M-53 carburator on a '62 ?

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dcmc
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M-53 carburator on a '62 ?

#1

Post by dcmc » Tue Mar 25, 2014 1:52 am

I had an M-53 that I was planing on using on a 1962 i am putting together from parts, I noticed that the bore is worn at the top, probably from the butterfly rattling on a worn shaft at idle. My understanding is that the bombsight venturi was intended to improve low end throttle response so I would like to use it, I do remember having one in an M-74B years ago. Question is what body to put it in ? Should I just put bushings in the worn M-53 and see if it will idle ?
or will the bombsight venturi actually improve throttle response in an M-74B if I can find out how to properly set it up in an M-74B.



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Re: M-53 carburator on a '62 ?

#2

Post by Cotten » Tue Mar 25, 2014 2:09 am

dcmc!

Installing an OEM "bombsight" into an M74B won't work without boogering one or the other quite a bit.

There would be no air correction circuit to the main nozzle.
It would have to attempt to run upon air leakage around a shrunken venturi alone.
And there is no easy way to make a shrunken bombsight fit, as swageing would be an R&D exercise in itself, that nobody I know has attempted. Even though most every OEM bombsight has shrank.

Borewear can be fixed if you really feel the need to use the M53, but I hear there are "hybrid" modern bombsights available for non-bombsight bodies.

...Cotten
Last edited by Cotten on Tue Mar 25, 2014 2:17 am, edited 1 time in total.

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Re: M-53 carburator on a '62 ?

#3

Post by 1950Panhead » Tue Mar 25, 2014 2:15 am

An M53 is an 1.5" carb same as an M54.
Typically the throttle shaft wears more then the bushing, replace the throtle disk if that is worn.
Keep the bomb site in the M53.
Jerry

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Re: M-53 carburator on a '62 ?

#4

Post by RUBONE » Tue Mar 25, 2014 2:49 am

M53s are designed for Flatheads and have multiple features that differ from the other carbs. The bombsight venturi was an experiment carried over from racers, and was not significantly better in street use so was dropped within a couple years, the M53A1 does not use it and performs better. M53 carbs also do not have an enrichening circuit on the low speed making them poor on Big Twins in cold start and cold idle situations. The idle air screw in the body is also designed for the demands of Sidevalve engines.

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Re: M-53 carburator on a '62 ?

#5

Post by Cotten » Tue Mar 25, 2014 3:04 am

RUBONE wrote:M53s are designed for Flatheads and have multiple features that differ from the other carbs. The bombsight venturi was an experiment carried over from racers, and was not significantly better in street use so was dropped within a couple years, the M53A1 does not use it and performs better. M53 carbs also do not have an enrichening circuit on the low speed making them poor on Big Twins in cold start and cold idle situations. The idle air screw in the body is also designed for the demands of Sidevalve engines.
Curiously Folks,..

The M74 used a similar stepped-tip needle, as compared here:
STEPTIP.jpg
And upon my own OHV machine, the M74 with the "flathead" needle worked fabulously.

....Cotten
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Re: M-53 carburator on a '62 ?

#6

Post by RUBONE » Tue Mar 25, 2014 3:08 am

An M53 is an 1.5" carb same as an M54.
Sorry, but they aren't even close...

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Re: M-53 carburator on a '62 ?

#7

Post by Cotten » Tue Mar 25, 2014 3:15 am

RUBONE wrote:
An M53 is an 1.5" carb same as an M54.
Sorry, but they aren't even close...
Folks,...

I think Jerry meant an M53A.

We all push a wrong button sometimes.
Some of us go out of their way.

...Cotten

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Re: M-53 carburator on a '62 ?

