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I'm getting crap mileage after my tuneup. Bike is tuned to Clymer spec, timing is spot on, carbs synched. My plugs are tan, and the #1 and 3 plugs are blackened.

Do i need to turn out my idle mixture screws to make it run leaner? Oddly enough, before i tuned it the bike was getting 40mpg!
 

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The idle mixture isn't in play once you open the throttle so that shouldn't be part of the problem if the bike idles properly. The two black plugs are saying things are way too rich in #1 and #3 cylinders. Time to reconsider exactly what you worked on during the tune up. The float valves in those two carbs could be leaking or the level off, something is causing too much fuel to flow through them. Possibly the choke plates are not fully opening in those two carbs?

:waving:
 

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OF course, idle mixture doesn't affect things after open throttle. I think you are correct about choke butterflies being stuck. The bike won't run at startup with just the choke -- it seems to die off after starting out at higher rpm for a while.
 

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Make sure the vacuum advance hose is in place and properly connected. Also, check your valve adjustment because if the valves are too tight you'll lose horsepower and gas mileage.

For the black plugs check the mixture setting or float level on the carbs affecting those cylinders.

There is a whole process you need to know to set the mixture on these carbs and, sorry,I'mtoo tired to type it all out right now. Get yourself a manual and follow the step by step instructions and synchronyze the carbs while you're at it.

Vic
 

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thanks, Vic. I'm thinking it may actually be the valve adjustments. Maybe too tight. Also, the cam belts are whining a little, which probably isn't good either.

I have already set up the carbs as per manual spec. This year doesn't do idle rpm drop, just 1 1/2 turns.

It must be the valve timing.
 

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DUH! This bike needs idle drop adjustment on mixture screws, being a 78 model. The mixtures are all off. I'm sure this is my problem with black plugs, and hopefully poor mileage. Bike is running well otherwise.
 

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I dont have a g/w anymore but I can tell you that on my 79, I went through all that stuff and more. I would say that more than likely your carburation is at fault even you have gone thru it. These bikes are notoriously fussy about carbs. Trust me on this one. Check re-check and clean, clean, clean those carbs. By the time you are done, you should be able to pop them suckers off in 20 minutes or less. Make sure they are completely clean even the little screens. Take nothing for granted, take your time and do them right. Also consider the age and wear on the carbs. I got a set of 1100 if you are interested and they need a set of float valve and seat.
 

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Thanks, Gilles. Well noted. They are fussy all right.
 

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i'd like to add alittle here . i just had my carbs done and an experience guy did them and said ,it was most headache project he'd ever done chasing down the glitch that never was done quite right and he came to believe the plenum(that bottom section where all 4 tie together )hair line cracked at the cast molding and are seeping slightly. i asked if he would sync them one more time because i was getting 10 mph and now i can get 30 which is good enough till i can really learn what to do about it.i think maybe the larger jetting also done may be contributing to the mileage problem but i figured rejetting is going to bea must with one day soon having to go to an aftermarket exhaust of some kind of cobra pipe system.

the whole reason i bought a parts bike was also because these carbs are non exsistant in the market and who ever is selling any on ebay and tells you they need to be rebuilt is questionable,you take good look at that junk they're trying to pass off as rebuildable.

i'd only learned from this site that 1100's are the way to go . thats some good looking out!!!:clapper:

i was hoping that conpany that was making the manifold and single barr. carb kit was still in bussiness . but no their not even answering their message board anymore. till they can find a company that will make the carb for their unique manifold they are forced out of bussiness at the moment. i wrote them and told them price would be no object to a guy with acarp for carbs wing. and i would pay almost anything to have a good running fuel delivery system.

i also do not have the return throttle cable and i have heard that not having it makes the idle higher than its meant to be.

the suggestion on the valves is swell and true.intake and out take specs can count for alot.
 

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ofuami:

#1 GET and INSTALL a return throttle cable! (whether you go single carb or not)

#2 Watch eBay for that single carb set-up you want. I've seen several of them in the last year. Set up a search that will email you when one shows. Bid what you are willing to pay, not what you would like to pay.
 

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I'd like to repeat that, GET A RETURN CABLE! The bike isn't safe without it. If the throttle spring broke or the shaft jammed it's possible you couldn't close the throttle. If it happened it most likely would do it when you have it open a good ways. I don't think it would be much fun if it happened in close quarters somewhere.
 
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