The Motorbike Thread

[Boumsong]

What carb and engine is it? Got quite a lot of experience with mikuni vm but the principles can be swapped from one to another. What do you want to know?

GSX1100F carbs, early 90's I believe. Currently being worked onto a cammed ABF (2.0l 16v mk3 golf engine). I basically want to know how best to weak them for accurate fueling. When I did a conversion like this before it'd do a tank of fuel in 80ish miles around 3mpg..Hoping for a bit more than that this time. But also trying to get it to be a reliable (ish) starter too.

[bing]

You'll need bigger main jets definitely. Best thing to do is go on ebay and get a dynojet kit, it comes with loads of stuff, needles, main jets, pilots, stronger springs, all that shiz. Starting is all to do with the pilot jets and the air screw, midrange is needle, high rpm is main jet. You need a baseline setup. Personally, after experience with gsxr1100 engines, I'd say get some carbs of one of those, the ones off an 1127 engine. The gsx1100f was only a 1052, and was tuned for midrange rather than all out power.

[forteh]

I'm not completely familiar with running the CV carbs on the gsxr/bandit/gsxf lumps but as bing said you will need to up the jetting all round; essentially you are putting a set of 1.1 litre carbs on a 2.0 litre motor. The revs are a lot lower though so it probably won't need to be a huge increase.

Effectively, closed to 1/4 throttle is controlled by the pilot jet, either a fuel screw (wind out to richen the mixture ) or an air screw (wind out to weaken the mixture). Too weak and it will spit back through the carbs when you blip the throttle, too rich and the exhaust note will be dull and smokey.

1/4 to 3/4 throttle position is controlled by the needle position in the main jet, you can raise or lower the needle relative to the slide (move the circlip down/needle up to richen the midrange and circlip up/needle down to weaken the midrange).

3/4 to full throttle is purely main jet control, obviously changing the main jet will affect the midrange needle.

As I said I've not spent much time on CV carbs but the above should still hold true. I'm more familiar with direct pull throttle slide carbs like the mikuni vm, the only real difference is that the CV carbs rely on inlet vacuum on the diaphragms to raise the slides.

There are numerous tweaks you can apply to the CV carbs to get then to respond better, increasing the size of the pilot air hole in the slides will cause them to lift faster and give better bottom end fuelling if you jet accordingly. Also don't forget that the old oil burner gsxr motors were jetted richer on the two middle cylinders to compensate for heat build up, on a water cooled engine this might not be so applicable.

I always thought the gsxf1100 was the long stroke 1127 motor as opposed the short stroke 1052, being a sports tourer, the extra torque of the bigger motor would make more sense. Fwiw the 1052 gsxr motor used 34mm carbs.

[Boumsong]

Would you recommend a different set of carbs over those then? I'm not set in stone on which ones to use just yet.

[Muel]

They can all be made to work. A carb is a carb really!

I'd go for Mikuni flatslides though. Expensive and not awesome (but not shocking) on fuel, but very tuneable. Bike carbs are often very sensitive to the airflow, so if you take their standard airbox away and put them on a bigger engine, that's 2 issues you'd need to address. Honda CBXs are famous for being utter bastards to get the fueling right if you make any changes to the air/exaust flows.

[forteh]

For base line getting it might be worth talking to Allens Performance, they know their onions with mikuni and keihin and might have already sized jets to sit that size of motor

Edit: http://www.allensperformance.co.uk/

Edited May 5, 2013 by forteh

[bing]

If it were me doing this, I'd try and find the carbs from either a gsxr1100K, L, or an M. The were the semi flat slide ones, and were the most tunable. And you don't need a massive air box with them, as most people jetted them to run with individual filters. I'm curious as to why you want to run bike carbs though, as opposed to a set of weber 40/45s.

[Boumsong]

Because webers go for upwards of £500 and bike carbs can be had for £60ish... Space is another isue too, theres not really a great deal of room at the front and the bike carbs are shorter/smaller and tuck behind the radiator nicely.

