High Rise Cam

[THUMPY125]

I currently have the 140cc oil cooled lifan in my pit pro with a few small mods and was wanting to put a new 7.1mm high rise cam into it. When i do this do i also change the valve springs to a heavy duty one, do i change the piston. Is there anything else that needs to be done like porting the head? Any help from cactus jack and mack would be great or anyone that has done this mod before.

 

[THUMPY125]

bump ^^^^^^^

 

[Sidney]

hey man email sa fun bikes they do all that type of stuff

www.safb.com.au

 

[born2slide1980]

Hey Thumpy, not sure if I will be much help but will try.

From my limited knowledge... I think you will need to upgrade your springs If you go any bigger than standard with the cam, to help prevent valve float and more importantly binding of the springs... Depending on how much lift and "duration" the cam has, it may also be necessary to cut a relief in the piston for the intake valve and deepen the exhaust a fraction, or purchasing an aftermarket piston might be an easier option. Porting the head is not necessary, but it will help the motor breathe a lot easier and make better use of your new cam, so you would be crazy not to think about doing this mod also.

Mack has also pointed out that most of the exhausts coming out, or being run with these motors are extremely restrictive, and with a big bore pipe or custom (if ya can afford it) and just standard ports/cam, there is a lot of room for improvement over the old system... I know from my own experience that my hi-flowed H-Bomb has woken up my 119cc and made a lot better use of the porting etc. So you may also want to look at upgrading in the future if you are chasing more power.

Hopefully one of the Gurus can confirm this or point out any better options.

Cheers

 

[Cactus Jack]

Born2slide has just about covered it ..... with the 7.1 mm lift cam you'd need springs with wider spaced coils to prevent coil bind and you'd also need to check piston to valve clearances and retainer to guide clearances as well .

Generally with a longer duration high lift cam it helps to bump the compression up a bit to boost bottom end torque . Properly done porting makes everything work that much better , along with a bigger carb (26 to 28 mm) and to top it off ........ a pipe with at least a 28 mm header stepping up to a 32 to 34 mm mid pipe and an even bigger diameter muffler will really get things responding well . Another trick I do is to lighten the rocker arms on my XR's to eliminate valve float at high RPM ..... the XR rockers are very similar to horizontal motor rockers so they should cope well since even stock XR valve springs are stacks stronger than any pit bike race spring I've seen so far .......

S&W XR75 racing valve springs are insanely strong compared to the 7.1 mm lift springs for Lifan big valve heads ...... I also have S&W SL70 race springs which are basically like stock XR75 springs with alloy retainers and a steel base cup .....

 

[Mack]

You should port the head and have a three angle valve job done. The longer duration cams really want good flow past the valves at low to mid lift for best performance and reliable running. I know most don't have the tools required, but you should find out what amount of advance/retard the cam is designed to run and check/degree it accordingly. There are a lot of china copied cams floating around now and who knows if they are even close to running where they should in your particular engine by just dropping one in and hoping for the best. Might still seem to run ok but starting and running will not be what it should. If you can not find this out, then check and set the cam very close to or only a few degrees either side of zero. For any one who has modified the deck height and/or shaved the head definitely degree your cam properly or have someone do it for you.

BTW- When getting valves and seats cut, have the machinist check stem heights against the cam you will be running. If he doesn't know what you are talking about find one who does. If you have ground out the guides, excessive side thrust on the stem in your now very short guides will accelerate wear.

 

[Cactus Jack]

The only real way around the rocker geometry and guide side loading problem is to run roller tipped rocker arms with lash caps on the valve stems or overhead cam with buckets . Rocker arms move in an arc whereas valves move in a linear fashion so the rocker wipes across the valve stem throughout the range of lift . Sink the valves or alter the stem height and you could end up with bad problems if you don't take care of it as Mack has pointed out .......