Main jet sizes

[George Willer]

I've never bothered to check main jet sizes before, but the question recently came up. Not very scientific, but I measured several I have access to. The results were surprising!

I measured 6 main jets by using the shanks of number drills and here's what I found:

1 #57 = .043"
1 #60 = .040"
3 somewhere between #63 = .037" (no go) and # 66 = .033" (loose) (some klutz broke or lost my # 64 and # 65 drills )
1 #67 = .032 new as received from C&G.

So... herein may lie the answer to why so many Cubs run differently??? It seems reasonable to suspect that the larger the hole, the richer the mixture. The answer to rich running may very well be to replace the main jet.

The 4 different jets listed in the parts manual make no sense to me whatever, since the numbers on the carburetors aren't all legible, and the jets have no marking at all. The parts manual isn't helpful regarding size.

I'd be interested to hear what others think.

[Dale51]

George I thought that what guys were referring to as the main was the venture tube but was not sure!
The size variance I was referring to in the other post was to the venture sizes.

I just went & tried the 2 main jets I have (as i have one missing & need one where can I buy one if you know) my two sizes are as follows
#59 .041" power unit
#63 .037 47 cub
My last one is on the cub & I am not messing with what is running well.
Do you know which year the jets are each from?
Wondering if this has any thing to do with it.

[George Willer]

Dale,

No, I don't know. The carbs have been swapped around a lot over the years and I tore 8 of them down and mingled all the parts.

Some of the throttle bodies have legible part numbers, but most don't. None of the other parts are numbered, as you know.

I haven't investigated whether the venturi nozzled are all alike or not.

I do know that the idle tube also has a very small orfice in the upper end that has to be there, whether the rest of the tube is there or not. Those orfices may also vary in size.

Armed with the knowledge the jets are not all alike will help some in future tuning.

[400lbsonacubseatspring]

I've never bothered to check main jet sizes before, but the question recently came up. Not very scientific, but I measured several I have access to. The results were surprising!

I measured 6 main jets by using the shanks of number drills and here's what I found:

1 #57 = .043"
1 #60 = .040"
3 somewhere between #63 = .037" (no go) and # 66 = .033" (loose) (some klutz broke or lost my # 64 and # 65 drills )
1 #67 = .032 new as received from C&G.

So... herein may lie the answer to why so many Cubs run differently??? It seems reasonable to suspect that the larger the hole, the richer the mixture. The answer to rich running may very well be to replace the main jet.

The 4 different jets listed in the parts manual make no sense to me whatever, since the numbers on the carburetors aren't all legible, and the jets have no marking at all. The parts manual isn't helpful regarding size.

I'd be interested to hear what others think.

Well, George,

If we assume that the "new" .032 jet is rated at 100% of desired flow, and it's area of fuel passage then is .000802 sq inches.

at .033, the area of passage is .000854 sq inches, or 106% of desired flow...not too bad.

at .037, the area of passage is .001074 sq inches, or 133% of desired flow. not great, but probably not bad enough to cause problems yet. You could probably run a high ethanol blend with this one, and not notice a performance difference.

at .040 the area of passage is .001256, or 156% of desired flow. I imagine this one is a problem causer, as it's in the ballpark of the jet size you'd need to run LP gas at near-atmospheric pressure.

at .043, the area of passage is .001451 sq inches, or 180% of desired flow. This one is wayyyyy rich, and undoubtedly should cause problems. It's in the ballpark of what you would need to run methane or nat gas at near-atmospheric pressure.

IMO, your #63 bit is probably a good "fail" indicator for what would be acceptable when trying to put a cub back into specs.

of course, with all the variations as far as carb air leaks, quality of spark, minor manifold leaks, valve lash, etc, some carbs "need" to run richer in order to cover for these other failures. It's not that they are running richer, but just have large orifices, which mask other problems.

Putting a new .032 jet in a tractor with a lot of other problems such as these would probably cause it to run too lean.

This might explain the phenomenon, though, of the number of posts from guys who have done extensive work to their cubs, only to find that the plugs are now fouling.

Thank you, George, for the data. I think it will help a lot of people with this particular problem.

[WKPoor]

Now we are getting somewhere!
I am surprised to know that not all Cub Carbs are the same. Cant' figure why there would be so many jets sizes but it now stands to reason that what anyone person has is a crap shoot.
My approach would be for George to measure his best running tractor jet size and possibly others also and then compare notes. The data my help those in the future.

[400lbsonacubseatspring]

Bill,

It may be that they all start out within a certain range, and simply wear, from fuel and impurities therein to the larger size.

If water can carve out the grand canyon, crap in our gas supply can enlarge a jet orifice a few thousandths of an inch.


