Swapping M88 for M51 venturi

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old man emu
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Swapping M88 for M51 venturi

#1

Post by old man emu » Mon Jun 10, 2019 1:12 am

The WLA was fitted with the M88 carburetor and a fixed high speed jet. It is possible to fit an adjustable High Speed jet assembly to the M88.

The venturi in the M88 is 15/16" diameter and the venturi of the M51 is 1-1/16" diameter. The M51 venturi has a 21% greater cross-sectional area than the M88, meaning more of the fuel/air mixture can be put into the cylinders, which ultimately means more power.

The question is: Can the M51 venturi fit into the M88 throttle body without major modification to the throat of the carburetor?

Just realised that if it was possible an M51 throttle butterfly would be needed too.

OME



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Re: Swapping M88 for M51 venturi

#2

Post by RUBONE » Mon Jun 10, 2019 1:19 am

Easy to do but one thing to remember. The m** has a vented venturi, the M51 has a vented body. So either the vent notch needs to be added to the venturi, or the body must be drilled like an M51. When running an adjustable high speed needle plug the hole where the fixed jet was.

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Re: Swapping M88 for M51 venturi

#3

Post by svkiwi » Mon Jun 10, 2019 2:01 am

Or...simply bore out your existing venturi to M51 size.

old man emu
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Re: Swapping M88 for M51 venturi

#4

Post by old man emu » Mon Jun 10, 2019 12:01 pm

RUBONE wrote:
Mon Jun 10, 2019 1:19 am
Easy to do but one thing to remember. The M88 has a vented venturi, the M51 has a vented body. So either the vent notch needs to be added to the venturi, or the body must be drilled like an M51. When running an adjustable high speed needle plug the hole where the fixed jet was.
Rubone,
I don't understand what you are saying in the last sentence.

It sounds as though the effort in making the change would not be paid for by a useful change in performance, unless you were racing with the engine.

My enquiry was really just made out of curiosity. Thanks for your replies.

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Re: Swapping M88 for M51 venturi

#5

Post by liberator » Mon Jun 10, 2019 12:54 pm

The bottleneck in a WLA's inlet is not the venturi...but its nipples.
If you want some performance you'll need to install WLD nipples and Y-manifold.
Anything else is a waist of time, money and effort

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Re: Swapping M88 for M51 venturi

#6

Post by kitabel » Mon Jun 10, 2019 1:50 pm

The M-88 military venturi size was chosen in part to allow for slightly better response when equipped with the side-car. Ordinary 45 solo WL models all used the M-51 size. The power increase is modest, nothing like 21%, but the top speed will increase if your gearing is tall enough.
The venturi (and all Linkert, Schebler, etc.) is a complex shape with leading and trailing angles and a radius joining them. Not easily modified even with a good lathe.
The notch isn't critical, a razor saw will do it.
The greatest increase in air flow using stock parts is to install a (used) original or aftermarket (some quality control problems) M-41L, M-51L 1-1/8" venturi which requires the same work. An Indian Bonneville (M-640 Sport Scout etc.) venturi is the same size but already internally vented; probably much more expensive and harder to find.
The throttle disc is the same. The main jet hole is the same size (8-32 thread) as the existing plug, #27327-24.
If you find that the adjustable high speed needle doesn't supply enough fuel in its range of travel, the new plug can be drilled to a small hole size (smaller than any original jet) to add fuel; start conservative - like .020".

old man emu
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Re: Swapping M88 for M51 venturi

#7

Post by old man emu » Tue Jun 11, 2019 7:19 am

This topic sounds like a good one for theoretical discussion but in real life, not practical. Still, we learn a lot from discussing the ntheoretical

Thanks for your contributions.

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Re: Swapping M88 for M51 venturi

#8

Post by kitabel » Tue Jun 11, 2019 3:11 pm

It appeared to me that your interest was to make a WLA with an M-88 faster.
The M-51 venturi will do that. It's not theory, it's H-D factory policy.

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Re: Swapping M88 for M51 venturi

#9

Post by old man emu » Tue Jun 11, 2019 11:21 pm

I was wondering if converting an M88 to an M51 was a simple matter of swapping the venturi. It seems that it is not as simple as that, and the most efficient way is to swap the whole M88 for an M51.

The advice you experts have given has answered my curiosity. I know how to get my M88-equipped WLA to go faster - shave 75 lbs off the nut holding the handlebars.

OME

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Re: Swapping M88 for M51 venturi

#10

Post by RUBONE » Tue Jun 11, 2019 11:53 pm

M51 carbs are rare, expensive, and in high demand. Converting an M88 is far cheaper with the same end result! Linkert made tens of thousands of M88s for the military, during those years they made practically no M51s.

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Re: Swapping M88 for M51 venturi

#11

Post by svkiwi » Wed Jun 12, 2019 1:43 am

Indeed! My M88 runs a bored out venturi.

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