M74B linkert high speed needle does nothing

[flatheadDave]

Hi all
I’m running an M74b carb on my 63 panhead for a bit now. And the power needle doesn’t really do anything. The low speed needle when adjusted changes the idle speed like it should and will kill the engine when I step it down. The power needle however doesn’t effect the engine at all. I can be on the highway or side roads going 45 mph and turn the needle all the way down and the bike runs exactly the same as if I have it 3 turns out. My friend tells me that I need to replace the jet. The plugs are getting fouled and I always usually have a back fire when riding down the down. I think from manuals, the jet is the high fixed jet #19 part number 27318-51

Does that sound right ? Or do I need a new needle too?

Here’s a pic of my bike

[panhead_kicker]

... the power needle doesn’t really do anything...

You can search the KB and find lots of musing about this, but I think the consensus was that the high speed does little if anything , with the correct jet.

... I think from manuals, the jet is the high fixed jet #19 part number 27318-51

Does that sound right ?

Yes, that is the correct jet for your M74B
In regard to plug fouling, that could be several things...Check the orifice of your #19, I *think* it should pass a drill shank no larger than a #48. The orifice is NOT a straight through bore, there is a narrowing step at the end of the threaded portion. So if yours lacks the step, someone could have previously opened the bore.
Also, what float are you using?

[flatheadDave]

I’m using a rubber ducky float. I’ve had a couple of linkerts before. M18. M51. M88. I’ve tuned these high speed needles while the bike was running and could defiantly tune the high speed with the needle. So somethings wrong with this one. I have to disagree to the statement that the high speed doesn’t really do anything. But I appreciate the suggestions and input.

I guess it could be I have the wrong length needle for the high speed or the 19 jet is messed up somehow. Like I said, for like a year now I’ve been running this way without being able to adjust the high speed needle. The plugs are dusty black. The bike starts fine and I have power and can ride no problem. Other than the back fire and sooty plugs and as mentioned, the high speed needle doesn’t change the bikes performance when seated.

Does the needle actually seat INTO the #19 jet? Or just above it blocking the flow?

Does someone have a measurement of how long the needle should be?

[panhead_kicker]

Oh, I'm sure on particular L&L models the needle performs as expected, but on the M74B per the topic...
On my 64 FLH, I too experience no noticeable high speed needle effect on my M74b with #19 jet. I run with needle closed.
For your float, did you set the height in bowl? I only know what the book says, and I use one of Tom Cotten's floats, which are manufactured to be set per HD info (1/4" below lip). But its possible an incorrect setting could contribute to rich running.

As you can see, the high speed needle does not interact with jet...
The needle shaft length from first thread to tip, is 2-7/8 "

M74B

IMG_4187.JPG (183.27 KiB) Viewed 891 times

[panhead_kicker]

Correction, the high speed needle is 2-3/4"

[RUBONE]

The high speed needle on an M74B is merely a "trimmer" and barely affects the overall running. It is more noticeable at elevation. The M74B uses a fixed main jet, usually a 19 but 17 and 18 are also available. In many cases the best overall adjustment ends up being shut off. A smaller main jet may help with that.

[flatheadDave]

Oh, I'm sure on particular L&L models the needle performs as expected, but on the M74B per the topic...
On my 64 FLH, I too experience no noticeable high speed needle effect on my M74b with #19 jet. I run with needle closed.
For your float, did you set the height in bowl? I only know what the book says, and I use one of Tom Cotten's floats, which are manufactured to be set per HD info (1/4" below lip). But its possible an incorrect setting could contribute to rich running.

As you can see, the high speed needle does not interact with jet...
The needle shaft length from first thread to tip, is 2-7/8 "
IMG_4187.JPG

I did set the bowl correctly. It is worth double checking the bowl float height and the intake for leaks which I’ll do. Thanks for the measurement. The needle I have might be incorrect then. Check out this pic I took.

