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K100-forum.com » Technical, repair and troubleshooting » Engine and transmission » Carburetor recommendations / thoughts please

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tinyspuds

tinyspuds
Life time member
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Afternoon all,

Title was a bit blunt wasn’t it? I want to put carbs on my K100 (lightly modified RT with sidecar).

Inviting suggestions for suitable Carbs, Configurations, and Manifold solutions (Your reasoning would be good too, so that I can sound very knowledgeable when I’m accosted at all the rally’s I will reliably attend).

I have no preconceptions beyond thinking that a performant 1 into 4 solution would be both heroic and beyond my maths and practical abilities (unless you know better).

I am hoping to still have a safe/usable (UK M’way) outfit at the end. I don’t expect to save fuel or improve peformance.  I have no better reason for doing it beyond I want to, I have the down time and I expect it to be fun.

I don’t want to spend a fortune or hire a team of engineers. 

I expect to delete or relocate to the sidecar:

Radiator, Fan, All things EFI, Battery, Maybe OEM HES, Possibly tank and fuel pump and FPR.

I expect to have a working test bed built from those boxes of bits Kpox promised me I would need.

In return, I’ll document the progress of any actual building that goes on. Everything I’m fed back will be collated and organised for publication at the back of the site.

I’m aware its been done before, and I’m not above nicking anyones ideas directly but I also thought it might be interesting to poll insights from a wider base that also know stuff but would otherwise never dream of doing this particular thing.

I’ve been down for 3 months. I’ve binged (and conquered):

- West Wing
- Star Gate SG1
- Star Gate Atlantis
- Deep Space 9
- Time Team
- Voyager
- Yes Minister

I expect a further 3 months that starts with a relocation to the Shed and a major reorg in preparation for the Big Project. Please consider your ideas as an act of Mercy.

TIA, Stu


__________________________________________________
1985 BMW K100RT + Hedingham HUB and LL’s. VIN 0028106.
1986 K100RS in boxes. VIN 0141918.
1954 Royal Enfield 350 Bullet. Original.
2000 Hayabusa with Charnwood chair, Wasp forks and EZS wheels.
    

Laitch

Laitch
Life time member
Life time member
tinyspuds wrote:I’ve been down for 3 months. I’ve binged (and conquered):
- West Wing
- Star Gate SG1
- Star Gate Atlantis
- Deep Space 9
- Time Team
- Voyager
- Yes Minister
Please consider your ideas as an act of Mercy.
Take on Battlestar Galactica if you haven't already.


__________________________________________________
1995 K75 90,000 miles
    

jbt

jbt
Life time member
Life time member
Yes, carbs have been fitted to K100 before.
And the average result is...not very enthusiast. Some tried to fit Mikuni carbs ramp from a Yamaha Genesis engine to keep a quasi vertical intake, some used a common manifold with a car carb.
And some fitted Keihin CR expensive carbs.
Don't expect a better output, but a bigger input with a lowered MPG.

And in your particular case, think about the particularities of a side car: the motorcycle carbs are design to cope with gravity and centrifugal forces that are always parallel to the motorcycle.
On a sidecar, this does not applies: gravity and centrifugal forces are not coherent as you can't tilt.
This caused big problems of fuel starvation on racing sidecars, and led to fit twin bowls, pretty high to sole this issue.
I'd be glad to try carbs on a K100, but on a K100 sidecar, I'm convinced that keeping a fuel injection is the best option.
If you relocate the fuel tank, think about creating a pit for the fuel pump at the bottom, and some walls to keep it always under the fuel level in curves!
I'm often suffering this issue on my K's rigs.
But if you want to use carb, so choose a car carb, with a bowl designed to work in a flat position.

That was my 2 cents!


__________________________________________________
Let us enjoy the transient delight
That fills our fairest day.
    

firstle

firstle
Life time member
Life time member
if you want a easy life and for the conversion to work , single SU every time , get a old type of the Austin and you can run at most angles .

    

Suzi Q

Suzi Q
Life time member
Life time member
Single SU would be my (laughably ignorant) suggestion, because that's what knowledgeable turbo people use. The issue would be the 4-1 manifold. If you could conquer that, then a single carb has got to be the thing.


