Author Topic: Carb air leak - struggling to identify exactly where  (Read 299 times)

Offline kryptek49

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Carb air leak - struggling to identify exactly where
« on: June 10, 2019, 07:25:32 AM »
My brothers bike suddenly died while out on a ride, as soon as you twist the throttle it bogs down & dies.  The bike idles fine and choke works correctly.

We've cleaned the carbs fully, but there appears to be an air leak around the intake boots on the engine where the carbs sit.

I've also got a GS500 so we've swapped some parts over and have worked out the following:

The rubber intake manifold boots are not the issue.
His carbs on my bike, and the problem is still there.
My carbs on his bike run fine

This seems to show that his carbs are definitely the problem, but doesn't help explain the air leak.

I'm happy with all of the carb internals as we've replaced everything, so what else could be the cause? I'm really struggling here and will have to give up and take it to a garage if we can't sort it today unfortunately.

We have also swapped my Airbox & Petcock over and verified these are not the problem.

To test the air leak, we've sprayed brake cleaner around the engine intake boots, and the revs jump up really high, indicating there is a definite leak.  My question is how is there a leak? If the manifold boots are not the issue, as mine appear to be the same? The clamps are secure, we even tried jubilee clamps and tightened them all the way but the leak was still there!

Where else could the carbs be sucking in air from??

Any help would be greatly appreciated, we are really stuck with this one.

Thank you

Offline mr72

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Re: Carb air leak - struggling to identify exactly where
« Reply #1 on: June 10, 2019, 12:08:23 PM »
The leak at the boots on both bikes is probably the intake boot o rings. Did you replace those? Almost definitely bad if they haven't been replaced.

When you tore down the problematic carbs did you replace every single  thing?

Also you are assuming an air leak but there is no indication that this is the only possible cause. I'd day stuck float needle is just as likely or the jet/needle/slide/diaphragm assembly has an issue from being assembled wrong, missing top o ring, leaking vacuum port, or a pinhole or tear in the diaphragm. float or float needle seat o ring missing or leaking could do this. Bad pilot needle o ring is a very likely candidate, done this exact thing to me before. But really it could be a dozen things, all of which have the same remedy which is to tear the carbs back down and recheck everything until you find the problem.

Offline kryptek49

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Re: Carb air leak - struggling to identify exactly where
« Reply #2 on: June 10, 2019, 01:18:50 PM »
Thanks for the advice mr72

We took the intake boots from my bike and put them on his - the leak was still there on his bike, and it's not there on mine, so I assume the boots & o rings are fine.

We replaced every single part on the carbs - including diaphragms, idle mixture screws, jets, floats etc. which I temporarily loaned from my bike's carbs.

We have also recently replaced all the o-rings and gaskets + needle valve with a repair kit.

May I ask about the missing top o-ring? Is this the tiny o-ring that is next to the diaphragms at the top? If so then yes this in place on bot carbs (although missing on my bike which runs fine?)

I'm also happy that the carbs are assembled correctly. We've torn my working ones apart so many times now we're pracitically experts! And my bike always starts first time meaning we must know what we are doing.

We have pulled the carbs apart at least 5 times now, and cleaned them as thoroughly as possible. We are happy that all the jets are clean, the float is set and operating correctly, that the diaphragm is in good condition, and that all o-rings and gaskets are brand new.  Nothing we have done to this point has had any effect on the original issue.

I understand that the obvious answer is just to continue pulling apart the carbs until we find an issue, but if we have already done this, replaced all parts with ones in known good condition, and installed everything correctly, then surely something else must be the answer?

Offline mr72

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Re: Carb air leak - struggling to identify exactly where
« Reply #3 on: June 10, 2019, 01:49:44 PM »
Thanks for the advice mr72

We took the intake boots from my bike and put them on his - the leak was still there on his bike, and it's not there on mine, so I assume the boots & o rings are fine.

no, both sets are probably bad.

You have to actually remove the o-rings and inspect them. Are they round-profile, soft rubber? or are they flat/square and hard?

