Another Combat inlet port question

hybridracer

hybridracer

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Feb 21, 2013
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Sorry to bring up combat inlet ports again but I don’t know what exactly makes them bad. Results of searches I have made only stated that the 32mm port was made too low in the head leaving a sharp turn to the valve seat which is bad but my two combat heads are not like that at all. The ports are 32mm at the manifold face and taper down to 28.5mm just before the valve guide (the same diameter at that position as in my 650ss head) so there should be plenty of velocity there and scope for widening around the guides?

Am I missing something?
 
hybridracer said:
Sorry to bring up combat inlet ports again but I don’t know what exactly makes them bad. Results of searches I have made only stated that the 32mm port was made too low in the head leaving a sharp turn to the valve seat which is bad but my two combat heads are not like that at all. The ports are 32mm at the manifold face and taper down to 28.5mm just before the valve guide (the same diameter at that position as in my 650ss head) so there should be plenty of velocity there and scope for widening around the guides?

Am I missing something?
Click to expand...
I'm going to say you're not missing anything. Not going to expand on it though. I don't have the technobabble vocabulary to back up anything I believe. :)
 
I'm surprised team leave it stock hasn't provided links to charts proving why you should leave it alone. :)

I would open it up a little without worrying about it if that is what you are thinking about doing. 30mm wouldn't hurt at that location. I do not port heads for a living though or have any charts or flow bench data. I just happen to have an earlier head that is opened up there and was also widened to 31.5mm at the entrance. It does neck down a little until it reaches the valve guide area. It works fine for me, but my engine, carburetion, and exhaust are not typical. I don't expect the engine to perform well at 30mph in 4th gear.
 
Anything which reduces vacuum or impedes flow in the port is bad. If there are changes in internal dimension, the taper needs to be slow. The needle jet and needle are critical for performance, and the taper on the needle compensates for loss of vacuum. I suggest that most people who road race, do not usually work at getting needles which work better. They just use the needles which are supplied with the carbs, and for road use, it usually does not matter.
I have not seen a chart which compares Amal needles with Mikuni needles. They all have the same dimension on the parallel part and the carb's choke dimension determines the lengths for both types. However I think there are a lot more variations available with Mikuni needles. Needles which are the slightest bit too rich will slow the motor.
When I tune, it is always with fuel type, ignition advance, port type and compression ratio fixed. Changing jetting to suit engine modifications can be a lot of work - it becomes easier with experience.
 
Last edited:
Some comments have been made about poor throttle response with Commandos. If the mixture richens too quick as you open the throttle - might cause that ?
 
acotrel said:
Anything which reduces vacuum or impedes flow in the port is bad. If there are changes in internal dimension, the taper needs to be slow. The needle jet and needle are critical for performance, and the taper on the needle compensates for loss of vacuum. I suggest that most people who road race, do not usually work at getting needles which work better. They just use the needles which are supplied with the carbs, and for road use, it usually does not matter.
I have not seen a chart which compares Amal needles with Mikuni needles. They all have the same dimension on the parallel part and the carb's choke dimension determines the lengths for both types. However I think there are a lot more variations available with Mikuni needles. Needles which are the slightest bit too rich will slow the motor.
When I tune, it is always with fuel type, ignition advance, port type and compression ratio fixed. Changing jetting to suit engine modifications can be a lot of work - it becomes easier with experience.
Click to expand...
My combat motor was for racing only and all that mattered was that it started straight away when push starting, worked well at full throttle and didn’t misfire or cutout while getting to full throttle.
 
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