Jet sizing and Carb questions

[mohill]

I have a set of carbs in question that are on my Johnson 40hp. I installed boyesen reeds with the recommendation of re-jetting. Took carbs apart and checked jet sizes.

My results: fixed idle jet #32
High speed #59D

This is on a big bore, not stock for the 40hp, 1.5 inch throat carb.

Stock jetting for the johnson 40hp has a high speed #39 I believe

I did some researching and the closest stock set up i can find to this (32 and 59d) is a 1979 70hp Johnson that stock was (32 and 57d)

My question is, are my carbs set up ok with the current jet sizes? I have no issues keeping the plugs wet, so thats good. I plan on shaving an undecided amount from the head to raise compression to around 160 or so, and was planning on getting some 56D jets from some reading I have done to "wake up" my motor, but 59d is larger than 56d so I think I am going to stick with the 59d but am unsure how the idle jet should be sized since this is not even close to the stock carb and jetting set up for a 1984 Johnson 40hp.


Any help and/or insight will be greatly appreciated as I am self learned when it comes to modifying motors.

[mohill]

With all the knowlege on this site, no one can shed light on the subject?

[omcstratos]

I am right behind you in the 2 cyl mods playing with jets. I haven't gotten my boyesen reeds but plan on it soon. I can't remember exactly but I think I have a "57 something" for the idle jet and a 56C for the main jet on my 50. But to answer your question, you will have to be the judge of your setup because it is hard to fully understand your whole setup by reading here. For a 2 stroke, light brown plugs are usually even too lean so find what jets will lean it out just enough with out drying out your plugs. If you are still running with the factory air box, from what I understand is you can free up some more airflow by removing it. I'd try that and jet the motor accordingly. That's all the light I can shed.

What's the difference in the big bore carbs, I don't know the size of my carbs right now but I wonder what the difference is in your big bore 40 and my stock 50?

[mohill]

50 something for idle jet seems rather large. Should be more like a 30 something. My air box is gone. Don't know what year your motor is but the 40 and 50 block is the same for many years. I think the stock carbs have a 1.25 inch throat and these I have now are 1.5 ill try and get a pic. I can tell you just by looking at them side by side these bigger ones allow a lot more flow. Much less restrictive.

[omcstratos]

I'll look at mine and re check my jets again, but I am about to begin playing around with the jets and saw that the idle jet is indeed larger than the main jet. I am curious to see about swapping the two. My motor is a 2000 and the carbs have the plastic top lid (whatever style that is) anyway, keep posting!

[shenders]

Ok. maybe I can help you out. First let me say this, IF I was going make a change in the
jets I would only go up or down one size. The boyesen reeds are not going to change fuel
flow but very little. The main benefit is if you brake a reed it won't damage the engine.
Keep in mine that idle jet is an air jet not fuel. If you go up in jet size that lets in more air
and gives you a leaner mix.
Put in new plugs and make a good long run at wot. Come back to bank and cut the power off.
What I’m trying to say is don't idle the motor to much. You want to see what the plug color is
at wot. It should be a nice medium brown. Not black or too light a brown.

Question, what color are they now?????

[mohill]

Shenders- plugs are staying wet and are a medium to dark brown color. I have not changed jet sizes, I have just obtained the larger throat carbs with the 32 idle and 59 high speed jets are what came with it. My original question is I am wondering if this jet set up is ok for my motor or should I change it up to run leaner etc. motor doesn't seem to bog down at any speed. Just trying to get advise to get the most out of my motor.

Also, the top idle jet is behind a screw in the top of my carb. If I take the screw out and run the motor fuel will come out of the top orifice. I am novice and ready to learn all the time so why is this if it is an air jet?

[LittleCharger]

Shenders- plugs are staying wet and are a medium to dark brown color. I have not changed jet sizes, I have just obtained the larger throat carbs with the 32 idle and 59 high speed jets are what came with it. My original question is I am wondering if this jet set up is ok for my motor or should I change it up to run leaner etc. motor doesn't seem to bog down at any speed. Just trying to get advise to get the most out of my motor.

