One thing about the 851 that has annoyed me for many years is the way it idles - best described as crap. As a 1989 model it has the early soft closing springs - the springs used to (try to) keep the valves closed as per #5 the picture below. They're not valve springs in the typical sense, just a helper.
If you run the engine without them, they tend not to idle very well. Once the revs are up it's not an issue, but at idle too much cylinder pressure leaks past the valve seats. In use that translates to a lot of misfiring and oxygen and hydrocarbons in the exhaust mixture, which makes it unpleasant to be behind. Even with the soft "Corsa" springs as this bike had they're not a lot better.
I had hoped that adjusting the valve clearances would fix the issue. When I first purchased the bike I had given it a big service, adjusting the valve clearances and replacing the original stainless steel collets with steel ones. The new collets tend to settle (wear) into the shims and valves, so you always need to adjust again once that has happened. Over the 6,000km I had ridden it in the 3 or so years I had it on the road it had become slowly worse, and parking it for 13 years was the most simple solution.
During the "back on the road" service recently I had found a lot of clearance - most around the 0.20mm range - and 3 pairs of broken collets. Once adjusted though, it wasn't a lot better, still having a fairly ropey idle with a dirty mixture - lots of O2 and HC to really get up your nose. And that was probably the thing that was annoying me the most now - being embarrassed at how horrible it would have been to be around the bike while sitting idling at traffic lights, etc.
I recall servicing an early 851 years ago at Moto One, pulling the closing clearances down nice and tight and it was nice and clean at idle, so was hoping for a similar result again. Not to be, unfortunately. To give you an idea, this is what a dirty mixture looks like on the gas analyser. 5000ppm HC and 5% O2 makes for runny eyes.
I had come to the conclusion the fix was the heavier closing springs fitted from 1991 models onwards. Confirmation of this came from the early ST3, which had a recall involving replacing the exhaust rocker closing spring with a much heavier part. The very soft inlet springs stayed as they were, and it took them from having a mixture like the above to the expected clean.
All the later model 4V Strada cam bikes tend to idle very nicely, and I could see no reason why my 851 should be any worse. It had good leakdown and cranking compression, both of which would indicate satisfactory general health. And there wasn't a great deal of wobble in the valve guides.
Ideally, I would have pulled the cylinders and heads and fitted the new pair of cylinders and pistons I have, along with 748 heads modified as per the 853 kit procedure and fitted with the heavy springs. Would have been an absolute treat. But, I'm not really interested in that sort of thing these days - really, I just want a bike to be able to ride, and the 851 was the only complete, club reg eligible bike I had available at the time. The time investment required is more than I have available to spend.
The only other option was to swap the closing springs with the heads in place. I wasn't looking forward to it. There's a French fellow named Patrick who did his years ago and has been encouraging me to do the same via the 851 forum, but I was still reluctant. Until the last time I rode it and decided I'd had enough.
So I dug the set of springs I've had for 15 or so years out of the tub and put the bike on the bench. Everything externally removable has to come out of the heads, as below. That's the easy bit.
You can see the difference in the springs clearly in a side by side photo. The original are 2.0mm wire diameter with 7 coils. The replacements are 2.7mm wire diameter with 5 coils. Using the spring rate calculator I usually use for fork springs (I think it'd work for rotating coils) and guessing the coil diameter at 15mm give rates of 1.0 and 5.5 kg/mm. Not sure that I believe that, but the spring rate is proportional to the wire diameter to the power of 4 and inversely to the number of coils. Thicker wire and less coils both give a higher rate. In terms of how hard it is to push the rocker down, there's a significant difference. Let's go with that.
The worst part of the replacement procedure, as it turns out, was getting the springs and rockers back into place and the pins in. The closing rockers and pins are at the very outer corners of the above photo, and getting in there with limited access - vertical exhaust and both sides of the horizontal with the radiator still in place (frankly, I've had the f#*%ing radiator out of this thing so many times in the last few months that it was not coming out again. Funny the jobs you absolutely hate) and with everything coated in Motul 300V it was as slippery as the proverbial two eels in a bucket of snot. I resorted to zip tying the springs in and then poking the rockers in. There was a fair amount of cussing - you work blue with this kind of job.
I started on the vertical, as I figured the exhausts would be the worst. As a clarification as to why this is a crap job, the tail of the spring has to go under a lug cast into the inside of the head. In the above photo, the lug is between the two closing rocker pins - the tails go under one from each side. The spring tail is behind the zip tie, so you have to pull it down maybe 70 degrees. It's not a fun job, even when the head is on the bench. But my home made tool can work an absolute treat.
Speaking of my home made tool, I used it last week on an ST4S head, the frustration of which reminded me after the fact that I was using it wrong. You'd think I'd remember how to use my own tool. Ideally, this tool works best from the opposite side - you work the exhaust spring tail under the lug from the inlet side. There's no access from the vertical exhaust side, so I popped the inlet spring in first from the inlet side then tried the exhaust. The first one went in so easily it could only give me a false sense of success. Working around the radiator and thermo fan on the horizontal sucked. But, overall, it wasn't as bad as I was fearing.
