Surging only at full throttle

Wed Aug 07, 2013 10:26 am

Hey guys! I've been lurking around here for a while, and let me say this site is an absolute treasure trove of information for a new cub owner like myself. :worthy:

Anyways, I can't seem to pin down the source of my current troubles, so I thought i'd ask for opinions.

My 1955 cub idles fine at low to medium rpms, and it'll even run smooth at high speed if I open the throttle very gently. If I crack it open fast or if my mower runs into some thick grass, it'll start surging up and down. I know this is either lack of fuel or a problem with the governor, but how do i tell for sure which it is? and if it's the governor, is there any external fiddling i can do before taking the thing apart? I've got a lot of mowing to do...

Re: Surging only at full throttle

Wed Aug 07, 2013 10:59 am

On the very front of the governor is a jam nut and bolt. loosen the nut and turn the bolt in about a half round or so. that moves a spring in toward the governor to limit the surges. It may need a little more than a half turn, but only do it a little at a time. If it is turned into far it will restrict the governors ability to respond to loads. the full instructions are in the IH service manual.

Re: Surging only at full throttle

Wed Aug 07, 2013 11:15 am

If this is a relatively new problem, check fuel flow through the carburetor and check for partial obstruction in main jet.

Old problem, check linkage adjustment between the carburetor and governor arm.

Re: Surging only at full throttle

Wed Aug 07, 2013 11:21 am

Follow Eugene's suggestions before doing anything with the surge limiter I suggested. I should have mentioned that, especially with you mentioning that you are a new cub owner. I tend to look at a problem and think someone working on it has gone over the other things that can cause it, and not think about the fact that someone new does not know the other problems to check. the rod connecting from the governor to the carb binding from paint or dragging on the block where it passes the front ear is a frequent problem on a new purchase.

Re: Surging only at full throttle

Wed Aug 07, 2013 3:26 pm

Thanks guys! I'm not certain that it's not the carb, but When i advance the throttle manually (post governor) it runs smooth, so that's why I'm thinking it's the governor. I did however have some concerns about the carb because the idle screw doesn't seem to do anything at all.. I can unscrew it all the way out or screw it all the way in and it doesn't make a difference. Oh, and it's the IH carb btw. I also noticed that the fuel flow was getting pinched off when the strainer bowl was screwed on tight, so I put a new gasket on it but i'm not sure if it solved the problem or not. it still wants to leak unless i tighten the dickens out of it.

I guess i'll try the governor screw and if it doesn't solve the problem I'll rebuild the carb (that's on my to-do list anyways!).

Re: Surging only at full throttle

Wed Aug 07, 2013 4:05 pm

Hey Neighbor! Wow, you're about the closest I've seen so far. I'm in Grand Island, but my cub came from Oakfield, where it spent it's life on a small farm across from my hunting camp. I own some property out there on Fisher Road, maybe 10-minutes from you.

Sounds like the guys have got you covered for the governor surging. Before trying to adjust the governor, I'd double check the fuel flow to the carb, and also double check that the linkage to the governor is not rubbing somewhere. It's really common to see it rubbing on the block between the governor arm and the carburetor. I've seen many this way. Also check the inlet to the carb where the second filter is. Those are real bad about plugging up.

Welcome to the forum.


Re: Surging only at full throttle

Wed Aug 07, 2013 5:33 pm

Eoghan wrote:the idle screw doesn't seem to do anything at all.
The idle screw only adjusts the air flow through the idle circuit. You should notice a change in rpms and/or idling performance at low engine idle when adjusting the screw.

Carburetor and fuel system, first check.

Re: Surging only at full throttle

Wed Aug 07, 2013 7:24 pm

Sounds like lean high speed mixture to me.

Re: Surging only at full throttle

Wed Aug 07, 2013 9:32 pm

alright, i'm going to pull the carb off tomorrow, give it a good cleaning and double check that the fuel flow from the tank is strong. I didn't realize there was a second filter at the carb! that's the first thing i'll check. there was a lot of gunk in the fuel strainer when i cleaned it, so i'm sure there's gunk in the carb too.

gitractorman: wow! you're just up the road. I actually bought this tractor out your way in Tonawanda from a guy that was using it to move a trailer around his back yard and plow his tiny driveway... seems to be in decent enough shape despite the rusty tank and lack of paint.

Re: Surging only at full throttle

Thu Aug 08, 2013 8:28 pm

pulled the carb apart today, it was FULL of rusty gunk. I was amazed that it even ran before. So I started cleaning it out (I don't have a rebuild kit just yet), everything went well except that i couldn't figure out how to get either of the "tubes" out.. I think one is the discharge nozzle and the other is the idle tube. I put the thing back together anyway and now when i decelerate or accelerate there's this weird spot that the engine hiccups and wants to die. I assume something is blocked in one of those tubes. is there a trick to getting them off? the one has a hex piece on it but i can't imagine fitting a wrench down there... the idle tube i have no clue..

oh, I also noticed that all of a sudden it's making a weird suction cup noise when i shut the engine off. never done that before..

Re: Surging only at full throttle

Thu Aug 08, 2013 8:45 pm

Eoghan wrote:is there a trick to getting them off?
Deep well socket.

Re: Surging only at full throttle

Thu Aug 08, 2013 8:51 pm

That's what i figured, but i don't have any that small! I guess I'll be headed to the hardware store again.

Re: Surging only at full throttle

Thu Aug 08, 2013 8:53 pm

As eugene said, the discharge tube in the carb throat can be removed by a deep well socket, or a nut driver if you have one that is hollow.
The idle tube is a bit more tricky, and if you can't get it with a modest attempt, it's probably best to leave it be. Spray carb cleaner up the tube, and look for two streams of the cleaner exiting two holes in the upper carb throat (wearing safety glasses of course!!) behind the throttle plate.

Re: Surging only at full throttle

Thu Aug 08, 2013 8:54 pm

If you have a set of nut drivers, those also work well for the discharge tube and main jet. The idle tube is threaded in about 1/4". If you're sure you need to remove it, spray it for several days with a penetrating oil and then slowly wiggle it back and forth with a good pair of needle nose pliers until it will let you unscrew it. You risk breaking it even if you are careful, and that repair involves drilling and chasing the threads. A rebuild kit probably isn't needed, a new "OEM" body gasket may do it. Check to make sure the top isn't warped by placing the halves together without the gasket and see if it fits evenly all around.


Re: Surging only at full throttle

Fri Aug 09, 2013 5:09 am

The two holes Raymond mentioned, in the carburetor throat at the throttle plate, are often plugged; a piece of copper wire will open them right up and make a marked change in idle characteristics on some cubs. Also make sure all loose debris is flushed out, or you will immediately stop up the main jet; it also can be cleaned with a strand of copper wire.