Carbs

[dopeshots]

Years ago I had a fast Lammy it ran an AMAl 34mil power Jet.

Today it seems that people are running Dellorto flat slide carbs or Mikuni carbs.......

What is the difference in the carbs?

Why flat slides?

With or without power jet?

What are the benefits of the modern carbs?

Any info would be greatly received as I'm trying to work out what he best route for building another fast Lammy TS1 or RB.

[coaster]

Others will give you better explanations but my understanding is that flat slides give better atomisation and throttle response as well as being easier to fit under the panels due to being shorter....bound to be other benefits too.

[nelson pk]

Yep, as above.......cleaner carburation lower down as the air flows better through a flatslide at lower throttle postions.
Using a roundslide can often mean overjetting at lower openings/revs to compensate for lean spots.
Thats how i see it anyway
I still use a roundslide at the moment but hope to upgrade to a VHSH 30mm dellorto soon.
Flatslides are more expensive and if you need a different number slide for jetting its twice as much to buy than a round slide but they are superior.
I would use dellorto flats every time over mikuni as there are more adjustment options (fixed atomiser in mikuni, changeable in dellorto) but this is down to personal preference.

[Adam_Winstone]

There is much talk about performance benefits (e.g. oval bore at low throttle openings gives better pickup) but I never found my flat slide Dellorto to give better performance than my MKII 34mm Amal. Indeed, I was frustrated to find that 'chucking on' the Amal, with guessed jetting, actually improved my 1/4 mile times

However, that didn't stop me from using the flatslide as the narrow size, and lack of fuel banjo poking off the bottom of the Amal, allowed me to fit the dellorto under the panel of my GP without needing to cut a hole. That said, I did still need to fit a short manifold, machine off the face of the carb and trim the brass overflow pipe off the float bowl of the Dellorto.

Adam

[GP Kevo]

Mikuni carbs have needle jets (atomisers, as you call them) in the TM flatslide range. The TMX35 and 38 modified D slide carbs have no needle jets. Mikuni have excellent casting and machining way superior to Delorto. Keihin are said to be even better. The good aftermarket flat slide Mikuni's to use on tuned Lambrettas are: TM 24, TM 30, TM 32, TM34 and in the fixed needle jet range: TMX 35, TMX 38 and the main jet-less TMS 38 which does all it's metering from special Jet Needles.
I switched from a Delorto VHSA oval bore flat slide to a Mikuni TMX 35 and got better fuel economy and performance from my TS1 and low engine speed running is much better. Set up was easy as well.
Amals are kind of "bronze age" in comparison.

The TMX 35 is remarkably compact for its size. You won't need to do anything but maybe file down the brass overflow pipe that points to the front of the carb. They are tall, though, so you will either have to tilt the carburetor to clear the tool box on a TS1 or with a cut out or Diablo fuel tank, you can stand it straight up providing you trim back the floorboard beneath it.

[soosh]

I dug my old 34 amal out a while ago and stuck it on my iron barrel 200 and was surprised at how responsive it was,i have used flatslide dellortos for a few years now but this still was good.
I didnt have a a 34mm manifold which was a shame as i bodged it on the 28/30 so didnt get the performance benefit but keen to see how the carb performed.

[dopeshots]

What about power jets do they make a difference over std carbs?

[roadrunnerian]

Hi
at the moment i have a tmx 35 on my ts1 have a slight problem with it hitting the engine mount or the tool box it has a mbd
inlet manifold been told to switch to lth inlet manifold will being doing this next month at the mean time may switch back to my amal mk2 34mm carb the tmx was well better on fuel.

[Yanker]

What about power jets do they make a difference over std carbs?

Allegedly they can in the right hands.

I have steered away from them for the following reasons (in addition to incopetence)

They are simply powered by vacuum in the venturi. To me this factor is too susceptible to other factors to provide reliably consistent fuel metering. Air pressure (also therefore temperature) slide opening, engine attitude (acceleration/cruising/deceleration), engine loading (up-hill/flat/down/hill) have put me off even going there. There is a formula for power-jet size (25% of 'normal' main size and decrease the main by the same ammount). Does this mean setting up the carb with the PJ blanked then re-adjusting?

