Alternate Carb Carburetor Conversion- SU

[Carl]

Hi All,

I brought my first RE5M in around the late 1990’s and after several frustrating years trying to get the carburetor flat spot sorted/eliminated I converted it to an SU carburetor.

When I brought it, it was a non runner and I brought a variety of spares from SAM at RR (carby kit, water pump rebuild a few seals etc) and over a year or so managed to get it running overall reliably but I would get it running almost perfectly one week only to have the intermittent flat spot return a week or so later.

In 2003 I converted my RE5 to an SU carburetor and it remained on this RE5 until it was sold and replaced by a slightly better RE that I purchased around 2008 (both machines were original paint/chrome).

The SU was transferred over to the new bike where it has remained since then. My original bike was refitted with the original carburetor and sold to a friend (with the offer of assisting with an SU when he got tired of the endless carburetor/cable fiddling.

So I have been running an SU continuously on a RE5 for over 10 years, I still have an original carburetor so I can change it back to original if I want to.

The bike has been ridden extensively over the past 10 years and the only grief it has given me was a stripped wire within the headlight case on a two day event occasionally earthing and popping fuses, I couldn't locate it at the time but was quickly fixed the following week.

My bike is a rider and I often ride it using all of the revs through to orange/red line, most are impressed with it acceleration and cruising speed.

This is a link to a youtube video of my bike:



The conversion took quite some time but since then I have a bike setup that has never had a flat spot, I have never had to adjust the carburetor (other than the idle speed occasionally) and the bike runs extremely well, slightly lumpier on the overrun and less of a surge as the port valve opens (as it is pulling stronger leading up to this opening). Most importantly is that the bike is always ready and in tune for a ride with no preparation/pre-checking required.

Overall I believe that the bike runs stronger and accelerates quicker than it used to, several RE5’s owners have ridden my bike and they I believe would agree with this (Andy, Steve, Paul, Gordon??).

Anyway I have been asked a number of times how I converted my bike and I was recently contacted by 2 other RE5 owners wanting to also convert their bikes. With this in mind I thought that I would join this board and create a thread and cover every aspect of my conversion (as best as I can recall). There was a thread on another board covering a summary of my conversion but i have long since lost that link.

I will add and post at regular intervals and each post will document an additional stage of my conversion.

Apologies to the purists out there that might be offended by the concept of an SU on a Suzuki.

[Carl]

Stage 1 – Selecting a Carburetor.

Norton used SU’s on some of their Rotaries so I assumed that an SU could work.

I recall seeing a brief hand written/scanned document concerning an SU conversion (in the UK I think) and this was with a 1 ¾” carburetor, this got me started so I brought a second hand 1 ¾” SU carburetor and decided to fit it.

Once I got the carburetor home and I looked at it and it was huge so I decided to measure and compare the original combined throats to that of the SU.

I calculated that the 1 ¾” was going to be almost 50% bigger than the combined cross sectional area of the existing 2 throats. Based on this I looked at 1 ½” as an option.

Carburetor Bore Comparisons are summarized in the attachment



After considering the above I decided that the 1 ¾” was too big and that I would go with an 1 ½” that I subsequently brought (at 7.7% bigger cross sectional area).

At this stage one might argue that the SU being CV will sort out the required throat size and that the extra 50% could remain unused, however given that the throttle cables operate both the port valve and oil pump I wanted as similar travel on the cable to standard as possible so that oiling and port valve timing would remain close to standard.

Put another was if the 1 ¾” red lines at half/two thirds throttle then the oil pump will not be oiling at the correct ratio (under oiling) – the solution would be to adjust the pump fully open to the new red line part throttle position but then the oil pump would not return as far at idle and over oil at slow speeds.

Anyway I went for a 1 ½ and it has proven to be a good choice.

I will put together a post on the adapter plate over the next day or so.

Attachments:

[Carl]

Stage 2 – Making the Adapter Plate

I selected a piece of 12 mm thick aluminium.

I used a RE5 inlet spacer and an SU gasket to mark all 3 bore holes onto both sides of the plate, this gave me areas common to both the inlet bores and SU and areas that needed to be flowed from the 2 smaller openings into the larger opening.

