New Mikunis. Dodgy idle, no power.

Hi guys,

I've all but given up on this and I'd love some advice.

I just stuck some new Mikuni VM34's on my '81 XS, along with some aluminium intake manifolds and K&N filters. Bought the whole kit from 650Central, pre-jetted for my stock setup.

I can get them to fire and idle very rough with the chokes on, but as soon as I switch the chokes off, it dies. If I give it some gas, it dies. No amount of adjustment seems to make much of a difference.

My vacuum gauges are reading 'bad valve timing or manifold leak', so I'm thinking it's a timing issue, but if it was running fine for a month before the swap. How is it possible for my timing to suddenly change?

Thoughts?


Some back story: The bike was running fine on a full stock setup (including original BS34's) until about a month ago. I was out riding and just about ran out of gas. The bike started struggling and coughing at the lights, but I managed to turn it around and fill up. Once i got some gas back in the tank though, the shitty idle remained.

I figured the BS carbs had had it, and I had the VM's anyway, so when I finally sourced some new lines, I fitted them. But the shitty idle never went away.

So I'm starting to think it's a timing issue. I was going to strobe it today, but the battery's too flat to turn it over, so it's on charge overnight. I tried to kick it over and it fired halfway down the kick and nearly snapped my damn foot off. What's the story there?

Otherwise, I've got Hugh's PMA kit sitting here on my bench awaiting the delivery of my new Pamco kit, so hopefully I should have a good shot at sorting it out when I rip all the old crap out and replace it.

Thanks for your tips and your time, friends.

Cheers.

1st thing to check is flow out of the tank. then check float height. best guess would be crap in your tank, in tank filters, fuel tap or fuel line.
a good, well charged battery is needed for these bikes to run!
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Yes, as yamaman said the fuel system is most likely the problem. When you keep a bike filled up, any crud builds up in the tank. When you run out of gas this crud gets stirred up. This crud plugged the idle circuits of your stock carbs. It sounds like the crud was still stirred up enough so it plugged up the new carbs as well. I might staert by cleaning out both sets of carbs and the tank.
Try the stock carbs again with good inline fuel filters.
Might try new plugs too.
Leo

On the battery, yes it needs a good fully charged battery to run well.
It seems aparent you are having charging issues. You are putting on a Hughe's PMA kit.
The stock TCI don't like low voltage. Crappy idle, no power are two syptoms of low voltage.
Leo

Leo, us convicts wern't considered "advanced" enough for the TCI (ha ha). So I suppose he is running points. I'd remove the fuel tap and clean that up 1st

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Wouldn't hurt to make sure your carb boots to the head are in good shape too. All else is covered above.

Cheers guys,

I actually figured crud was the issue earlier on - should have added this bit to the background info.

When i first put the new carbs on, I had to run a ridiculous adapter setup to go from my single vacuum petcock to the two new carbs. When i first hooked it up, the overflow hoses on the carbs started spewing fuel. After some head-scratching, I realised the pilot circuit was probably blocked full of crap. And it was.

So i took off my tank and the carbs, cleaned both, and then went and bought a new Pingel dual-outlet petcock (which is totally sweet). Had to modify the tank ever so slightly, but it's now attached and flowing freely with inline filters on each line.

So assuming the carbs are clean, the flow is strong, next thing to check would be the charging system. Plugs are looking a little ugly, so I reckon I'll just wait til I get the new ignition system this week and do the lot.

Still doesn't explain why my vacuum gauges read 'bad valve timing' or why i can get an idle from the carbs (albeit a shitty one) which dies as soon as I take off the choke, or give it some gas.

Would dirty, misfiring plugs affect the vacuum reading?

check float level and needle seat. double check the new manifolds for leaks. check valve adjustment/cam chain. if it runs on choke, and not without, then that suggests a lean condition. either not enough fuel (blockage somewhere or wrong jets) or too much air (manifold leak or valves not closing)

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