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WHY WONT IT RUN?? (carb question)

  • Thread starter Thread starter BGrey
  • Start date Start date
B

BGrey

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I just rebuilt the carbs on my '81 750L and my bike still won't start. If I hold my hand over each carb as the engine turns over it pulls gas into the carb, starts, revs up to about 5k and dies. The diaphragms seem to be in good shape, but are not moving when the engine turns over.. Seems to be a suction issue but I am at a loss... Please help the poor new guy before I get fed up and use a mechanic....](*,)
 
How is the petcock? Is the selector in the on or prime position? Functioning properly?, In tank screen not clogged with corrosion particles (dirt)? Vacuum line from petcock to carb intact and no cracks or leaks? Vent in fuel tank cap clean?
 
Petcock flows fine in the prime condition and flows in the on position if I apply vacuum to the vent line. Don't know about the screen or vent though...
 
I don't know about your bike, but my 850 doesn't want to run without the airbox on. Put the whole thing together (I assume its stock?), then try it.

What's the story? Did you have problems before? When you rebuilt the carb did you do a complete rebuild?
 
I got mine to run well enough to test in the garage by folding a shop rag in half, then stretching it across all the carb intakes and tie-wrapping it to the outside carbs. Provided just enough of a restriction for them to work.

Never did take it out on the street like that, so I don't know what the throttle response might have been, but it worked well in the garage. :)

.
 
These carbs need big time vacuum to operate the slides correctly. The air box needs to be installed before any tuning, all clamps tight, etc... even the slightest air leak close to the airbox side of the carb will cause problems.

Greg
 
That may be part of my problem. I am trying to start it with no filters or airbox on it. When I bought it it had foam pod filters that more or less fell apart when I took them off. I'll try the shop rag trick and see if that is my problem. If not, then maybe I screwd up the carbs in my first attempt at a complete rebuild... I guess buying a bike that had been sitting for 3-5 years might have been a bad plan...
 
I'm not at all sure about the "shop-rag" technique, however, I can assure you this: until you get the air-side right for the carbs you are spinning your wheels.

I have an 80' 750, an 81' 750 and an 80' 1100 which all are identically carb'd. You absolutely must get the intake air correct or nothing else matters.

Chances are that someone changed a couple of important factors within the carbs when they changed from the original set-up (air-box) to the pods which fell apart. The most logical thing to do is replace the pods with the same type (K&N, Unis', etc.) BECAUSE........if the pods were foam the person may have changed the mains, the air jets and may have drilled the slide holes. Or they may have changed nothing or any combination above. The original mains for your machine were more than likely 112.5. If they are still there then its probable that no alterations were made. However, if the pods were K&N then you are definitely looking at a set of altered carbs.

Just remember, if you try to set the carbs using a shop-rag technique or any other unconventional method, all your adjustments will be off when you switch to your permanent filter system.

Good Luck......be patient.
 
Oh yeah:

I would bet a substantial amount of legal tender that you did not screw up by gettin' this bike. I predict that once you get the carbs right you will be pleasantly surprised at what a jewel you have brought back to life.

All three of my GS's are incredibly dependable, torquey, and visually impressive machines, and all three had been setting for more than 7 years before I started working on them.

You are in the right place...these guys have the knowledge.......you must have the patience. Treat your machine like a kid.......don't work on any part when you are aggrevated/frustrated. Walk away and come back later..remember, it is just physics and once you get the correct balance the bike will run and more than likely it will run for years without a problem.
 
Almost every "restoring-an-old-bike-that's-been-dormant" story on this site has a happy ending. And almost all have the same typical problems - incorrect or leaking airbox/air-filter is common to almost all (besides a host of other annoying issues - like cleaning the carbs thoroughly).
I've restored four non-running bikes over the years and every one of them had this problem.
By the way, in my opinion, you're probably better off getting a set of used stock carbs and airbox from eBay than trying to get a carb with non-stock modifications to work. It's just not worth the head-ache.
For my friend's GS, I got a set of carbs for $15 from eBay. Between the eBay carbs and the carbs he had on the bike I was able to get the bike back to stock settings with little pain, and the bike has been running well for the last three years.
 
Hopefully these carbs will be OK now that they are cleaned and rebuilt. With the exception of the air screws being, well, screwed and being dirty (needle jet was covered in hard black crap) nothing else had been changed. Heres to hoping!
 
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