It is important to make sure that the SU is mounted high enough so that a cap screw can pass below it to mount the plate on the engine. My first plate was about centered and this did not allow enough room for the cap screw, my second plate had the SU mounted a few mm higher and this was good.

I drilled/machined out the common areas and then used a porting tool to blend in the 2 different sides.

I drilled and tapped the upper two engine mounting points and all four SU mounting points the lower engine mounting point was drilled (no thread).

Two studs were mounted facing towards the engine for the upper 2 mounts, four studs facing away from the engine for the SU and the lower hole had the cap screw, the studs were loc-tited into place

I now had an SU that could be mounted on a plate that flowed as best as available space allowed from a 38mm hole into both a 18 and a 32mm hole.

I welded a cable support bracket to the side of the plate and attached the throttle cable, I just used the pull cable as the SU had a good return spring, the return cable is unused and cable tied underneath the tank. The throttle has proven to be both light and responsive.

The port valve and oil pump cables were left connected.

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[goandy]

Nice of you to post Carl. I can attest that these SU carby swaps make a world of difference to the bike. Responsive! Easy to set up and can actually look as if it were meant to be there from the factory. When I put the su on my bike I'll also chip in with any useful observations.

[Carl]

Stage 3 – More thoughts on the SU and selecting a needle

SU carburetors are beautifully simple, a single needle, single jet, a constant depression piston that controls mixture by reacting to vacuum of the engine. The piston rises (carrying the fixed needle) as vacuum increase thereby increasing both air and fuel, the piston movement is dampened by oil in an upper reservoir (dashpot).

Once setup there are only two adjustments for the idle mixture and speed.

The variables are:

Bore Size
Needle profile
Main Jet Height (sets idle mixture but as the needle is raised it effects the mixture over the whole range – so the needle needs to be correct for the application)
Oil damping viscosity
To a lesser extent piston springs can be changed.

So it’s all in the needle, you need the correct needle to get the best from a SU and then the only maintenance is idle speed and mixture adjustment occasionally.

1 ½” SU’s have several hundred different 0.100” needles available (smaller SU’s have several hundred different 0.090” needles), these needles are used on a wide variety of machines, so when there are so many differing profiles it becomes clear that this is a critical step to getting an SU running at its best.

For my conversion I opted for a thicker needle than I expected to use with the intention of polishing/tailoring the needle progressively from slow speed through to high speed with the use of an oxygen sensor to determine the correct mixture. This sounds like a lot of work and expense but it cost around $150 to setup the sensor and gauge and an afternoon to tune the needle.

By selecting a thicker needle the mixture starts lean and by polishing off a few thou at predetermined measuring points you richen up the mixture, as each speed range achieves the correct mixture the next point speed is tackled – that way you don’t run the bike fast/hard when it is too lean.

I tailored my needle to a good mixture for around 75% of the throttle travel and left the top end a little rich.

Future posts will detail the oxygen sensor setup, customising my needle and then my needle sizes and what needle most closely matches my custom needle from the standard SU range.

[wayne]

Brilliant. Thanks Carl. Looking forward to the rest.

[Carl]

Stage 4 - Connecting up the SU.

Attached is a photo showing a few connections.

Petrol - as I recall the original petrol intake was too short and pointing the wrong direction, I removed it and used copper tube oversized and sanded it on a lathe until it was very slightly oversize, the tube was bent and then put in a freezer and the carburetor body placed in an oven, the slightly shrunk tube was pushed into the expanded hole where it remains todate - make sure that the tube wall thickness remains thick/strong enough after it has been resized to fit the carburetor hole - may have to enlarge the hole to keep the wall thickness up.

Oil Injection - this is the banjo fitting that is behind the petrol inlet, there was a suitable recess in the carburetor body that had a face suitable for sealing and when drilled out and tapped provided an oil inlet to the float bowl above the petrol level. The banjo is one of Jess's new ones.

Piston Vent - the rubber tube above the petrol inlet is a vent that allows air to be vented/replaced behind the piston as it moves up and down - this has been extended and vented beneath the tank/seat - to keep water and crap out of the space. 

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[Carl]

Stage 5 – Oxygen Sensor & Mixture Gauge

Most commercially available fuel mixture sensors require calibration and as such are of little use for the SU. Back in 2003 I found a great Autospeed article using a self-calibrating digital voltmeter with a simulated analogue reading and a range of 0-1 volts.