__________________________________________________
Sometimes I'm not really Suzi Quatro.
    

tinyspuds

tinyspuds
Life time member
Life time member
Laitch wrote:
tinyspuds wrote:I’ve been down for 3 months. I’ve binged (and conquered):
- West Wing
- Star Gate SG1
- Star Gate Atlantis
- Deep Space 9
- Time Team
- Voyager
- Yes Minister
Please consider your ideas as an act of Mercy.
Take on Battlestar Galactica if you haven't already.
Always good to hear from Vermont; Home of the ‘Winters that binge watching was invented for’ Smile. We talking Classic or the modern remake?



Last edited by tinyspuds on Fri Mar 01, 2024 4:59 pm; edited 1 time in total


__________________________________________________
1985 BMW K100RT + Hedingham HUB and LL’s. VIN 0028106.
1986 K100RS in boxes. VIN 0141918.
1954 Royal Enfield 350 Bullet. Original.
2000 Hayabusa with Charnwood chair, Wasp forks and EZS wheels.
    

tinyspuds

tinyspuds
Life time member
Life time member
jbt wrote: wrote:
And in your particular case, think about the particularities of a side car: the motorcycle carbs are design to cope with gravity and centrifugal forces that are always parallel to the motorcycle.
On a sidecar, this does not applies: 

Ooh! That’s interesting.


__________________________________________________
1985 BMW K100RT + Hedingham HUB and LL’s. VIN 0028106.
1986 K100RS in boxes. VIN 0141918.
1954 Royal Enfield 350 Bullet. Original.
2000 Hayabusa with Charnwood chair, Wasp forks and EZS wheels.
    

tinyspuds

tinyspuds
Life time member
Life time member
firstle wrote:if you want a easy life and for the conversion to work , single SU every time , get a old type of the Austin and you can run at most angles .

I like the single carb idea because it looks cool (so what, I’m shallow) and also because of the workspace it creates as opposed to shoehorning a set of 4.


__________________________________________________
1985 BMW K100RT + Hedingham HUB and LL’s. VIN 0028106.
1986 K100RS in boxes. VIN 0141918.
1954 Royal Enfield 350 Bullet. Original.
2000 Hayabusa with Charnwood chair, Wasp forks and EZS wheels.
    

tinyspuds

tinyspuds
Life time member
Life time member
Suzi Q wrote:Single SU would be my (laughably ignorant) suggestion, because that's what knowledgeable turbo people use. The issue would be the 4-1 manifold. If you could conquer that, then a single carb has got to be the thing.

The thread so far (thanks for the nudge) has made me realise that a car solution does provide access to a much larger talent pool iro the gas flow calculations. As for being laughable, you’ve come on a lot since your time in the German bunker during the height of Madass Madness.


__________________________________________________
1985 BMW K100RT + Hedingham HUB and LL’s. VIN 0028106.
1986 K100RS in boxes. VIN 0141918.
1954 Royal Enfield 350 Bullet. Original.
2000 Hayabusa with Charnwood chair, Wasp forks and EZS wheels.
    

Dai

Dai
Life time member
Life time member
tinyspuds wrote:
Laitch wrote:
Take on Battlestar Galactica if you haven't already.
Always good to hear from Vermont; Home of the ‘Winters that binge watching was invented for’ Smile. We talking Classic or the modern remake?
The modern remake if you can get passed the first six or so episodes. It's much darker than the original and confusing, but once you start to figure out the storyline it gets pretty damn good.


__________________________________________________
1983 K100 naked upgraded to K100LT spec after spending time as an RS and an RT
1987 K100RT
Others...
1978 Moto Guzzi 850-T3, 1979 Moto Guzzi 850-T3 California,1993 Moto Guzzi 1100ie California
2020 Royal Enfield Bullet 500
    

Laitch

Laitch
Life time member
Life time member
I never have viewed the original but Battlestar's remake almost got us all the way through mud season once.  Carburetor recommendations / thoughts please 112350


__________________________________________________
1995 K75 90,000 miles
    

firstle

firstle
Life time member
Life time member
i try to fit single SUs to any old cars i get , also fitted one to a 1958 triumph and it was the best running/starting triumph i had , just fitting one to a midget , if you go down this route take some pics as i maybe interested in trying also , having said that the fuel injection on the Ks i have had has been faultless , i did try a single carb on a goldwing (solex) and it was rubbish so went back to standard carbs

    

