Dellorto flaw

[Deleted member 12676]

Trying to perfect my race engine I found it almost impossible to get the jetting right. It seemed like there was a small air leak. I pressure tested the crankcase and there were no seal or base gasket leaks. My intake and reed valve were tested to be leak free. I messed with how the Dellorto carb mounts on the reed valve and finally got that perfect. But still the problem persisted. Then I took an investigative look at the Dellorto carb and found an air leak path designed into it! Taking the carb off and putting the slide in place and shining a light into the carb I could see light between the front of the slide and the carb. Air comes in over the slide from the air filter (and also from where the throttle cable connects to the top of the carb), goes past the front of the slide, and then gets into the air/fuel passageway through that small space between the slide and the carb. I can't believe they designed it that way. The design should be that the slide slightly scrapes the carb to seal and prevent any major leak. So now I have a Mikuni 18mm carb on order and until then I will put JBWeld on the slide and sandpaper it down till it just barely touches the carb. That should allow me to finish testing on a new pipe design.
I will probably have to let the machine shop lathe a new aluminum piece to replace the manifold piece of the Rock Solid reed valve to match the carb. Once I get the carb I can tell you all the inner diameter of its clamp so you'll know what size to tell Rock Solid to make the manifold on the reed valve you're ordering.

 

[Bob K]

A Mikuni 18mm ? Why not a Kehien ?
Just asking. I mean I like Mikuni but I know you tend to think these things through.
Your thoughts?

Bob K

 

[Deleted member 12676]

near impossible to find a 16mm-18mm clamp-on Keihin. But they are good carbs. I had one on my CR250 but it took using a computer program to get it jetted perfectly. Stupid Honda couldn't get it right from the factory.

 

[Deleted member 12676]

The inner diameter of the clamp-on part of the Mikuni is 23mm. So either I need a plastic sleeve of 2mm thickness or I need to have the reed valve part made from scratch to match.

 

[roughrider]

Well, this is a bummer. I just ordered the performance carb from JNMotors. Not sure if I'm visualizing the problem though. I don't have the carb in my hands to see what you are talking about. But, anyway, thanks for sharing your discovery, Jaguar.

PS: Just had a thought. Have you/Did you test the carb when it was hot?

 

[Deleted member 12676]

The two sections of metal that should be mating perfectly to disallow air leaks are thick enough to not change with heat. Maybe the space between them varies from one carb to the next due to manufacturing tolerances. Like I said, it can be mostly fixed with JBWeld. But to get jetting really perfect you need a carb that allows adjustment of all three ranges (low, mid, high RPM) which is what I'll have with the Mikuni carb. After correcting the air leak the Dellorto should be fine for most people though. I am aiming at absolute maximum performance from mine though.

 

[Bob K]

I just had a flash back from my past . When putting too big of a carburetor on an engine the airflow was too low through the carb to draw fuel reliable.
On trick to enrich the idle mixture was to take .020" off of the back side of the slide. This caused the emulsion tube to get a stronger signal and it helped
get a proper idle and more importantly, a good off-idle throttle response. I do not recall if I had to fuss with the needle height ( hey 30 years ago...)
this was a way of enriching the idle with out affecting the main or mid range. This was with Mukuni's.

 

[Bob K]

http://www.iwt.com.au/mikunicarb.htm

My old memory was correct. See the above link for a pictorial explaination.

Bob K

 

[Deleted member 12676]

the ideal carb size for my 60cc engine with reed valve reving to 8300 rpm is 17.5mm, so 18mm can't be considered "oversize".
The carb I have on it now is a Dellorto 12mm bored out to 14mm.

 

[Bob K]

That's what I like about you, you run the numbers! My point was/is, by altering the effective slide cutout height, you can independently alter
the idle/off idle mixture without affecting the other jetting circuits. I just shared an (old) experience with a hopped up motor and a slightly oversized
carb. This does not alter the fact that slide cutout geometery is an effective tool for tuning.(even with a "correctly sized carb)
Possibly, ( imagine that) the chinese did not get the
carb slide cutout correct. Or at least not in the 'sweet spot". I'm at sea level and you are at high altitude. Our results may/will vary to some degree.
I look forward to reports from your experiments with your new Mukuni. If you have not yet received your new carb, You might entertain the idea of
taking a bit off of the back side of your exisiting slide to richen up the idle/off idle and then backing off the main jet and see if it works.