GS550 revs to 3,000 rpm at idle intermittently, with no throttle input

[StevieB]

Hi it's been weeks of trying to get my 1980 GS550E running correctly. Still not 100%.

Firstly had fuel tap, carb rebuild and running / fuel leaking issues. Carbs cleaned as per forum guidance. Lost count of times taking carbs in and out and thinking I've got it sorted- emotional!I

I have set the mixture with a colourtune and balanced the carbs with a Morgan carb balanced (brilliant kit).

I am now at the stage where it runs and starts on the button - but get an intermittent fault.

Randomly the revs will rise from idle to 3,000 with no throttle input - stay for 10s to a minute or 2 (sorry neighbours) then settle to idle. If you rev it from idle it does "hang" back before settling. From forum posts I would think an air leak, stopped filter lid and metal airbox to plastic airbox join leaks but still the same fault (standard airbox).

I've done some fault diagnosis - with the revs sat at 3,000 spayed wd40 silicone spray round carbs ect. No leaks around engine/exhaust areas. The revs rise on no1 and 2 cylinders, carb to airbox inlet rubbers - possible slight rise on 1&2 carb to head rubbers (so slight not sure). Also if I pull the plug caps, 1 & 2 both drop the revs individually, but if I pull 3 & 4 it makes no difference.

I'm no expect but I suspect air leak on 1&2 inlet rubbers. But can't get my head round why it's intermittent. Surely an air leak would be constant?

But reading forums the best explanation to intermittent fault would be electrical. Suggested coil failure which leads to unburnt fuel, which when coil fires again burns excess fuel explaining rising revs. But surely this would be 1 & 4, or 2 & 3 due to coil firing?

I'm getting a bit confused and need some guidance before I start spending money on component changes in possibly the wrong direction. It's doing my head in, just wanna ride it!

Thanks.

 

[wraith]

Sounds like you have a strange one. As you say air leakage should be a constant. 

You say when you remove the plug caps off number 1 or number 2 cylinders you see a difference but not when you remove 3/4, which you would think that 3 and 4 cylinders or not running, are all exhaust down pipes running hot ever when it's running right or when it's rev high?

Have you tryed swapping the HT leads around ie putting 4 onto 1 and 3 onto 2? 

Then see if it still does the same thing on the same cylinders when you remove the caps.

[StevieB]

I was thinking as removing the plug caps on 1&2 it dropped the revs showed these cylinders were at fault, and 3&4 running as they should  be  showed they were good. Confused!

And didn't think of trying down pipe temps? They're wrapped so may not work but will try.

Didnt know u could swap plug leads like that, thought would be totally different firing sequence.

A couple of ideas to try.

Was thinking to undo all carb clamps and reseat them, hopefully stop any inlet rubber leaks.

One of my work friends has a smoke tester, thought maybe try that through the airbox to test for leaks.

Still not convinced if I have an air/fuel problem or ignition. Think it will be a longer project to resurrect than I first thought!

Seen there is a few similar problems on here for other people. May end up having to renew all carb inlet and outlet rubbers long term.

Thanks.

[Gixer1460]

As cylinders 1 & 4 go to one coil and 2 & 3 go to another, both plugs on one coil fire together - its called 'wasted spark' - one cylinder will be on power stroke the other will be on exhaust. Swapping the leads between paired cylinders is useful to see if a problem moves without a bunch of work.

Any intake air leaks in the airbox side of the carbs is immaterial - only leaks on engine side affect running. The inlet rubbers / inlet rubber O rings could be hard / cracked - usually a good source for inlet air leaks, replace O rings or a smear of RTV can affect a temporary seal.

Wrapped pipes will be a bit cooler but should still show a difference with an IR temp gun if there is a duff cylinder!

[wraith]

15 hours ago, StevieB said:

I was thinking as removing the plug caps on 1&2 it dropped the revs showed these cylinders were at fault, and 3&4 running as they should  be  showed they were good. Confused!

And didn't think of trying down pipe temps? They're wrapped so may not work but will try.

Didnt know u could swap plug leads like that, thought would be totally different firing sequence.

A couple of ideas to try.

Was thinking to undo all carb clamps and reseat them, hopefully stop any inlet rubber leaks.

One of my work friends has a smoke tester, thought maybe try that through the airbox to test for leaks.

Still not convinced if I have an air/fuel problem or ignition. Think it will be a longer project to resurrect than I first thought!

Seen there is a few similar problems on here for other people. May end up having to renew all carb inlet and outlet rubbers long term.

Thanks.

Simple basics.

You say you have a 4 cyclinder GS that rev up for now reason, you say if you remove cyclinder 3/4 plug caps there is no change to it running.

That say to me your only running on 2 cylinders and when it revs up its when it fires on all 4.

