Crude Inlet manifold mods

[morganbike]

My son races Grasstrack here in the UK using speedway type chassis with suspension on a 400 meter oval .We run a 2001 CR 125 and it is not possible to fit the carb onto the standard inlet manifold as it fouls the frame due to the angle it comes out at.

The norm is to fabricate an inlet manifold with 6mm ally plate,cut a 38mm hole in the centre and then weld on a 38mm tube so it allows the carb to be attached in a more central position.Then attach a 38mm Mikuni TMX carb as close to the reed cage as possible with a length of rubber hose and clips.This set up brings the carb closer to the reed cage than standard.We then fit the biggest K+N filter we can find and jet it up.Standard reeds and cage are used and as the throttle is pinned wide open most of the time the motor is tuned for gating,top end and overrev.

My questions

What difference does the distance the carb is from the reed
cage going to make.
Can this home made tube type inlet manifold restrict the engines breathing capability.
Are there any improvements/mods I can do to make it breathe better ?.

How can I adjust the timing when the stator plate is fixed.

Thanks

 

[marcusgunby]

The angled cr intake manifold is actually one of its worst design aspects, it seems to me you could make up a much better system than honda due to the rear shock location on the cr.

generally having a carb closer to the engine yields more bottom and better throttle responce(seen this on many bikes in supercross)

If you dont want much bottom end the cr is actually a good engine, with the exhaust port opened up like shown in eric gorrs book, it will produce good top end and overrev-the 38mm carb will help here as well, one more thing is the ignition holds back the cr a little, i used a wolf programmable setup and gained 1hp, you also want a pipe tuned for top end -you can achieve this by shortening a pipe by 5-10mm -i guess you dont use std pipe setups as the shape needs to be different?

to alter the timing you need to remove the plate and file out the holes-honda fit a little plate to discourage you doing it , but its easy to file it.

Hope that helps
marcus.

 

[steve125]

What difference does the distance the carb is from the reed
cage going to make.
Can this home made tube type inlet manifold restrict the engines breathing capability.
Are there any improvements/mods I can do to make it breathe better ?.

How can I adjust the timing when the stator plate is fixed


The angle is a factor, but the length plays a larger role in performance IMO. The closer the better in your application. This will actually help top end and over rev. As long as your home made tube has the same inner bore diameter as the carby it will flow fine. Just try and taper it "out" as best as possible into the reed cage, try for a smooth transition into the reed insert.

On the stator, just file or grind the adjustment slots enough to give you the range of adjustment you need.

 

[morganbike]

Thanks guys for the advice.I have read in a thread that mirror polishing the areas from carb bellmouth along carb inlet and beyond cage will reduce the performance as the fuel / air mix will not attomise correctly.I find this hard to believe.I am a big believer in polishing all areas that require good air flow.Any views.
We do not need bottom end just a good dolop of half throttle mid range pick up when we dump the clutch in 3rd gear off the gate.Full throttle clutch dumps cause too much wheelspin and there is a fine line between an engine stall / wheel spin / engine bog when gating.Our rules will not allow traction controll.Once out the gate its up to top gear then down one for the bend.We dont even shut the throttle or use the clucth when this down change takes place and we are very hard on engines ! .

Marcus- Is the wolf you used the one with 10 settings or did you get the hardware and programme it yourself.We are not allowed to use the one with the 2 way launch switch on it.Thats cheating ! .

On the plus side we can run up to 150cc so we will be having the EG 144 big bore done at next engine rebuild.I wonder if this big bore conversion would be able to take the punishment of 4 laps Grasstrack racing ?.

 

[steve125]

Thanks guys for the advice.I have read in a thread that mirror polishing the areas from carb bellmouth along carb inlet and beyond cage will reduce the performance as the fuel / air mix will not attomise correctly.I find this hard to believe.I am a big believer in polishing all areas that require good air flow.Any views



Forget the polishing, you want a smooth and slightly rough surface anywhere the air fuel mix travels. Polishing the exhaust port is fine to reduce the build up of carbon deposits.

IMO the 144 as long as its properly tuned, can take all the WFO throttle abuse you can give it, reliably.

 

[viking20]

If you use the 2001 cylinder , it can only be bored to 133 because of the exhaust valve....Earlier cylinder will fit , and can be bored to 144 , but the exhaust valve uses a different linkage. If you dont need the valve it can be plugged , and maybe Erics shifter-kart porting is just what you need ?

 

[steve125]

If you use the 2001 cylinder , it can only be bored to 133 because of the exhaust valve....Earlier cylinder will fit , and can be bored to 144 , but the exhaust valve uses a different linkage. If you dont need the valve it can be plugged , and maybe Erics shifter-kart porting is just what you need ?

This is all true. You can get to 144 by a 2mm bore and 2mm stroke. Its good you have a few options here.

Here's a link to look over the earlier CR mx engines used for shifter kart. The rumor is, this company built James Stewart's KX 125 engines over the last few years.
http://www.swedetechracing.com/

 

[marcusgunby]

This will actually help top end and over rev

LOL got that wrong then-i need a rest brain fade:(

 

[steve125]

This will actually help top end and over rev

LOL got that wrong then-i need a rest brain fade:(

Yes! you need to stop thinkin about 4 stroke valves........LOL
see what happens when you go over to the dark side??? ;)

 

[usgpru27]

Would an intake manifold from a Honda RS125 work? It is my belief that it is the same bolt pattern as the CR and has a straight track..

Just a thought.

 

[morganbike]

This is something I havn't thought about but will be looking into it.Not sure what size carb RS runs so inlet diameter will be the deciding factor.I know of a few who have bored carb out to 39.5 mm on CR 125 but still run standard inlet 36mm.Surely to increase the carb diameter and not the inlet manifold would have an adverse effect.
Further to my post on a tune for UK grasstrack racing and the replies I have had it seems that the way to go for a 2001 CR 125 engine would be to:

Throw power valve in bin and plug the space in exhaust port.
Obtain a 54x58 piston etc and bore out barrel to @144cc
Raise Cylinder off crank cases to let it rev and modify head to suit squish and compression.
Port cylinder for top end.
Fit a carb inlet manifold which is the same diameter as carb and get it as straight and close to the reed cage as possible.
Put a 38mm carb on and attach the biggest K+N filter we can get in the space available and go 5 sizes bigger on the main jet to start.
Put bike on dyno and alter ignition by modifying stator back plate for max HP.
Try V Force 3 versus standard reed cage.
Have an exhaust pipe specialist manufacture a one off rev pipe to suit tune.
Estimated power is 36.5to 38 HP at 11,000 and tailing off at 13,000 RPM :yeehaw:


Main rivals have CRF250 or YZ250F engines which make 32 to 33 HP but make that all the way from between 6500 and 10,000 RPM ! with torque to match :ohmy:
Yes we like two strokes and playing tunes on the gearshift, but their top end rebuild when it pops is expensive. :ugg: They will probably out gate us but we will out drag them to first corner !, Cant wait. :eek:

 

[marcusgunby]

If tuned for top end you should get more than that-i got 36hp with a small amount of bottom end left for emergency;) if you need a dyno contact let me know.

 

[Pete Payne]

You can actually bore in Eric's 144 4mm over piston , with the right equipment. I know of a guy in Virginia that does it , PM me for more info :cool:

 

[morganbike]

Air chunk,
Can you let me have the info on the 144 big bore cr 125.
Eric assures me that the cylinder is too thin above the power valve to bore out more than 2mm !.

Phil

 

[bclapham]

i think what you have to do is an offset bore- to get the head to match, you have to index it in a 4-jaw chuck and then put a 1mm shim behind one of the jaws for the offset machining.