Adjustable main jet

[ndutton]

I don't have one so I'm not experienced with it but what is a good baseline setting for the adjustable main jet? Number of turns off the seat?

[Dave Neptune]

FWIW

Neil, I tried one once I got my "stock carb" clean and the float properly adjusted. I was told to start at about 1 1/2 turns open.

I used it to check for lean and rich spots through out my boats range of speeds with an Indigo prop. I found the carb to be a bit lean at off idle to the lower mid-range and a bit rich at WOT. I used thid info and my knowledge of carbs from considerable experience to make a few changes to get better economy and performance across the board without messing with the jet. They do work if you are only setting for one speed quite well though. Personally I don't really like them in a boat because of the possible fuel leak from tweaking.

It did take me a while to get the carb "balanced better" and at least a dozen times breaking it open and changing jets, air correctors and emulsion well
(float) levels. The adjustments really made a big difference at idle and slow speed too as what I did quickened the "leaner" hi-speed transfer from idle. I could run a leaner idle and absolutely no bog.

BTW as you know I do love to tinker with carbs especially Webbers since there is so much info in their manuals.

Dave Neptune

[ndutton]

Thanks Dave. I was able to find the Moyer instructions on this website and your advice matches theirs exactly. I asked on behalf of another boat owner who has yet to join the forum but I'm sure he will soon.

[hanleyclifford]

I love my adjustable main jet but I have to say that it is only practical if you also install an air fuel ratio gauge so that you can zero in on the ideal stoichiometric ratio. The boaters who can benefit the most are those who do long range cruising under power, and the benefit is significant. Of course the ideal ratio will only be achieved thru a narrow rpm range which is why I always adjust fully warmed up at cruising rpm. If we want ideal ratio throughout the operating range we will have to go to electronic fuel injection. I'm game. Now who has the engineering and technology?

[Dave Neptune]

The carb on these A-4's is almost triple the size required as they work with tractor engines almost 180 cu in and larger. A perfect carb for these would be a Zenith off of an old 1300 CC VW but not good for our application as the A-4 is an updraft and the VW's are a down draft. I can barely get my finger in the barrel of the VW Zenith's. Ours are a compromise at best. If you look at the old cast iron carbs on the early engines you will notice they are considerably smaller and rated for the same power and those even have a restrictor plate for the Stevedore mode.

The std 4 and 5 screw Zeniths we use work far more efficiently on a larger displacement engine.

Dave Neptune

[hanleyclifford]

The carb on these A-4's is almost triple the size required as they work with tractor engines almost 180 cu in and larger. A perfect carb for these would be a Zenith off of an old 1300 CC VW but not good for our application as the A-4 is an updraft and the VW's are a down draft. I can barely get my finger in the barrel of the VW Zenith's. Ours are a compromise at best. If you look at the old cast iron carbs on the early engines you will notice they are considerably smaller and rated for the same power and those even have a restrictor plate for the Stevedore mode.

The std 4 and 5 screw Zeniths we use work far more efficiently on a larger displacement engine.

Dave Neptune

So are you suggesting that using the early carb with adjustable jet would give me a greater range of stoichiometic ideal ratio? I have a few I could put into service.

[hanleyclifford]

BTW, the stoichiometric ratio for gasoline engines is 14.7 : 1 At present I have an led analog gauge which I'm not happy with; next move is to install a digital gauge.

[Dave Neptune]

Hanley, if you could get the old smaller CI carb balanced it would be a better "fit" and even at that a bit to large. As the 4 & 5 screw Zenith are later models the metering for the transfer from primary (idle) to secondary (hi-speed) is probably much better as far as balance is concerned. And the idle transfer within the idle circuit (the slots above and below the butterfly at and just off idle are better to. The later models of most any carbs were a constant improvement as machining techniques as well as tolerancing steadily improved.

My point is that either carb is capable of flowing far more than necessary for 35 HP let alone the 18 to 20 something the direct drives can make do to prop restrictions of the direct drive in a displacement hull. A proper carb to displacement should yield about 2 to 2.5 inches of vacuum at WOT to give ideal velocity through the venturi thus creating a better mix. I have worked and been involved in many engine developmental projects and carb size is of utmost importance. In shops I dealt with the most common mistake of "home builders" was choosing to big of a carb or to much cam for the flow specs of the head. Those created to little velocity for good overall performance.

Getting a carb balanced with a good O2 sensor would make things much easier. It took me 5 days of nothing but testing using a good tach and vacuum gage and many pages of handwritten graphs to get my carb the way I wanted it. I only did it because I enjoyed the tinkering. On some of those days I would remove and crack the carb open many times then go back to my machine shop an make new jets, air correctors and emulsion tubes. I would work the carb down to where it would begin to bog lean in a range then bring it to better RPM's by adding more fuel via fuel levels air correctors or jet size. The tubes were a real PIA and I wound up using the stock tube but a different float level and air corrector. I put as much time into it as some racing engines I worked on. When I found the average of what worked best in my boat I stuck with it. It would idle all day or run WOT and in between with a milky cocoa colored plug. It was slightly slower to start when cold and did require a bit of choke if it sat for more than 20 minutes after warming up. I worked the tuning in 200 RPM increments from 1000 to 2500 RPM's which I could get with a clean bottom and prop. I decreased my fuel consumption in all of my slower cruising as well as when I wanted to hurry all with better performance and power.

Dave Neptune