750M Brad’s 600 Gets A Bigger Motor and Played With - Written 03/08

Summary: 750 carburettor 2V motor, base lined with slip on mufflers, jet kit and open airbox lid.  Dyno runs for jetting changes, bigger valves, 900 carburettor model cams and high compression pistons.

Special thanks to JD Hord for supplying (at no charge) the Ferracci high compression pistons used in this project and Bruce Meyers for some specific carb info.

So I finally got the 750 motor into Minnie, and then over many months played with it.  Not that I did too much playing, due to the usual family time constraints – half of what you’ll read about here was done in two days.  Although there was lots of other time spent learning about the Tech Edge Lambda Controller / Data Logger and Ignitech ignition control unit using Minnie as the test subject.

The engine is a 750 carburettor motor, which spec and most parts wise is the same as the 750ie motor anyway.  Mine was bought second hand with approx 80,000km on it I believe.  Someone had been inside it before and it was a bit messy and had some crank float.  So I replaced all the bearings, rod bolts, etc in the bottom end and reassembled it (I won’t say properly because I’m not that delusional about my own ability) with the original pistons and rings and didn’t do much to the heads.  Might have checked the valve clearances, I forget now.

I did check the squish though, and with the std 0.3mm base gaskets it was just over 1.0mm.  So nothing to be gained there.

Given that I knew from the outset my time would be limited, base line spec was cam timing @ 114 degree inlet C/L, Megacycle slip on mufflers (local ones that work very well on the twins), Dynojet jet kit (I had a couple I bought very cheap) and DP open air filter kit (came with the jet kits).  Initially I didn’t tie the bike down on the dyno, figuring a 750 2V couldn’t cause too many wheel spin issues.  However, as the second set of runs seemed a bit off to me, it now gets tied down for consistencies sake.

For jetting I oscillated between 40 and 42.5 pilots, needle on the third or fourth notch and 132.5 or 135 mains.  It had new needle jets, being Y-0 size, or 13240041E from a Cagiva Elephant 900.  They were very cheap.  750SS models had Y-6 needle jets (13240041B) and 750M had either Y-2 (13240041A) or Y-4 (13240041C I think), depending on year.  Y-0 is smallest, Y-6 largest, not sure exactly how much difference there is.

It had a bit of an off idle flat spot with 40 pilot jets and the third notch combo when cold, but was nice when warm.  Raising the needle to the fourth notch fixed the flat spot, but dropped the fuel economy from 190 km before the light came on to 160 km.  Although it only got 160km when it was a 600 anyway.  42.5 pilot jets were a bit rich down low, even with the needle on the third notch.  Mains I think 135 was the best option, although I didn’t try 137.5 which may have given me what I wanted to see air/fuel ratio wise.  See the bit below in blue text too.

The results of the dyno runs are offset a bit by the fact I ride this bike with standard mufflers, as I’m getting old and don’t like the noise any more and don’t want to annoy the neighbours.  The changes you get for various mods when running the slip ons can be somewhat muted by the std mufflers (especially the mid range), so the before and after feel from me may not really confer with what the dyno charts show.  It’s certainly a lot more responsive with the slip ons fitted, but the noise coming from the open airbox lid means that it’s not too quiet when riding with the std mufflers.  Sounds kind of cool when you wind it open.

First up we’ll compare it to a similar spec, but lower mileage 750M we did a job on recently.  It had a Factory Pro kit, which we’d normally use, but otherwise the same cam timing, air filter and main jet size.  Mine, in green, is a bit lower, but not too bad.  Power first, then torque and air/fuel.  One thing that does surprise me is the variation in shape of the air/fuel curves, as they both should be very much the same from 6,000 or so RPM up at least.

A side issue here, as pointed out by Bruce Meyers: apart from jets, needles and springs, there are small, but important differences between the Mikuni BSDT38 carbs that come on the various capacity engines.  The main system air bleeds in the carb inlets are larger on larger motors, and there are some other passages that vary in size.  Bruce pointed out that the variation in my WOT air/fuel could be reduced by increasing the size of the main system air bleed.  I never got to try it (getting drills of small sizes and increments to drill them out is rather hard here in Australia).  So keep that in mind if you’re fitting a bigger motor and jetting doesn’t work like you thought it might.

