Intake tuning, length and where to extend.

[RustiePyles]

From a performance standpoint and flat out speed the DellOrtos will be an improvement over the stock Keihin CV's but will not produce as much bottom end grunt (not that the 350T had much to start with). As far as intake length goes the only time you are going to see marked improvement on a dyno is with a bike that spends a lot of time at a specific RPM range i.e. a drag bike, land speed bike, or flat tracker. If its going to be street ridden its going to see broad use of the entire rpm range so tuning to a specific one won't be much help.

 

[Jim]

(Although my RD is also lightened quite a bit, has chambers, a single big K&N in the stock rubber "Y-boot", YZ125 fiber reeds, and a Powerdynamo CDI ignition. Plus rearsets of course.

Pics?

 

[arcticXS]

Here's the 'ole RD: Bought new by my oldest cousin in 1975, then used by his two younger brothers as well, as they turned 18 and got their unlimited motorcycle licence.
(16 to 18 year olds were limited to 100 cc until the mid 90-s when the limit was raised to 125 cc and 15 hp. So there was a shitload of CB100 and LS2 Yamahas on the road in the 70s. And some Kawa G7/ KH100 and Suz A100/ GT100)
I bought it from him some 5-6 years ago. Still on the original 64.00 mm bore. I overhauled the crank for him some 25 years ago, as the main bearings were shot.
I have another RD, which I already had converted to rearsets with cable actuation, so I just swapped the rear brake over. Not exactly clean and shiny, but goes and rides really well.

 

[arcticXS]

Then back to the subject of this thread: A couple of pics of my XS with RS34 carbs and stacks. Still waiting for the K&N filters to fit those bellmouths. Intake length from head flange to end of bellmouths is 185 mm or approx. 7 1/4 inches. Then a pic of the CB350 (twin) setup I'm working on. The velocity stack/ filter adapter is machined from POM plastic, and is only bored out to 37mm, and machined to fit the DellOrto 30 mm carb and the UNI filter, so far. My buddy is doing some test fitting, and figuring out a new battery location, before I will make the second one, machine down the outside, and machine a bellmouth radius on the filter end. POM is a nice plastic to machine !

 

[arcticXS]

Someone may be interested in an update on the CB350 (twin)/ DellOrto PHBH30/ UNI filter velocity stack project. Finished the first one tonight.
For the bellmouths radius, I used a router radius bit with 10 mm radius, as a lathe bit. I just made a split aluminium holder for it. Worked great in POM, but may not work in harder materials. It will be interesting how much of an improvement this setup (hopefully) gives, compared to the previous stock CV carbs and K&N pods.

 

[jay760]

I thought the point of the length was the head to venturi to improve performance not from the carb to the filter, I had mine made originally to try and compensate for the start stop effect of the valve,
I know one guy who says his are equally length, valve to venturi then venturi to end of velocity stack,

 

[gggGary]

on subject?
https://www.xs650.com/threads/helmh...applies-to-your-motorcycle.58407/#post-664340

 

[Max Midnight]

In his book 'Tuning for Speed' Phil Irving (he of Vincent fame) makes the point that changes in cross sectional area of the inlet tract will alter the result obtained from any formula.
Instead he suggests that good results are obtained with a total inlet length of 90" divided by the r.p.m. in thousands which, for example, would give 15" for 6K and 13" for 7K.

I'd assume that the above would be for maximum revs for engines of the period and intended for racing. On that basis a lower revs would probably be chosen for street use.
the measurement is applied from the centre line of the valve to the mouth of the inlet.

 

[hovel]

In his book 'Tuning for Speed' Phil Irving (he of Vincent fame)

Excellent reference. Maybe not contemporary but excellent none-the-less. Thanks for the reminder !

 

[arcticXS]

I just finished the second velocity stack/ UNI filter adapter for those DellOrto PHBH 30 for my buddy's CB350 twin. They will lengthen the intake approx 85 mm, compared to clamping a pod filter straight on to the carbs. They are 37 mm ID, for a near perfect connection to the carbs, without any step at the interface.
Motorcycle season is still a couple of months away here at 70N, so I'll have to wait a while for any results. For now, I'd say they at least look ok.
We may also make a couple of manifold extensions later, for moving the carbs back a bit. But for now, the intake setup is done. The exhaust also needs attention, as he is still using stock headers (tiny ID and HEAVY.....)
I have a couple of 1 1/4" OD J- bends that should be suitable for a 350 twin, with 28.5 or 29 mm ID.

 

[arcticXS]

Another pic, before I machined the radius. No step between stack and carb mouth. For the two small air ports, I asked him to gently file two shalllow grooves, to unmask them completely

(I believe they are idle air bleed, and needle jet air bleed. Not that familiar with DellOrto..),

 

[arcticXS]

BTW the 350/360 twins had super tight valve lash when set to factory spec. You can increase compression (and oil film dispersion on the valve stem face) by increasing valve lash. This reduces effective duration, which at OEM lash is longer than necessary for optimal flow. So by increasing lash you close the valves earlier and increase compression and reduce valve overlap. I don't remember exactly what I set them to but I will look in my notebook this weekend.

Would that be intake valves only, or both intake and exhaust? Any idea how much clearance is safe with a stock cam before they get slammed open?

 

[RustiePyles]

Would that be intake valves only, or both intake and exhaust? Any idea how much clearance is safe with a stock cam before they get slammed open?

Yes both intake and exhaust, I set em both to 0.1mm on my 360’s OEM is 0.08 & 0.05mm

 

[arcticXS]

Thanks for all tips and inputs on this subject. When or if I have the chance to test different intake/ air filter configurations, I will try to post my findings. Either it is proper dyno runs, or maybe an accelleration smartphone app/ dedicated accellerometer device. XS650 or CB350. In any case this will not be before July