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900SS with FCR and High Comps - Written 06/11

Summary: Tuning a 900SL fitted with FCR41 carbs, JE high comp pistons, Ignitech TCIP4 and Staintune SL high mount mufflers.

This 900SS (SL actually) had had an engine rebuild elsewhere and had been fitted with JE high comp pistons due to the cost of original ring sets. It had been purchased by the current owner with the FCR41 fitted and was running an open airbox lid and Staintune high mount SL mufflers. I believe it had untouched heads and the cam timing was “as delivered”. Apparently it hadn’t run well since the rebuild or even that well before, so I went through the carbs to check them out and see what jetting it was running. Based on my experience with these carbs I made a few changes (all but one of which was off) and fitted some of the Exactfit ignition coils.

Jetting was as delivered – 155 mains, EMT needle on the 3^rd notch, 60 pilot jets and slow air screws at 1 ½ turns out, except for the idle mixture screws. They were both over 3 turns out, and one of its traits had been poor running down low and fouling plugs. I left the mains, lowered the needles 1 notch, fitted 52 pilot jets and wound the idle mixture screws in to the spec of ¾ turns out. I was told the ignition timing had been retarded 2 degrees from std to allow for the high comps, but when I checked it with a timing light it appeared to have been advanced 2 degrees. So to give more flexibility of adjustment (and save having to pull the alternator cover off) I fitted an Ignitech TCIP4 unit with a map I’d done previously for a 900 with high comps.

At that I headed for the dyno and found a few things wrong. It was obvious on the ride down that it was too lean on the pilots, and the dyno showed it was quite lean at WOT and leaner at part throttle. Plus it pinged under 6,000 rpm, that really nasty pinging that makes your skin crawl. So we didn’t do much WOT running. Surprisingly I couldn’t hear it pinging on the road, but from memory it was a windy day so that may have made it hard.

So back to work for a few changes. Mains up to 162, pilots back to 60 and needles back to the 3^rd notch. I replaced the fuel with some fresh 95 unleaded – I usually use 95 for tuning as it is a generally sufficient octane rating and is consistent across brands in how it works - the 98’s tend to be more variant in my experience. In hindsight maybe it needed 98 simply due to the compression.

I didn’t play with the timing at the dyno as my new laptop doesn’t have a serial port and typically I couldn’t find the driver disc for the USB – serial convertor I have. So for the next dyno session I took a spare Ignitech with another map to try.

The jetting revisions made it much nicer on the road, pulling much better from a roll on at 4,000 rpm especially. On the dyno it made more power and didn’t ping as badly as before. Fitting the other Ignitech helped somewhat, but there was still some under 5,000 rpm. Based on these results I went up to 165 mains, raised the needle from the 3^rd to the 4^th notch, wound the slow air screw in ¼ turn and left the pilot jets and mixture screws as was. I took some more timing out after riding and that took most of the pinging out, although it would still give a single tink when you hit it really hard under 6,000 rpm. Which is perhaps a little unrealistic in terms of likely use out on the road. The curves I ran are below. “Dyno 1” and “Dyno 2” are the curves used in the fourth graph down. In comparison to the TCIP4 unit I used on my 750M, this unit and another that I’ve sold recently don’t have any of the delay with increasing rpm spoken about in the 2V Ignition and 750M reports. They are later production version 75 and 80 units which require different versions of the software too. Spark plugs used were NGK DCPR8E (as always), with talk on the Ducati Monster forum recently highlighting the fact that the TCIP4 requires resistor plugs to avoid undue failure of the units.

So, to some dyno runs. The first runs are against road speed, as the rpm pick up seemed a little touchy leading to some gaps in the curves when plotted against engine rpm. Green is the first run with the 155 mains, blue is with 162 mains fitted and red with 162 mains and less ignition advance.

I’ll break the changes up so you can see them a bit more clearly. The next graph shows the difference between 155 and 162 mains. I’ve been surprised at how little difference each jet size makes to the air/fuel ratio with the FCRs.

The next graph shows the difference with less ignition advance – 3 degrees less at 3,000 rpm tapering to 1 degree less at 6,000 up to 8,000 rpm. The dip around 110 km/h is 5,500 rpm. Whether or not this dip would come out with less advance again I don’t know. I did take another two degrees out after the dyno runs were done based on the road testing, but I couldn’t justify the time to do more dyno testing.

The final graphs compare this bike to the 41 FCR equipped bike from the previous FCR report. Power first, then torque. The dip at 5,600 shows up in the torque curve as a hole in an otherwise smooth curve, which is a bit odd. Again, it may come back to the decision to run the 95 octane fuel instead of the 98. When I first ran the JE pistons in a 900 engine I’d take a 3mm cut off the top of the crown to lower the comp – dropping it from 11.5 to 10.8 on my measurements and calculations. I did a couple without that cut, one of them a 900SSie that pinged using the same set up as we’d run previously without any pinging. Maybe that little bit less is all they need. And it seems to be regardless of cam timing, as I have a customer with a 900M running the machined JE, FCR41, 106 cam timing and 3 degree retarded ignition timing on original ignition units without any issues.

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