New MK3 Project for NC Rally, Maybe

I'm finally getting far enough along in this project to post some pictures. This is a MK3 that I bought from a friend who had it for years, and finally decided he was never going to get to it. It was missing a few bits, like tank and seat, and in need of a rebuild, but mostly there. I bought it so I could put another MK3 together to go with the one I already had, so my grandson and I could both have one for this years rally. I already had my street MK3 torn apart for a serious resto mod sort of build. I was a little too optimistic, so now we're looking at just getting the new one finished, and taking some other sort of bike along to the rally so we both have rides. This is how it looked when I got it last year.



I've since rebuilt the engine with a set of my 79.5 mm JE race pistons, cut down for a lower CR. Turns out the head had the famous intake port crack at the guide, so I replaced it with a ported head from an old race bike, to go with a Webcam 312A from Jim Comstock. The head had been skimmed long ago by .080", so I cut the piston down as much as I could, but also had to use one of Jim Schmidt's .021" copper base gaskets along with an .040" copper head gasket to get the CR down to 9.2. I'm hoping that combo will work with whatever premium pump gas I can find near the rally. If not, I'll retard the ignition a little, and plan on reducing the CR some when I get home. I'm going with the Trispark ignition, which I've never used before, and a pair of 36 mm Amals off of my other (now in boxes) MK3. Also using the non-crossover exhaust from the other MK3 and a new pair of peashooters that came with the new bike.



This is a picture of the bike as of tonight. Still a long way to go, but I'm confident we'll somehow have it ready in time.

Ken, that's a whole lotta MK3 you have going on there. Like you, I see the MK3 s as a good project machine, a strong base bike to improve upon. It can be made to go as fast as any roadgoing Commando and maybe hold together longer due to the stronger crank and cases.
Because they were so stifled with the stock mufflers, it might be the easiest of all the Commandos to get more power from. Just removing the stock mufflers adds a huge amount, then one can go from there as you have.
The wetsump issue seems to be a non issue with MK3s even if left to sit for a month or more, although this info is based on my own bike only.
Also, if you happen to twist your ankle by stepping in a pothole, the electric leg is pretty nice.


Glen

Hi Ken,

I love my MK3. Now I am excited about getting the 71 together and its coming along. The biggest thing is the cost of everything these days, as you know. I am jealous of your workshop. Curious what head you are using now with the 36MM carbs? RH10 maybe?

Cheers- Jerry

Jerry Doe said:

Hi Ken,

I love my MK3. Now I am excited about getting the 71 together and its coming along. The biggest thing is the cost of everything these days, as you know. I am jealous of your workshop. Curious what head you are using now with the 36MM carbs? RH10 maybe?

Cheers- Jerry

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Fullauto?

Jerry Doe said:

Hi Ken,

. The biggest thing is the cost of everything these days, as you know. I am jealous of your workshop. Curious what head you are using now with the 36MM carbs? RH10 maybe?



Cheers- Jerry

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Speaking of the cost of things and RH10s, one just sold on eBay for $643 plus shipping. For us canucks that pretty much $1,000cdn for a used core to send off to Jim for treatment or possibly use as is.
Getting costly alright.


Glen

The original RH4 was cracked in the port along one of the intake guides. The replacement head is an RH10, with exhaust thread inserts by Jim Comstock. Jim also ran it through his vibratory cleaner, so it looks better than new. It's an old race head I had on the shelf. It has stock valves with mild port cleanup and multi-angle valve job, RD spring kits, and lightened and polished rockers. I installed Timeserts for the studs, and shortened some Dorman studs to fit. I shortened the push rods to suit the skimmed head and new cam, and had the tappets slightly radiused by Jim. I'm hoping for a good compromise between top end performance and putting around smoothness. The 36 mm carbs might seem a little large to some, but I was using them on my stock MK3, and they worked just fine, so I swapped them over to this build.

Ken

Jerry Doe said:

Hi Ken,

I love my MK3. Now I am excited about getting the 71 together and its coming along. The biggest thing is the cost of everything these days, as you know. I am jealous of your workshop. Curious what head you are using now with the 36MM carbs? RH10 maybe?

Cheers- Jerry

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Couldn't agree more about the cost of things. I'm using parts I already have as much as possible, but it's still adding up. I'm trying to sell my CRF450X dirt bike and a Model A hot rod to raise some cash for the Nortons, but not having much luck. Economic times are still pretty tough for most people.

Ken

lcrken said:

The original RH4 was cracked in the port along one of the intake guides. The replacement head is an RH10, with exhaust thread inserts by Jim Comstock. Jim also ran it through his vibratory cleaner, so it looks better than new. It's an old race head I had on the shelf. It has stock valves with mild port cleanup and multi-angle valve job, RD spring kits, and lightened and polished rockers. I installed Timeserts for the studs, and shortened some Dorman studs to fit. I shortened the push rods to suit the skimmed head and new cam, and had the tappets slightly radiused by Jim. I'm hoping for a good compromise between top end performance and putting around smoothness. The 36 mm carbs might seem a little large to some, but I was using them on my stock MK3, and they worked just fine, so I swapped them over to this build.

