Desperate for a solution, GS750 not running

[Guest]

So here's my story, I need some help from the GS gurus here.

My bike: 1978 GS750 EC model 17k miles or so on the engine, with Mikuni VM26SS carbs - bought the thing in summer of 2012, ran great for about a year, until August of 2013. Now the weather's getting warm and I want to ride again!!!

Problem: the darn thing just wont run right! I can get it to start, but it wont really idle, or run at all without constant manipulation of the choke. Putting it into gear and trying to move it results in an instant stall, seems to be starving for fuel. Additionally, the bike will normally stall when you roll on the throttle in neutral, revs up to maybe 5k rpms, but then seems to starve out for fuel. I took it to a bike shop before I dismantled it, and they diagnosed it for me as definitely a carburetion, or fuel delivery problem. The bike also drips oil, I attached a photo of the spot I believe it leaks from - I saw it drip while running, just under the circular cap that attaches to the valve cover on the very top of the engine (removed in photo). There appears to be a rubber seal there that I would suspect is the source of the leak.

What I have done: Replaced the valve/head cover gasket, as well as the oil breather gasket, but not the head gasket. I completely tore down and rebuilt the carburetors. This involved dunking each one in the Gunk paint can of carb cleaner, 3 or 4 cans of carb spray, accompanied with a new set of seals from Robert Barr @ cycleorings.com. Additionally, I purchased the intake boot o rings and installed those new. pretty much followed the VM carb rebuild manual from this website to the letter.

I tried many different settings of both the air and fuel mixture screws. For the air mix screw, I mostly had it at one turn out from seated, but the bike seemed to respond better when it was turned even farther inwards, it would even run with the air screws completely seated, turned all the way in . That further made me suspect the bike is running extremely lean and has some sort of air leak problem, not drawing enough vacuum to suck the fuel. The fuel screw on the bottom of the carb, I have tried every interval in 1/2 turn increments from 1 turn out, to 4.5 turns out. Nothing seems to work.

Here are some of my questions - There are a few things I suspect could be the problem, but I'm really at a loss as to what I should do

1) the oil leak - people I have talked to say that if the problem is mechanical, IE a head gasket leak or valve problem, I wont be getting the proper vacuum from the cylinders - that being said, the oil leak is above the head gasket, and on top of that, I did a compression test and it seems to be fine. The manual says that your cylinders for this bike should run at minimum of 100psi. My test yielded these results:

cyl 1 - about 133psi, cyl 2 - 126psi, cyl 3 - 135, cyl 4 - 134psi

This would lead me to believe the head gasket is fine, but would that oil leak up by the valve cover still cause me to lose vacuum or something? the rate of this leak is about one drop every 3-5min or so while running

2) When reassembling the carburetors, I noticed that the joints between the carburetors did not all seal very well. In the "literature" online, I have seen these referred to as carburetor vents, joints, connectors, and nipples. They are essentially the fuel tubes that connect the carbs, allowing gas to flow between them. The one in the center, between carbs 2 and 3 is a T joint, and is where you connect the main fuel line from the gas tank.
The parts I have are old school, they have raised rubber ridges and not replaceable O rings, one of them was clearly replaced before I owned this bike, because it looks newer, and makes a snug fit between carbs 1 and 2. The central T joint one between carbs 2 and 3, as well as the last one between 3 and 4 have rather worn down rubber ridges, and dont seem to fit as snug

so I'm wondering if that could be the source of my leak? I have seen some posts about using gasket maker compound to build up these ridges, does anyone have any experience with that? I'd just buy some new ones but theyre a discontinued part, and none are currently for sale on ebay. Anybody know some newer compatible parts?

3) Air leaks on the Airbox side of the carburetors - from what I read on this site, people seem to agree that unless you have a well sealed airbox (for a bike with stock jets and pipes, which I believe mine has) you will not be getting the proper air mix. Other folks I have spoken to tell me that having absolutely perfect seals on the airbox end will not make or break the performance of the bike, and I dont know what to think.