Quote
We replaced every single part on the carbs - including diaphragms, idle mixture screws, jets, floats etc. which I temporarily loaned from my bike's carbs.

there's an o-ring on the pilot needle ("idle mixture screw") that you have to remove with a pick. It doesn't come out. It is probably hard and flat and will not seal. This allows fuel, which is supposed to go into the jet, to leak. It also allows air, which is not supposed to go into the jet, to leak in. That part will stay in the carb unless you specifically remove it, and believe me you'd remember if you did. They are not easy to replace.

Quote
We have also recently replaced all the o-rings and gaskets + needle valve with a repair kit.

Check the pilot needle o-rings. Most folks leave those out. Most kits don't include them.

Quote
May I ask about the missing top o-ring? Is this the tiny o-ring that is next to the diaphragms at the top? If so then yes this in place on bot carbs (although missing on my bike which runs fine?)

If that o-ring is missing or leaking then you will have a massive vacuum leak. Of course you can make the bike "run fine" with a massive vacuum leak. But it will run much better without a vacuum leak. There should also be caps on the tops of those vacuum ports which are important to not leak only if the o-rings are in place. Otherwise there's no point. You won't generate enough vacuum to lift the slides without these o-rings and vac ports sealed.

Quote
I'm also happy that the carbs are assembled correctly. We've torn my working ones apart so many times now we're pracitically experts! And my bike always starts first time meaning we must know what we are doing.

You sound like the expert then. Why are you asking here?

Quote
I understand that the obvious answer is just to continue pulling apart the carbs until we find an issue, but if we have already done this, replaced all parts with ones in known good condition, and installed everything correctly, then surely something else must be the answer?

Barring something truly extraordinary like a crack in the body of the carb or one of those little brass tubes being totally missing and you didn't notice or someone drilled out a cast-in covered port or whatever, then you have just overlooked the same thing each time you tore down and reassembled the carbs. I speak from experience, I did exactly the same thing, even to the point of replacing the entire carbs.

Truly though there are numerous things that could cause this, and if you haven't noticed I think now it is down to the pilot needle o-rings but who knows. There is likely something you think is irrelevant that you are overlooking or haven't mentioned, like oh he has pod filters and blah blah or we put in a fuel filter and blah blah... Or there is something that you think is a coincidence which is actually causal or you are making a false assumption about the cause, which are extremely easy to do and maybe impossible to avoid which is why I advise against trying to diagnose these things and instead just be absolutely 100% sure every single thing is done exactly right with the carbs.

Replace the intake boot o-rings, sounds like you didn't do that and I promise whether you know it or not, this will cause problems. And replace those pilot needle o-rings, being 100% sure you assembled them correctly (order top to bottom: o-ring, washer, spring, needle).

The fact that you have missing "little o-rings" under the carb tops tells me there's a lot more potentially wrong than even you realize. With a vacuum leak that big you can't diagnose anything properly. It will completely undermine every bit of work you do. Believe me, I learned all this the hard way and I'm trying to save you the year of trouble that I went through. That's why I wrote all this up on my blog, going to be hard to follow this procedure exactly and still have carb issues:
https://joshkarnes.blogspot.com/2017/09/fixing-common-gs500-carburetor-issues.html


« Last Edit: June 10, 2019, 01:51:14 PM by mr72 »

Offline kryptek49

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Re: Carb air leak - struggling to identify exactly where
« Reply #4 on: June 10, 2019, 02:07:28 PM »
1 - The intake boot O-Rings were definitely flat/square.  I will replace them, thank you.  I had assumed if it works on mine it must be good, but I understand what you mean now.

2 - When I removed the pilot needle (the one set to 3 turns right?) and there was an o-ring on the end of it.  IIRC it was set up as Screw->Spring->Metal washer->o-ring.  the o-ring came out very easily though, so this doesn't sound right does it? Is there another o-ring seated further inside the hole?

3 - The kit included an -ring for the pilot needle, but based on point 2 i'm not sure if it's the right one, or if it's installed correctly.

