Number 2 Cylinder Still Running Cold

[DaveP]

Still struggling to get my 1981 GS650 running well again and I am pretty much at a loss for next steps now. The engine has experienced a full re-build and is running again, but the number 2 cylinder is running much colder than the other 3 cylinders. It is also running a bit erratically now and not holding a steady idle. Too low or revving too high...

Here are the spark plugs in order from cylinder 1 to 4:


I have taken the carbs back off, torn down carb 2, and soaked it for one week, but no change. It didn't look bad before the soak either.

Compression is ok:
1 - 145 psi
2 - 133 psi
3 - 140 psi
4 - 140 psi

Spark is good. I previously did the coil relay mod and the stator/regulator/rectifier have all been replaced.

Carb boots are in good shape with new o-rings. When running, I have sprayed carb cleaner around the boots with no change in engine speed to show any air leaks. The air cleaner box and connections are original.

Any thoughts on where to look next would be appreciated.

Thanks,
Dave

 

[Brendan W]

Swap HT leads 2 & 3 and see if the coldness moves.

 

[Steve]

The first thing that I would do is to start with a new set of spark plugs to get some fresh readings. :-k

#2 is either running DRASTICALLY rich or has an oil leak. Your compression numbers do not suggest an oil leak, at least not one that also affects another cylinder.

One of the typical culprits of #2 looking like that is the petcock on the fuel tank. There is a hose that goes from the #2 carb to the back of the petcock, where it pulls on a diaphragm to open the fuel flow. If that diaphragm has a leak, you will get extra fuel into the mix for #2, giving those extra-rich mixtures.

To test the petcock, you will need to have two hoses that are long enough to reach to the ground, and two containers. Either glass or fuel-resistant clear-ish plastic are best. Run the two hoses from the two nipples on the petcock into the two containers. Leave the petcock pointing down in the RUN position. Check the containers in an hour or two. If there is nothing there, leave them overnight. If you end up with ANY fuel in either container, you need a new petcock.

Yes, there are petcock rebuild kits available, but the success rate with them seems to be about 10-15%, so by the time you get a kit, find that it didn't work, then get a new petcock, you will have wasted aboutt $25 and a week or two of riding time. Best to get OEM, it's 'only' $67.65 from Parts Outlaw. I believe that a new o-ring (part #2) is included, but you might want to add two of the gasket #4. Don't forget to enter the code (printed at the top of every one of their website pages) to get $5 shipping.

.

 

[fastbysuzuki]

1,3,4 look like they are running on the weak side 2 looks like it's running rich or burning oil, the petcock could be faulty and passing fuel down the vacuume line into #2 cylinder that is assuming it's like the 850 and the vac line is on #2 carb.
take the vacuume line off the petcock and see if there is petrol in the line if so that will be the cause of poor running on #2
if so a new petcock is needed there are repair kits but the consensus on here is they have a high failure rate.
i belive there is a ebay seller called georgefix ( I think) that sells them a a decent rate.

 

[DaveP]

Swap HT leads 2 & 3 and see if the coldness moves.

Good idea, but cold cylinder stayed at No. 2.

 

[Steve]

..., the petcock could be faulty and passing fuel down the vacuume line into #2 cylinder that is assuming it's like the 850 and the vac line is on #2 carb. ...

Just for clarification (on a couple of points):
1. Not all 850s have the vacuum port on #2 carb.
2. In general, the VM-series carbs, which ended in the US with the 1979 model year, have the vacuum port on the #3 carb.
3. In general, the BS-series carbs (also known as CV carbs) have the vacuum ports on the #2 carb.
4. Production of 850s started in '79, so the '79 850s have VM carbs, all the others have BS carbs.
5. Production of the 650s started in '81, so they all have BS carbs.
6. Cylinder (and carb) numbering is from left to right, as you sit on the bike. #1 is under your clutch hand, #4 is under your throttle hand.
7. All the twins had BS-series carbs, even before 1980. I believe their vacuum port is on #1.

.

 

[DaveP]

Steve and FastBySuzuki, I did replace the petcock with OEM early on. The original was probably fine too, but I didn't notice until swapping in the new one that the PO had used star washers and no gaskets to make the connection to the tank which was the cause of the leaks

The tank has been off and on the bike during this process and is at least half full. No signs of leakage from the fuel, vacuum, or vent connections while the tank is off the bike.

 

[tom203]

Plug the vacuum fitting on #2 carb at carb...run bike with petcock on prime position and hit the road..see if #2 spark plug clears up. Later remember to turn petcock back to "on" spot

 

[fastbysuzuki]

Yes but the petcock is not operating when the tank is off the bike or when it's not running so the fault may not be apparent follow Tom's advice and see what happens.

 

[DaveP]

Thanks for the replies. Probably next weekend now before I get to try again...