Confusing VM carb issue

[nert]

Nessism
Looking past the end result of "fixing" the hanging idle.
When turning out the mixture screws, what affect did it have on the engine idle speed?
Increase, decrease or stay the same.
EDIT
And was you idle staying elevated until you bliped it?
Did it slowly return to idle on its own?
what does blipping do to an elevated RPM to bring it back down?

 

[almarconi]

I still think that there is probably a vacuum leak somewhere. A leaky vacuum petcock diaphragm could be a possibility.

The PO also stated that a previous exhaust leak had been fixed, how was this repaired? Did the problem occur prior to the leak repair?

 

[dbarnes42]

First off, THANK YOU to everyone who has posted something in response to this issue. I do have good news and bad news to report.

In response to the exhaust leak, there was a small dime sized hole in the premuffler. The bike has a 80 exhaust on it, even though the bike is a 79. I fixed this problem by removing the premuffler and welding pipe into the area where the premuffler was. I basically turned the 80 exhaust into a 79. This fixed the problem of popping on decel.

The bike has a Dyna ignition, coils and wires with the mechanical advance. I will check the function of it tomorrow. I am using the stock plug, NGK B8ES, gapped at .25. I had read somewhere on this forum to gap it at .31, which I will do.

Now for the good news. Looking at the carbs today, I saw what could have been the problem. On the inside of the #1 carb is a bracket or clip that has a screw hole in the top and is "U" shaped on the bottom. The "U" part sildes over the rod that runs through the carbs that the slides bolt to. Well, this one was bent! And not just a little bit. I have a spare set of carbs so I pulled that one off and replaced it. I then turned to pilot fuel screws at 1/2 out and the pilot air screws at about 1 turn out.

The hanging idle is gone - mostly. Now it hangs up at about 1.5 to 2 K instead of the 3K it was doing.

But now the bad news. Bipping the throttle brings the idle back down, but now when the rpms drop, it falls below where I have it set and the bike wants to die. It did die twice on the way to work.

So, where do I go from here?

As a side note, I am positive I did the plug chops wrong and I only pulled #4 because it was easy to get to and I was running late for work.

 

[almarconi]

But now the bad news. Bipping the throttle brings the idle back down, but now when the rpms drop, it falls below where I have it set and the bike wants to die. It did die twice on the way to work.

Try opening the air screws a 1/8-1/4 turn and see if this improves the hanging idle. I think mixture wise you are pretty close. Make small adjustments and keep checking until the rpms rise quickly(when you turn the throttle) and then fall back to the baseline. Your baseline idle should be right around 1000 rpms or slightly less.

 

[Nessism]

Try opening the air screws a 1/8-1/4 turn and see if this improves the hanging idle. I think mixture wise you are pretty close. Make small adjustments and keep checking until the rpms rise quickly(when you turn the throttle) and then fall back to the baseline. Your baseline idle should be right around 1000 rpms or slightly less.

These are VM carbs, not CV. The pilot fuel screw needs to be opened up to allow extra fuel in.

 

[rustybronco]

I do have good news and bad news to report.

But now the bad news. Bipping the throttle brings the idle back down, but now when the rpms drop, it falls below where I have it set and the bike wants to die. It did die twice on the way to work.

So, where do I go from here?

I then pulled the #4 plug and it was black, like it is running rich. The sysmptoms show lean but the plugs show rich.

I was wondering if the position of the clip on the needle has anything to do with this problem. I currently have the clip in the 3 notch from the top. Stock position is 2nd from the top.

GS850 (77-79) the specs are for the VM's

idle r/min........... 950-1150
carb................... mik vm26ss
id no................... 45060
bore size ............ 26 (1.0)
float height ......... 23-25. (0.91" - 0.98")
fuel level ............ 3.0 - 5.0 (0.12" +/-0.20")
pilot air screw ..... 1.25 turns out
pilot screw........... 5/8 turns out
pilot air jet........... 1.2
pilot jet............... #15
cut away............. 1.5
jet needle........... 5DL36-2 (the -2 2nd notch)
needle jet .......... 0 - 6
pilot outlet........... 0.6
by pass ............. 0.8
main jet............. #102.5

My opinion? put the clips back to the 2nd notch and check the float levels by the wet method (a piece of clear tubing connected to the nipple on the bottom of the float bowls)

 

[Big T]

I think you need to do the last 3 posts

1. Fuel screw needs to be 7/8 to 1 turn out
2. If that doesn't work, check fuel level
3. Reset needles to stock

It's always some little thing, the problem is finding what that little thing is

 

[dbarnes42]

OK, I rode the bike to work and everything was ok, until I got to the first stop light off of the interstate. The rpm's hung up around 1800 to 2K and I barely blipped the throttle and they came down to idle. The bike did not want to die.

