Gregory Bender

Carburetor conversion

Moto Guzzi Quota 1000 and Quota 1100 ES models

Created:

Updated:

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Dave Keeling's conversion

Thanks to Dave Keeling who sent me this information about how he converted is Quota 1100 from Electronic Fuel Injection to a Carburetor. In Dave's own words:

After putting up with the poor running of the Quota engine, due to the simple fuel injection system and reading on the Quota forum that the prototype was developed with a Weber carb, I decided to try it.

I strobed the rotor, removing the various inputs to the ECU, they appeared to have no effect on the advance curve. The only one I couldn't check was the T.P.I. of which I could set later if necessary.

The throttle body was removed and an ex Ford 3L V6 38 DGAS Weber was offered up. The first problem was the auto choke mechanism; it made the carb too wide to fit through the frame rails.

I removed the choke body, but retained the back plate, fitting a cable operated arm to the original linkage. The carb now fitted the manifold.

The next step was to make new throttle and choke cables. I used bicycle rear brake cables cut down as a temporary measure. The choke and throttle linkage now operate from the handlebar controls.

The next step was to jet the carb. One concern was the forward inclination of the carb on the manifold. I considered modifying the manifold, but decided to keep it standard in case the carb conversion failed.

To get around the problem, I raised the float level by 2 mm so that the fuel level at the emulsion tubes was to original spec.

I then ran the bike changing jets as necessary. The biggest problem was the accelerator pump jet was causing too much fuel to be delivered causing a flat spot. I reduced the jet size, but it was still causing a problem, so I removed the linkage and it ran much better.

The next mod was to fit the airbox to the carb. I made a glassfibre box, which was riveted and glued to the airbox after the lower section had been removed. This then bolted to the top of the carb. Now the induction system was complete, it was back to the final setting of the carb.

With the carb set, the electric fuel taps were removed and replaced with manual ones, the pressure regulator was removed and the hole in the tank blanked off, as were the injectors.

All the excess wiring was retained but the plugs were capped and clipped out of the way. The fuel pump and filter were removed and will be replaced by a storage tray.

The bike is now much smoother to ride, it pulls well from 2,500 RPM with no hesitation at 3,500 RPM and revs all the way to the red line in the lower gears. It doesn't pink on overtakes and the fuel consumption seems reasonable, although not checked on a long run.

The initial jet sizes for the carb were:

  • Fuel level raised 2 mm
  • Main jets 150
  • Air corrector: 200
  • Idle jet: 50
  • Emulsion tube: F50 modified
  • Accelerator pump removed and blanked off

I made some further modifications and the final settings are:

  • Main jet: 140
  • Air corrector: 200
  • Idle jet: 52
  • Emulsion tube: F50 with lower holes blanked
  • Float level: 10 mm from top of float to body approx 3.5 mm higher than std, to counteract the forward inclination of the manifold

The performance is good and engine is so much smoother with no steps in the power and fuel consumption works out at mid to low 40's MPG (imperial) with the bike mainly used two up with luggage.

Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.
Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.

Photo courtesy of Dave Keeling.

Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.
Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.

Photo courtesy of Dave Keeling.

Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.
Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.

Photo courtesy of Dave Keeling.

Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.
Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.

Photo courtesy of Dave Keeling.

Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.
Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.

Photo courtesy of Dave Keeling.

Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.
Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.

Photo courtesy of Dave Keeling.

Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.
Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.

Photo courtesy of Dave Keeling.

Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.
Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.

Photo courtesy of Dave Keeling.

Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.
Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.

Photo courtesy of Dave Keeling.

Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.
Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.

Photo courtesy of Dave Keeling.

Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.
Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.

Photo courtesy of Dave Keeling.

Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.
Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.

Photo courtesy of Dave Keeling.

An update from Dave:

After replacing the original exhaust collector box with a Y piece, the lack of accelerator pump on the carb was more apparent, so I reduced the cam size that operates the arm that operates the accelerator pump diaphragm to roughly half of it's original size, therefore delivering less fuel...and it works well.

Another update from Dave regarding his second conversion:

I used a pair of LH Suzuki DR800 carbs. They are 33 mm bore, I measured the inlet tract in the head, 35 mm, and looked for something around that size without spending too much, but wasn't successful and as I had some spare DR 750/800 carbs, I used those to experiment with, so much so that I am pleased with their performance and for me, there is no need to go larger...35 mm would be ideal, but space is very tight on the 1100 ES, not wanting to cut the sidepanels, and keep the bike looking original.

Economy wise, we did a long trip last Summer from England to Croatia to the Euro Quota Treffen and back over the Alps and the bike averaged between 48/52 MPG (imperial) over 2,844 miles, fully loaded, two up.

Installing Mikuni Constant Velocity carburetors on a Moto Guzzi Quota 1100 ES.
Installing Mikuni Constant Velocity carburetors on a Moto Guzzi Quota 1100 ES.

Photo courtesy of Dave Keeling.

Installing Mikuni Constant Velocity carburetors on a Moto Guzzi Quota 1100 ES.
Installing Mikuni Constant Velocity carburetors on a Moto Guzzi Quota 1100 ES.

