3020 gas

[jbwalton74]

Hi iam looking for help, i just bought a 3020 gas, its been sitting for a couple years but doesnt have alot of hours on it. It runs good half throttle to wide open but will not run under half throttle inless the choke is all the way out. Ive drained tank and flushed it, flushed lines , soaked and rebuilt carb and put a new fuel pump on it. it still runs like crap what am i missing please help me before i pull all my hair out.

 

[msb]

Check the centrifugal timing advance in the distributor to see that it is free to rotate.Remove the rotor and put a couple drops of oil on the center of the shaft.Could also need the points & condenser replaced. Then run a compression check. You could have a sticky intake valve or maybe it needs the valves ground. Did it set inside all that time?

 

[jbwalton74]

i forgot to say i put new points ,condenser,plugs, wires and cap on too. i think it sat outside, motor and tranny were rebuilt and it was used very little after that. the valves were way out of adjustment that was the first thing we did. when it is idling with choke pulled out it wont idle down it is running at around 1200 rpm and thats as low as it will go. if i play with idle screw it speeds up more. it has a zeinth carb

 

[Boog]

Did you set the timing?

 

[Bob M]

Presuming the ignition is correct, problem is a lean condition at lower RPM’s. Look for two possible causes:

1 – Vacuum leak:

Could be leaking carb to manifold, or manifold to head gasket. If your 3020 distributor has a vacuum advance might also be a leak in the vacuum line or a ruptured advance diaphragm on the distributor.

2 –Idle fuel circuit is plugged:

The Marvel carb used on the 3020 has several small orifices and bleed ports in the idle circuit. Any blockage anywhere in the circuit will cause exactly the lean problem you describe. Even a tiny bit of rust, varnish, crumb of gasket material, etc. in the wrong spot will ruin low speed running. Fix is to THOROUGHLY clean the carb idle passages with fine wire then blow out with compressed air. (Just soaking in solvent and blowing is frequently is not enough...)

I’ve never messed with a 3020 with a Zenith carb but I'm sure it's similar.

 

[jbwalton74]

thanks for the advice someone else told vacum leak ,i replaced the carb to manfold gasket, i will check the other gasket to see if its leaking. The carb i cleaned reallg good. we are going to check the timing this weekend. iam not sure if my dist. is vacum advavce or not.

 

[Bob M]

If pulling the choke helps the engine run at lower RPM's it's virtually certain you're dealing with a lean mixture problem and not timing.

I've done a lot of carb/ignition work on a friend's 3020G over the years. But for the life of me cannot recall whether there's a vacuum advance on the dist. (If it DOES have a vacuum advance I know I've never had to mess with it!)

 

[Anonymous-0]

The Marvel carburetor used on the 3020/4020 is a dicast abomination. Not even Deere shops can tune it. Never could. There's an early 70s service bulletin that says put on a Zenith.

The M-S carburetor uses a large rectangular diaphragm for the accelerator pump and it fails along with some ball check valves part of that circuit. The 4020 shop manual has many pages on the M-S carburetor, but only a couple on the Zenith.

The Zenith used looks like the M-S used on most other tractors, oval float bowl, etc. Its far easier to tune.

Virtually NO gas farm tractor has used a vacuum advance. It would benefit part throttle economy but its not there.

Gerald J.

 

[Bob M]

Gerald - Your observations on the Marvel USX square aluminum carbs is right on! Miserable things to adjust/keep adjusted. The oval, cast iron Marvel TSX carbs are much preferred.

Re vacuum advance on tractors, both my Oliver 1650 and 1850 are so equipped. However I think all it does is reduce backfiring out the exhaust when you snap the throttle shut from full RPM, and possibly improve fuel economy a bit at very light load.

 

[Anonymous-0]

From the engine design books I've read, the gurus say there's 10 to 20% improvement in fuel economy with 10 degrees more advance at part throttle.

I know when I put a Mallory dual point distributor without vacuum advance on a Pinto wagon, that it ran fine at full throttle but was doggy and had poor fuel economy at part throttle.

On my 4020 which is know to be thirsty, I've wished for a vacuum advance on the distributor, but there's no proper vacuum tap in the carburetor. It needs to sample the vacuum in the region of the throttle plate, but when the throttle plate is closed it wants to see higher pressure than the intake manifold, e.g. to be separated from the intake manifold by the closed throttle plate. I've thought the "easy" way to get a vacuum tap might be to run 3 horizontal dual barrel carburetors (maybe from an early 6 cylinder Corvette) on the 4020. That would get away from the mixture unbalance caused by the yard long intake manifold, but sure would be a tuning nightmare.

As far as I can tell the cast iron TSX M-S was never used on the 3020 and 4020. The Zenith that was used looks like that though. Probably the M-S TSX family didn't have the flow range needed for the 4020.

Gerald J.