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Almost there...1981 GS550 help

  • Thread starter Thread starter khardrunner14
  • Start date Start date
K

khardrunner14

Guest
I did some more work to the bike this weekend. Your suggestions proved invauable as the bike now runs like a champ....for the most part. Here are the last few problems (I hope) before I can call it finished for now...

- I am still struggling with getting the bike to start/idle properly. It will start and idle pretty much on PRIME and with at least some choke on. I tried the air mixture screws at 1 turn and 1.5 turns with the 1.5 turns being a little better. Either way it will not idle anywhere below about 3 grand. Is this related to fuel, air, or even the new pilot fuel jet? What do I do?

- The bike will only run/ride with the petcock on PRIME. If I am careful with the throttle, it will limp along on run/reserve. Does this mean I have a petcock problem, or is it more likely a problem with the vaccum?

Again, here is an abrieviated list of what I have done:
- bike is bone stock
- rebuilt and cleaned the carbs (most expensive rebild kit from oldbikebarn.com)...checked float levels (22mm with gasket which is within the range)
- New O rings for intake booties
- New battery

Thanks again for your help!
 
Did you actually change the intake boots as well as the o-rings?
 
Could be that common problem of a leaking diaphragm in the fuel tap. Either that, or a leak in the vacuum hose that runs from one of the inlet manifolds (#2 or #3 cylinder, depending on your model of GS). Or, it could even be that the vacuum hose has been disconnected altogether.
 
tfb said:
Could be that common problem of a leaking diaphragm in the fuel tap. Either that, or a leak in the vacuum hose that runs from one of the inlet manifolds (#2 or #3 cylinder, depending on your model of GS). Or, it could even be that the vacuum hose has been disconnected altogether.
Click to expand...

Definitely worth checking. In fact, definitely worth replacing the fuel and vacuum lines if you haven't already.
 
where do I go to pick those up? Do I have to order bike bandit or will a dealer work?

Sounds like it should be a vacuum problem then, rather than petcock?

No I did not replace the boots because they looked fine.
 
khardrunner14 said:
where do I go to pick those up? Do I have to order bike bandit or will a dealer work?

Sounds like it should be a vacuum problem then, rather than petcock?

No I did not replace the boots because they looked fine.
Click to expand...

It does sound like a vacuum problem, and so could be any or all of the three: petcock, vacuum line, boots or more. You can pick up lines at any auto supply store. I think you need 1/4" & 5/16".

"Looking fine" and "fine" are very different. Did you squeeze them and check for cracking?
 
Yes I checked them over pretty well to make sure they were still good. I will perform the WD-40 test to make sure though.
 
All of the above advice is good.
Also:
You haven't mentioned whether or not you've checked compression on each cylinder. Low compression (most commonly due to insufficient valve clearances) will cause the symptoms you describe.
Also, you haven't indicated wheteher or not you've verified that ALL cylinder's plugs are firing - don't assume until you know for sure.
The above procedures are described in detail in many other posts on this forum.
 
All 4 of the exhaust headers were warm so I assumed all plugs were firing. Valve clearances sound like a big job, but if they have to be done, then that's life. I REALLY don't want to have to pull the carbs again!

Thanks for the input, I'll keep at it.
 
khardrunner14 said:
All 4 of the exhaust headers were warm so I assumed all plugs were firing. Valve clearances sound like a big job, but if they have to be done, then that's life. I REALLY don't want to have to pull the carbs again!

Thanks for the input, I'll keep at it.
Click to expand...

Not a big job at all.

But first things first. Replace those hoses and test the petcock. Put her in the "ON" position and remove both hoses from the carbs. Put the fuel line in a glass and the vacuum line in your mouth. Suck. What you want to have happen is the glass to fill up. As long as you keep suction on the vacuum line, the gas should flow.
 
If it doesn't flow...then replace the petcock?

If it does flow then I have a vacuum leak/problem?

That makes sense.
Thanks
 
Why did you replace the pilot jets? Use the stock Mikuni jets unless the jets are somehow damaged.
Sounds like you have a petcock issue but I believe you said it runs poorly/not at all under 3K rpm, even if on prime?
Verify correct flow first, that's easiest. Is the petcock clean inside? Any rust in the tank? Is the vacuum line in good condition and not kinked?
 
Well I performed the vaccum test suggested by uncle mike. I sucked until I was blue in the face with the petcock both on reserve and on with nothing. Gas only flows on prime.

So...I take it my petcock is bad. Is that something a cleaning can fix or should I just buy a new one?

That still doesn't explain why it struggles to run correctly even on Prime. It does idle much better on prime, but it isn't quite right. I don't know much about the Prime setting, but here is my theory...correct me please if I'm nuts. Since Prime is gravity feed, there is no control over how much gas reaches the carbs (save for the overflow valve). Because of this, the bike runs lean or rich (not sure which) on Prime. This causes the bike not to idle well.

I used the new fuel pilot jets because they looked cleaner (though I cleaned the old ones) and I thought they would be an improvement. Although I read elsewhere KeithKrause that you said to use the origionals. If I have to pull the carbs again (DAG!) then I'll put the old ones back in.

Here are the things I know I need to fix...
- petcock (replace or clean?)
- check vacuum hoses (the looked fine)
- use a reliable tach (mine is a reject)
- check compression
 
There's your answer, Fishbulb.

Seriously though, replace your petcock with a Pingel, and you won't even have to think about it for another quarter century. Of course, you will have to remember to turn it off,...

I just replace the hoses as a preventative issue. It's cheap and easy and I KNOW I don't have to worry about it.

Once you've done that, you know it's no longer a consideration. Then we can go from there onto compression, valves, etc.
 
Maybe I should have been more specific...

Which petcock and which adapters do I need. They have a ton of H-D stuff and drag stuff, but I don't need that.
 
Spend the $20 on a petcock rebuild kit. Or take yours apart and clean it.

You can also easily test your current petcock with some vacuum line and fuel line. Set the petcock to on, point the fuel line into a cup, and suck on the vacuum side of the petcock. No fuel? Take it apart and clean it. If that doesn't work, a $20 rebuilt kit will make itwork like new. And you don't need to futz with adapters.

The only reason I've ever seen a petcock fail is from crap building up in it. That only happens if you let a bike sit for a decade or so. :-)
 
khardrunner14 said:
That still doesn't explain why it struggles to run correctly even on Prime. It does idle much better on prime, but it isn't quite right. I don't know much about the Prime setting, but here is my theory...correct me please if I'm nuts. Since Prime is gravity feed, there is no control over how much gas reaches the carbs (save for the overflow valve). Because of this, the bike runs lean or rich (not sure which) on Prime. This causes the bike not to idle well.
Click to expand...
Prime doesn't deliver any higher flow rate than on or reserve. The amount reaching the carbs is controlled the same as any other position. All positions are gravity fed. Only difference is that when on/res, the vacuum overcomes the spring keeping the diaphragm closed. On prime, the fuel goes straight through because it bypasses. It can't run richer or leaner because of petcock position. The flow is ultimately controlled by the float needle valves that regulate the fuel level in the bowls. The sole purpose of a prime position is to fill the bowls after the carbs have been serviced and there's no fuel in them yet. Makes initial starting a lot easier.
Before replacing or rebuilding, be sure the petcock is clean. If it's dirty, it can fail.
 
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