Low RPM issue

[jimbyjimb]

Recently I have:

Replumbed fuel system/new filters; replaced stock air filter with K&N; auto cct; Mikes XS coils/8mm MSD wires; synced carbs.

After this was done, the bike ran great. About two weeks ago it started hunting so bad at idle it will shut itself off. Today I was riding, so I came home and started messing with the idle speed. I raised it up to around 1200-1400 and synced the carbs, just to see how she idled. Idled fine, BUT now under 1600-2000 RPM the bike bogs like she's too rich with a very occasional light pop on accel. I messed with the idle and such a bit more, always keeping the carbs synced. The bike is ridable, but will buck and jerk hard if I goose it down low in any gear, 1st-5th. Soon as it gets to roughly 2k it runs like the champ it's always been. No lack of power, no smoke, no hunting, no indication of any problem at all.

Bad gas? Any guesses? I have not messed with any idle screws but #4 and have richened/leaned it little by little at every sync/idle speed adjustment to no avail. Plugs all look clean.

edit- the bucking problem started today right after a fill-up at 76. The hunting idle has been a problem for 3 fill ups at various stations. Fuel lines are clear, so I can see no blockages. There are a couple bubbles that I bled out with no change in low-rpm running.

[mack]

Plugs

Try increasing the gap on your plugs closer to .08mm.

[BA80]

Could be a fuel problem but it sounds more like your having pickup coil wire issues. You better check them out.

Those can seem a LOT like a carb issue.

[motoman]

Priimary, secondary ignition sources check ALWAYS before goin to possible fuel related.................can save you from 'chasing your tail'........and never catching it.....always applied in the 'old school' automotive world, and also applies here......for starters, what BA80 stated!....and that the advance mechanisim(not the vacuum advance) rotates EXTREMELY freely with your hand, disconecting the rod from the vacuum advance first......

[jimbyjimb]

Well gents, after spending 2.5 minutes following your advice I discovered the vacuum advance is not hooked up. Either it was ripped off or the hose rotted. Could have heppened on my last vigorous washing. After fixing that, I'll report back. Thanks guys!

[jimbyjimb]

Ok,

So I have no idea where to plug in the vacuum advance. I can't find a port anywhere except the one for the octy on #2. 1980 SG.

I ran the bike again, using the vacuum advance with my mouth. It did improve the hesitation/bucking and especially cold idle with choke applied. I obviously couldn't road test.

I checked the ignition coils, wires, connections, no visible issue. Pickup coil wires look fine from inside the distributor and out and the advance mechanism is smooth as silk vacuum and mechanical. Fuses and wires to fuses also look fine.

Where the heck was this advance plugged in?

If it became disconnected this could cause a vacuum leak and explain my idle problems, right?

I have the tank off currently and am poking around checking things out.

[crazy steve]

The vacuum advance hooks to the number two carb on the motor side at the top of the carb; you'll have to look for the connection, it's kinda hidden in there....

[natemoen]

The octy is plugged into the intake boot where you would hook up to sync the carbs. The.vac advance is hooked to that nipple on the #2 carb body.

[jimbyjimb]

That explains that.

Can't have been my problem then. In the four years I've had the bike the octy has been hooked up to the carb and there have been four caps, one on each boot. The whole time I've ridden it it's had no vacuum advance hooked up. I have it hooked up now and we've eliminated that possibility for a leak.

I think I'll put it back together and ride it, see what happens. No loose wires/connections anywhere I can see. Which is certainly not definitive. I've been reading like crazy and I know the electrical system sucks on this pig, I just really want to blame the fuel system. Shame on me.

It's just odd that it hunts so bad at idle but runs so good above idle. Could tightening the cam chain be causing this? It seemed to occur right after putting on the cct as I did the other mods first with no issue and the cct last.

[jimbyjimb]

I just rode for 20-30 minutes. No weird power loss, but it still hunts like crazy at idle, stalling itself out. It seems to run perfectly fine 1500 and above. Maybe I'll look more in depth at the wiring. I just wonder if I got some crud in an idle circuit and maybe one or two cylinders cut out. I'll hook up a timing light to a wire from each coil and see if one of the coils is intermittently cutting out as well.

Grrrrr

[jimbyjimb]

Oh, another thing I noticed. If I have the idle set at 1100 and I crack the throttle plates just barely it runs perfectly smooth, regardless of whether it's above 1500 or not.

[BA80]

Oh, another thing I noticed. If I have the idle set at 1100 and I crack the throttle plates just barely it runs perfectly smooth, regardless of whether it's above 1500 or not.

That sounds a little bit like you may have a carb throttle plate/shaft hanging, not closing, throwing your sync off at idle. JAT

[jimbyjimb]

I just checked the coils and they're firing along just fine at any RPM. I also rode it with no air filter, no difference.

I hear you loud and clear, but I've synced the carbs twice in a month. They all seemingly adjust. The first time the bike ran fine. After it started running funky I rechecked to make sure they were still synced. They're all at 10 in/hg, according to my napa vacuum guage. This is at 1100 RPM. All idle screws are 2 turns out except for #1 since it's stripped.

For now I think I'll rule the ignition out (since the coils are both seemingly firing as they should and the wiring looks okay, without ohm testing) and try to find a vacuum leak. I'm going to get some brake cleaner tomorrow and run it while spraying down the boots. They're pretty cracked, which I know is normal, but it's worth a shot.

I also haven't done any carb work at all in the four years of ownership, so if I can't find a leak I'll just do the rebuild I've been putting off. I always keep my bike ready to ride and start it once a week, but it has had fuel in the bowls even while sitting for the last four years. It's possible the idle circuit is gummy, especially with all the wonderful ethanol enriched gasoline we enjoy so much these days.

If that still doesn't work I'll pull it apart and break out the ohmmeter. Ew. but, at least I'll have nice clean carbs.

[BA80]

Before you get all carried away with carb tear down, try running a can of seafoam through it or if you can get your hands on some, some professional strength fuel rail cleaner. You might get lucky.

[cywelchjr]

And you need to ohm out those pick up coils and tug on the wires as just looking at them doesn't actually tell you if their good. And 1100 is proper idle speed, so if everything works with idle set there, it's working properly. If your trying to get it to idle lower, it's not supposed to idle lower than that, although some have gotten them to idle lower, but the spec is 1100 RPM.