Photo courtesy of Dave Keeling.

John Sartorius's conversion (based on Dave Keeling's original conversion)

Thanks to John Sartorius who posted this information on the Yahoo! Guzzi_Quota news group. In John's own words:

After many months of frustration, I am pleased to report success (finally...I think...) in the Weber downdraft carb swap battle, to include keeping a working accelerator pump on the bike.

Final settings and comments/pitfalls:

  • 150 main jets
  • 200 air corrector jets
  • idle jets are now approximately 60 (approximately because 55s were the largest I had -- had to drill them out after blocking off the auxiliary accelerator pump -- see below)
  • emulsion tubes are F50s, modified as per Dave Keeling's bike.
  • Auxiliary accelerator pump is blocked off.
  • Main accelerator pump is still in the circuit.
  • Fuel level is set at about 36 mm
Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.
Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.

Photo courtesy of John Sartorius.

There were two issues on my bike that made it impossible to get the jetting correct. The first problem has been discussed in some detail by Dave Keeling, and that is the over-supply of fuel due to the accelerator pump. The problem is not just the pump itself -- it's the fact that in high-vacuum situations (such as chopping the throttle closed from high RPMs), raw fuel is drawn through the accelerator pump circuit and dumped into the intake manifolds.

I tried Dave's solution a few months ago and removed the pump and blocked the circuits completely. This solved the richness problem, but then I couldn't whack on the throttle at low RPM like I could with the FI -- the engine would stumble. Rolling it on smoothly was no problem, but I wanted the quick response that would come w/a squirt of raw fuel from a functional accelerator pump -- I just didn't want the over-rich condition.

Working with a local shop, we hit on the idea of bypassing only the auxiliary pump (and blocking off its circuit/fuel passageways), while leaving the main pump and circuit in place.

Perfection!

Well, almost perfection. Now the bike wouldn't idle. And I couldn't figure out why, as it had idled fine previously.

After riding the bike a bit (40-50 miles), and then pulling everything back apart, the problem was clear. Although the Weber carb is a straight bolt-on to the stock FI throttle body manifold, don't let that fool you into thinking the carb gaskets from the stock Guzzi FI application can also be reliably used.

It turns out the FI base gasket originally used on the throttle body for the fuel injection is too small (and too thin) for the carburetor base. This permitted a major vacuum leak to develop. I have a homemade thick gasket there now, but have ordered a much thicker automotive one from Redline Weber, made specifically for the 38/38 DGMS carb, and will replace the homemade one with that one next time I have the carb off the bike.

Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.
Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.

Photo courtesy of John Sartorius.

Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.
Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.

Photo courtesy of John Sartorius.

Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.
Installing a Weber carburetor on a Moto Guzzi Quota 1100 ES.

Photo courtesy of John Sartorius.

Now the bike idles and runs well. The accelerator pump is still working, so it snaps off idle very well.

Some additional comments. You'll note my final jetting settings are a bit different than Dave Keeling's. I suspect each bike will be somewhat different here; but here are some possible explanations:

  • Dave's carb was a converted 38/38 DGAS from a UK Ford (if memory serves); mine was a 38/38 DGMS ordered from Redline Weber in the US. In theory, that shouldn't matter, but who knows what undocumented differences there might have been between the two carb bodies, passageways, etc.
  • I have a modified airbox (Fast by Ferracci open top) and a Mistral exhaust. Dave's are both stock, if I understand correctly, so my bike would be expected to want a slightly richer mixture.
  • Also, in the UK (until recently, at least), they use proper gasoline -- not the 10% or 15% alcohol mix swill now sold in almost all stateside pumps. I am certain this must have some affect on the jetting.
  • I have a fairly large diameter balance tube between the two intake manifolds (tapped in where the injectors used to bolt on). Note sure what affect this has (or doesn't have) on the overall jetting, but it smoothed out the idle considerably.

I am going to run a couple tanks of gas through the bike this Saturday and Sunday on Skyline Drive and the Blue Ridge Packway, and will let everyone know about gas mileage when I get back. But as of now, I can say honestly that it is far superior (from a smoothness and throttle response at low RPM standpoint) than it ever was with the FI.

Another update from John.

I have now run two tanks of gas through the bike since deciding I am done fiddling with the jetting. The average was 43 MPG. Not fantastic, but better than the FI normally gave me (40 MPG was good when the FI was on; 37-38 MPG was the norm).

A bit more detail on the final (for now) jetting settings:

  • 150 mains
  • 200 air correction jets
  • The 55 idle jets were drilled to .024 (should be about the size of 60s)
  • Emulsion tubes are F50 modified:
    • The original air bleeds were .035
    • The current air bleeds are now .055
  • The idle mixture screws are about 34s to 1 turn out
  • The float level is 36 mm, measured from the tip of the float to the gasket surface, with the carb top inverted, NOT COMPRESSING THE SPRING
  • The auxiliary accelerator pump is disconnected & the vacuum signal has been blocked off