Determining the best fuel mixture setting

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Model T Ford Forum: Forum 2017: Determining the best fuel mixture setting
Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Patrick in Florida on Monday, February 06, 2017 - 05:16 pm:

I have a Holley NH carburetor that I just rebuilt and I am having trouble determining the proper fuel mixture setting for my T. I started out at 1 1/2 turns from closed, and the car started but ran rough. I then closed it 1/4 turn so that the setting is 1 1/4 turn from closed and it sounded great, but today, during a drive, it sounded very rough and as I drove, I tried to set the fuel mixture with a range of between 1 1/2 to 1 and could not get it to run smoothly.

I have always determined the proper setting by the sound of the engine, but my hearing is not what it used to be and I am unable to pinpoint the best fuel mixture setting for the most optimum performance. Is there a diagnostic meter you can hook up to the T to determine the best fuel mixture setting by which one gets the maximum performance? Thank you. Jim Patrick


Top of pagePrevious messageNext messageBottom of page Link to this message  By James A. Golden on Monday, February 06, 2017 - 05:55 pm:

I have always wondered if tapping the intake manifold and adding an in-dash vacuum gauge, with a hose or pipe, would show the best spark and mixture setting for a Model T engine.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mark Gregush Portland Oregon on Monday, February 06, 2017 - 06:03 pm:

You can buy the vacuum adapter from the suppliers that fits between the manifold and carburetor.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Patrick in Florida on Monday, February 06, 2017 - 06:44 pm:

Is the highest vacuum reading the best way to determine the most optimum fuel mixture setting? I would have thought the highest engine RPM at idle would be the best indicator. Jim Patrick


Top of pagePrevious messageNext messageBottom of page Link to this message  By Peter Claverie, Memphis TN on Monday, February 06, 2017 - 06:58 pm:

The old adage, "most fuel problems are electrical" may apply in this case.

If you've fiddled with the mixture without improvement, it probably is a missing cylinder.

Of course it could be a bit of trash in the carburetor's passages, but I'd look elsewhere first.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ted Dumas on Monday, February 06, 2017 - 06:59 pm:

Did you polish the float needle before you installed it? The new ones are not finished correctly. Maybe you have some small piece of trash left over from your cleaning.

It could be your timer or the coils loose in the coil box.

Are you getting good gas flow out of the tank?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bob Coiro on Monday, February 06, 2017 - 07:12 pm:

I find that sometimes, a few pieces of crud get into the fuel line and make their way into the carburetor, where they mess up the needle-valve setting and the engine starts running rough. _The remedy, for me anyway, is to pull over, open the mixture until the roughness disappears and drive on. _Eventually, the crud gets washed out of the carburetor and the engine starts running rough again from over-richness. _Again, I pull over and re-adjust the mixture.

Ideally, you want to adjust the mixture for fastest idle and then richen it up a bit more for smooth acceleration. _A little test-driving helps find the sweet-spot.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Patrick in Florida on Monday, February 06, 2017 - 07:22 pm:

My float needle is fresh neoprene rubber so the seal there is great. The carburetor has been soaked in lacquer thinner, then Xylene, rebuilt and cleaned out and adjusted. Float is gas tight. The gas tank was flushed out with 2 gallons of fresh gas when I changed the sediment bulb valve screen. Coils are tight in box, timer has been lubricated. All cylinders are hitting. Everything else checks out.

Note the car runs good and steady (not rough) with a slight, occasional miss. Just having trouble getting it adjusted so it runs as good as I know it can.

When I had my 1974, 350, 4 BBL Camaro (before they came with computers), I used to work on it referring to the settings in my Chilton's manual, and using a compression gauge, vacuum gauge, timing light and a diagnostic tester, get the timing, dwell and RPM's just right, in order to get it running perfectly. I just figured there might be a simpler version of a diagnostic tester that could be used in adjusting the T for maximum efficiency. Thank you, Jim Patrick


Top of pagePrevious messageNext messageBottom of page Link to this message  By Fred Dimock, Newfields NH, USA on Monday, February 06, 2017 - 07:36 pm:

I am with Bob on this except it is not always crud.
The optimum setting changes with temperature, humidity, and phase of the moon.
It is not a set it and forget it.

I fiddle with the setting to find the sweet spot as the vehicle warms up.
Me and the car communicate better than me and my wife


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Patrick in Florida on Monday, February 06, 2017 - 08:26 pm:

Don't you think that an RPM gauge would tell you when the mixture was at it's optimum? Since I can't hear it, at least I could see it. Thank you. Jim Patrick


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ken Todd, ............Red Deer, Alberta on Tuesday, February 07, 2017 - 10:59 am:

Have you checked the spark plugs recently?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Norman T. Kling on Tuesday, February 07, 2017 - 11:12 am:

If it runs smoothly with an occasional miss, I would suspect one of the coils. Or a dirty spark plug.