Checking that all 4 down pipes are at the same temperature will tell you if all 4 cylinders are fireing.

If not swap the HT leads from 1 to 4 (same coil) and 2 to 3 (same coil) this will help tell you if it's a ignition or carb/inlet/fuel problem

[StevieB]

Thanks for both of your replies. Has given me a lot to think about and plan some further fault diagnosis.

Curious if intermittently running on 2 cylinders, should be easy to prove if the exhaust down pipes are all hot (simple basics - I hadn't considered). Does make me wonder how people tune by ear if I can't tell between 2 and 4 cylinders running! 

As been looking into the fault for so long been a bit blinkered - just focused on vacuum leak. Most posts on hanging idle mention running lean or vacuum leak.

And the info on swapping plug leads to prove ignition or carbs will be another fault step.

Didnt know about wasted spark principle, thought they all fired singly in sequence.

It's an American import so Suzuki electronic ignition and no points - so thought ignition issues would be down to coils or leads (tried multiple sets of plugs).

So good to get some help from another perspective, will really help trying to pinpoint where the fault lies. I will let you know how I get on. 

Thanks 

[StevieB]

Sorry just thought also - if it was only firing on 2 would it still indicate 4 cylinders on vacuum guages?

Would the down stroke piston still produce enough vacuum to register on the gauge even though it's not firing?

Got me thinking maybe it is an ignition fault.

Cheers

[wraith]

You should still get a vacuum/compression even if not firing.

If, like you say you can remove 3 and 4 plug caps and no difference in engine running, it won't be the ignition itself but more like bad HT or caps fault.

I had on one of my bikes, two plugs go on me, the engine was still running and the down pipes where getting worm but not hot like the other two, Evey now and then they would kick in and make the engine run sweet.

So always worth starting at the start and making sure the basic is there. 

You could always apart some Duck oil around the HT and plug caps, if there is a crack the duck oil will smoke at that point.

[StevieB]

Tried swapping plug leads to start some fault diagnosis (1 &4, 2&3).

It wouldn't start! Swapped em back to original - still no start - various choke settings and plenty of fuel in tank.

Last time I ran it apart from high revs it was ok started and ran - stopped with ignition switch so there should be plenty of fuel to start in carb bowls.

But did prime again incase.

So against my initial thoughts of fuel air problem do I have a possible ignition issue as suggested.

With plugs removed and ground via cylinder there was only a spark on number 1. Not good but a constant failure should be easier to find than intermittent. Disconnected plug caps - resistance on each 4.5K - 4.9K ohms.

Coils primary windings both 3.7 ohms.

Measured inner core of each HT coil lead together (think correct way to measure?) for secondary got 14.5K left, 15.3K right ohms. Ignition fuse good and 12V at fuse contacts.

So my fault diagnosis is beginning down a different path. Just going through methodically.

Was going to clean up all ignition connectors next.

And thought could put a voltmeter on each coils feed from loom to see if get 12V when cranked over. It's an American import with factory signal generator so don't how else to test it?

Also can you test the actual cdi unit? Don't want to short anything out and cause more problems. I've got a basic understanding of electrics but no expert!

[StevieB]

Ordered a Clymer manual for the GS. Got one for my other bike and fault diagnosis really good and structured.

Gonna start from scratch with all fault diagnosis on ignition, fuel/air and compression.

Get it sorted logically without spending loads of time or money. Been ongoing problem for months.

Will update when sorted (or any unanswered issues!)

[wraith]

You can check if the coils are getting a pulse by getting a side light bulb and holder, contact it to the wires going to coil (one coil at a time) turn the engine over, if the bulb flashes you're getting a single to the coil. Quick and easy way 

If you only got a spark on number 1 cyclinder, it could be bad plugs or HT leads. Try it again but use the same plug for each cylinder to keep a constant leave.

[StevieB]

Just an update, in the shed making some progress.

I have been waiting for my Clymer manual to arrive past couple of weeks - nothing's ever easy. After all that got delivered the wrong book - so another on order.

Any one got any use for an American religious book - trial of a mind!!

Anyway thought try advice from here - invested in an IR heat gun as suggested by gixer1460 - got a way forward.

350 degrees on 1,2 and 3 downpipe. 40 on no 4.

Brilliant.

So gonna swap 1 and 4 plug leads and see if fault moves to no1.

Thinking if fault on 1 cylinder unlikely to be coil or signal generation as the same coil feeds no1.

Will update - but fantastic idea using a thermometer gun. Cheers

[StevieB]

"4 cylinder unlikely to be coil fault as fed by same coil as no 1"

[Gixer1460]

17 minutes ago, StevieB said:

"4 cylinder unlikely to be coil fault as fed by same coil as no 1"

It is unlikely but it could be a faulty coil or a failing / failed plug wire - never say never! But a swap of 1 & 4 is still a good check!