The next graph is three different jetting combinations for my bike, with the graph against road speed as I’m a bit suss of the RPM trace accuracy for the first couple.  Green is Dynojet 750 “stage 2” (the whole “stage X” thing just makes me laugh) kit with the needles on the third notch and Mikuni 132.5 main jets (Dynojet’s wacky sizes just shit me).  Red is with the needle raised one notch to the fourth notch and no other change.  Blue is with the needle back to the third notch, the Dynojet springs replaced with Factory Pro springs and a Mikuni 135 man jet.  This is what I settled on, due to both result and laziness.  95km/h is about 5,500 RPM.

The reason I fitted the Factory Pro springs is that when logged on the road the mixture would go quite lean after a gear change as the throttle was snapped open again.  Using a spring rate calculator I worked out the Factory Pro springs are about 33% stiffer than the Dynojet ones – 29 N/m compared to 21 N/m.  Original springs are around 93 N/m if my calcs are correct – you can see how much softer the jet kit springs are.  Realistically, reducing the vacuum bleed holes in the bottom of the slides may have been a better way to go about solving this (it’s really at least partly a damping issue, not a spring rate issue), but I just didn’t get enough time to play with that side of it suitably to make a decent experiment of it.

Also bear in mind that I didn’t actually test to see if fitting the Factory Pro springs made any difference – I got distracted by other stuff, as usual.

The graph below shows this, which is a section cut out of the TEWBlog software display we use for the Tech Edge units.  Green is the air/fuel ratio as sampled by the Tech Edge 3A2 unit with the wide band sensor in the header pipe to the muffler just after the stamped cross over, yellow is the RPM and red is the manifold vacuum (which is pretty much zero at WOT).  This log was with the needle on the fourth notch from memory and 132.5 main jets.

As you can see, in the on road data logs the WOT mixture takes a real swing from rich to lean (comparatively) from 5,100 to 5,600 RPM, so the dyno trace lag is as expected here.  I did do a couple of dyno runs with the air filter removed and the tank up at the end of one session just so I could see how the slides acted and when the slides were fully open.

About 5,300 RPM or so was the answer, which corresponds to the change in jetting quite neatly.  It’s quite surprising how much fuel comes up the inlets into the airbox when you open the throttle, how much the slides vibrate up and down and how much fuel actually comes out of the jet and down the carb – it looks like a lot.  All up an interesting thing to do.

Another thing I tried was a Ducati Performance 2 into 1 system, one of the old Gio.Ca.Moto ones.  Sold as a 600M system, the pipe is around 43mm with the collector around 48mm from memory.  It’s actually a very nice looking header set, but one that’s just too big for a 600.  I figured it’d be too big for a 750 too, and my dyno test confirmed it.  Blue is std headers and the ever trusty Megacycle mufflers, red is DP 2-1.  Might be nice on a 900 or 944 though.

Around this point I also tried something else I’d wanted to do for a long time – find out a bit more about how the ignition timing works.  I had been told that the lump on the flywheel defined the full and idle ignition advance – the leading edge giving the full, the trailing edge the idle.  This would be rather useful on bikes with high comp pistons, as you could adjust the maximum advance by modifying the leading edge of the lump without changing the idle advance and without having to move the pick ups.  Marvin Jensen tried this experiment and said it worked, but another fellow in the UK also tried it and said it didn’t, so I was keen to try it myself.

To check this I removed the flywheel and filed down the leading edge of the trigger lump, as seen below.  The red line is where the lump edge was originally, the blue line where I filed it back to and the yellow line is the trailing edge.

The amount I filed off was calculated to be about 5 degrees.  This is worked out by measuring the length on the original lump, dividing that by the calculated circumference of the flywheel at the diameter of the lump top (not the flywheel outer diameter, although it’s only a small difference) and then multiplying it by 360 to get a factor for mm of lump per degree.

So I refitted the flywheel and checked the advance with a timing light and the full advance was about 5 degrees back from the full advance mark on the flywheel, whereas previously it had been on the full advance mark.  The idle advance had not changed, which was what I expected and hoped would happen and what Marvin had said.  So, this is a simple and easy way of changing the maximum advance.

Although, I’ve realised now there was something I didn’t expect here.  The Ducati spec for these motors has always been 6 degrees before top dead centre (BTDC) idle advance and 32 degrees BTDC max advance, meaning the lump on the flywheel corresponds to 26 degrees of rotation.  When I was marking out some more advance marks on my original 750 flywheel late in the piece I found the dot corresponding to full advance was at 36 degrees, not 32.  The idle advance dot was still at 6 degrees, as expected.

