The Ford Barn

Maurer1929 · 07-18-2011, 05:28 PM

My 1929 Model A engine (hasn't been rebuilt in 20 yrs) runs very well up to 45 mph or so not ready to rebuild it. I have a new rebuilt carb Zenith 1 rebuilt by an expert. But it still runs rich, no popping or gas smells but even with the GAV almost closed (less than an 1/8 turn open) the plugs still have good coating of soot. Gas mileage is not that good either. Any suggestions on getting it to run leaner and improving gas mileage? Thanks

Tom Endy · 07-18-2011, 06:01 PM

Gas mileage is predicated on the flow of gas through three jets. Each has a window for flow rate. A good restored Zenith should have had the jets checked on a flow rate rig. Most reproduction jets (and many originals) are sized way out of the window. The procedure is to solder the ends closed and very carefully re-drill them and re-check the flow rate. It is a tedious trial and error effort to get them right.

The three critical jets are the main, cap, and comp jets. The idle jet does not affect gas mileage. The comp jet is critical and has the narrowest window for flow rate. Its flow rate is much less than the cap jet and contols the amount of gas sent to the cap jet. To richen the mixture you open the GAV valve and it ports gas around the comp jet.

If the flow rate of the comp jet is the same or larger than the cap jet the GAV has no control and the amount of fuel spewing from the cap jet is then at the rate of what ever the cap jet is sized at.

Attached photos of a flow rate rig.

Tom Endy

Tom Endy · 07-18-2011, 06:09 PM

My apologies, I posted the wrong photos. They are actually of a test rig for setting the float level. I will come back onto this thread with the correct photos. The trick is to find them.

Tom Endy

Tom Endy · 07-18-2011, 06:22 PM

I think I got it right this time.

Tom Endy

V4F · 07-18-2011, 07:09 PM

tom , how do you know the correct flow rate for a particular jet size ? is there a flow chart ? .. thank you

Tom Endy · 07-18-2011, 07:54 PM

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There is a flow chart. It was put out some years ago by Double D Productions. These were the folks in the Dallas MAFCA club that put out a very excellent video on Zenith restoration. The video is called Carburetors and Carbohydates. It is a color video on VHS or DVD. Bratton's carry it under part numbers 37990, 37991, $39.95.

Our club bought a copy years ago. It came with a chart that specified drill sizes for the jets. The problem is that drilling is not accurate. You can drill half a dozen jets out to the same size and they will all flow at different rates.

The flow chart I use I believe was specified in the video. I cannot attest to its authenticity or if it was sanctioned by Henry, but I have been using it for years and it seems to work.

Main - 140-150

Cap - 150-185

Comp - 138 - 142

Idle - 44 - 48

The values are in CC's per minute.

When I flow test I have a valve I turn on to allow water to run through the jet. It has to flow in the direction that fuel would flow in the carburetor. I time it for one minute while capturing the water in a chemistry lab beaker with CC graduations on it. It is a time consuming and frustrating procedure. You may end up flow testing, re-soldering and drilling the same jet out three or four times.

The best way to go about it is to have a seminar with three or four guys and set up an assembly line. One guys does the testing, another does the soldering, another does the drilling. You can spend the whole day at it. The trick is to do them as sets, mark the flow rates on a card and put the card and the jets in a plastic envelope. Then when you rebuild a Zenith all you have to do is reach for one of the plastic envelopes.

My process is to drill the jets out to about two sizes smaller than what is specified in the drill chart, then flow test it. It should flow test low. By running the same drill bit back through it it usually makes it larger. The plan is to sneak up on the flow rate.

Tom Endy

Pilotdave · 07-18-2011, 09:17 PM

Tom, thanks for posting an explanation of your process - I was hoping you'd do that when I saw the pictures of your set-up.

ctlikon0712 · 07-18-2011, 11:11 PM

If you flow the jets with water, isn't that using a medium that is a different viscosity than fuel? Which would give a different flow rate?

MrTube · 07-19-2011, 07:19 AM

Quote:

Originally Posted by ctlikon0712

If you flow the jets with water, isn't that using a medium that is a different viscosity than fuel? Which would give a different flow rate?

I'm betting he has some kind of conversion chart if this is the case.

Maurer1929 · 07-19-2011, 03:18 PM

Tom, thanks for the pics and great advice on flow rates to check. That will be next on my list to look into.

SANTA Ray · 07-19-2011, 04:09 PM

I don't disagree with everything said. I had similar issues and changed to mid-grade gas and it helped considerably.

