Gas in airbox

[don]

Hello all,

Need some words of wisdome from the all knowing forum guys. As the title suggests, I'm getting gas in my airbox on my 83 gs 550e,(4cyl-2carb). The only way I know of that this can happen is if an intake valve is open when the piston comes back up.

Please elaborate on tha vallidity of my thaughts.


Thanks in advance.

 

[tkent02]

Check the petcock, check the float valves. Very common problem on 1980 or later CV carb equipped GSes, the EPA mandated the overflow tubes go away, so now any overflow due to petcock or float valve problems has no place to go except into the engine or airbox, or both... Easy fix.

 

[don]

Stripped carbs COMPLETELY, dipped for like 400,000,000 hours in "Berry juice", all new o-rings seals and gaskets, new needle & seats, floats adjusted to within .003" of mid spec, new pilot jets, new intake o-rings, new intake boots, new intake and boot clamps, new throtle shaft seals, sealed up all of the BB's they use to seal up the drill holes.

The old girl runs sweet untill the raw gas in the airbox starts spashing into the carbs. While you're under way it's not too bad, gets just a touch soft, but when you come to a stop you can tell it's getting "choked out" and idles poorly.

I have run one tankfull of Yamaha Ring Free through it. Sometimes it runs PERFECT and sometimes it runs lik I have described. Hence, my thaught of carbon on an intake valve.... or something like that.

 

[don]

Oh, forgot to mention, the overflow/vent tubes on this engine just go to atmosphere.

 

[Don-lo]

Unfortunately, all it takes is one speck of dirt on a needle seat to cause an overflow. Can you push the carb-to-airbox boot up enough to see the carb throats? If you can, then turn the petcock to prime and see which one overflowing. If both carbs are overflowing, then float height is suspect. Someone also lately posted that his floats weren't floating. Can you pull the bowls without removing the carbs? I once found a speck of dirt on a needle tip causing a leak, but it was almost invisible on the rubber tip (3rd time checking).

 

[don]

OK, took the tank off and ran gas line from a bottle straight to the carb...... no gas overflow, floats ar good. I have removed the diafram from my petcock and plugged the vacuum port on the carb and the petcock. I remember seeing gas in that vaccum line. In the shed seems to run OK and no gas in airbox boots, (raining out today. Will not ride right now. Too old. lol). Planning to give it a whirl tomorow and see if a load on the engine makes a difference. I'm really glad I asked you guys about this, I was ready to take the head off and do a valve job on her. Any other thoughts with this new info are more than welcome.

 

[Don-lo]

Hmmmm, interesting symptoms. Gas in the vacuum line would not feed back into the airbox because it is too far downstream. You didn't need to take out the diaphram, you could have just pinched the the vacuum line or plugged the vacuum port. Backfiring or hesitation can load some gas into the airbox. How does it run now? Give it a ride tomorrow and see if there is a difference.

 

[don]

Ready for this....? Gutted the petcock as described before, took it for a 40 mile ride, and poof, runs like her old self without gas in the airbox. Obviously I have to go back and retune the carbs now that the fuel is actually being metered in all cylinders.

And here is the answer to the million dollar question.

Q: How would gas come out of the carb through the petcock vacuum line?
A: The four screws that hold the back of the petcock were dead loose. When the motor pulled a vacuum on the diafram, it would pull it away from the "gas side" of the assembly and gas would flow straight into the engine just like a stuck float.

Going to put the petcock back together, with the diafram, and see if I still have a problem. I suspect the loose screws were the issue all along.

Once again, you GS guru guys saved the day! Thanks!!!!

 

[Guest]

You guys are gooood!

Have you been watching NCIS or DrHouse on TV?

 

[don]

Thanks for the vote of confidence McLoud. I'm an old stock car guy. These cv carbs do the same thing as a Holly but they are a completely different animal. They make my brain twitch. With the help of these guys who know these things inside and out, I'm learning.

Thanks again.

 

[don]

Update. Reinstalled the diafram and tightened the petcock screws. As before, runs good in the shed. Raining out again. Planning on giving it a whirl either this afternoon or tomorow.

 

[don]

The update

The update

OK, here's the latest update.

Took it out for a ride and still ended up with gas in the airbox.

I am 100% sure the floats/needles are fine. I made a 1 quart bottle with just a gas line at one end and a vent at the other. I hung the bottle over the bike in my shed from the ceiling.( About 5 feet above the carbs for about 45 minuits.) No gas overflow. The floats and needles are fine. I use this bottle to tune the carbs with the tank off.

