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Suddenly only one cylinder fires

Started by Ryguy, July 11, 2023, 11:10:30 AM

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Ryguy

Hey guys. I am unfortunately back again seeking wisdom.

On my ride to work yesterday the bike stalled. Then when i started it up again only one cylinder was firing.

Upon further inspection at work, it seems like the other side will fire initially, but you have to give it throttle and hold the throttle or else it cuts out

When they both do fire, it is very rough in sound and vibration.

I also noticed the choke pretty much has no effect on the bike idling anymore

I bought new plugs, and rotated the spark plug wires to no avail.

I would love to hear some thoughts.

Ive uploaded a video below which shows everything.

Thanks, as usual gents.


chris900f

#1
To quickly test spark: remove plugs, connect them to their respective caps and lay them on the engine block, so that the base of the plug is grounded to the block. When you hit the starter, you should see the spark across the gap of each plug. You can do this procedure one plug at a time. If you see spark you can assume for the moment that ignition for that cylinder is ok.

You can do a seat-of-the-pants compression test to make sure the valves are ok. (key on/kill switch off) With the plug removed (and not connected to the cap) Place your thumb lightly over the plug hole and run the starter briefly. You should feel fairly strong air pressure coming from the plug hole. Do both sides, they should feel about the same.

If the ignition/compression checks out you can move on to fuel delivery(carbs). You can do a simple flow test to see if each carb is getting fuel. Connect a small piece if hose to the fuel drain on the bottom of the dead cylinders carb bowl. Run the hose to a fuel can or catch basin. Switch the petcock to the Prime position. Loosen the screw on the side of the carb, just above the drain nipple. Fuel should flow from the tank, through the carb and out the drain hose. Check the condition of the fuel that comes out for small debris that may have plugged up the pilot jet. If no fuel comes out, try tapping on the float bowl with the back of a screwdriver...sometimes a float gets stuck.

If fuel flows well from of the carbs, it is most likely that a pilot jet is plugged up, so next steps involve removing the fuel tank, air box and carbs for cleaning. But try these simple measures first and report your findings.

ps. It's not a good idea to try to run the engine on one cylinder for very long.

Ryguy

Thanks man. I will give that a go and troubleshoot.

Cheers!

Quote from: chris900f on July 14, 2023, 06:21:07 PMTo quickly test spark: remove plugs, connect them to their respective caps and lay them on the engine block, so that the base of the plug is grounded to the block. When you hit the starter, you should see the spark across the gap of each plug. You can do this procedure one plug at a time. If you see spark you can assume for the moment that ignition for that cylinder is ok.

You can do a seat-of-the-pants compression test to make sure the valves are ok. (key on/kill switch off) With the plug removed (and not connected to the cap) Place your thumb lightly over the plug hole and run the starter briefly. You should feel fairly strong air pressure coming from the plug hole. Do both sides, they should feel about the same.

If the ignition/compression checks out you can move on to fuel delivery(carbs). You can do a simple flow test to see if each carb is getting fuel. Connect a small piece if hose to the fuel drain on the bottom of the dead cylinders carb bowl. Run the hose to a fuel can or catch basin. Switch the petcock to the Prime position. Loosen the screw on the side of the carb, just above the drain nipple. Fuel should flow from the tank, through the carb and out the drain hose. Check the condition of the fuel that comes out for small debris that may have plugged up the pilot jet. If no fuel comes out, try tapping on the float bowl with the back of a screwdriver...sometimes a float gets stuck.

If fuel flows well from of the carbs, it is most likely that a pilot jet is plugged up, so next steps involve removing the fuel tank, air box and carbs for cleaning. But try these simple measures first and report your findings.

ps. It's not a good idea to try to run the engine on one cylinder for very long.

Ryguy

#3
So im back with the results lol.

The plugs fire.

I bought a compression reader. both cylinders have 130psi.

Fuel runs nicely from the carburetor drain. And I just finished cleaning the carbs and none of the jets were plugged

I fiddled around some more with running each clyinder individually. And here is what i Have concluded.

The working cylinder starts and runs without issue. idles at around 1400rpm with choke open as for normal driving.

The faulty cylinder starts and runs only when the choke is closed as you would for a cold start, and idles at 500rpm




Quote from: chris900f on July 14, 2023, 06:21:07 PMTo quickly test spark: remove plugs, connect them to their respective caps and lay them on the engine block, so that the base of the plug is grounded to the block. When you hit the starter, you should see the spark across the gap of each plug. You can do this procedure one plug at a time. If you see spark you can assume for the moment that ignition for that cylinder is ok.

