Linkert m74b rebuild

[T. Cotten]

Doesn't this mean that the bike will never really operate with a throttle disc fully closed

This is why I delete my posts...

It frustrates me too, Kitabel!

But if novices cannot get the basics here,.. The AMCA mandate dies a little more.

(With DLX/Linkerts, I guess its my 'cross to bear'.)

....Cotten
PS: Everyone should get their questions answered, even if the Divas won't answer mine.

[kitabel]

Doesn't this mean that the bike will never really operate with a throttle disc fully closed

This is why I delete my posts...

[T. Cotten]

I s'pose, Louis,..

If there was enormous daylight around a disc when fully closed (first attachment), perhaps it might idle, but that's certainly not the design; Fully closing the disc should shut her down.
(Note the plumes of carbon formed upon the disc from poor mixing. How it would be burned within the carb is mysterious.)

The more daylight, the more closed it must be to direct air at the bleeds, and the less control over the idle speed and mixture.

Beware also that when enough RPM is reached to cause the venturi to draw at the main nozzle, the idle circuit reverses itself, no longer metering fuel emulsion to the bleeds, but meters air to the bottom of the nozzle to emulsify fuel there. So an idle needle setting set rich for idle will lean the high speed circuit, and vice versa. The happy median that will give the best performance, fuel mileage, etc. is a lot easier to find with a happy bore and disc.

No wonder Linkerts are called 'crude' and 'simple', huh.

....Cotten

[lrcormier]

Concerning a tight fitting throttle disc, something that just occurred to me and got me wondering why is this so critical because when adjusting the idle you will need to screw the idle adjuster in which will slightly open the throttle disc. Doesn't this mean that the bike will never really operate with a throttle disc fully closed?

[lrcormier]

Went to home depot for some emery cloth, they don't have that. Purchased a 7/32 round chain saw file and thought maybe I could wrap a piece of wet sandpaper that I already had, the file would provide good gripping for the sandpaper. I cut a 1/2 in strip of 320 wet sandpaper and spiral wrapped it around the file, this made a tight fit but managed to twist it through both bushings. I spun and pushed and pulled by hand and after about an hour and a half sheet of paper I got a shaft that fits snug and spins freely. Bowl float from Cotten is on it's way so now I just need to find a reasonably well fitting throttle disc and I should have a working linkert m74b. As the old saying goes, more then one way to skin a cat, thanks to all who provided input.

[lrcormier]

Thanks Tom, that sounds like something that should work and will give it a try. I also got a suggestion from someone outside the forum that a chainsaw file use lightly would also work. I was prepared to buy a ream but these inexpensive alternatives could do the trick. If I somehow end up with to loose a fit bushings are also cheap enough.

[tfburke3]

I have an adjustable Starrett reamer I use but before I got it my method would be to tightly roll a long strip of about 180-220 grit cloth backed emery paper so it would just push in the bushings,the roll should slightly expand to snug,then spin in same direction as rolled and push and pull the paper up and down to lap to size.Usually there is not much to remove and the proper fit is as tight as possible but turns free.
Tom

[T. Cotten]

Does the shaft start to insert at all in either bushing, Louis?

....Cotten

[lrcormier]

The bushings were pressed in using a vice, for removal of old bushings I used a 7mm tap and then inserted a 7mm bold and tap it out using a drift pin. Also had applied some liquid wrench to bushings, they came out without very much effort. Apparently the bushings are designed to compress slightly upon insertion and went in without much difficulty, entire procedure (remove old insert new) took less then 15 minutes. I guess I'll try the method mentioned in the service manual and use a 1/4 in drill but will purchase a new one for this use.

I've read a lot of post about replacing the bushings but no one states how they then fit the shaft by either using a drill bit or ream. After all, these carbs were designed in the day to be rebuilt by your average home garage mechanic without the use of any fancy machines, that is unless the body is still in serviceable condition.

[T. Cotten]

See post #3, Louis!

Did you beat them in?

...Cotten

[lrcormier]

Got a new throttle shaft and bushings today. I installed the new bushings but what do I need to hand line ream. Although the bushings seemed loose on the shaft prior to installing, they are now tight and need to be reamed in order to install the shaft. Need recommendation for a reamer?

[T. Cotten]

As I posted earlier, Louis,..

If the groove does not extend to the idle bleeds, they are 'forgiving'. If it does, the air directed in and out of the bleeds by the disc at idle and low speed is diverted, and the transition to high speed becomes difficult to tune: An evil "flat spot", such as has given Linkerts and DLXs a maligned reputation in modern times.

And yes, if the bore is enlarged, the venturi must be as well. They can be swaged oversize, and then cut to a light press fit.
A great many potmetal venturies have shrank and distorted to where they are loose in the original bore anyway, allowing for their own 'daylight' to confound tuning.
Ironically, a potmetal venturi swaged and cut to a light press fit will return to a nice slip fit after several months, but at least it will stay 'round'!

Aluminum venturies are not subject to shrinkage, however are far more difficult to swage. COLONY offers .025'' oversize venturies that can be turned down for most repairs.
Huge oversized discs would have to be re-cut as well, but by that point you might as well just cut one from scratch.

....Cotten
PS: It should be noted at this point that the idle bleeds are a very delicate orifice, metering not only by the diameters of the holes and width of the slot, but by their depth through the casting. Most start with ~.030" wall, and an eight or ten thou enlargement of the bore would reduce that by four or five thou, and hasn't proven detrimental, so far.
Twenty-five over would cut the wall depth by almost half, and most likely would risk affecting metering function, for better or worse, we do not know.
(It would certainly rob the casting of future re-freshenings by generations to come.)

[lrcormier]

My eyebrows are very shallow and are barely felt with my finger nail. I guess to be perfect the carb bore would need to be honed, but I would also think that some wear is to be expected and the carb should still function reasonably well. My first order of business is to purchase a descent throttle disc, can anyone recommend a vendor that they sourced a good fitting disc from? After installing a new disc I'll have my machinist take a look and determine if honing should be done. I honing is done does that also mean the venturi will need to be replaced?

[T. Cotten]

Actually I did get a big laugh out of it. Actually, a lot of manufacturing for a simple cable pull. Lol

I appreciate it, Ryan,..

Anybody catch the Laurel and Hardy shtick?

(I was bored and razed in Peoria.)

….Cotten
PS: Back to topic, most carbs clean up at four to six thou over. Some not of course, but twenty over goes nearly into the idle well, thirty to oblivion!

[ryan]

M841L&R (and probably M17L&R) also used the nozzle seal, Louis!

If you have an 'eyebrow' of wear in the carb bore from the throttledisc, just throwing in a fresh disc will not close up the daylight around it, even if it happens to be "NOS" size. They are "forgiving" if the groove does not extend all the way to the idle bleeds, but will not be all it could be, unless honed and properly fitted with a disc.

(Beware of commercial 'oversize' discs that are grossly oversized!)

....Cotten
PS: Since the disc should never operate fully closed (as Kitabel noted), the cause of wear must blamed upon a combination of slop in the bushings, and vibration.
An aggravation of this wear is caused when over-torqued to the manifold. Distortion occurs at any fastener torque over 13 ft-lbs (7 ft-lbs for 1/4" fasteners).

PPS: Nobody got the mousetrap joke, huh. I might as well go back to Peoria.

Actually I did get a big laugh out of it. Actually, a lot of manufacturing for a simple cable pull. Lol