martymoose
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- Location
- Sydney, Australia
You want to ride an 81 jota in the city?
Replacing Dellorto's with Mikuni or Keihin. Was it worth it?
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chrisk
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- Location
- Brisbane, Australia
Maybe it will stop the Flying Kangaroo from boiling the fuel in the bowls in city traffic......
Laverda SF
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Always wanted to replace my open mouth Dellorto 30A's with Mikuni 34's so I could put an air filter on my 70 750SF engine but I never did - The Bike is sold but still have them MIC's in a Box - Anyone interested ?
Not exactly a bolt on. It will require a little modification on the throttle links ;o)
Not exactly a bolt on. It will require a little modification on the throttle links ;o)
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hooksey
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- Location
- Vic , Australia
In Short..............Yes!
martymoose
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- Location
- Sydney, Australia
34mm? Im running 36 dellortos on the sf2
Thomas aus LAU
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Rob
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- Location
- Midlands, UK
Yes they will. A 25-30% improvement in fuel consumption demonstrates decent carburation across the rev range. Makes no difference to max power. Only downside (apart from cost!) I've found is difficult cold starting on choke...it needs just a bit. Not too much and not too little. Bit like Goldilocks.
Rob
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- Location
- Midlands, UK
lawrence
Hero member
- Location
- Idaho, USA
I have never found my Mirage difficult to start with the Mikuni carbs. Quite the opposite, actually. Red published a sheet that contains recommendations for jetting. It makes for an easy set-up and fine turning. I came away with the sense that the Mikunis are precision instruments and the Dellortos -- are not. As such, they respond well to careful tuning and once done properly, they don't require attention. I don't know how many miles one would have to ride to recover their cost in fuel savings, but the immediate reward of a better running Laverda is enough for me. I'd make the change again in a heart beat.
Comparing apples with pears imho...
The PHF Dellortos and the RS Mikunis are from entirely different eras. The RS arrived when the PHF was around 20 years old. The PHF was Dellortos first big (>30mm) carb with a central fuel bowl, following the SS1. In fact, the Laverda triple was one of the first production bikes to be equipped with these carbs. #1001 from Cors' museum carried sandcast PHF (prototypes?). The RS have the benefit of 2 further decades of development, a time during which the requirements placed upon carbs in general had dramatically changed, from making maximum power with little regard to consumption to making even more power with drastic emmission reductions in mind. While originally being a racing instrument, emmissions reduction wouldn't have been high on the Mikuni RS priority list, but efficiency certainly would have. Better efficiency=less consumption=less emmissions, simple.
Of course, product development has to show some benefit, otherwise it would have been a waste of time. The RS can meter fuel far more precisely than a PHF and therefore can reduce consumption, but not to a great extent. To produce any given number of ponies, an engine requires a given amount of fuel, and it really doesn't care if it is passed through an italian or japanese carb, or dribbled from a dented Coke can, before it gulps it up. I seriously doubt that anybody could measure a noticable difference in WOT consumption between the 2. It is only in the low- and mid-range that the RS would have a distinct consumption advantage over the PHF, but not by much. I achieved a ca. 5% reduction with my switch to RS36 from PHF36, nothing to write home about.
Yes, the Mikuni RS is the better carb, but more through virtue of age than design. The PHF is not a bad carb, and can be set up to work surprisingly well, if the base isn't too knackered. In 10-15 years, when the now-new RS get a bit long in tooth, they'll have similar issues to what the PHF have now. Hopefully we'll still be able to buy spares for them, along with a tank-full of fuel, without first having to sell a kidney.
piet
The PHF Dellortos and the RS Mikunis are from entirely different eras. The RS arrived when the PHF was around 20 years old. The PHF was Dellortos first big (>30mm) carb with a central fuel bowl, following the SS1. In fact, the Laverda triple was one of the first production bikes to be equipped with these carbs. #1001 from Cors' museum carried sandcast PHF (prototypes?). The RS have the benefit of 2 further decades of development, a time during which the requirements placed upon carbs in general had dramatically changed, from making maximum power with little regard to consumption to making even more power with drastic emmission reductions in mind. While originally being a racing instrument, emmissions reduction wouldn't have been high on the Mikuni RS priority list, but efficiency certainly would have. Better efficiency=less consumption=less emmissions, simple.