#8

Post by kitabel » Tue Mar 25, 2014 5:48 am

RUBONE wrote:M53s are designed for Flatheads and have multiple features that differ from the other carbs. The bombsight venturi was an experiment carried over from racers, and was not significantly better in street use so was dropped within a couple years, the M53A1 does not use it and performs better. M53 carbs also do not have an enrichening circuit on the low speed making them poor on Big Twins in cold start and cold idle situations. The idle air screw in the body is also designed for the demands of Sidevalve engines.
I disagree. The bombsight was intended to improve response in a small engine (45") without the flow restriction of the *-1939 carbs or the poor air speed of the straight venturi 1940 carbs. The SV construction wasn't a factor. The bombsight became superfluous for 1954 when the engine was stroked to 54", and was discarded. The factory found it easier and cheaper to simply size the venturi to engine displacement.
It was only an "experiment" in terms of not becoming a factory production part (except for 1952-53).
The booster concept was already in automotive use much earlier, and used in most automotive carbs for 30 years. Every 4 bbl. has this, the QuadraJet has multiple boosters.

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Re: M-53 carburator on a '62 ?

#9

Post by 58flh » Tue Mar 25, 2014 1:00 pm

The Bombsight as Kitabel said in Rochester carbs. is correct & it actually works on them!--I believe it works as theres more CFM & CYL.s to feed,So airspeed is a factor in play.I know from personal exp. on RQ-carbs. that Milling the airhorn off & leaving the -(bombsights in)-WAS a instant increase in better 1/4-mile times.As far as removing the BOMBSITE & plugging the hole,Speed was lost! & that was tested with quite a few passes down the track.Im not saying it will make a bike run better--Just that Larger engines benefitted from it!--So much so that a new QUADRAJET/by EDELBROCK have them Shaved & installed as per/customers preference.-----RICHIE

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Re: M-53 carburator on a '62 ?

#10

Post by Mr.Big » Tue Mar 25, 2014 1:05 pm

I've used this one in an M51 and believe it improved the performance. Never seen another one so I'm assuming they did not sell well?

Image

Image

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Re: M-53 carburator on a '62 ?

#11

Post by 58flh » Tue Mar 25, 2014 1:17 pm

MR BIG---I have seen many & If you can speed-up the airflow down the throat IT WILL improve performance!---Many Crower & Hilbourne constant velocity systems Experimented with runners up to 9-inches long!/I believe that is what HD intention was.But not enuff runner to make it effective in a small cfm set-up!.If you run a vel-stack of 6-inches It will improve airspeed down the throat,Maybe then the bombsight would make a difference.All this is theory ,A 1/4 mile track & a few passes makes it reality!----Respectfully---RICHIE

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Re: M-53 carburator on a '62 ?

#12

Post by Cotten » Tue Mar 25, 2014 2:02 pm

Mr. Big!

The Frisco has an annular groove cut into its outside surface, like conventional venturies, thus retaining the air correction circuit.

OEM bombsights were solid,..
HDSIGHT.jpg
..thus dropping one into any body but a body designed for one (M-53, MR-3, etc.), would disable the main nozzle.

Here is another "hybrid" that tried to overcome this issue in a unique way:
344SITE3.jpg
I have no idea how it performed.

...Cotten
PS: 'Eliminates "spit" from OHV models' is humorous from today's perspective of sealed and tested manifolds.
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Re: M-53 carburator on a '62 ?

#13

Post by dcmc » Tue Mar 25, 2014 8:47 pm

Thanks for all the responses, I can't remember what was done to the m74b I had with the bombsight venturi, i do remember at the time i got the bike listening to all the mods that had to be done to the carb. I don't remember the bike as being especially fast. I just wanted to run one, since I had it left over from parts i got for my 48wr that is long gone.

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Re: M-53 carburator on a '62 ?

#14

Post by Cotten » Wed Mar 26, 2014 12:49 am

Future "boosters", Folks!
THIMBLES.jpg
If dimples on a golf ball give it lift, then why not on a secondary venturi?

....Cotten
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Re: M-53 carburator on a '62 ?

#15

Post by kitabel » Wed Mar 26, 2014 5:20 am

A bombsight is a restriction, and reduces air flow in all cases.
What is increased is the pressure drop across the nozzle tip, which improves response.
If the motor is big enough to have good response with a straight venturi, it will produce less power and less mileage with a bombsight.

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