Sunday, December 16, 2012

Bob Brown's race 851

A member of the 851 Yahoo list Posted a link to Bob Brown's 851 race bike that is currently for sale on Bikepoint: 851 Racer.  I've seen the bike in person and think it's pretty cool, so I thought I'd put a little bit up on it.

Originally an 851 kit, it had lots of mods over the four or so seasons it was raced (88 - 91).  Bob had cylinders made up to allow it to go to nearly 100mm bore for a full 1000cc, and the frame had extra bars welded onto each side of the vertical cylinder that Bob says made a big difference to crankcase life.  It cracked a few sets early on, as did all the 851 that were raced.  Roche used 36 engines in 1990: Bob witnessed the last one letting go rather impressively coming on to the main straight in the last race of the season at Manfield, after he'd won the title in the first race of the day.  From memory the 916 wasn't any better, I'm sure Corser and Fogarty used a similar number of engines in the mid 90's.

Duane Mitchell, who worked with Bob in the era, told me years ago it was very quick, and went past the factory bikes with ease in a straight line.  Until it just shut down, which it apparantly did with monotonous regularity.  So they pulled the Weber injection and Bob had some manifolds cast to fit a pair of FCR39 to each cylinder and a custom alternator cover to allow a Krober ignition to be fitted to it.  Which made it much more reliable.

It's obvious how much time and money went into it, with lots of very cool period race bits like radial master cylinders, etc.  And lots of custom/one off work as well.  Clearly a money pit, and very cool.

He also has his new in glass case 851 Tricolore Kit listed too: 851 Tricolore Kit

And also this TT2, which was displayed a few (several) years ago at the FOIM at the Museum.  It's just gorgeous: TT2 

Friday, December 14, 2012

Keihin FCR main air jet test: post 2

I was going to put more info in the other night, but it was late and I had work to do.

The graph below shows three different bikes all running the same 152 mains (just can't get rid of them) and how the curves taper (or don't).  Blue is M750 with FCR39, Red 900SS with FCR41 and green 900SS with FCR39.  The green curve is by far the most consistant.

It also seems that the richer my M750 gets, the less consistant the WOT mixture becomes.  In particular, the more prominent the (comparatively) lean peak at 7,200 becomes.  Blue is 152 mains, red is 158 mains and green is 165 mains.  Also, the three step richer change from 158 to 165 has a much bigger impact on mixture than the two step 152 to 158 change.  I have done a few FCR main jet changes that have struck me as having less effect than I anticipated.

Although, the Mikuni 600 and later 900 carbs I had previously been running on Minnie could also show a lean peak at similar rpm depending on inlet and exhaust configuration.

I also ran some part throttle checks.  Initially when I fitted the FCR I tried 52 pilots, then went down to 48 after the first ride and down to 45 before the last dyno runs to try to cure the 1/8 or so throttle richness.  It's possibly still a little on the rich side.  I have left the idle mixture (slow fuel) and slow air screws at their spec settings, just because they seem to be a bit more stable that way in my limited experience.  And some playing with them didn't seem to help.

The next graph shows 1/4 throttle, which has remained surprisingly stable.  I say that because both pilot and main jets have been changed, and I'm told that the main does have an impact on part throttle.  So I'm not sure if the return to smaller mains will have an impact on the 1/4 throttle mixture.  Hopefully not.  Blue is 52, red 45.

1/2 throttle shows a richer mixture with the larger mains, blue 158, red 165.

Similarly, 3/4 throttle also shows a richer mixture with the larger mains, blue 158, red 165.  The sudden step at 4,000 rpm is a bit wacky too.

Given Minnie is now unregistered, I won't get to try out any more changes.  But I think I'll set the FCR up with 160 mains instead of the 158 and leave all else as was.  Hopefully the jetting will work ok in an SS, as that's where the engine is theorectically going.

She was running very nicely, although the 1/2 throttle wheelies when taking off from the lights were getting a little too easy and frequent.  Started well too with the "two pumps, wind in idle speed screw, press button" procedure.

Wednesday, December 12, 2012

Keihin FCR main air jet test

Today was Minnies last day of rego, and at this point of time I'm not planning to re-register her for the minimal use that she gets.  But I had one test I wanted to try with the FCR, and that was larger main air jets.

The idea is that the main air jet sizing wil influence the shape of the fuel curve.  Read Patick Burn's tuning guide for a much better explanation.  I have found that with the FCR on Ducati engines you often get a mixture that tapers rich as rpm climbs. Specifcally it seems to be with larger carb size to engine capacity ratios.  Or, more simply, 41 on a 900 as compared to 39 on a 900, or 39 on a 750 compared to 39 on a 900.  Comparitively, the 900SS I fitted FCR39 to gave a relatively flat curve.

The FCR for Ducatis come with a 200 main air jet, the largest Keihin make.  So the assumption was I could get some 200 and drill them out.  As you do.  With my jet drill kit the 200 was between 1.90 and 2.00mm, and I drilled them to 2.10mm.  I also went up to 165 main jets from the 158 that I had fitted previously on the assumption that I'd need more fuel.  Turns out, perhaps, that the reason the biggest main air jet available is 200 is because that's as big as the carb will respond to.  That's the theory I'm running with at this point, based on tody's runs.

If I told you the graph below was for 158 mains in red and 165 in blue you'd probably believe me.  I would.  Same shape, just richer.  So, no joy in main air jet land, but at least now I know.  And that has to be worth the effort.

Now I have to put the 158 mains back in, and probably some undrilled 200 main air jets, just to be sure.