Need opinions on fine tuning.

howie123

howie123

Senior Member
I am nearing completion of a Model A speedster. The engine is 1928, and old rebuild. The carburetor is a Zenith that I rebuilt. The gas tank is behind the seat and gravity fuel feed was not adequate for going up hills. I installed a low pressure in-line fuel pump which caused the engine to idle very rough. I then installed a regulator. The regulator came pre-set at 4.7 psi. I adjusted it down only by watching the fuel flow. I blieve the carburetor is running rich as there is a light coat of black soot on the spark plugs. The engine oil has remained clean. I made a sight gauge for the carburetor which shows the fuel level to be 1 in. below the mating surfaces. I have no gas gauge so I calibrated a did stick. It appears I am getting about 16 MPG. I have done most of my driving at 30-35 mph. Today I took a drive and at about 40 mph I heard a couple of small backfires from the area of the muffler. I readjusted the GAV from nearly closed to about 1/4 turn open. I readjusted the spark to 6 clicks or about 2/3 of the way down. I gradually increased my speed to 40-42 mph and it seemed to run pretty good. The temperature held at about 170 most of the trip but when running at a constant 40 mph for about 5 miles it did get up to 190. How hot is too hot? I think I am getting close to having a good running Model A. Any suggestions on a systematic approach to fine tuning would be appreciated. Thanks. Howie
 
With a newly finished modified car there are lots of issues to sort, and of course with multiple potential issues you can go mad adjusting the wrong thing...
Couple of thoughts: Electric pump/regulator coud easily be a source of problems...The Z is not going to like much pressure at all (and don't forget, retarded spark can minic rich in plug readings...)
Since it runs all right without pump on level, try sorting that issue in a level place.
Get a vac gauge hooked up, multiple uses in this sorting. Runnit at steady speed on level, switch between pump and no pump (splice in a temporary Tee and extra line if needed), see if vacuum changes in steady run. Also check plugs both ways. Try to prove pump is or isn't a problem before you get into other stuff, as excess pressure will mess up anything else you try! (if pump is a problem and cannot get down to a tolerable level of pressure, it can feed a small secondar tank hidden in cowl...there are ways to handle this for continuous feed with gravity delivery.)
Next, running at steady speeds on your favorite level place, sort out advance setting and GAV setting for max vacuum...they affect one another, so go back and forth between the two adjustments until you feel both are happy together. Sort your advance for useful cruising speeds, then as you drive retard for hard pulls that would cause pinging at cruise settings. Low vac situations want less advance, high vac wants more...
 
In reply to howie, you didn't say whether it was a model A or B Zenith carburetor. I set my float by eye but one inch is a bit too low. According to the carb books that I have say the model A float level should be 5/8 . The backfiring that you describe is caused by fuel starvation or a too lean condition caused by a too low float level. A too Lean condition will also result in overheating. Running with the gav set at about 1/4 turn off the seat is good for the model A zenith. The model B Zenith should run with the gav closed after warm up. I run my model a's with original heads with the spark fully advanced at speed. I think that 190 degrees is too hot and is probably caused by a lean float setting and the spark too retarded. I would try raising the gas tank for better gravity flow and lose the fuel pump. I run my gas tank behind the seat on my speedster and it works good with gravity flow.
 
Bruce and Purdy, Thanks for the input. You have both given me some very helpful information. The carburetor is a Model A. I plan to recheck the fuel level with the sight gauge. I will then remove the carburetor to make the adjustment, check jet heights and sizes, etc. I will also try running with more spark advance. When time allows I will recheck the timing, point gap, plug gap, etc. As I said, I feel like I am getting real close. Removing the fuel pump would be a last resort, but I may have to install a pressure gauge so I can really check the adjustment. Any other tips would be most appreciated. Thanks again. Howie
 
The fuel pump can be kept without pressure to carb if you have a bit of room...put a small secondary tank under cowl to gravity feed carb. That tank, which could be very small, wcould then have a large return hose to main tank near the top so it could not build pressure or its own level regulator...OR a flat end on tank with the float bowl from a Holley Four barrel would give it its own float level regulator capable of handling high pressure and providing a steady fuel level in the secondary tank. I've seen that system on racecars providing starting feed to a Hilborn pump!
 
I would try a regulator that will adjust down to 2LB I would think the float would be seeping by at 4lb pressure.
 
Very low regulation seems to be a problem...you are about at the limits of the design range. And you need a gauge even if there are numbers on the regulator...some like the little round chrome ones are extremely unreliable. I think the low range Holley may have 2 at the bottom of its range. Lots of A carbs seem to have trouble against gravity alone...
I think it would be good to test actual performance with temporary gravity feed, just a lawnmower tank or can of some sort sitting up top connected by hose.
If that cures the soot, then see if a quality regulator can go low enough for the carb.
 
Back in the early seventies I put together a sort of sporty home made strip down- speedster. I rigged up a 20 gallon drum gas tank and mounted and braced it to the rear crossmember. The tank was a little higher than the carburetor and worked pretty good, unless the hill was long and steep and if the tank was less than half full. I installed an electric fuel pump and hooked it up through a toggle switch. I wouldn't have to run it unless it was necessary on steep hills. The gas would flow through the pump by gravity when the pump was off. I had the pump mounted near the carb, so the pump was pulling from the tank instead of pushing, like it would do if mounted at the tank. I didn't have a pressure regulator and it seemed to work good with a tillotson model X carb. I ran it on six volts.
 
Thanks for all the input. I removed and cleaned the plugs. I also checked the timing using Les Andrew's method with a test lamp. I was able to adjust the timing much closer than by eye, and I think I could do even a little better. I am using a Holley low pressure regulator. I turned it down when I installed it, but if necessary I will add a pressure gauge so I know exactly where I'm at. The car now starts quicker and runs cooler. It is not quite as smooth at low speeds but seems to have more power and gains speed quicker. I know I have to remove the carburetor to adjust the float level so will do some further checks on it at that time. Also I am shopping for a Marvel carburetor since they seem to get good reviews. All in all I feel like I'm making good progress. Thanks for the help. Any other input is appreciated. Howie
 
I have several marvel carburetors. These are complete untested cores that appear to be pretty good, have the needle driver, special fuel line fitting and the threads appear good in the flange. if interested pm me.
 
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