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38mm Dellortos

Marc Brown

Just got it firing!
Joined
Aug 29, 2013
Messages
3
Location
Vashon
I have a set of 38mm Dellorto carbs from a BMW I plan on putting on my 1000sp
I've already machines some spigots to fit the Guzzi rubber intake.(BMW's are female style)

Trying to find a starting point for jetting

Only other engine mod is a B10 cam

Any help would be appreciated

Thanks
Marc
 
Here's what I ended up with in a 1000SP engine installed in a loop frame with 36 dellortos and a Megacycle X10 cam. Should be a reasonable starting point:

PJ - 57
Slide - 60/3
NJ - 265AB
Needle - K5 (bottom / richest notch)
MJ - 135

This bike lives mostly at sea level & that's where the dyno testing was done to arrive at the settings above.

I had an 850T w/ 1000SP Gilardoni top end, 36's and a B10 & here is the jetting it ended up with:

pilot jet- 60
atomizer - 268AB
needle - K-18 (clip in the top (leanest) groove
slide - 60/3
main jet - 135
 
Rough Edge Racing said:
Do you have small or medium valve heads?

Small valve

Just a little blending at the beginning of the intake port to match the manifold diameter. Not very scientific, but has to be better than a lip at the manifold - head joint. Other than that, didn't touch the ports on either bike.

Manifolds were the Delrin ones from MG Cycle on the 850T and home-made steel ones on the Loop to allow use of the original airbox.

manifolds2.jpg
 
Thanks
That will at least get me started in the right direction
Luckily I have access to a dyno to get it sorted.

Marc
 
And does a stock Guzzi twist throttle have enough travel to open the slides completely?
Generally speaking, bigger carbs can work on a "matched" head port can work ok depending on the port air flow and velocity.
 
Rough Edge Racing said:
And does a stock Guzzi twist throttle have enough travel to open the slides completely?
Generally speaking, bigger carbs can work on a "matched" head port can work ok depending on the port air flow and velocity.

For the installation on the Loop, the stock splitter did not. I got a different splitter from Flanders that allowed a bit more travel. The Super Practic throttle had just enough travel available to pull the 36mm slides to full open.

I like 36's on small valve engines, installed a few now. perfectly acceptable low-speed and way more umph after 5000 or so RPM.
 
For what it is worth I put a small valve and mid valve heads on my flow bench and the small valve heads actually made slightly more flow. This is not to say that the mid valves can't be made to flow more with proper porting (as done by a smart person with a flow bench) but in reality the increase in valve size is marginal.

From what I can see the Leman's power gains are mainly from the increase in compression and carb size.

The small valve heads are good place to start if you are porting as it has extra meat on the small turn radius where the problem is (the casting has the same number and differs only in the that combustion chamber is machined about 2mm deeper to clear the piston).

38 is actually a pretty optimal size for a properly ported head - 40 is definitely pushing it.

Get the Dellorto tuning guide and read it and buy a good selection of jets. Idle and WOT are easy, in between takes time. I monitored exhaust gas temps as the easiest way to get a handle on where I am.
 
From what I understand, current porting technology is concerned with with flow and velocity at less than full lift...The valve spends a lot more time open at less than full lift.
 
I typically plot from 0 to full open. What I have found is that if your porting works at full open it will in %99 of cases reflect improved flow everywhere else. Porting for flow and velocity is another matter - you are really looking to maximize both. I use a pitot tube attached to a manometer to probe the port to see where the velocity is - this is of course intrusive but it gives one an idea.

This is why simply making a big port with a big carb doesn't work - the velocity goes down and so does momentum and the signal to the carburetor.
 
Just thought I would post a THANK YOU

Last weekend finally fired up the bike after many delays..

I was expecting to spend some time fine tuning the jetting, but it was spot on based on Georges settings with a B10

Thanks Again
Marc
 
Once its srted I think you'll be happy. While I was building a LM engine, I stuck a Call II motor in the frame with a B-10 cam and the 40 dels tacked on. Very noticeable improvement. Buona Fortuna,
 
Well...
I put the bike on a dyno and got a whopping 47hp.
I've double checked the cam timing, compression is 157-159, Ignition timing checked at full advance.
I'm not sure why it's so low, my little Honda 350 race bikes getting 38 hp

Marc
 
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