Carb jets Questions

[sphanna]

Reading Tom Endy's EXCELLENT study on the carburetor. I sincerely thank Tom and several other generous and knowlegable contributors to our hobby for teaching us commoners so much. We owe so much to you all.
I have a few questions about carb and jets.

1. At slower speeds but above idle, is the cap jet the only jet in use or is the main jet also in play?

2. As speed increases, at what aproximate speed does the main jet " kick in "?

3. When the thottle plate is opended beyond the closed position, does the vacuum vent for the idlejet still draw some fuel through the idle jet or is the idle jet completely without flow as the throttle is opened?

[Mitch//pa]

ck out this site it will answer some of your questions

http://www.modela.org

[BILL WILLIAMSON]

Originally Posted by Mitch//pa

ck out this site it will answer some of your questions

http://www.modela.org

Yes, STUDY, STUDY, STUDY, this, until you get the operation of the carb clear in your head. It will answer many of your questions & take the "mystery" out of carbs. These are about the most simple, basic carb out there. About as simple as a pop can & a couple of straws.
Bill W.

[sphanna]

Mitch & Bill: Thanks for you input. I appreciate it.

Bill, i am glad you are back. I wish you health and happines. Sorry for your ordeal.

God bless

[johnbuckley]

Looks like we don't know the answer to your questions Sphanna, surprising with the depth of knowledge of this forum. I hazard a guess to give the following ... but the kicking in of the jets really relies on the vaccuum rather than the revs and thus these are only rough and ready approximations
1)450 rpm-1000 rpm approx, cap jet, no main
2)800-1000 approx
3) idle only up to 450 rpm approx. When the throttle plate is opened a little there will still be a flow ( though diminishing with opening) through the idle; thus at approx 450-650 idle and cap,
PS the link for www.modela.org doesn't seem to be working for me - is it my computer link or is it down?

[Tom Endy]

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From what I have read, this is my understanding of how a Zenith carburetor works:

At a nominal idle of about 450 RPM fuel is being drawn from the secondary well through the idle jet and into the small hole in the intake throat of the carburetor. The throttle plate is cracked open just enough to expose part of the hole. The cracking open of the throttle plate allows air to rush past it causing fuel to be mixed with air and drawn from the small hole. The idle speed has to be as low as possible in order to adjust the fuel\air mixture screw. If the throttle plate is cracked open too far it over rides the idle circuit and there is no affective adjustment of the air\idle screw.

As soon as the throttle plate is opened air rushes past the main jet and the cap jet and fuel is drawn from both of them. Both jets are sitting with their tops just above the reservoir level, so that there is fuel inside the two jets just below the tops ready to be drawn out as soon as a rush of air goes by when the throttle is opened. This is why it is important to set the float level accurately in the reservoir.

The main jet has a direct passage to the fuel reservoir and the more the throttle plate is opened the more fuel will be drawn and the richness of the fuel mixture will increase. You can think of this as a straw placed in a glass of water the more you suck on the straw the more water you will get.

The cap jet does not have a direct passageway to the fuel reservoir. It gets its fuel from the secondary well, which is open to atmosphere. You can think of it as the same straw in a glass of water, but with a hole punched in it. The more you suck on the straw the less water you will get.

The combination of the two jets, one drawing more fuel when speed in increased and the other drawing less fuel when speed is increased balances the fuel mixture (richness) over the speed range.

In addition, the cap jet is controlled by the compensator jet. The comp jet is much smaller than the cap jet and sits in the base of the reservoir and allows fuel to enter the secondary well. The gas adjust valve (GAV) allows the driver to control the richness. By opening the GAV valve it allows fuel to flow around the comp jet and into the secondary well and increases fuel to the cap jet and increases richness.

The secondary well also acts as an accelerator pump. At idle it fills up, when accelerating the cap jet has an abundance of fuel for acceleration from the secondary well. When rolling down the road the secondary well is mostly empty except what the comp jet lets through and the GAV lets though. As soon as the throttle plate closes the secondary well fills back up.

As soon as the throttle plate opens the secondary well empties and the idle jet is no longer a factor. When the throttle is closed the secondary well fills back up and the idle is again controlled by the idle jet from the secondary well.

Tom Endy

[sphanna]

Johnbuckley

Thank you, you have understood my questions and i appreciate your answers. When I was still flying, I made many Atlantic crossings to your fine country. Enjoyed every trip and layover.

Tom Endy. I knew you would be able to answer my questions. I really appreciate your knowledge and willingness to share. You have helped so many of us in this hobby.