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Running Lean - How to correct ?

  • Thread starter Thread starter pac315
  • Start date Start date
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pac315

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Hi other GS lovers,

My name is Paul C. I live in Georgia and ride all year! I have recently purchased a 1980 GS750L (TSCC engine). The original mileage was 9,215 when I picked her one year ago. This is my first bike since I sold my 1980 Yamaha 850 Special in 1986 with 45K miles (all mine from new). I took off riding for all these years because ? well, ahm, humm ? I do not really know why! Anyway, I?m back! I really like this old GS, and I must tell you that I like fixin things, and bought this bike with intentions of getting her as good as new and using my mechanical aptitude (I get little chance to do so since I have a basic desk job)! Of course I immediately bought a Clymer?s and then replaced the following items right after I purchased the bike: brake shoes, tires, battery, chain/sprockets, handle bars, mufflers (4 into 1), and cleaned everything and replaced all the fluids & filters (forks, brake, oil). I rode the bike for the last year with no real problems (except a ?flat spot? between 1,200 & 4,000 RPM and a few oil leaks) until about a month ago. I was tired of getting oil all over everything, my leg and boots, so I decided to tear her down and install a complete new gasket set in the engine, and fix that ?flat spot,? which seemed to be getting worse. I just completed the engine tear down/gasket replacement and carb rebuild. The engine now has 11,721 miles. While I had the engine apart, I had the head checked and flattened (the machine shop took off 0.014 in). The valves and guides looked great. The pistons and cylinders walls were very clean and the engine basically looked fine inside. Of course I honed the cylinder walls and cleaned everything spotless. I reassembled the engine and at the suggestion of my local Suzuki mechanic, used two head gaskets (since so much was removed from the head to get it flat). I also used copper spray on the head gasket parts except the corner ?O? rings. Because I did not have a good manual on how to rebuild my Mikuni BS33SS Carbs., I let the local Suzuki shop do them (ouch).

Well, I assembled everything, primed the carbs and hit the starter button and she fired right up! I then took the bike in to the dealer and had the carbs synchronized. After about a week of riding, I adjusted the valves and retorqued the head (one or two bolts needed additional turns, the others were fine). This is only a partial success story. The bike does not leak a bit of oil, starts fine, but still has that damn ?flat spot.? It just bothers me! The bike shop mechanic thought the bike was running OK and just told me to keep the choke on for a while longer when it is cold and when it finally warms up it will run better. Well, I?m just not satisfied with that answer. I did some reading up on carburetors and tuning and I have come to the conclusion that my bike is running lean! I looked at the plugs, they looked lean. I ran the bike with out the air filter, and wow, the problem got much worse ? the bike really ran poor! So, I am convinced that my Mikuni Carburetors are running lean, especially in the lower RPM range (it seems to pull fine at high RPMs so I think the main jet size is OK). Therefore, given all that has been done in the past, what are my next options? Can I adjust the jet needle? Are there other things I should check out? My local bike shop mechanic will not return my calls. I can not blame him, he got to make a living, but I really want to figure this out on my own. If I throw in the towel and take it to the shop, he wins and I loose, plus I have not learned anything.

Thanks in advance for responding, Sincerely, Paul C.
 
When the carbs were rebuilt did you replace the intake boots or the intake boot o-rings? Is the air intake system stock? Unfortunately we don't know exactly what your mechanic did. Also you probably need to resynch after adjusting the valves (but that is not the cause of the lean condition, I suspect).
 
The aftermarket exhaust may have leaned it out a bit. You may need to adjust the needles to rise more in the carb to allow more fuel into the engine at mid speed.

You did not mention any airbox modifications, I assume it is still stock, and has a standard foam filter. KN or other filters may require rejetting.



Are you sure the floats are at the correct height? Since you did not rebuild the carbs you may want to pull the carbs and check. This site that you found has a section on carb rebuilding on the home page that details how to check them
 
Lean Mikuni BS Carbs, how to move the jet needle up ??

Lean Mikuni BS Carbs, how to move the jet needle up ??

Thanks for your replies. The air box is standarn and unmodified. It has a new filter. I did not resync the carbs after adjusting the valves, mainly because they were not off by much.

The bike shop that cleaned my carbs did replace the main jet (standard size) and replaced many "O" rings, and gasket. He said basically they were in good condition, with the normal gunk, (mostly in the bottom of the float bowls) and that the CV boots looked fine. The rubber manifold boots are in great shape.

I really think I need to "modify" (or move it up) the Jet needle. Can this be done on the Mikuni BS33SS carbs? I found the wonderful Carb Clean & Repair Pictorial by John Bloemer (thanks) but it does not really show if the Jet needle can be moved up or shimmed (and how). Any help here would be apprecriated before I go tearing into the carbs.

Thanks!

Paul C.
 
Paul,

Did you replace the o-rings under the intake boots? Between the boot & head. PM me if you need a set.

I know there are a few threads in here about shimming the needles with small washers but I haven't had to do it since I was lucky enough to get adjustable needles in mine.

Are you anywhere near Atlana? I'm going to be living down there in about a month and I'd be glad to give you a hand sometime. I've got a Carbtune II and you'd be suprised what a difference synched carbs make!

/\/\ac
 
TSCC bikes don't have the o-ring on the inlet manifold, they clamp on instead of the 2 bolts.

Paul are you sure you have BS33SS carbs? :? I only know 32,34 and 36, the '82 GS750T (TSCC) I worked on a while ago had the 32's.

If the bike idles well and starts good without an overly long warm-up period I would raise the needles, overhere we get 5-way adjustable ones stock but yours don't I think in which case you would need small washers from a shop like Radioshack IIRC?
 
Try richening the mixture screws a little. There is some overlap effect with the jet needles that may have contributed to the lean plug reads. At least it's a simple thing to do. Keep a record where they're at so you can put them back if you want. The 4-1 will contribute to a lean mixture.
Turn them out up to 1 additional turn from where they are and test. They are generally 1 to 2 turns out on a stock bike, 1 1/2 to 2 1/2 for a pipe.
If this doesn't help and you believe the carbs are clean/synched well and no intake leaks and the air box is SEALED well, then you'll have to raise the jet needles and test.
If you don't have adjustable needles, you'll have to shim.
Hard to say how much to raise them but I'd probably get a spacer close to .023". That would be about the same as raising the needle "1/2" of a position.
Depends on how flat the flat spot is. You may need more.
 
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