Using 34mm mk2 Concentrics the best way

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I've just about finished a full rebuild on a 1974 mk2A 850 and now have to make a few hard and fast decisions. The previous owner had done most of the work and had purchased a pair of new 34mm mk2 Concentrics then fitted them ready to go. Now I know I can't use a standard airbox with these so I'm left with a choice of using velocity stacks, K&N filters or the left field choice is to change over to a single carb and keep one as a spare. I've been assured that a single 34mm Concentric will supply enough fuel to run the bike properly and I'm not really after a performance increase so that's no big deal but (there's always a but) he also fitted a PW3 camshaft so would the single carb still be ok with this fitted?

Right now I'm leaning towards fitting velocity stacks and going up a couple of sizes on the main jets but it never hurts to get other peoples opinions and you guys seem to have tried most things with these bikes.

Go ahead, persuade me one way or another :?:
 
One thing for sure if your on the "street" and riding regularly you must have an aircleaner..

I dont know your "cam", but a 34mm Mikuni will run a 750/850 with great street performance no problems!
 
Dont be a coward , fit the pair . :D

Rock Guards arnt a bad idea . Anywhere you wouldnt take your chain , you wouldnt take your carbs . :o

If its damp and grassy , they mightnt need filters . Dusty deserty stuff , off road , gravle roads and beaches
might make a Air Box ( custom ) a good idea . But if Socks fit on them , go that way . Custom Cool .

Smoothing the ridge where the curved intake bore mates the carb bore is o.k. too . Good Carbs .
need the enricher for cold starts . My 32s on 750 cammed ran W H I T E everywhere on 200 mains.
Id go 240s for starters , were hard to read . got 70 mpg includeing 80 + touring on 23 teeth , and
sustained ton plus . To work was 50 mpg ( city ) though still often confused as I thought the road
was the Isle of Man . Or Barcelona . Or Italy at least . :D

Twin Cable twist grip best . Wont get onto JUNCTION BOXES . :( :x .
equivilant of 36 mm Mk 1s , so theyre About RIGHT . unless your a suberbian trundler .

PS , put a BAR across the two , where theyre tapped up there , to make as one piece . Saves the mounting rubbers straining .
Some put a Rubber o ring ( vacum cleaner belt ) around intakes to frame tube , to neutralise weight . JPN 72 even .
 
I'd silver solder some screens at the backs of the velocity stacks before fitting them no matter what else I planned and given that neither stacks nor K&N filters are massively expensive when taken as part of the total rebuild I'll more than likely try both if I keep twin carbs.

It'd be a shame to leave a brand new carb sitting in a box so I'll almost certainly go with both but I was just curious as to whether it'd work well with only one.
 
Genuine Artical . Forgive the advertiseing .

Using 34mm mk2 Concentrics the best way


looks like these ( K&N ) are made to FIT too .

Using 34mm mk2 Concentrics the best way


and theres a Dual one , maybe in that size ??

Using 34mm mk2 Concentrics the best way


though some prefer washable foam sock type things .

Using 34mm mk2 Concentrics the best way


might be ok if the bike was painted REd , :? :P :x
 
A PW3 is a pretty big cam. It performs best at high RPM.
A single carb is going to perform best at low RPM.
When you put the two together you get a bike that runs OK but does not perform it's best anywhere. Jim
 
Run both carbies for the best performance, remember you are running a hot cam and you will need the fuel, never run velocity stacks on a street bike as it will be sucking a lot of dirt and grit that is in the air, put K&N filters on, they work very well, as for the velocity stacks, I ran a set when I first built my 850 in a Featherbed frame, had a slight leak on one of my fuel taps, went to start my motor, it kicked back and a flame blew out the back of the velocity stack, slight fuel leak, bang major fire, lesson learned never run without air filters, plus dirt and dust will do damage to your motor.

Ashley
 
ashman said:
Run both carbies for the best performance, remember you are running a hot cam and you will need the fuel, never run velocity stacks on a street bike as it will be sucking a lot of dirt and grit that is in the air, put K&N filters on, they work very well, as for the velocity stacks, I ran a set when I first built my 850 in a Featherbed frame, had a slight leak on one of my fuel taps, went to start my motor, it kicked back and a flame blew out the back of the velocity stack, slight fuel leak, bang major fire, lesson learned never run without air filters, plus dirt and dust will do damage to your motor.

