New year, same old carb issue. (idle rpm)

[gorbys]

Last year was the first season properly running the gsx1100 engine in my gs750 and I spent so much time troubleshooting heaps of stuff without ever finding the root cause as to why I can't get a stable low idle on it. Yesterday I had it running again and I was reminded of this very annoying problem. It seems like every time I ride I have to adjust the idle rpm, sometimes even during a ride as it will be fine when I take off and then when I pull over it's too low, or to high.

Yesterday I got it running, warmed it up, bike idles high as it should on choke. After it's warmed up, choke off,  it's idling at around 15-1600 rpm, and I want it to sit at 1000-1100. So i adjust it down with the idle screw until I get to around 1000, where it will sit for a few seconds before dropping to 6-800rpm struggle and eventually die. Now if I instead of letting it die I try to save it by giving it some throttle and revving it it will rev up and then it will usually settle at 1500 again for a short while before dropping down and eventually dying.

When adjusting it it seems like there's not enough "range" on the adjuster to get it to sit where I want. Like it's either 1500+ or 7-800 and subsequent death. Nothing in between no matter how minute of an adjustment I make on the screw.  It's just so infuriating, I had a gsx750 for a couple of weeks last year and there was nothing like this, I adjusted the idle and thats where it sat. Granted that was a mostly stock bike. The 1100 has a full exhaust and K&N filters. But still, the 850 8v engine that was in it before also had exhaust/pods combo but idled like stock.
Any ideas or tips to try out?
-cable linkage is working smoothly
-carbs have been cleaned and synced
-rubber boots and clamps are new
-can't find any air leaks when spraying around carbs and boots

Also to note: sometimes if I its idling at 1500 if I put it in gear and relase the clutch in to the friction zone and back in the idle will drop down to less than 1000.....

[TonyGee]

first off set the idle when the engine is at working temp.  if the rev's are hanging and then dropping is sounds like its running lean (to much air), have you tried the mixture screws ? 

[gorbys]

31 minutes ago, TonyGee said:

first off set the idle when the engine is at working temp.  if the rev's are hanging and then dropping is sounds like its running lean (to much air), have you tried the mixture screws ? 

If the idle is at 1500, and I rev it, it will rev and then settle at 1500. If I try to lower the idle to say 1000 and I give it a rev as it's about to die it will rev, hang at about 1500-2000 and then drop down and die again. 
You suggesting I give it some more fuel and then see if I can manage to drop the idle a bit to?

[TonyGee]

2 hours ago, gorbys said:

You suggesting I give it some more fuel and then see if I can manage to drop the idle a bit to?

rev's that hang is a symptom of an air leak or a lean mixture when it comes to carbs.  

[PanzerWomble]

As Tony says you will need to back the mixture/pilot screws out on all 4 carbs as they control idle circuit - the knob in the middle (57) just sets the throttles to idle. 

Turn pilot screws in GENTLY until they bottom. Turn all out 1/4 turn at a time (wait 15 seconds between each 1/4 turn adjustment to give engine time to adjust) until highest RPM idle is obtained. Normally do not turn the pilot screw out more than 2.5 turns. If you must turn it out more than 2.5 turns go up to the next larger pilot jet. I've not an ET manual to hand so check that as it should give the the factory setting .

Once you get to high idle you should be able to turn the one control screw ( 57)  in the middle of the bank to get the 800 revs.

 

Pilot screws are the ones in the turret at the front of the carb nearest the engine

Pilot jets sit below part 38 

 

 

[Gixer1460]

51 minutes ago, TonyGee said:

rev's that hang is a symptom of an air leak or a lean mixture when it comes to carbs.  

Or sticky throttle cable 

[imago]

46 minutes ago, Gixer1460 said:

Or sticky throttle cable 

So often overlooked (I'm as guilty as anyone else) as you're tempted to dig in before checking the cable.

[gorbys]

3 hours ago, TonyGee said:

rev's that hang is a symptom of an air leak or a lean mixture when it comes to carbs.  

3 hours ago, PanzerWomble said:

As Tony says you will need to back the mixture/pilot screws out on all 4 carbs as they control idle circuit - the knob in the middle (57) just sets the throttles to idle. 

Turn pilot screws in GENTLY until they bottom. Turn all out 1/4 turn at a time (wait 15 seconds between each 1/4 turn adjustment to give engine time to adjust) until highest RPM idle is obtained. Normally do not turn the pilot screw out more than 2.5 turns. If you must turn it out more than 2.5 turns go up to the next larger pilot jet. I've not an ET manual to hand so check that as it should give the the factory setting .