Now the boots between my carbs and my airbox are certainly not new. It looks like a few of them have been replaced, but some of them are old, hard rubber that I know for a fact arent making a great seal. As far as the airbox itself goes, on the inside of the covers, it still has the original rubber strip probably from 1978, which I assume is probably not airtight anymore. all in all I can probably assume the airbox end of the carb system is not 100% airtight. All that being said, before my bike starting running poorly, I was using this airbox with the same carb boots that probably werent 100% sealed, and the bike ran totally fine.

after doing all this work on the thing, I have absolutely no idea where to go, and I figured I'd post my story and hope for some suggestions before I bring the old girl to a shop and drop the big bucks.

I need some expert advice here, so thanks in advance to any responses I receive.

[twip52]

additional photo

here's the carb connectors / vents Im talking about in 2) of the OP

[chuck hahn]

Oil leaks most likely the cam end covers ( black half moon things ). Usually you need to wipe the head with alcohol so its oil free and apply a thin coat of RTV in the U shaped groove and on top where the gasket crosses it.

Carb intake manifolds..have you replaced the orings?

Carb dip..how long were they in there? 24 hrs per body is the acceptable norm to, be sure all is cleared out in there. 2 baodies will fit in the bucket if you remove the plastic basket.

Have you poked ALL the holes in the emulsion tube and the pilots..to include the tiny one on the very tip end?

[chuck hahn]

The TEE goes in between the 2 and 3 carb and the transfer tubes are obviously between 1 and 2 and 3 and 4. Also have you verified the floats hts are correct?

[greg78gs750]

those rubber half moons can be replaced with a valve check, plenty of info on those out there.

i 'don't think" it is likely you are getting an air leak from the fuel t, i am pretty sure that part is now available from z1 enterprises as a brass with oring replacement. A pretty good fix has been accomplished by some (including me) by giving those rubber-coated tubes a couplea wraps with teflon tape, then reinstalling. Worked great on my old 750, no leaks in the 4 or so more years i owned it.

those airboxes do like to be sealed up tight, from the sound of it you have found basscliff's website? good info there, to be sure. I sealed all the seams on mine with some of the 2 part epoxy that comes with the spiral mixing tip, worked great.

you replaced the intake boot o-rings, but if those 30+ year old boots are cracked it won't make a difference. Those are available, for a price. Not TOO bad, if i recall, and worth replacing.

the airbox to carb rubbers are, as i recall, a harder part to find....but i know someone here will know where to get them.

it sounds like you are being pretty thorough, i would STRONGLY ADVISE AGAINST TAKING THEM TO A SHOP. the money would be better spent on new rubber bits.

have you done a valve adjustment yet? cause all this work could be just chasing your tail without that......

welcome to the site, sounds like you have been lurking for a while, those that know more than me will be along shortly. Be patient, sounds like you have the right attitude.

greg

[chuck hahn]

Hard intake manifolds wont seal well, so try a shot of starter fluid at the point the carbs go into each manifold and see if the RPMS jump..if so theres a leak that needs fixed. Clamps also stretch and periodically need replaced...obviously they wont squeeze tight enough anymore.

Get some Wintergreen oil and soak the rubber in it for a few days and it will again be rather pliable. I got a 16 OZ bottle on Amzon.com pretty cheap. But be sure its the COMMERCIAL grade..same effectiveness but way cheaper.

[twip52]

What has /hasnt been done

I did not check the float heights, or do a valve timing job. I guess these will be next on my to do list. I've never done the valve timing on this bike, I did it on my old Honda I had, an 80' cm200t, but from what I've read its not that difficult for this engine either. I just didnt do a valve timing adjustment cause when I took it into the shop, I asked the mechanics if they thought it needed one and they told me they didnt think so.

I'll also replace those rubber half moons that seal the valve ends ASAP to see if that solves my leak.

