750SS IE with 900 Carb Cams

Some time ago I decided that the old carb model 900 cams would work well in the smaller engines. This was based on the timing specs I had for them at the time, that were given in the manuals from 906 to 900SS. However, after I tried them in the 600M I found some new specs that were quite a bit different, and not what I wanted to see. I had initially thought the inlet timing was 20/60, but it turned out to be 24/70. More duration, with earlier opening and later closing. This has a couple of effects. The earlier opening meant I would have less piston – valve clearance on overlap than I expected, leading to the cams being retarded from where I thought I could set them. The later closing shifts the torque and power peaks up the rev range, and can even lower the power peak on some of these engines.

So, this lead to a couple of things. One was these cams not giving an improvement in the 600M, secondly it reduced my enthusiasm to fit them to a 750. I was, however, still curious, although this curiosity will get the better of me some day.

We have a 750SS ie customer who was pretty keen to play a little. I ran this as a bit of a side project – he couldn’t afford to pay retail Moto rates for the job, but having access to a bike I’d never played with before was something I didn’t want to pass up. So I played with it in my own time, and he payed for the bits. A nice compromise. I still wasn’t sure if this was going to work, but I gave him the usual guarantee – if it doesn’t work, we’ll put it back to the way it was. He was happy with that, so off we went.

I’ll give the cam specs again in this report, although they are all the same as the 600M fiasco. I’ve included the 800 cam specs, just to show how they relate, and what I really wanted.

The problem with 750 engines – they have used the same part number pistons and cylinders and heads from at least ’91 onwards, maybe even earlier – is that there isn’t a huge amount of piston to valve clearance std. Whereas most models have at least 1mm or more over the 1.5mm "minimum", the 750, with the std cams set to std spec, only have about 0.5mm (about 2.0mm total). The 1.5mm minimum is a long time industry std. You can go lower - I have seen std "as assembled" 900 engines with well under 1mm, but when we have to guarantee a job, it’s best to stay on the conservative side. It’s no fun pulling heads to replace valves you’ve bent. Engines with Titanium rods can go under this, as the Ti rods really don’t stretch very much at all. Not this engine tho.

Given the 900 cams open the inlets 12 degrees before the 750 cams do, this means you use up all that piston – valve clearance and some more. So, I had to retard the cams from the original 900 spec of 113 degree inlet centreline, to 119 degrees, to get the required clearance. This put us behind the 8 ball from the start. I was also looking at the V2 210 grind as a possibility, but it would have to be retarded over 15 degrees (spec inlet centreline 105 degrees) to have any hope of working, plus it would possibly have too much maximum lift on the inlet for std 750 heads, causing rocker – valve stem seal interference problems. And I didn’t want to complicate things any further.

I did some baseline runs with the bike, just to see how it was going before I started playing. It wasn’t exactly std, and quite surprised me with the way it went. A ’01 model 750Sport (the "dark" SS), it had DP low mufflers, DP air filter kit (open lid, BMC filter), Ultimap (FIM) flashload and a 14t front sprocket. The owner had mentioned that it would lift the front wheel pretty easily, but I was a bit sceptical, frankly. I’d never ridden a 750 that was that enthusiastic. Turns out he wasn’t lying. Although the power of this bike wasn’t any better than they usually are, the mid range and response was much better, and it would indeed wheelie its heart out in first gear. I dyno’d it with both the Ultimap flashload (a Moto One customised version of the UMF031 map) and the DP ECU, as we had one lying around. Just for comparison. I had installed the DP ECU not long before I did the cam swap, just to see what the owner thought of the difference between the two. Overall, not that much he said, although it wasn’t quite as snappy down low with the DP ECU.

Below are a couple of graphs. First is a comparison between this bike (with the above listed mods) with Ultimap flashload in red, DP ECU in blue and another, all std 750SS ie in green. The all std bike is a ’99 model, so its slightly better power from 8,500 – 9,000 may have to do with the "as assembled" cam timing prior to the use of adjustable pullies std, as the cams were generally a bit retarded back then. Second is power and air/fuel curves for this bike. Red is Ultimap flashload, blue is DP ECU. The slightly leaner air/fuel of the DP ECU is why it makes a touch more power in the upper RPM. The fact the Ultimap flashload is a little richer up top is more than likely due to how Duane likes to set things up for best on-road performance.

Setting 1
Allowing for the compromises I was working under, I was losing confidence that this mod would be of use, but got the cams in and the bike running. It needed lots more fuel at idle – the idle trimmer was increased quite a bit for the right mixture, but the air bleeds also had to be wound out to get the idle high enough. Both of which I would expect to see when fitting bigger cams. Off I went to the dyno, and the first runs – with the same Ultimap flashload as before – gave the result below. 900 cams is red, 750 cams is blue. The power gain or loss corresponds pretty much to the volumetric efficiency, which is inversely proportional to the air/fuel mixture. Richer air/fuel is due to less volumetric efficiency, usually meaning less power.

