Carb Info

Mini Bike & Go-Kart Parts

I had mentioned a new carburetor in my first post that was given to me with the Fox mini bike. It does look like the same carburetor but i can see a difference in the choke arm and the fuel inlet. Would this carb work as a spare and any information on what this carb was used on.
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Well-Known Member
Its possible the body could be used, the thing to do is if you disassemble the stock carb your Fox has on it to rebuild or service you would want to open that one up and compare a few things.

1 would be the bore (where it chokes down in the center of the body), you dont want that any smaller than it already is.
2 the porting internally
On the diaphragm carbs used on the HS40's, They feed the low speed circuit directly off the diaphragm reservoir, where the smaller displacements they feed it through the high speed circuit/inlet via a small passage drilled from high speed chamber to low speed chamber.

I would say in theory you could plug that passage and leave off the welch plug covering the low speed chamber if the main bore did measure out to be usable, but not something I have ever actually tried.

Unless your original carb is really toasted with corrosion internally though, those are usually pretty savable. Its not really the body that causes problems (if its actually cleaned correctly), its usually the throttle and choke shafts since the soft brass wears and causes air intrusion as well as erratic throttle response.

This is some information I had collected when I was doing some early slanted engines and the 2 known slant specific carbs that were made that may help:

Thanks everyone :thumbsup: That silver is kinda hard to photo, It has a different (I think better) look in person. I was pretty relieved it came out alright, this was my first time doing a single stage paint with metallic in it and I was kinda sweating it. It actually laid down and covered thinner than some of the solid colors I have been spraying from the same brand/paint line (Nason Fulthane).

Re guarding the Diaphragm Carbs, as capgun said they were used till early 1971, These were specific to Slanted engine applications only which began when Fox corporation introduced the angled engine plate design on their 1969 bike lineup.....Everyone quickly jumped on the bandwagon for 1970. In 1971 they created the slanted intakes so the bowl carbs could be used and they got phased out. I am not sure why the change was made at that time, from what I can tell a choke lever and air cleaner placement dilemma may have played a part in it, they had the same problem with standard horizontals and bowl carbs at the same time and made a revision during 1970 on those to better accept Header type systems that were becoming the norm.

There were only 2 versions of this carb that I can find, one for the H30/H35 and one for the HS40 and again their soul purpose was for the slanted application so you wont find them setup like this on anything else. Looking at Tecumseh service carburetor lists, they never offered a suitable replacement for it either. These mini bike/cycle specific carbs look virtually identical externally so you want to look at and decipher the stamped numbers found on the intake side flange of the carb body to know which one you are working on.

The H30/H35 part number is 631595, the stamp number on that body will start with 379
The HS40 is part number 631588, the stamp number on its body will start with 356

One rather unique thing that makes them stand out over most others and easier to spot is the choke lever and detent, Rather than the common spring rod detent it is a stainless sort of clip looking bracket that screws into the body, the lever is pretty standard but it has a flat spot on the upper shaft to help lock the shutter in the open position.

This is a 631588 version I just completed for a 1969 HS40 that I am currently finishing up:

Here is that carbs body from above in comparison to the H30/35 631595 version on the engine I just did:

As you can see, your really cant tell them apart externally, so it is important to refer to the stamped numbers on the flange. Only other difference I see (and I dont know if you can depend on that) is the cast number on the left (needle) side just in front of the air cleaner base mount lug which you may be able to make out in the one photo above. the H30/35 had a 21 cast in where the HS40 has a 22

I had intended to do an actual comparison build thread of the 2 carbs but that didnt work out and I needed to get the H35 finished and out of my way. Hopefully the shots I took though as I rebuilt the HS40 version the other day may help show what I did find as differences between the 2 carbs.

