Carb Angle

D

Dale

Senior member
Location
Malta
One for the more techically astute:

I am about to fit a pair of Mikuni's to my SF2, and will need to make up some new manifolds.

The standard manifold sits at 90deg to the cylinder head, and so the carbs sit on an upwards slant, possibly interfering with the bottom of the fuel tank.  (Espcially as I will have cone filters extending their length).

Is there any advantage/disadvantage in making the new manifolds come from the head horizontally?

Your thoughts would be apreciated.
Dale
 
I seem to recall the Boss  telling me they won't work  correctly at an angle any more than 30 degrees

Red has manifolds
 
Yes, they have their limitations... but if the stock PHBs work, so will most any other side-draught carb with a central float chamber.  Just need to get float height right.

IIrc, the cylinder angle of the twin is 25? from vertical.

piet
 
I read somewhere that the Mikuni RS Flatslides are not recommended for an operating angle greater then 30 degrees.
On a triple engine, if using straight manifolds they sit at 28 degrees, work fine, not too many issues.
We started to use 10 degree angled manifolds that we made initially to try and drop the back of the carb rack down to line up better with the 120 airbox, but found that when running the angled manifolds on my own Jota the carbs performed way better.

On the Twisted Twins, one of the 270 degree engines has an ex Phil Todd head that has been ported to the maximum.
That engine shown below is going to be running 40mm Mikuni RS Flatslides so we had to make custom Inlet Manifolds for it.
Because there has been so much porting work done on the head we had limited room left to provide a sealing O-ring so we could not just make a round hole as shown in the attached photo, we had to work out the "length" of the O-ring and make a custom ring groove.
These manifolds are also tapered 10 degrees as I worked out that the carb angle on the twins is more then a triple but cannot remember what the actual angle is.
This is going to be one tough twin if it all works.

Custominletmanifolds.jpg


Custominletmanifolds2.jpg
 
Red

You say that using angled 10deg manifolds made the carbs work better, but did it noticeably affect the engine performance as Phil has suggested?

Dale
 
redax5 said:
We started to use 10 degree angled manifolds that we made initially to try and drop the back of the carb rack down to line up better with the 120 airbox, but found that when running the angled manifolds on my own Jota the carbs performed way better.
Click to expand...

 
I would always add 10 degrees rather than take it away - i've done this on my own 750 - it does cause problems even with the std 36mm d/o's - but I just angle them in a bit. This motor does go extremely well for what it is - there is a photo in this month's "Classic Bike Guide" that clearly shows carb angle. The photo below does show the true shape of the 750's inlet port - the later (big valve) head is the same shape - but bigger diameter port. - Regards - Phil
 

Attachments

  • 750 inlet.jpg
    750 inlet.jpg
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Is the engine sitting in the frame for checking it?
There is very little room to manoeuvre with carbs on the twins. The frame gives very little clearance to the top of the carbs, as well as on their inlet (airfilter) side.
 
Thanks Phil
That photo make it obvious.
If the cabs are angled down, then the airflow will be forsed to follow a U shape.

I guess it becomes a compromise between engine eficiency (very important) and physical space for the carbs (unavoidable).

Tippie
I can't check the frame myself right now, as it is off at the painters.
Hence he question to the forum.

If I had the smallest amount of organisation and foresight, I would have thought of it before I dis-assembled.
(That is what you get with amatures!)

Dale
 
that photo does not make it obvious, tilt the photo 25 degrees which is how the head is sitting on the engine and it becomes a bit clearer, there is no u-turn involved when using angled manifolds
I'm not interested in getting into a pissing match with this.

Phil, you are correct, in theory you should always try and straighten out the inlet tract which is what modern bikes are doing now, some of the designs you see now have a near vertical flow path to the inlet valve.
Dells may work okay with the standard set up but I don't run Dells, haven't run a Dell now for close on 10 years nor could I be stuffed working on them, generally point blank refuse to even touch them.
Sounds a bit harsh but my take is I can spend hours trying to get a worn out 35 year old set of carbs working again and the cost at the end is nearly the same as a brand new set of Mikuni flatslides.

After riding with Mikuni's for so long, and just from the feel of the engine response, etc, I find that they work the best when I drop them 10 degrees
When we get back from USA I plan to do the modifications to my water brake so we can get our Dyno operational fully, it works at the moment but we have one design issue to overcome which is around 7 days work.
Once that is pulling full load I'll do the runs and see if there is an actual difference in mounting a set of Mikuni's on a triple engine at 28 degrees or 18 degrees
Red

 
Might be the difference comes from a bit longer inlet? I think to remember that some people already found out that a slightly extended inlet improves breathing on these engines. I would assume that an angled manifold is slightly longer than a normal one, but you might have considered that and have made them to match the original length? Could you maybe just measure the inner length at top and bottom of your angled manifolds and compare them to the original ones, please? Just curious, not contradicting with what you say. ;)
 
Dells may work okay with the standard set up but I don't run Dells, haven't run a Dell now for close on 10 years nor could I be stuffed working on them, generally point blank refuse to even touch them.
Sounds a bit harsh but my take is I can spend hours trying to get a worn out 35 year old set of carbs working again and the cost at the end is nearly the same as a brand new set of Mikuni flatslides.
Click to expand...

  That's OK - but most of the Classic racing regs insisit on keeping the original type carbs - so work has to contnue on improving them - Regards - Phil
 
yes, that is also correct here in Aussie and if I was going to set up a race engine for someone where they wanted me to supply the carbs for a triple or a twin I would go and buy 3 or 2 brand new Dellorto 36mm carbs and run them with straight manifolds
use a doner set of 32's to supply the mounting system and throttle shaft linkage, few small modifications and job is done

 
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