My carb intake boots on the carb to engine side seemed to be in good shape. dont know if they were original or not, but the rubber was still soft and pliable, with no visible cracks or deformities. I considered replacing these, but the only ones Ive found on the net were a set of OEM suzuki parts on ebay for like 175$ for a set of 4, and for a bike I paid a little over 1k for this seemed like a lot. The carb to airbox boots on the other hand I saw on ebay were only 75$ shipped for new OEM spec, and since those were in noticably worse shape than the engine side boots, I was considering buying a set.

In response to Chucks post, could you elaborate a bit more on what you mean by poking the holes in the emulsion tubes (not familiar with what emulsion tube means) and pilots? I took out every pilot screw and jet that was removable, shot carb spray followed by compressed air, and looked through every hole I could see to make sure they were clear. Would this require something other than toothpicks? like one of those carb cleaning rod/ tool kits I see on ebay?
I dipped the carbs in the Gunk bucket overnight, I did them one at a time, and the amount of time they probably spent soaking varied a bit, but probably no less than 18 hours each.
- another thing I left out of my original post, is that the fuel mix / pilot fuel screws had their tips broken off. I had new ones in the rebuild kit, but when I tried to install the first one I broke the tip off like an idiot. I figured I'd just use the old ones, which ALL had their tips off, and prev owner had filed them down and just adjusted them accordingly, and like I said ran absolutely fine with that hardware.

Greg, regarding how you used epoxy on the airbox. Did you glue the seals of the boots? the seals around the covers? I saw that the prev owner did try and seal the covers with some type of silicone or caulk type material.

Thanks again

[mrbridgeport]

With the engine running you can spray a little starting fluid at each carb manifold boot.if the bike surges when you spray it you will know that you do not have a good seal. You can do the same thing to the airbox boots to check them.. Also check the float heights on those carbs. Adjust valves and then syncs the carbs with a carbsynce tool. Bench sync is not good enough.

[eil]

That would be a valve clearance adjustment, not a valve timing adjustment.

Have you seen the mega-welcome? http://www.thegsresources.com/_forum...d.php?t=196338

Issues like this are solved through the process of elimination. So, from your description of things, the next steps are:

1) valve clearance check/adjustment (although this probably won't fix these issues it still has to be done unless you enjoy burnt valves)

2) Get new airbox boots.

3) Seal up the airbox (how is your air filter, by the way?)

4) You said your intake boots are good. I'll take your word for that. But have you checked the o-rings underneath? They may not be making a good seal.

Personally, I would get new pilot mixture screws as well. The old ones may technically work but you'll have an easier time adjusting the mixture with ones that have good tips on them. (Especially since breaking the tips tends to enlarge the hole, making adjustment even more difficult.)

[chuck hahn]

OK....take out the pilot screws on the bottom and shine a flashlight in them to see if theres any hint of brass tips stuck in the holes..if so, use the search feature and look for broken tip removal tutorials.

Emulsion tube is the one that the main needle sl;ides in and out of..it comes out from the bowl side and has a series of holes in it that must be poked clean and so do the pilot jets. They are the little ones to the side of the big ones in the center. You need to find a post by NESSISM and click on the links at the bottom of his signature and do the carb rebuild tutorial to the letter.

[chuck hahn]

Go here and choose the 8 valve service manual. And do the carb rebuild tutorial as mentioned...they arent clean right. get the VM carb kit from cycleorings.com and also tell Robert you want the intake manifold orings as well.

Do the valve adjustment, get the carbs rebuilt and bench synced, then vacuum gauge sync them when the bike is back and running. Check the timing also. Put in a new air filter so it is all new and right and you should be fine.

And the BEST ADVICE is this..NEVER NEVER believe a damned thing any previous owner says when buying an old bike..They want it gone for a reason and arent gonne tell you why.

http://members.dslextreme.com/users/bikecliff/

[allojohn]

OK....take out the pilot screws on the bottom and shine a flashlight in them to see if theres any hint of brass tips stuck in the holes..if so, use the search feature and look for broken tip removal tutorials.

Most likely your problem, and would explain why nothing changes when you adjust the mixture.