Board index Flathead Power-Technical Questions, Answers, and Suggestions 45 Flatties CLASS C RACE NOTES

CLASS C RACE NOTES

Post Fri May 11, 2018 8:43 pm

Posts: 814
Location: Pa. , USA
Ah the mighty MR4, the short answer to both questions is whatever works, of course the internals of a motor come into play, cams, valves, ect.. personally i've been in the presence of these rare gems only a few times and have seen them run strictly off the needle or the adequate fixed jet and using the needle to fine tune (which is what i prefer if it cooperates), never seen them run off a fixed jet only, What method are you using Dave and how is it working ?
Vintage roadracing, Class C, AHRMA # 335

Post Sat May 12, 2018 11:02 am

Posts: 3405
Location: Central Illinois, USA
Nobody has trusted me with an MR-4, Folks!

But it would not surprise me if it was designed like the M-53, M-53A1, and M74 (no"B"), in that it used both a fixed jet and an adjustable HS needle, with the smaller stepped tip.

The -53 series also used a LS needle with a longer stepped tip,
53LSNDL.jpg
and I would suspect the MR-4 did as well.

I have little or no literature for either, as they rarely if ever cross my benches.
(But I seem to remember there was a deviant 3/16" float setting somewhere, apparently to accommodate a very thick gasket.)
The "drool pit" beneath the venturis still confounds me!
53A-1 channel.jpg


....Cotten
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Post Sun May 13, 2018 2:14 am

Posts: 61
Hi Guys

It runs well and pulls nicely. I havent touched it since I bought the bike. On the low speed side it appears to have a no1 jet and the needle is screwed all the way in. It starts and idles fine. On the high speed side it has a plug and the needle looks normal (I'll post a pic tomorrow) and is about 3.5 turns off fully closed.


Cheers
David

Post Sun May 13, 2018 5:59 pm

Posts: 61
And here's the pics of the needle

Image

Image

Image

Post Sun May 13, 2018 9:35 pm

Posts: 814
Location: Pa. , USA
Thanks Dave for the great MR4 info, I vagley recalled the hi speed being tapered and the lo speed being stepped but needed a confirmation, if it's not classified info what other mods are done to your motor ?
Vintage roadracing, Class C, AHRMA # 335

Post Mon May 14, 2018 1:19 am

Posts: 61
Hi Tim,

No secrets because I don't know. I purchased the bike about 12 months ago and have raced it at a few meets. Its not the fastest and not the slowest but I cant tell if I'm down the back because of bike pace or my riding abilities.

The guy I purchased it from raced it regularly, including a trip to Dayton and then stopped using it for about 5 years. Because it goes I haven't pulled it apart but I'm starting a WR engine so I'll know a lot more about that.

Here's a couple of pics of the bike

Image

Image

The only thing I did was put this Velocity stack on it that made it run worse!

Image

Regards
Dave

Post Mon May 14, 2018 10:28 am

Posts: 3405
Location: Central Illinois, USA
Dave!

I would ink up the tip of both needles to see where they actually make contact with their seats.

....Cotten
PS: I see by your last pic that your LS needle has an earlier HS knob.
Why are the ends of the bolts holding the carb to the manifold not visible? Are they heads of reversed bolts,.. or welding..?

Your airhorn is longer than mine.

Post Mon May 14, 2018 5:55 pm

Posts: 814
Location: Pa. , USA
Very nice bike Dave , appears well built, I would like if possible to keep the MR4 information rolling, I have a MR4 that won't tune, it came without a nozzle and tried a couple various mods with no luck (including the M53) and put it on the shelf. As I recall from a few that passed thru the shop over the years they looked similar to the M74 nozzle, Was told recently that it may be possible to create one from an M74 nozzle but of course I have nothing to pattern from, if possible and at your convenience could you drop the bowl and nozzle and post some pics of the nozzle next to a measuring device ???
searched many times for an MR4 nozzle image and all I got were pics of the M53 nozzle which I know isn't correct, your efforts may help more enthusiast , thanks, Tim
Vintage roadracing, Class C, AHRMA # 335

Post Mon May 14, 2018 8:33 pm

Posts: 61
Cotton - I think it must be the pic as all four are there and poke though

Tim - I've had the bowl off a couple of times and I think the nozzles are different. There isn't a spring in the bottom of the bowl and the nozzle doesn't drop out like a normal carb. When someone saw me playin with the bowl off they said "don't take that nozzle out or you'll never get it back in" and I just said OK and didn't ask why. I'll see that guy again in a couple of weeks so I'll ask the question.