Also, the top idle jet is behind a screw in the top of my carb. If I take the screw out and run the motor fuel will come out of the top orifice. I am novice and ready to learn all the time so why is this if it is an air jet?

What are you running the motor on? Tell us about your set up.

[shenders]

I am with charger. lets hear about your set up. speed--rpm, where is the engine setting. how deep? pictures if you can. what kind of prop.
are you still going to cut the head???

as far as the idle goes omc made 3-4 sets of carbs. all were a little different. now I am 71 trying to remember things from the 1980's..its hard so may be
off on some thing a little. sorry.

[mohill]

Motor: 1984 Johnson 40hp
Currently running big bore (1.5 inch throat) carbs and boyesen reeds
Future mods are to shave the head to 160-165 psi (now both are at 125) and possibly look into opening up partially blocked ports or anything restrictive

Prop: standard 11 3.4 x 17p omc sst (I think 11 3/4 could be 11 1/4) either way I am prop limited in my opinion because this is not a performance prop and I have gotten all that I can get out of it. Running 5600-5700 rpm at 36mph with 2.42 gear case. Gives about 3 % slip. Had this same motor and prop on a flat bottom and got 6200 rpm out of it.

Boat is a 2013 alumacraft 1546 welded jon wieghs about 300 lbs. Antivent plate is 2 inches above the bottom of hull on a 5 inch setback due to aftermarket t/t unit. I get fine water pressure now and can probably go up some but not much on mounting height.

I am not trying to break the sound barrier as I use this boat for all purpose riding/fishing/hunting, but I know i can get more speed out of this set up. I think I am prop limited as far as performance goes and am looking into a performanced based prop but am getting mixed info on chopper style, cleaver or semi cleaver, over hub or through hub. I feel like the boat needs a prop that will "lift" it up. With my current prop I can trim up some but still alot of wetted area causing drag. I also am trying to make sure the carb is ok set up with jets the way it is because it is not the stock carb set up for this motor. I have a feeling i may be running too rich.

No pic readily available at the moment. Will get back with one

[omcstratos]

You need to talk to Ron about some of his props. I use my boat for the same purpose as you, hunt/fish/go faster than my buddies while doing so, lol. With this style of riding, Ron told me to try one of his Yamaha style semi-cleavers. They are a good surface piercing prop that still has good holeshot for when your boat is loaded down a bit. My buddy has a 02 alumacraft 1546 with a 3cyl Merc and a 18" thru-hub true cleaver and from testing his boat, I would prob say it is too heavy a hull for a true cleaver. You can a find my boat setup on the Help from Ron Hill thread. My 22" semi-cleaver planes off 3 times faster than his true cleaver and runs slightly faster.

I went from a 10.75x17 stiletto uncupped sub-surface prop turning 5500 rpms to a Signature 12.5x22 turning 5000 rpms. Propshaft about 2in from the bottom of the boat. I need to bring the rpms up a bit but I hope that may give you some sort of a baseline.

Bring up the compression in that motor, mine is 148 on both and runs fine on non ethanol 89.

[shenders]

why don't you mill the head next. leave the jets as they are for now. try for around 145---150 comp. you can get the comp. to height.
I would take .020 th at a time until it came in where I wanted.. make a run at wot and see how it goes. you can always take another .020
if you need to....

ps.. keep good notes on everything your doing and how the boat is running..

[LittleCharger]

why don't you mill the head next. leave the jets as they are for now. try for around 145---150 comp. you can get the comp. to height.
I would take .020 th at a time until it came in where I wanted.. make a run at wot and see how it goes. you can always take another .020
if you need to....

ps.. keep good notes on everything your doing and how the boat is running..

Great advice...if you cut the head to the sst45 head spec you should be in the 145-150 ish area on a good powerhead or find your self a 55hp or 60hp head they are roughly the sst45 head spec right out of the box

See post #60 http://www.boatracingfacts.com/forums/showthread.php?10179-Set-Up-Help-11-Tunnel&p=98945&highlight=#post98945