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Marvel Carb throttle shaft cause this?

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Jeff in ND

07-07-2014 07:33:28

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Last summer I posted about the 1948 Z on the farm.

I only get a chance to play with tractors a couple weeks a year on vacation so it takes a while to get back to it!

It will idle OK, but starts to run rough and cut out some with any sort of load, even just driving up a light grade. Can"t trust it to stay running to pull a empty wagon even.

The tractor had been sitting a few years and I thought maybe old gas. The gas has been changed out and I put new plugs in but there was no change.

For good measure I cleaned out the air cleaner and gave it a oil/filter change for probably the first time in >15yrs. Fortunately, I found a couple of the NLA cartridge oil filters in the shed. I didn"t expect that to have any bearing on this problem but it made "me" feel like I did something, haha.

Any rate, I wondered if there was a vacuum leak someplace as I could hear some hissing when I had the throttle opened up over idle and stood nearby the intake. I vented some propane torch gas around the intake to head gaskets but that didn"t have any effect. Trying to track the hissing down I discovered that it was not there at idle but when the throttle was higher, the governor was having a bit of a problem: When the engine would run rough and cut out, the governor would try to keep it up by pulling the carb throttle arm (as expected). Then I could hear the hissing get worse.

So, I manually wiggled the carb throttle shaft and discovered its really floppy in the carb. Could the air leakage here be causing this problem? I remember years ago we had a old mower that would race speed up/down because the throttle shafts flopped in the casting.

I see from some searching there is a bushing available for this. Was/is there one in there originally (I haven"t tried to clean or take the carb apart yet)? If not, I presume I need to have the casting bored out by a shop to press this in? What about any seals?

Thanks in advance.

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07-30-2014 07:58:58

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 Re: Marvel Carb throttle shaft cause this? in reply to Jeff in ND, 07-07-2014 07:33:28  
Sounds like your are shooting in the dark. The kits I sell have throttle shafts seals and instructions. Anyway. Float level should be set as follows. Turn the top half of the carb upside down, leave the gasket off, install needle and seat, and float. Take a 5/16 stock round rod and place it under float directly over the throttle shaft, both sides of the float should lightly touch the rod. The Idle screw and the load screw should be turned in to lightly touch the bottom of the seat and then turned out 1 to 1 1/2 turns. Adjust from there to fine tune with tractor running.


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Jeff in ND

07-29-2014 12:49:48

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 Re: Marvel Carb throttle shaft cause this? in reply to Jeff in ND, 07-07-2014 07:33:28  
Back yet again!

Put the carb back on the tractor last week but could not get it to run at all. The best I could get was about 3sec. Other than that an occasional putt. It would start right up before I took the carb off so not a ignition issue.

This is not helped by me having to hand crank it :( since I have the starter and gen off and don't have a battery anyway in addition to the wiring being all crumbly and corroded. I am not quite in the shape I used to be so lots of cranking in a hot shed does not make my mood improve, haha. Quite frankly, I can turn it over faster w/crank than when it was last starting with the starter. I am sure that problem due to the poor connections in the old cables and grounds. I had a heck of a time getting the starter out due it being rusted into the bell housing and that also means the ground was rusty.

But to get back off that tangent....
I did note the gasket between the upper and lower casting of the carb was getting damp after my efforts. Float too high?? It was not dripping out on the ground though so I don't think the float needle is sticking (it was replaced with the kit).

Also, the crappy instructions with the parts kits didn't have anything on where to set stuff. I suspect that is my main problem. Where should the throttle shaft stop screw and idle mix screw be set to start with? The original manual for this tractor is around someplace but I can't seem to locate it right now. Probably got put away for "safe keeping".



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Jeff in ND

07-17-2014 06:59:32

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 Re: Marvel Carb throttle shaft cause this? in reply to Jeff in ND, 07-07-2014 07:33:28  
Back again.

I bought 2 different kits from different vendors. One kit just to get a new shaft (no shaft seals in that kit at all) and the other kit for the seals. It has both the felt and the rubber to chose from.

For the seals, should I put any oil or grease on them? The instruction sheet doesn't say anything. Seems to me that if the seals are dry the wear will be high? (not a poet, haha).

Got the carb soaking now in solvent to get the hard crust off it. I want to get this back on the tractor and try it out next week when I am back at the farm.

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Alan K

07-07-2014 07:43:58

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 Re: Marvel Carb throttle shaft cause this? in reply to Jeff in ND, 07-07-2014 07:33:28  
Worn throttle shafts can cause hunting and erratic rpms. I believe there wasn't a bushing in them originally, but not sure.

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Jeff in ND

07-08-2014 17:21:58

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 Re: Marvel Carb throttle shaft cause this? in reply to Alan K, 07-07-2014 07:43:58  

Partially cleaned it up just now and removed the throttle shaft. Does not appear there is a bushing in there now. The shaft is egg shaped about 0.006" from what appears about 0.278" dia as measured near the unworn end by the lever arm. I stuck a 9/32 drill bit in the carb casting (0.281") and its a free fit that does not have a lot of slop. But not completely snug either. Not sure how tight is this supposed to be?

There is a brass ring pressed into the end I suppose from looking at exploded pix I found to be for keeping a felt washer in there. I think that felt must be long gone.

The drill bit fits with less slop than that shaft so maybe a new shaft and felt seal will take care of this w/o needing to bore out the casting and press in a bushing? It can't be any worse at least I suppose.

BTW, there is a brass tag with "TSX 97" and below that a "1" on one side of the rivet and "51" on the other. Is that a date code for Jan 1951? This is a '48 Z and originally had the louvered side panels. I don't have any recollection of the carb on this being changed but my memory only goes back to the mid 70s (born '66). This tractor was bought new by my grandfather. Could well be a replacement from back then.


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07-15-2014 11:43:21

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 Re: Marvel Carb throttle shaft cause this? in reply to Jeff in ND, 07-08-2014 17:21:58  
Most likely the shaft is the problem rather than the body. I would try to find a replacement shaft and put a kit in with rubber seals in place of the felt ones. Some kits may include a shaft but if not try McDonald Carb & Ignition or Matt at Motec

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Alan K

07-09-2014 07:17:40

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 Re: Marvel Carb throttle shaft cause this? in reply to Jeff in ND, 07-08-2014 17:21:58  
Yes that is the date on the carburetor so if it is a 48 Z it more than likely was changed. Generally the throttle shafts are supposed to fit snug yet move freely. A slight bit of play in it is better than not enough.

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07-08-2014 19:37:23

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 Re: Marvel Carb throttle shaft cause this? in reply to Jeff in ND, 07-08-2014 17:21:58  
A good machinist could put bushings in your carburetor, but that would be a pretty big expense.
Coming up with a new throttle shaft may be a problem. Herschel-Adams (I think it was) used to have a kit with the throttle shaft in it, back in the '90s. but I haven't seen a source for that kit for years.

What I suggest is get the carburtor kit for that TSX 97 from NAPA. Their kit has a rubber boot to take the place of that felt seal. I am pretty sure with one of those in there performance will improve. I don't think you can use the boot and the brass retainer in there together, though.

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