Commando 750 Dellorto

Hi, I have acquired a dellorto phf34 and want to try it on my 750 commando. has anyone tried this and can anyone recommend a good base line for jetting? There really doesn’t seem to anything on the net, or I just can’t find it, which is surprising. I Am going to mount it with a single mikuni manifold. Any help greatly appreciated!

coldfish said:

Hi, I have acquired a dellorto phf34 and want to try it on my 750 commando. has anyone tried this and can anyone recommend a good base line for jetting? There really doesn’t seem to anything on the net, or I just can’t find it, which is surprising. I Am going to mount it with a single mikuni manifold. Any help greatly appreciated!

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Is it new ?


Standard settings for that instrument
here

Hi, no, not new. Came from a Husqvarna 350.

I saw that post but I was a bit confused about the slide, he says he uses a 54" slide. That's a 60 slide with 0.6 mm removed off the bottom. But I thought that would make it a 66 slide? Maybe he meant a “60” slide, being a 54 with 6mm removed? Anyway I hoping someone who has a Dellorto on their commando might tell Me what works for them… we live in hope….

Ken used a pair of Dellorto 34's, and disabled the accel pump actuators, so nothing about his jetting is very relevant to using a single. You have your test and tune work cut out for you. I hope you have big bag of patience and a lot of tuning skill. They are not that easy to dial in without a known to work base set of jetting. If they were easy to tune, this thread would be 10 pages deep with repeated advice by now.

When I was building my hot 850 motor for the Featherbed frame back in the early 80s I brought a new set of 32mm Dellorto's with pumpers, made manifolds using the Norton and Dellorto's manifolds, one day before fitting them I adjusted the pumps right up to see how far they pumped the fuel while opening the throttles man did they squrt that fuel, way to much for the motor but be instant power increase, but think how much fuel be washing down the bores, even adjusting them right down, but getting the Dellorto's dialed in was a task in itself, got the Norton running good but was a bitch to start just way to much fuel and after changing jets sizes I decided was to much for my little knowledge and brain to handle and went back to new Amals jetted for my motor and simplify, but when I did open the throttle up those pumps made the bike jump up and go, I was close but the hard starting was a big put off for me and finding someone to dial them in wasn't around.
But in away replacing them with new Amals saved my licence as well my motor as with the Dellorto pumpers my Norton would have been trashing machine and keeping the front wheel on the road opening the throttle would have been a task by itself.
You can shut the pumps off but why put pumpers on and not use them, but I lost patients with them and just wanted to ride.

Pumper carbs are probably used on MX bikes to give better throttle response when there is a sudden loss of vacuum. On a road bike, the circunstances are different. The shape of the needle compensates for loss of vacuum, and you do not usually whack the throttle open. Pumper carbs on a road race bike would be more difficult to control. Mikuni needles come in more tapers than Amal needles, and geting that right is critical. The difference in performance between just right and slighly too rich on the needles, is very significant. If you are slightly too rich, you can feel the motor is very powerful, but when you gert it right the difference is a lot.
If you are using methanol fuel, with Amal carbs, you can make your own needle jets - But with petrol it is amost impossible to get it right with a normal carburetor - ask any of the two-stroke guys. A four-stroke is no different.

Thanks for your responses. I think maybe I’ll ditch the pump and see if I a get it sorted without. I had an old moto guzzi that I ran without the pumps and that ran very well.

I have a pump on my mikuni TM
I use it for starting
4 or 5 pumps then hit the button
It fires up straight away
this is with the enrichener on
In the summer it's not necessary to pump first

coldfish said:

Thanks for your responses. I think maybe I’ll ditch the pump and see if I a get it sorted without. I had an old moto guzzi that I ran without the pumps and that ran very well.

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I suggest maybe looking for a FCR 35 as parts to re jet those are easy to get (Honda shop) and reasonably priced ....just a thought....

I have Dellorto jumpers on my Seeley Mk3 850. Dellorto in the UK supplied me with gaskets & jets as fitted to Ducati 850 of the period.
The bike went well but it was well rich. Never did sort it or turn down the pumpers as it was fun to ride. However I only got 30 to a gallon & it's a small tank. I ran out a few times lol.
I will sort it when its next reincarnated.

Off topic: I get over 60mpg with dual FCRs and the accel pump active on a not stock 750 Norton motor that sits straight up in the frame. I like them, but that doesn't mean much.

Less off topic: I put a 36mm Dellorto on a 604 ATK, which is a Rotax thumper powered off road bike. It worked OK, but was far from perfect, so I went back to a 39mm TM Mikuni (36mm bored to 39.1mm) and put up with the hiccup WOT from a standstill. Once underway the TM was a better carburetor than the Dellorto for making seat of the pants HP. The Dellorto required more hours than I could dedicate to tuning it and an abandoned runway for test and tune. All that left me thinking I'd never use a Dellorto carburetor again. Once bitten twice shy. Probably not fair to Dellorto, but that is where my negativity is coming from.

acotrel said:

Pumper carbs are probably used on MX bikes to give better throttle response when there is a sudden loss of vacuum. On a road bike, the circunstances are different. The shape of the needle compensates for loss of vacuum, and you do not usually whack the throttle open. Pumper carbs on a road race bike would be more difficult to control. Mikuni needles come in more tapers than Amal needles, and geting that right is critical. The difference in performance between just right and slighly too rich on the needles, is very significant. If you are slightly too rich, you can feel the motor is very powerful, but when you gert it right the difference is a lot.
If you are using methanol fuel, with Amal carbs, you can make your own needle jets - But with petrol it is amost impossible to get it right with a normal carburetor - ask any of the two-stroke guys. A four-stroke is no different.

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Did you ever find the Mikuni "ideal needle" for the 34 and 36 VM carbs? The 6DH3 is the usual one provided in carb kits.

Though I have never worked with pumper carbs my theory would be to initially disable the pump until the overall tuning is done.

As you lean off on the needles and needle jets, you reach a point at which the motor will cough (miss). Very slightly richer is the point at which you get best power. But when using petrol in a normal carburetor. the available adjustment is probably not fine enough. If you get the jetting lean enough to provide best power, it will probably become more weather sensitive.
Has anybody on this forum tried using the leanest Mikuni needles in their Amal carbs ? In 1970s carburetors, all the needles are the same diameter above the shoulder where the taper begins, and Mikuni needles come in lengths which suit Amal carbs. I suggest Amal needles might be a bit crude. When the Japanese jet their carbs, they always seem to get better power. Every model of Japanese bike of the 1970s had different needles and needle jets - they did not do that for nothing. With large size Mikunis and Mk2 Amals, when you use methanol fuel, you can make your own needle jets using a combination of metric and number drills. But with petrol the adjustment you need is too fine. Methanol jets are larger, so the tuning errors are relatively smaller than when you use petrol. It is the main reason thet motors are faster when you use methanol. It hides-up the tuning errors.
Your Commandos might perform better if you fitted 6D Mikuni needles - they have the slowest taper. They are the correct length for a 32mm Amal carb.
You will probably find that when you are forced to feed the throttle on slower, the bike will be faster.