Cylinder # 3 spark plug fowled despite no vacuum valve.

[Christianncg]

I know, I know... Probably gonna have to tear into the carbs. But out of curiousity if any one has experienced this too, I have a spark plug that for the past 1000 miles has remained sooty compared to the brown of the rest of the plugs. I suspect a stuck float or a bad float needle, but dont want to overlook an easy fix. Oddly this is the same spark plug that had a fried spring inside the boot, and nigh infinite ohms.

After riding around 210 miles the fuel light came on. Going 75 most of the way, this equated to about 35 mpg, which sucks. After filling up and running the numbers, as soon as I turned in near my neighborhood, I felt a cutting out of sorts, One could say bogging too. Revving the motor you could clearly hear a lack of a cylinder/ bank. It held Idle fine however, just any throttle or load and it was gutless. Disassembly lead to #3 spark plug sooty..

THATS NOT ALL. Cycling the key (and consequently fuel pump) 6 or so times lead to a gurgling coming from cylinder number 3's carbs, lasting about 20 seconds with 6 pulses. . . .

 

[dwalby]

I can't recall if the ST1100 plug caps can be easily unscrewed from the ignition wire or not. If they can, swap the cap that you recently repaired over to another cylinder and see if the problem follows the cap. Better to rule out weak spark before tearing into the carbs. Also, since you say you don't have a fuel vacuum valve, make sure the vacuum line from the #3 cylinder is properly plugged off and not leaking. And, you are cleaning the dirty plug to look like the others before each troubleshooting attempt, right??

 

[Christianncg]

Yes sir. New plugs installed last time, vacuum caps capped nice and snug like. Will swap plug wires when the reinstall happens.

That being said, I can rule everything out already. I attempted to crank the bike with no plugs and was greeted with a shower of gasoline from cylinder number 3. It must have been borderline dead rich since this is the first time I have ridden in the heat for this long. When I rebuilt the carbs I overlooked the needle and seats in hopes that they would be ok. I did adjust the float to 7mm just like the rest. Maybe the needles are just tired. A disassembly will probably lead me to either,

A. Polish the seats to get better contact between it and the rubber tips.

B. Replace all seats and needles with a kit and be done with it.

C. Find I have a sunk float and that an expensive parts journey begins.

D. Find a foreign piece of material inside of the carb body and clear it out.

E. Find The float spec to be off (Doubtful.)

 

[dwalby]

glad you were able to easily identify the problem, good thinking.

C. I don't know if a leaky float could be patched or not, but if you can't find a replacement float a little dab of epoxy might seal the leak (if you can find the source of the leak).

D. foreign material sometimes sticks to the shutoff needle and it doesn't seat properly so you get gas overflowing out of the bowl onto the ground. You don't have that, unless your overflow tube is plugged, might be worth checking to make sure the overflow tubes are all good. A quick turn on each of the bowl drain screws should come out through the same hoses, right?? Not sure how easily accessible they are though with the carbs mounted, never had to do that. Other than that, foreign material would only plug the fuel passages, not cause excess fuel flow, or am I missing something?

 

[Christianncg]

I dont think these carbs have overflow tubes. Just drains. But I could be wrong. I'm at dinner right now but will take the carb apart when I get home and inspect the seat, needle and float.

 

[dwalby]

its been many years, knock on wood, since I had to look into such things, but I think the way the bowls were always setup was a thin tube that normally sits above the fuel level has a small hole in it. If the fuel gets too high it flows into that hole and down through the same passage that flows when you crack the drain screw open. Never had the ST carbs apart to verify they are the same way, but I suspect they would be. Also, going by memory from the mid-80s when I did this last, so take that into account as well.

 

[TerryS]

I bought some new float needle valves from the supplier below in Taiwan at a fraction of the OEM price. They've been in service for 10,000km with no issues, certainly looked the same as the OEM parts.

https://www.ebay.com/usr/damineding?_trksid=p2057872.m2749.l2754

That fuel sprayed out of the #3 cylinder certainly suggests you have a float needle that won't shut off. I'm of the opinion that removing the fuel cut-off valve leaves more of a possibility of fuel entering the carbs and overflowing if the needles aren't perfect, just due to heat soak and trapped pressure when the bike is stopped.

 

[SMSW]

If you are gettting that much fuel in the cylinder that it 'sprays' out when you pull the plug and crank it, might you be in danger of a hydraulic lock - i.e. too much raw gasoline in that cylinder?

 

[bdalameda]

I would also check to make sure the fuel enricher (choke) valve is closing fully. If it does not seat down fuel will dump into the intake and load up the plug.

 

[Christianncg]

If you are gettting that much fuel in the cylinder that it 'sprays' out when you pull the plug and crank it, might you be in danger of a hydraulic lock - i.e. too much raw gasoline in that cylinder?

Yes and no. Gravity fed systems are more susceptible to it because the "pump" (gravity) is always on, depending on the condition of your vacuum valve/ petcock. With a pump containing a restrictor/regulator the chances are far less, pressure is relatively weak in the lines to cause a hydrolock once shut off. The reason gas sprayed is I cranked the key 7 times, running the pump for more then the usual second and filling the cylinder with gas. Under normal circumstances the pump only runs full on with the motor running, and when the motor runs, consumption of gas occured enough to hide the symptoms I assume. No white smoke but certainly drinking the gas and fowling the plug. It's only been today that I had the cylinder shut off due to too much fuel, and how hot it was. I will check The enrichener circuit too.

I will probably shotgun (an educated shotgun though the needles as they were original and had a slight bit of wear if memory serves correct. Time to tear into the carb to peak...

