Carb to suit 600cc Single Road Bike (Honda GB) Perhaps Mikuni VM48?

**sammcj** · 14th November 2009, 15:12

I'm looking for an after-market flat-side carb for my Honda GB500 that's been bored out to 600cc that's being a right bastard!

A suggestion from another KB'er was perhaps one of a Honda Dominator / XR600/650 Engine?

I read a review from a guy with a 600cc GB saying the 41mm Mikuni was a good fit.

Thanks in advance.

**xwhatsit** · 14th November 2009, 16:27

Don't take what I said about the VM48 too seriously -- I have no idea what is a suitable size for a 600cc single. Given you're currently running a CV carb designed for a 500cc single, something around the same size (slide carbs are more efficient for a given size) wouldn't be a silly guess, surely.

However the VM (of suitable diameter) would be a good carb; cheap, simple to tune, uses common jets.

A Dommie is a 650 version of the same engine, albeit probably lower compression than what you've got, but would probably be pretty close too, although probably a little harder to get just right given it's a CV carb.

Worth noting the VM isn't a flat-slide carb (a bit snatchy off idle for road use, tend to be expensive bits of racer's kit), it's a simple old round-slide carb. The NX650 Dommie uses a CV carb like the one you already have.

**Kickaha** · 14th November 2009, 16:34

Originally Posted by sammcj2000

I'm looking for an after-market flat-side carb for my Honda GB500 that's been bored out to 600cc that's being a right bastard!

A suggestion from another KB'er was perhaps a Mikuni VM48 or one of a Honda Dominator Engine?

I read a review saying the 41mm Mikuni was a good fit.

Thanks in advance.

How much do you want to spend?

I'd be looking for a FCR41 or even a FCR39 and then taking it to Dynoworx to get it tuned properly

Bigger isn't necessarily better, depends on what you're trying to achieve

edit: is it just because it's being a right bastard you want to change the carb?

**sammcj** · 14th November 2009, 16:40

Originally Posted by Kickaha

How much do you want to spend?
edit: is it just because it's being a right bastard you want to change the carb?

Not sure how much I want to spend, If I could get something straight of a 600/650 I'd probably have a go at installing it myself. So maybe around $80-$120 for a s/h carb in good cond.?

Yes, it's only because it's being a right b!@#h that I wanted to try swapping the carb, KG motorcycles has tried just about everything (in his words) to get the thing to run right, lacks power and doesn't like idling when hot.

The idle air adjustment at the bottom of the current carb seems to be far too fiddly to get right, with it running - adjusting this seems to make no difference weather its fully wound in or out, but after some fiddling today I noticed it did backfire slightly more on down-rev after making an adjustment and once it was hot with it adjusted incorrectly it did rev up and miss a beat over and over (so I put it back!)

I'm so sick of spending money and time on this bloody bike I'm ready to throw the damn thing over a bridge!

see: http://www.kiwibiker.co.nz/forums/sh...d.php?t=111405 and http://www.kiwibiker.co.nz/forums/sh...d.php?t=107205

**Kickaha** · 14th November 2009, 16:47

Originally Posted by sammcj2000

Not sure how much I want to spend, If I could get something straight of a 600/650 I'd probably have a go at installing it myself. So maybe around $80-$120 for a s/h carb in good cond.?

Ah well the carbs I was talking about would cost 8-10 x that

Originally Posted by sammcj2000

Yes, it's only because it's being a right b!@#h that I wanted to try swapping the carb, KG motorcycles has tried just about everything (in his words) to get the thing to run right, lacks power and doesn't like idling when hot.

The idle air adjustment at the bottom of the current carb seems to be far too fiddly to get right, with it running - adjusting this seems to make no difference weather its fully wound in or out, but after some fiddling today I noticed it did backfire slightly more on down-rev after making an adjustment and once it was hot with it adjusted incorrectly it did rev up and miss a beat over and over (so I put it back!)

I'm so sick of spending money and time on this bloody bike I'm ready to throw the damn thing over a bridge!

Save some money and take it to dynoworx in Ashvegas to run up on the dyno, very knowledgeable guys and I would be extremely suprised if they cant either sort it or diagnose it

Waste of time tossing another carb on it when you'll have to rejet it anyway and with an exhaust gas analyzer a dyno run will tell you if it's a fueling problem

If you're going to toss it off a bridge let me know which one and I'll park my ute under it

**sammcj** · 14th November 2009, 16:49

Fair enough, thanks for the input, I'll flick dynoworx an email and see what their rates are.