I have an M53 on my 38WLDR and an M51 on my 44WL and both have the spring and the nozzle drops out without too much pressure.

Cheers
David

Post Mon May 14, 2018 9:02 pm

Posts: 814
Location: Pa. , USA
Yes there is a screw with a shorter spring holding the nozzle in place, never had a problem with re assembly as long as the venturi lines up, just note the position it comes out so you re install it the same way, but if you want to talk to your friend first that's fine with me, worth the wait..
Vintage roadracing, Class C, AHRMA # 335

Post Tue May 15, 2018 10:52 am

Posts: 3405
Location: Central Illinois, USA
Dave!

Its gotta come apart for cleaning and inspection at least.
Please look for daylight around the OD of the venturi when installed.

The only carbs I know of where the nozzle was not retained by just a spring were Type A DLX Scheblers, so I would love to see pics of your assembly.

Please inspect as to where the top of the nozzle spigot protrudes into the secondary venturi. (I would presume it should be just a hair short of dead center.)
Since your nozzle is retained below the bore instead of above it, spigot length is your first concern.

A contemporary nozzle that may give a clue is the M74, compared here with an M74B.
M74vsBnzl.jpg


Linkert made a great many running changes from '48 until about '55, and then returned to previous designs, abandoning many "innovations".

....Cotten
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Post Tue May 15, 2018 5:52 pm

Posts: 61
I have a race meet in a couple of weeks so I wont be taking it apart before then but I'll be having a play after that.

So did we have any consensus on the jet situation, is 3.5 turns too many?

Post Tue May 15, 2018 6:24 pm

Posts: 3405
Location: Central Illinois, USA
Dave!

Just so we understand each other,
I call it a 'fixed jet' when its a precision-drilled gallery plug on the bowlstem, such as your #1.
It feeds both HS and LS circuits, plus extra from the HS needle (metered also by the bowl level itself.)

Your needles are 'adjustable jets' in reality, but not commonly in HD literature.

Without an autopsy on an unmolested example, we can only conjure if we have the original needles, by very close inspection of their intersection with their seats for clues.
(The LS is nigh impossible.)

And I would be conjuring to suggest that somehow your non-stepped needle may obstruct flow on a smaller seat, requiring extra turns out.
Or that an undetected vacuum leak demands it. The tiniest ones are the most evil.

....Cotten

Post Tue May 15, 2018 7:41 pm

Posts: 814
Location: Pa. , USA
DaveAus wrote:
I have a race meet in a couple of weeks so I wont be taking it apart before then but I'll be having a play after that.

So did we have any consensus on the jet situation, is 3.5 turns too many?

Best way I know to answer your question is to do an @ speed spark plug check, you want to know what your motor is doing at hi rpm so you go out on a back road or practice session at the track and run it at hi rpm for as long as possible come off the track shut it down asap push it back to your pits (pit crew is nice here) let it cool down then do a plug check, you will want color on your plugs, a nice tan is preferable but that can change depending on the fuel or additives used, point is if you have color and the bikes running strong I wouldn't mess with with the carb setting, if the plugs look clean or white it's running too lean and you need more fuel or you have an air leak or both, hope this helps..
Vintage roadracing, Class C, AHRMA # 335

Post Tue May 15, 2018 7:45 pm

Posts: 61
Thanks for the feedback guys. Not sure why I'm getting so tied up about this stuff considering the bike goes well..........I just want more :D

Just a point of clarification Cotton, I was always of the understanding that it didn't matter which side you put the fixed jet on but I was told recently that it does. Ie if you put it in on the power side of that carb that impacts the power side and visa versa. Is there any truth to that theory?

