Hi Guys,
we are rebuilding a 93 Big Boss 350l. The job is being done in Africa and it has been a hard work to get all the missing parts from the US. But the parts arrived somehow and the job is moving on.
What I don't really understand is the way how the radiator functions. In my eyes the radiator looks like the type which gets its cap on top to close it proper. Only because of overpressure it will open. But my ATV is having this small expansion tank on the left side, which should be connected with the radiators top. I'm wondering why this tank is fixed lower than the radiators top. I only know these expansion tanks from other vehicules, been fixed at the highest point . How can it work proper? Or is it not really a expansion tank? Only a tank for the overflowing coolant? At what pressure does the original radiator cap opens, ours got lost and I don't want to order again from the US.

Thanks in advance for your respond.
Regards buschmaschine

Welcome! 13 psi is the radiator cap rating. This is a recovery(surge tank) and if you look inside the radiator you will see the how close the exit tube leading to the recovery tank is related to the cap. It's designed that way to use vacuum to bring coolant back in if needed from the bottle. The MOST important thing a lot of people forget to do is to bleed all air from the small 10mm bleed bolt at the rear of the head that's located close to the temperature sensor.Just fill the radiator,don't have the engine running,crack open the bolt,let the coolant flow until all air bubbles are gone,then close the bolt,fill the plastic tank to the top level,then the job is done! If you don't bleed this system it will over heat purge coolant out of the coolant bottle and can cause engine damage.Warped heads,cylinder,etc. You ought to be able to down load a good pdf manual on this engine that will help you on any problems. OPT

Thanks for your answer OPT!
Yes I know how to bleed the system before running the engine.
Concerning the radiator cap, I still have a question: It seems like the original radiator cap is having valves which allow the movement of coolant in both ways? Isn't it? What means the cap opens because of overpressure, leads the coolant to the bottle and it will move back via Vaccum when the system gets colder? Maybe it should be advisable for me to get the original cap.

Thanks and Regards
buschmaschine

Yes! The cap allows movement both ways.Some people call the tank a surge tank,others call it a recovery tank. Both terms are correct!. Use only a 13 psi rated cap. OPT

Dear OPT,
thanks for your ideas.
I have put a 13PSI cap and it works fine.
Right at the moment we are trying to figure out another problem.
The engine will start while cold, but after some running time its difficult to restart. There is maybe something wrong on the carburator. It has been ultrasonic cleaned and works not bad, when it works. Another thing is that the screw with spring to adjust the idle speed is fully entered and it feels it could even enter more to keep the engine running. Any ideas? Let me know.

Best Regards buschmaschine

A lot of people over look the float arm and pin itself when they either rebuild the carb with a kit or just replace the needle and seat. If the float arm is worn and wobbly best to replace it along with the pin. These are dealer items only,not available in any kit I've found. You can look here on your model under carb for the part numbers. Browse. Also if the carb slide gets worn where the idle screw end make a deep groove in the slide,sometimes screwing the adjuster all the way in doesn't help. You can remove the washer from the spring and screw assy and this allows the idle screw in a little deeper. (I've had to do this before) If after you replace the float arm and pin(IF needed) plus the idle screw assy washer and it still won't idle well,then it could be starting to have low compression or crank seal leakage that would affect the idle.Compression should be a minimum of 115-120. Also piston wear itself can cause this problem. Cranks when cold,but as the engine warms,the cylinder expands and if the piston/rings are worn you may be loosing compression,rings not sealing good against the cylinder walls. This was another common thing to happen on two strokes. OPT

We had rebuild the engine. New crank seals, rebored cylinder with new piston kit. I did not check compression, but it should be good. I will remove the carburator tomorrow and check it again. The carb I bought used, because the one on our ATV was missing. While cleaning the carb I did not change the position of the air screw (spedd screw). Maybe its wise to check even this position? Could this be a reason too, while the carb is not working well? My clymer manual talks about 3/4 turn out for this screw, is this right?

buschmaschine

Yes recheck the carb and also the jets sizes that are listed for your machine in the parts breakdown versus what's in the carb,could be different! Normal position on the air/fuel screw is between 1 to 2 turns out.Not as sensitive as the fuel/air screw on a four stroke engine,but just start out at 1 1/2 turns and see how it runs.I'd check compression any way to eliminate this as a problem! OPT

Ok. I checked the carb again.
I found a main jet with 220 in my carb and replaced it with a 200.
The 200 was coming with the rebuild kit but I had not yet changed because of the old one's good condition. Even the manual is talking about 200, so I found it as right. I found the air screw 2,75 turns out and have put it now at 2 turns out. Even I worked on the carb piston where the idle screw touches. Now the engine will start well and even when hot there is no problem to restart. Adjusting the idle speed is even possible. How do I adjust now the air screw in a right way. It should be in concordance to the spark plug picture, isn't it?
Regards buschmaschine

If you're between sea level and 2000 ft you can actually drop the main jet down to a 190! I had to do this on a bunch of 350 and 400 atvs down here. They would tend to run a little boggy,too rich. On the air screw just start at 2 turns out,turn in until engine starts to slow down,then out to where engine does the same and set in between these points. Like I mentioned,it's not that critical on the air screw compared to a fuel/air mixture screw on a four stroke. You should end up between 1 1/2 -2 1/2 turns out from a light seated position. Just make sure it takes the throttle and doesn't hesitate or bogg down. Check the plug after a fast run,should be medium tan on the electrode. OPT