Hanging revs/Carb/Vacuum issue?

[CrazyCloud]

1978 GS 750 (VM carbs). So I took the carbs off to put different jets in (for 'new' V&H 4 into 1 pipe, old exhaust was swiss cheese except for down pipes). The end of the pilot jet in carb 3 stayed in the carb body when I unscrewed it. I think I got it drilled out, carefully with 5/64 drill bit until no more brass chips, the new jet seated just fine. But now my #3 cylinder is cold during warm up (does get hot after a while, choke passage in bowl is tested & clear). But the big issue is that my throttle is hanging now when I rev it at all. I thought this would mean a lean condition, but my plugs are sooty black except for #3, which looks normal. Stars and idles great.

My pilot jet was 15 stock, now 17.5; went from from 102.5 to 115 mains (per V&H instructions, mikuni jets from z1). Pilot fuel screws 1 turn out, air screws is ~2 turns out. Carb 3 is unresponsive to air idle screw. I have a clear gas line to the carbs, can see that it is full of gas. Petcock flows fine when I suck on the vac line. Carbs dipped in last month, new Barr o-rings, z1 rebuild gaskets, new intake o-rings. I moved the needle clips down from the 2nd to the 4th from top to trouble shoot and saw no change. Intake boots are nice and pliable. I don't hear or see any exhaust leaks from head or muffler junction, new exhaust gaskets. Bike was running just great before put on the exhuast and put the jets in.

So my question is, is it likely that the goofed up pilot circuit on #3 carb (master) is causing the hanging revs? Plugs say pilot circuit is quite rich (except on #3), so I'm thinking some kind of vacuum leak if that's not it. I sprayed starter fluid around the intake boots and got no rev increase. Carbs were bench synched, but not vac synched. I know vac sync is indicated, but it was running great before so that doesn't seem like a likely cause. Thanks in advance for the collective wisdom!

[CrazyCloud]

Stock air box, valves adjusted recently, float heights 25mm. The throttle cables and linkages all operate smoothly, no binding, snap closed quickly.

[tom203]

Stock air box, ......

You went up a ways on mains, not clear that this was needed with stock airbox setup- V & H probably figured you were going podding at same time.
maybe drop back to stock main/ needle stuff and go from there.

[GSX1000E]

It sounds like you only have four head pipes as an exhaust system. Is that correct?
It also sounds like you have either metal bits in the carb body of #3 after drilling and/or you have drilled too far into it.
Definitely go back to new #15 pilot jets after you have thoroughly cleaned the carbs again.
No recommendation for the main jets can be determined until the exhaust system details are given in better detail.

[Steve]

It sounds like you only have four head pipes as an exhaust system. Is that correct?

That was answered in the first line of the first post.

1978 GS 750 (VM carbs). So I took the carbs off to put different jets in (for 'new' V&H 4 into 1 pipe, old exhaust was swiss cheese except for down pipes). ...

My pilot jet was 15 stock, now 17.5; went from from 102.5 to 115 mains (per V&H instructions, mikuni jets from z1). Pilot fuel screws 1 turn out, air screws is ~2 turns out.

The main jetting sounds OK, but why did you go up on the pilots? Usually, all that needs to be done is to turn the screws out another 1/8 to 1/4 turn, then tweak the air screws for best mixture.

The end of the pilot jet in carb 3 stayed in the carb body when I unscrewed it. I think I got it drilled out, carefully with 5/64 drill bit until no more brass chips, the new jet seated just fine.

Did you verify that the tip of the needle was removed? You may have gotten out all the brass chips, but there is no drill bit in the average tool box that will safely remove a stuck tip. They are usually removed by pushing them from the carb throat, not drilling.

Carbs were bench synched, but not vac synched. I know vac sync is indicated, but it was running great before so that doesn't seem like a likely cause. Thanks in advance for the collective wisdom!

It is more likely than you think. ANY time you take a set of VM carbs apart, you need to re-sync the carbs. Although there are dimples in the throttle rod for the bellcranks to seat into, you can not guarantee that the setscrew will land EXACTLY where it did last time, so you need to do another vacuum sync.

I am guessing that the carb sync is off a bit, #3 (the one that's not getting hot) is open a bit farther than the others, which makes you think it's running lean, but the other cylinders are running rich, due to your large pilot jets. That is masking the lean condition a bit, but not quite enough.

.

[GSX1000E]

???

Originally Posted by CrazyCloud
The end of the pilot jet in carb 3 stayed in the carb body when I unscrewed it. I think I got it drilled out, carefully with 5/64 drill bit until no more brass chips, the new jet seated just fine.


"Did you verify that the tip of the needle was removed? You may have gotten out all the brass chips, but there is no drill bit in the average tool box that will safely remove a stuck tip. They are usually removed by pushing them from the carb throat, not drilling." - Steve

He can remove the tip of the pilot jet from the carb throat? Learn something new everyday around here. Hmmm.