Once you get to high idle you should be able to turn the one control screw ( 57)  in the middle of the bank to get the 800 revs.

 

Pilot screws are the ones in the turret at the front of the carb nearest the engine

Pilot jets sit below part 38 

So I did that, I turned them all in, started it and set the idle adjustment screw so that it would idle without me holding the throttle. So now we're at 1500 or so rpm's. I turn them out 1/4 turn and wait for the rpm's to rise and they do and I continue to do so until its screaming at the neighbours at 3.3k rpms at which I then adjust the idle rpm down again. Perhaps this was wrong? should I just have continued upping the mixture until it doesn't go any higher? I have neighbours all around and 3k + rpms is frigging loud on a 4-1 with no db killer so I'm genuienly concerned about complaints at this point. Regardless, back to somewhat reasonable rpms and I try to adjust some more but it doesn't get much response.

And now begins the weirdness: I adjust the idle to a damn steady 1000rpms and it holds it for i dont know a minute or two and I'm all giggity then all of a sudden there's a little pop and it starts to race off up up until its now at 2200 at which point I back out the adjuster to bring it down again, then it almost dies and I  turn it in and get it sitting at 1000 again but that only lasts for a short time before it dies. 
And several times I got it sitting at a lower idle, 1000 one time, 1200 another time, but all of the times it only last a short while before it either races off the moon or just dies. 

I did all this with the tank removed and i watched the adjuster sometimes as i turned it and i turn it a little bit; nothing. A little bit more; still nothing, just a hair more: too much! 
Would be pulling my hair out if only I had some left!

2 hours ago, Gixer1460 said:

Or sticky throttle cable 

 No that's not sticking, returns as it should and there's freeplay

[TonyGee]

4 minutes ago, gorbys said:

I did all this with the tank removed

did you have the vacuum pipe plugged when you had the tank off ? and did you have some sort of fuel supply as its running ?  got to keep the float bowls full at all times

[gorbys]

56 minutes ago, TonyGee said:

did you have the vacuum pipe plugged when you had the tank off ? and did you have some sort of fuel supply as its running ?  got to keep the float bowls full at all times

Yes. And yes. 

[TonyGee]

did you check the condition of the diaphragms ?  

[gorbys]

8 minutes ago, TonyGee said:

did you check the condition of the diaphragms ?  

They where fine before winter and it had the same idling issues then so

[TonyGee]

9 minutes ago, gorbys said:

They where fine before winter and it had the same idling issues then so

I think the next step is to check the engine, do a compression test just to rule out a mechanical issue. 

[bluedog59]

Give the engine a service, oil/filter/tappets etc. Pointless trying to set carbs otherwise.

[gorbys]

4 minutes ago, TonyGee said:

I think the next step is to check the engine, do a compression test just to rule out a mechanical issue. 

I did that today as well. Looked alright. Checked valves probably a couple of times last year they're all within spec. Checked float level when they where apart last time so that's in spec. Also replaced all o rings on carbs.  

[PanzerWomble]

It's a carb issue not a blocked oil filter ....

 

OK so got a high RPM using  the pilots .... adjusted down with the  throttle slide control and yet still hunting. ....and you reckon the diaphrams are ok ( how have you checked this ? ) 

 

POPs = Air leaks ....exhaust or manifold ....or switched ignition leads possibly . 

 

I'm deja view on similar issues a few years ago n here ...which was a crappy throttle cable sticking . 

[TonyGee]

14 minutes ago, gorbys said:

I did that today as well. Looked alright. Checked valves probably a couple of times last year they're all within spec. Checked float level when they where apart last time so that's in spec. Also replaced all o rings on carbs.  

what where the compression readings out of interest ?  

[gorbys]

3 hours ago, PanzerWomble said:

It's a carb issue not a blocked oil filter ....

 

OK so got a high RPM using  the pilots .... adjusted down with the  throttle slide control and yet still hunting. ....and you reckon the diaphrams are ok ( how have you checked this ? ) 

 

POPs = Air leaks ....exhaust or manifold ....or switched ignition leads possibly . 

 

I'm deja view on similar issues a few years ago n here ...which was a crappy throttle cable sticking . 

Was checked when the carbs where disassembled and later when I adjusted the needle height.

