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ET: Engine (Twin) Pilot jet


Magnetoman

Well Known and Active Website User
VOC Member
Those flat slides look the part and would be great in an appropriate installation, but the two turns in that tube play hell with the flow. Although longer inlets can boost h.p. at low rpm, it doesn't take a dyno to know that any help the extra length might give is more than eaten up by the loss of h.p. at higher rpm due to the constricted flow.
 

vibrac

Well Known and Active Website User
VOC Member
I seem to remember back when I had a passing interest in racing parallel twins ( Oh! wasted youth) That right angled bends in inlet tracts gave more power than gentle tapers because of better mixing....
 

erik

Well Known and Active Website User
VOC Member
What is about the original small bellmouthes on the Amal 276 and 289 carbs?Any influence on power delivery?
 

Magnetoman

Well Known and Active Website User
VOC Member
What is about the original small bellmouthes on the Amal 276 and 289 carbs?Any influence on power delivery?
For what it's worth, on the flow bench adding a bellmouth to a 1038 Concentric increased the flow by 0.8%, but on a 1036 it decreased it by 0.3%. However, the differences are small enough that I'd have to say the bellmouth made essentially no difference within experimental error.
 

vibrac

Well Known and Active Website User
VOC Member
tant the bellmouth shape or even its length its the whole length of the inlet tract (and a whole host of related factors)
 

Little Honda

Well Known and Active Website User
VOC Member
Yes... but did you test it on your dyno? #cashinthat401k

It should be noted here, that the study covers the total length of inlet between valve seat and air entry, not
the outer trumpet on the carb before the needle valve. Moreover, the shown bellmouth form is not the best
possible one: The entry profile should be surrounded by a 180° radius to get less entry turbulences.
 

oexing

Well Known and Active Website User
VOC Member
I like to have long bellmouths on my carbs, performance-wise a good total length of the inlet is no bad idea, look at Manxes and Goldstars with their substantial carb spacers plus bellmouths to achieve this. Modern fuel injected car engines have funny "bunch-of-bananas" induction pipes to get a real long effective length from plain air to inlet valves. That length is depended on the speed range of the engine, the higher revving the shorter the system can be.

Vic

IMG00013.JPG
 

Magnetoman

Well Known and Active Website User
VOC Member
It should be noted here, that the study covers the total length of inlet between valve seat ...
The situation with our engines is different than modeled by that study. The study assumes an unobstructed inlet tract from the trumpet to the valve but, other than full throttle, the slides of our Amals and Mikinis present a very significant obstruction roughly halfway along the tract. This makes modeling of the full h.p. vs. torque curve for our bikes more of a challenge.
 

Cyborg

Well Known and Active Website User
VOC Member
It should be noted here, that the study covers the total length of inlet between valve seat and air entry, not
the outer trumpet on the carb before the needle valve. Moreover, the shown bellmouth form is not the best
possible one: The entry profile should be surrounded by a 180° radius to get less entry turbulences.
Duly noted. I should probably also note that I just attached the first thing I came across. I know this makes me a bad person, but it was just a thinly veiled attempt to rattle his cage.
 

Cyborg

Well Known and Active Website User
VOC Member
It should be noted here, that the study covers the total length of inlet between valve seat and air entry, not
the outer trumpet on the carb before the needle valve. Moreover, the shown bellmouth form is not the best
possible one: The entry profile should be surrounded by a 180° radius to get less entry turbulences.
At least for me, a good example of that was (back in the dark ages) I was asked to jet a CB750 with a “custom” air box and headers that had been installed by the customer. Long story short, it simply wouldn’t behave until I took the rubber stacks out of the stock air box and installed them in his custom contraption. Seems like velocity stacks are more critical on some carbs than others.
 

Bill Thomas

Well Known and Active Website User
VOC Member
They had trouble with one of the Jag's , Many years ago, And the factory cure was a small plastic bellmouth,
Pressed onto the airbox.
 

erik

Well Known and Active Website User
VOC Member
Today the Pilot jets in different sizes arrived.But weather changed to really bad and it can take time until I can test it.but I will tell you my experiences.Erik
 

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