Confusing VM carb issue

[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 ½ 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-...Q5fAccessories

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/storag...nual_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.

[dbarnes42]

Ok, so today I spent the better part of the day working on this thing. Here is a rundown of what I did:

Pulled the carbs off and checked the float level dry and it was withing specs.

Changed the clip on the needle back to the #2 position.

Set the fuel screws at 5/8 turn out and the air screws at 1. 5 turns out.

Bench synched the carbs. This became somewhat difficult because no matter what I did with the screw on the slide for #1, the slide would not go up or down. Not really sure what I did to solve this, but eventually it started moving.

Cleaned out the airbox. It seemed to have water in it, so I resealed it. I am not sure if it was water, but it didn't have any odor. Plus, I had riden in the rain a couple of days ago.

Performed a wet fuel level check and it was within spec also.

Got everything back together and started it. Started fine, idled good, but the problem was still there. I then vacuum synched the carbs, but the problem remained. I was pretty much ready to put a for sale sign on it at this point. But I am persistant. So, I unhooked th throttle cable and started over. But this time I set the fuel screws at 1/2 out and the air screws at 1.25 out. I vacuum synched the carbs again. At this point, the bike was idling very smooth, and without the throttle cable hooked up, I moved the slides by hand. The problem WAS NOT there.

Out of curiosity, I put caps over the vacuum port on the carbs and the petcock and left the petcock in prime. I was trying to rule out some type of vacuum leak in the line from the carbs to the petcock. It did not make a difference, so I put a new piece of line on just in case. It seemed to have no affect on anything.

So, I rerouted the throttle cable, made sure that is was pulled tight with no slack and hooked it back up. Once it was hooked up, I adjusted it so it had a little play in it.

As of 9 pm eastern, the problem was not there. But I have not had a chance to ride it any distance because of rain. However, in the garage things seem ok.

I know that the fuel screws are not close to the stock setting, but this is what the engine seems to like. Once I can get the bike out, I will do some pkugs chops and see where I am at.


Thanks to everyone who helped me on this.

[dbarnes42]

I finally have good news! I was able to ride the bike to work today. It started up fine and after about 30 seconds, I started closing the choke. Within a minute or two, the choke was fully closed and the idle was good and smooth.

Jumped on and off I went. Before after about 10 miles or so the rpm's would hang when I stopped. NOT TODAY!!!

After my 42 mile ride to work, when I pulled in the parking lot, the bike still idled fine. Performance on the interstate was great. There was no surging/wandering at highway speed. The acceleration was even better!

I am not really sure what cured my problem, but for the time being it is running great. I just hope I can keep my hands off of it for a while and not find something else to mess with!

Here are the carb settings I ended up with:
Pilot fuel screws 5/8 out
Pilot air screws 1.25 out
Clip on needle in second notch from top
Bench and vacuum synched carbs

Thank you very much to everyone who gave me advice and pointed me in the right direction.

[almarconi]

Glad you finally got it sorted out

[Wallowgreen]

Glad she's running well for you now, enjoy the ride

[KEITH KRAUSE]

Interesting experience you had. Sorry I arrived late to add my thoughts.
Two best things you did was put the jet needle position back to where it should be, and pulling the slides out (which was necessary anyway).
On these older carbs, the slide(s) will expand a little after full warm up and cause a hanging idle. "Blipping" the throttle or letting the throttle "snap back" on its own from 1/2 open or more can sometimes help if the slide isn't too bad or there's no serious build up of fuel varnishing or mechanical issues. Snapping the throttle isn't good for the carb synch though.
The slides benefit from a good polishing of the slide bores. A little Semi-chrome or Mothers will polish up the bores nicely and clean them well too. Your bike was running rich as far as the jet needle was concerned (approx' 1/5 to 3/4 throttle position). With poor fuel/air ratios and added mechanical isues, all kinds of bad stuff happens.
The side air screws should be adjusted using the highest rpm method, not just set at a ballpark point.
The little dimple in the brass plate goes down. The dimple actually fits in a little indentation in the plastic ring for the jet needle. Hard to line them up though during re-installation. A dab of grease helps. Some 750 carbs don't use the plastic ring ('76/'77?) above the e-clip but the brass plate still has the dimple.
Glad you got it running right.