Bad misfire / cutting out.....carb related?

[BVMark]

My fairly new to me 02 ST1100 has developed a bad stumble/misfire that sure appears to be a fuel starvation issue, but maybe not. I'm hoping to get some diagnostic help here before I start opening things up for a look.

The symptoms:
* Under constant throttle, at approx 3500 - 4000 rpm there is a noticeable stumble / misfire.
* Under hard acceleration in the lower gears the engine revs freely until approx 7000 rpm and then falls on its face. It misses and cuts out until it eventually reaches 8k.
* When doing a 5th gear full throttle roll-on from approx 3000 rpm the bike bucks, stumbles, cuts out, etc and won't go over 4200 rpm.

The bike starts easily, and cruises smoothly under 3500 rpm, or above 4500 rpm.

Where do I start diagnosing this?

 

[ST1100Y]

Diaphragm in the auto-fuel valve thorn?

 

[vinny]

The fuel diaphragm would be the first obvious place to check. It could be a minor vacuum leak in another area. It could also be ignition related. I would start with the vacuum/fuel system first as it is a little easier to diagnose and cheaper to fix. Sometimes, a major electrical failure in progress can cause similar symptoms. Check the main electrical connection to the left of the battery... the big red connector is a know failure item if not properly maintained.

 

[John OoSTerhuis]

+1 Auto Fuel Valve

 

[Mark]

It sounds like a diaphragm, either the vacuum fuel shutoff which you can route around quickly to test, or one of the carb diaphragms which are much more difficult to fix (and more costly!).

 

[BVMark]

My first thought was that one or more of the carb diaphragms was compromised, but the fuel supply diaphragm makes sense too. So, to test the fuel supply diaphragm I just re-route the fuel hose and bypass the fuel valve?

I bought this bike to be an antidote for my KTM 950, which is really wearing me out with all of the maintenance and repairs it requires, so I'm pretty bummed I'm having this problem after having only ridden the Honda a couple thousand miles. Hopefully I'll get this figured out and it will be a fairly painless fix and the bike will be the super reliable, low maintenance rig I was hoping for.

 

[Jim Van]

1.) Auto fuel valve and/or fuel filter ?? Replace filter and bypass the fuel valve.

2.) Torn carb diaphrams ?? Inspect for tears. See : Carb Diaphrams

3.) Marginal fuel pump ?? See: Fuel Pump Output

4.) Dirty carbs caused by dis-integrated sub air filter ?? See : Sub Air Filter . Replace sub air filter & run some Sea Foam .

5.) Verify gas cap pressure relief is working ( at top of gas cap ) AND that vent tubing isn't pinched where attached at front side of gas tank. Try blowing & sucking at the vent port - it takes some effort. Try blowing through vent tubing. Check for crud blocking opposite end of tubing, too.

6.) Doesn't sound like ignition to me, but I have been wrong before.... Replace plugs and check spark plug connector resistance if none of the above doesn't cure the problem. Connectors have about 10K Ohms of resistance. Good Luck.

 

[Mark]

I just re-route the fuel hose and bypass the fuel valve?

Yes.
Well 'yes' if you re-route *and* plug the vacuum hose that goes to the 'fuel shut off valve' that is powered by engine vacuum!

 

[ST1100Y]

So, to test the fuel supply diaphragm I just re-route the fuel hose and bypass the fuel valve?

There is a quite easier test:
remove the carb-sided vacuum hose from the rear (inboard side) of the fuel valve, place another hose (like 10" long) onto the stub instead, place the free end between your lips, draw vacuum and "plug" the end of the hose with the tip of your tongue... if it holds the vacuum, the diaphragm is OK, if not its obvious that there is a crack.
(another clue is collection of dust around the small vent hole at the bottom of the fuel-valve body; when the diaphragm leaks, air therefore also dust gets drawn in, thus a visible accumulation of crud there...)
Reasonable priced, and very well made replacements can be found at NRP Carbs.

A leak there can obviously exists unseen for quite a while, before the tear finally gets wide enough to cause trouble...
Often the unnoticed leak gets "compensated" by (mis)adjusting the butterfly-flap on #3 during a carb sync, so checking the adjustment after fixing the fuel-valve is strongly advised.

Below pic taken 2011 shows the torn fuel-valve diaphragm of my '00 ST, which had no issues or symptoms whatsoever, revved bravely to the red-line, etc...
I located that culprit more by coincidence and out of curiosity by above "suck on a piece of hose" method, during one of my annual pre-flights...
After fixing it (by installing an NRP rep-kit, after the K&L kit showed being poorly(!) made...), I found carb #3 being off, new sync, runs perfect ever since...



The ST1100 is pretty bulletproof and reliable, as long as it sees its proper, precautionary maintenances, which, in comparison isn't a lot to do...

 

[BVMark]

Well, I finally got some time to open up the bike and take alook at the fuel valve, and discovered a previous owner had already removed it. I guess it's time to pull the carbs and look for a problem with one or more of those diaphragms. I'm bummed.