[StevieB]

Hi just been running it again,swapped plugs and plug leads multiple times.

And with original plugs In case us on fit new plugs.

No 4 cylinder just doesn't run up to temperature, no matter what plug or lead (1 or 4) is fitted.

When the revs have risen uncontrollably I think as suggested that is when it's running on all cylinders.

So there's an intermittent fault. Logically I wouldn't think valve clearances as would be constant and on vacuum guages readings across all cylinders the same.

But I will do a compression test to eliminate that.

Will check no 4 carb bowl for fuel, plug dry. And try fitting clear tubing from bowl drain to check fuel level in bowl.

Unfortunately I think the carbs may have to come out again for further investigation.

Not what I was hoping but progress thanks to reading the tips on exhaust temp.

 

[wraith]

Try Duck oil, spray it on all the electronic parts and start the bike. If there is a electric fault you will see a puff smoke from the duck oil wherever the fault is.

If it is a intermittent fault they are a pain to find but this duck oil makes it a bit easier.

[StevieB]

Cheers.

Got fault diagnosis steps in mind - compression,ignition,fuel.

 

Test the compression first.

Test ignition - will try the duck oil method, also fit colourtune plug in no4 - run it and visually confirm spark if there is one.

Test fuel delivery to no4 carb - check actual fuel level in bowl with clear tubing method as per carb rebuild link from here. This will confirm float and needle.

If it's got correct fuel level suspect no fuel through idle circuit.

Can confirm by taking it for a ride, using high throttle input so using main jet not idle. The no4 exhaust pipe should give a high temp.

That 's my theories!

Will keep updated, if my tests work may help someone else.

One day there will a posted pic of it running!

[StevieB]

Hi just found out after a lot of investigation why my GS is not running on 4 cylinders - and unfortunately as not a quick fix know what the cause of the problem is 

Ignition and compression good as previous.

Looking into carbs fuel supply first. Fuel from tap good.

Have now checked fuel level in carbs. With a tube from bowl on no1 and no4 (as 1,2 & 3 cylinders running as they should) and can only get to 1 & 4 carb when on the bike.

No1 fuel level at 4.5 mm as specified in manual (as I set them up), but no4 over 6mm (even though previously set at 4.5mm on rebuild).

 

No 4 was the one I had trouble setting up heights as had repair (weld) to one of the float posts previously snapped by a former owner.

 

Anyway trying to cut a long story short - took off the bowl of no4 in situ on the bike - the repaired no4 carb post has given up the ghost and totally snapped off. Explains why it would'nt achieve the fuel level to supply the idle fuel circuit.Mr

 

So I need to find a no4 bs32 CV carb for a 1980 GS550e (or other bike fitment) or a full set.

Will post on the wanted site here first as everyone on here really helpful before I start searching Eblag etc. Hopefully someone may know of a source to buy from.

Cheers

Edited August 2, 2021 by StevieB
Found set of carbs

[StevieB]

Just found a set of carb bodies on Eblag. Will c what they're like and update progress 

[StevieB]

Continuing carb saga, wrong ones for a kawasaki. Totally different. Good seller - full refund.

Managed to get a set of bs32's for a 750 gs from a friend. Stripped down except air mixture screw is totally seized and rounded off. It's  soaking overnight in anti seize compound.

The number 4 carb for my 550e is listed as part no 13204-47090.

 

Just got a couple of questions - apart from measuring carb intake bore are there any part nos on the carbs? Never been able to see any.

 

And are bs 32 CV carb bodies the same on the 550,650,750 and 850 ? - with just all internal components (jet sizes etc) being particular to each cc model.

Just curious as tried all components from my 550 carb into the 750 and they all fit. The throttle butterfly valve on the 550 says 140 and the 750 says 135 but identical in size?

 

If the 32 mm bodies are identical will make sourcing another no4 carb a lot easier if can't get the mixture screw out.

 

Edited August 8, 2021 by StevieB
Wrong pt no.

[StevieB]

It's been a while but the engine is now up and running well. Ended up replacing no4 carb body - all fuel levels good, starts and runs fine.

Learnt a few things about gs mikuni carbs getting it sorted.

32 cv carb internals are interchangeable despite different part numbers (in my experience - except mixture screws).

Learnt how to balance carbs.

Found a good supplier of second hand carb parts - Mikuni man on Eblag - really helpful 

Vapour and ultrasonically cleaned, carb body and other parts look like new. May help someone else.

Thanks for all the advice on here - got there in the end, so nice to get it back on the road.

[wraith]

Nice to hear you've got it up and running 

[StevieB]

Cheers - nice to be riding old skool again