4020 Running ? ( Gas )

I did this right after we bought our 4020 GAS around 1993 and it has been pretty much trouble free. Only modification I can reccommend is to make sure you have a Zenith carb.

I have rebuild many, many carbs. over the years and the only one that ever gave me the most fits and I finally gave up on was the one MS on our 4020. I finally took the advice from others about getting a Zenith. I didn't want to spend big money on an experiment so I lucked out and found one cheap that came off a 3010 or 3020 ? same bore and stroke just 4 cyl. instead of 6 so I figured it couldn't be too far off. I rebuilt it and stuck it on the 4020 and that made the single biggest differance across the throttle range of anything else. I wouldn't hesitate to spend alot of money on a good carb. now !

Firts off run a compression test,adjust your valves and check for little things like cleaning all the wire connections and check for worn dist. parts. I'm talking about a complete and thorough tune up is all. If something isn't within specs. it never will work right.

The electric pump (ignoring the metal work I did) was under $20 and I like to let the electric pump fill the carburetor before cranking which saves on battery and gets me a much quicker start.

The MS carburetor gets 4 or 5 times as many pages in the 4020 shop manual as the Zenith. But there is a 70s vintage JD service bulletin that says scrap the MS and put on a Zenith, so its sure that they often couldn't tune it either. I suspect the most common problem is a break in the vacuum operated accelerator pump diaphragm but my 4020 came with a Zenith and I'm not going to change to MS for the education of the experiment.

A worn distributor top bearing could easily lead to poor running especially if the rattle off the shaft is a significant fraction of the point gap speecification. But that shouldn't show up after 20 minutes, that should be poor all the time.

With the time frame of this particular complaint, I'm more inclined to believe its fuel supply rather than coil or ignition failure. The solid state ignition probably isn't going to come back to life after failure, nor is the condenser in a normal ignition (which has many fewer parts to fail compared to a solid state ignition), though coils can develop a shorted turn when warm and work again when cool.

With this time delay, a lack of tank vent can be a problem. There's a service bulletin about that, recommending that the gas cap not quite bottom against the hood grommet because the passageways are on the bottom of the cap and being against the grommet can block them. So when it sputters, pull the choke on to see if that help. If that does, take off the gas cap to see if that helps. Then adjust the gas tank neck extension to raise the cap above the grommet.

But do the simple things first.

Gerald J.

Funny thing showed up on parts picture is a kit to add a fuel filter. Looks like some new lines and filter in the kit AR50139 for $30.99 I sure would of thought it would of been alot more ?

Then check the fuel lines and any added filter for being plugged and check the fuel lift pump to see if its any good. I put an electric pump on my gas 4020 last year and it now runs all the way to the bottom of the tank instead of quitting with a third of a tank left.

The carburetor float needle valve is probably worn badly from decades of vibration and that may be letting it run rich. Replace that before you dig into the rest of the carburetor.

Gerald J.

Gerald J.