Of course, product development has to show some benefit, otherwise it would have been a waste of time. The RS can meter fuel far more precisely than a PHF and therefore can reduce consumption, but not to a great extent. To produce any given number of ponies, an engine requires a given amount of fuel, and it really doesn't care if it is passed through an italian or japanese carb, or dribbled from a dented Coke can, before it gulps it up. I seriously doubt that anybody could measure a noticable difference in WOT consumption between the 2. It is only in the low- and mid-range that the RS would have a distinct consumption advantage over the PHF, but not by much. I achieved a ca. 5% reduction with my switch to RS36 from PHF36, nothing to write home about.
Yes, the Mikuni RS is the better carb, but more through virtue of age than design. The PHF is not a bad carb, and can be set up to work surprisingly well, if the base isn't too knackered. In 10-15 years, when the now-new RS get a bit long in tooth, they'll have similar issues to what the PHF have now. Hopefully we'll still be able to buy spares for them, along with a tank-full of fuel, without first having to sell a kidney.
piet
Rob
Hero member
- Location
- Midlands, UK
Just to clarify my consumption figures relate to a stock RGS, so swapping from 32mm PHF with 70,000km on them to new RS36. Repeated the exercise with a big bore and hi compression RGS. Similar results - improved from 40mpg to c52mpg. I'm guessing my Dells were rather more neglected than Piet's!
Laverdalothar
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- Location
- Germany
here is where I disagree a bit with you, Piet. Especially on the 32mm PHF, the place where the highest velocity should be (around the needle) they have actually the lowest, as the "tunnel" opens from 32mm to 38mm (slider diameter). This causes the air-speed to drop around 20% I would say (at least at WOT, but also before...). Further to that, there are tons of edges in the carb that - knowing any edge in the inlet causes the air to either stop, turn into other direction or do other funny things - cause loss of power/torque.
The RS and especially the FCR are smooth bore carbs that are - almost - like a constant tube. This gives better controle of the airflow which is the reason you can adjust them better (asside of the fact that they do have way more adjustment points, as they also have Idle- and main air jets).
Next that adds to the better design is the linkage between them. The linkage on the PHF's are very fragile and make the carbs get out of synchronization even by putting them off or on the bike, as there is a big chance to bend the linking plates...
The RS can be adjusted off the bike, they are so stiff that they do not change even if you wiggle them into rubbers.
So - from my point of view, the RS and the FCR's have huge (!) design advantages over the PHF. Nevertheless, I agree that it is less so much top power where they are better than fuel economy and throttle response. Potentially, they have a bit more midrange torque - haven't measured it in direct comparison. But defenitely they improve throttle response.
piranha-bro2
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- Location
- Melbourne
You've hit the nail on the head with the reference to 'smooth bores' Lothar. When we were fitting CR Keihins to big Jap fours in the late 70s they were always referred to as smooth bores, and this seemed to be the major benefit offering, especially in the mid range. I've ridden bikes after swapping from std carbs to CRs and the difference is immediately apparent.
chrisk
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- Location
- Brisbane, Australia
I wonder if Laverda continued to mfr, what carbs they would have changed to?
If at all.
If at all.
Dellortoman
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- Location
- Tasmania, Australia
I know it's getting more into a philosophical argument than a technical one, but neither PHF nor RS carbs actually have a smooth bore. At partial throttle there's a bloody big sliding blockage that the air has to find its way around, and at WOT there are recesses all around the bore where the slide was before it got pulled out of the way. I doubt that one type is any better than the other as far as air flow is concerned. Air flow through both types would be a turbulent mess at any throttle setting.
I think calling any slide type carby "smooth bore" is misleading. The closest thing I've seen to smooth bore is a set of Lectron carbs. Butterfly carbs are the only type that have anything actually approximating a smooth bore.