George,

Are there 4 new jets available currently?? Are they all still available?? I would be interested in buying these and having you measure all 4, if you're amenable, so we have a reference point for the new jets, and hence a reference for their degradation specs.

[cowboy]

I was at my IH dealer two weeks ago and he said none of the main jets were avialable but you know how that goes anothee dealer or salesman may deal with it more and know how and where to find them. I do hope those that can will check their main jets for size and how the engine runs at that jet size and if it dosent or complictating problems. I wish I could cheak my main jet size but it might be time for me to buy #drill index.

PS it always amazes me how knolage there is on this site

Billy

[George Willer]

I was at my IH dealer two weeks ago and he said none of the main jets were avialable but you know how that goes anothee dealer or salesman may deal with it more and know how and where to find them. I do hope those that can will check their main jets for size and how the engine runs at that jet size and if it dosent or complictating problems. I wish I could cheak my main jet size but it might be time for me to buy #drill index.

PS it always amazes me how knolage there is on this site

Billy

Billy,

The main jet I got recently from Burmuda Ken at C&G was PN 354192R91 and measures .031"... so we know it's available. It's listed for engines from serial numbers from 501 to 172438, but that's assuming the carb and all its' parts are the original ones.

The parts CD and TC37-F list the same additional part numbers for more jets for other serial numbers or carburetor numbers:
359551R91
364610R91
404945R1

I have no way to know what the sizes would be for the others... and for sure the parts man won't know or care.

There are also two different discharge nozzles listed. I would assume they are intended to be used with specific jets. The plot thickens!

If you get a number drill set, be aware that these sizes are smaller than is usually found in sets. Commonly found sets run from sizes #1 to #60... larger to smaller. Only two of the jet sizes I found were within these sizes. My drills from #61 through #80 were bought individually. The #80 is finer than frog hair... .0135".

[Donny M]

Just went out to measure some jets I have. Can't find my little packet with my small number drills I do have a full # set and the jets I have are smaller dia. than .040" or #60. I'll keep looking for the packet of larger numbers

Interesting thread, Mr. GW. Thanks for starting it

[Lurker Carl]

Blue Ribbon Service Manaul/ GSS-1411/Section 2/Fuel System/Page 2

Here are the specifications for the two versions of the IH carb. The air bleed, main jet and discharge nozzle are calibrated sets. Dan Overcash had problems with his Cub running too rich - it turned out he had the larger size main jet and getting the smaller one fixed the problem.

[moe1942]

Any jet with the same body profile and thread pitch will work. They are simply metering orifices. I have fine tuned all manner of engines by swapping jets.

Altitude plays a big role in fuel jet size. Here where I live I am 89 feet above sea level. Air is denser so I need a tad more fuel to get the proper fuel/air ratio. Someone in Denver may need the opposite. I dare say that most Cub owners have the time to experiment..

If you don't want to get a handful of jets to play with, get one several sizes smaller than recommended and broach the hole slightly larger until the Cub hums...


And yes, I know many factors besides sea level affect air density.

[Phillip W. Lenke]

Having read the post. I decided to check the main jet dia, checks at .039
So I went rogue. I drilled thru from the hex in and tapped for a 6-32 set screw. ,punched the oriface out ,made a new one, drilled to .031. Identical to the old one except that it is about .007-.009 smaller. I hope this takes care of my carbon problems or at least helps. Plan to run tonight as we are getting snow right now 3-5 inches. SO I hope this works. Will let you know. George did you say how much a new main jet cost?
Phil

[Donny M]

Phill,

I asked Mr. GW that question last night he said "$18.00 puls shipping" I think I can make them for much less.

[Phillip W. Lenke]

So it would be about $22-$25 shipping being what it is .

[George Willer]

Having read the post. I decided to check the main jet dia, checks at .039
So I went rogue. I drilled thru from the hex in and tapped for a 6-32 set screw. ,punched the oriface out ,made a new one, drilled to .031. Identical to the old one except that it is about .007-.009 smaller. I hope this takes care of my carbon problems or at least helps. Plan to run tonight as we are getting snow right now 3-5 inches. SO I hope this works. Will let you know. George did you say how much a new main jet cost?
Phil

Phil,

I like your idea better than what I was planning. I had planned to solder the orfice shut on the oversized ones and drill the solder to size. I think I can do the drilling and tapping in the lathe without disturbing the hex end. The stub of a screw will be locktited in place before drilling the new orfice.

Next I have to make some sense of the differences in venturi nozzles. Maybe if we all compare notes we'll be able to identify which nozzle goes with which jet? Part numbers are NO help at all!

[/url]