Linkert 74B - Fixed high speed jet or not

[craig60pan]

I am in the process of rebuilding 2 Linkert 74B carbs and have run into a dilemma.
One is for a 51 FL and the other for a 60 FL.
One has the fixed high speed jet #19 installed and one has a plug in its place.
Both were running pretty well when they came off the bikes but since I am at a point where I can add the jet that was missing or remove the jet that is there, I though I would solicit opinions on which way to go.
Both motors are basically stock 74s with stock/mild cams, 2 in to 1 exhausts.
Any thoughts would be appreciated.
CJ

[Bigincher]

I recently went through a few of my M74B Linkerts, and found about the same thing; one had a fixed jet and a body plug, one had two body plugs. I chose to go with the #19 fixed jet. But I am no expert....

[craig60pan]

Big
I stumbled onto your blog over at Classic American Iron.
Nice job. My inclination is to add the jet but figured I would get a couple of opinions first.
It was running without it but I can't say how well as I went from a S&S B to the Linkert to an S&S E now back to the Linkert.
Been a while
CJ

[NightShift]

Dear Craig,
The book says a jet so you aughta put one in. The needle tip is sharper than one with no jet.

R'spectful,

[craig60pan]

NS
I am sure you are right, if I did not put it in I would be second guessing myself from that point forward.
In it goes.
CJ

[Bigincher]

Nightshift has it right. I didn't know about the different sized high-speed needles until I read it here a few weeks ago. It may be in The Linkert Book, too.

[Kuda]

I am in the process of rebuilding 2 Linkert 74B carbs and have run into a dilemma.
One is for a 51 FL and the other for a 60 FL.
One has the fixed high speed jet #19 installed and one has a plug in its place.
Both were running pretty well when they came off the bikes but since I am at a point where I can add the jet that was missing or remove the jet that is there, I though I would solicit opinions on which way to go.
Both motors are basically stock 74s with stock/mild cams, 2 in to 1 exhausts.
Any thoughts would be appreciated.
CJ

Just another data point to add to the confusion: 74" FL (but running 8.5 to 1 and an FLH cam), stock other than mentioned, ran an 18 fixed jet with a WL air snout fitted with a K&N cone filter. Had a bitch of a time tuning it, idled OK but would start to load up if it idled for more than a minute or so, plus it had a nasty stutter just off idle and at low steady speeds, and worst, was only getting 28 or so MPG. High speed needle was always all the way in. Plugs always black and sooty. Finally pulled the 18 jet and replaced with a OEM 17 jet. The difference in the size of the hole was suprising, looked like 1/4 of the size. Got a few miles in last weekend, had to open up the high speed needle to 3 turns or so, power seems down slightly, but stuttering is better and MPG seems a bit better too. The fact that I'm now able to trim the mixture with the needle makes me think it was way too rich before. Next I'm going to replace the air filter with a repop oiled brass one and see if that clears it up any more...

-Kuda
'49 panchop

[Cotten]

Sadly,

Because modern fuels have a different volumetric efficiency, and our motors are often far from stock, the original fixed jets may only be a starting place for tedious trial and error.

But Kuda is dead right about two things:
You want the needle to have some effect,
and mileage is an important measure of tuning.

We can no longer rely upon plug reading, as modern fuel additives clean them too well.

Before attempting fixed jet trials, please dress the needle point and seat, as well as the usual scrupulous pressuretest of the intake tract and pan cover screw over each port, etc.
And the float circuit and idle circuit must be spot-on as well.
(Please remember that a fat idle circuit leans the power circuit.)

Begin your trials at the lean end, as your first runs will probably be short. Your runs will get longer as you approach tune, and it is safer to error too far on the rich side than the lean.

Don't trust a number on a jet; always measure it with a drill set.
Never drill a jet without defiling the number.
You might as well drill a common #10-32 screw, as the bore must be precise in length and finish to be within original specs.
Your copy of Palmer's has these specs.

Patience, and good luck!

....Cotten

[craig60pan]

Cotten
Does it matter which of the two holes has the fixed jet?
CJ

[Cotten]

Craig!

Both holes dump into the same main nozzle 'well', as does the seat for the HS needle. It should make no difference.

On bronze castings, this seat is usually only de-burred by a swipe of a fine file through the mill cut to remove any puckering around the seat. The exception are such models as the M45A with a raised lip that are better off left un-molested.

One must also wonder if such a boogered fixed jet affects its metering. The MOCO literature suggests that it does. At that point one must then wonder if the blast finish affects it as well!

....Cotten

[craig60pan]

Thanks Cotten
Although the carb I rebuilt is clean, it does not sparkle like yours
CJ