I also checked a Ducati Performance aluminium flywheel for the earlier 2 phase alternator bikes and the full advance dot on it was also 36 degrees.  On that particular flywheel the idle advance dot was at 6 degrees, as checked against the degree wheel when I was resetting the cam timing.  But when I checked the ignition timing with a timing light the idle advance was around 3 degrees BTDC.  So I would assume on those flywheels the lump is 33 degrees long.

All of which meant that the modified lump on my flywheel was still giving 31 degrees at full advance, not the 27 I imagined.  Although, the runs I did with this flywheel fitted showed no loss of power due to the 5 degrees less advance, nor did it appear to affect the fuel economy.  The graph below shows the initial run with this motor and the final “before big valves” run with the modified flywheel and 5 degree less maximum advance.  As you can see, no difference.

I should also point out here that I have also been playing with the Ignitech http://www.ignitech.cz/english/aindex.htm  Sparker TCIP4 ignition unit on this bike, so there was a bit of to-ing and fro-ing with ignition control units and flywheels that might confuse some.  Check out this unit – it’s the way to go if you want to get into the ignition for high comps or the like.  And for what it is it’s amazingly cheap.  There’s a whole lot of info (15 pages at writing this) about it here - http://www.ducatimonster.org/smf/index.php?topic=93255.0  I’ll condense a lot of it into a report very soon, as I think I’ve learnt all there is to know (that applies to these engines anyway) that we need to be aware of.

I asked Bruce Meyers if it was worth fitting the 900 carb cams to std 750 heads.  His reply was that it depended on the castings and if I sent him some photos he’d give me his opinion.  Basically a typically thorough and experienced Bruce long version of probably no.  So I didn’t bother with this step, as it just seemed like time wasted.

Next up I fitted some new 750 heads I had with some bigger valves – DP 42.5mm inlet valves and DP replacement 38mm exhaust valves.  I had the exhaust valves cut down to 37mm and the heads ported to suit the bigger valves.  The work done was quite minor, but was basically the same as that done to my 94Hp ST2 heads (same company), concentrating more on finish texture, port shape transition smoothness, guide boss and short turn than major changes to the ports.

I shimmed the heads up for both 750 and 900 cams on the bench to save time later, then fitted them to the bike with the std 750 cams first.  Cam timing was set to 114 inlet C/L, the same as the previous setting.  Chamber volume I didn’t measure, but it didn’t look too much different to me, if at all.   Off to the dyno we went and got the result as below.  Green is before, red after.  Power first, then torque and air/fuel.  Not much change in any area really.  Pity.  The only side issue here is whether or not the 5 degrees of ignition advance missing from my modified flywheel would cost any power.  I don’t know.

Next I fitted the 900 cams – these are the cams fitted to all ’90 to ‘99 carburettor model 900SS and 900M, as well as the 906 and 907 models.  These have more duration and more lift than the 750 cams.  Interestingly enough, the valve event end points – exhaust opening and inlet closing – are in about the same places.  It’s the later exhaust closing and earlier inlet opening points that give the extra duration, along with it more overlap.  Doug Lofgren sent me a cam doctor chart of this some time ago which shows what I’m talking about very clearly, but I seem to have lost it somewhere.  The table below gives some more info on the specs and settings used.


Inlet C/L

Inlet timing

Exh. timing


Inlet lift

Exh. lift


119 (spec)














113 (spec)




















When I fitted the 900 cams I set them to the 119 degree inlet C/L that I found was about the most advanced I could set them with std pistons and still have 1.5mm piston to valve clearance on the last 750 I put 900 cams into – the 750SS ie.  I then drilled a hole in the Vee Two adjustable pullies so I could use a drill to realign that setting, and repeated the process for 107 degree inlet C/L setting.  As I’d had the inlet valve reliefs in the pistons modified when the engine was apart, I could run 107 without any issues.  Well, I assumed I could, I didn’t bother actually checking, and it’s been to 9,000 RPM so many times since I figure there’s no piston to valve clearance issues.

The “drilling the holes in the pulley” thing would have given a much better result had I plugged the round holes Vee Two drill between the slots first.  As it was I had to find the small solid bits between the holes and slots and it got a bit messy with holes drilled that ended up finding a hole or slot behind the outer washer piece.  But apart from aesthetics, the result was as functional as I required.  If you have no idea what I’m talking about get a Vee Two pulley and remove the screws and front plate, then you’ll see the holes and slots and what I’m talking about.  Or not.

