I just dont get it !!!

Took my old engine out cos it only ran on 3 cylinders when hot....replacement engine does exactly the same - shared parts are carbs and electrics.Ive thoroughly stripped, set and cleaned the carbs, ive put on new coils, ht leads and plugs, but its just the same. If you leave it 5 minutes it will restart and run for a few more minutes till first one cylinder gives up and then another. Float levels perfect! New coils. I just dont get it!!!

 

Somethings breaking down!
More often than not it's a too Rick Pilot Mixture fouling the Plugs.
But, you say you've checked that.

You have to Check the Ignition Wiring all the way back from the Pick-up Coils.
You probably have a dirty contact or something silly you overlooked that's your Gremlin.

If you swap the Coil Leads does the Missing Cylinder change to an outside one missing?

 

you just might have something there.....i know its running megga rich...would that explain the strange behaviour of stopping..cooling...running? I do hope so!!!

 

That didnt work...nothing makes sense! Jets are al new, float levels spot on, runs on full throtthe for a minute then one cylinder paks up...then another - wont start. Leave it 10 minutes and it starts and runs perfect for another minute. Fuel flowing perfect. Heres an odd thing i just noticed - whenb it wont start the choke makes no odds, but put your hand over the carb mouth (no filters!!) and it will fire up - has all the signs of fuel starvation but why stop and why take so long to restart - fuel line is good, free flowing???

 

Have you removed the float seats and checked for garbage under them?

 

That didnt work...nothing makes sense! Jets are al new, float levels spot on, runs on full throtthe for a minute then one cylinder paks up...then another - wont start. Leave it 10 minutes and it starts and runs perfect for another minute. Fuel flowing perfect. Heres an odd thing i just noticed - whenb it wont start the choke makes no odds, but put your hand over the carb mouth (no filters!!) and it will fire up - has all the signs of fuel starvation but why stop and why take so long to restart - fuel line is good, free flowing???

 

the carbs have been cleaned, soaked, jetwashed and polished - inside and out..they are like new! its not that - thanks

 

See if the bike will run on Starting Fluid or the Gas from an un-lighted Propane torch.

Remove the seat.
Spray Starting Fluid into the Airbox.
Have a towel or a fire extinguisher at the ready.

If the Bike runs on an external source of Fuel ...
You have missed something in the Carbs or your Pilot Jets are too small or
the Passages from the Pilot Jet to the Pilot Mixture Screw are clogged.

First do the external fuel test and report your findings!

 

OK...NO AIR BOX...JUST OPEN MOUTHS (FILTERS TO FOLLOW)...IT FIRED UP USING EASYSTART SO ITS THE CARBS...BUT WHAT?? THEY ARE CLEANED THROUGHT!

 

With NO Airbox those Carbs would be in an extremely Lean condition.
You would have to REJET in order to get a Mixture that would sustain Ignition.

What the Values are for NO Airbox is something I do not know. I can only advise you on the Theory.

The Pilot Mixture Jets would have to be larger.
The Main Fuel Jets would have to be larger.

The Pilot Mixture Screws would need to come out more.
The Bike will probably Start on Choke ... if the Starter Jets are clean and clear and for a limited time until the reservoir of Fuel in the Start Jet Well of the Fuel Bowl is depleted.

Then, after a period of recovery ... start again, cycling as before.

Since I don't advise Members on how to ReJet for NO or Pod Filters ... you'll have to begin another Thread and seek the advice of those who have been moderately successful ReJetting.

Please bear in mind ... the Carbs were not designed to run without the Airbox restriction of a limited amount of Cubic Feet per Minute.
The process of rejetting your Carbs might be Trial and Error or Experimentation to get it to run somewhat decently.

There will always be some performance factor that will not be met.
Depending on HOW your specific Bike responds to the rejetting ... you might suffer low, medium or top-end performance challenges that are difficult to overcome.

At least we know what you are battling, now and can put the word out that you need assistance rejetting for wide-open intake.

 

Ive rejetted to a std primary and a 107 main. 100 and it wont run, 120 and it will run but is way too rich - both combinations have same stopping effect. I dont need the choke to start it, so i dont utilise those jets at all...still has me baffled!

 

Hook up clear plastic lines to the carb drains adnd open up drain screws and watch you float levels. You may have a carb issue with fuel tube between carb bodys. Is it #4 giving you trouble? Check to see if that carb is low on gas when you run for a bit.

 

no.4 is the first to fail...

 

Ok I don't know if this will help but I will suggest anyway, My Black 750 was running like crap. I pulled the carbs and cleaned and cleaned untill I thought they were perfect. I never thought to pull all the needles and seats and blow through the fuel line and see if anything came out. I couldn't get the float level to stay right with engine running in that #4 carb. I took off the carbs again, and pulled the floats, needles & seats and I found a piece of rust up in that tube that goes between ths carb bodys. I worked it out of there and now they work fine. As another note I also sync'd the carbs at the same time and it runs so good I can't beleive its the same bike. So I would for sure check your float levels . The fuel level should be about at the lock washer that goes under the float bowl screw. It will go down when you gas it and come back up.

The only other thing I can think of is if for some reason you are loosing spark to that cylinder, Have you looked at the plug wire where it connects to the end. I heard that sometimes the wire can get corroded in there.
Good Luck

 

when you describe the fuel level is this with the bowl on or as it apears off....i just pulled all the bowls off...all 4 were full to above the bowl gasket...petrol pissed out as i undid the screws - keeping them perfectly upright as i did so..does this sound like way too much fuel?? Its getting vert close to pyro time!!

 

Fizzy - -
You can do this on the bike, but most people do it on a bench, carb rack level. All 4 drain screws in the bowl have to be freed up. A small fuel supply is taped up one foot above the carb rack, like a small funnel, and a clear plastic tube. Another small clear tube goes into the bowl drain.
This tube is held next to a bowl mounting screw, and the gasoline should rise above the screw head, but not above the gasket. A photo would help. Search topic : float level.

So - yes - way too much fuel.

If the carbs are off, and bowls are off, invert the rack and sight down the 4 floats, and see their visual alignment. They should all look the same.

 

Ok...I set all the float levels and went one grade colder on the plugs. This seemed to help but not cure, so I set off with screwdriver and plastic pipe in pocket - flat outish (85mph...on a quad!) till is went into shutdown - wuickly tested float level on no.4 and what do you know...no fuel!! Ive stripped the carbs, done away with the gause filter and opened out the fuel way by 1mm. Took it for another test ride, lasted far longer than previously before running dry - now is it just possible that the stock fuel system cant supply enough fuel to my straight thru system, remember, no air filter and straight through big bore exhaust?? Would a fuel pump cure it??

 

ping

 

Well, ... it might.
But, I don't know how well the Carbs are going to tolerate the pressure on the Fuel Rail O-rings and other than gravity might force a float open to flooding.

But, Mod-away!
Haven't you already done all the Mods you can do?
I can't think of any other way to increase supply or lessen demand.

Other than just not revving the thing very high! : ))

 

and on that note...how high is high? I have a digital dash, i tend to cruise at 7K but it will happily rev past 10K - am i revving too hard?