GS650G Katana rebuild - firing on 3 cylinders - driving me crazy!!!!!

[Guest]

Morning guys,

I promised myself I would not need to ask for help, because there is so much information on this fantastic site that with a bit of digging I could find the answer to any question. However for the sake of my sanity I have finally given in.

Ok, I have spent the best part of two years turning a load of rusty parts from an assortment of dead bikes into what I think is a pretty nice GS650GX Katana. See the picture below.



As part of the re-build I have done the following:

Stripped and cleaned carbs, replace all O-rings (as per carb re-build tutorial).
New valve guide seals.
New rings, (very) lightly honed bores, all dimensions within spec.
Re-conditioned crankshaft.
Tappet shims adjusted, and set to within spec (when measured correctly!)
Checked cam chain, set valve timing as per manual.
Valves checked by leaving paraffin (kerosene) overnight in upturned head. No valve re-seating necessary.
etc etc etc etc.

So on the big day, when I had finished the project and tried to start her, I could only get her to fire on 3 cylinders. (Cylinder 4 won't fire). Ok, lets think about this logically.

Have I got spark? Yes. Tried swapping 1 and 4 HT leads, no difference. New plugs all round, no difference.

Ignition timing correct? Well, it is on 1, 2 and 3 so it has to be on 4. (?)

Is #4 plug wet? No. looks dry to me. Ok, strip the carbs down again, repeat all the cleaning, still no firing on #4 cylinder.

Has #4 float bowl got fuel in it? yes. Removed drain plug, fuel flows freely.

Ok, what happens if I remove the carb balancing screw from the inlet manifold, and spray easy start in the manifold? Wait, its BLOWING out of that hole. That can't be right. So ... what could that be?

Maybe the intake valve is not closing? Or exhaust valve not opening? Has the exhaust shim fallen out of its bucket somehow? Or the retaining clips fallen off the inlet valve? (You can see I'm getting desperate now!)

So off comes the tappet cover. Everything seems to be ok. Valves go up and down as advertised. Valve timing is still correct.

So, compression. Don't have compression gauge, so may have to hire one. But if I run with #4 spark plug out it certainly blows hard through the spark plug hole.

Hey, maybe the exhaust is blocked and somehow I didn't notice! No. Took it off, de-coked it (again) re-installed. No difference. But holding my hand over the spark plug hole there doesn't seem to be any fuel vapour there.

Ok strip the carbs for the third time. (At least I'm getting better at getting the damn things off and on). Still no difference.

Give up in despair and ask for help.

This brings us to today.

Any suggestions will be gratefully received!

Cheers,

Ian

[roeme]

Impressive progress and work!

Where you stand currently, I'd buy a compression gauge off the interwebs. It doesn't have to be a expensive one; cheap ones go for around 25-40 bucks. Even though these can be off - mostly when using an adapter - by around ~0.5-1 bar (~10-20psi), thats more than sufficient to determine wether your cylinder is feeling well
(unless renting is cheaper, then rent...).

But before that; have you tried dropping a teaspoon of gas into the cylinder prior to starting?

[Brendan W]

Could it be that the throttle plate is completely shut and a tweak on the balance screw might get air flowing ?
I second the impressive work comment. Great looking bike - a phoenix.

[IanW]

Don't think it's the throttle plate, I did a static balance (can't do a proper carb balance until the bike's running!) but mostly it's because I have had the throttle in all positions while doing the tests. However I'm not ruling anything out!
On the bright side, I have managed to source a pressure gauge at a good price, so I'll be picking that up next. Lets see what I can learn from the test. Doubt I'll be able to get to it for a couple of days though.

Thanks for the input and your comments,

Ian

[IanC]

Nice looking scoot. I didn't even know there was a shafty Tuna.

[jdvorchak]

Just looking those carbs seem to be BS34 Mikuni. The enrichener circuit starts in the float bowl. Pull the bowl off and you'll see a hole near the front of the carb on the bottom of the float bowl. That is a V shaped milling that goes into the hole in the top of the float bowl through the gasket. A pipe from the carb body fits down that hole. That hole has a very small jet in it. I can get a #10 guitar string down through the hole. Squire carb/brake cleaner into both holes, the one in the bottom of the float bowl and the one that goes through the gasket. You should see a good stream squirting both directions.

Now onto the idle jet. Remove the rubber plug and wrap electrical tape around the straw on your carb/brake cleaner so it's a tight fit. Squirt into that hole. You should see it exit above the throttle butterfly in a good stream. some will also come out around the needle main jet.

My thinking is that if you have compression and spark the only thing missing is fuel. when you had that carb apart you did clean all 10 pairs of emulsion tube holes? They are on the side of the brass thing that holds the main jet. They are how the idle jet gets its fuel.

Good luck!

[IanW]

Yes John. I agree with everything you have said (except they are BS32 Mikunis).
And I have done everything you suggested.
And then doubted myself, removed the carbs, and checked again.
3 times!!!!

