M88 Modification to use larger M51 venturi

[RvP]

I have a stock WLA engine in a civilian frame with an M88 carb and #5 heads. The M88 is stock except for having an adjustable high speed needle and the high speed jet is replaced with a solid plug.

I want to know if I can go to the larger venturi as used on the M51 carb to get a little more power for riding 2 up. I know the M88 venturi is 15/16” and the carb is internally vented for the nozzle and the vent is provided by the venturi. I also know the M51 venturi is 1 1/16” and the carb body has the vent for the nozzle.

1) Is it possible to modify the M88 carb body for an external vent to use the larger 1 1/16” (or even 1 1/8”) venturi as used on the M51?

2) If so, can I use the same nozzle?

3) Is there a noticeable improvement in power going from a 15/16” venturi to a 1 1/16” venturi whether or not it is possible modifying the M88 or using the M51?

[Ohio-Rider]

I’m not sure the modifications your considering would gain you much performance from what the M88 is giving you now. If most of your 2 up riding is below 45mph then maybe you could get that little extra pickup by playing with different sprocket arrangements.

But don’t take my word for it because I have yet to win any races with mine yet. Lots of second place showings though. LOL

[RvP]

More Power for hills

Steve,
Thanks for the quick reply. The 45 is quite happy on flat ground at 50MPH riding 2 up. I do have a 34 tooth sprocket on the front which is a little tall but it keeps the RPMs down when cruising. I am looking for a little extra oomph for going up hills and still retain the tall gearing. If no one can answer my question about modifying the M88 then I will go with one tooth less on the front sprocket and see if I gain that oomph without gaining too much RPM at cruise.

[Ohio-Rider]

Yep, I know what you mean about needing a little more oomph for getting to the top of some hills. There are hills around here that with out a good Ľ mile running start, I’d be holding up traffic by the top. My bike would either be screaming in second gear or lugging in third gear. That’s just one part of the fun of these old machines to me.

I think one of our members was looking into producing a 4 speed for these old 45’s, but I don’t think that has happened yet. Boy a “hill-gear” for a 45" would be sweet. Till then, Ride it like a sports bike.

[T. Cotten]

RVP!

Swapping venturies is only a matter of just that,... If you order Indian varieties.
Otherwise, you can quickly achieve an identical vent slot in a larger H-D unit by hand with a file.

So please do not alter your original body casting by drilling it!

Beware of one obstacle to success: Almost all vintage potmetal venturies have shrank over time.
The I.D. isn't the issue: It is the O. D.:
Daylight around the venturi allows air to defeat the draw upon the main nozzle (among other problems.)

Sadly, the largest producer of modern units has copied shrunken originals.

We all would love to hear results from trying different nozzles! Please let me know if you want different varieties and one-offs to try...

Whether or not a larger venturi will help your riding enjoyment is subject to a lot a variables, but I doubt it will hurt.

....Cotten

[George Greer]

Just to add what I did for my WLA and M-88 with a 1 1/16" venturi that I added.

I just pressed out the old venturi, measured the width and depth of the slot, matched the measurements to the 1 1/16" venturi with a jewlers file.

Installed it but I had a loose fit as Tom's photo shows..(was reading info/tech at FHP).

Made a tool that matched the taper of the venturi, and "swaged" it to fit the bore of the M-88 tightly...No visible gaps with a strong light.

Got one of Tom's Dura-Floats, (that I suggest everybody use).

Have not changed any nozzles. Standard one installed.

Tom, what does changing the nozzles out with other's do for the engine? And I would take you up on a sampling's to try.

George

[RvP]

Mystery M88

Thanks to all of you for your replies. A closer examination of what I have has led to a mystery. I have 2 M88s, one in a box for another WLA I am putting together and one on the WLA I am riding. I have been studying the M88 in the box since it is not attached to anything. Tonight I looked at the M88 on the WLA I am riding and there is an external vent in the side of it with a plug with a screwdriver slot in the top of the plug. The carb body is stamped M88 and the part number is correct but there is a plugged vent hole in the same place as on an M51. Here’s my first question:

1) Did Linkert ever make an M88 with an external vent and plug similar to the M54 & M54B? I have no clue if the external vent on this M88 was put there by Linkert or some backyard mechanic. Anyone seen this before on an M88?

Tom, we can both rest easy since I have one M88 that someone has already performed surgery on that I can use for my venturi experiments. This also eases my mind that a wayward drill in my clumsy hands is not going to ruin an M88 that is working perfectly now. I can remove the external vent plug and put it back as is when I am done. (I still want to know how the external vent got drilled in this M88!). When I take the carb off to get the venturi out I will check for daylight as your picture shows. Thanks.

Thanks to everyone else for the tips, it looks like I have many options.

Ronald V. Papasso

[bmh]

Cotton, I have noticed most venturies to be under sized too and wonder if you had a fix for that? I've been very carfully sealing them with a silcone. This is not very fun or professional looking but seems to work.

[T. Cotten]

George!

I have no idea what sticking an Indian nozzle (two varieties, both with bigger air holes than H-Ds) into your M88 would do, but I have encountered 'all kinds in all kinds', so I am certain that it has been tried.

My only personal observation was with an Indian that stuck valves at highway speeds with an H-D M-51 carb, but ran great with a proper M344. The only significant difference is the nozzle.

Ronald!
I have encountered many boogered vents (both inside and out), but none that have been threaded and plugged. My military carb experience is limited to the common M88, so far.