**sammcj** · 14th November 2009, 16:57

email to honda country:

Hello,

I have a Honda GB that's been bored out to 600cc by KG motorcycles here in CHCH.

I've had nothing but problems from it ever since he touched it, I swear its running slower than it was as a 500, it pops and stops on idle and has nothing after 6000rpm (that is probably the stock headers).

I've thrown so much money at it I'm ready to throw the damn thing off a bridge.

I was wondering if you'd be able to give me a rough idea of how much it would cost for you to tune her up / find out what's wrong.

KG has:

-Swapped the carb with another of the same
-Swapped ignition / CDI over with two other units
-Swapped voltage regulator
-Installed new generator
-New plug

After the rebuild it had been leaking oil around the gasket join, took it back to him a week ago and he managed to snap a head bolt! ended up costing me around $400, I'm not going back there again!

If you could give me some (any!) idea how much it might cost for you to tune / diagnose the issue I'd be very greatful, I love the GB but can't afford to have it parked in the garage with issues all the time.

**sammcj** · 17th November 2009, 12:25

Had a good reply from Honda Country / Dynoworx

Hi Sam,

Sorry to hear about your dramas with the GB. Your tech guy seems to be heading in 2 different directions with electrical and carburetion fixes, so it is hard to know what is wrong with the bike.

I would suggest we first test the ignition system to ensure it has the correct spark voltage, and then we put the bike on our dyno to check the carb jetting which I suspect is out. The dyno uses an exhaust analyser to measure the air fuel mixture accurately that allows us to jet the bike correctly.

Approx cost would be $300 for the first hour of dyno time,then $100 per hour there after, but we should be able to jet a single in an hour.

Murray Sexton

Workshop Manager

Honda Country

Ph/fax 03 308 2030

workshop@hondacountry.co.nz

**vifferman** · 17th November 2009, 12:58

How come, if KG were the ones who snapped the head bolt, you were the one who had to pay for it? If they had any integrity, surely they'd admit liability and suck up the cost?

**sammcj** · 17th November 2009, 13:02

Originally Posted by vifferman

How come, if KG were the ones who snapped the head bolt, you were the one who had to pay for it? If they had any integrity, surely they'd admit liability and suck up the cost?

He said it was just one of those things, 'it was seized and when I went to un-do it it snapped of straight away!'

When I first got the bike bored out he quoted me around $1200 and it should 'want to lift up on the back wheel!', which turned into $1500, which turned into $1800 which turned into $2100....

Not going back there again...

**Kickaha** · 20th November 2009, 22:48

Originally Posted by sammcj2000

Had a good reply from Honda Country / Dynoworx

So will you be taking it down to them?

From my experience and that of my friends they are very good to deal with

**Dodgy** · 21st November 2009, 13:32

You will need a sidedraft carb - I put on a Sudco special Mikuni 41mm flatslide. It made a huge difference.. It looks to be a Mikuni 38mm offset bored to get 41mm

I doubt that you will be ok with an FCR as the carb will not be at the right angle to work. On my old 630 Honda racer, I had the inlets welded up and recreated to be at a 45 deg angle. The old twin Mikuni 34mm carbs didnt work quite right, but when I bored out the inlets a bit more and put on a pair of 38mm FCRs - what a difference! Too extreme for the road tho...

**sammcj** · 21st November 2009, 18:31

Thanks Dodgy, I'll note it down

Kickaha - Yup, I waiting on an email call back with a time / date, aparently their only dyno guy is out of town next week

**Kickaha** · 21st November 2009, 18:59

Originally Posted by Dodgy

I doubt that you will be ok with an FCR as the carb will not be at the right angle to work.

You can get sidedraft FCR carbs so there should be one that would be suitable

**Motu** · 21st November 2009, 20:09

I saw a brand new in the box 36mm Blue Magnum for $50 at the Claudelands swapmeet a couple of months ago.I've got a 36mm blue Magnum (and a 30mm,and two 34mm's) that I ran on a TM400,and it came off an XR600.Most easy to tune bolt on carb I've ever used.