Topic: Honda CB550 '78 sohc carburettor problems - Help!

[sosolidpoo]

Call me stupid..... Ive just bought a cb550 which someones decided to fiddle with in the past. It has 'bolt on' air filters which look good but are a nightmare to get working properly. Its been stood for a while so it was a pain to start as you would expect. So i completely re-conditioned the carbs and fuel tank, cleaning everything, but it still would only just start on full choke and die without the choke being on full. To cut a long story short, and after hours of messing about with mixture screws, new spark plugs/leads and even new air filters, it still will only run on full choke. So i took the bull by the horns so to speak and bought another set of carbs from my local scrap yard, and have given them the once over like the last ones. Ive put them on the bike and i now am able to get it started. Hooray. BUT... they seem to be slightly different to the originals. They physically look the same, front and back screws on the slider caps, they fit without a problem, the main jet sizes are the same (k.90) etc. but there are 2 main differences. The air intake (manifold) is slightly bigger which means my new bolt on filters dont fit, not a big problem, and there are 2 pipes which look the same as the inlet pipe for the fuel (slightly smaller and are made of black plastic, not metal, situated in between the carbs. What do they do? are they overflow pipes in case of too much fuel? are they vacuum pipes for air/fuel tank? Help! the bike seems to run ok(ish) without touching them or puting a pipe on each of them, do they need to be linked together like a balance pipe? i think i may have the k series engine carbs, or maybe ive got carbs off a larger engine. Dont know. If anyone can throw some light on this subject i will be forever in there debt. Please respond with your email address if you want to see a picture and i will try and send you one asap. 

[SteveD CB500F]

Hi John.  That was quick!  The HOC board is a good "recruiting ground" for these forums!

Where are you?  I have a '77 K3 and will try and help where I can.  In the meantime, here's the 500/550 carb data that I have at the moment (couldn't post this on HOC as I couldn't work out how... 

Steve in Wokingham, UK

[sosolidpoo]

Hi Steve, thanks for the response. I dont seem to have a link or anything to click on..... is it me? i am in salisbury which is near southampton ish. 

[SteveD CB500F]

Oops.

Here it is...

[sosolidpoo]

Thanks Steve. I should have PD46A's on it. The originals are 46a's but they wont run on the bike. The one's that ive fitted are a PD51A. Any idea which these are off of? cb650, 750 maybe? Ahhhhh!

[SteveD CB500F]

Here's the list for 750s.  PD51 doesn't appear on it.

http://hondachopper.com/carb.specs.html

Anyone?

[TwoTired]

Call me stupid.....

I don't always do requests.  But, I'll make an exception in this case.
You're stupid. 
Bear in mind, I'm not really qualified to make the evaluation.

but it still would only just start on full choke and die without the choke being on full. To cut a long story short, and after hours of messing about with mixture screws, new spark plugs/leads and even new air filters, it still will only run on full choke

I'm guessing you didn't pull out the pressed in slow jets, cleaning them and the passageways behind them during your cleaning.  This will leave you with very little fuel for low throttle positions.  And, the choke will bump up the throttle and increase vacuum to draw bonus fuel from the main throttle valve to get it running at all.

there are 2 pipes which look the same as the inlet pipe for the fuel (slightly smaller and are made of black plastic, not metal, situated in between the carbs. What do they do?

Could these be float bowl vents?  Your original carbs should have bowl vents, too.  And, interlinking tubes for this purpose, also.

I have no hard data for support.  But, could these PD51A carbs be from a CB500 K3?  We didn't get those in the states.  But, it seems odd that the original air box couplers wouldn't work.  I dunno.

Don't call me stupid.

Cheers,

[SteveD CB500F]

Hey Lloyd, we have discovered that these PD51s are from a 650!

Maybe the extra pipes are for the fuel tap vacuum ??

[TwoTired]

Hey Lloyd, we have discovered that these PD51s are from a 650!

Maybe the extra pipes are for the fuel tap vacuum ??

Yeah, I read the other posts after I shot my mouth off.

If the extra pipes are for fuel tap vacuum,  shouldn't they be capped off?  Also, you could probably feel the vacuum with your finger.

I give up...