[CrazyCloud]

I pulled the carbs again last night, this time the slides did seem to be sticking. Found a few black gummy streaks on the slides/mating surface on carb bores. Not sure what it could be, thought I was pretty meticulous about cleaning everything but maybe I contaminated them somehow. Everything had moved smoothly when I put them together inside, maybe the cold in the garage made it worse? I poked out all of the passages on carb #3 again, blew some compressed air in from different directions. I also flushed some kerosene through the pilot circuit with a syringe and and it flowed just like the other carbs. I will button it up tonight after cleaning the slides/bores but wont get to run it till tomorrow.

My reasoning on upping the pilot was due to long (5+ minutes) warmup times, I had read on here that +1 size on pilots can help with long warmups. That and the V&H instructions suggested it (probably assumning +pods from the sounds of it). GSX1000E, sounds like I need to go back to stock on pilots for sure on those from your experience though. Steve, thanks for the thoughts on vacuum synching, that sounds logical and will be my next step if it persists tomorrow.

I will try and update tomorrow, fingers crossed.

[CrazyCloud]

Still hanging revs after cleaning slides/bores. I took the airbox off so I could see what the slides are doing when it's running. When running, the slide for carb 3 is not moving with throttle, but the slides for all of the other cylinders did move with throttle. Likely due to lack of vacuum in that carb? They all move together when the bike is off and I work the throttle.
I did get the pilot circuit dialed in closer, plugs are looking much better after turning gas screws in 1/4 turn, should be able to fine tune with air screws. I will have to wait a while before getting a manometer, so I think the troubleshooting is is at a standstill until then. At least I've got a while until spring! If anybody is in cental/southern Indiana and would want to trade a case of beer for use of a manometer let me know...

[Steve]

Just so you know, removing the airbox so you can see what's happening introduces a bunch of other problems, but it should still allow you to see something.

Apparently you have not yet done a carb sync (needing a manometer was the clue). If carb #3 was closed a bit more than the others, that would explain its lack of movement. The slides do not respond to engine vacuum, they respond to the amount of air that is going through the carb. If that carb was out of sync, it might be enough of a difference to not lift the slide.

There are plenty of GSers not that far from you, up in the Indy area, so help is not all that far away. I am about 3 hours east of you, but that "case of beer" has absolutely NO appeal or value.

.

[CrazyCloud]

I understand with the airbox Steve, just trying out all of the diagnostics/information gathering I could think of with what I had on hand. As far as the beer goes-- could easily be root beer or sarsaparilla. I'm going to buy the synch set, just have to move some things around with the other hobbies to make it happen. Thanks for your patience with all of us goofballs.

[BassCliff]

Hi,

Did somebody say "stuck pilot screw tip"? Just in case, here's a PDF file for further reference:

Remove Stuck Pilot Screws
(by Mr. Hoomgar)


Thank you for your indulgence,

BassCliff

[CrazyCloud]

Think I found it

I synced the carbs today with no improvement. #4 was was quite a bit high (on vacuum), and 3 was pretty low. After that I tried some starting fluid around the intake boots again in several spots and found a big vacuum leak on #3 and a small one on 4. It feels good to have a likely diagnosis after so long. I think taking the carbs off multiple times made the boots seal worse and worse. Looks like I'll be saving for some new intake boots!

On the positive side, I haven't thrown any wrenches and the cursing was limited to a sheared off screw on the choke lever (which came out easily with an easy out). I'm kind of surprised that six months into this bike I'm still having issues like this, hopefully by our 1 year anniversary we'll have most of this sorted out!

[cowboyup3371]

Great job for finding the issue. Remember these bikes are usually very neglected by the time we get them and it takes a lot for everything to run properly. However, once you do they are a blast.

[londonboards]

We started talking about VM carbs then went onto talk about vacuum lifting the slides.

I assume you mean this type of carb and not the CV type:

VM Carburettor Pic

Am I not right in thinking that VM carbs throttle slides are directly operated by the cable and don't have vacuum slides?

If so, and no.3 slide is not opening when the others are lifting, then they are out of synch.

Greetings

[CrazyCloud]

Yes, VM carbs. I think you are mostly right Richard, carbs 3 and 4 were out of synch quite a bit and that was probably part of the problem. But even though the slides are all mechanically linked to the same rod, there is a spring/space in the linkage that attaches the slide to the shared "throttle rod", this allows for independent movement of the slides under some conditions. I wouldn't have believed it if I didn't see it myself; without the the bike running they were all opening together, but while running the slide for 3 was "sticking" down. I think Steve describes how air volume can affect slide position above. That is why I think the vacuum leak is affecting the position of the slide also-- as even after the synch the problem persists.




Photo 2 From Paul Mussers VM rebuild guide on BikeCliff site (thanks again Cliff!).