Suzi Q

Suzi Q
Life time member
Life time member
So here's a daft one.
The Mini cylinder spacing must be similar to the K, because fifteen million K's have been slaughtered so's their heads could be grafted onto asthmatic old rustboxes. Why not try the reverse route? - maybe a 4-1 inlet manifold from a mini could be made to........jeez I can't believe I'm typing this nonsense...Carburetor recommendations / thoughts please 72


__________________________________________________
Sometimes I'm not really Suzi Quatro.
    

Dai

Dai
Life time member
Life time member
Well that's your spring project sorted, Chris. Let us know how you get on Carburetor recommendations / thoughts please 44271


__________________________________________________
1983 K100 naked upgraded to K100LT spec after spending time as an RS and an RT
1987 K100RT
Others...
1978 Moto Guzzi 850-T3, 1979 Moto Guzzi 850-T3 California,1993 Moto Guzzi 1100ie California
2020 Royal Enfield Bullet 500
    

tinyspuds

tinyspuds
Life time member
Life time member
Suzi Q wrote:maybe a 4-1 inlet manifold from a mini could be made to........jeez I can't believe I'm typing this nonsense...Carburetor recommendations / thoughts please 72
Funnily enough, that is where I started. S


__________________________________________________
1985 BMW K100RT + Hedingham HUB and LL’s. VIN 0028106.
1986 K100RS in boxes. VIN 0141918.
1954 Royal Enfield 350 Bullet. Original.
2000 Hayabusa with Charnwood chair, Wasp forks and EZS wheels.
    

tinyspuds

tinyspuds
Life time member
Life time member
firstle wrote:i try to fit single SUs to any old cars i get ….

Hi F, Thanks for these feedbacks.

Would be interested in understanding the form here. The SU was once the SI unit of car carbs. I imagine that there are thousands of 1:4 inlet manifolds designed to accept a single SU mounted vertically and the same number again horizontally (my tech speak on this subject will veer towards Early Primate).

Do you track down a carb of the flavour required considering:

mounting orientation,
engine capacity,
Rev characteristics
Venturi capacity,
Engine interface diameter (as it relates to the bikes inlet size).

Do you then track down a manifold that is close to spec/config and (have someone skilled to) modify it to fit the physical application

Or order one custom made, Where do you find such people?


Thanks and kind Regards,

Stu


__________________________________________________
1985 BMW K100RT + Hedingham HUB and LL’s. VIN 0028106.
1986 K100RS in boxes. VIN 0141918.
1954 Royal Enfield 350 Bullet. Original.
2000 Hayabusa with Charnwood chair, Wasp forks and EZS wheels.
    

jbt

jbt
Life time member
Life time member
If you can tune any carb to fit the engine specs about AFR and these sort of issues, the manifold is another question.
First: because of the geometry of the K engine, any manifold design for a single carb will suck because it will create angles that will limit the gas speed into the hoses.
The inner diameter, the changes of inner diameter, the length (and equal lenght) of the intake hoses are very important, so is the lenght before jonctions of the hoses if you use a single carb. As for the exhaust, this needs to be tuned finely to prevent bad resonances. Using a car manifold, designed for an engine that runs at low RPM is not necessarily adequate with a motorcycle engine with twice max RPM.
You should search for engines with the same geometrical specs, and similar perfs to borrow the manifold.
Or create a new one from scratch, welding hoses. The most difficult may be to achieve a manifold with equal lenght of the 4 intake hoses.
Another option would be to use carbs with top down bodies, designed for flat engines, as VW flat 4 airheads or Alfa Romeo boxers. Better design, but short intake is mandatory.

It's an interesting project!


__________________________________________________
Let us enjoy the transient delight
That fills our fairest day.
    

firstle

firstle
Life time member
Life time member
this one is going on the midget later this year with the gearbox od change
https://ibb.co/LPx6Wvp
 "Using a car manifold, designed for an engine that runs at low RPM is not necessarily adequate with a motorcycle engine with twice max RPM" ........... this is where the SU wins , it has no fixed mechanical mixture position , the problem maybe finding the correct needle and spring , the carb will handle high rpm and the load , not all out performance carb (unless dashpot is removed) but well suited for road use .

    

tinyspuds

tinyspuds
Life time member
Life time member
jbt wrote:Another option would be to use carbs with top down bodies, designed for flat engines, as VW flat 4 airheads or Alfa Romeo boxers. Better design, but short intake is mandatory.

