venturi question

[Anonymous-0]

got a 409 farmall in our super m with a w 9 carb, been running 2800 rpm . starts out strong but fades under increasing load, i suspect 35 venturi is to blame. advice please thanks.

 

[Underdog Moline]

http://www.ytmag.com/cgi-bin/viewit.cgi?bd=pulling&th=162498

might help a bit

 

[Dblair]

hook a vacuum gauge to the manifold above the carburetor and check it out .
might have to build more muscle in .

 

[Gary Baker]

Are you using a fuel pump?

 

[Anonymous-0]

yes we are using a pump with 5# pressure, we burn 110 octane fuel, we've been playing with the timing. itdoesn't seem to be lacking on fuel according to the plugs and the exhaust. maybe we are just expecting to much?

 

[Mike in SD]

Have you tuned it on a dyno? What kind of HP are you getting? Is your cam cut for the 2800 RPM? What else has been done to the carb? What kind of pump are you running, a rotary or rattle pump, a rattle pump may not be enough? Are you still running the fuel through the sediment bowl?

There are alot of factors that could be causing you problem try and rule out the easy ones first.

 

[Anonymous-0]

no way to dyno this tractor. cam torque curve is suppose to peak at2650 or so. main jet isdrilled to 131 , emulsion tubeis drilled too. no sediment bowl. carb bowlis vented to the outside thru the top

 

[Superih]

carb bowl is vented to the outside thru the top

What does this mean? The fuel bowl is under a vacuum.

 

[Anonymous-0]

no the fuel bowl is just vented to the outside so it can breath, when this was done we had to turn the load screw in because it ran way rich. we can reverse this by putting a plug in and start over. the reason we tried this was to see if we could keep from using fuel faster than we could deliver it from the tank

 

[Anonymous-0]

sorry, i had to read your ? again , the answer would be yes to the vaccum in the fuel bowl or we wouldn't have changed the load screw after the vent install. i'm ?ing why this would happenif fuel comes in as it is being used, but we're new and thought we'd try maybe bad advice?

 

[Superih]

I know that the fuel bowl has to stay under vacuum, I have tried venting to atmosphere before, it over pressurizes the fuel causing more to flow to the load screw, which is fine, but it throws the proportions of the rest of the circuits in the carb out of balance (idle, main air bleed, etc.)

The imulsion tube controls the fuel delivery rate as rpm's change, as rpm's increase an engine requires a leaner mixture than lower rpm's. You must be careful not to over drill the top and middle hole, as these let air in as demand on the carb increases, if they are too big the engine will go lean.

One of these days I need to dig up a book I have that contains a very good description of how these carbs work and scan it onto this forum.

 

[Anonymous-0]

no problem, i can plug the hole and move on. i have been looking at the emultion tube, i did not personally do any drilling on it, the machinist that assemblied our motor said he drilled the bottom hole bigger but i don't see any differance between the bottom hole and the middle hole, i am going to go to a friend with a indexed drill set and see what's what, try to get a # or a size. i have a emultion tube here out of a 715 gas combine, but i don' want tojump of the deep end and not be able to go back if it's a mistake...

 

[Superih]

If you go too far simply soldeir the hole up and re-drill.