M won't lug up date

Thanks for all the input and ideas. Had a chance to get the M in the shop today and checked a few things. Compression test 1= 180psi 2= 180psi 3= 185psi 4= 210 psi. Timing at idle is 2 deg BTDC, full open throttle is 23 deg BTDC. Advance is progressive through the RPM range be starting just off idle and reaching full advance just before full RPM. Installed a vacuum gauge and has 20 inches vacuum and steady. Plan on pulling tuesday , will see what it does under load. I forgot to mention in my first post that it has a bad stumble when you open the throttle. Thanks.

adjust the carb

yes, main jet could be too lean(not drilled big enough or partially blocked) or maybe if its design has several holes drilled in the actual side of the nozzle that carries the fuel up to the low pressure area in the center of the venturi from the fuel bowel....if you screw that nozzel out of the carb casting alotta times you'll see there is an accelerator well formed around the nozzel that fills with gasoline when your NOT under fuel throttle conditions and when you suddenly goose the throttle fuel comes up through nozzel from the main jet in an increased amount but even more fuel gets to the venturi by that fuel in the accelerator well gushing in through those holes in the sides of the nozzle.

If your problem was it running outta fuel after having performed well for a couple seconds under full throttle the you want to remember that BOTH the gas tank and fuel bowel in addition to having pools of fuel must be vented to the atmosphere at least in an equal amount to the amount of fuel your expecting to flow to the carburetor.

Sounds like your low speed needle is set too lean.

(quoted from post at 18:26:23 09/07/19) Thanks for all the input and ideas. Had a chance to get the M in the shop today and checked a few things.......Installed a vacuum gauge and has 20 inches vacuum and steady. Plan on pulling tuesday , will see what it does under load. I forgot to mention in my first post that it has a bad stumble when you open the throttle. Thanks.

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You state the vacuum is 20 inches and steady...I assume this is also true at full throttle RPM? Checking the vacuum as RPM increases is a good check for exhaust restrictions. If the exhaust is restricted the vacuum will drop rapidly as RPM increases. This is because the exhaust restriction places a load on the engine and the throttle butter fly must open to increase RPM.

Will you be able to monitor the vacuum reading during your pull on Tuesday? As the engine is loaded the governor will open the throttle and the manifold vacuum will begin to decrease. The engine must be able to pull enough air flow through the carburetor to maintain a level of manifold vacuum; for the venturi to operate thereby maintaining the fuel air mixture. Without venturi action the engine will stall from lack of fuel. This can occur when the carburetor is too large for the engine. Another way an oversize carburetor can manifest itself is a stumble as the throttle is opened from idle.

Vacuum is steady from idle to full RPM. Yes I will be able to watch vacuum gauge as I pull. Hopefully no rain Tuesday.

I know you're going over the carb adjustments and all, but on my old M, I was having an issue with power as the weight would come on, even in light classes. I cleaned the carb, the jet was drilled out, it had an 806 venturi, all that. My fuel tank was clean inside, and my sediment bowl had been replaced by a 90degree fitting, valve and inline filter with a new steel 3/8 line. I was at my wits end figuring it out. I had it all apart and back together multiple times. I could blow through the line, everything was clean. I then took an air hose to the new line. I blew out some chopped up leaves or something that some bug or something had packed in there while it was in the garage. I could blow through it and fuel would run through it, but apparently not fast enough. Put it together, and it ran great from then on. Just something else to look for.

(quoted from post at 10:13:19 09/10/19) I know you're going over the carb adjustments and all, but on my old M, I was having an issue with power as the weight would come on, even in light classes. I cleaned the carb, the jet was drilled out, it had an 806 venturi, all that. My fuel tank was clean inside, and my sediment bowl had been replaced by a 90degree fitting, valve and inline filter with a new steel 3/8 line. I was at my wits end figuring it out. I had it all apart and back together multiple times. I could blow through the line, everything was clean. I then took an air hose to the new line. I blew out some chopped up leaves or something that some bug or something had packed in there while it was in the garage. I could blow through it and fuel would run through it, but apparently not fast enough. Put it together, and it ran great from then on. Just something else to look for.

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Here at YT the standard fuel supply test which is advised 2x /week is to remove the drain at the bottom of the carburetor and verify that the gas will flow a steady stream for at least a minute. if it slows to a trickle find where the obstruction is.