So I rode to the dyno without belt covers and ran with the preset 107 degree inlet C/L setting, then cracked the pulley half screws, lined up the 119 degree holes with a drill bit, tightened the half screws and away we went again.  Very easy.

The result for 107 inlet C/L is shown below, as the blue line added to the previous graphs.  Another fairly minor result.  It is a bit leaner overall, so maybe going up one notch with the needle and one size larger on the main might be a good idea.  Maybe.  Also interesting to note that we haven’t changed the peak power RPM, so we know it’s not heads or cams that are defining that.

The difference between 107 and 119 inlet C/L is shown by the next graphs.  Green is 107, red is 119.  As you can see, there’s no reason to run 119 if you don’t have to.

The next step (time line wise) was to advance the pick ups to get more maximum advance, given I was still running the modified flywheel with retarded maximum advance.  Actually, somewhere around this time I fitted a 600 engine flywheel I had.  The old two phase alternator 600 engines use a stamped steel flywheel that weighs 690 grams, as compared to the 750 and 900 machined flywheel that weighs 1890 grams.  A very cheap light flywheel, shown in the photo below.  You need to drill and tap two extra holes in the 600 flywheel to take the extra bolts the 750 and 900 flywheel has, but that’s easy enough.

However, this is partly why I discovered the 32 and 36 degree flywheel lump thing.  The 600 lump is 26 degrees long, as is stamped on the flywheel, so in actual fact this gave about 1 degree more full advance than my modified 750 flywheel.  I expect the 600 flywheel has 26 degrees instead of 30 as the 600 engine is a bored and stroked 400, and even with dishes in the flat top pistons still have around 11:1 compression.  The 750 and 900 in comparison have around 9.2:1 compression.

Also at this point I was also running the Ignitech box and looking for more maximum advance for other reasons, but if I had been running the original control units I believe the next result is the result I would have achieved when the 900 cams went in.  This result was also complicated by another characteristic of the Ignitech box I discovered later.  So the difference between the green and red curves on the next graph is 4 degrees of ignition advance only, the red curve having more.  The runs were also done on a different day, but I don’t see that as being an issue due to the general consistency this bike shows on the dyno.

If we go back up 5 graphs to the one with the addition of the 900 cams at 107 centreline, and then add the yellow curve (run 080, the red line from the above graph) we get a better idea of what the change of bigger valves and cams can bring, and in this light it’s quite good.  Green is the starting point from the beginning, red is bigger valves and blue is bigger valves, 900 cams and 5 degrees less ignition advance.  We’re up over 7Hp (not much in number terms, but over 10% for this engine) and haven’t increased the maximum power point – it’s still 7,500 to 7700 RPM.

Next was the final step – high compression pistons.  The pistons I used were Ferracci 12:1, provided by JD Hord after he pulled them out of his 750 in favour of a big bore kit.  As delivered these pistons have a sharp ridged area around each valve relief, which looked to me like a good way to generate hot spots and detonation.  I removed these ridges and smoothed (to some extent, I was in a hurry) the crowns to give the shape as below.  Ideally I would have polished them, but it’s the old time thing again.  The photo shows the side view of these pistons as compared to the std piston – there’s a rather large lump on the high comp jobbie.

Advertised as giving 12:1 compression, with my mods these pistons may have been around 11.5:1 to 11.8:1 as a guess.  I did measure the lump and a quick calculation indicated they would be in the range of 12:1 anyway.  They were also lighter – 408 grams complete as compared to 460 grams for the std pistons, a difference of 52 grams or 11%.  Many people will say this would lead to engine destroying vibration, but if you could pick a difference I wouldn’t believe you – I certainly couldn’t.  It’s as smooth as it ever was.

So I nailed them in with new rings, drained the std unleaded from the tank (91 octane in our system) and filled it up with the 5 cents or so more expensive 95 octane, our old style premium unleaded.  Which, for a tight arse like me, really hurts.  There are also 98 and 100 octane unleaded fuels available, but I tend not to use them because some are denser or contain some ethanol or other stuff like that which can lead to variations in performance based on the fuel alone.  The 95 seems to be much more uniform in it’s make up.  I wasn’t sure if 95 would be enough with 12:1 comp, but figured I’d give it a go and save myself the 4 or 5 cents a litre extra that 98 or 100 octane costs.  Turned out to be OK.  I have toyed with trying 91, but haven’t as yet.

