Inlet tract length

[Lazyeiger]

What effect does shortening the inlet tract on a CR125 have? In other words moving the reed cage closer to the piston? I know it increases crankcase compression, but what does this do to the performance? The rules I race to state that the inlet must not be shortened and I just wondered why!

 

[marcusgunby]

I think you will gain a small amount of bottom and throttle responce.

 

[jaguar]

I saw a spacer advertised that goes between the reed cage and the engine and they said it increased bottom end power. So if they are right, then shortening that distance would increase top end power.

 

[marcusgunby]

Not really,each engine has its own sweet(tuned length) and you need to experiment to find out, the 02 cr had a spacer and it had no bottom so it doesnt prove it gives more-doesnt disprove it either.

Alot of sx bikes have them moved closer which leads me to think it does give repsonce and bottom.

 

[DEANSFASTWAY]

Yeah Ive seen quite a few carbs where the front of the carb body is turned off on a lathe so far that it has no choke, moves the throttle plate closer and gives more crancase comp and more hit. To start you gotta lean the bike over till fuel dribbles out the vent tubes a bit.

 

[Brewster]

Yeah Ive seen quite a few carbs where the front of the carb body is turned off on a lathe so far that it has no choke, moves the throttle plate closer and gives more crancase comp and more hit. To start you gotta lean the bike over till fuel dribbles out the vent tubes a bit.

Why would shortening the carb increase the crankcase compression?? The reed valves close the crankcase area, not the throttle plate.

Ride on
Brewster

 

[bclapham]

Yeah Ive seen quite a few carbs where the front of the carb body is turned off on a lathe

Hey Dean:

how would i go about mounting up a carb in the lathe to do that mod??????????

that 38mm PWK striker i put on the RM144 was about 10-15mm longer from the slide to the end of the front spigot, i managed to draw file the spigot and remove about 3mm's but i think if i can mount it in a lathe i can get it a bit closer and put a new sealing ring on as well- i dont need to go that far though and loose the choke- there is plenty of meat on them PWK carbs and ive got an old one to play with and a good lad that is magic on a lathe- i just need an idea of what tool (mandrel????) we would need to beg, borrow, steal.

the reason i want it closer is that the back spigot/airboot just touches on the shock spring when the bike is on the stand- it would be nice to get a little more zip from it as well!!! :clue:

cheers mate.

 

[jaguar]

I think moving the reeds closer to the cylinder would increase crankcase pressure which would aid power at all revs. The downside would be decreased lifespan of the crank seals and reed valves.

 

[DEANSFASTWAY]

Brewster: My bad, sorry, it may not actually increase crankcase compression ratio at a mechanical level but it pulls the throttle plate close for a better signal at the jets. BRUCE ,just clamp the back side of the carb in a 3or4 jaw chuck and get it centered good then turn it down . your friend should have some good cutters and he can even custom grind a tool to match your shape or groove for the airboot.on some bikes and motors though if you turn the front down and the carb is on an angle the bottom of the float bowl can get real close to the engine cases and touch . this is bad and can cause fuel frothing, which can give you jetting problems or starvation problems.Bruce , is that the RM144 with the jetting problems you had before/ maybe the spring jiggling and vibrating was adding to your nightmare a bit? Husky 250/360 motors with mikuni carbs had the float bowl nut resting on the top of the engine cases , on the later models they had a sort of rubber cushion glued there. I used to :thumb: like to add a reed spacer (CR250 honda) on these motors to add space , it also seemed to give them better pull.Good Luck

 

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[bclapham]

thanks Dean, i never thought about mounting up that way! :silly:

ive got this thing running well now on the 38mm PWK, the jetting issue was the slide- 6 wouldnt run well, 5 was better but still bogged and wouldnt jet out and the 4 made it run real well. on the PWK, the slide is about 15mm further back than the TMX- maybe this is where my loss in signal was- next time i have the bike apart, i will try and get it a few mms closer. i can remember they were hot rodding the PWK airstriker on the yz250 but they were loosing the choke and the mods were expensive

cheers

 

[billyghog]

Hey Dean:

how would i go about mounting up a carb in the lathe to do that mod??????????

that 38mm PWK striker i put on the RM144 was about 10-15mm longer from the slide to the end of the front spigot, i managed to draw file the spigot and remove about 3mm's but i think if i can mount it in a lathe i can get it a bit closer and put a new sealing ring on as well- i dont need to go that far though and loose the choke- there is plenty of meat on them PWK carbs and ive got an old one to play with and a good lad that is magic on a lathe- i just need an idea of what tool (mandrel????) we would need to beg, borrow, steal.

the reason i want it closer is that the back spigot/airboot just touches on the shock spring when the bike is on the stand- it would be nice to get a little more zip from it as well!!! :clue:

cheers mate.

Hmmm... This would be possible if you want to do it Bruce - Let me know.