Ashley

Me too Ashley... open carbs, very minor kick back, fire, etc. 2 years to get it back on the road. Not good.

But, why do filters prevent such a fire? The engine will still kick back (for whatever reason) so what is it the filters do to prevent the fire taking hold?
 
comnoz said:
A PW3 is a pretty big cam. It performs best at high RPM.
A single carb is going to perform best at low RPM.
When you put the two together you get a bike that runs OK but does not perform it's best anywhere. Jim

Andy-_B, I tried a single 34mm Amal MKII some years back. It was the weakest of many senarios I had tried. The 36vm was good, the Mik 34 flatside was suprisingly better than the bigger vm.

Although the JS stage one cam is suppose to be less radical than the PW3. it is made streetable by detuning in the carb area. Some added compression may be needed to make that cam come alive as expected but it is a game of give and take to make it streetable and still have that underlying go go go power.

I know you are just asking about air filters and a couple of UNI filters will be just fine, but you may want to concider trading those 34's for a couple 32mm Premiers. Some of us have had great success with a big single flatside.

Getting a big cam to idle at a stop light is tricky enough with a single carb. She's a bit lumpy, but just off idle it's butter. Hopefully the pa-plunking at the light will be the worst of it. For some of us, that lumpy idle is significant and welcome and does not mean a poorly tuned machine for those who recognize such sounds (motorheads).

There will be trials and tribulations, but the light on the other side is quite bright.
good luck.
 
Using 34mm mk2 Concentrics the best way


You can see the two threaded holes on the flat face above the intake bore .
A piece of say 1/2 x 1/8 Dural across , same centers as head ports ( if the manifolds are parrallel ) gets them steadied up .
Drilled and bolted on , of course .

Socks could be used with the stacks . Grills are regarded as ' flame traps ' . as the suckers dont have ticklers to LEAK ,
blowback is unlikely to find spilt fuel , like a Mk 1 . :P This is why a count on the tickle gets them ( mk 1 ) rich
without setting the world on fire . :oops: Usually bike vertical ( not lent over ) fuel on , gloves arraged etc ,
a rock with the knees and push with fingertip , mental count one - two - and at 2 1/2 smartly withdraw digit .

It is not usually neccessary to make the fuel external .

Dual cables avoid the rock in the hiddeous plastic junction box . The inner wotsit cocks and the cables dont draw even
consistantly . as in it draws one slightly earlier off the free play , alternately and erraticcally . Ancient chromed brass
or steel ones ( junction boxes ) are more precision , if you must use one . The longer ones have longer junction slides
that have miniscule rock . But have to be clean , unworn & lubricated . A more precision item .
however the dual load through the one primary cable gives twice the weight at the curves - thus More Drag / resistance
to microscopic throttle changes , makeing it appear you cant ride precisely . :P :lol: :twisted:

Dual Cables from Twistgrip Have One Return Spring Load on entire length ( half of two :shock: )
With open curves the throttle is noticeablely lighter and more precise .

Though if you ride through a hedge the ferrals at the twistgrip can fall out and need assistance to recover . a bit of silicone in clean would stop that .

The Dual 34s in a well set up devise should get comfortable cruise at lower settings and better economy , as its more powerfull you dont need
a lot of throttle to maintain the pace . They should give accurate reliable jetting . The Breather tubes are for ' overflow ' and Fumes . Enviro stuff .
so could go to a carbon canister if youve painted it Green . :x :P

People made there own cables & shops carried all the fittings & wires , before electric starters became common .

Should be a bit livelier of the line , and pull nice wheelstands with dual carbs , through 3rd gear . :? 8)

afterall , something made on this HAS to be Fine ,
Using 34mm mk2 Concentrics the best way

or are they Mk 1s in there somewhere . ? :( :?
 
