766 gas

I am going nuts-----tractor has run like a bull for several years--now bucks and jumps under a load after an hour.
Fuel tank has been cleaned and flow is great to sediment bulb , carb cleaned, wiring checked------anyone have similar problems?

If you push the clutch when it starts to lose power, the engine comes back up to speed??

thanks

Fuel tank vent, or ignition coil, but I'm leaning towards the fuel tank vent. Might want to check the clearance gap on the valves also.

If it runs fine while cold , but starts to act up after it gets warm, it could well be the coil. Easy to swap it out to test, and not expensive to buy. Had one do this very thing on my gas powered swather two years ago.

If all else fails, try a fuel with higher octane if you haven't already.
C

This is just a brain storm but does that carb have an electric fuel shut off and if so could it be giving you trouble. Also do you have a hot spark all the time ?

Well you won't like what i have to say on this subject , But here goes , what is happening is you are having peringtion and your pistons are swelling and starting to guild to the sleeves as they are super heating and they can swell above the top compression ring over .025 of and inch . The area above the skirt of the piston where the rings live is approximately .019 to .021 smaller then the skirt area and your skirt to wall clearance usually is someplace between .003.5-.004.5 when new . The use of 87 octane gas will cause this to happen as 87 burns hotter and faster as told to me by the Chemist that ran a test on gas that caused and engine failure , ing. timing is critical as ing. full advance should be 18 degree BTDC at rated RPM . Myself i have fought the issue on gas in tractors in this area for well over 20 years and had a pretty good handle on it till three years ago when they went and changed gas once again . In my area of this rock in space we had a lot of small dairy farms and they all had gas tractors of some flavor or other . Like most people they never thought about GAS , they just needed it and wanted the cheapest they could find . Well CHEAP was not the way to go . Myself i farmed with GAS tractors and a GAS combine , WHY well i am CHEAP and i could buy gas tractors for the low end of nothing at the sales , same as my Combine . Myself i got along just fine sort of . When i went to the gas station to fill the truck i would get gas for the tractor at the same time . I ran high test in my pick up and so the tractor got high test . My late 706 ran fine on it and never gave much thought to it . A close friend just bought a 766 gasser and he was one of them that would drive fifty miles to save ten cents on a gallon of gas . Ronny called and asked if i could give him a hand getting the plowing done as we had a wet spring and we all were running behind , told him i could so he and i went at it and he started to have problems of loosen power , sputtering back fire and die and at times lock up and would not turn over till it cooled down . So we started to see what was going on with his , So this is what i found , He had D21 plugs way to hot so we put C 86 A/C's in next we found that his timing was set wrong so we reset the timing to spec.'s we checked fuel flow and the carb for any blockage and all was good there . It helped some but still was having problems myself i was having no problems . To keep from getting his to warm he was forced to run low third where i was running first high . ANOTHER friend and customer has a 460 gas bought new on the farm in 1961 as the main tractor back then there is a lot of history on engine problems on that on most with valve issues till the correct oil was used . BUt now it was starting to have the same issues that Ron was having and we went the route of checking this and that and did this and that . Then i sold a 706 to a close friend and more problems till he locked up the engine and when it cooled down and restarted it was knocking . When we pulled the head all six pistons had swelled and took out all six sleeves and tis is the one we really dug into as i knew it was a fuel problem and got digging deeper and found that minium octane for these tractors is 93 not 87 on 460-560 it is 89 not 87 and old H and M's well what ever . we rebuilt the engine and changed fuel suppliers and it ran fine till the fuel supplier hired a new driver and brought out the wrong gas and we ate a piston just after the tractor was gassed up and it did not make the end of the field when it locked up . I know the EXPERTS are going to jump down my throat on this BUT i just work on I H tractors for a living and fix the EXPERTS problems . So all i can say is get the operators manual and READ under fuel requirements . And good luck finding gas that will run your tractor while tryen to work it as the 93 they are tryen to pass off now is NOT getting the job done .

May not be related, but when was the last time you adjusted the valves? People are so spoiled now a days with the hydraulic lifters in their vehicles they forget about having to adjust their valves as part of a FULL tune up.

I have a IH 656 that acts like that, on a hot day mowing and low on fuel. I believe the fuel is getting too hot and is vapor locking.

Thanks to everyone who replied-------changing the coil today----before anything else-----and the solenoid on the carb that allows fuel to flow is working fine-----so I have kept all the suggestions and will begin again. Many thanks!!!

Does it have an adjustable load needle ? like on that solenoid ? sounds like it is running lean.

I also wonder about the main jet solenoid on the carb. You say it works fine, but maybe when it is good and hot it gets weak and can?t hold the valve open. Just a thought sometimes the fluke items get you.

Just last week the neighbor’s 656 was acting like that. A new fuel shutoff solenoid solved the problem. Not saying that is your problem. I’m surprised no one has mentioned the condenser though a bad condenser usually starts acting up in minutes instead of waiting an hour or so to get cantankerous.