Just like the 600m all over again. But, I have the bonus of total electronic tuning with this bike, so I went to work. The photo below shows the set up. The monitor on the left is for the dyno computer, the laptop is mine with the Ultimap custom mapping software.

As the air/fuel wasn’t too bad, I went for ignition advance changes first. I tried more and less advance, and got the result below. Where I started is green, with the final ignition advance settings is red. You can see that changing the ignition advance hasn’t changed the air/fuel at all, and the mixture is the same even though it is making more power.

Once I was happy with the ignition timing, I played with the fuel a little. There wasn’t much to be gained here either, but it did respond a little. Red is the final setting, green the best before the fuel changes (after the ignition timing changes). The air/fuel curve is a little flatter, and tapering to richer at the top end.

The final comparison from before to after is shown below. Power and air/fuel first, then torque. Before with std cams is green, after with 900 cams and tuning is red.

So it certainly wasn’t what I was hoping for, and not what I’d seen other people claim. Although, some of the other curves I had seen were for bikes with other mods, and this is where some other details come into play. If the bike was fitted with high comp pistons, the result would probably have been better. Most high comp pistons also have deeper valve reliefs as a std feature, allowing for high lift cams to be fitted without extra machining required. I know of one person who had fitted 900 cams without even checking the timing, so he may have been quite lucky he had the high comp pistons as well, as he may have had piston – valve problems with std pistons. Because I like to check every detail, the details often catch me out, whereas some people don’t bother and get away with it just fine (that never works for me).

The owner came to get the bike, and I told him that when he had given me some feedback, I’d show him the dyno curves. Just because I didn’t want the dyno results to influence what he felt when he rode the bike. This can be quite important – I know of results for other people where the owner is very happy with a tuning job, until someone shows them a dyno curve, and says there’s something wrong with it. Duane has had it a few times, and has been crucified by dyno operators who really don’t know what they’re doing. I told our owner to ride the bike around, and come back with his feedback.

He came back a couple of days later and said it was very nice. Not a huge improvement, but very good in the mid range – smooth and responsive – although about the same at the top end. So I showed him the WOT dyno graph, which disagreed with his opinion totally. Which is just the way it goes sometimes, clearly illustrating the dyno's importance as a development tool, not as an overall yardstick. Even after seeing the graph, he was quite happy to pay the $500 for the cams and other bits used, so was obviously happy enough with the job. It needs to be remembered the dyno graph is WOT only, and often a cam swap to bigger cams can have quite pronounced improvements in mid throttle areas that are not so expected.

Also, not all the tuning was WOT. I did quite a few part throttle air/fuel runs before the cam swap, and ran the same runs afterwards to compare the mixture changes. This is where an indication of the mid throttle improvement showed. The first graph show air/fuel before and after for 10 degrees throttle and 25 degrees throttle. The runs don’t cover the full RPM range, as at 10 degrees throttle the engine would only accelerate to just on 6,000 RPM. At 5 degrees throttle, it would only accelerate to 4,000 RPM. This is due to the power output and throttling effects – and represent fairly accurately what happens on the road.

For the graph below, 10 degrees throttle air/fuel before is green, after is red. 25 degrees before is blue, after is yellow. As the after mixture is richer at 10 degrees throttle, the volumetric efficiency is obviously lower, so it wouldn’t be quite as strong as before. It wasn’t noticeable when riding the bike, but I would expect this from longer duration cams with a later inlet closing. The 25 degrees throttle before and after are much closer together, with the after now a little leaner. The variance isn’t big enough for me to make much claim here (you need to allow a little for dynamic variations and systematic error here), but I would be happy to say the volumetric efficiency was about the same.

Next is before and after air/fuel for 5 degrees and 50 degrees throttle. As you would expect, the volumetric efficiency after the cam swap at 5 degrees is less, so the mixture is richer. What I didn’t expect was the leaning of the mixture at 50 degrees throttle. Here the longer 900 cams were working better as low as 4,000 RPM, so the improvement the owner felt in roll on performance in the mid range I understood from this result. This echo’s a comment the owner of the Cagiva 900 Gran Canyon had made when we replaced the "750" cams it came with std with those from the 900SS ie (longer again than the 900 carb cams). He felt the power came on lower down after that cam swap, even though, on the WOT dyno graph, there was clearly no improvement until well up in the RPM range.

I used the results of the after air/fuel part throttle runs to modify the part throttle mapping quite a bit, and I was very pleased with the result. Cam swap aside, it was very nice to ride once the tuning had been done. Which bought me to the next question – was the result the owner felt due to the cam swap and tuning, or would I have achieved a similar result with dialing the original 750 cams and the same style of tuning. Dunno. It’s a question that is starting to annoy me a little now, and I would need to play with a std cammed 750 to solve it. Unfortunately, someone has to pay for the dyno time, so it’s not quite that simple.

