VFR 750FS Run Problems

Hi Guys
I am new to this forum, but need help as I am completely out of ideas. This will be a long post as need to give full history. I am located in UK.

The bike has 55k miles on it and was laid up for a couple of years until summer 2012, but was started periodically every two months to run fuel through and warm the engine. I have been running it normally since them until September this year, when I replaced a leaking rear shock and spring unit. At the same time I noticed some corrosion on left rear chassis from the battery box to the spark unit. I removed fuel regulator the pump and spark unit (did not disconnect spark unit) to clean and spray chassis rail with rustoleum paint. At the same time I also noticed corrosion on radiator so repainted and replaced that. I also replaced air filter and plugs. When I came to restart it in early November, it would start on choke but would not rev as soon as I twisted the throttle it died. It wasn't runnning as fast as usual on the choke alone. Eventually when warm I started it with difficulty with the choke off. It would rev almost nornally but wouldn't tick over properly; as the engine warmed it eventually died and couldn't restart it. There was whisps of white smoke from excess fuel coming from the exhaust. At firtst I thought it was flooding but later believe this was due to my attempts to keep it running and the needle valves allowing excess fuel into the carbs. I then; checked fuel relay and fuel pump were working normally; replaced fuel filter and reverted air filter and plugs back to the originals as I firmly believe the problem was something I have done; checked the mopunting and connectors on the rear coils as these had been off. All to no avail.
Believing this was a carb problem and potential this short lay up had exacerbated some fuel gumming that mioght have happened during its previous long lay up, I removed the carbs, removed an cleaned the jets and carbs with carb cleaner and blew out with compressed air. Changed all float needles and checked the cut off heights. Generally the carbs were spotless before and after cleaning with no signs of gumming. rebuilt and problem still evident with no improvement.
Scratch head time, and still back to must be a carb problem, purchased a set of carbs on ebay 'in perfect working order, 27k miles and stored dry' put thes on and the exact same symptoms remain. Tried bypassing fuel relay and then bypassed fuel pump allowing gravity feed only. No change. have also put new petrol in and checked for any earths I may have left off. All breathers seem connected up OK. Checked rear two sparks and it seems to be sparking OK, at least when cold. Here are my current thoughts:

Is there an earth anywhere I may have left off, the only one I remember have found is a thin one to the left side of the radiator? Are there any other breathers I may have cocked up on? Could it be a spark problem? Note I am back on new plugs and air cleaner now. I am convinced it must be something I have done but can't figure what. Also there are four carbs, so surely if there was a problem with one it wouldn't affect all four. Does the breather on the carbs just go to the tank and nowehere else

My next plan is to, go back again to original plugs and air filter. Put original carbs back on and ultrasonic clean the 'new' carbs.

Any help is much appreciated as I am stumped, note the bike is UK based so doesn't have a load of emmission type vacuum pipes.

 

Following on from this. There are two breathers that exit near the left footrest; one is connected and the other finishes (with a hose clip on) just in front of the battery box, ie it is not connected to anything, nor does there seem anything for it to connect to. can anyone shed any light on that please.

 

Did you say you had a breather line from the carbs to the fuel tank? If so that is not correct. The carb breather is just open to the atmospheric and sometimes functions as a fuel overflow if all the carb internals get really gummed up. The same is true for the fuel tank vent--it should just be open to the atmospheric.

That un-connected breather by the battery box may be the fuel tank vent line that you have mistakenly connected to the carb breather, or it may be the carb breather line.

Your symptoms seem carb related. The choke jet and the pilot jet have very tiny openings and require verification that they are open to the correct size, usually chemicals are not effective. A number 13 guitar string works well. Do a forum search on 'cleaning carbs' for a good pictoral by the one who smells of prunes and is missing a toe.

 

The carb breather pipe has a reducer on it and then connects to a breather on the fuel tank, that is the same as always and it won't reach the unconnected breather and the diameters are such that they don't mate. The fuel tank drain goes to the second pipe that vents at the footrest. Although I am confident these have not changed whilst I have had the bike - about 8 years - I am not discounting anything unless proven and so would be interested from anyone telling me where the small diameter breather ffrom the bottom of the fuel tank goes to, as I say mine goes to the carb vents. Do you still think it wrong? I will try it just vented to atmosphere and see if it makes a difference.

I have just changed the spark plugs back and the air filter back to original and got the fuel pump and reklay connected. It woudl start on choke, but not rev. If I backed the choke right off when warn it stopped running. I could however start it using the throttle and no chiokle when warm and it would run almost normally for about 45 secs to a minute and then it would die, it did not die through shortage of fuel as there was plenty going through as evidenced by opening the needle valves and plenty of fuel in carbs. It seems to me there is an electrical issue killing it, but I cannot figure out what.

I had all the pilot valves out, inspected them through a strong magnifying glass and there was no sign of any gumming, sprayed carb cleaner in and blew them out with an air line after soaking. I like the guitar string approach and will try that. I would appreciate explaining that though even if one pilot valve was manky, surely it would just misfire on one cylinder and not affect all four. The bike was only laid up for four weeks max, which I would have thought hardly enough time for gumming to occur.

Will check out the carb cleaning link you mention and thanks for the response.

 

Check to see if the unconnected line goes to the radiator overflow tank.

i looked at a set of 4th gen carbs on the bench and the carb air vents all come together and out thru a short stubby tube that goes in to the air filter housing according to the FSM--and the fuel tank vent line runs to a carbon cannister. So not sure about what is your configuration with tank vent going to carb air vent.

The stall after choke start warm up sure sounds like clogged pilots to me, and the air vent routing may be shutting off the fuel flow if the tank is not venting. If it were an electrical issue then it probably would not be healing itself--once they break they are done.

 

Thanks. The carb canister must be a us emmissions configuration, nonetheless will try just venting to atmosphere to see if it makes a difference. I checked out the carb cleaning and it was just as I did, except for the centre hole on pilot - didn't realise there was one, so will do later today. I do though feel it must be something I have done during the work, but......

Thanks for input and will let you know how I get on.

 

Kennybobby thanks. I have got it running, but am still a little confused. I have taken the carb vent off the small diameter vent on the fuel tank and just open air it and the engine runs fine; thats the good news. The bad news is that I am 99% sure it originally came off the fuel tank connector, as I definitely took one off and it is the only pipe of the correct diameter that could go on. So here is the question: If anyopne out there witha UK spec or non US emmission spec VFR750FS can tell me where the carb breather pipe goes to and also where the small diameter vent from the fuel tank goes to, I would much appreciate it. It would probably help if I had a pucka Honda Manual rather that the UK Haynes one.
I guess I was convinced it was something I'd done and that seems the case, just need the fianl piece of the jigsaw.