BB1 SUPERMONO - Updated 11/06

Oh yeah. One of those jobs where someone comes up to me and says "there’s a guy out the front who wants you to hot up his……".

I must preface this by saying I have previously been involved with a Bimota Supermono. It was one of the least pleasant bikes I have ridden, mainly due to the vibration. It just didn’t feel right either. I had had to rebuild the engine, and was then trying to solve a problem generated by someone’s misinformation, and hated every minute of it.

Anyway, after a little while I started to look forward to playing with this little bike. The engine is from BMW’s Funduro, an altogether enjoyable little 650 single road/trail thingy that we have sold quite a few of. Just a little slow. As with any Bimota, any work more complicated than turning on the key is a real hassle, and involves dealing with the inevitable results of a small manufacturer making something as small and light as they can.

Taking this expectation on board, I went ahead when it arrived. A recent private import by a fellow who had transferred from New Zealand to Australia, it is his play thing, next to his 916 SP. We found a local custom exhaust man ( Neville at Moorabin Motorcycle Engineering ) who removed the std muffler and made up a new dual muffler set up incorporating the rear bodywork mounting points. The owner picked up the bike from Neville, stopped in at Dynobike for a dyno run and then dropped the bike off to me. He thought it was much better in terms of feel and response, as you would expect. It also had the expected "brap brap" noise that a loud single makes.

Pulling it all apart, I found it wasn’t really that hard to get to the carbs. I later found I could reconnect the electrical components board and pull it out of the way, allowing me to play with the carbs with the engine running. Not that I could get to anything worth adjusting, just get it running.

When Antony had dropped it off, I noticed the bike wouldn’t idle at all. It was throttle open and run, or close and stop. Checking the CO I found it was very lean at idle. Imagine my joy then, when I found the jet kit contained some larger pilot jets. I fitted the pilots, needles and springs and the smallest main jets and went for a ride. The mains weren’t wrong enough to make the engine breakdown, although it did feel a little flatter than it had before the jet kit went in. The needle setting was satisfactory too. The idle, however, was way weird.

Back to the workshop, where I began the experimenting. First step was to get the idle happening. I had to wind the mixture screws out quite a long way to get a result. This was exaggerated by the fact that to change the screw setting I had to remove the carbs and tip them up. This made the usual mixture screw in-out impossible, so it was extended trial and error. After a few different settings, I had something to go out on the road with. Next was the needle, dropped a notch to see what would happen. The mains went up by 2 sizes and off we went again.

This was repeated until all the needle notches had been tried and one chosen, the idle experimented with in and out and the mains for the dyno settled on. Off we went to the dyno. I did find that the needle setting I went with gave the best rideability, although it was a little more responsive on the lowest notch. Compromise once again. I did entertain the thought of rubbing the needle down at the top, but decided against it for the common good.

The dyno chart for the final setup ( jet kit, open pipe ) is shown below in red, with the std jetting and open pipe in blue and original in green. Quite a nice improvement in the midrange, and certainly a much nicer bike to ride around. I was amazed just how nice it was to ride in the end. Even with the open pipe it was quite quiet on cruise, rather smooth and very responsive.

Dynochart courtesy of DYNOBIKE (03) 9553 0018

Now our man Antony needs to decide how much further he wants to go. From the torque curve below, you can see a steady fall beginning at 6,000 RPM. To make any real top end gains, the shape of the torque curve will have to be changed. This is usually the expensive part. Airbox mods or removal may lead to a overall lifting of the curve to some degree, but the top end is not going to be as easy or cheap to find. His original aim of 10 Hp may also be revised now he has some response and rideability.

Dynochart courtesy of DYNOBIKE (03) 9553 0018


The original bike I had been involved with, which Moto Italiano had imported for compliance back in ’96, had since been sold a couple of times and ended up in the hands of a BEARS racer.  He had modified it considerably, including having JE make him some high comp pistons.  From memory, he had to order 4 for JE to do them.  Antony bought one of these and had us fit it.  An engine out job, as you can’t even get the head off with the engine in the chassis.

The piston was very high comp.  I’m sure 14:1 had been mentioned to me at some point.  Antony also had found a German company named Team Pami who has a long history racing these bikes.  You can buy from them hot up and big bore kits of various stages along with head work, that sort of stuff.  From them Antony had bought a pair of Mikuni 36mm flat slide carbs.  These carbs did away with the airbox also, being supplied with two uneven length carbon fibre bell mouths.  The jetting and bell mouths are based on what Team Pami have found works, so I was happy to leave it at that.

So we fitted the piston and carbs and got it all running and Antony took it to the dyno for a run.  As this bike is track only, and I’ve never even seen it with its current revised fairing, I haven’t ridden it or had the chance to assess the tuning.  But Antony thought it ran fine and didn’t ping.

Unfortunately, it came back a couple of track days later, having spat an oil hose off the dry sump tank and covered much of the bike in oil.  Not what you want when you’re tracking.  It was assumed I’d left a hose clamp undone and I had no proof otherwise (as you don’t) so I wore it and we got it going again.  Next track day out it did the same thing again, although this time it also expanded the dry sump oil tank enough to pop the battery out of its recess and deform the tank.  At this point I figured it had an engine pressurizing issue so we went looking.

