I had to change the jet on my '01 325 Trailboss last week. What a pain! Anyhow, after changing the jet and putting it all back together I couldn't get it to start. The carb just wasn't getting any fuel from the tank (3/4 full). After checking it out, it appears the fuel system uses some sort of vacuum valve which just wasn't allowing fuel through it. I guess there was to much air in the system to purge and therefore gas can't get pulled through. To get this bike running again, I had to remove the carb again, fill the bowl with fuel and then reassemble once again. I had hoped with a full bowl of fuel, the motor would run long enough to evacuate the fuel system of air and then work normally. Luckily it worked out fine and the bike is still running fine. I'm guessing this system is all in the name of safety to prevent a fuel spill in case of a tip over accident but after all this extra work, I really long for a plain old gravity feed fuel system. Has anyone ever bypassed this rediculous setup and if so, are there any drawbacks? If I can't remove the vacuum system, does anyone know a better way to recharge the vaccuum than what I had to do? This is the 2nd time I've rejetted the carb to allow for changes in elevation and the 1st time I had no problems restarting the bike. Also, is there an easy way to access the carb jets? This thing is a pain in you know what to work on.
Max

 

It is pretty necessary to have the accelerator pump on a 4stroke, without it, you would run impossibly lean uin some areas. You could buy a 2 stroke carb that does away with the accelerator pump and go to gravity feed, but I am afraid you would be asking for problems. You can prime the pump somehow, but I can't think of how to do it right off hand, I will have to look in my repair manual for the instructions and I will get back with them. Alos, there is no real easy way to change jetting components, the carbs are hard to get to and the process is slow, you just have to be patient. You can purchase a main jet tool from several companies that makes the main jet change easier, but the other jets, no.

 

You mean that thing up front that the fuel lines run to before they get to the carb is a accelerator pump? Max

 

No that is a fuel pump, the accelerator pump is built in the Carb itself. The reason 4 strokes have to have fuel pumps and accelerator pumps is because the engine does not have the intake pressure that a 2 stroke does, thus the fuel pump and accelerator pumps help get the mixture into the engine. The vacuum pressure from the intake stroke is much less between a 2 stroke and 4 stroke, that is why a 2 stroke doesn't have to use the fuel pump or accelerator and the 4 stroke does. I am sorry, I responded to your question without even giving thought to the fuel pump, but it is like the accelerator pump on the carb, almost a necessity.

 

I had a hard time restarting my scram after I changed the main jet. I turned the choke on, sprayed a bunch of starting fluid in the intake, and had to crank on it for a while. It took about 10 minutes but it finally did start and has run great since

 

Try starting them fresh out of the create. Actually if you just keep trying it, it will eventually start. If we have one that just will not start, we usually do the starting fluid method. this way only takes a few seconds.

 

What drives the fuel pump then? There's no wires connceted to it, only the fuel lines from the tank and input to carb. So it has to be driven by vacuum which means that this 4 stroke has intake pressure to run the pump so it should also have enough intake pressure to pull fuel from the tank using only the gravity assist. My YZ400F 4 stroke doesn't need a fuel pump and neither do lots of other 4 strokes. Is this just a Polaris thing? I'm still not certain that this isn't just a safety feature to prevent fuel from spilling continuously if the bike is turned over. I've had Hondas and Yamahas forever before this TrailBoss so I've got a lot to learn still I guess because this bike is definitely different. I do like it though, just can't stand working on it. Max

 

Yes it is run by pressure, but beyond that I don't have a clue. I know that most or all of the Polaris 4 strokes have fuel pumps just like all or most 4 stroke car engines have fuel pumps. I will see if I can find anything on the internet to help explain things better.

 

I don't have accelerator pump on my 700 and don't know any stock polarises that do I do have the fuel pump that they all have.