The problem with RH4 heads

Everyone already knows that the 850 guides are too big and impede the flow.

Below is a diagram showing how the port was simply bored out to 32mm up to the guide leaving a choke down ridge at the middle of the guide where it necks down to 30mm. This is the opposite of what you want because the guide area is the restriction point. This is why R10 850 heads run better with their 30mm ports. Its better to have a venturi leading up to the guide with the port at least the same size or a little wider at the guide to help breathing. Both the RH4 and the RH10 heads have the same port diameter between the guide and the bowl. The bowl also being the same size. You can feel a 1mm ridge of reduced diameter if you run your finger down an RH4 port all the way into the guide area.

The best option is to sleeve the RH4 ports down to 30mm ID all the way to the guide. You can DIY with the right size tubing or machine your own.

jseng1 said:

Everyone already knows that the 850 guides are too big and impede the flow.

Below is a diagram showing how the port was simply bored out to 32mm up to the guide leaving a choke down ridge at the middle of the guide where it necks down to 30mm. This is the opposite of what you want because the guide area is the restriction point. This is why R10 850 heads run better with their 30mm ports. Its better to have a venturi leading up to the guide with the port at least the same size or a little wider at the guide to help breathing. Both the RH4 and the RH10 heads have the same port diameter between the guide and the bowl. The bowl also being the same size. You can feel a ridge of reduced diameter if you run your finger down an RH4 port all the way into the guide area.

The best option is to sleeve the RH4 ports down to 30mm ID all the way to the guide. You can DIY with the right size tubing.

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Hi Jim , which glue did you use to fit them ?

When I installed the JSmotorsport sleeves, I used Loctite 510 as others have recommended and it worked great.

marinatlas said:

Hi Jim , which glue did you use to fit them ?

Click to expand...

Just about anything will work including plain old Hi temp silicone gasket sealer.

Flow might not be the most important factor. 30mm ports probably cause more vacuum which affects the needle jet and causes more fuel flow. To get similar fuel flow with bigger ports, we need higher revs. When you jet carbs using smaller ports, it is probably easier to get the jetting right. The shape of the carb needle compensates for loss of vacuum. I use 6D Mikuni needles in 34mm MK2 Amal carbs with 30mm ports. Peak revs for me is 7,500 RPM. If the ports were bigger, the motor might not reach that rev level so easily. To get a four-stroke motor going quick, the mixture must be as lean as possible right down the needle. Vacuum is more important, when the motor is designed to pull, rather than rev to get power. The internal shape of my inlet ports are a continuous curve from carb almost to valve seat.

When I was racing my 500cc Triton, I was really dumb. I enlarged the inlet ports as much as possible. I crashed a lot. A Commando engine is very different from most others. I have never believed it could be so good. With most motorcycles, when the gearing is lowered, they accelerate faster.

The large 32mm port slows down the air velocity whereas the 30mm port accellerates it and gives better cylinder filling at medium RPMs.

What about blending the bottleneck at the roof around the guide and down the sides into the bowl and raising the floor with epoxy to meet the bottleneck cross the bottom of the port. Sort of the mild D-shape arrangement. Not simple, but might work well with the 32mm entrance to the ports, a bigger than stock cam, and some high compression pistons.

Have you or someone else done pre-sleeved and sleeved intake port dyno runs with the same head, carburetion, and stock cam to see what the gains of the port size reduction with your sleeves are like?

The 30mm port sleeve inserts up to the bottle neck and pretty much takes care of the problem. The port seems well shaped with the sleeve installed.

Ideally you would want to open up only the top 1/2 of the reduced diameter of the bottle neck going around each side of the guide and leading into the bowl but not too far into the bowl. See image below of where to take out about 1mm max depth each side of the guide with a 1/2" diameter sanding cartrige roll mounted on a die grinder. This puts in a "cobra head" or "M" shape going around the guide.



This shape is taken further as in the Nascar port below



I haven't made the dyno tests but have measurements of Proven C.R. Axtell ports which were the best dirt track motors in their day.

Getting ahead of installing these in my '73 850. The pics of them installed on your site Jim don't have the sleeves flush with the carb manifold surface. Does it matter?
I have RH10 carb manifolds that would mount 32mm Amals to a 30mm port, so I'm thinking that would give an optimized intake flow from the carb to the bowl with these sleeves in my RH4 head. Any experience with that configuration?

Deckard said:

Getting ahead of installing these in my '73 850. The pics of them installed on your site Jim don't have the sleeves flush with the carb manifold surface. Does it matter?
I have RH10 carb manifolds that would mount 32mm Amals to a 30mm port, so I'm thinking that would give an optimized intake flow from the carb to the bowl with these sleeves in my RH4 head. Any experience with that configuration?

Click to expand...

That photo was from a customer who just happened to install them about 1/16" too deep. It really doesn't matter as long as they don't protrude outward towards the manifold. Either the 32 manifolds or the 32 to 30mm manifolds should work well but you might get a little better momentum effect with the 32 to 30 manifolds. I'm not sure its enough to notice a difference between the two.