What's the general consensus on 2 into 1 intakes?

[xswillie]

Hey folks,
What's the general consensus regarding two into one intake manifolds versus a two carb setup? I kinda like the one from rebel spirit. I think joewiseguy used to make one too.

Experiences? Recommendations?

I'm considering one of these with a VM32 for ease of operation and tuning, or I'm considering dual VM32s.

What would years of experience say about this?

 

[gggGary]

Second hand: different length manifolds and somewhat convoluted routing make getting it running even, and fuel puddling typical issues.
As always making stock parts work right is less effort in the long run. look up the Kawasaki carb thread for an option.
kwacker karbs

 

[atom4488]

Would you accept years of automotive experience, but not motorcycle-specific experience? Years of modding VW, Datsun and BMW 4-cylinder engines that typically had single-barrel or two-barrel carbs mounted on log-type manifolds. Nothing wakes an engine up more than a carb-barrel-per-cylinder induction setup. Think Weber DCOE's on a Datsun or BMW, Weber DCNF's or IDA's on a VW or a throttle-body injection system with one throat per cylinder. IMHO, going to a single carb/split manifold is a performance downgrade.

 

[xswillie]

Ok, so mikes sells what looks like a good VM32 kit, but there are no vac ports on the intake boots.
How do I do the carb sync and how do I open my petcocks up?

 

[Grewth]

I'm sure that you can buy non-vacuum petcocks, just off, on, reserve.
Not as safe as vacuum items but they don't play up.
You can balance your carbs the way they used to do the old British twins when twin carbs first came in.
I think you have to run one side at a time to set the tickover, ground out the spark plugs.
Larger throttle openings by ear/feel.
Helps if you have a 2 into 2 exhaust.
Alternatively, you could modify the manifolds to add vacuum ports.

 

[Grewth]

Would you accept years of automotive experience, but not motorcycle-specific experience? Years of modding VW, Datsun and BMW 4-cylinder engines that typically had single-barrel or two-barrel carbs mounted on log-type manifolds. Nothing wakes an engine up more than a carb-barrel-per-cylinder induction setup. Think Weber DCOE's on a Datsun or BMW, Weber DCNF's or IDA's on a VW or a throttle-body injection system with one throat per cylinder. IMHO, going to a single carb/split manifold is a performance downgrade.

If pure performance is what you want, that is probably true.
The more fuel/air you can ram into an engine, the more power you will get.
If you look at the difference in performance on the old British bikes that had twin carb and single carb options, the single carb bike would typically make 85-90% of the peak power, torque dropping behind at high rpm.
Flexibility and traction at low revs was generally better, and the big bonus was fuel economy.
Single carb 650's would generally return 55-60 mpg, twin carbs 40-45.
That's on imperial gallons BTW.
In affluent 1960's America, with gas at $0.25 a gallon, that didn't really bother anyone.
Buyers wanted maximum performance, the cost of the extra fuel was irrelevant.
Unfortunately for us, Yamaha never considered a single carb option, or they wouldn't have designed the frame with that vertical tube so close to the head.
It would be interesting to see what an XS engine would do, suitability equipped with an equal length manifold and a good CV type carb, like the SU conversion for the Norton Commando.
Unfortunately you would need a special frame to achieve that.

 

[Raymond]

I agree with what @Grewth says. Ran a Triumph TR6 650 twin. The Bonneville was a twin-carb special version made for the US market. Have looked at performance reports which say the Bonnie made maybe 1, 2 extra hp but lost torque lower down the rev range. So, the area under the torque curve is greater for the TR6 making it more of a rider's bike? Plus the better fuel economy and easier to keep in tune.

Though IMHO the XS650 with twin CV carbs doesn't feel like it lacks torque. I guess the CV carbs were new tech for motorbikes back in the '70s?

 

[Grewth]

Though IMHO the XS650 with twin CV carbs doesn't feel like it lacks torque. I guess the CV carbs were new tech for motorbikes back in the '70s?

I've read that the original XS1 prototypes were actually fitted with British made Solex carbs, but for production, manufacture was licensed to Mikuni.
Hence the Mikuni - Solex cast into the early model carb tops.
So if these excellent carbs were available to the British industry, why weren't they used ?
The only logical answer is the basic tight fistedness of the British industry.
To the best of my knowledge the only British twin ever fitted with a single CV carb was the Triumph 6T Thunderbird in the 1950's
That had a SU carb fitted as standard.
But Triumph dumped that and went back to a slide type carb.

 

[Jim]

I guess the CV carbs were new tech for motorbikes back in the '70s?

Earliest use that I'm aware of is the '65 Honda 450. 32mm Keheins iirc... CB350's had similar.
I'm sure there were others earlier, but that's probably the first large production run that used 'em.

 

[BluzPlayer]

I have a Joe Wiseguy 2-1.
Have yet to run it, but fully intend to.
I have ZERO concerns about how it will perform in my 750, Shell 1 motor.
They've been around forever.
I ran one on a different bike long ago, also unequal length runners without issue.
There may be more unequal length runners on vehicles running around than there are equal.
However...
@Grewth is correct about TOP horsepower in a N/A engine will almost always result from having a single well matched, balance tuned carb per cylinder mounted to a straight intake runner that has also been tuned for length.

Of course I'm not building a race bike.
And I have some VMs if I change my mind or run into fabrication issues.