Post Wed May 16, 2018 9:07 am

Posts: 3405
Location: Central Illinois, USA
Dave!

Both gallery plug holes lead from the bowl to the same chamber (as does the HS needle seat), so the "theory" is baseless.
But it does tell you something about its proponent.

And Tim!

Where do you find fuel that still colors plugs like the good ol' days?
Mid-Western P4gas burns so clean that you have to over-enrichen to get any color at all, and then its black and not brown.
Perfect tuning often produces white plugs, due to deposit inhibitors, injector cleaners, and a hundred other additives.

Anybody remember the "Colortune" gadget?
Mine's lost in the shop somewhere....

....Cotten

Post Wed May 16, 2018 11:58 pm

Posts: 814
Location: Pa. , USA
Correction ; earlier I mentioned the MR4 nozzle being similar to the M74 nozzle, wrong, meant to say M75 which is also similar to the M45 I'm told, I purchased a new M75/ M45 nozzle from a gent at Oley who's specialized in Linkerts and had a stand loaded with parts, also you will find in Panics Linkert book diagrams displaying the similarity of the 2 nozzles, now all we need is the real deal dimentions to pattern from...

Cotten, sorry to hear about your P4 gas dilemma in the Mid West, here in e. Pa we have what is called RG91 that several local stations have as a specialty, it's a hi octane non ethanol fuel that I use in my street bike, street rod and the racebike (w/ lead additive) if the track I'm going to doesn't supply n.e. racing fuel, track tested I can say it works great ! I'm very interested to hear what works in your race bike..
Vintage roadracing, Class C, AHRMA # 335

Post Thu May 17, 2018 9:02 am

Posts: 3405
Location: Central Illinois, USA
Here's an M75 nozzle on the right, Folks!

KNUKNZLS.jpg
An M5 is on the left, and other Knuckle models in the middle, differing only in the length of shoulder beneath the spigot.

Note the M75's larger holes (#54 instead of the typical #57), and wider exit gallery at the top ~.090" instead of ~.050"). Bores are #28.

....Cotten
PS: I've never raced, Tim!

PPS: Dave!

Bombsight models retained the nozzle at the top within the body, whereas conventional venturies retain it.

This one on the left (back and front) rattled so badly that it is 'hour-glassed".
SQARNZZL.jpg

I believe its an M53 nozzle. Maybe A-1?

PPPS: I'm so confused.

Here's what I labeled 'bombsight' on the left with the common Knuck nozzle on the right.
bmbnzl.jpg


Who's got literature?
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Post Thu May 17, 2018 7:00 pm

Posts: 1705
Location: Interlaken, NY USA

As for plug "chops", I just installed a wideband O2 sensor and gauge on my 88" flathead, (Christmas gift from a Very thoughtful Son), just 'cause inquiring minds want to know. For several years now have been observing plugs running almost white, but the engine seemed to not be hurting itself. Turns out, mixtures running 11.5 to 12.5 or so under power, obviously quite safe. Sorry, carb under test is, dare I say, a VM38 Mikuni. Quite surprised at uniformity of ratios under a variety of conditions. Don't know exactly what the setup cost, just brazed a bung in the front pipe, 'bout halfway from either end, hooked up the gauge, and done.
DL

Post Tue May 22, 2018 2:34 pm

Posts: 3405
Location: Central Illinois, USA
Literature, Folks?

Somebody has to have a K Parts Book, and can at least tell us the numbers for the nozzle(s)?

(Can't trust the 'catalogs'...)

Thanks in advance,

....Cotten
PS: If anybody's got a K Service manual, I'm dying to know what the 'drool pit' is supposed to do!
Last edited by Cotten on Tue May 22, 2018 3:15 pm, edited 1 time in total.

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