Broadly speaking voltages from oxygen sensors at the correct temperature that are below 0.2 volt are lean and above 0.8 volt are rich, voltages from 0.25-0.75 volts are stoichiometrically correct. So with a 1 volt range the first quarter is lean the last rich and the middle two quarters sweet.

Note – that purists will make the point that elevation and temperature effects these readings slightly but for what I wanted to do the accuracy of the ¼, ½ and ¼ was fine. If you are trying to tune the last fraction of a horsepower out of a competition engine then you would be running it on the lean side and very close to too lean, our bikes we just want to run well and ensure that we are neither too rich nor lean.

At the time Autospeed were onselling the units and I brought my first from them (at a significant mark up), since then I have purchased a couple more direct from Lascar (as I then had the manufacturer and part number)
.
The original article is at the following link – I now note that they no longer resell the meter but now indicate the manufacturer and part number in the updated article.

www.autospeed.com.au/cms/A_0959/article.html

This is a link to the meter

www.lascarelectronics.com/temperaturedatalogger.php?PHPSESSID=ie2meuaun9ekbcup36l99h6s26&location=uk&datalogger=111

The meter runs with a 9 volt battery and I have mounted it in/on a self contained plastic box that is attached to the petrol flap with elastic bands and a foam backing. Two wires come out one to earth and the other to the centre of the oxygen sensor.

When riding the meter is easy to read with a slight glance down and can be seen in full sunlight.

The Oxygen Sensor was just a commercially available single/two wire sensor that I recall costing around $80 . This sensor is self-contained and it produces a voltage when at operating temperature. The other option is a 3 or 4 wire sensor that has a heater element in it and would require 12 volts to heat the element – I choose the 1 to 2 wire sensor and placed it where it would get hot quickly.

Placement of the Sensor – I drilled and tapped a hole in the upper rear of the exhaust manifold, this allowed me to screw the sensor in directly into the manifold and preheating was not an issue. When I had finished with the sensor I removed it and plugged the hole with a shortened bolt (so as not to protrude into the manifold).

Having had success with this measurement I have used the system on several other bikes and most times I grind a nut to suit the exhaust pipe radius and bronze weld the nut (over a drilled hole) to the pipe beneath the engine and out of sight, I can then either fit the sensor or plug the hole.

The 1 or 2 wire sensor needs to go in a header pipe as the muffle will most likely not get to a high enough temperature.

Some years ago I used this method on a 1927 BSA Flat Tank Side Valve 500cc that really should have had a top speed of around 80 km/h. It was totally standard but I picked the best options for the carburetor based on the oxygen sensor/volt meter, I was able to increase my countershaft sprocket from factory standard of 21 teeth to 24 teeth. On several occasions I rode the bike up to 100km/h and it was still pulling, it wasn’t over geared as it still climbed my test hill in top gear, vintage brakes and beaded edges tyres prevented me from pushing it any further, but other club members were always impressed with how good the bike went and the only explanation was the carburetor setup.

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[Carl]

Stage 5 – Oxygen Sensor & Mixture Gauge - part 2

I could only attach 3 photos to the last post so here are the other 3 photos that belong to the post.

The first photo shows my test setup on the bike and the second the location of the sensor in the exhaust manifold, my manifold had less fins and a little more space than this spare, anyway this is where the sensor went - I may have had a little more space, squeezed it in or filed off a couple of out of sight fins. The sensor was fitted to my first RE5 that was replaced some years later with my second RE5 and this one has the customised SU fitted but no sensor. 

The third photo shows where the sensor would be when fitted to a RE5, for some months I left the sensor in and removed the wires so I could on occasion go back and verify my readings, it was then removed and replaced with a plug, nobody ever noticed it unless it had wires going up to a tank mounted gauge.

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[pmcburney]

Fantastic stuff Carl!


I am really enjoying this, especially the depth of info related to the O2 sensor stuff... I'm going to make me one of those I reckon.

I don't suppose you'd be able to post up a pic of the adapter plate itself off the bike at some stage?
The description is very good, but a picture speaks a thousand words...

Looking forward to the next chapter!