As long as the nozzle is vented one way or another, or both, I doubt that it makes any difference which way or how much. My experiments with choking the side vents by installing drilled rivets did not enrichen things at all.

BMH!

Most USA fuels will take silicone right into the combustion chamber.

As George suggested, I use a swage, which is a simple screwpress with two cones of appropriate size to gently push the fore-and-aft of the venturi back out oversize, so that I can then lathe-cut to a concentric press fit.

Potmetal is really a super-cooled liquid, like window glass, that is slowly moving and crystallizing over the decades. This is the root cause of the shrinkage.
The older and more brittle Schebler venturies often 'resist' swageing, resulting in breakage.

Sadly, a perfectly-fitted potmetal venturi will continue to shrink.
A light press fit can become a slip fit after a year of sitting on the bench, even without any exposure to motor heat.

Although modern aluminum venturies will not shrink, swaging under-sized ones to a proper fit requires enormous pressure. It is almost better to cut one from scratch.

....Cotten

[RvP]

Time to get busy

Regardless of what appears to be an externally drilled, tapped and plugged external vent got into my M88, it’s time to see what happens….for free. A good friend of mine has a 1 1/16” venturi laying around and this afternoon we will remove the stock M88 15/16” venturi, remove the external vent plug, and insert the 1 1/16” venturi as used on the M51. I am also keeping my fingers crossed the 15/16” venturi comes out easily or with a Craftsman 1” socket and there are no air gaps when the 1 1/16” venturi is inserted. Won’t know if it gives the oomph I am looking for other than how I “think” it feels. Thanks again to all for the responses.

[RvP]

Job done, the jury is still out

During the venturi transplant, the brass plug on the side vent of the M88 would not come out. We then moved onto removing the 15/16” venturi using a 1” Craftsman socket. Turns out the external vent on the M88 is drilled all the way through but there is no way to tell if it was put there by the factory or someone else. Since I could not remove the external vent plug, I took George’s suggestion and filed a vent in the 1 1/16” venturi. I should say my friend Dan did the work, I watched. The 1 1/16” venturi Dan had had was too loose, daylight all around. We also found that the 1” socket can also be used to gently expand the venturi so it fit tight, no daylight.

The results, not as impressive as I thought. I did pick up 10MPH more in second gear going from 45mph to 55mph. Third gear picked up 5mph going from 65mph to 70mph wide open, it would go no more nor would I want it to. I can’t say I picked up anymore of the extra oomph I was expecting since I was after power and not speed. All of this was riding solo and due to the severe down pour we are currently having in South Jersey I could not get a break between down pours to do a final tweaking. Tomorrow, weather providing, will be a 2-up ride and I will see if I feel a difference. The final answer may in dropping one tooth on the engine sprocket.

Thanks again to every one.

Ronald V. Papasso
AMCA #3129

[bmh]

Cotton, Loctite has many products wich are totally imune to modern pump gas. You should pay a visit to your local distributor, they have products to do almost anything these days. Loctite 510 is an anerobic gasket making compound that is resistant to modern gas. The use of the actvator will speed cure time especially on larger gaps.

[T. Cotten]

Brian!

A visit to a Loctite distributor would be a 75 mile round trip, so I must first ask: Is the #510 "immune", or just "resistant"?
I hope it's not the red stuff, it proved no match for Shell Premium with ETOH.

Hell, neither is powdercoating.

What a goober says on its label means nothing to P4gas; I already have a shelf full of tried-and-true gas-proof sealers that aint' no mo'.
(Seal-All, Aerogloss, Indian Head, POR-15, Caswell's, 3M #800, etc., etc., etc.,...)

Somehow, cementing in a venturi seems barnyard, even if the goober stays put. J-B Weld works, but it makes the next servicing a real chore!

....Cotten

[bmh]

Cotton,You can check the data sheet your self on Loctites web site ,the claim is 60% of original strength after 100 hours of immersion in unleaded petrol. It is important to remember this is an anerobic and not an RTV. While this is listed as an automotive product you won't find it at the auto parts store. They also have many solutions available for restoration problems through their industrial division. I'm going to try some of their Norback ceramic coating to seal the inside of a set of crankcases, their tech dept. assures me nothing short of actual rocket fuel as used by NASA will desolve it once cured. You can browse most of the catalog online but for specific apps you should talk to a rep or someone at tech. I agree this is not the way to go for venturis that is why I was curious about how you handled the issue as you do far more carb work than I would ever hope too. Appearently you have no concerns that swaging larger and turning down has an effect on internal venturi shape and air flow at the nozzle. Thanks again for all your help and expertise.

[T. Cotten]

Brian!

Sadly,
If only "unleaded petrol" were our concern, then there would be dozens and dozens of great products that would (still) work. Associated with ethanol, added detergents and injector cleaners make many USA brand-name pump fuels the most corrosive and digestive in history. Goobers aren't just desolved, they are eaten.

P4gas will even eat brass.
Seasoned tank coatings can suddenly "fall out". (See attachments.)

So please forgive me if I don't run out and stock up on another arsenal of bottled elixirs to bench test. It takes six months of exposure to make some of the best fail (such as Caswell's two-part epoxy.)
And the real horror is that tomorrow's fuels may be a whole different alien spit anyway:
Even the supplier of my float material has been forced to reformulate over the seven years that I have been cutting them. What was absolutely fuel-proof in 2000 is threatened today!

So I just stay away from sealers, when I can.

.....Cotten
PS: The change in the internal airfoil of the venturi when properly swaged is very small.
Hell, they run with the venturi in backwards!