Fast Eddie said:
ashman said:
Run both carbies for the best performance, remember you are running a hot cam and you will need the fuel, never run velocity stacks on a street bike as it will be sucking a lot of dirt and grit that is in the air, put K&N filters on, they work very well, as for the velocity stacks, I ran a set when I first built my 850 in a Featherbed frame, had a slight leak on one of my fuel taps, went to start my motor, it kicked back and a flame blew out the back of the velocity stack, slight fuel leak, bang major fire, lesson learned never run without air filters, plus dirt and dust will do damage to your motor.

Ashley

Me too Ashley... open carbs, very minor kick back, fire, etc. 2 years to get it back on the road. Not good.

But, why do filters prevent such a fire? The engine will still kick back (for whatever reason) so what is it the filters do to prevent the fire taking hold?

Good question. Examine any air filter, ( EXCEPT A Notrun roundtoovaloidpaperone, lol) and you see a metal mesh on the inside. When a backfire flame happens, the metal conducts away the heat, helps extinguish the flame. Study a marine flame arrestor on a gasoline fueled marinized car engine, resembles a Suppertrapp stacked plate baffle. Taking time to fit air filters, even when tuning/repairing/troubleshooting is GOOD PRACTICE to reduce the likelihood of fire.
 
pete.v said:
comnoz said:
A PW3 is a pretty big cam. It performs best at high RPM.
A single carb is going to perform best at low RPM.
When you put the two together you get a bike that runs OK but does not perform it's best anywhere. Jim


Although the JS stage one cam is suppose to be less radical than the PW3. it is made streetable by detuning in the carb area. Some added compression may be needed to make that cam come alive as expected but it is a game of give and take to make it streetable and still have that underlying go go go power.

.

JS stage 1 cam (Axtell #3 profile) compared to the JS stage 2 (PW3 profile) in the graph below. From bottom to top are the JS0 JS1 JS2 JS3

JS0 is near stock

Using 34mm mk2 Concentrics the best way
 
I run two 34mm Mk2 Amals using on my Mk2A 850 using methanol fuel. However I think you will find even with petrol you actually need to go down a size in main jets using the bigger carbs. I know it doesn't sound right, you will simply have to do your hi-speed slightly uphill runs and do plug chops and read the plugs. If the 2mm black ring is not there on the porcelain where it meets the metal down inside - watch out ! The main jet size is not as important as you might think - how often to you hold the throttle wide open for miles on end ? Your main problem will be getting the needle jets and the needle tapers right. One thing that I carefully stayed away from was over-porting the inlet tracts - I only ported the first half inch of the head to match up with the manifolds - there must be no steps. I use velocity stacks with no air cleaner or mesh. At one race meeting a stone flicked up and got through a gap under the seat. It held the slide up, but didn't go further - happened in the pits. So I fitted a rubber patch under the seat to close the gap. If you've got mudguards, it shouldn't happen. I don't like mesh on velocity stacks, sometimes when fuel blows back, it can be seen hanging in the mesh. It must change the mixture. I'd only ever use an air cleaner on an off road bike, unless I had the habit of chucking the bike into the kitty-litter on a road race circuit.
 
Echoing a few of the recommendations and some extras

A brace across the carb bodies is a good idea, I relieved the load on the rubbers by threading a piece of bungy cord through the bar and looping around the frame tube under the tank but I run an air filter that adds some weight.

Not had much trouble with the cable splitters but standard cables don't fit. If you sketch up what you want someone like Venhill will make them up.
The cable choke conversion is worthwhile.

There wasn't a standard dual inlet K&N available when I put them on, not sure about the current situation. I got K&N to make one based on the Mk1 type with bigger inlets, works really nicely and didn't break the bank.

The biggest problem is trying to set up a reliable idle, I would add a pair of deep floats and No. 3.5 chromed brass slides to your shopping list.

Using 34mm mk2 Concentrics the best way
Using 34mm mk2 Concentrics the best way


There's an old thread somewhere with this lot in,

Happy plug chopping

Rich
 
I think I can get away with a dual inlet if I either machine a couple of millimetres off the outside diameter of inlet stubs on the carbs or bore out the filter flanges by a similar amount. It might be a case of taking a small amount of material out of the bores as well as machining down the stubs rather than weaken one part too much but I've got access to a decent machine shop so anything can be done.

The one I'm looking at is 1mm out on the centres but I can adjust that on the brace I made today because the rubber inlet manifolds will easily take that.
 
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