1984 VF700F rear cylinders not firing, bike won't idle

1984 VF700F rear cylinders not firing, bike won't idle. I'm getting spark to both cylinders. I cleaned the carbs today and still no change. I have replaced some diaphrams over the years from donor carbs (not sure what year bike, I've had them for years). I have taken the diaphrams with the sleeves and needles in all and put them in my bike. I bought the donor carbs several years ago because of the difficulty in getting diaphrams anymore. I was never sure of the position of the diaphrams I took out or put in. I have now read where some of these bikes have different needles between front and rear. Could this be my problem? Any other ideas? Any idea where I could get needles?

 

Sorry, it's the rear cylinders that aren't firing.

 

Also it will idle with the choke on.

 

Did this problem just start or has the bike been sitting up for more than a month? When you cleaned the carbs did you just spray carb cleaner thru it, or did you remove all the brass bits and soak them and then rod them out with a stiff wire such as #12 guitar string?

The diaphragm and needles are not involved in the idle circuit at all, but the pilot jets and mixture screws definitely are. The choke circuit bypasses the pilots and mixture screw, so the fact that it runs with choke but not without indicates the carbs need cleaning.

Next time you have the carbs open be sure to write down all the sizes of your jets and needles. If the main jets are different front to rear then the needles may have been different also. But these carbs will idle and run with either set of needles in any of the slides--maybe not optimal efficiency but you would hardly notice under normal riding...

 

Thanks for the reply.

The bike had sat for the winter. I only took the rear carbs apart. They really didn't look gummed up at all. I took all the brass parts out, sprayed carb cleaner through them, blew compressed air, and ran wire through them too. When it idles with the choke, there still isn't heat on the pipes coming out of rear cylinders. Once it warms up, it will almost idle without the choke. it idles for a few seconds, then just stalls out. pretty much need the choke to restart. The wire I used was the center wire from a TV "cable" wire which is very stiff. I like the guitar string suggestion, I bet it can make it through bends. Maybe I should try that....??? I wonder about the "mixture" screw. Can anyone tell me about that?

 

mixture screws....plural... each carb has one... if you have taken them out then you need to have the carbs synched....you can bench set them, but to have it really done right you need to have it synched while running... and ..did you take off the air plenum when you removed the carbs? if you did chances are you have lost a spring or two...... don't remove the air plenum rule number one in carb removal on these little finicky bitches...

 

http://vfrworld.com/forums/1st-2nd-...tor-fuel-air-circuit-identification-porn.html

The mixture screws are covered by a thin metal cap which must be center drilled and pulled out to get access to the screws. If the carbs have set up, then the orifice thru which the mixture exits the carbs and enters the intake may be clogged and will need cleaning before the bike will ever idle.

The center wire of a piece of coax cable is way too big to clean the pilot jets which are only 0.013" in diameter. It will work for the main jets, but that is not what supplies the fuel for idle.

Do a search on this site for "carb cleaning" and you will find some articles with descriptions and photos of how to do it properly.

If you don't twist the screws that adjust the throttle butterfly plates then you may not have to worry about carb "synch". These are the screws with the little springs on either side that are a pain if you lose any of them. Carb synch is a fine tuning procedure after you have the bike running on all the fuel circuits properly--yours just need a good cleaning...

 

Thank you all for your replies. Hopefully I can find time to look at again today.

 

Go take a look at this--it is great info

http://vfrworld.com/forums/mechanics-garage/22449-cleaning-carbs.html

 

People are ALWAYS saying they cleaned their carbs but have no idea how to do a thorough job.....sometimes they only clean the float bowl and think they're done !!