I took the carbs off and completely apart again. I gave them to my buddie to do a once over on them. (Number one, he works at a Suzuki dealership, number two, remember when Sterling Marlin was kicking everyone's but back in the 80's with the Kodak #4? This guy was doing the carb for that car! So I trust him.) He put them through the ultrasonic cleaner 6 or 7 times. We reassembled,synched, and tuned them. Well, what do you know............ still gas getting into the airbox. And as before, runs nice until the gas starts building up in there.

Back to my valve theory.

I took the motor out of the bike and gave it to my buddie to take the head off and see what he can see. When I took the carbs off to do this I noticed that there was oil in number one carb. The plug for that cylinder was also oil fouled. Cool, there's the answer!!! .....................Well, To my surprize, when he took the head off, nothing real exiting. The only thing he could find was all of the intake valve springs were about 10% under service limit spec.

All of the seat patterns were free of carbon and nice and shiney. He filled the combustion chaimber with gas and checked for leaks, he filled the intake ports with gas and checked for leaks, and he filled the exhaust ports with gas and checked for leaks. NO F-BOMB LEAKS! The valve seals were even sill soft. He checked the cam chain tensioner setup and checked the cam chain for stretch. The cylinder walls don't have one blemish,scratch, nick, bug tird, nothing on them. All looks good.

We're just going to put new valve springs and seals along with all of the gaskets etc on it, put it back together and give it another whirl.

I'm at a loss here fellas................................

 

[Larry D]

I'd say it's still the petcock. There are mixed results with rebuilding them. I would recommend getting an entire new one.

 

[don]

Sory Larry-d, Didn't post it, but the reason I ended up doing everything in my latest post was because a second test ride with the petcock gutted gave me the same results as all others. Still gas in the airbox.

 

[Guest]

If you'r getting weak spark or none at all the fuel fill flow to the air-box.

 

[Colorado85gs550]

there was some information that this bike of ours being a "buzzy" little motor causes the fuel to froth up at sustained high rpm........I think that given the right outside air conditions/altitude, etc you could be experiencing some of this.....just a thought!

 

[Colorado85gs550]

DATE: September 27, 1998
QUESTION: Performance info needed - 1983 GS550
I have a 83/84 GS 550 es that I am racing in AMA/CCS sportsman competition here in Fla. Do you know of any place that I can get tech tips for engine updates, help on those miserable dualthroat carbs, cam degreeing numbers, performance parts availability, etc.? I can find listings for headers, jetkits, and such, but I would like to know what was done to these bikes (the 550tscc)back when they were new. Do you know if Yoshimura, or Yoshima, ever fiddled with them? Any help that you can give me would be GREATLY appreciated!
I have the chassis working pretty well at present, with Katana forks, brakes, and wheels, and GSXR shock/w modified linkage. I would like a little more guts in the engine though. At present the carbs are set up with 118 mains, 37.5 pilots @2-3/4 turns out,and sc15 needles. I'm running a yoshi header, a k&n filter, and undercut trans. I have the ignition advanced 4deg.On the top end it's good for about 125mph as geared. But I would like more grunt in the mid-range. I realize this is all pretty confusing, but if you have any insights, I would sure be interested in hearing them.
RIDER: David Starling
REPLY: Zack Schultz
I have an 84 GS550 myself. I'll start with your last comment first. These things were supposedly good for 125 new. I saw this myself (indicated) several times. And I'm glad someone is campaigning one. Tuning the carbs. Think of them as siamesed singles and the problems minimize. Everything is separate except the float bowls. Hence, the bowls are undersized. Make sure that there is enough fuel flowing to feed the motor.
Kevin Cameron once wrote that some buzzy engines (and this qualifies0 have trouble with fuel frothing at sustained high R's (like Daytona). Foam doesn't lend itself to accurate jetting. Adding weight to the carb dampens it's vibration and frothing is eliminated. I think he said something about a pound of welding rod wrapped around the float bowl. I did speak to someone a few years back in the Galveston area that was racing one of these. I don't keep up on the class rules, but he was using 600 Hurricane pistons (63 mm) in the engine. These are the same size that Wiseco was using in their big bore kits, so there's metal to go that far over (+3mm). Good luck and pass along results.

 

[don]

It will do this even if I just take it out around the block. 40mph or so.

 

[Colorado85gs550]

I definately think something didn't get put back on properly inside atleast one of the carbs........have you removed the air box and run it static to see if you can see fuel sputtering out of the carbs in a mist or if it is simply running out of one?

I think your next step would be removing the airbox and running it at rpm in a static state to see what you can see coming out of the carbs and where......

 

[Dave8338]

If the petcock (once back together) still flows fuel down the vacuum line...I'de suggest a complete new one. I put a rebuild kit in mine and after futzing with it for several hours, I used it to test my throwing arm!

If need be...buy a new one and save yourself a TON of PITA.