[Andygears]

In my opinion, and only that, I feel that the high speed needle works in conjunction with the high speed jet. Putting a #17 in my M74 B helped my pan be adjustable with the high speed needle, replaced the #19. For a trial, you could put both jet holes with a blank, open the high speed needle more and try it. Some Linkerts for flatheads run both jets blanked. Anyway, room to make it leaner and maybe get adjustability.

The other factor on that main jet is the main nozzle. When you remove the Venturi tube, there is a groove around it that lines up with the bowl vent, hence, outside atmosphere pressure when assembled. IF, IF, that main nozzle does not seat in the counterbore of the Venturi tube, by spring pressure, and flat sealing, then the vacuum formed by the nozzle/Venturi to pull the gas up the main jet into the carb becomes spoiled with inconsistent results. Sometimes carbon keeps the nozzle off the seat in a carb that ran for a long time then was disassembled and reassembled.

Stuff to try
Andygears

[flatheadDave]

Oh, I'm sure on particular L&L models the needle performs as expected, but on the M74B per the topic...
On my 64 FLH, I too experience no noticeable high speed needle effect on my M74b with #19 jet. I run with needle closed.
For your float, did you set the height in bowl? I only know what the book says, and I use one of Tom Cotten's floats, which are manufactured to be set per HD info (1/4" below lip). But its possible an incorrect setting could contribute to rich running.

As you can see, the high speed needle does not interact with jet...
The needle shaft length from first thread to tip, is 2-7/8 "
IMG_4187.JPG

Ah ok. I hear you about the 74b is different. I appreciate the help.

[flatheadDave]

The high speed needle on an M74B is merely a "trimmer" and barely affects the overall running. It is more noticeable at elevation. The M74B uses a fixed main jet, usually a 19 but 17 and 18 are also available. In many cases the best overall adjustment ends up being shut off. A smaller main jet may help with that.

Wow so your saying this is normal being that if I can close the needle down all the way and it still running, that’s normal with a 19 jet?

[RUBONE]

The high speed needle on an M74B is merely a "trimmer" and barely affects the overall running. It is more noticeable at elevation. The M74B uses a fixed main jet, usually a 19 but 17 and 18 are also available. In many cases the best overall adjustment ends up being shut off. A smaller main jet may help with that.

Wow so your saying this is normal being that if I can close the needle down all the way and it still running, that’s normal with a 19 jet?

Yes, because all the fuel it needs is being fed by the fixed jet. It is also entirely possible that the jet has been enlarged by a previous owner. It should be .082. (That is a 45 number drill size).

[kitabel]

As a very rough guess, the main jet size can be estimated by dividing the displacement in inches by 11,000 for a 1½” 4-bolt carburetor. The square root of the result is the main jet ID.
For a 74" engine this works out to .082", which is (mere coincidence) the ID of the #19 (OEM) M-74B main jet.
The factory #18 at .0795" is the next smallest factory jet, about 6% leaner. Almost a full A:F ratio: 13.0:1 (safer than 14.7:1 for power) becomes 13.8, etc.
5/64" is .0781", even leaner but closer than the factory #15/17 @ .076".

Has anyone seen a comparo of "so many clicks or turns of the Power needle equals .001" main jet ID"?

[panhead_kicker]

@kitabel & @RUBONE
Where do you guys get your jet ID info from?
I have two NOS #19 jets, and a third used on the bench carb, all three of which will only pass a #48 drill bit shank @.077 by my calipers.
The one NOS #17 jet I have will only pass #49 bit shank @.072.
All of these jets have a smaller step at the threaded end of the orifice, which is the limiting factor. The hole on the slotted end of the jet is closer to.092
(googling these drill sizes , it appears my calipers are off by .001)

27317-50 & 27318-51

IMG_4195.JPG (36.34 KiB) Viewed 776 times

[kitabel]

Factory service literature, Palmer.
A big secret from the parts dep't.: if a customer wants a .090" and they (Ghost) don't have one, they take the closest size and write .090" on the envelope.