Dellorto jet size markings

J

jdugen

Junior member
Location
Manchester UK
Hi all, me again.
The delli's fitted to my 1978 Jota are, as far as I can determine, all original and as supplied. (PHF 32 D). Taking them apart for a thorough dose of ultrasonic cleaning I found that the main jets were numbered 446, no other marks or numbers.
The actual 'hole' is pretty damn large and would equate with the large marked number.  Am I reading this wrong or do these actually have a sinkhole for a main jet?

Cheers
 
I would suggest it was more like 145.
Could you read it as 145?
That was the stock Slater main jet recommendation back in the day, along with K1 needles, 55 or 58 pilots, 60/2 slides and AB265 emulsion tubes.
 
Should be around 125-170, depending on what the DPO thought when doing all the "maintenance" and/or "tuning".

Standard Jota with Jota pipes and big-bore collector should be around 135-145-ish.

Hole in Dellorto jets is actual size, ie, a 120 jet will have a 1.20mm hole.  Has nothing to do with flow volume or such fancy stuff :D

piet
 
...and just to add to the fun, some folk have jet drills to make their own adjustments!  I feel sorry for the subsequent owners trying to make sense of a jet marked with one size and actually being something else...

AFAIK, as supplied, PHF jets don't go that big (446) ....the float valve wouldn't keep up!!
 
Damn old age, fading eyesight etc.  They are 146's the one I looked at had a nice little scratch mark.
Now.... what is the equivalent Mikuni jet size....  I have a set of 34mm CV carbs from a Yamaha 1000.  To tempt me, the carb / head rubbers fit straight onto the Laverda's stubs.  They also split easily to allow correct spacing and have a single bar choke mech operated from a side lever.  All that is needed is a link arm from no.3 carb to the center carb linkeage, easy enough.  If these carbs can supply a 5 valve 1000 engine revving to 1000+ they should be more than adequate for the Lavs inneficient old engine.  ANYTHING has to be better than the cranky Dellorto's.
 
jdugen said:
ANYTHING has to be better than the cranky Dellorto's.
Click to expand...

Dunno about that... all they need is a bundle of cash and a quiet afternoon to get them right again for a few years!

Quite happy with my RS Mikunis, but I shudder at the thought of overhaul time coming around, compared to them, Dellortos are peanuts! :o

piet
 
http://www.breganzane.com/workshop/carbs_cv.php  Steve Battison did a Mikuni CV carb install, with 36 mm CV carbs, way back in 2001, and kept them running for years, ultimately developing a fuel injection system for his RGA. I would suspect he would be the one to talk to, he posts here as Steve B, or you could contact him through his web site, link above.

Lots of accumulated knowledge amongst forum members, and especially Red, on jetting Mikuni RS 36 flat slides, not so much on a CV carb conversion.

it is not just main jet size, don't forget slide cutaway, jet, jet needle, idle circuit, etc

that said, I would bet that 34 CV carbs would be close just as they came to you, especially as they are so forgiving, I would just inventory what is in them for jets/needles, mount them on the bike, run them, then figure out whether you need richer/leaner from what you have. I doubt trying to cross match from a round cable operated slide 32 mm Dellortto to a 34 mm CV vacuum operated  slide Mikuni will be of much use....

Paul LeClair
 
Thanks again Paul.
I'll need to make up the bracing bars for the new spacing and then the throttle lever for no. 3 carb so I'll busy myself with that come weekend.
I'll try them as is, but the main jet is about half the size of the bullet hole in the Delli's jet.  Out with the jet drills untill I get there.

I like these carbs as they are actually designed for downdraft mounting and, they are considerably lower in height than the towering inferno Delli's.  As the weather is so diabolical I might as well use the time productively......
 
I think I ended up with 130 mains and the needle on the bottom clip and they ran well on my RGA (1130cc, high comp, etc).  I then changed them onto Christiane's RGS (pretty stock 1000) and funny enough they ran lean down low.  Not easy to tune them if they aint right, since you can't really buy anything other than main jets for them.
Anyhow, every engine is different, you have to find your own way I'm afraid.  If you get them running well the result is very good, you lose a bit of top end but it's much better at low and mid range and just all round "better" to ride.  You also get a huge improvement in fuel economy.
Cheers
SteveB
 
Me and Marty have both got 38mm Kehan Cv carbs off a Honda Big,thats a 1000cc UJM.They were supplied with 145 mains but after some dino time mine is now at 172.The only difference that made to performance was fuel consumption going from 55mpg to 49 thats Imperial gallions and Ozzie miles.The only other possible adjustment is wahers under the needle mine has had up to 4 and its best at stock which is 1.At 34mm it might be a bit restricted,they have a friggen big butterfly in the throat.These type of carbs are very forgiving as to jetting,as 145 to 172 might indicate.BTW don't bother trying a Dynojet kit for the bike the carbs are off expecting much with there triple taper needles and 5 notches,been there with not much success.I like what I have but if I was going again I would have gone with FCR type.Way more bits avalable to confuse the mind
 
When it had Dellortos on it, my Jota ran nicely with 120 mains (4C cams, HC pistons, big bore exhaust and stock air box). The PO had 135s in it when I bought it and it was running too rich at full throttle - about 11:1 AFR according to the dyno. 120 mains brought it down to 13:1 and gave it a few more horsepower. Dunno how some folks get away with 145 mains, maybe their fuel is heavily dosed with ethanol.

Jota has got Mikunis on it now. Doesn't go any better but used less fuel.

 
Mikuni's fitted.  Took an hour or so to make up the bracing bars, fuel lines and no. 3 connecting bar.  Very straight-forward for a moderately equipped workshop.  Throttle cable fits straight in to the Mik's linkage. Chokes operate from a side mounted lever bolted to the no. 1 carb body, the through rod was shortened slightly.
70 main jets replaced with 125's I had in stock, still to be road tested.  Pilot jets are too small as the air-screw is out about five turns, my jet reamer's will have to come out to get that right.

These are off a Yamaha FZ 750 (also known as the Fazer circa 1989)  but were fitted to the FZR 750's of the same era.  Quite a few available on the 'Bay for ?40 - ?60.  Carb head rubbers ?26 for 4, Main jets ?3 each.  (I had all this in stock from past rebuilds.)
They are designed to mount nearly vertically so some adjustment to the floats was necessary, took a few minutes....NOT days!

Rivet counters will note that the carb to air-box rubbers are reversed, but fit and look OK.

To say the difference is amazing is an understatement.  Easy start, a lively engine that responds immediately to throttle input, (oh, and the throttle action is as light as a feather!)  I am chuffed as mint-balls.
 

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