 

[gggGary]

Here's' a big 1 into 2 thread https://www.xs650.com/threads/2-into-1-intake-manifold.49
There's a huge thread on a one into two conversion think it's a Joe's manifold. TeeCat and went on a TX650. He was east coast and nice fella physiologist IIRC.
Found it! https://www.xs650.com/threads/single-carb-conversion-thread.3647/
When he sells the bike https://www.xs650.com/threads/1973-yamaha-tx-650-cafe-roadster.44625

 

[WideAWAKE]

Ok, so mikes sells what looks like a good VM32 kit, but there are no vac ports on the intake boots.
How do I do the carb sync and how do I open my petcocks up?

Manifolds have a threaded hole with an Allen plug.

Unscrew it and screw in a barb to synch with.

Suppose you could permanently mount a barb to it and run your petcock off of.

I’d ditch the vac petcock but that’s me.

Obviously this is if you are gonna run 2 carbs and not a single.

 

[badlegbill]

I have run single and double carbs on British and Japanese twins and have found that keeping slide carbs exactly synchronised was slightly less trouble than balancing CV carbs acceptably. The behaviour of air and mixture is different and generally dealt with by manufacturers well, incidentally the issue was sorted by Belfast University in Northern Ireland for the 650 twin amongst other Yamaha models of similar vintage.
I have some news for Raymond, all Bonneville models were made and sold throughout Europe although what were called US specification models were exported they were also available in the UK, as an option for both T120 and T140 with the styling differences being mostly cosmetic although early US spec. had handlebar controls reversed at manufacture and larger tanks. Thruxton models produced slightly more power but needed frequent vacuum balancing to run consistently.
I still have my balancing tool, which works on my XS, is basically a double ended pipe with a small window showing a caged red ball and a scale. You clipped the pipe to the inlet each side and adjusted to set the ball in the middle of the scale, to check you simply swapped sides.
At my age I no longer chase power and am happy with broad torque curves so a single carb is my preference if on a same length inlet, I do not have any experience with unequal length inlets and am a bit suspicious about getting stoke (stochiometric ratio) on both cylinders with balanced heat generation.
It is not so much an issue in the UK because vehicles more than 40 years old can be considered as "Historic" if not significantly modified from original spec. Historic vehicles are exempt from the annual roadworthiness test, there is no way for them to pass the emissions check, do not pay Vehicle Excise Duty, which is an annual payment to the state for the privilege of using the roads and are cheap to insure.
I have several bikes which have virtually no fixed costs and my favourite car is a 50 year old Pinto engined Ford with a single carb.

 

[Raymond]

I have some news for Raymond, all Bonneville models were made and sold throughout Europe although what were called US specification models were exported they were also available in the UK, as an option for both T120 and T140

Yes, I knew that, what I meant is, often said the impetus to pursue the extra power came from the requirements of the US market. And of course selling the souped up version as a Bonneville.

 

[Grewth]

If it's of any interest to anyone, there's a sort of strange conundrum with the 750cc Triumph twins.
The detuned 750 twin launched for 1973 didn't actually go any faster than the 650, although bottom end and mid range torque was improved.
And one British magazine actually got a 750 Tiger (single carb) that was 1mph faster than the 750 Bonneville they tested.

 

[badlegbill]

The rumour at the time was that the oil in frame T140 cracked enough to leak if tuned for higher revs, without expensive dynamic balancing, so back to Mr Shelby.
Horse power sells cars (bikes) torque wins races.

 

[Grewth]

The rumour at the time was that the oil in frame T140 cracked enough to leak if tuned for higher revs, without expensive dynamic balancing, so back to Mr Shelby.
Horse power sells cars (bikes) torque wins races.

The early ones seem to have cracked anyway, around the swingarm pivot centre section.
Then they found some way to prevent this.
Just about.
I can well believe that a higher state of tune could easily induce more cracking.
These frames are incredibly light for a 750, especially a non-balencer shaft 360° twin.
Maybe BSA-Triumph thought that they would wow the world with their super light excellent handling rolling chassis.
And this would somehow compensate for their aging. out of date, OHV engines.
Too little, too late.

 

[badlegbill]

The best of both worlds in those days often resulted in tuned Triumph 650 and 750 twins being shoehorned into a Norton Featherbed frame with Triumph twin leading shoe front brakes.
I was once the proud possessor of such a machine built by John Tickle.
Unfortunately the frequent servicing persuaded me that an XS 650 would more easily fulfil my needs.
The Triton, for this is what such a combination was labelled, handled and rode beautifully but I could not afford both otherwise I would have kept it.
Some tried widening the Featherbed bottom rails to accept the XS650 twin but never really successfully as far as I know.
I might look at having the frame copied with enough width in the lower rails and matching it with modern suspension and discs.
Has anyone got a successful Featherbed conversion. Oh the reason for dropping the XS within the frame is that the heavier engine if sat too high prejudiced the handling.

 

[bosco659]

Would you accept years of automotive experience, but not motorcycle-specific experience? Years of modding VW, Datsun and BMW 4-cylinder engines that typically had single-barrel or two-barrel carbs mounted on log-type manifolds. Nothing wakes an engine up more than a carb-barrel-per-cylinder induction setup. Think Weber DCOE's on a Datsun or BMW, Weber DCNF's or IDA's on a VW or a throttle-body injection system with one throat per cylinder. IMHO, going to a single carb/split manifold is a performance downgrade.

I second that opinion from an automotive perspective. I had a ‘71 Datsun 510 that I fitted with a pair of Weber side draft carbs and boy did that wake the engine up. Cools to look at with 4 SS velocity stacks too, oh and the sound

 

[AUTOMAN]

2 carbs into 1 inlet? WTF.

Why?