Now either it's something in the carbs that is so obvious that I can't see it, or it's something else entirely.

(Even checked if I had left the masking tape on the inlet to #4 cylinder after painting!)

By the way, the jetting is slightly lean, with 102 main jets (instead of 110) and 45 pilot jets (as per standard). However, I'm at an altitude of 1800 metres so that's probably about right. I think.

[roeme]

Have you achieved at least some initial firing through the method I suggested?

[IanW]

Hi Roeme,

Not yet, I unfortunately haven't had chance to get to it - I coach the local kids' soccer team and also play in a band during the evenings, so time is a precious commodity! But a teaspoon of fuel is definitely going to be tried next. On the other hand, I did manage to get a compression tester, and have called in a few favours to get an adaptor turned up at work to fit my spark plug size. I'll let you know how the raw fuel in the cylinder turns out.

[jdvorchak]

Just a sanity check, because this happened to me once with a GS850. Spark plug wires in correct position? Left coil should be wired to 1 and 4, right side coil to 2 and 3.

[Wallowgreen]

Take this as intended, a data point and possible option, but here's a post on my experiences from years ago...

http://www.thegsresources.com/_forum/showthread.php?218056-1-pipe-cold&p=1997369#post1997369

I wrote up a post with more details and pictures I thought but can't find that now.

I mentioned in the longer post I changed all the internals from each body into the replacement ones. The 850 was running sweet when I sold it, still have the 550 and that's been running great ever since.

Good luck!

[roeme]

Hi Roeme,

Not yet, I unfortunately haven't had chance to get to it - I coach the local kids' soccer team and also play in a band during the evenings, so time is a precious commodity! But a teaspoon of fuel is definitely going to be tried next. On the other hand, I did manage to get a compression tester, and have called in a few favours to get an adaptor turned up at work to fit my spark plug size. I'll let you know how the raw fuel in the cylinder turns out.

No worries mate, I just wasn't sure you saw what I wrote, that's why I reiterated. Best of luck!

Compression Tester Adaptor! Be aware that an adaptor changes the volume to be compressed, throwing off the reading. The extent depends on how it's made and cylinder volume.

Also, Wallowgreen's link brought up another question; did you put the carbs in an ultrasonic bath at some point? I don't know your carb model, but mine had to go through both ultrasonic and manual cleansing to free up all crevices.

[jdvorchak]

With all due respect to Roeme. Don't worry about the adapter throwing off the numbers. The adapters I've seen could contain no more than a couple of cc of volume which would be insignificant. What you're really looking for is all within about 10 to 15 PSI of each other regardless of that actual number as long as it is above 100 psi for a bike that has been sitting for a few years. Typically I'll see 110 or better on old bikes that have been sitting for years. Any reading below 90 would probably not support combustion. Unless your bike all read 90, then I'd doubt the gauge or your technique.

Remember these steps for a good compression test:
1. All plugs removed and plugged back into the wires and grounded.
2. Battery fully charged.
3. Choke off and throttle wide open for the duration of the test. <=== IMPORTANT FOR ACCURACY
4. Spin the engine and continue spinning until the number on the compression gauge stops rising. May take 6 or 8 or more "thumps".

[IanW]

Well guys, thanks very much for all the advice, it's much appreciated.

And now, I have just taught myself a lesson in humility. I connected up the compression tester and got 3 cylinders reading all the same, and cylinder number 4 reading absolutely nothing. Nada.
Combined with the "blowing" through the inlet manifold it sounds to me very much like an inlet valve stuck open.

The lesson of course is that I could have solved this two months ago, except I "knew" that the valves were ok because everything was checked and measured and tested while I was assembling the engine, including a static leak test on the head.
I'm sure I would have done a compression test if I had a tester available, but I didn't think I needed to buy one to test something which I already "knew" was ok.

Still, you live and learn. So, as the Queen of Hearts said to Alice, "Off with her head!"

Thanks once again everyone, and I'll check back in a couple of weeks.

[2BRacing]

Sorry to read about this problem. Before you remove the head, first remove the valve cover and inspect #4 inlet valve bucket and clearance again. In your original post you mentioned that the valves passed the kerosene leak test when you had the head off, so it means that the valve was able to close properly. Turn the engine so the #4 intake cam lobe is pointing straight up. Put your finger in the notch of the bucket, and see if the bucket can turn freely. If it does not turn freely, the valve clearance is zero or the bucket is sticking in the head.

Either correct the valve clearance with a thinner shim, or try and free up the bucket with penetrating oil. A 50%/50% mixture of acetone and automatic transmission fluid is a very good penetrant. If the bucket is sticking, apply the mixture as often as you can for a few days - do not rush things!

I really hope you can solve your problem without having to remove the head, as this will result in having to replace the head gasket and maybe the base gasket as well (not sure if disturbance of a new base gasket on a newly-overhauled engine will also result in leaks, as happens with engines that have been run - maybe someone more knowledgeable can comment on this?).

Good luck!