 

[Christianncg]

Well, I adjusted the floats to 8mm, polished the offending brass fitting, and took some 3k grit to remove the shiny spot on the rubber. Wired the fuel pump to 12v and let it run for 5 minutes. Not so much as a drip. Where as before it would take about six seconds of fuel pump to cause a problem. Everything shows good for now, just gotta hook everything back up and install some new plastics that came in today. Also, a new fuel line is in order between the carb fuel filter and the metal line going to the pump. it's beginning to swell shut. I will probably go pick some up tomorrow and then give it a sync, since it could be synced to a rich condition... fudge.

 

[dwalby]

glad its sorted, what do you think actually fixed the problem, float adjustment, or something else?

 

[Christianncg]

I want to replace them but cannot afford it yet. It would appear to be fixed. Setting the floats a little higher probably did it, as the spring tension on each individual float is different. (why i will probably replace the float needles anyways, different tensions = different float levels when filled.) I also kinda needed to lean out the cylinders as it's a smidge rich on the rest of them.

I disassembled each spark plug boot and found out all have been arcing, and won't make continuity when reinstalled. So more work for me. That has to be fixed before the carbs go back in..

 

[Christianncg]

Found out where all my gas is going. Lol I'm glad it evaporated and didn't catch fire.

 

[bdalameda]

Unless you've already done it, while you're in there I would replace the o-rings, spigots, clamps and hoses in the valley between the cylinders. - Known coolant leakage points. It's not a matter of if they will leak, but when the leaking will start.

 

[Christianncg]

Unless you've already done it, while you're in there I would replace the o-rings, spigots, clamps and hoses in the valley between the cylinders. - Known coolant leakage points. It's not a matter of if they will leak, but when the leaking will start.

It's been done. This is the swapped motor project a few posts back I made.. all coolant hoses, o-rings and fluids replaced. Still can't believe enough gas leaked to leave a stain...

 

[Christianncg]

Well, New needle and seats from K&L Supply will be ordered. After reassembling I found out the hard way they were still leaking, not nearly as bad as they were before but enough to fowl the #3 spark plug. When I can order the new seat/ needle set I will perform a diagnosis on the fuel system, hooking up a spare car battery to the fuel pump and running it for an hour monitored. Shouldnt be any seepage as the pump is regulated for this kinda stuff, me thinks. only puts out friggin 2 psi passively.

 

[red one]

I bought some new float needle valves from the supplier below in Taiwan at a fraction of the OEM price. They've been in service for 10,000km with no issues, certainly looked the same as the OEM parts.

https://www.ebay.com/usr/damineding?_trksid=p2057872.m2749.l2754

That fuel sprayed out of the #3 cylinder certainly suggests you have a float needle that won't shut off. I'm of the opinion that removing the fuel cut-off valve leaves more of a possibility of fuel entering the carbs and overflowing if the needles aren't perfect, just due to heat soak and trapped pressure when the bike is stopped.

Plus one on that. I know many run without it but Mother Honda made that part for a reason.

 

[afrymoyer]

First of all, there are NO overflow tubes in the ST1100 carburetors. If an ST float needle/seat fails it will fill up the bowl and flow out the carb vent at the front of the carb bank(49 state non-emission bike). It also runs up the main jet and emulsion tube, trickling fuel out the main jet circuit and seeping into the cylinder.

The spring tension on the needle plungers has absolutely nothing to do with the float level. Once the float is seated it stays seated, even if the springs are un-equal or weak. The springs are there to absorb the float shock from the fuel sloshing in the bowl while underway. You've purposely leaned out an already lean running bike from the factory. Set them back at 7mm.

The shiny ring's you noticed on the rubber tips of the needles, means that the rubber tips are worn out and grooved. Sanding them down with sandpaper has removed the groove, but made them uneven. They are STILL hardened and worn out. Replace the needle and seats...

While you're at it, check the fuel cap and make sure that it isn't varnished and sticky. Service the cap. Make sure the entire tank vent system is working properly. Also Check the fuel pump FLOW as per the manual specifications. Your auto fuel vacuum valve has been removed (which is ok), but there is potential that the pump can overpower the floats.

Don't try to fix those stupid resistor spark plug caps. They will fight you till the bitter end with drivability issues. They are only $20 a piece. Replace them. This is probably one of the causes of the "rich" condition.

 

[Christianncg]

I intended to change them back to 7mm once the k&l kit arrived. I always suspected with a weaker spring there would be less resistance for the pump to push the float needles down. #3 was the weakest, but I suppose that's coincidental.

I do normally replace them, it's one of those items like the carb bowl gaskets, rubber carb boots and rear drive o-rings that gets replaced automatically, for piece of mind. This however was intended to be a budget built and it would seem I mistakenly bet on black, thinking that they would hold. I have seen worse needles hold lol. I am slowly finding out budget bike Building is not an actual thing.

Since ownership the cap has had positive pressure when opening. Not violent but never suction. I have read here that this is normal. I'll check it anyways.

Fuel flow is 1800cc per minute, or just under 150cc per 5 seconds. Performing the test on ST riders yields 450ccs per 15 seconds with the cap off. Seeing that 500ccs was considered common place during this test, that rules out the pump.

The problem with the caps was the top portion was corroded. I took them to work and matched a drill bit to the diameter of the cap spring. Drilled just enough to give a new face of metal to bond with, then some de-oxit for good measure. There isn't really anything wrong with them other than a corroded top piece. That being said I will probably end up replacing the caps within the next few weeks when the budget allows it, along with spark pluggies too but not for another few weeks. . The coils measured fine heat stressed with 25k on secondaries and 3.5 on primary windings.