easily. Had this problem with motorcycle engines. Also, running premium fuel (93 octane) in an engine that is set up at the factory for 87 octane can cause a problem with idle if the carb doesn't have an idle mixture adjustment screw. Premium fuel does not light as easily or burn as well at lower compression ratios,which will throw off the air fuel ratio. You shouldn't need 93 octane unless the compression ratio is above 10:1. The old belief that higher octane is better is just not true. Octane must match engine design, especially compression ratio. Been there, done that, found out the hard way.

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AC917Hydro
Depending on the carb, there should be a screw to adjust the low speed, or idle jet. The trial and error method I use is (with the engine running) adjust this screw until it bogs down with too much fuel. (you'll get black smoke from exhaust) Note the position of the screw. Then I turn it all the way back until its running too lean.(missing and sputtering) Remember to count how many turns you give it. I then set the screw at the half way point between these two settings. It seems to work well for most lawn mowers, tractors, weed-eaters, etc. If you need to take the air filter off, the extra airflow might also affect the mixture. I'd dial the screw a touch towards lean if that is the case. Good luck!

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Mike
I tried spraying carb cleaner around the carb tonite, no air leaks. They are mikuni carbs so I don't think it's the diaphram. Wehaven't really had any snow the past couple years and these machines hav done alot of sitting. I did add some seafoam tonite and I'll try and run them tomorrow nite to work the seafoam thru the system. If that does'nt work I guess I'll be going thru the carbs. Thanks for all the advice. Mike

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Mike
Well I took all 4 carbs apart and gave them a complete cleaning. Had to replace 2 float bowl gaskets (9.00 each) ouch. Everything looked really good, so I just put them back together and lo and behold they run perfect. I did notice most of the problem was at the fuel screws, so next time I would try just cleaning these without removing the carbs. I have actually never had much luck rebuilding carbs so I really feel good with this outcome. Thanks for all the advice. Mike

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HubbardRA
Justin, You are somewhat correct. The burn rate of a fuel is related to its octane rating and the pressure it is under when it is ignited. Premium fuel contains more energy, but burns slower. Regular fuel has somewhat less energy, but burns faster. If you increase the the pressure at which it is ignited (higher compression ratio) the fuel will burn faster. If you run low octane fuel in a high compresssion engine it will burn too fast causing pre-ignition, with a pinging sound that is really audible detonation. This is bad for the engine. If you run premium fuel in a low compression engine, it doesn't burn fast enough resulting in unburned fuel going out the exhaust. Approximate usage: up to 8.7:1 87 octane 8.7:1 to 9.5:1 89 octane 9.5:1 to 10.5:1 93 octane higher ratios AvGas or CamII A friend of mine has an engine with 13.5:1 compression. He has to use at least 110 octane. My two cents.

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