Then it was just a case of putting everything back in, and then everything back on (replaced what looked like a leaky timing shaft seal too) and it was time to fire it up and see what happened. It was a satisfying pile of debris once done.
When I had the plugs out I noticed the vertical was whiter than the horizontal, so I adjusted the throttle linkage rose joints out of alignment to close the vertical throttle blade a little. I couldn't be bothered doing it properly, it's such a pain access wise with it assembled to a run-able state. As I hoped, it was idling higher. At least that was reassuring, the valves were holding compression better. I didn't replace the eprom first as I had planned, and to my surprise it was a lot leaner than it had been previously. I guess the holding compression relates directly to trapping more air. I wound the idle trimmer on the ecu a couple of turns counter clockwise to richen the mixture, and we went from around 1% CO to 4% CO. It wasn't a lot cleaner mixture wise than it had been previously - it's in the 2500 - 3000ppm HC and 2.5 - 3.5% O2 range around 4% CO, but it idled so much lower and smoother that it was clearly better.
For an eprom I compared the 009B based eprom I was running with the 035 eprom from the 1992 851. The 89 - 91 bikes all used the 009/009B - the B refers to a software change that Duane told me related to how the engine was "turned off", which was done to improve the life of the starter clutches. It was done well before my time, and I've never found a service bulletin relating to it.
The 1991 model used the heavy closing springs with the 009B eprom, but I don't recall how they ran. Don't know if I've ever ridden a 91 now that I think about it.
Comparing the 009B to the 035 gives the % differences as below. Negative numbers mean 035 is leaner. Blue along the bottom is RPM, red up the side is throttle opening in degrees. P7 ecu starts at 0 degrees, which is the lowest TPS output voltage it sees every time the key is turned on. It's not actual throttle opening, just opening from closed. The implication here is that if you change the closed throttle position on the idle stop you change the fueling at low throttle openings. Can be a frustrating system, as the only way to adjust idle speed is with the idle stops as the P7 throttle bodies have no air bleeds.
The 035 eprom is a lot leaner at low throttle than the 009B, and that is the direction I was heading with the eprom I had been running which was my mapping from when I was using an Ultimap UM011 based eprom dropped over the 009B base software with some changes to the environmental trims. The 009B is also richer on the air temp trims, so in use it will be richer than the above comparison indicates. I leaned those trims off and reduced the range of them in the eprom I'm running now.
Comparing my eprom to the 035 is as below. The temp trims on them are much closer. I made the lower throttle lines the same as the 035 pretty much, and the 035 idle fuel was spot on with the idle trimmer set to the mid point - you remove the top of the ecu and measure the voltage at the trim pot outputs and set them to 2.5V, which is midway between 0 and 5 (oddly enough). I like to start at the mid point if I can.
The larger positive differences at the top up to 7,000 rpm are for the revised cam timing - closing the inlets earlier traps more air and needs more fuel. Although after the torque peak, you need less fuel as the volumetric efficiency is dropping faster. So 8,000 rpm and up it's getting less fuel. I have the rev limiter set to just under 10,000 rpm from memory, so the point at 10,500 is redundant.
The -100 on the bottom line is simply due to me turning the fuel off on overrun. You just make the map value 0.
After all this I put it back together and went for a ride. It was pretty hot here today - 37 degrees - and that may influence how it ran a touch. Possibly influenced how it started too. When I turned it on the temp gauge moved off the stop even though the engine was "cold". I have made the coolant trims at 5 and 17 degrees coolant temp richer from memory, but once running it doesn't need more enrichment as it warms up through the 29 and 41 degree points. But they do like more fuel for the initial start, and the ecu from 1.6M onwards have a starting enrichment table as well as a coolant trim table, where they have a decaying enrichment that works on rotations since started.
It almost gave me the impression it was a little off a few times, but the more I rode it the harder it was to find any proof of that. It had a point at 3,500 rpm just off closed throttle that may be too lean, as it shuts down a little there at times, but it's not really an issue. It really is a lot nicer at low throttle openings, which was the point of this.
I was going to check the idle mixture after riding it in case it'd cleaned up a bit more due to valves seating better with a run, but forgot.
It did cut out a few times as I pulled the clutch in coming to a stop. Usually that's ignition timing based, but this eprom has modifed spark map breaks that allow more advance at 1000 rpm to try to stop that, and a plateau from 1100 to 1500 that gives you good control of the idle speed with the advance at those points being the same as the original eprom value - 6 degrees from memory. But I may put some more advance in - it makes them idle nicer if you can add advance without losing control of the idle speed. That part is annoying as I've never had this bike do that before, and it's another reason to have to remove the seat base to get to the ecu. Usually more than once. I'm so over replacing eproms - flashing ecu is so much easier.