Problem servicing Mikuni carb on Hondq GCV520

Hi Guys,

I was servicing the Mikuni twin-choke carburettor from a GCV520 engine (GJAKM-1053057) which is installed in a Honda lawn tractor HF2315 HME.

After disassembly I noticed that the two rubber caps on the main bleed pipes were broken off - perhaps it happened when I inverted the carb to remove the air vent cover. (I found the remains of them on my bench and the rubber was very brittle.)

Problem is there's a bit of the rubber cap still stuck in the main bleed pipes which I can't get out.

The main bleed pipes are not removable so that's not an option and I don't want to poke the remains down with cleaner wire because I'm not sure where they'll end up. It looks like the bleed pipes go into the centre of the carb and curve out into the venturis - can anyone confirm this? If so, it might be worth taking a chance on poking the remains straight down the tubes into the venturis where I can easily extract them. My concern is that there could be passageways not visible that they could get jammed in.

Can anyone advise me on this please?

Thanks,
Chris

maxrevs, Have you looked for a cutaway drawing? How about motorcycle applications> Those tubes are also known as emulsion tubes, admitting air to mixture?

Have you tried blowing some carb cleaner up through the hole in question?

motoman said:

maxrevs, Have you looked for a cutaway drawing? How about motorcycle applications> Those tubes are also known as emulsion tubes, admitting air to mixture?

Click to expand...

I'll have a look this evening for a cutaway drawing - good idea.
Yes, I always call them emulsion tubes too, but the parts list called them main bleed pipes so perhaps the terms are interchangeable.

BlazNT said:

Have you tried blowing some carb cleaner up through the hole in question?

Click to expand...

Unfortunately the only end of the tube that's accessible is blocked by the remaining stem of the rubber cap.

I think I might try a tiny drill bit to see if I can fragment the rubber stems enough to remove them as debris …hmmm…

How 'bout a stiff wire with a little, deburred hook on the end What is the role of the rubber - dirt block??? I remember Weber emulsion tubes , but cannot remember if the little brass caps had a hole in them. If not functional how 'bout a chemist here suggesting a softening agent? Recently learned about nitro methane softening lock tite. Try googling "agent to soften rubber."

Edit. Still on the motorcycle subject...Mikuni was a widely heralded Japanese carb before fuel injection so it would seem there is an extensive literature somewhere. Perhaps antique motorcycl forums, archives , etc. You might be surprised what you find.

I would find something I could get in there. The tube from carb cleaner spray can. Vacuum hose. Doesn't have to be a sealed just as long as you can spray something through it. Hell even high pressure air pointed at it would work unless its just jammed in there.

Yikes, I looked around online and Mikuni is big on aftermarket Harley carbs and silent on the one you have. The drawings are fuzzy and not complete. I did see a single throat model where the emulsion tubes were threaded. Have you tried to unscrew them? Jacks parts has some parts , but not the tubes.

Edit. I saw a good up close overhaul of a single throat Mikuni on youtube. The guy also drilled out a brass welch plug to get at a jet. The only Mikuni website I found catered to after market performance...Harley

Re: Problem servicing Mikuni carb on Honda GCV520

Thanks for all the input guys, much appreciated.

There's a dealership not far from me who I get along very well with and buy a lot of parts from. I took the carb to show them and to get the benefit of their experience.

Well, first off it was clear they did not have a high opinion of the GCV520 engine in general - I think the exact phrase was: "pain in the arse". Then on to the carb itself: "crap", "piece of s**t" …

It turns out that even if I get the old rubber stems out it won't do me any good - there are no replacements available. So, the carb is junk! :shocked:

They said they've had to junk lots of those carbs, often finding the rubber inserts in the float bowl. Seems they go brittle with age and ethanol and that's it.

Unbelievable that the lack of two tiny rubber thingys can turn an entire carb into an expensive paper-weight - and shame on Honda/Mikuni for not making them available as spare parts. Not untypical though, not long ago I had to junk a Walbro carb that was less than three years old because replacement valve seats were "not available".

Yes, the whole emissions malarkey has cost manufacturers a lot of money, but they're sure clawing it back through the "parts not available" policy. Instead of £10 for parts it's going to be £175 ($250) for a new carb. :smiley_aafz:

I think you have been misinformed about the rubber caps.

Part number is 16168-Z0A-004 price around £3-4.

Yes its not the best engine but if carb cleaned properly and removing the carb mixture screws then usually it works well.