Overall, the result wasn’t very good on paper, but the owner was quite happy. The main issue was the piston to valve clearance (or lack there of) and the cam timing settings it dictated.

There was a solution to this however, one I found after the dyno runs. This bike had an oil leak. Not a simple one either. It had a leak just behind the starter motor from between the crankcase halves. And the only way to fix that is to pull the engine and split it. Luckily, the warranty on this bike still had a couple of weeks to run, so we submitted a request and gained approval to fix the leak. Not a very hard job, just one that required much disassembly. And, it does involve removing the cylinder and pistons, giving me the opportunity to have the inlet valve reliefs machined a little deeper to give us more piston – valve clearance. Cool. Some may consider my motives somewhat sneaky (admittedly true), but an oil leak is a legitimate warranty claim. And if we can fix this sort of stuff under warranty, it makes for much happier customers. All of which gave rise to cam setting 2.

Setting 2
With the valve reliefs machined 1.5mm deeper (there’s plenty of meat in these pistons to allow this) I could then advance the cams quite a bit to close the inlet valve earlier, which I expected to help considerably in the cam swap vindicating stakes. The std spec inlet centerline of the 900 carb cams is 113 degrees. Setting 1 was at 119 degrees, or 6 degrees retarded. Setting 2 was at 107 degrees, or 6 degrees advanced. 12 degrees change in cam position should have a fairly noticeable effect, and so it did here. We first gave the bike back to the owner with the same tuning as it had after the initial cam swap, as I didn’t have time to get to the dyno and our man needed his bike back. Although the idle trimmer was again increased quite a bit, so it was catching more air at idle, making it richer to some extent overall. Even in this state, he came in the next day to let us know how much better it went. A good thing, to be sure. He was quite amazed at the difference 12 degrees of cam timing could have (12 degrees is a long way), and was very happy with the result. It apparently wheelied exceptionally well now.

When I did get to take the bike to the dyno, the improvement was quite clear. The graphs below show the initial, std 750 cam run in green, the "setting 1" 900 cam final run in blue, and the "setting 2" 900 cam initial run in red. This initial red run is with the same mapping as the blue run, simply a cam timing change. The fact the engine has been apart should not have any effect in the power dept. I did reshim the crank bearing preload a little looser, but I’m not convinced this would really show up. Power is first, then torque. The small loss at the top end is due to the cam timing, but the difference is small, and I’ll take a couple less over 8,000 RPM for a mid range improvement like we got any day.

I did a full remap pretty much at this dyno session, the first time I had seriously tried using the air/fuel readings to do part throttle and second cylinder offset mapping. As expected, I had some unexpected dramas, from both equipment and procedure. But you always get that first time round. I have a couple of things to make before I have another go and a quicker way of doing it, so next time it will be far less frustrating and much cheaper. Given I was paying for the tuning (I’ve been subsidising this job, and I’ve spent quite a few hundred dollars of my own in dyno time), I was the one forking out for my "suck it and see" problems, which is fair enough.

I did lots of runs at part throttle, making changes to the maps and re-doing. I didn’t get as much done as I wanted, as I had a dead line (not a bad thing) and ran out of time. But I was happy with what I got, and so was the owner. The full remapping wasn’t so much about power, as about smoothness and fuel economy. The little things that ultimately mean more, in normal use. I did spend a few hours after I got back from the dyno analyzing the results (written out on sheets of paper for comparisons) that lead to another custom map that I’ll get the owner to try when he’s had the current one for a while. Just to see if it helps the smoothness and economy any more.

Which brings us to the end of this report. While not a big improvement on paper, the owner is more than happy with the way the bike rides. He is quite surprised by the minimal change in dyno terms, given how much better the bike feels on the road. Especially in response and general ride ability. The bike feels faster now in that not quite so obvious way we often get with these sorts of jobs, and is very smooth.

Whether or not the job is worth paying for is another matter really. Having to machine the valve reliefs in the pistons makes this a far more expensive job than just a cam swap, which is expensive enough to start with. Adding engine removal/splitting/reassembly/refitting to the cam swap job really blows it out.

If you were fitting hi-comp pistons, the machining wouldn’t be needed I’d expect (depends on the actual piston) due to the way hi-comps are usually machined as delivered. And the labour to remove and refit the engine would be compensated for the greater increase in output that the hi-comps will give. If you took the time to fit bigger valves while you had the engine out and apart, the result would be better again. I’m not sure if you can fit the 900 sized 43/38mm valves on the original 750 valve seats or not, but 42.5mm inlets must work fine, as that’s what DP sells (used to?) for the 750. With bigger valves, you’re looking at an engine more in line with the 800. And the combination of bigger valves, higher compression and 900 cams would really work well then. Too expensive for most to bother with no doubt, but that’s the reality of the job.

Curiosity satisfied for me at least.

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