Dellortto carb jetting for factory 3 into 1 exhaust - 79 1200

[Paul LeClair]

Hi all

79 1200 with 4C cams, IIS ignition, back on 32 mm Dellortto's.

the bike has been running big bore collector and "Jota" mufflers. Exhaust needs to come off for some re conditioning, which will take a little while, especially if I send the Jota mufflers and the header pipes to be re chromed, if I can find anyone that still does that.

the carbs are presently at the bare castings stage on my bench, with the bodies being run one at a time through my small ultra sonic.

pilots 60, slides 60/2, needle jets AB 265, needles K1, clip set one notch below middle, and believe it or not, it has been running 145 mains for years with excellent results, clean plugs, pulls hard at wide open throttle.

I am thinking I may temporarily fit the Laverda factory 3 into 1 exhaust, any ideas on jetting changes as may be required? I have main jets in sizes up to about 160, 165, just have to dig them out of storage, or stay at 145's?

what will the 3 into 1 do to carb requirements? I am assuming it MAY flow more air through the motor, but maybe not, I have never played with a Laverda factory 3 into 1 before.....

Paul LeClair

 

[Dellortoman]

Hi Paul

More learned people than you and me could probably theorise on jetting changes till the cows come home, but you won't know what works until you try it.

I'd suggest trying the carbs with their current setup. If the bike ran well with the previous exhaust, the chances are it still will. In my experience, changing exhausts doesn't make a huge difference to carb settings, especially at partial throttle openings. At anything less than full flow, pretty much any exhaust will be more than adequate. The subtleties of exhaust tuning and resonance really only come into effect at full noise. So you might need to tweak the main jet for when the engine is operating at WOT in the region of max power.

On that subject, 145 main jets seem way to big. I'd suggest that they're probably doing nothing to control fuel metering. The bike might run just the same with no main jets at all.

When I bought my Jota, the PO had put 135 mains in it. I put it on the dyno which showed it to be running rich at WOT. Reducing the main jet to 130 and then 125 made no difference to the A/F ratio. It wasn't until we got below 125 that we started to notice a change. As an experiment, we gave it a run with no main jets at all and it ran just the same as with the 135! We ended up with 120 mains for a 13:1 mix at max power. Could have gone a touch leaner. The original spec is 118, which would have been perfect, but I didn't have any with me.

That experience lead me to believe that the 32mm Dells as fitted to Laverdas with K1/AB265 needle/jet won't flow enough fuel for anything bigger than about 125 or 130 mains. So I don't think your 160+ jets will be of any use.

 

[Paul LeClair]

I knew I was going to take some grief for the 145 mains......

carb jetting was set up on a DynoJet 250i with a sensor up the tail pipe. I started at 118's and worked up, about ten steps, burning my hands each time I swapped the main jet, at 135's it was still lean at wide open throttle, finally at 145 the engine "woke up" and runs at about 13.2:1 with clean plugs. Strange as, I agree, but it works on this particular bike. On my RGS 1200, set up on the same dyno, I was running 118 mains at about 13:1 at wide open throttle, with clean plugs. Then I switched the RGS to Mikuni RS 36's, years later I am still fiddling with jetting, still get trailing throttle popping and banging, about to raise the needles on the Mikuni's for the second time to try and get rid of that......

anyways, yes I will simply stick the 3 into 1 on to the 79 1200 with jetting as is and see what happens. Every other header I have ever tried on other bike's usually improved things a bit at wide open throttle and killed mid range or resulted in a flat spot somewhere. My issue with the 3 into 1 is it is just too bloody loud, will be my neighbours problem for a few weeks as I renovate the existing exhaust system......

Paul LeClair

 

[AndyW]

145 mains are the stock items for 1200 Slater Mirages...

 

[Paul LeClair]

145 mains are the stock items for 1200 Slater Mirages...

and this bike was verified by Slater's as a "Slater's Mirage"........ but I still have not figured out how adding 4C cams and the Jota exhaust would so significantly change the carb jetting requirements to these monster main jets.

Paul LeClair

 

[AndyW]

and this bike was verified by Slater's as a "Slater's Mirage"........ but I still have not figured out how adding 4C cams and the Jota exhaust would so significantly change the carb jetting requirements to these monster main jets.

Paul LeClair

I think Cam is probably right in suggesting they only come into play at WOT once 'on the cam' and was probably insurance against the carburation weakening and holing a piston at full bore.. . Mine runs just fine on 145s though can eat/foul plugs if idled around town for too long without a rip around the countryside... 