It goes without saying you need a reasonably good set up to begin with.

Good Luck - I just returned from an Alaskan trip from GA and got 20.4 avg. mpg. driving 45 mph to 50 mph. 31 Coupe.

07-18-2011, 05:28 PM	#1
Maurer1929 Member Join Date: Nov 2010 Posts: 47	Running Rich My 1929 Model A engine (hasn't been rebuilt in 20 yrs) runs very well up to 45 mph or so not ready to rebuild it. I have a new rebuilt carb Zenith 1 rebuilt by an expert. But it still runs rich, no popping or gas smells but even with the GAV almost closed (less than an 1/8 turn open) the plugs still have good coating of soot. Gas mileage is not that good either. Any suggestions on getting it to run leaner and improving gas mileage? Thanks

07-18-2011, 06:09 PM	#3
Tom Endy Senior Member Join Date: May 2010 Location: Southern California Posts: 3,134	Re: Running Rich My apologies, I posted the wrong photos. They are actually of a test rig for setting the float level. I will come back onto this thread with the correct photos. The trick is to find them. Tom Endy

07-18-2011, 07:09 PM	#5
V4F Senior Member Join Date: May 2010 Location: ca. Posts: 2,522	Re: Running Rich tom , how do you know the correct flow rate for a particular jet size ? is there a flow chart ? .. thank you __________________ V4f

07-18-2011, 07:54 PM	#6
Tom Endy Senior Member Join Date: May 2010 Location: Southern California Posts: 3,134	Re: Running Rich Sponsored Links (Register now to hide all advertisements) There is a flow chart. It was put out some years ago by Double D Productions. These were the folks in the Dallas MAFCA club that put out a very excellent video on Zenith restoration. The video is called Carburetors and Carbohydates. It is a color video on VHS or DVD. Bratton's carry it under part numbers 37990, 37991, $39.95. Our club bought a copy years ago. It came with a chart that specified drill sizes for the jets. The problem is that drilling is not accurate. You can drill half a dozen jets out to the same size and they will all flow at different rates. The flow chart I use I believe was specified in the video. I cannot attest to its authenticity or if it was sanctioned by Henry, but I have been using it for years and it seems to work. Main - 140-150 Cap - 150-185 Comp - 138 - 142 Idle - 44 - 48 The values are in CC's per minute. When I flow test I have a valve I turn on to allow water to run through the jet. It has to flow in the direction that fuel would flow in the carburetor. I time it for one minute while capturing the water in a chemistry lab beaker with CC graduations on it. It is a time consuming and frustrating procedure. You may end up flow testing, re-soldering and drilling the same jet out three or four times. The best way to go about it is to have a seminar with three or four guys and set up an assembly line. One guys does the testing, another does the soldering, another does the drilling. You can spend the whole day at it. The trick is to do them as sets, mark the flow rates on a card and put the card and the jets in a plastic envelope. Then when you rebuild a Zenith all you have to do is reach for one of the plastic envelopes. My process is to drill the jets out to about two sizes smaller than what is specified in the drill chart, then flow test it. It should flow test low. By running the same drill bit back through it it usually makes it larger. The plan is to sneak up on the flow rate. Tom Endy

07-18-2011, 09:17 PM	#7
Pilotdave Senior Member Join Date: Oct 2010 Location: Grafton, MA Posts: 1,229	Re: Running Rich Tom, thanks for posting an explanation of your process - I was hoping you'd do that when I saw the pictures of your set-up.

07-18-2011, 11:11 PM	#8
ctlikon0712 Senior Member Join Date: Feb 2011 Location: Cocoa, Florida Posts: 1,609	Re: Running Rich If you flow the jets with water, isn't that using a medium that is a different viscosity than fuel? Which would give a different flow rate?

07-19-2011, 03:18 PM	#10
Maurer1929 Member Join Date: Nov 2010 Posts: 47	Re: Running Rich Tom, thanks for the pics and great advice on flow rates to check. That will be next on my list to look into.

07-19-2011, 04:09 PM	#11
SANTA Ray Member Join Date: May 2010 Location: Peachtree City, GA Posts: 65	Re: Running Rich I don't disagree with everything said. I had similar issues and changed to mid-grade gas and it helped considerably. It goes without saying you need a reasonably good set up to begin with. Good Luck - I just returned from an Alaskan trip from GA and got 20.4 avg. mpg. driving 45 mph to 50 mph. 31 Coupe.