I set the cam timing to 113 inlet centreline before heading to the dyno so I could try that, 107 and 119 if I felt like it.  I didn’t drill any more quick set holes in my pullies for 113 – there wasn’t enough unmolested pulley space left to do that.  In the end I ran 113 and 107 and left it at that – the result was as expected and I couldn’t be bothered doing any more.  I also ended up running the maximum ignition advance I had available too – so far I haven’t heard it ping once, which does surprise me given the nasty results we’ve had with 900 motors and high comp pistons.  Maybe the 4mm smaller bore and smaller chamber helps there.

The next graphs show the improvement the high comp pistons gave.  Power first, then torque and air/fuel.  I was quite happy with the result.  I did change the jetting a little too when the high comps went in.  I raised the needle the equivalent of half a notch by replacing the std 0.5mm steel washer that sits under the big nylon washer under the needle clip with a washer 1.0mm thick.  I also went up 1 size on the main jet from 135 to 137.5.  Minor changes only, but noticeable on the air/fuel curve and more where I’d like it to be.  I don’t expect it made any difference to dyno power.  Green is std compression, red high compression

The next graph shows the difference between 113 and 107 cam timing.  113 is green, 107 is red.  As expected.  I was thinking 119 might make more top end, but didn’t see the point trying it as it would have less midrange again and that’s what I use most of.

So that’s the end of the 750 2V adventure pretty much.  All in all I was fairly happy with the result.  I thought the bigger valves may have had more effect, but there’s a possible side issue there that you often get when fitting bigger valves without opening the combustion chamber around them.  Opening the chamber costs compression, but not opening the chamber shrouds the valves more and reduces low lift flow.  Adding the higher lift, longer duration 900 cams would have overcome this and also added lift and duration (sounds like the same thing, but it’s a two fold effect as such) to generally lift the performance.  The valve sizes of 42.5/37mm are close to the 800’s 43/38mm valve sizes, but the 800 also has 900 sized chambers, which are more open and, even with the bigger valves, less shrouded as std than the 750 chambers are with the bigger valves fitted.

The next graph shows the starting point – jet kit, open air box lid and slip on mufflers in green and final - jet kit, open air box lid, slip on mufflers, bigger valves, 900 cams and high comp pistons in red.  With the steps along the way taken out, it looks pretty good.  The peak torque and power RPM have risen 500 to 1,000 RPM, but there’s more of both everywhere and it’s by no means peaky in its power delivery.  It has over 20% more power at the peak, and certainly goes much harder on the road when the throttle is opened at almost any RPM.

Next we’ll add the steps along the way.  Green is jet kit, open air box lid and slip on mufflers.  Red is with bigger valves added.  Blue is with 900 cams and some more ignition advance.  Yellow is with the high comps.

Speaking of 800, here’s a comparison with a few.  Green is my 750 in its final state, red is a S2R 800 and blue an 800M.  The 800s have more capacity, shorter inlet tracts and less compression, so overall it’s a pretty good result.

We’ll also compare it to a big valved carb model 900 (in this case 900SS) which also makes about 75 Hp, albeit in “std with slip ons” guise.  You can see the difference the capacity makes, and also the quite different top end characteristics.  The 900 hits a wall at 7,500 RPM, the 750 holds pretty well to 9,000 RPM.  On the road, once they’ve both reached peak power RPM in first gear, they’d be fairly even in acceleration.  Probably matched by the fact my 750 with these mods would cost as much or more than the equivalent 900 with slip ons.  But you get that.

I did some more dyno runs playing with the Ignitech box and std mufflers, because that’s how I ride it and I can tell the difference in power delivery.  But I wasn’t expecting it to be as much as the dyno showed.  The midrange hole is shown better on the torque graph, as is the strong torque rise from 5,000 to 6,700 RPM and fall away after 6,800 RPM.  Pretty much how it feels on the road, and certainly peakier than it is with slip on mufflers fitted.  It actually feels a bit slower with slip ons I think (I ride it that way very infrequently), just because it’s much more linear in the delivery.  Power first, then torque, slip ons in green, std mufflers in red.

I was thinking my next engine would be this motor stroked with a narrow 900SS crank to 827cc, but now I’m thinking I’ll sell this one (if anyone will buy it) and build the 600 motor I have in bits up with the 900 crank to give 674cc, 748 rods so I can rev it and big valved 750 heads modified to suit the 600’s smaller bore so it can make use of the revs.  Not because I think it might be a good idea, more to end up with less parts kicking around pretty much – I have almost everything I need.  And given my riding skill and ability is deteriorating due to lack of use these days, I can’t use big engines any more.  They actually scare me, and not in the way I enjoy.

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