Fuel in Exhuast Pipes '79 GS750L

[drivadesl]

Hi gang first post. THis site has been unbelievably helpful in getting me to this point, as I was able to search and learn so much. But finally think I should seek advice. I'm resurrecting the above bike ( which I am the original owner) from 7yrs of storage and started with a total and complete carb disssasembly,36hr soaking in Berrys, O-RIng replacement, jet replacements, float adjustment,fuel line replacement, and petcock replacement. Also replaced the air filter with new, new manifold Orings and SS bolts. The carb boots were flexible so didn't need anything there. After reassembly I was able to get the bike to start and idle, even without any choke. Engine would not respond to throttle, just bog down, and finally I realized all the white smoke I was seeing was due to fuel/oil leaking out around the center of the exaust pipes. It was enough to collect into a pan, and could clearly see it was a fuel/oil mix.

AFter additional research here it appears it is one of two issues. The petcock: I bought this used, so I have to htink this is the most likely cause. IT is correct for the bike having two positions, on and Reserve. No prime, which the original did not have. Thing is,there is no fuel leaking from the fuel port when the tank is removed from the bike so not sure why it could cause the carb bowl to overflow when idling. Also, there is no fuel leaking out the overflows at all, which I confirmed are clean.

Second possibility are the carb floats. I checked the heights when I first did the rebuild, but pulled the carbs again, and checked. Spacings are listed below ( this is with gasket installed)

1: 22.15 mm (thed spec is 23+/- 1mm.
2: 22.34 mm
3: 22.67 mm
4: 23.5 mm

The 4th carb seems higher than is should be, but seems to me a higher reading would cause a lower level in the bowl. In any case, I wanted a sanity check from anyone who has a feel for the numbers.

FInally I checked the port in the carb bowl that is tied to the starter jet/choke tube to be sure it was clear and decided to resoak the bowls for two hrs and blow these out to be sure no blockage.

Next I'm going to pull the petcock, but wanted to get feedback on the bowl levels first.

Sorry for the long story, but I have the impression you guys like details to be sure us noobs are doing things correctly. THanks for your time and any comments.

 

[drivadesl]

Oh, and to add to the above, I completely derusted the fuel tank, and coated with POR, so I'm sure there is no residual gunk floating around to clogthe carbs. THe bowls were spotless when I removed these following the rebuild. THanks!

 

[hannibal]

I'm struggling with fuel running into the engine as well, so I'll be following the responses on that front. I believe my overflow vents are clear, but fuel is still flowing into the engine even though no fuel comes out the overflow hose.

As for the petcock, I once verified that my petcock was working correctly after the bike had been sitting for days. I then started it up, and shortly therefater, I decided to pull the tank. This time when I pulled the fuel hose off the petcock, it was not working correctly and allowed fuel to flow out. I concluded that the petcock was getting stuck on, but after sitting for a while it would shut the fuel flow off (as it should immediately after the engine stops). The point is that your petcock must be bad in order for fuel to enter the carb when it's not running.

For fuel to go from the carb into the engine (which I'm dealing with now), I'm starting to think there must be two problems: 1) the float needle valve is not stopping fuel flow at the right fuel level (bad valve or float level set too high), and 2) a blocked overflow tube. I have to check this, but I don't think the fuel level can rise high enough to enter the engine without exceeding the height of the overflow tube.

Just my theories. Hope to hear more responses.

 

[drivadesl]

So finally had a chance to remove the petcock and dissasemble. Based on the broken diaphram, I think I found the problem.



As I mentioned this was used, so you get what you pay for. Still cost me $50.

Time to look for a replacement. I think I'm going to try and find a new one, and rebuild this one as a spare.

 

[drivadesl]

So I replaced the petcock and was able to start her up without fuel leaking all over. That's the good news.

otherwise the bike does not without at least 1/4 choke, and stalls with any kind of throttle applied.

I cleaned the crap out of these carbs, replaced all the orings, and sealed up the air box. The pilot screws are adjusted to 7/8 turns out if i recall correctly. Any suggestions? What do do/check next?

Very frustrated right now.

 

[chuck hahn]

Bottom screws (PILOT SCREWS) at around 7/8 out and leave them alone after that. Do all fine tuning via the side (MIXTURE) screws. If your getting fuel in the pipes i am suspecting a cylinder isnt firing. Have you checked for any fuel fouled out plugs? Sure all the wires are making spark by holding a plug to the block and cranking?

VM carbs have an overflow tube that would normally drip fuel if the fuel is getting too high in the bowls. Check the float hts again and set them at the middle of the "range" stated. This will for sure have the fuel level high enough for good operation but yet not at an overflow level.

And looking thru the STICKY at the top of this forum I found this chart. Note it goes off the engine number down at the top area of the clutch cover. Set the side ( MIXTURE ) screws accordingly.

AnonymousGuest

I found this in my service manual.
It is for GS750 1976-1979 ,GS550 1977 - 1982

Check the Engine no. ,not frame no. Look for the differences on the same model!!!​

 

[chuck hahn]

Try turning the big idle adjustment knob in a turn and see if it will idle off the choke at least. And note that the engine will get more responsive as the cylinders heat up a little so dont freak if it stumbles 15 seconds after you start it. Warm it up a little and then see if there some better throttle reply.

Have the valves and other maintenance been done? Little tip to cure a "no spark" at the plugs. First trim about 1/4 inch off the wire and rescrew the cap back on. If that doesnt work, look inside the cap and youll see the brass thing that grips the plug is slotted.

Unscrew it and dump the resistor out and replace it with a 1/8" rod cut to the same length of the resistor. real heavy gauge copper wire works too. If the resistors go bad they cut off spark no matter what you try.

1 and 4 do not have springs in the caps...2 and 4 DO have springs. So if youre doing a 2 or 3 cap, the spring goes in the cap first, then the replacement rod, and then the brass gripper thing gets screwed in.


EDIT!!!! A non firing cylinder will also make for bad throttle response.....

 

[drivadesl]

Thanks for your replys. I was able to get her to idle best after adjusting the mixture screws to 1 turn out, but she whould not take any throttle. Also adjusted the idle RPM, but the idle was pretty shaky so there are other issues.

There may be some slight fuel in the exhaust yet as after a few minutes the exhaust turns white and smokey and I see remnants of spray out the sides of hte pipes. Nothing like it was previously, but maybe I need to decrease the float height a bit to ensure that is not the issue. Hopefully I can do this this weekend and will report back.

Funny but I would think fuel would run out the overflows first before running into the cylinders wouldn't you?

You got to love these bikes to stay at this, I'm thinking of plunking down some cash on a new bike just to go riding, and not wrench turning!!

 

[chuck hahn]

How full is the oil...and twist off the filler cap and put your nose down the hole and take a good whiff. If it even suspiciously smells of gas then drain the oil. EVER leave it in the PRIme position either. I know you got a new petcock but if its left in the PRIme position it allows fuel to flow and never be shut off.

If it didnt smoke before, I suspect something is forcing oil past the rings...IE crankcase contamination.