Friday, October 5, 2012

Minnie gets some FCRs (aka, a day full of arrrgh)

So I bought a pair of FCR39 on ebay some time ago, and they've sat on a shelf ever since.  I found some time today to fit them and plough my way through the usual comedy of errors that arise whenever I'm trying to do something in a hurry.

I believe the carbs had come off a 750M, but the main jets were surprisingly big at 165 mains.  Otherwise they appeared to be as they are delivered, with the 60 pilots and EMT needles on the third notch.  I dropped the mains to my pair of ever accomodating 152 based on some advice from JD Hord along with 52 pilots, raised the needles to the 4th notch, set the idle mixture screws to 1 1/2 turns out and the slow air screws to 1 turn out.  I didn't bother checking the main air jets, and I would expect them to be the spec 200 as no one ever bothers changing them.  Although I do believe that Bruce Meyers recommends going bigger.  More on that below.

After a phone call to Tony (he of the short rubbers mod) I decided to route the throttle cables between the RHF indicator mounting block and dash mount over the headlight and down the LH side under the diagonal frame brace.  Which took the usual amount of routing and re-routing and induced crankiness.  "arrrrgh" moment #1.

I hogged out the airbox around the throttle cable mounting area and was proceding quite well until as I was about to refit the airbox I noticed fuel dripping from the inlet cross tube.  The second of my "arrrgh" moments, this required the replacement of the o-ring on the end of the tube.  Sounds easy enough, but this involves separating the carbs, meaning you have to undo the RH slide control from the throttle shaft to allow the RH carb to slide off the shaft.  No problemo there, but putting it back on involves refitting the balancing adjuster.  Frankly, I don't have any understanding of how this adjuster works.  It makes no sense at all to me.  I ended up using 2mm drill bits to set the slides theoretically evenly (or as close to is as the infuriating "moves as you tighten" system allows), but then set up some longer leads to allow me to run the engine with the airbox and battery out to check the balance.  As it happens, it was fine.  No idea why given the confusion, but there has to be a little gold in every bucket of poo to keep you going.

As an aside here, I did go looking for some info about this, and the best info you'll find for FCR tuning is Patrick Burn's FCR tuning guide:,Pat.html with now being a good time to mention it.  Read it and take note.

But I couldnt find it yesterday when I was fitting them up, which is a pity as I had the "synchronizing" (as the Americans like to call it) procedure totally wrong.  Getting it totally wrong leads to statements like "or as close to is as the infuriating "moves as you tighten" system allows", but I got there eventually.  As Patrick says, it does take 5 times as long as you expect.  And given the slide caps are covered by the airbox, you can't reassemble the bike then adjust without disassembly.  And that sucks.

Splitting the carbs also let me work out why the slides would jam at half open.  Seems the little wheels on the RH slide weren't pushed all the way on.  A good hard push to snap them back on and a quick clean and the jamming was gone.

I did forget to reattach the accelerator pump transfer hose from LH to RH carbs, but as it turned out that didn't influence the pumps (not) working.  Depending on the pump circuit design, if the circuit downstream of the pump isn't sealed the pump can't push and suck like it's meant to and nothing hapens.  The float bowl formed rubber gaskets had both been sitting incorrectly around the little pump circuit internal transfer passage, being pushed out into the float chamber.  I did manage to work them back into (soft of) shape and I assume they stayed where I wanted them when I jammed the float bowls back on.  But the pumps still didn't work and the ride to the dyno and dyno runs didn't improve things.  It's quite amazing how much they really don't need the pumps.  You can't snap the throttle open under 5,000 rpm, but it's not as bad as you might expect.  I plan to look at this issue in the future.  If it annoys me.  And I can be arsed.

