Friday, October 31, 2014

Vintage Dual Carbs, Part 2

Airflow.  That's what its all about, at least for the performance enthusiast.  More air (carrying the proper mix of fuel of course) = more power.  Even what is often the first modification, open exhaust, falls into this category as evidenced by the fact that a less restrictive exhaust will require richer jetting to go along with the increase in air intake.  Obviously the same goes for the air cleaner.  So what's next after that? 

Bigger carbs, bigger valves, and porting are all the radar screen for those in search of horsepower, and that's not a new development. If I am not mistaken both Chet Herbert in the late '40s and George Smith Sr. in the '50s ran Riley carbs on their famous dual carb Knuckle drag bikes.  The Riley carbs were originally a racing part for Model A Ford engines.  I can only speculate at this late date that the reason for their existence was for increased air flow. 

 So perhaps this is the appropriate point to mention a few fallacies.  One is that when running one carb one each head (aka individual runner), you need to use smaller carbs than when using one carb to supply both heads.  Think about this for a moment.  On a V twin engine, both intake valves are NOT open at the same time.  Ignoring for a moment any tuned length/ram effect, that means a carb that is too small to be a performance carb with a conventional intake manifold will also be too small to be a performance carb when used on a one carb per cylinder set up.  As a matter of fact it seems that the experts, i.e. anyone I regard as smarter than me (a long list to be sure), suggest that on a individual runner set up, the carb actually needs to be larger than for conventional set up due to the lack of a "plenum effect".  If I follow the reasoning correctly, it is because a plenum will actually help to dampen the large fluctuations in pressure in the intake tract from the valve opening and closing.  This effect would be smaller on a two cylinder than 4, 6 and 8 cylinder engines, but present none the less.

On the other extreme I have had someone with a dual carb set of heads, upon hearing my opinion that the carbs were too small, state that, "Yeah, but hopefully I can jet them up enough to work."  Well, sure you can jet them "up" to work.  All that takes is the right jet to maintain the correct fuel/air ratio.  The problem is very seldom getting enough gas into the engine for  high performance, the challenge is in getting enough air in.  That's what makes a carb too small for a performance application; lack of air flow, not lack of fuel flow.  But more on that later.

So, what was the hot set up for dual carbs "back in the day?"  Well, I suppose that would depend a lot on your exact definition of "back in the day."  Forty years ago (back in the mid '70s) a large round slide Mikuni was a common performance upgrade to replace the aging Linkert, as were SU carbs adapted from British automobile applications.  The Mikuni was a natural for dual carbs, though the popularity of said dual carb builds was on the wane primarily due to the more flow friendly intake tract of Shovelheads and Sportsters. I can't say I remember ever seeing a dual carb Harley with SU carbs, though the constant velocity design would seem to be nearly ideal for the job.

Going back a little further in time, it seems that Italian made Dellorto carbs may have been one of the most popular for use on dual carb heads.  Their SSI series slide type carbs were available in sizes up to 42mm making them a good choice.

highly modified Knuckle from the mid '60s sporting Dellorto carbs

Ron's vintage Knuck also features early Dellortos - more on this bike here

"Famous" Doug Gall used a pair of later Dellorto carbs

Now, cool as the Riley and Dellorto carbs may be, due to their scarcity one will probably not find many of them being used on modern "period correct" builds.  What you will generally find is Linkerts.  Yes, they seem to be constantly rising in cost, but compared to a Riley carb they are still dirt cheap. 

pair of modified Linkerts on The Knuckledragger

So, how does the Linkert stack up as a performance carb, you ask?  We'll look into that in part 3.

2 comments: said...

Looking forward to part 3 on performance Linkerts! George smith modified heads with dual MR-4s on an 84" stroker, trying to squeeze as much out of them as possible!

Milo said...

great post, looking forward to number 3!!