All systems go but will not run past a bad idle

This is very mysterious. Never had this much trouble on any other moto I've owned. First VFR though. 1990 750. All carbs cleaned and blown out with floats set at 9mm. I've had them off a dozen times to re-check settings etc. Compression is 170 all 4 cylinders. Fuel pump pumps tons. Good blue purple spark at all the new plugs. The bike wants to start and will idle badly for about 5 seconds then dies. Re start is easy but again it dies within a few seconds. If I try to throttle it even a nano amount it immediately dies and coughs up in a carb and lets out a sigh and a little puff of vapour. If I shut the fuel cock off and let it use the fuel in the carbs it will idle for a minute sometimes ...at about 850 though, not enough to keep it running to do a balance, but I have eyeballed the butterflies using a feeler guage and they should be close enough. I cannot get the idle any higher. In that minute that it actually idles I will open the fuel and within a second it will die. I have in desperation removed the muffler thinking it may be plugged but it's fine. A Valve adjust was done recently as well. I'm hoping someone has run into this type of problem. Let me know if you need more info. Much appreciated.

 

Carb vent line clear, unkinked, open to atmosphere??

Mixture screws set at 2 1/2 to 3 turns ??

 

Drain a bowl or bowls and check the amount of fuel to see if they are filling properly?Damaged boots?

 

Thanks for the reply..Vas ist carb vent line? There are just vacum screw plugs at the intake. Where is the carb vent line?..internal or external. I've had a feeling it's a vacum issue for a couple of days now. It feels like one but then...

 

mixture screws at 2 and a half...

 

boots are good and have been inspected as well as the piston diaphrams in the carbs. This is what makes this mind numbing after awhile.

 

Went out to check on carb vent lines..found them at the south end of the carb block..2 tied together? ..seem to be fine. Tried running it again and it's definitively starving for fuel. Plugs are bone dry. Checked fuel pump again and it's pumping fuel. Carbs were checked out by the mechanic before he sold them to me. They look real clean with what look like new floats..nice cream color. Who knows who lies these days to make a buck though. No constriction of the fuel getting to the carbs that I can see ...externally at least.. About to take them off yet again..arrrggghhh. Possibly fuel pump pressure?? Seems like enough but I'm no expert on these bikes. Float levels set as float arm just touched the needle valve and then set at 9mm to center of the float. Does this sound right? I can,t help but feel this is something real stupid that I'm missing..Thanks

 

Were valves actually adjusted or just checked? What I'm getting at is were the cams removed? Could the cam timing be off? Or out of synch from one bank to another?

How about the airbox? Have you checked the opening for obstruction?

Your bike doesn't rely on fuel pressure per se...just that the pump can pulse enough in the carbs to keep 'em full. I like Toe Cutter's suggestion...better yet...just open all the drain screws while you're turning the engine over. Fuel should be dribbling out the bottems soon after cranking the starter.

 

I am thinking you may want to jump the fuel pump and fully charge the carbs and drain a bowl to verify it is filling and possibly run pump to see if fuel delivery is restricted. It could be plugged screens in the seat.

 

Great suggestions from all....will try everything tomorrow...I'm thinking the valves were just checked as the covers are still dirty from natural road wear. Have not checked the screens in the seat ..good point. carbs are out again for another check..will open drain screws while pump runs to prime to full...good ideas..great forum help..thanks

 

It might be wise to 'bench test' your carbs before you go to the trouble of installing them completely. Use gravity feed fuel supply or hold the carbs close enough or rig a longer fuel line directly to the bike's system and charge 'em with gas to make sure they're filling.

 

Screens

http://vfrworld.com/forums/mechanics-garage/22449-cleaning-carbs.html#post170040

 

Flow the pump and those super mini screens inside the carbs . stupid quesion? did ya check your fuel filter???

 

what about fuel pump relay ? Try to eliminate it (jumper wire).
Ciao, hope this can help.
Lamps, Carmine

 

I have eliminated the relay at every attempt. All jets and especially the valve seat screens checked and blown out. To be extra thorough I took the carb block to the local Honda guy and had him double check everything and he said perfect..like new..so when I put them back on in a few minutes ...it will not be a carb issue if it does the same thing. I'm going to gravity feed the fuel this time to eliminate the pump to see if this works. For what it's worth I will report any progress or if this fails to start this machine ..a tearful plea for more of everyones' expertise will be in order..

 

yes..new fuel and filter..

 

Put carbs back on carefully...put airbox back on..gravity feed fuel and........the same. Idles like s#&@t and will not go vroom. The mechanic who sold me the new carbs says I'm crazy and that the carbs are like new (they are pretty clean). He will not help any longer unless I bring the bike in and you know what that will cost. $1000.00 for sure. I have told him everything that's happened and he's at a loss without looking at the bike. I'm wondering if the exhaust has anything to do with this..Front pipes are hot due to me starting..idle for a few seconds..then firing it up again over and over again. The rear pipes are not as hot.??? The engine seems to be only running on a couple of cylinders but as mentioned there is good spark and tons of compression as I've checked and re-checked all of this. Spark plugs are dry with no trace of gas when pulled after all this starting and re-starting. I hate to admit that it's got me stumped. Called the previous owners garage and they have serviced this moto since 1990 and left the owner with an estimate of $1800.00 to fix the carbs. I thought I'd be smart and find a good set of used ones (which I did ) and the world would be rosy and smell of flowers...but no such luck..Might be giving in to the mechanic and sign my assets over to him....Thanks for all your input anyway

 

so..I had a dream the other night and all I remember was the word MUFFLER (seriously)...so I'm at my wits end today with all this and was resigned to towing it to the garage and spending hundreds or more for the EXPERTS to look at it,,anyway..i decided what the hell, I've had the muffler off before but just eyeballed at the time..nice shiny Supertrapp...sooo I decided now that I had it idling like a pig..let's get paranormal and take the muffler off just for the hell of it..you already know where this is going. WOW..fired right up and is responsive as hell. Morale of this story is..listen to your Mother! I learned a lot during this ordeal and if anyone else has a problem they and others can't quite figure out..maybe the mufflers plugged. Shiny on the outside but crap on the inside..like some mechanics I've recently met..Hey thanks to everyone who responded to this post..Cheers

 

Muffler??

Here's what i shook out of a VFR can that had been sitting on a shelf for 2 years:





Mice, not a squirrel !!

 

LOL..now that I can. I'm sleeping much better tonite.