need to get to the bottom of this

[Knight_Rider]

Hey guys had my carbs apart today and noticed that one carb had a 152main and the other had a 155 is it common to have to different sized main jets? i would have thought i would have made one cylinder run leaner. The pilots were 55 they were both the same.

What i really want to know is there a basic formular for working out jet sizes?? was talking to a guy today who was trying to tell me that 2 strokes will melt down if held wide open throttle for 2 long he reckoned that the only way to stop this was by keeping the lid on the airbox and having a adjustable sccop which allowed air into the box, kinda like governing the motor bit like a diesel, if you restrict the airflow it wont rev out the same if you add more air it will rev out. By removing the airbox lid he reckoned that they can just keep reving till they self destruct, anyone got any thoughts to this??.

[Forcefed]

First, yes, the jets should be the same in both carbs unless something drastic to alter the airflow is different from one side to the other.

 

By removing the airbox lid he reckoned that they can just keep reving till they self destruct, anyone got any thoughts to this??.

 

Well, i don`t think the amount of air that the airbox can or can`t flow will determine the RPM limit. Things like tuned length of the pipe you are running - Port timing - C/R - Timing - and on and on will have a MUCH greater effect on the limit at which a banshee engine will rev.

 

Actually, if you think about it, the transfer ports to especially the duration & area of the port face is the smallest point within the whole pump - and the tightest bottleneck. Without the ability to introduce a continuously fresh charge (via the transfers) into the cylinder, rpm`s will climb and then plateau. Provided the pipe is doing its job as well.

 

True, the amount of air the engine is initially allowed to consume with the lid on could possibly limit rpm, and by removing the lid add a few more rpm`s to the equation, but will this keep it from revving to the moon ? No. If we however did think along those lines, then with out the lid their would still be an "RPM/Air governer" by means of the stock carbs. Put bigger carbs on and you still have a govenor - intake/transfer/exhaust timing.... raise the exhaust to 205* and you won`t be able to rev to the moon because not all pipes can operate properly at that RPM... see how it takes changes all the way across the board to net an effective RPM increase ? (over simplified of course)

[Knight_Rider]

Yea i can see where you are coming from, this guy was really negative about 2 strokes i think he had it in for them lol. I've done alot of straight line runs in mine WOT ive never melted a piston yet touch wood lol.

Do you think the 155 main jets are regarded as small? the motor is still a 350cc ive just had so many people give me different opinions i know the only way to do it properly is to do a plug chop but i just want a base to start with the motor has fmf gold series pipes pro design intake boyseen reeds standard timing and compression 35mm KEIHIN flatside carbs

jarrad

[Duner440]

What jets to run in your carbs all depends on mods and the elevation where you are riding. Make sure that if you are going to start jetting your bike that you start with a big jet and lean it out until you are happy with it. The best way to do it would be a plug chop. I would definitly recommend that. Good luck.

[Forcefed]

Since you have already made some WOT runs and haven`t melted it, then right there is your baseline....you can listen to a million people, but I only trust my plugs. There are sooooo many more viariables to concider other than just what you can just bolt onto the bike that influences -proper- carberation.

 

Ever heard anyone ask for jetting or timing help based on the weight of the rider ? lol, yup - even the load the engine is placed under during specific riding conditions factor in ! :wink:

[Animalman294]

Well, first off yes the jets should be the same and if they are not you need to figure out why there is a difference, unless whoever had the bike before you put those in, in a pinch and never switched them back.

 

Second you need to come on the HQ to find out answers to your shee problems.

 

Third you want to open the intake up to let the motor breath, and by leaving the box on it chokes the airflow down. For a stock motor the box is fine but if you start modding it then you need better flow. Plus if you are racing this guy he wants your shee to not pull as hard so he can win with whatever piece of crap four stroke he is on....................

[Knight_Rider]

thanks for the help guys i'm with you on this one if a 2 stroke has been setup right there is no reason why it cant be as reliable as a 4 stroke. Those new 4 strokes are highly stressed motors and to keep up with the 2 strokes they have had to spend big money on them to do so.

 

I think what i might do on my shee is put a EGT into each pipe and measure the exhaust temps under load and make sure both cylinders are running nice and even that way if i set it up on a egt i know what changes it will make if i change the jetting. Most of my riding is at sea level and our temps over here on average are around the 18-25 degrees with bugger all humidity very dry heat.

[Banchetta]

First off, DON'T every listen to your dealer again. Restrict the air box!!!!!! :laugh: Ditch the cover or cut the last 1.5" off the end of the lid. This will give you another 4hp. Next, since you already made numerous runs on it w/o blowing it up (knock on wood). Run it for 5-10 seconds wide open in 6th gear, pull the clutch in, shut it down, coast to a stop, pull the plugs, look at the ground strap. Light tan is too lean, chocalate is perfect, black is rich.