Hi all,

I've searched for this both here and elsewhere quite a bit. Why did Racing Beat put .8s in their Dellorto kits? Almost every other application uses main jets in the 130-170 range, whereas RB put in 230s. They also put another air bleed in the idle jet holder (basically the idle e tube), and used 75s, which are big, since they only go up to 82 (as far as I can find -- which sucks cause they are a bit too small on my street port). It seems to me that they are using lean e tubes and compensating for it by using big jets. Could someone explain what this would accomplish? Has anyone experimented with using different e tubes / idle jet holders?

Looking for instance here, it would seem that for a 4 cylinder engine with 573cc/cylinder (from the carb's point of view what a 12a should roughly look like), one would use .6 e tubes, and main jets around 155 or so.

I have an RB Dellorto on a LC manifold on my street ported 12a, as well as a mess of Dellorto jets, so I think I'm going to mess around with this unless someone can tell me why it won't work.

Best,
Sweblin

 

I don't know Dellorto carbs at all but I can tell you that you cannot correlate a rotary engine with a 4 cylinder engine with 573cc per cylinder. Rotaries act nothing like a piston engine and require quite a bit more fuel expecially with your street port.

 

Fair enough, but the .8 tube is much leaner than the .6 I was referring to. You can tell from looking at the pic:



http://www.dellorto.co.uk/merchandis...sp?PartID=5737

Holes in the tube create a leaner mixture, their location determines which rpm range is affected. Lower on the tube means higher rpms range, and vice versa. At least, I think that's right.

I contacted www.dellorto.co.uk, and they suggested I try a .2, which I will. He also said that the page I referenced above is full of it when they recommend tubes based on cylinder size, so that rough comparison was bogus to begin with.

-S

 

I suspect they came up with that jetting to address the transition from idle
to WOT. Rotaries breath differently thru the rpm range than pistons. Since RB
probably spent a few man years developing the Dell carb solution, I'll bet you'll
have a hard time beating it overall.

If you looking for jetting for a SP check out this page http://www.gruntled.com/Dellorto.

 

Keep the .8's in there. The leaner tube also adds more air for better emulsification of air/fuel. I've never had a need to change that e-tube. Yes they compensated with a huge main fuel jet. It works.

I had the same Lake Cities intake and a 12a streetport and got it to work very very well. My detailed jetting specs are here: http://www.gruntled.com/Dellorto/Brad-Jetting.html

80 to 82 idle fuel jets are the ticket for a streetport.

 

Yeah, I had considered the emulsification point. Ok, well, has anyone tried anything different? I'm still planning on trying it out and seeing what happens, but I'm guessing it won't be particularly successful based on what you guys are saying. If it sucks, I mean, I've wasted 25GBP or however many dollars that is.

I'm still too far out on the mixture screws on the 82s I have. I have no source for 85s or bigger. It definitely responds to bigger idle jets on the progression circuit so far, so I'm thinking about changing idle jet holders to go richer.

Drivefast7, I've read your page on gruntled, and I'm probably going to switch to 37mm chokes based on your experience.

-S

 

You can ream the idle fuel jets out as large as you need.. google 'jet reamer kit' and also get a set of jet measurement gauges. cbperformance.com used to sell these, others do as well.

37mm was much better on the street and it still revved. It's all about velocity!

 

Thanks, I'll check em out.