Paul McB

[Carl]

Hi Paul,

No problem, nice to know that I am not wasting my time.

Anyway regarding pulling the adapter off. I spent 2 years or so in the early 2000's taking off fiddling and replacing the standard carburetor (with the hard to reach bolts/fiddly cable attachments designed for Japanese double jointed mechanics) trying to get mine to consistently run well, I took great pleasure in not having to remove or fiddle with the SU carburetor for the past 10 years, so its probably about time to remove and see hows things have been going.

The removal is three hoses, one cable and four relatively easy nuts to remove the carburetor and then remove 2 nuts (easier access without the carburetor getting in the way) and a cap screw for the adapter, which I will do sometime over the next week or so.

At the moment I am rebuilding a Suzuki GT750J as well as fettling an old 1925 Scott for a weekend run.

The next stage is customising the needle and this may take a few evenings to put together.



 

[wayne]

Quite interested in this next bit Carl.

When we dynoed 5 RE5's last year, it was noted that they all ran well rich of what you'd expect from a regular engine. And that went for the two perfect runners using standard jets as well. I notice that even in a bike without hesitation, a richer main jet seems to make the bike run better at cruise speeds.

A couple of Mazda mechanics I've heard from indicate that the cars also run richer than you'd expect.

[Carl]

Stage 6 – Customising the needle to suit the SU

I purchased a book called “How to Build and Power Tune SU Carburetors” by Des Hammill, its a short but very detailed book, 64 pages and cost me AU$29.95, I brought it (I think) from “The Pitstop Bookshop” in Perth. 

Anyway this book details the process of selecting a start needle, dividing and marking it into 16 sections of which 11 are used on a 1 ½ SU.

These sections are then transposed onto a dowel stuck into the top the piston and then I was able to use the dowel position to transpose the needle position onto the throttle twist grip (I have attached a photo of a mock up showing this concept - in reality the measurements would be marked out more accurately and there would have been 11 on them).

Most of this method is described in more detail on the link  

www.terryhunt.co.uk/mini/pics/tech/picsb/pics.htm

This then enabled me to hold the twist grip at progressive needle positions and read the mixture, if rich I would then remove the needle and hold a fine piece of wet and dry paper between my fingers while spinning the needle in a drill press.

I would measure the thickness of the needle at this position before sanding and then remove around 2 thou before retesting. It took very little abrasion to remove 2-3 thou.

To get to a SU needle you remove the piston by removing the cap (2 easy to reach screws) and lifting the piston out, removing the grub screw that holds the needle and then putting the needle in a split collar (that I made) and then polishing it in a drill press, afterwards refitting it and all up I reckon that the test ride, stop dismantle, polish and rebuild would have taken less than 5 minutes.

I tailored the entire 11 positions in one afternoon, most positions required more than one polish.

More to follow

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[Carl]

My start needle.

I started with an “AEA” start needle, at the time I would have had a reason for this but 10 years later I can't recall exactly why.

On checking my excel spread sheet I have noted a Norton needle being an “ADR” and the “AEA” is similar to an “ADR” but with a leaner top end (i.e. larger diameter in section 8 -11), this would have enabled me to start lean and tailor the needle in the upper end whereas had I started with the “ADR” and found it to be too rich I couldn't go back.

I have attached my records of my starting needle, Norton needle, my custom needle plus some other needles that I added after I had customised my needle.

It can be seen on the graph that my custom needle is richer than the Norton needle through most of the range (and a range where I measured mine to be correct), the top end on my and the Norton needle are very close but I know that mine is a little rich here (I went 1 extra polish after I hit stoichiometric to give myself a rich top end).

Given what I know now I would most likely start with an “ADR” Norton needle and expect to remove a few thou from sections 2 -7 and this would be a closer needle to start with.

Alternatively there may well be a better needle out of the extensive SU data base, I have included all my measurements to assist others if they want to trawl through the data base looking for a closer start needle.

I would not replace any carburettor or needle without an Oxygen sensor to determine the correct mixture across the range.

We have a rare and hot running engine that we definitely don't want to run too lean, just because it runs on a replacement carburetor doesn't mean it's right, check it with a sensor it will be heaps cheaper than a cooked engine.

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[goandy]

Thanks Carl. I shall be ordering that adr needle...