I had the pilot fuel screws about 1/2 turn out and the pilot air screws at 1 turn out.

Just out of curiosity and because I can't leave well enough alone, I adjusted the pilot fuel screws to 7/8 out before I left work. When I started the bike, I noticed that the little hickup it had before when the choke was on was gone. So, off I went.

Now the bad. The first stop light off of the interstate, the rpm's hung up around 3K again. Blipping the throttle made the bike die. I should point out that I live 42 miles from work and 30 miles of it is interstate. So the bike is nice and warm at this point.

I appreciate everyone's help with this. My plan is turn the fuel screws back in to 1/2 out (maybe a little more but not 7/8), put the needle back in the second notch, install new plugs and synch the carbs. One of these has to help.

Also, how to I check the float level using the wet method? I will try this, but I am a little lost on what to do other than put clear tubing on the float bowl drain with the bike at idle.

 

[almarconi]

Now the bad. The first stop light off of the interstate, the rpm's hung up around 3K again. Blipping the throttle made the bike die. I should point out that I live 42 miles from work and 30 miles of it is interstate. So the bike is nice and warm at this point.

The fact that the problem got worse as the bike warmed up means that you are too rich.

But now the bad news. Bipping the throttle brings the idle back down, but now when the rpms drop, it falls below where I have it set and the bike wants to die. It did die twice on the way to work.

This is also a classic symptom of a bike being rich at idle.

Leave the fuel screw at 5/8 turn out and reset the air screw back to 1.25 turns out. Measure your service fuel level to make sure this is within spec.
Clean or replace the plugs and try again. Also put the needles back into the second notch. I would also inspect your carbs to make sure nothing else is damaged.

 

[Nessism]

I'd try 3/4 for the fuel screw and 1.5 for the air screw. Running at 1/2 and 1 is defenitely different than most people use with a stock bike. One turn on the fuel screw is typically on the rich side but it works for some people, and the air screw should be tyically twice the number of turns as the fuel screw - to be adjusted for the highest idle speed although sometimes the engine doesn't respond to these minor tweeks with changes in idle speed.

 

[almarconi]

These are VM carbs, not CV. The pilot fuel screw needs to be opened up to allow extra fuel in.

I'm thinking he's running rich at this point. Something is not right with those carbs. The OP said one of the parts he replaced was bent so its possible something else is wrong with them.

If the fuel level is incorrect, no amount of fiddling with the fuel or air screw is gonna fix it.

 

[dbarnes42]

I will set the fuel screws at 5/8 and the air screws at 1.25 and see what happens.

Next question, since I will be removing the slides to change needle positions. There is a thin metal plate that fits between the slide linkage and the slide itself and ihs held on with 2 small screws. On this plate there is a dimple. Does this dimple go up towards the linkage or down towards the slide?

Last question - could someone please explain the wet fuel level check to me?

 

[rustybronco]

Last question - could someone please explain the wet fuel level check to me?

When overhauling carbs, one of the things needed to be done is to set the float heights. having to adjust the float height by means of a height measurement, normally done with some type of measurement device such as a ruler, calipers or even one of those L-shaped cardboard "devices" that come in some of the carb kits you purchase; this method is called setting the float height "dry" I.E. no fuel in the bowl. normally this method of setting the fuel level in the fuel bowl suffices and no problems occur with the fuel level being to high (rich condition) or with the fuel level being to low (lean condition). manufacturers normally have a spec for the fuel level in the bowl (example... 2.5-3.5mm or 5mm +/-) normally called the "wet" fuel level.

the method of checking the wet fuel levels on VM carbs, is to attach a piece of clear tubing to the brass tube found on the bottom of fuel bowl and hold it along the side (usually the middle) of the fuel bowl. then it's a simple matter to open the drain screw allowing the fuel in the tube to attain the same height as found in the fuel bowl.

this method should be done with the engine running, but...

the wet method can be done as a bench check before the final installation of the carbs on the bike. just be sure to orientate the carb rack as they are found in there normally installed position.

the wet method of setting fuel levels is a nice way of double checking that things are as they should be.

don't leave home without it!

 

[Nessism]

Did you check the ignition rotor yet and are the carbs vacuum synced?