Ken

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The pictures show the head as if it were new. Very Nice.

Not that I know what I'm talking about, but my 40mm single with the JS1 may be comparable to your 2 36 amals and the 312a, mine a 750 and yours an 850 of course. It will be interesting on how your "around the town" functionality comes about.
For what its worth, I feel I relate to what you are doing, building a sleeper, so to speak. What will you exhaust be? With that cam and carbs, it may beg for something other than stock, or maybe not. I have a longish 1 1/2" exhaust. I guess I have tuned around it and or maybe just got lucky with the end results. It curtainly wasn't without effort.

I will be watching you progress with personal interest.

Thank you for sharing you stuff, Ken,
Sleepy Pete

What do you want for the model A hot rod

zefer said:

What do you want for the model A hot rod

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PM sent.

Making some progress. It's been a long time since I built a Mk. III from scratch, and I've lost a lot of time just figuring out routing for some of the hoses and wires. Seems like I assemble everything at least 2 or 3 times before I'm happy with it. Should have left my other Mk. III together so I could refer to it, but I got carried away, and it's now also in pieces. Oh well.



Ken

Starting to look like a motorcycle.



Ken

Looking good Ken.

A little more progress today. Sorting out the new wiring harness has been tough. When I bought the bike as a roller, the wiring harness was already removed. It was in such bad shape that I ordered new harnesses from Andover. Then I foolishly tore down my regular Mk III to get started on rebuilding it. So I have no reference for routing the wiring harness. The Mk III has a lot of wires and connectors not present in earlier harnesses, and a lot of them are connections I'm not using, since I've swapped over to a 3-phase alternator and regulator, as well as a Trispark with single coil. I'm finally down to one remaining routing question, but I've got the rest sorted out.






Now to the wiring question. In this picture, you can see where the main harness passes through a grommet in the plate just below the rear of the main frame tube. There's a leg of the harness that branches off at that point, and I'm not sure if it goes in front of the plate, as I have it now, or should be behind the plate, on the other side of the grommet. This leg has the wires for the neutral switch on the gearbox, the two wires for the stock alternator, a bunch of connections for the ignition warning module, and the leads for the power tap. It seems to me that it should be in front, as I have it now, but I'd really like to know for sure. If anyone here with a Mk III could take a look and let me know, I'd appreciate it. I'm still looking through service manuals and pictures, but haven't been able to find anything that shows that particular spot.



Ken

lcrken said:

A little more progress today. Sorting out the new wiring harness has been tough. When I bought the bike as a roller, the wiring harness was already removed. It was in such bad shape that I ordered new harnesses from Andover. Then I foolishly tore down my regular Mk III to get started on rebuilding it. So I have no reference for routing the wiring harness. The Mk III has a lot of wires and connectors not present in earlier harnesses, and a lot of them are connections I'm not using, since I've swapped over to a 3-phase alternator and regulator, as well as a Trispark with single coil. I'm finally down to one remaining routing question, but I've got the rest sorted out.

Now to the wiring question. In this picture, you can see where the main harness passes through a grommet in the plate just below the rear of the main frame tube. There's a leg of the harness that branches off at that point, and I'm not sure if it goes in front of the plate, as I have it now, or should be behind the plate, on the other side of the grommet. This leg has the wires for the neutral switch on the gearbox, the two wires for the stock alternator, a bunch of connections for the ignition warning module, and the leads for the power tap. It seems to me that it should be in front, as I have it now, but I'd really like to know for sure. If anyone here with a Mk III could take a look and let me know, I'd appreciate it. I'm still looking through service manuals and pictures, but haven't been able to find anything that shows that particular spot.

Ken

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It's behind the plate on my MK3.

Thanks, Jim. I was afraid of that! It's a PITA to re-route it, but I will.

Ken

Road blast plasters the fronts of the down tubes so makes sense to hide/protect behind things. Looks like the loose loom is wrapped in black electrical tape, if so the ends tend to unrwrap with sticky residue so might consider small zip tie or thread wrap/glue to keep end of covering wrapped longer. Best wishes on enough rerouted slack to reach terminals w/o strain.

The loom is wrapped in black vinyl tape, but it's not the sticky tape we normally call electrical tape, but a non-sticky wrap that only seems to stick to itself. I've seen it used on other commercial wiring harnesses on both cars and bikes. I've been using sticky electrical tape for years on harnesses, but I think I'll hunt up a source of the stuff Andover uses on their harnesses. I'm sure it's available from wiring supply houses.

Ken

lcrken said:

The loom is wrapped in black vinyl tape, but it's not the sticky tape we normally call electrical tape, but a non-sticky wrap that only seems to stick to itself. I've seen it used on other commercial wiring harnesses on both cars and bikes. I've been using sticky electrical tape for years on harnesses, but I think I'll hunt up a source of the stuff Andover uses on their harnesses. I'm sure it's available from wiring supply houses.

Ken

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No question, the wire wrapping tape works much better. The sticky stuff makes the loom too stiff and the glue makes a sticky mess after a while. Jim

I have been using this stuff:
http://www.painlessperformance.com/webcatalog/braids

Then close the ends w/ shrink wrap

Works well
Pete