Hey,

in an effort to understand exactly how a 4wheeler motor works and makes it's power, i've come across a lot of discussion about jets and reeds. i don't think these are in my Jeep engine, so they're foreign to me. can someone explain to me what they do, and how you can make them give you more power to changing them to your set-up?

many thanks,
bigmac

 

bigmackloud
Wow this could be a mouthful!!! The jets are for correcting your fuel and air ratio.( your Jeep may have these, depending on the year ) The reeds on the other hand have allot to do with how your motor "breathes". The Reeds in your motor act as a valve opening and closing as needed according to your piston location and inertia factor's that are being created by you particular engine. Ouch ! That will take some pondering ha!! Reeds are in there simplest form a one way valve, As the Motor turns over there is a low pressure or vacuum put on the intake system, this
low pressure opens the reeds. creating the intake stroke. This is the nice thing about a Reed Engine, it's very forgiving as fare as intake port timing is concerned. Reeds can make up for some pretty sloppy porting, not like the old days of piston port motors. A piston port motor didn't have the luxury of a reed, everything on the intake was timed from the piston itself. This made things allot more difficult. Oh well, that was then this is now! To make this short and sweet so I can go to bed. The Reeds control the intake of your motor, as the piston goes through it's up stroke it causes a vacuum underneath itself. As it does this the reeds open letting the intake process begin. As the piston reaches top dead center ( T.D.C. ) the intake will have an inertia factor depending on how fast the intake charge is being pulled ( allot to do with rpm )( volumetric efficiency can and will go up with rpm if designed correctly ) this will delay the closing of the reeds. As the piston rotates past T.D.C. the reed will close as soon as the vacuum becomes a pressure and the reeds will have done the first part of there job. The tension, quantity, tip area, and total volume of the Reeds all play in as a factor of how well this process is played out.

 

Main jets, Pilot jets, Needles & air correction screws all meter fuel at diffrent throttle settings.

These need to be changed when you change the air going in to or coming out of the engine (air box mods or better flowing pipe for example) to keep a proper air/fuel ratio in your engine. A too rich motor will foul plugs, have a faster carbon buildup & run poorly. Too lean can destroy your motor since the fuel charge on a 2 stroke cools & lubricates the top end, The fuel charge also acts as a means of cooling the top end on a four stroke.

Reeds act as valves on a 2 stroke engines. The vacume of the intake stroke opens the reeds to allow the fuel/air/oil mix to enter the combustion chamber, The compression stroke closes the reeds to keep the fuel/air/oil mixture in the cylinder & to keep air from going to the carb from the cylinder.


I tried to keep it simple to understand. Books have been written on these subjects. Hope this helps.

See my Raptor & other toys on
My Web Site

 

Damn Harry you type faster than me! You must use more than 1 finger!![img]i/expressions/face-icon-small-smile.gif[/img] [img]i/expressions/face-icon-small-wink.gif[/img] [img]i/expressions/face-icon-small-tongue.gif[/img] [img]i/expressions/face-icon-small-cool.gif[/img] [img]i/expressions/face-icon-small-tongue.gif[/img] [img]i/expressions/face-icon-small-cool.gif[/img]

Mabey you can get something out of mine too but it's hard to compete with Harry!![img]i/expressions/face-icon-small-smile.gif[/img] [img]i/expressions/face-icon-small-wink.gif[/img] [img]i/expressions/face-icon-small-tongue.gif[/img] [img]i/expressions/face-icon-small-cool.gif[/img] [img]i/expressions/face-icon-small-tongue.gif[/img] [img]i/expressions/face-icon-small-wink.gif[/img] [img]i/expressions/face-icon-small-smile.gif[/img]

 

I could have been in bed a half an hour ago if I would Have known you were going to answer this!!!! Sorry

 

Hey,

thanks. both great replies. i think i understand the principle behind them. are reeds only used in 2strokes? does that mean a 4stroke uses tradiational valves? the jetting also makes some sense. when you buy a jet kit, do you get a variety of different jets? how do you know what your jetting should be?

thanks,
bigmac

PS: my jeep is a 96' XJ, so it's got fuel injection. i guess the jetting would be for the carb engines right?

 

Yes 4 strokes have Cams,springs & valves to control fuel/air mix 2 srtokes has reeds.

Carbs have jets, fuel injection rates injectors by lbs of fuel per hour.

My dynojet kit had an assortment of jets & jetting recomendations for engine mods & altitude.

The only way to know for sure that your jetting is right is to use an 02 sensor.

See my Raptor & other toys on My Web Site

 

You are correct in that all 4-strokes use the traditional valves. Most have rocker arms that actuate them, "pushrod" engines have pushrods that push the rockers, the rods sit in a lifter the lifter is pushed by the cam, the cam is turned by the crank. This creates a lot of power robbing friction. (Overhead cam eliminates the pushrods and lifters (some four cyliners still use lifters, too many ideas over the years!!!)) This is why overhead engines are more efficient and make more power. Reed valve 2-strokes don't have all of these moving parts, which is a contributing factor as to why they make more power per cc. All atv 2-stroke engines are reed valve (with the possible exception of yamaha's zinger). There are 2 other types of 2-stroke engines that I know of. The piston port that Harry discussed and Kawasaki and Suzuki's great idea of a rotary valve (large disc with notch that turned with the crank and let air in as the piston went up).
Jets are used in carb applications. Jet kits include a variety of jets that used to get the proper air/fuel mixture. Harry posted a greatly informative article on this forum a couple of months ago on how to jet a carb.
I kept getting interupted and it took me over an hour to type this, so if someone already said these things before I posted, I appologize.

Hope this helps,
Richard