When the engine is cold, you should open approximately 1/4 turn for starting. Then when it starts, advance the spark and switch to magneto. Set the spark to where the engine runs smoothest. As the engine warms up, you turn the carburetor adjustment back to the sweet spot. Once you find out where that spot is, you can mark it. With the 26-27 there is a raised spot on the knob and you just turn it to the same direction every time. However on the earlier adjustment you will need to find the direction the rod is when it runs best and try to put it in that same position.

The mixture will also vary with altitude, but I wouldn't think that would be a problem in Florida.
Norm


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Patrick in Florida on Tuesday, February 07, 2017 - 02:41 pm:

Good idea. I will check the plugs. Does anyone know, offhand, what the plug gap should be on traditional Champion Spark plugs? I checked e-bay last night for a vintage engine analyzer, but they don't have them for a 6V system. I had a Sears Engine Analyzer in the 70's and 80's when I had my Camaro and it worked great! Thank you, Jim Patrick


Top of pagePrevious messageNext messageBottom of page Link to this message  By David Menzies on Tuesday, February 07, 2017 - 03:40 pm:

I read some where that one turn to start and seven eighth to run. I do not know what model of carb I have, but those settings do work well. It will not start with seven eights turn but will start with one turn when cold. With more then one turn it will not start.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Harold Schwendeman - Sumner,WA on Tuesday, February 07, 2017 - 04:15 pm:

Seems like my 3 Model T's with NH Holleys all run with the carburetor adjustment open about one and a half turns open. I think most will want to run somewhere between one and two turns open. Also, I'm pretty sure that with the adjustment anywhere between one and two turns open, the engine will at least start. Might require quick re-adjustment slightly toward leaner or richer, but should be close enough to at least start. Hope this helps,....FWIW,.... harold


Top of pagePrevious messageNext messageBottom of page Link to this message  By Roar Sand on Tuesday, February 07, 2017 - 08:43 pm:

I probably tend to run a bit on the lean side, but I listen to the sound of the exhaust, and I go by the feel of how well the engine pulls. You can hear when the engine is happy! I do quite a few small adjustments along the road, and I do go a tad towards richer on hills.
That's my story, and I'm sticking to it!


Top of pagePrevious messageNext messageBottom of page Link to this message  By John Doolittle on Tuesday, February 07, 2017 - 08:52 pm:

Jim- are you running a hot air pipe, a gas filter, or an air filter?

Please answer- this is critical to know.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Patrick in Florida on Tuesday, February 07, 2017 - 10:19 pm:

Air filter. Had always used a hot air pipe but was concerned about all the road dust getting sucked into the engine so I bought an air filter from one of our members and replaced the hot air pipe with it. No hot air pipe, but the temperature here has been in the high 70's low 80's since getting it running. The gas is filtered through a new stock filter screen inside the stock Ford sediment bulb that came on the car. Jim Patrick


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tim Lloid on Tuesday, February 07, 2017 - 10:28 pm:

It seems with temp changes and humidity changes they need I slight turn here or there but you normally feel it In The seat of your pants. You can just hear and feel with a slight twist one way or the other in the garAge or on the road if she likes it. Tim


Top of pagePrevious messageNext messageBottom of page Link to this message  By Gene Carrothers Huntington Beach on Wednesday, February 08, 2017 - 02:59 am:

A true way to know about fuel mixture is to drive it and then pull and read the spark plugs.


Top of pagePrevious messageNext messageBottom of page Link to this message  By John Doolittle on Wednesday, February 08, 2017 - 06:24 am:

Just as a matter of ruling out possibilities, you might try removing the air filter an installing an old hot air pipe. I fought with the same problem that you are having and found that my problem was solved 100% by running the hot air pipe (year-round).

My Champion X plugs run great set at .025".


Top of pagePrevious messageNext messageBottom of page Link to this message  By george house . . .caldwell county, TX on Wednesday, February 08, 2017 - 09:11 am:

Howdy Jim et al,
I'm interested in how you combined a late model U joint type fuel-air adjustment to that much earlier NH straight through carb. That's neat ! And a nice looking engine compartment.
BTW, I feel Ken has the right idea. My T ran real well for 7 miles going into town last Saturday but fouled plugs got me back home on 2-sometimes 3 cylinders. I put too much MMO in the gas tank..


Top of pagePrevious messageNext messageBottom of page Link to this message  By john kuehn on Wednesday, February 08, 2017 - 09:46 am:

Jim, when I was having an issue with my 24 Coupe similar to yours the problem turned out to be something real simple.

It was a loose nut on the no.1 spark plug wire connection to the coil box. It was just tight enough to hold the wire in place but after checking the connection I could easily move the wire back and forth. I had done the usual timer and carburetor checks and then some. I rebuilt another NH and installed it to no avail.

It took about 2 turns with a small wrench to tighten the nut on the no.1 wire that connects to the coil box and that was it.


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