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can't solve rough idle

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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.
 
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
 
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.
 
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.
 
flyingace said:
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.
Click to expand...

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.
 
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?
 
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Did you sync these?
I broke down and bought a manometer and it is amazing what it did for mine.
 
Dave8338 said:
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.
Click to expand...

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.
 
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.
 
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........
 
What part of Vancouver island do you live?
I am in victoria.
I could try to help out if you could come by.
 
Uhmmm. I didnt see anything about replacing the AIRBOX BOOTS.... They are rubber, they shrink, they get old, they dont seal...they make sucking noises like a kitten on its mothers teat...

Also, I know your floats are BRASS, but they will STILL develop leaks from time to time....If they fill with gas, they will sink, the fuel flow wont stop and the bike will drown. Did you check them? Youve done alot, and I applaud your efforts. Dont give up now! Its almost there. Its riiiiiight in front of your nose... You just have to uncross your eyes to see it. Bikes are simple when you get around the frustration...

It only needs three things
AIR
FUEL
FIRE

its getting too much of one or more of these things...
 
a rough idle has only a couple causes.

you have the mixture of fuel and air .

you have timed ignition .

you have the sealing integrity of the valve and seat.

it is a little oversimplified but essentially that simple.

if you are sure that you have checked everything:
I suggest removing a plug wire 1 at a time. when you do this you are looking for the cylinder where removing the wire does not change how it is idling.
what that tells you is that you have a mechanical sealing problem between the poppet valve and valve seat in the cylinder head. time to remove the head and get a valve job.
 
Yes the airbox boots have been replaced. As far as I can see the floats are working fine, but yes it is getting too much fuel. There is remnants of fuel in the intake boots, airbox boots. I've checked the spark, good, done a coil mod, good, checked the coils, good. Airbox has been rebuilt, twice, new airfilter, no air leaks. I'm sure I"m missing something simple (I hope). I did my compression yesterday 125, 125, 115, 125. Curious about the 115. I'll try removing a wire at a time, will be interesting to see the results. Hey gearhead13, thanks for the offer, I'll PM you.
 
At this point, if it hasn't already been suggested, I would make a tube to fit in the carb drains to externally check the fuel levels for each carb.
 
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Update. Tried taking off one plug cap at a time but didn't make any difference. Replace a shim that was just on the edge of being tight. Put the fuel and air screws at 5/8 and 1 1/4 (which was no way close to what the colortune had them set at, some were 1/8 and 2", go figure). The bike ran way better, sort of until I tried to run it hard then same old same old except a bit better rpm in lower gears...arrrrr...so...decided to pull it all apart again (why not) and fix the leaking and what apeared seized cam tensioner. Fixed that (nice to not have it leaking) but now its running a bit worse....ha,ha,ha....picked up a better voltage meter, checked the coils (with a bit of help from here) and it's showing both coils are no good (0.7 ohms on the primarys and 20'000 on the secondarys and thats with the stock points. So....if anyone has advice on where to get coils that won't break the bank that would be great.
 
update

update

Well it turns out the coils are fine. With the help of Gearhead13 leaned the floats out, all are set at 24. That seemed to help. Before they were more on the 23 side, and I was setting the floats by letting the float fall on the spring loaded needle by gravity, instead of measuring as it just touches. Valves are all set. Points are done and timing. I was told my dwell shoud be about 22.5 but I am getting 42 on both. I figure as long as they are roughly the same it should be fine. Seems most people on here don't use a dwell, feeler gauge is more than good enough. The air and fuel mixtures are at the stock settings. So..the bike is running better, pulls better from a stop and the idle is better than before, but still has a rough idle, stumbling, and anytime I try and push it past 5000 it bogs, doesn't matter what gear. While we had the carbs apart I finally tried moving the clip down, so its now in the middle, 3rd from the top. But no improvement. On Gearheads suggestion I've ordered another set of 102.5 mains as mine were looking a bit worn. I have another bank of good carbs, so when those main come in I'll have another bank to throw in and try. I'm going to leave those clips at 2 since moving the others didn't seem to help. ....the saga continues. But on a good note while muckin about I painted the rear caliper, looks way better. So she's on the road once again, ha,ha,ha.
 
I would say that the difference in spark performance between 22.5 and 42 is quite large, doesnt matter that they are the same, imho. Changes your timing as well.
 
I've got the points set towards the narrower side of the specs right now. If I try and bring the dwell down further I will outside of the specs. I'll try setting it again though with the dwell meter on while I change the points gap. I suppose I set both points and then do the timing. I looked around a bit and found some other specs that taked of 45-49 for the dwell. And actually it looks like a dwell meter isn't the most accurate as they are designed to work on a car which doesn't rotate as much as the DOHC.
 
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As I remember when i used a dwell meter, I think it had a selector for # of cyls, so your 42 could be 1/2 that at 21 degrees.:confused:
If it were me and I could afford it, I would go with a dyna-s on there, points are a pain.
 
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