Hard starting when hot.

[Klaus]

Hello all.

So I fitted my TEA-20-80 with a new Zenith. It works nicely, but I am struggeling setting it up just right. It seems to hesitate (with black smoke) when I open up the throttle, so i Screwed the main jet in - alot, but the result is pretty much the same. Air filter is newly cleaned. The other day when I had the tractor stopped for 10 minutes, it was very reluctant to start. I suspect that it is a carb setting issue.
Any ideas or pointers are very welcome.
Klaus

 

[Anonymous-0]

Black smoke at throttle opening hints that the accelerator pump is putting out too much fuel.

You need to adjust three things on that carburetor, idle mixture, accelerator pump, and main jet. They interact and the governor hides the effects of adjustment unless you watch the governor close the throttle as you improve the two mixture adjustments or you hold the throttle in a fixed position and adjust for maximum engine speed.

Then you adjust the accelerator pump for a good transition from idle to run without black smoke or stumble.

Gerald J.

 

[Klaus]

Thaks Gerald.
But now you have got me all confused.
I have main jet, idle mixture and idle speed. I am not sure what you mean by "accelerator pump". Maybe i've just got the different terms mixed up?
Klaus

 

[JDemaris]

I don't see any accelerator pump on the carb in the photos. I have several IHs with British Zeniths that DO have accelerator pumps. But, British Zenith is a totally different company then US Zenith.

Accelerator pump senses throttle-shaft movement and squirts raw fuel into the engine whenever it moves. Autos and pickup trucks back before fuel injection all had the pumps. Not real common on farm tractors though. American Zenith did make a few, but not the one you've got in the photo.

If you are sure that carb is jetted properly for your engine, you might want to check the float level. If it's too high, it will give those symptoms.

 

[Jerry/MT]

On a hot engine, hard starting can becaused by an overly rich mixture which occurs because the intake manifold is hot and the airflow into the engine is being heated, causing it to be restricted. (Rayliegh type flow in the intake manifold.)
Most of these carbs have a "acceleration well", an area that holds fuel that only is available when the venturi differential pressure is above a certain level. Holes in the metering nozzle allow the excess fuel for acceleration to enter the metering nozzle. At least that"s how it works on the Marvel-Schebler carb. I have no experience with Zenith carbs.
I believe your Zenith carb is a "pressure balanced" design. That means the fuel bowl is vented to the carb inlet, and so mixture ratio is independent of the air cleaner restriction.

The thing I would check is the fuel bowl level which is set by the floats. If the float level is too high, you can get flooding and mixture enrichment that can"t be controlled via the metering jet.

Another possibility is that your main jet is too large.

I would check the float level first bcause it"s the easiest thing to check.

 

[Anonymous-0]

Accelerator pump can be operated by falling manifold pressure as when the throttle is opened fast. Can be operated by a cam on the throttle shaft inside the carburetor housing. It only takes a pump piston an eighth inch diameter moving a quarter inch to supply enough fuel for an engine that size.

And adjusting the float level to make the gas level higher or lower does affect the mixture in the transition region independent of the two mixture screws. The idle speed screw just sets the minimum opening of the throttle, something you set after you get the mixtures right.

You have to set idle mix with the engine running very slow to keep from having a big contribution from the main jet that hides any changes you make with the idle mix screw.

Gerald J.

 

[altrip51]

Possibly it"s flooding some. Check the float level. Is your petrol clean? A bit of sand can cause the float to stick.

When you start the tractor warm you should have the throttle all the way off and no choke.

Also, do you turn off the fuel valve at the tank, even if you are only stopping the motor for 10 minutes to reduce the chance of flooding?

Is there any problem over time with continuous running of the engine.

Good luck, Al

 

[JDemaris]

Yeah, and the "back suction" economizer valve often used in USA Zeniths also responds to throttle position and/or movement.

Getting away from theory for a moment (espcially since I don't know the model # of the carb), a high float and fuel level certainly WILL affect mid-ranged performance and can cause excessive black smoke. It can plug air-bleed holes, it can permit fuel drops to accumulate in the air intake, etc. I've seen it happen on many tractors, and with some . . . the fix was to get the fuel lever corrected.

 

[Anonymous-0]

I used to have what the door tag said was a '65 F-350 dually. The rear axle had to have come from a deuce and a half, the front axle brake fittings were 1964. The guy I bought it from claimed it had a 390, but it had truck 352 heads which in those years also fit a 390. It had a four barrel carburetor that didn't match at all. It ran poorly for a long time until I dug into the sheet that came with the carburetor rebuild kit. It had different float levels for different sized and model engines. I presume to get the transition and the idle mix right. Until I increased the fuel level in the float bowl, it would sound like it was loading up while idling but actually it was leaning out. So float level may have to be tweaked to this particular engine.

Gerald J.