Main Air jet removal

[1980cm400t]

Has anyone ever removed the main air jets from their carb bodies? I have a rather challenging set up to tune and need the ability to regulate air flow to the main and pilot jet eductors. The pilot air jets are not a problem to remove but the main air jets do not appear to have any obvious removal mechanism.

[sparkfly88]

Main air jet... you mean the jet right by the pilot?

[skids]

They are pressed in I think. If you are bent to remove them, heat the body before you pull them out (although I never did that).

[natemoen]

Yes, they are a pressed in restrictor. They are NOT a jet like the rest are. If you remove them I would say you are just asking for trouble.

[TADracer]

Actually, if you have a 'rather challenging setup' to tune for then perhaps the Mikuni BS34 carbs are not your best choice.

[IanDMacDonald]

That, or a Stage kit.

[natemoen]

Has anyone ever removed the main air jets from their carb bodies? I have a rather challenging set up to tune and need the ability to regulate air flow to the main and pilot jet eductors. The pilot air jets are not a problem to remove but the main air jets do not appear to have any obvious removal mechanism.

Explain your setup for us.

[1980cm400t]

I have 4 into 2 exhaust no muffler and no baffles. On the intake I have velocity stacks and low pressure drop pod filters. The carbs have been completely gone over they are clean and have new floats and diaphragms. I have adjusted main jets, pilot jets, needle height and type, pilot screws, float level, and idle adjustment. No matter what I do it will not idle and usually will not rev properly. It is running extremely rich because the pilot jet and main jet have eductors in front of them and the increase in intake air velocity is causing the eductors to dump massive amounts of fuel into the engine, so much so that white smoke (i.e. unburned gasoline) and some liquid gasoline comes out both exhaust pipes. The best performance I have gotten so far was with size 30 pilot jets, size 105 main jets, the needle in the maximum lean position, and the lowest fuel level physically possible. The result was that it would rev at at a flick of the throttle but would not idle unless the petcocks were turned off and the fuel level allowed to lower. Even then there was a generous serving of black smoke out the exhausts i.e. still to rich. At that point the idle screw bottomed out and became the limiting adjustment. I plan on going back to my stock setup and begin reducing the pilot air jet size to reduce the pilot eductor performance. But I am afraid that I will need to do the same to the main jet and I am not sure how to remove the main air jet without damaging the carb bodies. If it becomes a problem I may just plug the main air jet and make the main jet bigger to compensate.

[natemoen]

I have to say that your setup does not make sense. You have mods that would require going richer yet your jets are extremely lean. I think you need to go back the the stock and stator over. What do your plugs look like?

[DGXSER]

To be honest, there are very few if any folks who get these engines to run worth a plugged nickel on open exhaust. Theses engines like backpressure. If you add some sort of baffles, restrictors in the exhaust, you will get much better results.

[skids]

I have 4 into 2 exhaust no muffler and no baffles. On the intake I have velocity stacks and low pressure drop pod filters. (snip) It is running extremely rich because the pilot jet and main jet have eductors in front of them and the increase in intake air velocity is causing the eductors to dump massive amounts of fuel into the engine.

I dont know what an eductor is. Maybe you could educate me. I see that you have pod filters. Are the carbs earlier model? If so, what did you do with the vent hoses? You can't plug those off, and I would not expose the ends to high velocity air from the stacks because that would pull vacuum into the carbs.

I think you would be sorry if you removed the pressed-in main air jet because you are trying to compensate for other things that are not right.

[motoman]

Basicly, no offense, but you have a fuster-cluck of assorted mods that'll never work right........startin' with the open exhaust. No help unless you change out that exhaust for a baffled system for starters.

[1980cm400t]

You are correct that with increased airflow and no back pressure the carbs should be running lean not rich, which is what has been throwing me off (by the way the plugs are black and covered with carbon). These engines do not like backpressure, the carbs are designed for a specific air flow rate. Every engine will be more efficient and more powerful with less pressure drop on the intake and exhaust. The reason that no one is able to run these bikes without backpressure is the air eductors on the carbs. A typical carburetor uses vacuum in the main air passage to suck fuel into the engine. These carburetors use the above but also use an eductor powered by the inertia in the velocity stacks to pull additional fuel into the engine. What this means is if you significantly reduce backpressure and increase airflow (which I have done) these carbs will run richer due to the increase in fuel flow from the eductors which is counterintuitive for carb tuning. I have also removed the original octopus fuel system and replaced it with a much simpler gravity feed system with a fuel filter. I have the newer model carbs with the plastic floats. What are the vent hoses you are referring to?

[IanDMacDonald]

Disregard the tip regarding vent hoses. Skids spoke of them if you had the '78/'79 carbs which you do not. Your carbs vent through the bell housing.

[natemoen]

I would say that if you are really going to go down this road then you need different carbs. Carbs that are fully adjustable in all aspects and not just some of them.

It just isn't worth ruining something for the sake of trying to make it do something it really isn't meant to do.