 

[trippivot]

VM carbs are not Geico brand. a cave man cannot do it.

 

[dbarnes42]

I did check the mechanical advacne and it sprung back rather quickly. I am doing a vacuum synch this weekend.

Does anyone know about the thin metal plate with the dimple on it? It fits between the slide linkage and slide. Dimple up or down?

 

[Nessism]

I don’t know about the plate but you might want to look for a matching dimple in the slide which would explain the story.

One other thing to check is voltage at the coils; if it’s less than 10.5 volts or so the spark will be a little weak. Also, check your plug caps for resistance and snip off about ? of wire from each high tension cable to eliminate any possible resistance build up there, assuming you are running the stock coils.

Last thing and then I’m done, the 5/8 fuel screw position is lean compared to the typical setting for most bikes. Sometimes the hole in the carbs gets hogged out some, especially if the fuel screws have been broken off in the carb bodies or crammed in too tight, so this comment is not absolute. Any time your adjustments stray too far from the acknowledged norm you need to ask yourself if this is correct – it may be, but you should always ask the question.

 

[Wallowgreen]

the method of checking the wet fuel levels on VM carbs, is to attach a piece of clear tubing to the brass tube found on the bottom of fuel bowl and hold it along the side (usually the middle) of the fuel bowl. then it's a simple matter to open the drain screw allowing the fuel in the tube to attain the same height as found in the fuel bowl.

this method should be done with the engine running, but...

the wet method can be done as a bench check before the final installation of the carbs on the bike. just be sure to orientate the carb rack as they are found in there normally installed position.

the wet method of setting fuel levels is a nice way of double checking that things are as they should be.

don't leave home without it!

Rustybronco, just checking I haven't gotten my knickers in a twist as someone who's had to wet check the float levels on my VM's somewhere between 1 and 1000000 times. You mention to attach the clear pipe to the brass pipe at the bottom of the bowls but that's the overflow. I think he needs to get some of those gizmo's you put me onto that screw in place of the drain plug with the pipe attached. Only other thing I found to work was some aquarium airline tubing cut to appropriate lengths then stuffed in the hole the drain plug came out of. Get's a bit messy as it doesn't seal entirely but workable. I'll see if I can dig up the link you sent me to the fleabay page for the drain plug / pipe gizmo and add it later on. Found it: http://cgi.ebay.com/ebaymotors/Carb...4547186QQptZMotorcyclesQ5fPartsQ5fAccessories

Next thing mate, if you don't have a workshop manual for the machine, go here and download one courtesy of you friendly neighbourhood BassCliff (not sure if he wears his underpants on the outside or not ):
http://www.mtsac.edu/%7Ecliff/storage/gs/GS850_Manual_HiRes.pdf
The process is shown on page 9-9 figure 9-26.

Advice I'd give is do this off the machine, I know RB suggested you do it with the bike running but when I tried that I couldn't see in there worth a toss. From trial and error I found one of the best ways to get the job done is to sit the rack on a five gallon bucket, if you deform the top of the bucket a bit, they sit on there nice and level. Then I put my gas tank on our wheelie bin with some 2x4 under both ends to spread the weight. Run an extended fuel line down to the T and let it rip. You'll be able to see all four at once like that. Saves a load of time.

Not difficult, just a bit messy. Good luck.

 

[rustybronco]

Rustybronco, just checking I haven't gotten my knickers in a twist as someone who's had to wet check the float levels on my VM's somewhere between 1 and 1000000 times. You mention to attach the clear pipe to the brass pipe at the bottom of the bowls but that's the overflow.

Wallgreen, I might have Honda/Yammie carbs on the brain. I'll check tonight if opening the drain screw also dumps the fuel out the over flow tube (I suspect you are, because of the drain screw having an o-ring on it). if you are correct I'll edit my post to reflect it.

 

[Wallowgreen]

Wallgreen, I might have Honda/Yammie carbs on the brain. I'll check tonight if opening the drain screw also dumps the fuel out the over flow tube (I suspect you are, because of the drain screw having an o-ring on it). if you are correct I'll edit my post to reflect it.

Here's a picture I found from when I stripped mine down one time. The overflow is the pipe standing up inside the bowl and if you look carefully, the drain plug outlet can just be seen at about 9'o'clock in the bottom recess. I'm sure they're seperate.

Also in the picture on the right is the linkage for the slide, there are three holes in the part the plate that was mentioned screws down to. I think the nobble goes in the middle hole on this part of the linkage, so upwards and away from the slide.