4 - I'll pick up some o-rings for my own bike to correct this then!  I did think it was odd they were missing.

5 - Sorry for sounding like I think I know it all! I was just trying to joke about the amount of time I've wasted on this project when it really shouldn't be this difficult.  I really do appreciate the time you've put into your replies, so thank you.

6 - I'm sure there is not a crack in the carbs or something ridiculous like that fortunately.  And I can understand how I have likely looked at the carbs so many times that if something were wrong I would likely think it was normal.  I've done my best to be comprehensive with the process I have been through to this point, but didn't mention the bike is stock - so yes it is, standard air box, air filter, fuel lines all normal etc.

Our next step will be to replace the intake boot o-rings, and hopefully you can give some advice on no.2 as to why the o-rings were so easy to remove?

The o-rings missing on the top were on my own bike, not my brothers which has the issues, I only mentioned this as I found it odd when I noticed it.

I'll have another read through your blog, i'm sure you're right it's just something simple I've missed.

And again, thank you for your help, it is greatly appreciated!

Offline mr72

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Re: Carb air leak - struggling to identify exactly where
« Reply #5 on: June 10, 2019, 02:45:57 PM »
if the pilot needle o-ring came out with the needle then that's not a problem, just I have found on both of my sets of carbs after numerous times swapping them, they required me to pick them out with a bent piece of wire. If you replaced them then you are in good shape.

Could be the intake boot o-rings and nothing more. And likely your carbs are set up extremely rich, which is why it "runs fine" even with leaking intake boot o-rings and missing top-cap o-rings, and maybe why those carbs work on your brother's bike.

Offline kryptek49

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Re: Carb air leak - struggling to identify exactly where
« Reply #6 on: June 11, 2019, 08:06:23 AM »
Just done a little bit of looking into why my o-rings next to the diaphragm are missing.

Does the GS500F even have these o-rings? The space for them is there in my carbs, but looking at parts diagrams it's not shown?

Here's a link for my brothers 02 GS500E: https://www.bikebandit.com/oem-parts/2002-suzuki-gs500/o/m22461#sch246381 - see part no. 42

And here's the link for my GS500F: https://www.bikebandit.com/oem-parts/2008-suzuki-gs500f/o/m17395#sch554389 - where you can see no o-ring is shown at the same place as no. 42?

Anyway I've ordered new intake boot o-rings! Thanks :)


Offline mr72

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Re: Carb air leak - struggling to identify exactly where
« Reply #7 on: June 11, 2019, 11:22:11 AM »
As you probably noticed, the 07 carbs are completely different. IDK whether they are supposed to have that o ring no must have missed where you mentioned the year models of these bikes. Everything I know applies to the early 2-circuit carbs.

Offline kryptek49

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Re: Carb air leak - struggling to identify exactly where
« Reply #8 on: July 02, 2019, 01:41:44 PM »
UPDATE: We gave up and finally took the bike to the garage, and the reason for the bike not running was actually completely separate to the air leak

His tank overflow wasn't working properly, and his tank was full of water - around 50/50 fuel/water actually which is ridiculous.

All sorted now and revs perfectly.  Also makes sense why my carbs work as the float bowls would have been full of decent fuel from my bike.


Offline herennow

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Re: Carb air leak - struggling to identify exactly where
« Reply #9 on: July 03, 2019, 10:14:42 AM »
ahh, the joys of fresh fuel.

Im not sure of the problem though,  the lid should seal the tank against water ingrreess. The drain/Tank overflow is primarily to drain spilt gas away, even if not working, water should not get in.

Might just be the gas had ethanol in it and after a loong time it absorbed a lot of water which sememd to be 50% (although that seems very high).

Id check eveything thorougly. Cap sealing, How much rust in the tank? etc.

Offline mr72

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Re: Carb air leak - struggling to identify exactly where
« Reply #10 on: July 03, 2019, 02:29:33 PM »
Glad you found the issue! I always forget to consider bad gas as the cause. It's far and away the most frequent cause of non-running issues with carbureted engines.

You should still fix your vacuum leaks though!  :thumb: