1937 ul intake manifold

[ldfly112]

should i be installing one or two intake gaskets or should i be using a insulator block between carb and manifold . cant seem to get the carb adjusted correct starts good and runs good at most rpm but has a flat spot when holding throttle steady if i accelerate it clears right up any ideas are greatly appreciated

[T. Cotten]

Two gaskets are shown in the book, Idfly,..

And I don't believe the factory ever made an insulator for 1¼" models, just a cast extension for some military models.

All of the wear points in a carb add up, but the most common cause for an off-idle 'flat spot" is borewear from the throttledisc:

BOREWEAR.jpg

When the eyebrow groove extends close to the idle bleeds, the transition from the idle circuit to power circuit suffers; since the needles overlap, you can only tune ideally one or the other.

Beware also of daylight around a shrunken venturi that will defeat the main nozzle:

DAYLIGHT.jpg

A high idle speed helps a little...

....Cotten

[ldfly112]

The venturi was changed from the M51 size to the M51L size due to 80 " pistons being installed so there should not much wear in the shaft should I change it back to the original M51 venturi

[chuckthebeatertruck]

If you installed 80” pistons it means you have 80” cylinders. If you bought new ones from friebus you had best check the intake nipples for leakage.

then understand the Venturi you installed was most likely a colony repop which doesn’t fit well in many cases and will leave the gap Cotten is alluding to.

there should be zero flat spots on your low revving flatty and eliminating all intake leaks or sloppiness is first.

[Steve Slocombe]

For 1937 the carb fits with two of the thin paper 1121-21 gaskets and the short 1123-30 screws. The insulating gasket came out in 1939 with three longer 1121-39 screws to fix it. The parts books show the insulating gasket continuing on the WLA/WLC small twins and the Big Twin flatheads still using the three bolt carb. For your bike I would tend to use the insulating gaskets and the longer screws, to save vapor lock.

There are good carburetor tips above. A number 42 drill pushed by hand through the idle passages can also sometimes work wonders in removing older crud from the idle circuit.

VL 74 pistons are the same as UL 80s, which may cause confusion. 1937 UL cylinders have casting numbers 120-37 and 120-371 and were used for both 74 and 80. Thinner wall cylinders came out in 1938 for use on the 74s, with casting numbers 120-38 and 120-381, with the 1937 cylinders then used on the 80 inch BT flatheads until they were withdrawn after 1941.

[T. Cotten]

The venturi was changed from the M51 size to the M51L size due to 80 " pistons being installed so there should not much wear in the shaft should I change it back to the original M51 venturi

Gosh, Idfly,...

How does a venturi swap guarantee the shaft to bushing fit? It is always best to double-check, especially with somebody else's "fresh rebuild".

The fit of the venturi to the carb body is actually more important than its bore size. The smaller has a performance advantage if you have a heavy payload (passenger, baggage, or a sidehack), yet larger is great for solo duty, and modern highway speeds.

Beware of venturies worn around the nozzle spigot, as it defeats its function as an 'accelerator sump',..

wornvent.jpg
(Repair is easy with a PEEK seal.)

Chuck is right to warn about repop venturies, although Colony's improved after I alerted them that originals have nearly always shrank and distorted. Both need to be swaged over-sized if the bore is honed oversize, and originals are much, much easier.

....A number 42 drill pushed by hand through the idle passages can also sometimes work wonders in removing older crud from the idle circuit.

I read about scraping "crust" a lot in vintage literature, Steve!

But I have never encountered anything that required more than a soak and brushing to remove, even in ancient barn finds. (Potmetal Schebs have their own special sort of "crust" of course.)

And to avoid confusions, the idle bleeds themselves are delicate metering orifices, and should never be attacked with more than a large safety pin point to burnish any burrs, and an appropriate sharpened feeler gauge for the slot.

.....Cotten

[ldfly112]

I took her out for another test run yesterday aprox. 100 miles adjusted the high speed jet slightly 1 click at a time it seemed that after 20 miles or so the hesitation was less obvious. The M51 was just rebuilt by ITW and he has done a few carbs,. for me in the past all worked great . The bike runs close to perfect starts on first kick after 2 prime kicks every time accelerates nice from 0 -60 mph spark plugs look good light grey just that one spot usually 40 mph in 3rd gear holding the throttle steady is where it hesitates and comes out of it as soon as you accelerate . i am going for another ride this morning and try playing with the carb adjustment a bit more THANKS for the help I will be going to Davenport week after next if i need any parts or a different carb. i can find there THANKS again

[T. Cotten]

Back when I could still ride, Idfly,...

Tuning the power needle was a matter of first having the idle needle dialed in with a warm motor, then getting on a long stretch of highway where at a constant cruise, the power needle could be slowly turned in until there was a result, and then turned out slowly until there was a result,... and then set in the middle. Pretty much like the idle.

When everything else was in order, the idle needle would then need another tickle,.. and if its a lot, the whole routine was repeated.
The needles over-lap, with a rich idle making the power circuit leaner, and a lean idle making it richer. (Don't let any "expert" tell you these things are "simple"; They are only forgiving.)

Part of the fun of these machines is that you 'engineer' them down the road, repeating the tuning to perfection. And then do it again when the fuels change.

....Cotten

[ldfly112]

could the exhaust /muffler play apart in this issue

[T. Cotten]

could the exhaust /muffler play apart in this issue

I'll have to defer to the experts on that one, Idfly!

A restrictive air cleaner can give fits, but the classic "flat spot" can happen even with a 'brillo pad' element.

Its easiest for me to blame it on what I don't understand:

accltxt2.jpg
(from the 1942 Armored School Handbook)

When the idle circuit reverses itself, does it always flush like a toilet,... Or will it gurgle and puddle if held just shy of accelerating?

My contention is that the transition is cleanest and sharpest when the carb is perfect.

....Cotten

[ldfly112]

I will be changing the venturi back to the smaller 1/16 size correct for 74 c.u.today just the process of elimination I am hoping that goes well please keep those ideas coming very much appreciated

[duffeycycles]

Cotton's point about restrictive brillo pad element is spot on.Tedd's are wrapped too many times & restrict flow.Also a bit too wide for proper fitment.

[T. Cotten]

Cotton's point about restrictive brillo pad element is spot on.Tedd's are wrapped too many times & restrict flow.Also a bit too wide for proper fitment.

I meant paper and foam elements, Duffey!

I often double-wrapped copper mesh with no problem, even my stroked Pan.

And Idfly!

The smaller venturi should lower the RPM at which the transition occurs, if everything else is in order, of course.

...Cotten

[duffeycycles]

I don't consider paper or foam at all !

[T. Cotten]

I don't consider paper or foam at all !

I took my last two foamies out of inventory, Duffey,..

And put them on my air compressor, with dual intakes and two seven-inch stainless covers.
Racing compressor.jpg
Its a racing compressor.

....Cotten
PS: Idfly!
Taking the carb off and on and off means fastener-stress upon both manifold and carb flange.
And a fat gasket means the flanges can distort even more than two thin ones, and they all distort.
(Please torque to no more than ten foot-pounds.)

Bubble-testing cannot inspect the assembled flanges, so the only test I know of (so far) is to start and idle the machine while 'snooping' everywhere with an un-lit propane torch. Throttleshafts can also be inspected at both top and bottom at the same time.