Adjustable Main Jet?

[edwardc]

I was wondering what the knowledgable folks here thought of the adjustable main jet (Product No. - FCAR_07_58) that Don sells?

I'm considering adding one to my late-model 5-bolt Zenith carb, but wonder if it's worth the trouble and cost. Is it really an improvement, or just a way to compensate for other problems?

[Jesse Delanoy]

I have a fixed main jet, and I'm very satisfied with it. Power is fine, starting is great, and I burn between half and three quarters of a gallon an hour, depending on how hard I'm pushing the old girl.

[hanleyclifford]

Ed - I made five trips to Florida on the fixed jet and one with the adjustable. I was pleasantly surprised at my ability to tune the mixture. But I also had the air/fuel ratio gauge to help me "dial it in". If you plan any long trips the combo is worth the expense. See my Engine Room album. Hanley

[ArtJ]

I noticed in another post recently that someone was able to clear a clogged
jet by adjusting it Rich temporarily.
Would be interested in what others
thought of this as a way of bleeding the line and carb while underway,
and( hopefully) avoid a shutdown and wait for cooling of the engine in order to bleed
safely by removing the fix jet's main passage plug.

It would be a great help if works even sometimes

Regards

Art

[Baltimore Sailor]

Any part that gives one a little more flexibility is OK in my book.

I added the adjustable main two seasons ago and it was as easy to put in as a spark plug. It did save me once when I got a clog in the carb after a long layup for repairs, and being able to rich up the main mixture was the difference between me getting her home and not getting her home.

[gary gerber]

I find ArtJ and Baltimore Sailors comments interesting regarding using the adjustable main jet to clear " partial blockage" temporarily ( so one can motor on ). I had never considered the adjustable jet on my 1970 Morgan 33.

This makes me want to consider the adjustable jet purchase also. What would the initial setting recommendation be, how many turns to back out the needle, an average... as everyones' engine is alittle different?

[hanleyclifford]

Not only are all the engines different, but mixture will vary with changes in rpm. I found that I could not set the ideal mixture at idle and expect it to remain stable at cruise rpm. I found myself adjusting each day as soon as the engine was running for about 1/2 hour. Atmospheric conditons also seemed to call for adjustment. But all in all I enjoy the exercise and it does yield noticicable fuel economy.

[ArtJ]

Not only are all the engines different, but mixture will vary with changes in rpm. I found that I could not set the ideal mixture at idle and expect it to remain stable at cruise rpm. I found myself adjusting each day as soon as the engine was running for about 1/2 hour. Atmospheric conditons also seemed to call for adjustment. But all in all I enjoy the exercise and it does yield noticicable fuel economy.

Hi Hanley

Just curious, does the fixed jet need adjustment as well, but can't get it?

Also, what do you think of the adjustable being used to bleed or clean the
carb?

Regards

Art

[hanleyclifford]

Art - I can only surmise that the fixed jet carb will not deliver a constant air/fuel ratio over the rpm range. The adjustable jet's apparent value in clearing blockages stems from allowing extra fuel to clear very small particles from the jet and discharge nozzle. If the adjustable jet is being used in this way then somebody else upstream is not doing it's job. Regards, Hanley

[ArtJ]

Thanks Hanley

Art

[Baltimore Sailor]

Art - I can only surmise that the fixed jet carb will not deliver a constant air/fuel ratio over the rpm range. The adjustable jet's apparent value in clearing blockages stems from allowing extra fuel to clear very small particles from the jet and discharge nozzle. If the adjustable jet is being used in this way then somebody else upstream is not doing it's job. Regards, Hanley

The only time I had to use the adjustable jet in this way, I had both a Racor and polishing filter upstream (and still do). I can't explain how the gunk got in there, I'm only presuming some did, since opening up the jet a half-turn let her run.

I ended up turning the jet the half-turn back the other way after a day or so of motoring, with no noticeable effects; I guess the blockage went away on its own. That couldn't happen with the fixed jet: you'd have to take it out and flush the carb into a jar instead. Much easier this way.

[hanleyclifford]

Another consideration: The fixed jet is smaller than the adjustable but has a single hole dead center. The adjustable jet is larger but has a needle dead center reducing the maximum crossectional dimension. This may account for the adjustable jet blocking a particle that would get thru the fixed jet and would account for the ability to dislodge the particle with a slight opening of the jet. I have experienced this myself, also having two filters upstream.

[ArtJ]

Another consideration: The fixed jet is smaller than the adjustable but has a single hole dead center. The adjustable jet is larger but has a needle dead center reducing the maximum crossectional dimension. This may account for the adjustable jet blocking a particle that would get thru the fixed jet and would account for the ability to dislodge the particle with a slight opening of the jet. I have experienced this myself, also having two filters upstream.

Based on the above,can we deduct that a fixed jet, which clogs with both filters in the line would
not clear itself with a adjustable jet, since it is already bigger?

Regards

[hanleyclifford]

I would put it this way. The fixed jet will pass a larger particle initially but since the adjustable jet can be backed out it should be able to pass the larger particle, but not in it's optimal running position.

[ArtJ]

I would put it this way. The fixed jet will pass a larger particle initially but since the adjustable jet can be backed out it should be able to pass the larger particle, but not in it's optimal running position.

I wonder how difficult it would be to design a jet which could have a wider
range and be able to pass bigger particles for flush out purposes, but be
able to be adjusted normally as well?