Antony also bought in one of the plugs it had been running to show me which was grey and in his opinion running ok, although to me it looked like it was covered in melted piston.  One of the plugs we removed (the outer one) had the ground electrode melted off as well, so I wasn’t that surprised when the leak down test showed lots.  Removing the engine and pulling the head revealed the piston had lost the top ring land on the inlet side, down to the valve reliefs.  There was no sign of any of the missing piston, and only one spot of head damage, where the remaining end of ring land on one side had lifted and hit it.  It had been run on 95 unleaded (our mid octane unleaded at the servo) so I really wasn’t that surprised given the alleged compression.

For a replacement piston Antony bought one from Team Pami, giving 12:1.  This had a much smaller dome than the previously fitted JE, making me much happier.  We also had to replace one inlet valve which had started to recess quite noticeably.  So we did all that and got it back together and headed off to the dyno to see what was going on.  At this point Antony was quite understandably becoming paranoid about the whole oil spewing out when riding thing, and I was becoming quite paranoid in general.

When Antony first bought the Mikuni carbs from Team Pami he also ordered a vacuum fuel pump, not unlike the pumps we see on Ducati Monsters.  I wasn’t too sure how it would go sucking fuel out of the under slung tank, but Gottfried said they’d never had any problems (while winning many Supermono championships) so I was happy with that.  I’d fitted a one way valve to the pickup in the tank to help with getting the system primed, and there didn’t appear to be any problems first time out when it was dyno’d with the carbs and piston fitted.

But this time we wanted to check the fuel pressure, so we fitted a gauge to the carb inlet and went to the dyno.  This generated a problem, as the pump would only pump fuel if it had a closed circuit on the supply side, something I experienced originally when I fitted it.  It wouldn’t pump fuel out of the tank with the fuel hose off at the carb.  Hook the hose up though, and it was soon happily supplying fuel.  I figured the gauge was giving the pump enough open volume to effectively kill the vacuum it needed so we dropped that idea.  So it would fill the float bowls slowly, and be fine for the first run, but the second run would suck the bowls dry about half way through and it’ll fall in a heap.

We tried opening the fuel cap to see if there was a problem there, but we still had the same thing.  So then we tried an auxiliary fuel tank above the carbs, which worked just fine.  At this point we assumed we had some sort of pump issue, but given it was OK initially – the bike had never had an obvious misfire causing fuel issue while melting the piston - it was all getting a bit confusing.

To try to solve it we fitted an electric pump, this one from a carbed ZX9.  A little pump that generated pressure and then stopped running, it worked a treat and gave us 1.5 psi at the carb from memory.  With this, the running out of fuel on the dyno issue went away and we got a nice succession of runs.

Another problem we’d experienced as soon as the JE high comp went in was quite a lot of oil coming out the engine breather.  Originally just a small tube on the front of the right hand (alternator) engine cover, it pushed oil out almost as soon as the engine was started.  We dealt with that using a catch bottle initially, then Antony had a very nice alloy breather tank/box made up that sat behind the carb inlets and also gave a back and floor of sorts to the area behind the carbs, creating an airbox of sorts in conjunction with the tailpiece at the sides and electrics plate above.

When we replaced the piston we fitted another breather outlet to the center of the alternator cover, where there is a removable plug allowing you to turn the crank.  This carries a M18 x 1.5 threaded fitting, with an internal hole of maybe 12mm.  We ran half inch or so tubing from this to a junction block with the original breather outlet and then up to the breather tank behind the carbs.

This larger tubing gives around twice the ID of the original, meaning 4 times the internal area and much less likely to be blocked by breathed oil which is then pushed out by the air behind it.  And with the main outlet maybe 10mm from the centre of the spinning alternator there would also be a centrifuge windage effect of sorts inside the cover, helping to keep oil out.  It certainly worked – we didn’t see any oil in the new (clear) tubing at all, even at the alternator cover outlet.  Keeping in mind this engine changes its crankcase volume by 650cc every 180 degrees of rotation, you can see how important a good breather system is when it’s revving to 8,000 RPM regularly.

The graphs below show the improvement from std piston to both high compression pistons.  Green is piped and jet kitted from the original report. Red is the JE very high comp piston, blue the Team Pami 12:1 piston.  As you can see, the piston, Mikuni flat slides and lack of an airbox have added some midrange with quite a lot more top end.  The dip around 4,000 RPM is just the Mikuni flat slides getting themselves together after the throttle is pinned.  Power then torque.

If you go back up to the last paragraph of the original report above, you’ll see I thought the aim of an extra 10 Hp was unrealistic.  At this point, the gain is about 8, which surprised me.  But one thing I noticed on the dyno was how the engine revved over 7,000 RPM.  We were keeping to a max of 8,000 RPM as best as possible, but the engine would slow a bit from maybe 6,500 to 7,300 then rev very quickly from 7,500 up.  If you look at the above curve you’ll see the power is still climbing at 8,000 RPM.  Not a great deal, but it is still heading up.

Based on this, if we wanted to keep to the 8,000 RPM red line I’d like to try advancing the inlet cams.  With the flat power curve above 7,500 RPM I think there’d be some more midrange for the taking without affecting the max power output.  Maybe we’ll get to try it one day, hopefully without having to pull the engine out again.

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