Plastic MKIII airbox on warmed up engine?

I know that the black cap mufflers and the pipes with the crossover hamper performance. But what about the airbox? I'm putting an engine into my MKIIA that has a bit more compression, a WebCam 312 cam, and one of Jim Comstock's flowed heads with reangled and larger valves. I wouldn't mind keeping the large airbox, but am afraid that it will choke the engine too much. The inlet horns are decent size without the rubbers that choke them down. Look at any modern bike and the airboxes are about the same size based on capacity. 2nd choice is the K&N type filter. No room for a ham can with the CNW starter. Has anyone actually done any testing of these?

milfordite said:

I know that the black cap mufflers and the pipes with the crossover hamper performance. But what about the airbox? I'm putting an engine into my MKIIA that has a bit more compression, a WebCam 312 cam, and one of Jim Comstock's flowed heads with reangled and larger valves. I wouldn't mind keeping the large airbox, but am afraid that it will choke the engine too much. The inlet horns are decent size without the rubbers that choke them down. Look at any modern bike and the airboxes are about the same size based on capacity. 2nd choice is the K&N type filter. No room for a ham can with the CNW starter. Has anyone actually done any testing of these?

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It is too restrictive. It was part of emission and sound requirements that NVT employed back in the day. What does Comstock recommend.

I don't think the airbox is going to hold the engine back at all.

The noise rules required restrictive silencers and for sure those killed power.

The big airbox is the opposite scenario.

To reduce induction noise Norton designed a big airbox with a large filter area. This meant reduced restriction.

Glen

I am with Glen on this one - that air box is not restrictive at all. No need to change it.

Norton installed the easily removable rubber inserts into the airbox openings to reduce noise. With those rubber inserts removed that airbox is great. It certainly is much easier to change out the filter element compared to the Ham Can filter.

I think I'll keep the airbox, at least for now. Jim would probably say get better carbs! I do have a set of MKII Amals and manifolds around here somewhere.

The airbox is not restrictive, my MK2a with peashooters and black box needs 260 mains for correct carb settings, which is the same as a MK2 with peashooters and Ham can filter. The restrictive part is the 2 rubber restrictors that get dumped very quickly, I have 2 in a box just for completeness but they will never be fitted.

I have them as well, from my MKIII and the IIA. Since the bike is just now going together, I wanted to get the box in place before the engine goes in.

milfordite said:

I think I'll keep the airbox, at least for now. Jim would probably say get better carbs! I do have a set of MKII Amals and manifolds around here somewhere.

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I have a JC flowed FA head with larger valves, a 312a and 9.5:1 pistons and a Maney repro exhaust. Jim and I talked about carbs and his advice was the Amals could push plenty of fuel for a hot streetbike. I did have to go up significantly in the jet sizes.

I know that the black cap mufflers and the pipes with the crossover hamper performance

The crossover doesn't hamper performance, why would it? Possibly improves midrange, though that was a Triumph claim. Sounds nicer than unbalanced with the peashooters too imo, less harsh.

Actually the crossover does slow my bike a bit. It goes up the hill faster with the balance tube blocked.
I don't know why.
That's with open peashooters.
With black caps and the balance tube open (stock MK3 or Mk2A exhaust) the bike is pathetically slow, I wouldn't bother owning it if that was all it had.
With the open peashooters it's a decently snappy machine. Closing off the balance tube made it just a bit faster. Couldn't feel it but it could be measured on every pull.


Glen

milfordite said:

I have them as well, from my MKIII and the IIA. Since the bike is just now going together, I wanted to get the box in place before the engine goes in.

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milfordite said:

I know that the black cap mufflers and the pipes with the crossover hamper performance. But what about the airbox? I'm putting an engine into my MKIIA that has a bit more compression, a WebCam 312 cam, and one of Jim Comstock's flowed heads with reangled and larger valves. I wouldn't mind keeping the large airbox, but am afraid that it will choke the engine too much. The inlet horns are decent size without the rubbers that choke them down. Look at any modern bike and the airboxes are about the same size based on capacity. 2nd choice is the K&N type filter. No room for a ham can with the CNW starter. Has anyone actually done any testing of these?

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Seems to me the best thing you could so is talk to the guy who did the head work!

No problem with the MK3 airbox. They flow great. About the same as the ham can.
Either are considerably better than the K&N . Jim

milfordite said:

I have them as well, from my MKIII and the IIA. Since the bike is just now going together, I wanted to get the box in place before the engine goes in.

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I can't speak for a MK3 but on a Mk2A the plastic airbox goes in no problem with the inlet manifolds removed (engine in)

SteveA said:

Seems to me the best thing you could so is talk to the guy who did the head work!

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Jim Comstock did the work, and it seems he is just fine with the plastic box. I was thinking of adding a CNW starter kit. If I do, the air box will have to go.

milfordite said:

Jim Comstock did the work, and it seems he is just fine with the plastic box. I was thinking of adding a CNW starter kit. If I do, the air box will have to go.

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Sad to have to make those compromises, but that cNw starter is worth giving up the plastic box and the ham can for us "older folks".

When I got my MKIII in 1988 it had a K&N on it and the starter blanked off. I never installed the stock airbox, though I acquired one and kept it. I always liked the look of the Ham Can and while working at the Norton shop I acquired all the parts to revert it to MKII specs. I hated changing the filter elements though. I eventually got the starter working with a 4-brush field that required a bigger battery. I have no experience with the CnW starter and maybe it doesn't need a bigger battery but clearly the factory Prestolite starter works with the air box and can be made to work with the Ham Can. A couple of seasons ago I installed the MKII battery tray and ham can filter parts. It was a bit of a fiddle getting the starter in also. I had to reclock it so the electric cable wouldn't foul the Ham Can.

My reason for doing all this was to properly fit a Honda GL1500 battery. You can't use the MKIII airbox with a bigger battery. Using the K&N with the remenants of the Airbox mounts, it was only just possible to fit the battery in sideways and the mount was never really right and just about impossible to service. With the MKII setup I can use the factory battery mount and retainer and rubbers. Much better. It's as though Norton planned to use this setup until they had to meet EPA regs and ended up with the airbox and an undersized battery. I can't believe such a jury-rigged solution as the MKIII used was driven entirely by cost, i.e. cheaper 2-pole starter and small battery. Aside from all the practical things the Ham Can also sounds really cool on full song and it runs much better than the K&N. FWIW the bike has peashooters and 260 mains in MKI concentrics with 3-1/2 cutaway and 106 needle jets with needles on middle notch, ("EPA" needles, which I believe is all that is available now in the States, please correct me if I'm wrong, I liked it better before); it has to run on the choke much longer than before the needle change. Can't find the old needles.

My MK3 starter works great with the Factory size battery. 4 pole conversion and heavy cables.

Glen

Thomasa said:

FWIW the bike has peashooters and 260 mains in MKI concentrics with 3-1/2 cutaway and 106 needle jets with needles on middle notch, ("EPA" needles, which I believe is all that is available now in the States, please correct me if I'm wrong,

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"EPA" needles are for Amal Mk2s.

There were no EPA needles for Amal Mk1s as far as I'm aware because Mk1s simply didn't meet the US EPA regulations which is why they were replaced with Mk2s (on US Triumphs from '78 etc.) but that was after Commando production had ended.

I doubt there'd be much of a problem obtaining the Amal Mk1 928/104 (4 ID ring) needles in the USA (for the standard 850 928/107 stepped spray tubes neither of which were anything to do with US EPA regs.)