Topic: Round Top VS PD42 Carbs

[TurboD]

I have been wondering about how the round top carbs would compare flow wise to the later PD42 carbs found on the F2-3 engines. I fashioned up a fixture to test both of them back to back.

What I found is some what expected, something that I suspected. The old round top carb actually outflowed the PD42, not by a lot but they do flow more. With all the carb talk lately, I figured this might be helpful.

Testing at 10 inches on MY bench.
Round top flowed 87.6 CFM
PD42 flowed 86.1 CFM

This is testing the number 2 carb on each. (not that I think it matters)

[scottly]

Have you seen this?
http://draftcycleworks.blogspot.com/2010/09/porting-honda-head.html

[TurboD]

Yes I have seen the article, although dated it has many good points that still hold up. I have never understood his numbers though, he is testing at 12 inches and the numbers are really low. Regardless if his bench reads high or low, the basics are the same.   

[scottly]

Remember, he was testing both the carbs and the port as an assembly. The gain from the 32 mm was significant, even with the stock port.

[MRieck]

Remember, he was testing both the carbs and the port as an assembly. The gain from the 32 mm was significant, even with the stock port.

Yes. And using 32mm valve....with no seat boring (which the 32 can use).

[MidnightLamp]

I wonder how something like a cheap OKO flat slide with a 4-1 cable pull would work on a head like this? Seems to work fine for most other bikes.

http://www.ebay.com/itm/OKO-32mm-Flat-Slide-Racing-2T-Carburettor-Kit-for-Vespa-Lambretta-Karting-etc-/321428054286?pt=UK_Motorcycle_Parts&hash=item4ad69ae10e

I've run sets on a lot of brit twins and singles with good luck, and they're relatively cheap. They're straight copies of keihin PWK's (which are also not that expensive) and use PWK jets for the most part.