It's an interesting project!
All valid points JBT. My preliminary reading was beginning to seem like manifold design might be a bit more forgiving on the SU vs a bike carb. But I’m not convinced.
2 carbs into 4 at least only needs a pair of matching manifolds, that said: I’m being told it is common place for 1 into 4 so I’m going to keep researching.
No question that the downward dog approach of the flatties is neater but boxers are a bit of a cheat because they basically use a matched pair with the carb at the centre. As you say ‘Interesting’ (esp. when the comprehension level is as low as mine).


__________________________________________________
1985 BMW K100RT + Hedingham HUB and LL’s. VIN 0028106.
1986 K100RS in boxes. VIN 0141918.
1954 Royal Enfield 350 Bullet. Original.
2000 Hayabusa with Charnwood chair, Wasp forks and EZS wheels.
    

tinyspuds

tinyspuds
Life time member
Life time member
firstle wrote:this one is going on the midget later this year with the gearbox od change
https://ibb.co/LPx6Wvp
Bingo First Person, 
Thats the pic I’ve been searching for. Do you imagine that the main duct gets a little bigger (mathemmatical expression) internally towards the two ends to provide for the extra juice/distance or is the SU just not that fussy?
Coupled with a downward feeding (you’d think they’d have a name for those) SU would get the job done perhaps.


__________________________________________________
1985 BMW K100RT + Hedingham HUB and LL’s. VIN 0028106.
1986 K100RS in boxes. VIN 0141918.
1954 Royal Enfield 350 Bullet. Original.
2000 Hayabusa with Charnwood chair, Wasp forks and EZS wheels.
    

firstle

firstle
Life time member
Life time member
the SU is just not that fussy ,  Carburetor recommendations / thoughts please 112350

    

Jordan53

Jordan53
active member
active member
I've had a little experience with SU, mainly with cars.
For sidecar use I would consider using an HIF type, that has a concentric float configuration and should be more consistent when cornering and braking.
SUs are nice and simple, the main challenge being to find an appropriate needle and spring.

I found this webpage helpful:
https://www.classicminidiy.com/technical/needles
The comparison chart is active, allowing comparison of needles live online.

A good resource is other people who have tried and reported on their experiences with needle selection on similar vehicles to your own.
You just have to find them. Smile
An air/fuel ratio kit based on an EGO sensor might be useful too.
Lots of spare time would help!


__________________________________________________
1989 K100 RS

VIN: xxx0149621
    

firstle

firstle
Life time member
Life time member
 HIF type very nice carb but maybe limited for the angle it can be used at , cornering and braking same as when fitted to a car , i like the earlier type which you can rotate the float bowl 

    

K75cster

K75cster
Life time member
Life time member
Removing the plenum and air box is going to leave you with a massive air gap that the radiator can use to clear the air from the radiator, using a four into one inlet and single carby across the engine is going to put that carby in the radiators air flow. A good deflector would allow some cooler clean air to roll into the carby area and save any chances of a boil up. Truth be told you can choose any tank you like with a petcock when this decision is made. I've no idea how you would plug the injector holes. I suppose you can duct some of that hot air from the radiator into the chair, should make winter rides a peach for the pillion.


__________________________________________________
Keith - 1987 K75c with r100rt replica fairing and half of a 1984 K100rt 1992 K1100LT a blue one

The Clever are adept at extricating themselves from situations that the wise would have avoided from the outset - QUOTE from david Hillel in Out of the Earth.
    

Dai

Dai
Life time member
Life time member
K75cster wrote: I've no idea how you would plug the injector holes.
Thread them but leave a ledge at the bottom. Use grub screws and o-rings which will force the o-rings to create a seal. You'd probably need to glue the grub screw in as a backup too. It's a job you'd need to do with the head on the bench to ensure that everything is seated where it should be.


__________________________________________________
1983 K100 naked upgraded to K100LT spec after spending time as an RS and an RT
1987 K100RT
Others...
1978 Moto Guzzi 850-T3, 1979 Moto Guzzi 850-T3 California,1993 Moto Guzzi 1100ie California
2020 Royal Enfield Bullet 500
    

Laitch

Laitch
Life time member
Life time member
Here Ernesto's engine. The result could be conforming to Dai's description.
Carburetor recommendations / thoughts please Screen12


__________________________________________________
1995 K75 90,000 miles
    

Dai

Dai
Life time member
Life time member
Whoever did that had a better idea than mine. Threaded the holes, yes, but took the easier route of an o-ring under the caphead.