The only advantage of the Mikunis is that they have better control over fuel metering through the throttle range. Even using a beat-up old second hand set of Mikunis off a wrecked Suzuki to do my conversion, my fuel range from a full tank increased by at least 60km. That's a 30% improvement, so a bit better than Piet's 5%. Maybe my old Dells were knackered, but the bike ran well enough with them. Evidently it just guzzled more fuel than it needed.
I reckon Piet's right about there not being much difference in fuel efficiency at WOT. But most riding is done in the mid-range anyway, where the Mikunis appear to do a much better job than the Dells.
The A/F plots from dyno runs before and after the carb swap on my bike showed a more even mix through the whole range with the Mikunis. The Dells were all over the place in the mid range and only flattened out to a sensible mixture from about 3/4 throttle up to WOT.
I think calling any slide type carby "smooth bore" is misleading. The closest thing I've seen to smooth bore is a set of Lectron carbs. Butterfly carbs are the only type that have anything actually approximating a smooth bore.
The only advantage of the Mikunis is that they have better control over fuel metering through the throttle range. Even using a beat-up old second hand set of Mikunis off a wrecked Suzuki to do my conversion, my fuel range from a full tank increased by at least 60km. That's a 30% improvement, so a bit better than Piet's 5%. Maybe my old Dells were knackered, but the bike ran well enough with them. Evidently it just guzzled more fuel than it needed.
I reckon Piet's right about there not being much difference in fuel efficiency at WOT. But most riding is done in the mid-range anyway, where the Mikunis appear to do a much better job than the Dells.
The A/F plots from dyno runs before and after the carb swap on my bike showed a more even mix through the whole range with the Mikunis. The Dells were all over the place in the mid range and only flattened out to a sensible mixture from about 3/4 throttle up to WOT.
GregT
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- Christchurch NZ
I've probably set up a wider range of carbs than most here. Started with Monoblocs and graduated to GP's - still working with GP's too.
Then VM Mikunis, through the real smoothbores to RS's. Keihins from road versions to CR's and FCR's.
Plenty of Dellorto time too.
In my experience the big advances have come in emulsion tube and air bleed design and layout. When they have a spray pattern suitable for the airflow through that size throat, they become very tolerant of jetting mismatches. The RS's are very tolerant in my experience. Probably more so than FCR's. The Keihins tend to fill in gaps with the accelerator pumps. The Mikunis with better design.
Mikuni had a serious go at a real smoothbore with the VM race series of 29 and 33mm bore. Old tech now and the 33's in particular are hard to set up. But they did - and still do - make serious power.
But for big post classics now, it's Keihin CR's all the way. Very easy to set up and they hold their tune well.
The GP and RN Amals are the real smoothbores with hollow slides and a solid piece filling the bottom of the slide area. But s*** they can be hard to get right.
Then VM Mikunis, through the real smoothbores to RS's. Keihins from road versions to CR's and FCR's.
Plenty of Dellorto time too.
In my experience the big advances have come in emulsion tube and air bleed design and layout. When they have a spray pattern suitable for the airflow through that size throat, they become very tolerant of jetting mismatches. The RS's are very tolerant in my experience. Probably more so than FCR's. The Keihins tend to fill in gaps with the accelerator pumps. The Mikunis with better design.
Mikuni had a serious go at a real smoothbore with the VM race series of 29 and 33mm bore. Old tech now and the 33's in particular are hard to set up. But they did - and still do - make serious power.
But for big post classics now, it's Keihin CR's all the way. Very easy to set up and they hold their tune well.
The GP and RN Amals are the real smoothbores with hollow slides and a solid piece filling the bottom of the slide area. But s*** they can be hard to get right.
There are thousands of mods done to FCR in the dirt bike world, to get reasonable driveability out of Massive bore carbs. My 250 ran a 37mm carb and with the accessory 3 taper needle adjusted to the correct one, they sell the kit with 2 different needles and lots of clip positions to choose from. You read the data sheet for altitude and ambient temp and fit the right needle to suit with fine adjustment via the clips. Plus The Power Now gadget that simulates a smaller venturi at small throttle settings and with the accelerator pump timing and volume adjustments possible. All that makes a world of difference.
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