AndyW 8)

 

[Paul LeClair]

I have been studying slide needle profiles (with a magnifying glass...), have a look at the needle chart of the K series needles, here:

http://www.dellorto-carbs.co.uk/graphics/need3.jpg

so K1 are currently considered the "best" option for a triple, I gather, not sure who determined that?
K1  A=2.45, B=1.75, C=37

K2 are currently considered the "best"option for the 750 twins, I gather, not sure who determined that?
K2 A=2.45, B=1.75, C=42

K14's came in a lot of 1200's from the factory, I am told. I have at least a dozen removed from various triple carbs.
K14 A=2.48, B=1.75, C=33

K28's also apparently came in some triples
K28 A=2.50, B=1.30, C=41

A is the top of the needle shaft diameter near the clip grooves
B is the pointy tip of the very other end of the needle
C is the taper area over most of the length of the needle

so A comes up out of the needle jet first as the throttle starts to open
then C has to come up next, over a wide range of throttle opening
finally, at wide open throttle B, the tip will beat its highest point either still in the needle jet barely, or all the way out (will have to look at a carb/slide/needle

by my logic, then, the thinner A is, the richer very initial throttle opening will be. So, K1 would initially be a tiny bit richer as the slide starts its rise.
then C starts its rise, the thinner it is, the richer the mixture will be. Interesting, the K14 appears from its measurements that it would be richer through the middle of its rise than the K1 would be

then finally B comes into play with the throttle wide open, with the K1 and K14 being identical, and K28 being slightly leaner.

so maybe a K14 is the needle to use for a triple if looking to richen the mid throttle openings?

Anyone?

Paul LeClair

 

[Harry Lime]

Slater Mirage? 3-1 with no baffling?

145 main 55 pilot.

It just is, OK? Well, it was for mine after much faffing and experimentation.

H.

 

[Grant]

I always have found that 65 pilots work best on the 1200s.
55s work better on the 180 1000s, again IMHO.
Of course tuning may change the jetting requirements.

When I ran a Harris 3:1 on my Jota it needed 165 mains and a cut down Jota end can before it ran anywhere like properly, but reverted quickly to a standard system.

Again, on triples I always go for K1s and throw away K14s.

 

[psg]

My 4C/Jota pipes 1200 Mirage runs 60 pilots with K1s and I still turn the idle mixture out 2-2.5 turns for best idle. I'm at PLCs altitude. When I had my Motodd 3:1 on I ran up to 140 mains and still could turn the enrichers full on with no effect at WOT, then began to question if there was enough vacuum to bring them into play at high revs and WOT. Left it there until I put the Jota pipes back on and went back to 128 mains. Gutted air box, still has BTZ.

Stu

 

[Dellortoman]

My Jota had K5 needles in it when I bought it, as well as the 135 mains. It had other "tuning" modifications like A11 sprockets on 4C cams and a 3-1 exhaust that sapped top end power. The bloke I bought it from said it went really well. I don't know what his baseline was for that assessment, maybe an old BSA or similar. Whoever tuned the bike for him (I think it was Stanco who fitted the 4C cams) made a right cock-up of it. When I put it on the dyno it was struggling to make 60 RWHP. Sorting those issues brought the power back up to around 80 RWHP and halved its fuel consumption.

 

[Harry Lime]

Slater Mirage? 3-1 with no baffling?

145 main 55 pilot.

It just is, OK? Well, it was for mine after much faffing and experimentation.

H.

Bad form to quote oneself, but I did about a gazillion laps of Cadwell, Snetts and commuted through central London to get those settings............still got a box full of jets!

 

[Paul LeClair]

Bad form to quote oneself, but I did about a gazillion laps of Cadwell, Snetts and commuted through central London to get those settings............still got a box full of jets!

I have quite a collection of Dellortto brass myself, there has been a lot of experimentation and fuel soaked hands over the years......

Paul LeClair

 

[aebbern]

Then I switched the RGS to Mikuni RS 36's, years later I am still fiddling with jetting, still get trailing throttle popping and banging, about to raise the needles on the Mikuni's for the second time to try and get rid of that......

I'm curious about this... assume by trailing throttle you mean with the throttle closed while decelerating.  If that's the case, wouldn't it just be a case of lean idle mixture (and not at all related to the needles)?  I had a similar issue on my LC8 and fixed it with a slight tweak (richer) of the fuel screw.

 

[Paul LeClair]

Hi Andy

the Mikuni flat slide idle system has a a pilot jet, and an air screw. I have fiddled with the air screw pretty much all the way in to three full turns out, changes the idle but doesn't fix the popping and banging on deceleration.

I have another issue with the Mikuni's on the RGS anyways, the floats need replacing, and I yanked the carb rack off the RGS tonight, the bike is going back to a freshly rebuilt set of Dellortto's while I obtain new Mikuni floats, a new center float bowl (mine is cracked and fixed with JB Weld), and a selection of additional sizing in pilot jets, I don't think the size pilots I have in there (the only set I have) are correct for the application.

the RGS "1200" runs really well with the Mikuni's at everything except idle and the popping on deceleration issue, but summer is short and I just want to ride the various Laverda's for the rest of the season, I am sick of working on them when it is a nice riding day outside, lots of time in the bitter cold of winter to sort this stuff, I just wasted several hours rebuilding and replacing yet another half axle in the 1972 Triumph Spitfire car, same story there, sick of working on it in nice weather, just want to drive it......

Paul LeClair

 

[Dellortoman]

My Jota (with Mikunis) pops and cackles a bit on overrun, but the bike runs and idles OK so it doesn't bother me. It did the same with the previous Dellortos, and I hear a lot of race bikes making similar noises, so maybe it's part of the territory.