You can do a seat-of-the-pants compression test to make sure the valves are ok. (key on/kill switch off) With the plug removed (and not connected to the cap) Place your thumb lightly over the plug hole and run the starter briefly. You should feel fairly strong air pressure coming from the plug hole. Do both sides, they should feel about the same.

If the ignition/compression checks out you can move on to fuel delivery(carbs). You can do a simple flow test to see if each carb is getting fuel. Connect a small piece if hose to the fuel drain on the bottom of the dead cylinders carb bowl. Run the hose to a fuel can or catch basin. Switch the petcock to the Prime position. Loosen the screw on the side of the carb, just above the drain nipple. Fuel should flow from the tank, through the carb and out the drain hose. Check the condition of the fuel that comes out for small debris that may have plugged up the pilot jet. If no fuel comes out, try tapping on the float bowl with the back of a screwdriver...sometimes a float gets stuck.

If fuel flows well from of the carbs, it is most likely that a pilot jet is plugged up, so next steps involve removing the fuel tank, air box and carbs for cleaning. But try these simple measures first and report your findings.

ps. It's not a good idea to try to run the engine on one cylinder for very long.

chris900f

So it's a tricky one eh? How about the carb diaphragm for the faulty cylinder. On these CV carbs, when you twist the throttle you are opening a butterfly valve, but the slide is lifted by the air being pulled into the cylinder. If the rubber diaphragm is damaged (hole/tear) the slide won't lift. You can access the diaphragm by removing the cap on the offending carb.

https://www.gomotoonline.com/oemparts/a/suz/508b06a7f8700235307234bf/carburetor-model-k4-k5-k6

Ryguy

#5
Damn! sorry dude, I goofed. Not sure what I did wrong the first time. Maybe the pressure didn't leave the gauge when I was testing them last night.

I did it again today

The working cylinder was 140psi. The faulty one was only 90psi.

So I guess compression is the issue. You were right.

So doing research it looks like it can be anything from valves, to rings to headgasket.

Any clever tricks you can think of to weed some of them out?

Thanks a bunch for your help. You've been huge.

Quote from: chris900f on July 16, 2023, 09:21:20 PMSo it's a tricky one eh? How about the carb diaphragm for the faulty cylinder. On these CV carbs, when you twist the throttle you are opening a butterfly valve, but the slide is lifted by the air being pulled into the cylinder. If the rubber diaphragm is damaged (hole/tear) the slide won't lift. You can access the diaphragm by removing the cap on the offending carb.

https://www.gomotoonline.com/oemparts/a/suz/508b06a7f8700235307234bf/carburetor-model-k4-k5-k6

chris900f

#6
So low compression is either usually cylinder wear or a valve problem.  Yes it could be a head gasket, or even loose head nuts.

Since it won't idle past 500rpm I'm thinking maybe a valve.

Pull the spark plug and pour a tablespoon of engine oil down the hole. Replace the plug and run the starter briefly--(key on/kill switch off and use an old plug for this step) Pull the plug and retest the compression. If it comes up substantially then the loss of compression is from a worn-out ring or piston. ie the charge is slipping past the piston. If it makes little or no difference then it's more likely a valve issue.

If it's the valve, it wouldn't hurt at this point to check the valve clearances. See BaltimoreGS on Youtube for some good videos on this. It's a bit of a longshot but if there is zero clearance on a valve, it won't be able to close all the way and the charge will escape on the compression stroke. You might get lucky and just need to get a thinner valve shim to fix the problem.

While you have the valve cover off also check the torque of the head nuts, supposed to be about 26fl/lbs, if you don't have a torque wrench just use a socket and medium strength, if they don't want to move, don't force them. You are looking for loose nuts, if they don't respond to a firm pressure they are probably not the issue.

keith

I might be having a very similar problem. And I have my motorcycle road test next week. Go figure!

https://gstwins.com/gsboard/index.php/topic,74218.0.html

Ryguy

Well it looks like your right again chris!

I followed that video you recommended and went through the steps. I got held up a bit because I had to buy a flexible clearance measurer. But it looks like the problematic cylinder's exhaust valve is super tight. Leaving it open probably.

