Board index Flathead Power-Technical Questions, Answers, and Suggestions Pans bombsight venturi on a pan

bombsight venturi on a pan

Post Sat May 19, 2007 6:52 pm

Posts: 3
Location: Santa Fe, New Mexico
Not sure if it's been done before. But was curious if the bombsight venturi would work on some of the later linkerts (m74's)? Or is it a waste of a part?
Would there be any enhanced performance from doing this?

Post Sat May 19, 2007 7:31 pm

Posts: 2688
Location: Los Angeles, CA
It is an old time speed secret. I think someone, possibly Colony, is reproducing thr bombsight venturi.

Post Sun May 20, 2007 6:01 am

Posts: 3158
Location: Central Illinois, USA
Linkert "Bombsight" models used an entirely different casting than the M74B.

There would be no way to vent or retain the nozzle (which was different also.)


Post Sun May 20, 2007 1:19 pm

Posts: 3
Location: Santa Fe, New Mexico
thanks for the info guys. So, is there any hop up enhancements that you can do to the m74's?
and Cotten are you still making you're floats?

Post Sun May 20, 2007 1:47 pm

Posts: 2688
Location: Los Angeles, CA
Cotten, I have seen an ad for new Bombsight venturis to fit the M-74B.

Post Sun May 20, 2007 6:15 pm

Posts: 3158
Location: Central Illinois, USA
So ask 'em.

But don't expect original parts to interchange.

(PS: Monger! I am still cutting floats as fast as I can, but my insurance company, their engineers, and construction contractors involved with my building's storm damage are making things worse, not better.)

Post Wed May 23, 2007 3:20 pm

The reason it's a secret is that it reduces flow.
If that's what you want there are ways to do it.
Are there things you can do to an M-74B?

Post Sat Jan 26, 2008 6:48 am

Posts: 12
Location: Switzerland
M74-B with Bombsight Ventury installed... carb not yet installed on a bike :mrgreen:

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Last edited by 58panheadfan on Sat Jan 26, 2008 8:24 pm, edited 1 time in total.

Post Sat Jan 26, 2008 8:38 am

How to explain this?
1. When did someone do this?
I would guess 1939, when the 1-1/2" M25 etc. were introduced (with a plain venturi), probably by adding a booster carved from an automotive application. I have heard (but do not believe) that Tom Sifton did this before the factory did, also credited to Brownie Betar (my 642 Sport Scout flat-track racer had an M340 supposedly modified by him with a completely fabricated brass booster - David Sarafan has the carb now).
2. the purpose of a secondary (or tertiary, as found in QuadraJets) venturi is to increase the vacuum signal to the main nozzle/emulsion tube/needle jet etc. at very low speeds, which makes the engine more responsive without resorting to richer jetting or a smaller primary venturi. As the size of the engine vs. carburetor throat goes up, it becomes less necessary. It would be mandatory to use a 1-1/2" carb on a 30" twin (Indian 741), very useful on a 45, somewhat helpful on a 61, and largely wasted on a 74.
3. the original 1-5/16" M74 venturi flows more air than the bombsight, due both to actual area, and less shape-based obstruction.
4. if the mixture be accurate in both cases, a 74" motor will produce more power with the larger venturi, since both are below the maximum area.
5. adding a bombsight changes every function, every circuit above tip-in, but not in the same proportion.
6. a possible benefit would be to lean out the low speed cruise mixture to extend mileage without losing throttle response. However, the bombsight will enrich this function - so the change must be made or mileage will suffer. Do you know how to compensate for these changes in the main nozzle air bleed hole pattern? The parts are cheap, your time may not be.

Post Sat Jan 26, 2008 4:56 pm

Posts: 12
Location: Switzerland
Hi Panic

Very intersting infos, thank you!

Installed bombsight venturi is a new bolt on production. I never run this carb on a bike yet, so I don't have any experience at practice. What I know from talking about a guy who has an bombsight m74-b mounted on his pan: quicker throttle response in mid to high range speed and improved increase in speed.

Post Sat Jan 26, 2008 5:40 pm

Response: probable.
Peak power: only if the change also improved his fuel curve.
That's the problem with results reported from complex changes like this - very, very few people enjoy taking the floor and saying "I had this great idea, but it didn't do any good and cost a lot of money". It's the same as "psychic research" - people are always going to find what they wanted to find.

Since a 74 is still under-carbureted with the big venturi (i.e., still pulling a few inches of vacuum at WOT) any increase in venturi area will provide a nice power boost.
Atmospheric pressure: 14.7 psi
Full throttle vacuum with original carb: 1.5 psi (3" vacuum)
Maximum power potential: ((14.7-1.5) ÷ 14.7)^.5, or (13.2 ÷ 14.7)^.5, or 94.76%

Full throttle vacuum with new carb: .5 psi (1" vacuum, about the low limit)
Maximum power potential: ((14.7-.5) ÷ 14.7)^.5, or (14.2 ÷ 14.7)^.5, or 98.28%

Added power: 3.7%, or about 2.5 hp on a good motor. Anything more than that means either:
1. the original carb was even worse
2. the original carb was not supplying correct mixture
3. the change in manifold length or volume etc. did something to the torque curve.

You can see immediately why an S&S G will be of no use on a 74: the difference between the vacuum with a slightly larger carb (let's say 40mm) and the S&S is really small, and it will also bring vacuum below the level that the carb needs to work well. If vacuum is too low, fuel stays in the bowl.

If you reverse this, and use a smaller carb (such as an M-53 compared to an M-74) you always reduce power, except 1. through 3. above.

Post hoc, ergo propter hoc.

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