Interstate Mk2A Question

The "A" suffix was always applied to the machines with the Annular Discharge silencers and black box air filter. This was a European (German, I believe) requirement and in general, European market customers including the UK received the Mk1A and then Mk2A models.

The U.S. mostly received the 'standard' type with the 750 style of silencer and air box. However, period adverts seem to suggest that small numbers of these were sold in the UK too and that the US had some "A" models.

There were in fact two versions of the 1974 Mk2A brochure, outwardly identical but with alternative photos !

The choice is yours really, but the silencer and air filter types should be consistent for an authentic look.

Richard, I always wondered where the “A” came from. I never noticed you could get a Mk1 A…

I guess it was related to early forms of emission controls.  It wasn’t until the early 80’s and the introduction of lean burn technologies which taxed the performance of carburettors that we saw real emission management.

They choke the life out of an 850 and were quickly replaced with peashooter and K&N’s to let the torque out. I run a 22 G/box sprocket quite comfortably.

Thanks for another gem…

Jon

I actually preferred the appearance of the 'Black-cap' silencers. When I bought my Mk2A Interstate in 1989 it had the usual Peashooters and when they started to look tired I fitted the RGM straight-through Black Caps. As the name implies, they are not as restrictive as the usual Black caps and sound fine - the only difference is that they don't have the actual black caps!

The black box air filter is not restrictive only the silencers, if you fit peashooters and keep the black box the best jetting matches the Ham Cam filter setup.

Depends how much your sucking....

Not sure what"Ham Cam Filter" is... I picked the needle up one notch for smooth run but that's free breathing both ends...

Jon

The road test extracts were different too, as they were in the foreign language versions, although the European brochures, as far as I can tell used the same photos as the UK edition. For the later Mk3, a completely different brochure was printed in the US.

Can you guess which brochure applied where ?

... that in fact the black caps didn't affect performance and may in fact have enhanced mid range. The fact that they're quieter (and I confess to fitting peashooters to my 2A) can mislead.......

As attached Jonathan......;0

Mine is a ‘73 Mk1A Interstate which was the first “noise control” model with black cap silencers and large plastic oiled foam air filter box. When I bought it in ‘77 previous owner had already fitted peashooters but still had the black cap brackets which were different from peashooter brackets. Changed brackets to suit and 45 years on plus a few sets of replacements peashooters bike still runs fine.

I can categorically say that the original black caps stifle the power and don't enhance the mid range. I did back to back tests a couple of years ago on my Mk 3 , first with genuine black caps and immediately after with straight through black caps, the difference was unbelievable. Bike has original black box air filer and twin Amals, so was conclusive (to me at least)

... was based on the Commando Service Notes page 32. That of course was comparing black caps with peashooters and other original equipment not straight through versions. I have great respect for the views of Tim Stevens who wrote the book.

A favourite Xmas treat way back. Thanks Mike, a very good likeness.  It’s staying on the shelf though…

:)

Jon

When you swap the silencers from Black cap to peashooters you must increase the main jets from 230 to 260 or the top end will be useless due to lean mixture. The needle needs moving one notch too but can't remember which way but upwards is likely the one to try first.

I’m on standard 230’s but lifted the needles.  The use rarely takes me over half throttle with the 22T sprocket.  But I’ll keep it in mind should I get a blood rush and a sunny day…

Thanks

Jon

Hi everyone

                  Just to add my twopenny's worth my '74 mk2a was brought back from the US around 1994 and had 14'000m on the clock when I got him in 1997' he was a bit of a time capsule as nothing had been messed with.  Mine has the earlier 750 airbox, balance pipe on the down tubes but also the peashooters were of a larger diameter at the join with the reverse cone due to being baffled which was part of the exhaust construction and not a 'mute' in a standard straight through peashooter however, within a short time I replaced the whole of the exhaust system with what the 750's had from toga.

...changed the exhaust system but how did it go?

The US got the MK2 for most of 74, only the last of the US 74's were MK2A's in preparation for the MK3. The check is the battery tray, MK2 the battery long side runs forward to back, MK2A the tray is per the MK3 and the battery long side runs across the bike. 

A lot better through the rev range Alan, I then followed this up with one of Mick Hemmings larger inlet valve mods and some gas flowing which improved things again including clutch slip in 4th at 70mph if a fist full of throttle was used too quickly, more frequent scrubbing of the clutch plates mitigated that, so thoughtful of Norton to design the primary cover to double as a degreasing bath eh?:-) 

... if only they'd given it a drain screw as well. I drilled and tapped my cover to provide one as otherwise when you take the cover off oil goes everywhere.

I ‘acquired’ one of the large tin baking trays from the kitchen cupboard many years ago to sort that little issue.

... the oil still gets all over the exhaust and frame tubes.

That's where a couple of sheets of  newspaper come in handy,  always have some, old habit from years ago when my back room was my work shop however that stopped after a nasty accident I had getting my third finger left hand stuck in a wedding ring.

Last year I took my grand-daughter to the NMM at Coventry.

I said to her, I'd love to be the guy selling baking trays to the Museum......there are 000's of them!

I went to the recent… open day. Whilst browsing Norton models a guy turned to me and said, “even the meat trays are clean under the Nortons”!   Then as you browse you start checking the hundreds of meat trays. Bet they were not British….

Jon