But she started with the first try (an excellent prompter for putting the little things off) and seemed to run ok.  I did wind the idle mixture screws around to see what it liked, and as it turned out it idled happily with them fully in, so I need to put smaller pilot jets in.  Given it has 52 now and I think I only have down to 48 hopefully 48 will give me a rich/lean reaction to the screw adjustment.  It should also cure the just off closed throttle richness that I'm getting when cruising in 2nd at 40km/h sort of thing.  I did try adjusting the slow air screw setting from the original of 1 turn in to 3/4 then out to 1 1/2, but that appeared to not make any difference.  Especially one of the desirable kind.  I didn't do any dyno runs around 1/8 throttle due to some time contraints at the dyno, but did some 1/4, 1/2 and 3/4 ish runs as well as the WOT.

The 152 mins were too lean, and combined with the lack of pump shot made the roll on at the start of the run a flat spotter's paradise.  And I noticed some light pinging in the 5 to 6,000 rpm range, which I haven't heard before.

First graph shows the mixture for the WOT (blue), 3/4 (red) , 1/2 (green) and 1/4 (purple).

The 1/2 throttle run really shows the tapering mixture affect that I've seen a bit with these carbs (and Dellorto PHF too). This would be helped with a larger main air jet and then chainging all the other jetting as required, but that's more jets to buy and a whole new day of tuning. The 1/4 throttle is a lovely flat line around 14.7:1 (could be a tad richer), and the tuning diagram shows 1/4 throttle as being needle root diameter and clip position related: so I might have to raise the needle. JD did recommend the 5th notch, I went with 4th up from the spec of 3rd so one more might help. But I have to remove the airbox to do that and the first requirement for that change to happen is a big bucket of motivation.

The only change I made jetting wise on the dyno was to try 158 mains.  this helped the mixture, but not the power.  You get that.  It did help the way it felt on the road.  Or maybe the way it sounded.  It had that "noisy, but not really going anywhere" feel to it with the 152.

The next graph is power, showing before (Mikuni 38mm carbs) in green, FCR39 with 152 mains in red and 158 mains in blue.  Not sure if the pumps working would make any difference to the power over 4,500 rpm.  It's not a big power increase, and I couldn't give it the whack test to see if it was responding any harder and I ride the bike so rarely that I have no idea what it really feels like anyway.  In fact the thing that stood out that most was the much reduced throttle effort.  That alone was worth the work.

The next graph shows the air/fuel ratio change going from 152 to 158 mains.  Blue is 158, red 152.  Better, but still lean around 7,000 rpm and not as smooth as I'd like it to be.

So some more jetting work to do.  We'll see how long it takes to get done.  As this bike has been ridden maybe 6 times this year (including twice to the dyno) it's not a big priority given how well it runs now.

I would like to do the larger main air jet thing, just for the experience.  I'd really like to do some extended testing of things like 1/4 throttle mixture variation based on main jet and needle notch changes, etc, but with the cost of dyno time and access (when Dave is working so am I) I'll have to wait unti I have my own running I think.

The last graph compares the same 152 main jets fitted to my M750 in blue, the 900SS with FCR39 in red and the 900Ss with FCR41 in geen.  As a very general rule the 750 has 16% less capacity than the 900, so on average the fuel flow for a given rpm should be 16% less due to 16% less air flow and therefore vacuum over the jet.  Which was similar to the flow area difference between the 39 and 41 too from memory.  I'm actually surprised the theory seems to be backed up, I wasn't really sure what to expect.

UPDATE 09/10/12

Today I solved the accelerator pump issue.  Not really sure how though, as nothing looked like it had major issues.  I replaced the float bowl gaskets, accelerator pump o-rings and diapragm and within 7 throttle twists had fuel spraying.  Which was nice, as it wouldn't start yesterday without the pump working.

I also dropped the pilots to 48 and wound the slow air screws back in to 1 turn out.  There still wasn't an overly definitive change in idle with mixture screw variation though once warm.  It would idle with them fully in and 2 1/2 turns out.  I settled on 1 1/2 turns out from memory.

But, if it has cleaned up the very low throttle richness, I'll be happy.  Time (as in, when I get to go for a ride) will tell.

I also found that the 200 main air jets fitted as std are the largest Keihin supply.  So I ordered 2 pairs of 200 with the other parts and have 2.1 and 2.2mm drills on the way to make them a bit larger.  I'm curious, and there's only one way to fix that.

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