I finally found the cause

[gustovh]

If you read my other thread on the lean condition that SuzieQ had, you know that I had been fighting it for a while. Spotless carbs with all new seals and rings and still a runaway idle. So I decided to start at the beginning and ran a compression check, not so good, 90lbs. Across the board. Adjusted the valves and got 150+ on all 4. So decided to start and run her before reassembly. Turned on the fan and let her warm up real good, good throttle response and steady idle at 1000. Blip the throttle and drops right back to idle without hanging. Took her for a ride after putting her back together and all is right with the world.

i never knew tight valves could mimic a vacuum leak.

V

 

[Cipher]

If you read my other thread on the lean condition that SuzieQ had, you know that I had been fighting it for a while. Spotless carbs with all new seals and rings and still a runaway idle. So I decided to start at the beginning and ran a compression check, not so good, 90lbs. Across the board. Adjusted the valves and got 150+ on all 4. So decided to start and run her before reassembly. Turned on the fan and let her warm up real good, good throttle response and steady idle at 1000. Blip the throttle and drops right back to idle without hanging. Took her for a ride after putting her back together and all is right with the world.

i never knew tight valves could mimic a vacuum leak.

V

In some far off land called Nihon legend has it that a magical tome called an owners manual had a matrix of necessary preventative maintenance.
When followed valves remain in proper design parameters.

 

[Big T]

Now, now

I don't remember how many bikes Gustav owns. 8? 13?
It's easy to forget the maintenance schedule of one when you have many.

Plus, he's a genuine geezer

 

[gustovh]

Now I resemble that. Last set 5,000 miles ago. Must have set them at the low end, been fighting it for a few hundred. Would disappear for a while then return…finally didn’t go away. Thus the previous thread.

i have 13 bikes now Todd, with the addition of the 650Savage, and I only 73. Want to take a tour next summer, I need someone to pick up my bike these days.

V

 

[Brendan W]

All those years I've been checking valves instead of replacing intake seals....... Duh!

 

[salty_monk]

They must have been pretty tight when last set to have that affect at 5,000 miles. How much movement did you find in there? (I always write it down but with 13 to look after maybe not!).
A good reason for writing it down - if a lot of movement in a valve suddenly appears that would generally mean something more serious going on.

Glad you got to the bottom of it!

 

[gustovh]

They must have been pretty tight when last set to have that affect at 5,000 miles. How much movement did you find in there? (I always write it down but with 13 to look after maybe not!).
A good reason for writing it down - if a lot of movement in a valve suddenly appears that would generally mean something more serious going on.

Glad you got to the bottom of it!

I believe that I just set them too tight at the last adjustment. She has over 40,000 on her and runs strong. Just a new one on me, I set them to the loose side this time. Cheers

V

 

[Hihowyaderrin]

I believe that I just set them too tight at the last adjustment. She has over 40,000 on her and runs strong. Just a new one on me, I set them to the loose side this time. Cheers

V

Bought a gs450 and have sorted everything carb, o-ring and boot/ air hose related to no avail. Every configuration of adjustment I can find in a manual and still, the dreaded hanging idle at 4k in my case. One thing I’ve yet to do is valve adjustment. No ticking, easy start, wonderful sound… so a valve adjustment can definitely mimic a hanging idle?

 

[salty_monk]

Logically I guess a valve too tight would not properly seal. That's why you risk burning the seat. Depending on which valve and which part of the cycle (intake or exhaust) I could see that having some weird effects for sure... Especially with our "wasted spark" system igniting anything left over in a supposedly empty piston.

Its the first time I've heard of it causing this though, I'd say it's a thousand more times common for it to be an air leak somewhere. Usually it's as simple as opening up the intake screws a lot further than any of the original specs and/or sealing the airbox / boots / pipework. I have heard of an airleak at the Petcock causing it before too.

Some of the reason the manual doesn't make sense any more is related to the Ethanol in the fuel I think. The fact they were set up so lean from stock compounded.

 

[gustovh]

Bought a gs450 and have sorted everything carb, o-ring and boot/ air hose related to no avail. Every configuration of adjustment I can find in a manual and still, the dreaded hanging idle at 4k in my case. One thing I’ve yet to do is valve adjustment. No ticking, easy start, wonderful sound… so a valve adjustment can definitely mimic a hanging idle?

Would like to know if that works for ya.

V

 

[Nessism]

Bought a gs450 and have sorted everything carb, o-ring and boot/ air hose related to no avail. Every configuration of adjustment I can find in a manual and still, the dreaded hanging idle at 4k in my case. One thing I’ve yet to do is valve adjustment. No ticking, easy start, wonderful sound… so a valve adjustment can definitely mimic a hanging idle?

Why haven't you adjusted the valves? The valves tighten with mileage, so the valves don't "tick". The clearance closes down as the valve pounds into the seat, decreasing clearance until they hang open and BURN.

 

[gustovh]

Why haven't you adjusted the valves? The valves tighten with mileage, so the valves don't "tick". The clearance closes down as the valve pounds into the seat, decreasing clearance until they hang open and BURN.

I have adjusted them. I know about valve burn, have been wrenching a long time. At 5000 miles they shouldn’t have been that tight. They are on the loose side now guaranteed.

V

 

[Hihowyaderrin]

Why haven't you adjusted the valves? The valves tighten with mileage, so the valves don't "tick". The clearance closes down as the valve pounds into the seat, decreasing clearance until they hang open and BURN.

Because I simply haven’t yet. Been sorting everything else. Only now finding that it’s very important to do first before carbs or other components

 

[Hihowyaderrin]

I have adjusted them. I know about valve burn, have been wrenching a long time. At 5000 miles they shouldn’t have been that tight. They are on the loose side now guaranteed.

V

Initially it did wonders but I made a slight adjustment to the idle screw and the high rev seemed to return again. I have to have something wrong here. I have done everything imaginable

Ive replaced all jets, o-rings in the carbs (including diaphragms)
bench synced the carbs
New Intake boots and rings for the head
new boots for the air box (lightly oiled filter and sealed box)
air mix screws 2.5 turns out
Did valve adjustment (had no clearance on exhaust #1 and intake #1) exhaust has .8 clearance and intake .5 now
new head gasket and gasket for air box
petcock doesn’t leak


mikuni bs34 carbs
If anyone can find me a photo of the fuel/vacuum line routing of the 450 setup I’d be forever grateful. It has to be something simple as that. I’ve tried a few different configurations at this point. I’m becoming discouraged

 

[Hihowyaderrin]

I see the line on top of carb two has an inlet that is capped off, however there are two lines that come from the inside. Are those supposed to be T to the vacuum line on the petcock?

 

[Rich82GS750TZ]

The line coming out of nipple on the front carb body # 2 that had a bolt stuck in the other end is supposed to connect to the smaller of the two nipples coming out of the petcock, if your bike has the original vacuum type petcock. The only reason this line should be blocked is if you are running a non vacuum type Pingel tap.

 

[Hihowyaderrin]

Where might the two in the middle “in between” section go? Would that be capped as well or just routed to an empty space just to exist?

 

[Rich82GS750TZ]

The two coming out the bottom in your pic are likely bowl vents that should be routed over the airbox and left open to still air. Not 100% as I’ve never had a twin.

BTW, your fuel filter, which your bike didn’t come with, looks awful. Are you sure it’s flowing fuel properly?

You really ought to start your own thread in this same carb sub-forum. Be sure to list your bike year and model in the thread title. Your issue, and any possible solutions to your issues will get lost in this thread which has very little to do with your bike, since you seem to have ruled out valve adjustment.

Also highly recommend creating a signature with your bike year and model in it (at the very least).