 

[Brant]

Well, I finally got some time to open up the bike and take alook at the fuel valve, and discovered a previous owner had already removed it. I guess it's time to pull the carbs and look for a problem with one or more of those diaphragms. I'm bummed.

Since you've eliminated fuel shut off valve as the issue, no different than 'old school' automotive world.......'secondary ignition before fueling'.....always
May consider plugs, plug wire ends where they screw into plug cap ends, coil input voltages and coil ohms resistance across pole connections that they ate still within specs. Carbs as last resort IF secondary ignition is all within specs.

 

[Grabcon]

Well, I finally got some time to open up the bike and take alook at the fuel valve, and discovered a previous owner had already removed it. I guess it's time to pull the carbs and look for a problem with one or more of those diaphragms. I'm bummed.

I just had this happen on my 1100 and it ended up being the fuel filter. My symptoms were miss firing and poor MPG. I have a car fuel filter with replacement cartridges it has a glass tube so you can see the filter. It looked clean but when I pulled it off and took it apart the filter was plug with really really fine stuff. Replaced the cartridge and it runs great. No missing and the mileage went up as well. At least 5 mpg

 

[kappy5003]

I have a similar issue with my AnaSTasia.

About 14 months ago while working in the city I took her off the road as she wasn't quite running as smooth as she used to.

I decided to do forks ,shims, radiator hoses, some leaks plus tyres and do all fluids.

Anyway around November last I experience a tragic family event that gutted me so everything went on hold, so a few months ago I put her on a trailer and brought her home as therapy.

Its been a long slow road.

After sorting out some battery issues I finally fired up AnaSTasia. It took a while for her to start but when I did I could tell all was not smooth and there was heaps off black smooth as I revved her, probably flooded her. I also noticed more tappety than before I did the shims.

When Idling and when revved slightly I could tell it was not quite smooth, like timing not right, so too hard leave for another day.

So the next day when and fired up 1 st time tappet noise almost gone but still not smooth.

I checked the spark plugs ( Iridium that I have had for about 20,000kms) they were all quite black

Since then I have started her a few times seems as if getting a little better. New fuel--carby cleaner.

Any way a couple of days ago I got to operating temp and noticed running better when warm, so did carby sync some minor adjustment not really a lot, less than I would have thought.

The following day I fired her up again, Wont fire without choke when cold and it not that cold really, anyway got temp up not as warm as the day before and checked carbs and discovered quite a bit of discrepancy. Thinks out loud, I look at that later.

A few days ago I started her with a little choke and notice a little backfire though carbs, even with choke off still a little backfire. Increase revs a little and can here a little popping, all with air cleaner cover off.

I also smell what I call a pepper type smell from around the intake area, I related this smell to a sort of coolant smell , takes me back to an old car I had with coolant leak car heater system.

Yesterday I decided to obtain some standard plugs and on removing Iridium plugs noticed they are still black and quite damp.

After installing new plugs when starting I observed popping through intake but also noticed when revving slightly popping from all intakes plus felt as though more sluggish than before but slightly smother ,does that make any sense???

So before I start ripping everything apart unnecessarily can I ask a few opinions factual would be a nice, I feel may be necessary to pull carbies out etc.

I know plugs should probably be greyish and not damp.

Cheers
Kappy

 

[Jim Van]

Kappy :

It's obvious that you have a very rich mixture, by the looks of the plugs. I'm assuming all four were black & moist.

I would first check to make sure the choke mechanism is not binding and it is shutting completely off and there is some slack in the choke cable when the choke is in the "OFF" position.

 

[kappy5003]

Almost just now.
Choke cable and operation, checked and OK.
Ran for short all pipes hot.
Pulled No 3 plug quite wet at base of thread.
Pulled No 2 & 4 both black on bottom electrode ,No 4 more so, all plugs are new,minutes old.
Side note prior to taking off the road the engine didn't feel as sweet and smooth as she did say 5000km before, hence the reason for doing shims.
She has sat and not been started for approx 14 month hence carby concern.
Shim settings double checked and ok.
On just starting convinced I have a coolant leak that I can smell but can't see, ----more exploration

 

[afrymoyer]

Kappy... You should start another/new thread. Your troubles differ from the original poster.

I hope you stumble across something quick and easy as a fix, however, I believe you have several things going on. Your bike has been in storage for a long time. I'm thinking that the slow speed / pilot circuits are plugged, causing the popping through the intake and liberal use of the choke to get the bike started and keeping it running. As also think that the fouled and wet plugs are due to the float needles being sticky (open) and flooding out the engine. I recommend pulling the carbs for a GOOD cleaning and inspection....

 

[kappy5003]

Thanks Adam,

I had thought about a new thread, but I will do that so I can post results/progress.

Cheers
Kappy

 

[jvick98]

BVMARK,

The symptoms you describe sound just like those of a 2002 ST 1100 owner in Monument, Co. He added a dose of Seafoam, let it sit for a few days and problem cleared up. Try the easiest things first!

Jim