My GS850G ongoing issues and maintenance.

[Rich82GS750TZ]

Dave, please pardon my ignorance, but what is this?


sounds bada$$, by the way.

[Grimly]

That's the AFR oxygen sensor. Once I finish off sorting out the mixture and can be sure it's stable, I can remove that.

[Grimly]

Fitted a Telefix fork brace the other day and tried it out.
Not bad; can definitely feel a difference, just like every other time I've fitted a brace to the spindly forks of this era. In this case I was definitely feeling the front squrming a bit with all the extra weight of fairing, lights and everything else, so it's tightened things up just a touch.

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[Steve]

Not bad on the muffler sound.

I do have a question, though. What's the purpose of the shorter pipe?

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[Grimly]

The design evolved.
Originally, I'd intended for the cone-ended (actually just flattened) pipe to run the entire length and the incoming gases would be forced on all sides of it to eventually make their way through the perforations into the centre then outwards at the end. I'd intended to partially flatten the three surrounding pipes to provide a gas passage and also a central locator within the outer shell to keep the main pipe in the middle.
After I'd welded on the central pipe to the exit flange, I realised it was a bit skewed, but that immediately threw up another idea - keep the skew , make sure it went well away from the AFR bung and simply use just one short piece as a packing piece and gas flow channel. All the gas has no lack of expansion and flow spaces down past the main pipe, and as it hits the scrubbers some of it will percolate through them to freedom, but most will likely just find its way through the front half perforations. The scrubbers are mainly there to damp down any tendency for a hard reflection from the end wall, at which they seem to be working well.

[Steve]

Fair enough. Seems to work well.

The baffle in my (unknown manufacturer) muffler is a bit similar. Rather than crimping in front end of the perf tube, it is welded to a plate similar to the one at the exit. If it stayed like that, it would be like most others, with a straight-through perf tube. However, there is a plug welded in the middle of the perf tube that forces all the gasses through the perforations, around the plug, then back through the perforations in the back half.

I got this exhaust system from my son before he sold his 850. He tried different packing around the perf tube. Out of sheer ignorance, before we noticed the plug, we had wrapped fiberglass around the tube, like any other muffler. The bike barely ran above idle. When we discovered the plug and found that all the gasses had to go THROUGH the fiberglass, he ran it for a while with NO packing. Not too bad, but just a bit loud. I have (loosely) packed it with 000 stainless steel wool and it does a fine job.

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[Grimly]

Yeah, I found similar baffles in the Dyna mufflers I fitted to the XJ. Because of the unequal downpipe lengths of the H-D, the baffle plugs were at different spacings within the, otherwise identical, mufflers. This causes an odd off-balance exhaust note, so I ended up with adjustable spacings on the baffle plugs and matched both of them fairly well. The structure of those is otherwise unaltered - just a perf tube the length of the muffler, wrapped with fibre.

[Grimly]

The ongoing saga of the SU carb. Regular readers will know I've been running one for decades, but it's always been not-quite-right.
A couple of years ago I fitted an HIF 38 (equivalent to 1.5"), but have been plagued with it running out of steam at 90mph; this wasn't really much of a problem, as more than 80 around here doesn't go on for long, and I don't do motorway journeys much, if at all.
Finally, the penny dropped - the needle jet on the HIF38 is (old school measurement) 0.075" orifice and the head of fuel from the tank is barely 6" above that, more when the tank is full.
A replacement needle jet of 0.097" has been obtained, and is going into the spare carb I will be gas-flowing for later fitment. This bigger jet will enable the 90mph barrier to be breached, but I don't think it will go a lot higher - the previous carb I had was larger, and likely had the larger needle jet too, but it topped out at 115mph. That was to be expected; after all, it's a single carb and there's a limit to how well it can supply four pots.
However, my goal here is to get the 38mm carb flowing and breathing the best it can in this application. So, just as a temporary measure, to rule out fuel supply issues, I fitted a cheap-ass electric pump and a couple of filters, along with a new manual petcock, as the old vacuum tap was guilty of also not flowing as much as it should.
To feed the 80 paper horses of this engine, there needs to be a fuel flow rate of ~66ml per ten seconds. A simple measurement maxed out at 44ml per ten seconds, so quite a shortfall.

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[Grimly]

Some more progress with the Harley muffler and Delkovic downpipes.
I re-configured the inside of the muffler to a more conventional arrangement with a perforated stainless tube running down the centre and surrounded that with Acousta-Fil fibre blanket, which expands on heating and beds in the silencing arrangements. This, on its own, was ok, but there was still an out-of-kilter pulse note which was still annoying me and apart from that it was causing engine vibrations which don't do anything any good. I'm convinced the downpipe length isn't quite equal enough, and combined with the slightly uneven cylinder filling of the SU+manifold, it just leads to slightly uneven combustion across the four. Sod's Law dictates that whatever unevenness is caused by one is exacerbated by the other.
So, back off with the muffler and stuffed the front part of it with two stainless pan scrubbers.
Hooyah. It works bloody well! Nice and civilised and smooth again. It's not perfect, but it's a lot better with the pan scrubbers disrupting the rogue pulse from whatever pipe it is.
I also rediscovered the sheer pleasure to be had from just riding around at sensible speeds on a quiet bike.
Who'da thought that?

[GS1150Pilot]

So interesting. All of my SU experience comes from BL A-Series motors.

[Grimly]

So interesting. All of my SU experience comes from BL A-Series motors.

I grew up with them on those. The carb was by far and away the best designed and engineered part of the package. Well, until the HIF series came out, then it was even better.

[GS1150Pilot]

True enough. I used an HIF44 on my last Mini Cooper

[Grimly]

Next move, improve the intake tract.
I already have a stubstack in the intake to the SU, and it definitely works, but a perusal of the mgbexp forum rang a bell with me, and I recalled that some MGBs came with factory stubstacks that were much better designed and simple to fit with modern K&Ns etc.
So, last week, scored a couple of these for very little money, as there's not much demand for them; the classic Mini tuners can't really use them because of space restrictions, I think.



They should clean up nicely and a K&N RU-4410 fits directly on with a clamp around the shoulder.
This is what they look like cleaned up and fitted to an MG....


... and I have a large filter with a big enough neck that isn't actually a K&N, but will look like this when done...



I'll probably get this done over the weekend.

[Grimly]

Finally got around to doing the MGB intake tract and eventually ended up forking out for a K&N 4410



It ended up a bit tight in there, as even with the short K&N filter, it was hitting the forward battery bracket. A bit of brute-force and ignorance sorted that out.



However, I'm not terribly happy with that, and I know there used to be an intake plate with the same flowed throat that allowed the filter centre line to be lower than the carb mouth. They were fitted to some Wolseleys and Rileys of the 1950s, so I'm on the hunt for one. Hen's teeth by now, but I just missed one on Australian ebay by a week or two, so they are out there.

[allojohn]

Ouch! .