What the f!@?! is wrong with my bike!? (Honda GB)

**grotto** · 4th November 2009, 16:01

I had a similar problem with my GB which was choke related. I took the choke cable out of the choke lever, and it was fine. I ended up making a new inner for the choke cable and re-fitting it. I think the old one was a bit short.
Have you checked the inlet manifold isn't leaking? I also had that with mine.
Good luck, and nice bike, worth sorting out, although it sounds like you've soent a fortune trying to get it fixed already

**sammcj** · 5th November 2009, 19:37

OK so...

I left the bike at KG today, he wanted to have a look at it before I go pulling things much further apart.

Came back to it and took it for a spin, seems to be all good!
Trotted back to KG to ask what he'd done turns out he's not 100% sure what actually fixed it. (It still pops at times, but it's manageable)
He said all he did was adjust the idle mixture screw on the bottom of at carb, which he has adjusted before and did not touch when he did the head recently.
The weird thing is, he recons he only screwed it all the way in, then back out to roughly where it was!
He said he may have knocked the choke cable but he checked that and he thinks the cable is fine (mind you he did make it up so might be trying to cover his ass?)
ANYWAY

As for the blinckers flashing too fast when under rev's that was due to him(?) mistakenly putting one bulb in that was the wrong wattage!

Took her for a scream at the port hill to watch the sunset and she went well, didn't stall on me once, is definitely lacking a bit of power perhaps due to the crappy stock pipes.

So I guess it's good and bad news, the good is it's fixed - the bad is that we still aren't 100% sure what was wrong.

I'm going to keep and eye on it and see how it goes.
Next step: Tighten up the steering a bit, replace front lamp and break shoes.

Thank you all for your input and advice, I'm sure you be using it when this happens again in the future (not doubt you'll hear about it!).

-Sam.

**dino3310** · 5th November 2009, 21:49

on ya mate

**laserracer** · 6th November 2009, 05:44

Sweet... glad its seems to be fixed

**sammcj** · 6th November 2009, 16:23

Little update:

Just went out and gave her a bit of shit in the countryside.
She popped once after pushing it up a straight then coming to a stop - I'm not too worried about this.

She's definitly lacking some power, it was actually somewhat better when it was running worse.

-There is big, disappointing _nothing_ between 6 - 8k RPM.
-It took roughly 4 seconds from sitting at 80Km to speed up to 100Km in 3rd gear.

Bit of a shame, mechanic said she'd really fly as a 600 but it seems slower than a factory 500 (except perhaps at the low-end, FYI Factory 500cc 0-60 is 5.4s)

If I had more money to throw at it I'd probably get another set of headers and take it somewhere else for a tune-up, but being a part-time student I'm out of luck there so I'll keep on plodding along with it as is I guess, I was hoping to take her down to Oturehua later this month but I don't think I'd trust her.

**CookMySock** · 6th November 2009, 16:32

Originally Posted by sammcj2000

he recons he only screwed it all the way in, then back out to roughly where it was!

If there was some shit in the the idle jet, that might clear it out.

Steve

**xwhatsit** · 6th November 2009, 16:57

Originally Posted by sammcj2000

Little update:

Just went out and gave her a bit of shit in the countryside.
She popped once after pushing it up a straight then coming to a stop - I'm not too worried about this.

She's definitly lacking some power, it was actually somewhat better when it was running worse.

-There is big, disappointing _nothing_ between 6 - 8k RPM.
-It took roughly 4 seconds from sitting at 80Km to speed up to 100Km in 3rd gear.

Bit of a shame, mechanic said she'd really fly as a 600 but it seems slower than a factory 500 (except perhaps at the low-end, FYI Factory 500cc 0-60 is 5.4s)

If I had more money to throw at it I'd probably get another set of headers and take it somewhere else for a tune-up, but being a part-time student I'm out of luck there so I'll keep on plodding along with it as is I guess, I was hoping to take her down to Oturehua later this month but I don't think I'd trust her.

So you bored out a big single a large amount (I remember reading you upped compression a good whack as well?) and now it doesn't idle well... hmmm....

I know it's a common modification on the RFVC engines but still, it's not exactly world-shattering to end up with those results.

Having flat spots in your rev range is surely an issue of inappropriate carburettion for the capacity increase. Put the fiddly CV carb away, get hold of a big simple slide carb that's of appropriate size (probably 2mm smaller or the same size as the existing CV carb?) that's easy to tune and have it put on a dyno by somebody with plenty of needles and jets. May even find the recalcitrant idling improves!

Automatic decompressors are an accident waiting to happen. I've got a generation older single than yours but my decompressor has caused exhaust valves to stay open when starting (where's all my compression gone!) and in other circumstances refused to work properly so I took the little cam on the kickstarter out. Simply roll the bike backwards in first gear til TDC before kicking. The RFVC decompressor might be very reliable but it's still another variable.

However, intake leak would cause all of those problems (idling, flatspots, poor general performance). I assume somebody's sprayed carb cleaner all around the intake manifold joints while it's running to see if the engine does anything?

**sammcj** · 6th November 2009, 17:03

Originally Posted by xwhatsit

So you bored out a big single a large amount (I remember reading you upped compression a good whack as well?) and now it doesn't idle well... hmmm....

Actually I didn't up the compression.

Put the fiddly CV carb away, get hold of a big simple slide carb that's of appropriate size (probably 2mm smaller or the same size as the existing CV carb?)

Who do you think I should speak to about procuring / fitting?

Automatic decompressors are an accident waiting to happen.

hmm I might just do this!

I assume somebody's sprayed carb cleaner all around the intake manifold joints while it's running to see if the engine does anything?

I'm not sure what KG did, put it this way... I wouldn't trust that he did.

**Ixion** · 6th November 2009, 17:04

Well, actually you DID up the compression, By quite a lot, unless you relieved the head.

**sammcj** · 6th November 2009, 17:12

Oh really, didn't realise it worked like that, Shows how much I know about engines.

**xwhatsit** · 6th November 2009, 18:59

Originally Posted by sammcj2000

Who do you think I should speak to about procuring / fitting?

Something like a Mikuni VM-series (VM48 too big?) could be ordered from a myriad of sources on teh interweb cheaply. Fitting it is something you could do at home very easily; may need to have a new throttle cable made up (then again the VM is cable-pull like a CV carb, wonder if you could get away with the same cable end) and perhaps a bit of swearing getting the intake manifold to fit over the carb stub but nothing a complete bodger like the average KBer couldn't handle.

Originally Posted by sammcj2000

I'm not sure what KG did, put it this way... I wouldn't trust that he did.

You can do that at home. Buy a can of carb cleaner, get the engine warmed up, spray liberal amounts on the interface between head and manifold and manifold and carb (less likely). If there's any change in engine note (the engine might pick up or suddenly miss or even stop -- you may see bubbles as the carb cleaner gets sucked in) then you have a leak. Go right around, check on both sides of the bike. You might like to try it with a bit of RPM too.

You might be able to tune the stock carb to match the increase in bore, but a simple slide carb is very easy to tune comparably. Surely the best thing is to get it onto a dyno once you've checked everything else (the valve clearances sound interesting, had no idea they could cause trouble like that), either with the stock carb or a new one, and get a proper expert to look at it under load.

Mebbe you could flog a carb off a Dommie, that might match well with minimal fiddling?