can't solve rough idle

[robertbarr]

What/where exactly is the fuel inlet seat o-ring?

Your '79 uses VM carbs, so you don't have an O-ring on the fuel inlet seat.

It's a very common source of problems on bikes that are just barely newer than yours.

[azr]

Thanks for that, I thought it was refering to the copper ring at the float needle.

[Steve]

... But I"m open to anything.

Lots of suggestions on the intake side of things, but this is what caught my eye:

The pipes are the original stock ones with the baffles partially rusted but there are big piles of crap in there.

Have your verified that your exhaust pipes are not plugged up?

Straighten out a clothes hanger and poke it through the header pipes.
Stock pipes are double-walled, so they might look great on the outside, but the inner pipe can be rusted away and clogging the passages.

.

[azr]

I checked the pipes and although I can see rusted away baffles at the end of the pipes there are no lumps of rust in the pipes at all. Hopefully that means the baffles aren't as bad as I thought.

[49er]

lol...wellll...I wound up making my own very fine tipped punch and hit it out with a hammer, ha,ha,ha,ha....since then I have replaced that carb with another. I'm going to pull the carbs all apart and double check that all the jets and needles are what they are supposed to be. I was looking at my bits and pieces box and noticed that my main jets are numbered 102 not 102.5 as per the factory spec, but shouldn't be a real problem I don't think. I've thought about moving the clip on the main to compensate for being rich..but...I don't really want to go off stock until I figure out what the actual problem is first.

Sounds like your ride has not been loved by it's PO. I'd check all the jets
to make sure that their sizes are, as you think they are. You really need a jet drill set to confirm that their hole sizes are as marked on the jets. Some bad people do actually drill them out.
Without this information, you're "prodding in the dark".

If you weren't careful when drifting out those broken mixture screw tips, you will have enlarged the orifice. This will severely richen your mixtures at normal settings.

You also said that you replaced one carb, but that two screw tips had been broken. Wasn't the second carb damaged? Are you sure that the replacement carb/carbs are exactly the same specs as the originals?

[azr]

I replaced the one carb with a body from a 79 750 VM. Should all be the same, and I swapped over the jets. I'm going to pull the carbs in the next few days and confirm all the jets and aswell I have a full rack from a 79 GS850 but it not as clean as mine and has bigger mains, but between everything I have I should come out with a perfectly stock rack. The second broken tip in #4 came out on its own so I felt there was no concern on overenlargement. As well I'm going to check the valve shims again. I did them last fall, correctly as far as I could tell and I've barely put 1000km on her since but I might as well check.

[blo]

Hi there
How is your petcock? I had a similar idle problem that turned out to be a pinhole in the vacuum membrane of the petcock. This would allow a drop of fuel to travel down the vacuum line and screw up the idle intermittently. Just my 2 cents worth.

-blo

[azr]

The petcock is new and seems to be working fine. The vacuum seems to be enough. I tried to hook up my vacuum gauge to it but the vacuum pulses too much so I could get a reading.

[flyingace]

If have VM carbs and your bike is running rich throughout the rev range, then it sounds like you are not getting enough air. With those carbs, you don't have to be so concerned about sealing the airbox, in fact, you may be too concerned about it.

Take off the airbox and see how it runs. Then check your plugs to see if they have leaned out. If they are too lean - at least you have a starting point.

Have you checked compression? Are you sure those tailpipes aren't clogged? The above won't help if these conditions exist.

[Dave8338]

If have VM carbs and your bike is running rich throughout the rev range, then it sounds like you are not getting enough air. With those carbs, you don't have to be so concerned about sealing the airbox, in fact, you may be too concerned about it.

Take off the airbox and see how it runs. Then check your plugs to see if they have leaned out. If they are too lean - at least you have a starting point.

Have you checked compression? Are you sure those tailpipes aren't clogged? The above won't help if these conditions exist.

I'll mention the exhaust for the third time. When all else fails on the intake side of an engine, it's time to look into the exhaust.

[azr]

So I redid the carbs, all stock with the exception of a 102 on #3 and #4 instead of 102.5. There were some jets/needles I couldn't check as there were no numbers but I have an extra 79 GS850 rack to compare with and they are the same. Chokes are all set "perfectly". Redid the valves and there were several tight so I reshimmed. I'm surprised by that as I did them last fall and have barely put 1500k on her since. Now I start it up and she won't hold an idle without the choke on and if it falls below 1500 she stalls. It won't start without the choke on. When she idles at 1500 with the choke on she makes a low level bogging noise, like a sucking noise in the carbs. If I try and give it any gas it stalls immediately. I checked all my hoses and everything is good. I must have done something wrong. Maybe the valves were not done right, but it's pretty simple, but you never know. Did a bench sync as described on Cliffs site, let them drop to the very bottom, all even both open and closed. I had the vacuum synch on the bike at the time and although I couldn't idle without the choke on they were out but not horribly (but maybe that doesn't matter?). Any suggestions?

[crapwacker]

Did you sync these?
I broke down and bought a manometer and it is amazing what it did for mine.

[rputney01]

I'll mention the exhaust for the third time. When all else fails on the intake side of an engine, it's time to look into the exhaust.

For what it worth, I had a problem with a wandering idle and no amount of carb fiddling seemed to solve it. Last weekend I repacked the baffle with long-weave ceramic wool. I had used stainless steel wool, which hadn't degraded from last summer, but it had an annoying resonance in the muffler between 2K and 3K rpm. The ceramic wool is used to insulate kilns and is pretty cheap, so I decided to give it a try. It not only smoothed out the idle, but the throttle response in the low range is much stronger. The bike is also significantly quieter, which my neighbors appreciate at 6:00 a.m. I don't know if this would help with your problem, but as mentioned above, it may be worth another look at the exhaust.

[azr]

Repacking the idle, yes, I've seen the links about that, but can't that only be done on exhaust that have the ability to remove the muffler? My bike still have the original 2 into 1 exhaust that is one unit (for the most part) the baffles are welded on, the only way to get at them is from the rear and as far as I understood you have to take the baffle off and wrap the cerramic wool around it. Enlighten me if I'm wrong about this procedure.

[azr]

So...pulled the carbs apart and repalced all the mains to 102.5 which is stock, repalced what looked to be a slightly bent main, still all stock though. Carbs back on, rechecked the valves just to make sure and other than one intake that is at 0.10 they are all within 0.08-0.03. The bike started up (go figure, have nooooo clue what I did to make it start). Vacuum synced the carbs, and used my new colortune to adjust the fuel and air mixture (not the greatest tool in the shed but interesting). For #2 carb it showed that I should have the fuel mixture screw all the way in to have a blue flame?? anyways. The bike seemed to idle better and throttled better 'in the driveway'. Took it out on the road, same old problem. Boggs when throttled up or pass 5000rpm. Now she's really popping on the decel and when I pull the plugs they are all soot covered...ARRRRRRRR....there is a nock up front like a tight shim or something. There is one that was 0.03 but it shouldn't be making this much noise I wouldn't think. It appears that my auto cam tensioner is broke as it won't turn at all, tight as all get out. So....I guess I'll redo the cam tensioner but other than that I am fresh out of ideas. When the plugs are clean she idles almost perfect and pulls well but it only takes a few minutes of riding and they become carbon fouled and she starts missing and running rough and she won't pull past 5000 and popping...a real mess. Helpppp. There is a mechanic in the area, as much as I hate to admit defeat........