Bighorn Premix

We ran full pre-mix on all the F5R/F9R bikes at Team Hansen/Team Kawasaki, 30:1, Castrol R30, but that was years ago. Today, Motul, Silkolene works.

You will also have to close the holes on both the rotary valve cover and clutch cover where the oil injection check valve installs between them. Do not just leave the brass valve in place, you must close these two ports off. If the valve is left in place and open on the clutch cover end, air will be pulled into the rotary valve cover when the valve is opened from the vacuum.

In the beginning, we'd run them with regular rods and F5/F9 rods, non-slotted, worked for 3 race meets. After that, we would swap the whole crank out and make something else out of it, like a flower pot, etc.

After that, we ran regular motocross cranks from F81M's, as they were lighter on the flywheels, accelerated better, and already had a slotted rod. Only things we did was take them apart, check them, deburr everything, and the rod beams for strength, set the side clearance to .030, and heli-arc (tig) weld the pins after truing them.

We never had any F series racer rod/rod bearing fail, slotted or not.At first, we just used the stock crank and non-slotted rod for the F5/F9, and fully removed the oil pump system.

It was three quarters of the year into the first year we decided to change the balance factor for better acceleration and slot the rod. To do this, we started with F81M cranks from then on. F81M cranks already had slotted rod for premix use.

Randy Hall didn't want to make the change, but Steve, Harold and I just went ahead and did it.

As we all know, the H series engines have oil receivers on their left crankshaft flywheels. These oil receivers are there to take the sling off oil from the left hand main bearings, on the flywheel side, and aim that oil down the crankpin for pin oiling. This was a positive method of centrifugal force oiling the big end bearing. This same system was also used on the slotted rod cranks, with the 4 hose oil pumps.

On the singles, F series, the oil was injected into the inlet stream only, NOT centrifugally pressure fed into the main and/or rod big end bearing. Not all these engines had slotted rods, especially the F1, F2, F3, F4 and F21M. The C and G series engines were lubed like the F's were. None of the C series had slotted rods, and if I remember correctly, the only G that had a slotted rod was the last year of the G31M. I remember using a cutoff wheel and port grinder motor to cut slots in stock G series rods, and the Suzuki rods we used in the G31M as they were stronger. My really nice G31M used the Suzuki rod and MotoBeta piston/bearing/pin.

The key to running a non-slotted rod without oil injection is to increase the rod side clearance, at least .026, better to .035 or so.

When you consider that the F series with oil injection didn't spin pressure lube the big end pin, it didn't suffer from oiling problems.

Slotting the rod does, in some aspects, weaken the rod, but that is why it should be built with the engineering beef to live with the slot. There are one heck of a lot of two strokes that run slotted rods, premix and injection, and they don't seem to have strength issues.

I've done a bunch of dirt bike crank rebuilds/rod kits for the local dirt bike guru here this last summer, all slotted rods on premix, and almost all of them had rod bearing rollers that were just fried, beat flat, smoked, squared off, dead.

All of them were on Motul dirt bike oil, as that is what is available from the bike shops around here. Funny, when I get them to switch to YamaLube R, all the piston seizing and rods going out stops.

I think the Motul street and roadracing oils are great, but they seem to not have the right formulation for a dirt bike engine right just yet.