[Dom_capper]

Hi,

I am looking into getting my bike license because cars are just so expensive.

I already have a full car license, so would i need to do a theory test still once i have done my CBT?

Also what bikes should i be looking at?

Cheers

Dom

[Ben Cox]

Hi Dom. Yep you will still need to do a motorcycle theory test. Bike wise is very much down to personal tastes, budget etc. I wouldn't jump straight on a zzr1400 or an R1 though, as you'll probs wrap yourself round a tree! Most things in the 400-600cc range will do you well.

BEn

Edited May 5, 2013 by Ben Cox

[bing]

So the old man is getting a new toy, after getting bored with his triumph street triple 675, I did tell him at the time he'd be bored of it inside of a year, having come from a gsxr750 k7, to this. He says he can't make it do what he wants it to. So step forward this thing of beauty

Should be a marked improvement. I went to look at it with him on Saturday, and within 20 mins he bought it. It is bloody immaculate, not a mark on it. Its getting the belts done before he picks it up. Proper tackle now

Edited May 17, 2013 by bing

[Muel]

WORDS! MANY WORDS!

To be honest first of all Dom, how old are you? Without that info I wouldn't advise anything lol. Edited May 18, 2013 by Muel

[Revolver]

So yeah, I bought a 1982 Honda CB125T. I'll get my hands on it on Thursday. I've been digging and apparently some CB125Ts had 17bhp, which sounds nice. They de-tuned it to 12-odd bhp around 1982 so I'll have to have a closer look to see what I've got.

Really looking forward to it!

[Muel]

They're fairly nice little bikes I believe, but unfortunately that year had the 6 volt electrics... Good enough if you keep all the connections clean and decent though.

I'm not certain but I think it was pegged at being a littler version of the CB250RS, which are fantastic bikes. Always wanted one of those.

Should be fun!

[Revolver]

6 volt electrics? Is there something I should know?

Yeah, what I read said that one of the the 125 versions was nearly as fast as the 250.

I wish I could remember where I put my dad's leather jacket. I've got another one here but it's a little bit tight.

[Muel]

6 volt electrics? Is there something I should know?

There's a reason that almost all bikes use 12v now. 6v is OK but not really powerful enough for a lot of stuff.

Yeah, what I read said that one of the the 125 versions was nearly as fast as the 250.

Maybe nearly as fast as the CB250T, but not the CB250RS if you ask me. The RS had 28bhp and weighed roughly the same but handled quite a lot better. (Not that the 125 handles badly or anything. They still handle better than CGs I believe).

[Revolver]

Maybe I can swap out the 6v setup for. 12v setup.

Can't wait! Need to get a CBT booked.

[forteh]

Maybe I can swap out the 6v setup for. 12v setup.

Can't wait! Need to get a CBT booked.

Nope, the generator will be 12v, you would need to replace the whole lot - probably cost more than the bikes worth to get a conversion kit if they're available.

6v is ok, you just don't get bright headlights

[Revolver]

Oh. Oh, that doesn't sound great! I wonder if brighter bulbs would help at all.

[forteh]

The concept of 6v has escaped you hasn't it?

It's like trying to ride a bike up a hill with 1 leg

As long as you don't expect to have a nice white beam to flood the road in front of you then you will be fine.

[Revolver]

I get it, I get it.

I'll carry a torch then

[Muel]

Soooo, 10 years ago today David Jefferies died at the Isle of Man. Still remember it like it was yesterday, which is wierd as I rarely "connect" with a particular racer over another. Meh, RIP.

[Revolver]

The bike is now in my garage! There's a few things to fiddle with, namely the foot brake light activation spring and the choke cable.

otherwise It's all good really.

[Dom_capper]

Pictures?

[Muel]

Yay! When's your CBT booked? Or have you done it already?