Pops, true, but the exhaust is a straight through exhaust so pops and bangs aren't uncommon since there is no dampening

3 hours ago, TonyGee said:

what where the compression readings out of interest ?  

around 100-110 psi

 

EDIT: okay, so I see in the service manual now on gsarchive that standard is 128-171psi. Limit is 100psi. Out of the many indicators of malconditions the only one that sticks out to me is "poor sealing of valves" as the piston rings where replaced and cylinders honed when I bought the engine but not the valves, they where just lapped. But they didn't look good and should have been replaced but time and money wasn't available at the time. 

Have we reached the consensus that low compression is to blame for this? Valves and poor sealing does pop up under "Engine idles poorly" and "Engine is hard to start"

Perhaps cams are misaligned? I Think I should have another look at that too

Edited April 24 by gorbys

[gorbys]

Gonna do another compression test today and try to add some oil to the cylinders see what happens

[Jonny]

This doesn't sound to me like an issue caused by compression.

If it were me, I'd try another set of carbs on it first. Sounds like you've already done all the things that would idenitfy the usual issues. If it works with different carbs, you know it's the carbs. You might have some sort of blockage in one of the fuel circuits responcible for idling. It take it you have completely striped and (ultrasonic) cleaned the carbs?

[gorbys]

12 minutes ago, Jonny said:

This doesn't sound to me like an issue caused by compression.

If it were me, I'd try another set of carbs on it first. Sounds like you've already done all the things that would idenitfy the usual issues. If it works with different carbs, you know it's the carbs. You might have some sort of blockage in one of the fuel circuits responcible for idling. It take it you have completely striped and (ultrasonic) cleaned the carbs?

I do have another set of cv carbs on the shelf from the older gs850, but they are in no shape to be run as they sit now. They need a complete overhaul. Maybe I should just order some parts and get to work on them.

They weren't ultrasonic cleaned as my cleaner is dead (just ordered parts to repair it though). But all the jets are new, and I made sure to blow brake cleaner through all the circuits to check that they where open. Which they all where.

I also posted about this in a classic motorcycle group on facebook over here and one guy replied that he had a 82 1100 gsx that would behave exactly the same! And his problems disappeared after refitting the original airfilter box. I don't have the standard box, is there some other way to simulate this? Socks over the air filters? Sponges inside? Taping off a portion of the inlet??

[Jonny]

27 minutes ago, gorbys said:

I do have another set of cv carbs on the shelf from the older gs850, but they are in no shape to be run as they sit now. They need a complete overhaul. Maybe I should just order some parts and get to work on them.

They weren't ultrasonic cleaned as my cleaner is dead (just ordered parts to repair it though). But all the jets are new, and I made sure to blow brake cleaner through all the circuits to check that they where open. Which they all where.

I also posted about this in a classic motorcycle group on facebook over here and one guy replied that he had a 82 1100 gsx that would behave exactly the same! And his problems disappeared after refitting the original airfilter box. I don't have the standard box, is there some other way to simulate this? Socks over the air filters? Sponges inside? Taping off a portion of the inlet??

I've heard many stories of bikes running poorly with pod filters (espceially single pods), but not at idle. The draw is so low it shouldn't matter. Plus there's no airflow issues because you're stationary at idle. I run an 1100EFE motor in a GS750 frame and also use K&Ns (twin ovals) and have never had an issue like this.

Maybe the guy with the 82 GSX hadn't rejetted the carbs to run without the airbox...

What are you using to balance them? Might be worth putting some gauges on when it's doing this and see if one carb is dropping when warm.

 

Edited April 24 by Jonny

[gorbys]

1 hour ago, Jonny said:

 

What are you using to balance them? Might be worth putting some gauges on when it's doing this and see if one carb is dropping when warm.

 

Just the standard set of four gauges with vacuum hose. Neat tip. Will try that.

[TonyGee]

having an air filter on or off won't make much difference at idle with the bike stood still,  the compression isn't the best but it should still run fine at that as they are all fairly equal.  a balance might help if they are way out.  but as Jonny said if you can, try another set of carbs.  

[bluedog59]

My friend had exactly the same problem ( on a single cylinder engine ) and went through 3 carbs before he stuck my "leak down tester" on it. Exhaust valve leaking due to poor condition valve seat and carbon build up. Pointless trying to set carbs if the engine is not in good fettle.

Who said anything about a blocked oil filter ?