Key difference i found between the 2 that you will spot is under the diaphragm. above the carb body has been gutted as far as you can take it (all welch plugs and needle assembly removed)

I found the H30/35 to have welch plugs covering both the High and low speed circuits, where the HS it was covering only the High speed circuit and the low was just left open. On the H30/35 they drilled a passage connecting the 2 reservoirs where the HS did not have the passage drilled

I put this cleaning needle across on the HS body about where that passage was drilled on the H30/35:

the passage was small and angle drilled but in about that same line. OEM tecumseh rebuild kits for these cover an array of versions, so there are more parts than needed, So don't go plugging holes just because the welch plug is in the kit, If that passage is not drilled you will take the low speed circuit directly out of the picture if you plug that port up. Low speed is the one to the left of center with nothing in it but a tiny hole, High speed is the centered one with the non serviceable brass check ball assy in it.

Parts listings show that both carbs use all the same rebuild and bolt on parts, I dont see anything else visually different between the 2 bodies other than the port passage, I did forget to measure the bore on the H30/35 though :doah: The HS40 version I got about 11mm at the choke point. Thats a little smaller than the stock HS bowl carbs, but dont let that fool you. When they are cleaned correctly, worn out parts like the throttle shaft replaced, getting clean fuel supply, and on a decently tuned engine they run pretty dam good.
Great information Markus and Thank you. The original carb is in great shape, I think the factory paint help it. My concern was since the bike sat for 45 years and not started what does the carb look like inside but it did start and had a nice idle with no bog at WOT. I forgot to mention on the original carb the choke shaft has some type of cover thats held on with two screws. I will take a picture of it and post it.
Markus I did mention two screws my bad there's just one, with the air cleaner assembly on it looked like another screw.
I did take some pictures of the original carb and noticed a hole on the left side and the new carb doesn't have that hole. Other thing i notice on the original carb that the throttle blade didn't open all the way do to the bend on the linkage, easy fix. 20190612_183455.jpg 20190612_183615.jpg 20190612_183756.jpg 20190612_183839.jpg


Well-Known Member
Yea all the throttle driven type carbs were setup so the throttle plate would only open about 75-80%. That replacement carb looks to be setup sort of a govenor controlled, where its set to run WOT and the gov is what controls how a generator operates. You can manipulate the throttle driven type a little though, It can be tricky because you need the length of it to hit the flange of the body to stop from over extending but when you come back to idle it needs to clear the center section of the body enough so you can still rely on the stop screw for adjustments. With a little adjustment of the governor system to lighten up its control you can get a little more out out of the top end and a little more throttle control (just remember to limit your WOT runs to bursts- don't just peg the throttle to full open and head out on the road to Aspen to return a briefcase or anything like that ;))

That setup on the choke acts as a detent to hold the choke open when warmed up and running. You can see all that if you expand that info I posted in my above post. That was specific to the mini bike diaphragm carbs only, Its tensioned onto the shaft so it stays locked open (or supposed to) when it meets the flat spot ground into the choke shaft. They dont like to be manipulated much if it gets loose, You can crack/break the tab easy. You would be better off modifying another choke shaft by filing a flat spot into it it ever came to that....which is most likely the cause anyway due to wear on the shaft.

Judging by the looks of your original carb though, you have a pretty clean time capsule like example going on there so I doubt you have any severe wear issues as of now though.

Have fun with it.
Thanks again Markus great info. I was think what you had said about a time capsule, taking detail pictures of the mini bike and parts on it since has never been taken apart for members and future members that are restoring one or has one in pieces and cant remember how the part goes on.


Well-Known Member
Thanks again Markus great info. I was think what you had said about a time capsule, taking detail pictures of the mini bike and parts on it since has never been taken apart for members and future members that are restoring one or has one in pieces and cant remember how the part goes on.

That would be a great thing to do, there is a thread called original survivors thread where at least a majority of the bikes are 90-100% accurate originals thats great for referencing that a good photos of it would be great in. photos Loaded into the fox category in the media section of this site are helpful as well for people searching.

From what I saw of the pic of your bike it looks like its the early small head/early shrouding engine so its one of the earlier made bikes.