420 c rough idle

[Paul Buhler]

Hi: My 420 c sort of lopes when idling. I rebuilt the carburetor earlier this year without changing the idle jet - it looked smooth and the seat appeared undamaged. The tractor runs well under load and has plenty of power and starts easily, so I'm assuming that the timing, plugs and wires - which I rechecked the other day, and load jet are ok. When I try to adjust the idle screw, the tractor runs smoothest with the jet completely closed leading me to guess that the idle jet is worn, or that I have an air leak between the carb and the cylinders somewhere. One other thought that I haven't check yet is that the valves need to be adjusted - the engine was run rich for a long time by the prior owner and the carbon build up is showing up in the oil; would loose valves affect the idle without affecting load? Any ideas on how I might isolate the problem would be appreciated. Thanks. Paul

[Ray III]

If you can turn the idle screw all the way in with the throttle fully closed and the engine keeps running, something is wrong with the carburetor. Take it apart again and find out where fuel is sneaking into the venturi.

[H-D]

With unleaded gas, some of the older engines with relatively 'soft' valve seats will eventually get into valve seat recession, makin' the valve lash too tight, but this usually presents itself as much harder starting in cold weather.

[Paul Buhler]

Thanks for the input. There's a lot of experience shared on this board and I appreciate everyones ideas; keep 'em coming. I plan on resetting the valves soon, and I'll take apart the carburator again and see what I find. The venturi is a press fit, and felt snug. Should it be bedded with something?

[snoopy]

You might want to check and see if the pivot shaft for the throttle butterfly is worn causing extra air to being sucked in. The idle jet controls the flow of air in the idle system. the high speed jet controls fuel flow.

Good luck
Bob

[Ray III]

You might want to check and see if the pivot shaft for the throttle butterfly is worn causing extra air to being sucked in. The idle jet controls the flow of air in the idle system. the high speed jet controls fuel flow.

Good luck
Bob

You sure about that?

When I took mine apart I thought there was a fuel passage to the idle needle.

[snoopy]

Not 100 %, but that is what it says in manual for MC carb. I think they are very much the same, but could be wrong. It says turning the screw towards it seat richens the mixture. Aproximate setting is 7/8 turn open.

for what it is worth
Bob

[Lavoy]

On the TSX series Marvel Schebler's, the idle jet is an air jet. I would guess you have a passage plugged. Did you remove the jets and blow through every passage?
Lavoy

[Paul Buhler]

I believe I have a TSX 688 carb. My reading of the manual says that the idle jet controls air flow - closed, richer mix, out leaner mix, as Levoy says too. I did use compressed air and carb cleaner on everything when I had it disassembled looking at the manual as I went. I thought that I had checked all galleries - but you never know those ports and holes are pretty small and I may have picked something up since. The leaking bushings around the throttle plate shaft sounds like something to pursue. I was thinking of applying grease around the outside to cut off any air leakage into the carb there. If I applied the grease while the tractor was idling and I noticed a change it would seem to me that I was affecting something. I'll be double checking the carb to manifold seal too.

This is more of an annoyance than a problem. I like the smooth running sound I get under load and would like to hear the same smoothness at idle too. Purring is good

On the other idea regarding valves and lead free gasoline, what is recommended for machines that will be run hard periodically, but not all that often? I need some free time to get to checking the gap.

As usual, thank for sharing. Paul

[H-D]

Since you asked, I 'm an advocate of Marvel Mystery Oil, use it in my ol' shovelhead Harley's to good results. Used it in my '58 Cadillac coupe (now the ol' lady's) in both the fuel & oil. Made the lifters quiet down remarkably quickly, it's good stuff for removin' sludge deposits too (removes gradually by detergent action, rather than by turnin' sheets of the stuff loose to clog the oil pump strainer like hot solvent products do. Since I'm repeatin' on this, I'll risk it again & suggest that you check your ignition timin' for function with increase & decrease of engine speed. Older machinery which has sat for a spell (especially these crawlers) is prone to stickin' of the advance (centrifigal) in the distributor. Stickin' in the retarded position robs power & makes for balky acceleration (sometimes even backfire), while stickin' in the advanced position makes for harder startin' & rougher idlin'. I ought make this into a macro, since I've spouted it so many times!!!

[Paul Buhler]

Well I guess that putting a Marvel oil through a Marvel Carb should produce some magic somewhere - hopefully good. I think I'll go other routes first - I'm a bit cautious about using additives, but since the non lead fuels have had something removed, I'll assume that there will be a reaction - like more valve seat wear, so using an additive designed for this specific deficiency makes sense to me. I'll double check the distributor advance/retard too. Thanks, Paul

[Ray III]

Actually I don't think these engines were meant to run leaded gas. They had different grades of gasoline then as now. The compression ratio is pretty low to allow the use of lower quality gas.

[snoopy]

They had different grades of gasoline, but they all had lead in them.

[snoopy]

The Facts: From the mid 1920's until the mid 1980's motor gasoline fuel contained an additive, tetraethyllead, that improved fuel performance by preventing preignition in the cylinders of the engine. Preignition resulted in damaging and efficiency and power reducing knocking.

for what it is worth

from a google search on leaded gasoline.

[H-D]

Tetraethyllead had the beneficial side-effect of lubricatin' the valve & seat contact 'patch'. An additive to 'help' the valves & seats IS beneficial! The possible exception to this are some air-cooled engines (Corvair for sure) which had harder than average valve seats. Did y'all know that the EGR valves which were initially employed by the automakers to lower combustion chamber temps (which lowered NOx) also had the beneficial side-effect of allowing more total spark-advance & a more agressive spark timin' curve? Ya never know what 'good' will come outa the Yankee government!