__________________________________________________
1983 K100 naked upgraded to K100LT spec after spending time as an RS and an RT
1987 K100RT
Others...
1978 Moto Guzzi 850-T3, 1979 Moto Guzzi 850-T3 California,1993 Moto Guzzi 1100ie California
2020 Royal Enfield Bullet 500
    

tinyspuds

tinyspuds
Life time member
Life time member
Thanks all for such a great level of engagement.  It occured to me that SU enthusiasts might have similar forums to us and equally keen interest in the unexplored.  There’s hundreds! I’ll do some analysis tomorrow and establish contact with a couple of likely candidate groups.


__________________________________________________
1985 BMW K100RT + Hedingham HUB and LL’s. VIN 0028106.
1986 K100RS in boxes. VIN 0141918.
1954 Royal Enfield 350 Bullet. Original.
2000 Hayabusa with Charnwood chair, Wasp forks and EZS wheels.
    

tinyspuds

tinyspuds
Life time member
Life time member
Laitch wrote:Here Ernesto's engine. The result could be conforming to Dai's description.
Carburetor recommendations / thoughts please Screen12

I had loosely supposed I would use some form of the exising arrangement e.g. injectors (maybe modified) and a bolted down rail, in the first instance at least during proof of concept testing.

    

tinyspuds

tinyspuds
Life time member
Life time member
Found my styling cues.

Carburetor recommendations / thoughts please Img_1310


__________________________________________________
1985 BMW K100RT + Hedingham HUB and LL’s. VIN 0028106.
1986 K100RS in boxes. VIN 0141918.
1954 Royal Enfield 350 Bullet. Original.
2000 Hayabusa with Charnwood chair, Wasp forks and EZS wheels.
    

firstle

firstle
Life time member
Life time member
another car engine in a bike , never that good imo

    

tinyspuds

tinyspuds
Life time member
Life time member
Does anyone have any decent drawing with measurements of inlet diameter and spacing?
Cheers.


__________________________________________________
1985 BMW K100RT + Hedingham HUB and LL’s. VIN 0028106.
1986 K100RS in boxes. VIN 0141918.
1954 Royal Enfield 350 Bullet. Original.
2000 Hayabusa with Charnwood chair, Wasp forks and EZS wheels.
    

Ringfad

Ringfad
Life time member
Life time member
K75 for sale in Ireland with a carb conversion. May give you some more ideas

BMW k75 for sale in Co. Meath for €1,500 on DoneDeal


__________________________________________________
Carburetor recommendations / thoughts please Ir-log10

   ;BMW; K100RS Style Black 1987 105K Km     ;BMW; K1 Black 1993 60K Km     ;BMW;  K1100RS Black 1996       ;BMW; K1 Blue 1990 25K Miles
    

tinyspuds

tinyspuds
Life time member
Life time member
Ringfad wrote:K75 for sale in Ireland with a carb conversion. May give you some more ideas

BMW k75 for sale in Co. Meath for €1,500 on DoneDeal
Carburetor recommendations / thoughts please 112350


__________________________________________________
1985 BMW K100RT + Hedingham HUB and LL’s. VIN 0028106.
1986 K100RS in boxes. VIN 0141918.
1954 Royal Enfield 350 Bullet. Original.
2000 Hayabusa with Charnwood chair, Wasp forks and EZS wheels.
    

tinyspuds

tinyspuds
Life time member
Life time member
Carburetor recommendations / thoughts please Img_1416
Carburetor recommendations / thoughts please Img_1415
No draughtsman, but drawings for the project were a must, so here are a couple. I’m using these as a path to informed conversations with fabricator/carb folk rather than a set of numbers I’d die for. TB manifold dimensions, for example, are sus due to deteriorating rubber of the example I measured. Whereas the detail around intake ports, spacing and centres were double checked by cutting out a cardboard template using my numbers.

The story continues.


__________________________________________________
1985 BMW K100RT + Hedingham HUB and LL’s. VIN 0028106.
1986 K100RS in boxes. VIN 0141918.
1954 Royal Enfield 350 Bullet. Original.
2000 Hayabusa with Charnwood chair, Wasp forks and EZS wheels.
    

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