Idle Jet # and size

[BadaBing]

For search- ability I am posting this in its own thread.

Zenith Carburetor Idle Jet part number T55-6-14

according to zenith specs this should be an 8-32 thread. More as news developes

[hanleyclifford]

This is Bill's Fault

I had a look at that GT Enterprises Zenith carb page and learned some interesting things. http://gtgravelyparts.com/zenithcarbs.html For one thing they offer a new carb for $325, $21 more than Moyer Marine. In addition, their carb is a rear facing unit. They claim the forward facing unit is no longer available but that is sort of a whopper. They also claim the carb cannot be reversed to face forward but a cursory study of a few of my spares leads me to think I could indeed turn an A4 carb around and draw air from aft. This offers some intriguing ideas, especially for us adjustable jet guys. Even the fixed jet installation would face the idle adjuster forward where it is more easily accessed. Of course, that would mean no more mechanical fuel pumps. The idle mind is the devil's workshop.

[edwardc]

... a cursory study of a few of my spares leads me think I could indeed turn an A4 carb around and draw air from aft. This offers some intriguing ideas, especially for us adjustable jet guys. Even the fixed jet installation would face the idle adjuster forward where it is more easily accessed. ...

Of course for us V-drive guys, with our "backwards" engine, the adjustable main jet and the idle adjuster are already conveniently accessible!

[Al Schober]

The thread on that jet is #8-32. The nut I tried was in my #8 bin, but was actually a #10-32 - no wonder it was loose on the jet!

[BadaBing]

main and vent jets

While messing around and in the interest of perfection AND after trading idea with Capt. Neptune I decided to attempt to measure my Main and Bowl Vent Jets. My findings suggest very strongly that my carburetor is an after market, replacement, carburetor.

If you look very closely at the two jets you can see the number 21 stamped on both jets. My best measurement, using available types of wire and needles is that both of these jets are slightly larger than .039 and smaller than .045. Based on feel I am inclined to think, now, that they are both .040 or .041. However, the vent jet does look slightly larger.

As an interesting side not re. jet sizes. The number stamped on the jet does not seem to correlate to any actual measurement. I began wondering IF possibly the # is a "gauge" size. I just happen to have a #21 surgical needle on hand and trusting that the medical community is careful to stamp needles accurately I tried the needle to size the jets. The needle slid very easily into the jest. A measurement with of the needle dia reads .039. Allowing a little margin for error in my micrometer that would explain the numbering.

However it has been suggested by one wiser then I that the "factory" carburetors came with these two jets differing in size. Possibly .0421 for Main and .029 for the vent jet.

I know this is getting rather picayune, almost like counting angles on the head of a pin, but it is winter and I need some puzzle to keep me away from daytime TV.

Hanley you seem to have a few jets on hand. Can you measure a few of them and let us know what sizes you find?

[hanleyclifford]

The short idle jets I have are .043" which is a #57 drill bit. The long version of the idle jet is also .043" until just near the end which appears to be knurled to a smaller size. I have some small drill bits on order and will report back on this.

[BadaBing]

While preparing your report

If you can please measure a vent jet or two. There is a theory about that a smaller than .042 vent jet, in the range of .028 cause the main just to "kick in" sooner during the opening of the throttle. Theoretically causing a smoother transition from idle to mid range to wot. This allows a leaner overall mix, better economy better performance. At least that is how I u understand the theory.

Supposedly the older accelerators had the smaller vent jets.

With regard to the actual idle jet. On my carb, which had the short version of the idle jet I see a small brass tube sticking out of the oriface here the idle mix adjustment needle it. I think the newer versions have this time pressed into the passage while the older version incorporated it into the jet itself.

[hanleyclifford]

The thread on that jet is #8-32. The nut I tried was in my #8 bin, but was actually a #10-32 - no wonder it was loose on the jet!

I was able to get one of these things out real clean but an 8-32 screw would not go in by hand and I don't force things into an aluminum housing. I remain unconvinced this is an 8-32 and I'll wait for my small metric taps for further testing.

[Al Schober]

Hanley,
Just try the jet in a #8-32 nut. The jet I tried is unused (& not corroded) and fits a #8-32 nut just fine.

[BadaBing]

idle jet tap size

I received a call today from Todd at BY Enterprises 're the jet and tap. Todd said he called the factory and spoke to the makers of the zenith 68 carbs. They told him that the tap/ thread is metric. Size 4P .75
4P being the pitch of the thread and .75 mm he said the standard conversion works out to .1 5 0 to '154 dia.

[hanleyclifford]

Getting Impatient Waiting

for my metric tap set, so I went out and bought an M4x.70 screw and ran it thru the idle jet seat in the housing. The jet then turned in to seat. Interestingly, however, the OD of the screw is .149" whereas the OD of the jet (on thread) is .154". The fit is snug but satisfactory; do not crank excessively. It pays to take a moment and grind the top of the jet where the slot tends to spread the jet and make insertion difficult because the well in which it lives is tight. I am about 90% certain M4x.70 is the ticket; BTW I cannot find there is any such thing as M4x.75 - can anyone help on that?

[romantic comedy]

as far as I know, the 4 x .7 means that the screw is 4 millimeters and the pitch is .7.

Thinking about it... if the screw was .7 mm it would be .027 inches. That is smaller then 1/32

[ndutton]

he called the factory and spoke to the makers of the zenith 68 carbs. They told him that the tap/ thread is metric. Size 4P .75
4P being the pitch of the thread and .75 mm

Now that you have the size you can get the proper tooling and put this issue to rest. The tap is available from McMaster-Carr, their #26015A154. Diameter is 4mm, thread pitch is .75mm

http://www.mcmaster.com/#standard-metric-taps/=10t9h6c

[BadaBing]

Now that you have the size you can get the proper tooling and put this issue to rest. The tap is available from McMaster-Carr, their #26015A154. Diameter is 4mm, thread pitch is .75mm

http://www.mcmaster.com/#standard-metric-taps/=10t9h6c

Thank you

[hanleyclifford]

Now that you have the size you can get the proper tooling and put this issue to rest. The tap is available from McMaster-Carr, their #26015A154. Diameter is 4mm, thread pitch is .75mm

http://www.mcmaster.com/#standard-metric-taps/=10t9h6c

Yup. I should have checked McMaster first. Time to get more familiar with metric because our carbs have several different size jet seats and I want to confirm them all.