Even so those factors have put me off them.

Imagine scenario, you are doing say 70Mph into slight headwind, powerjet is in play, you go into a tunnel and need to maintain 70, touch the brakes to do so, start draughting the 4 wheeler in front, close throttle some, powerjet drops fuel metering by 20% while you are at the same rpm and speed on a hot motor. This scenario also applies to non PJ carbed engines, but they have a bigger safety margin. (bigger main and the possibility to blip the throttle with more effect in deceleration if required). IMHO, for this reason ,PJs are probably better for acceleration and the circuit - but even current race scoots use a manually adjustable power jet as state of the art fine tuning over the few miles they do per meeting.

A good few years back an old club mate got his GP200 back from a tune + dyno-set-up. It went very well for what he wanted Original Italian GP200, S4 tuned, AF Clubman, Mikuni TM27 PJ. As it was already run in we had a look at the PJ function from a layman's terms: have a gandar revving it on the stand to feel the response: well at WOT the PJ wass cutting in and out with no other factor involved: presumably over fuelling, stuttering, gasping then overfuelling again. Ridden the PJ had no discernable effect, probably never cut-in, and was an over complication

I'd like to be proven wrong on this

Best carb? I'm 'graduating' to Keihin PWK for the responsiveness and fuel efficiency: 1 x 28mm and 1x 33mm, phasing out Dell PHBH30 and VHSB34

[nelson pk]

What about power jets do they make a difference over std carbs?

Allegedly they can in the right hands.

I have steered away from them for the following reasons (in addition to incopetence)

They are simply powered by vacuum in the venturi. To me this factor is too susceptible to other factors to provide reliably consistent fuel metering. Air pressure (also therefore temperature) slide opening, engine attitude (acceleration/cruising/deceleration), engine loading (up-hill/flat/down/hill) have put me off even going there. There is a formula for power-jet size (25% of 'normal' main size and decrease the main by the same ammount). Does this mean setting up the carb with the PJ blanked then re-adjusting?

Even so those factors have put me off them.

Imagine scenario, you are doing say 70Mph into slight headwind, powerjet is in play, you go into a tunnel and need to maintain 70, touch the brakes to do so, start draughting the 4 wheeler in front, close throttle some, powerjet drops fuel metering by 20% while you are at the same rpm and speed on a hot motor. This scenario also applies to non PJ carbed engines, but they have a bigger safety margin. (bigger main and the possibility to blip the throttle with more effect in deceleration if required). IMHO, for this reason ,PJs are probably better for acceleration and the circuit - but even current race scoots use a manually adjustable power jet as state of the art fine tuning over the few miles they do per meeting.

A good few years back an old club mate got his GP200 back from a tune + dyno-set-up. It went very well for what he wanted Original Italian GP200, S4 tuned, AF Clubman, Mikuni TM27 PJ. As it was already run in we had a look at the PJ function from a layman's terms: have a gandar revving it on the stand to feel the response: well at WOT the PJ wass cutting in and out with no other factor involved: presumably over fuelling, stuttering, gasping then overfuelling again. Ridden the PJ had no discernable effect, probably never cut-in, and was an over complication

I'd like to be proven wrong on this

Best carb? I'm 'graduating' to Keihin PWK for the responsiveness and fuel efficiency: 1 x 28mm and 1x 33mm, phasing out Dell PHBH30 and VHSB34

All jets are powered by the vacuum in the venturi! All jets are effected by temp change, altitude etc, the power jet is just another adjustment.

An adjustable power jet, set up the by the right person will give you more of a safety margin than standard jetting as it cuts in at just under half throttle to supplement the main and if you think its running a bit lean or hot you just turn the power jet up a bit rather than take your carb off, undo the float bowl, strip the jets and change.

A power jet doesn't really give you more "power" as such but you can lean the main jet off to give cleaner running lower down the rev range and then when you increase revs or the opening of the throttle the power jet cuts in to supplement the leaner main jet.