So from what i gather, I will have to replace a 'shim'. I wont be able to tell how thick of a shim is in there until I yank it, and I will have to order that tool. But then i gather i just order a thinner shim.

You are a legend sir.







Quote from: chris900f on July 17, 2023, 09:16:03 PMSo low compression is either usually cylinder wear or a valve problem.  Yes it could be a head gasket, or even loose head nuts.

Since it won't idle past 500rpm I'm thinking maybe a valve.

Pull the spark plug and pour a tablespoon of engine oil down the hole. Replace the plug and run the starter briefly--(key on/kill switch off and use an old plug for this step) Pull the plug and retest the compression. If it comes up substantially then the loss of compression is from a worn-out ring or piston. ie the charge is slipping past the piston. If it makes little or no difference then it's more likely a valve issue.

If it's the valve, it wouldn't hurt at this point to check the valve clearances. See BaltimoreGS on Youtube for some good videos on this. It's a bit of a longshot but if there is zero clearance on a valve, it won't be able to close all the way and the charge will escape on the compression stroke. You might get lucky and just need to get a thinner valve shim to fix the problem.

While you have the valve cover off also check the torque of the head nuts, supposed to be about 26fl/lbs, if you don't have a torque wrench just use a socket and medium strength, if they don't want to move, don't force them. You are looking for loose nuts, if they don't respond to a firm pressure they are probably not the issue.

chris900f

That's excellent news!

Shims are sold in 0.05mm increments. So if, for example, you pull the shim and it's a 2.50
you can replace it with a 2.40 which should give you 0.1mm clearance. It might be slightly
different, because "zero" might actually be in the minus range, so you might calculate for 0.1mm
and measure .08mm with the feeler gauge--but as long as you're in-spec the compression should be restored.

You can order shims, but a lot of shops will just sell you one; as they buy bulk assortments--take your old one with you to make sure the diameter is exactly the same.

Ryguy

Hmmmm

So, I took out the 240 size shim and ordered a 230. I thought my math was good, and that should have been plenty.

But zero change! Lol. Darn it.

Still can't get my thinnest gauge measurer in there and still compression at 90. So it did nothing unfortunately. I guess I should try again but even thinner. 220, or even 210??

Quote from: chris900f on July 20, 2023, 10:18:47 AMThat's excellent news!

Shims are sold in 0.05mm increments. So if, for example, you pull the shim and it's a 2.50
you can replace it with a 2.40 which should give you 0.1mm clearance. It might be slightly
different, because "zero" might actually be in the minus range, so you might calculate for 0.1mm
and measure .08mm with the feeler gauge--but as long as you're in-spec the compression should be restored.

You can order shims, but a lot of shops will just sell you one; as they buy bulk assortments--take your old one with you to make sure the diameter is exactly the same.

herennow

Sounds like a plan, remember that most of us run our exhaust valves at wider then spec clearances to keel the valve boiler and reduce  valve seat recession.

Running with zero clearance might have burned your valve seat. Only way is to get clearance right and test compression. If not good you could get away with a lapping of the valves, or possibly require them recut. But let's go step by step. Do you hava the proper shim tool? If not, be very careful with homemade tools, easy to damage things.

Bluesmudge

Some people keep the thinnest shim (2.15) in the toolbox for just this sort of scenario. That way you can swap it in, do the math, and order the right shim rather than hoping the next size down is right.

Ryguy

Damn!!

So I bought the smaller shim. 215. And it worked, it allowed for clearance. And it's in spec.

But it wasn't the problem I guess!! There still is the same low compression! Crap.

Looking for suggestions on what might be  the issue or ways I can attempt to weed some of them out.

Pretty bummed. I thought those shims would have certainly been the cause.

Not even sure where to begin now.

Bluesmudge

Did you do this yet to determine if it is a valve vs the cylinder rings?

Quote from: chris900f on July 17, 2023, 09:16:03 PMPull the spark plug and pour a tablespoon of engine oil down the hole. Replace the plug and run the starter briefly--(key on/kill switch off and use an old plug for this step) Pull the plug and retest the compression. If it comes up substantially then the loss of compression is from a worn-out ring or piston. ie the charge is slipping past the piston. If it makes little or no difference then it's more likely a valve issue.

My guess is burnt valve and it's time for a top end rebuild. Doesn't matter either way really, since if you are that far in you might as well freshen up the hone and put in new rings anyways along with lapping the valves and new valve seals.

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