Sunday, April 26, 2015

Scratching my (cylinder) head.

I feel like I've spent enough time working on this engine for how little I've spent riding. Yes, I can hear hordes of Norton riders laughing hysterically (and weeping, too) as they read this, but I'll stand by what I've just said.

So, when I took the cylinder head off the other night, I went at it with little ceremony and tore off all the many, many parts that need to come off before you can get to the actual head. That, in and of itself is annoying, but lifting the head off while keeping the pushrods from falling, trying not to nick anything, looking for any spare nanometer you can get to clear the cylinder block is absolute bloody drudgery that made me really hate this bike in the moment.

Anyway, once I managed to get the cylinder head to one side, I took a look at the cylinder block and noticed something I wasn't expecting:

Not the usual oil seepage by the oil return passage at the back of cylinder 2, but what looks like blow-by to the left of cylinder 1.

A compression leak? Is that the noise I was hearing? Could that mean that there is actually nothing wrong with the head? I'll still check everything of course, but if I can't find anything obvious, could this be the culprit?

Perhaps all of this happened because I didn't re-torque the head (which I should have done, but didn't do, don't ask me why), or maybe I got the torque settings wrong on the wrench, or maybe still I messed up something else I'm not even aware of.

Still, the thing to do now is to proceed with exploratory surgery on the cylinder head and figure out what's what. Stay tuned, but don't hold your breath...

Thursday, April 23, 2015

Here we go again...

You'd be forgiven for having forgotten my Fastback: I nearly forgot I have this thing myself, that's how rarely I get to see it, let alone ride it. Truly, what a shame.

Anyway, as you may recall, I rebuilt the top end last year, and although it works, there is a problem that needs fixing if I want this to be more than a 1:1 scale static model of a motorcycle.

Without going into every single step, here's a photo of where we're at:

What I need to examine more closely are the moving parts within the cylinder head, plus take a look down the cylinder bores to check for any nasty surprises: the latter was easier to do and it would appear that everything is fine. The bores look clean and free of any scoring or marks of any kind, so I can just check that all fasteners are properly torqued down and leave it at that. This is proof that SRM did a very good job, and also that I didn't completely cock this up when I put it back together. I'll allow myself an imperceptible pat on the back for that one.

As for the cylinder head, other than a visual check to see if anything looked out of place, it's time to disassemble everything once again.

(Sometimes I really hate these damn bikes...)

Monday, April 20, 2015

Firm footing.

The left-hand pillion footrest on Paralever GS BMWs is bolted directly to the exhaust silencer, as is the pannier strut. This is a strong and reliable arrangement, but it does nothing for the comfort of the person sitting at the back, who will soon wonder why their left foot has gone numb and what that army of buzzing fire ants is doing crawling up their leg.

So yes, there is a lot of uncomfortable, high-frequency vibration that gets through to the footrest. We had tried curing the problem with a huge, industrial silent-block, but all that did was make the footrest fold downwards, which is unstable, dangerous and ultimately just as uncomfortable.

There is an after-market bracket that's available from a few retailers, we bought one to try, and while the concept and execution are sound, it does require (I think) stock rider footrests, which we don't have, and possibly removing the left-hand side panel, or a different exhaust silencer altogether.

Now take a look at this:

Based on the concept of the aforementioned commercially available bracket, this one was custom made by MARCELLINI M. & I. METALLICHE COSTRUZIONI of Aprilia, and they did one hell of a good job. They can make one for your BMW too, or pretty much anything else you can think of: their plasma cutters, benders and CNC mills are chomping at the bit...

Tuesday, April 7, 2015

Dynojet kit: lessons learned.

Let me be clear on one thing before we get into this: the stage 1 Dynojet kit that I've fitted to my Honda is great. Combined with the new exhaust headers, the result is awesome!

Simply put, there is now more of everything, everywhere in the rev range. More power, more torque, more acceleration, more top speed, and all of it is smoother and more tractable than before.

Now, here's what came with my kit: a selection of main jets, let's call them small, medium, and large for the sake of argument. The small one is for high-altitude applications, very cool but we're not riding up the Andes so... nevermind. The medium one is for use with stock (i.e. restricted) exhaust headers. The large jet is the one I used, and is for the larger, unrestricted exhaust headers.

Then came an adjustable needle, with an E-clip and plain washer for fitment, and instructions for how to set the air/fuel idle mixture screw.

The shape of the needle is very different to the original, it has a completely different taper, and that's how the magic happens.

Installing the kit is dead easy and all you need is a couple of screwdrivers to get the job done. However, after I initially fit my kit I noticed a drastic worsening of the mid range, especially when downshifting, total lack of tractability and a horrendous increase in fuel consumption. Clearly something had gone wrong.

I'll explain what happened and how I corrected the problem, this is probably tedious reading but it could be useful to anyone out there fitting the kit to their bike.

When I reset the pilot mixture screw, and replaced the needle, I noticed two things.
First, the screw was set at more than four (!) turns from seated, whereas the factory setting is about two turns. That is a massive difference, and not even the Dynojet kit is set to be that rich.
Second, there was a small rubber O-ring under the needle.

Initially I didn't give these details too much attention, I just set the screw at three turns as specified by the instructions, and replaced the needle including the O-ring, since the instructions say to include all stock spacers. Then, when I noticed that the engine was running worse than before in some respects, I started to put the pieces together. The mixture screw set so far out (i.e. rich mixture) and the needle raised by that O-ring (again, rich mixture) must have been an attempt by someone in the past to richen the mixture, which on these bikes is very lean as standard, to comply with emissions regulations. Since the standard needle is not adjustable, the only way to raise it (thus richening the mixture) is to put a spacer underneath the head. Crude, but effective.

So, I took the carburetter slide out again, and took a few measurements to compare the stock needle with the Dynojet one. Long story short, the little O-ring (see photo below), which I had kept with the Dynojet kit, thinking it might have been a stock spacer, was in fact raising the needle by just over 2mm... that's huge! No wonder it was running like crap and the fuel consumption was through the roof!

With the O-ring removed, and the needle set as intended, throttle response is, in a word, crisp.
There are no signs of hesitation or flat spots anywhere to be found, it just responds instantly, ready to go.

What a difference one little O-ring can make. Background is wall, not cottage cheese.

Incidentally, there is an interesting write-up on carb cleaning at this link, with some excellent detail photos. You know, since I didn't take any photos while I was doing mine...

Thursday, April 2, 2015

Something nefarious is afoot...

...we'll find out soon enough.