open throttle - bogs down

Having finally got the last XJ550 purring wonderfully , by figuring out that the pilot jets were indeed blocked, it idles like a new bike and runs when not pulling back too hard on the throttle really nicely. However pulling the throttle open for that burst of accel - the bike completely bogs down, ease off a bit and it does take off ok and I can build up to 6k and gets up to 120km easily. I have cleaned every other bit of the carbs, emulsion tubes, enrichment circuit its all shiny and clean and the clunk test passes with flying colours. It does it on PRI or any petcock setting and yet I know the fuel flow is good. Any ideas? Its like opening the throttle wide starves the bike of fuel?? I have new airfilter and no vacuum leaks anywhere.

 

If i remember right from one of ricks descriptions it sounds like you are running to rich. Just been tweaking my mixture settings plugs looked good and it would pull like a train upto 6k then stumble and struggle to get past. Leaned the mixture out slightly and it now pulls like a train all the way to the redline.

 

I'm not clear on just what causes the symptom:

Does this occur when you suddenly open the throttle from something like 25% to WOT?

How about if you ease into it? Gradually rolling the throttle open as the revs pick up?

 

Thanks Guys.... I am not sure, other than the pilot screw if there is anything on the mikuni carbs for adjusting the mixture... There is no mixture screw that I know of and you cannot change the needle height at all. Micarl, if I roll the throttle on gradually it will pull all the way steadily up, its opening it wide in any gear and at any speed that causes engine to bog down quite badly as if the fuel was just turned off. I know that the pilot screw does somewhat affect the mixture but with the butterflys wide open it should just create vacuum to pull the sliders up and let fuel come on down. I am going to try the air filter out and see if I am air starved - what dyou think? The other thing I am thinking is the timing... since I rebuilt the engine I am not totally sure because i didnt strobe the engine, if it is spot on, the other thing might be cam chain slack I suppose, but that wouldnt affect timing right? Also havent ruled out electrical issue, bad plug or bad plug cap.

 

When you snap the throttles open air velocity drops, causing a pressure increase in the carb throat and reducing the amount of fuel drawn in. Causing a lean condition.

That's why cars had accelerator pumps, to get over that hump. Many motorcycles still use them.

The CV carb reduces the leaning effect by maintaining a constant velocity air flow under the slide and keeping fuel flowing through the needle jet opening.

You could be a bit lean on the idle screws. I'd think though that if they were that lean you'd notice at idle.

I think the most likely scenario is that the little holes in the emulsion tubes aren't fully open. That'd make you a bit lean in the mid range and top end. Add a little bit from the throttle snap and it becomes critical.

A final possibility is that the springs in the slides are shot, letting the slides open too fast. OR extra or larger holes have been added to the bottom of the slide making them too responsive.

I'd pull the emulsion tubes and make sure they are spotless.

 

Micarl - emulsion tubes were pristine, shiny and clear through all 16 holes.. the springs were also in very good shape. I will give the pilot screws a turn out on all four tomorrow, it has to be a fuel air mix issue as electrical would not kick in with throttle actvity, the other thing I will check is float level again, but I am pretty sure that all four were just level with the seam.

 

I believe 550 level should be 1-3 mm below seam.

 

Had a similar problem that was fixed when I adjusted the floats to proper level. Might want to double check those.

 

Bogging-out would be too Rich.
Too much Fuel for the Off-idle Mixture to Ignite.

Based on a Plug Read ... you would see evidence that they are too Rich
Probably fouled or wet. Too dark.

But, you may not be a whole turn away from where they need to be.
I'd try adjusting them IN an 1/8th of a Turn.

 

OK - so pulled carbs off AGAIN!!! - checked float levels and PERFECT ! exactly 2mm from the seam on all 4 carbs. Emulsion tubes were spotless. Changed pilot screws a half turn and made NO difference to the running.

In fact look at these plugs, mixture seems to pretty much right on all 4 cylinders - perhaps a little rich but nothing alarming. So why on earth does the bike struggle when I open the throttle full. I can pull all the way to 8000 rpm by not over opening the throttle but open too far and ...cough, strangle, bog..bog.... bog ... I am not going any faster than 100k (60mph) mate !!!

TCI - I gather this controls timing advancement on higher RPM, but it cant be that or I would never would hit 8k rpm
Air Filter - brand new
Vacuum leak - none wasted half a can of propane - cough cough - looking for a leak
sliders and springs - perfect - drop clunk like a bag of wet concrete like RicKco says
Petcock - cant be does the same on PRI RES every setting
faulty coil
Fuel flow rate ?? Hmm would an inline filter improve it? A petcock that is not built for the flow rate that a 550 needs on full throttle? But PRI seems to flow out on the garage floor like Niagara falls.
Aaargh - I love a challenge
Any other suggestions guys (and gals)

 

With those Plugs looking as good as they do ... you might want to check to see if your Diaphragm Pistons are free to risie and fall within the Bores.

A hesitant, binding, sticking or lazily responding Diaphragm Piston will rob you of the Main Jet Fuel Supply until it manages to rise within the Bore and raise the Needle to supply the aneeded Fuel.

Easily enough checked.
Remove the Tank and the Tops from the Carbs
Safely secure the spring and Diaphragm Units aside.
Raise the Piston and release.

It should be EASY to Rise and Fall within the Bore.

If not. Scrub the Bore with a Fine ScotchBrite Pad (Gray)
Shine the Inside Diameter until the Piston has NO resistance!

See if that changes the way you are running a little bit!

 

ive known this happen in race engines where the spark has been weak, opening the throttle is too much for it and it splutters...quick free test is to close the gaps on each plug to just a few thou or 0.1 of a mm...then see if theres a difference. This is purely for indication purposes.

 

Oh Boy
New plugs made no difference nor adjusting them - carbs off again- this time spent an hour polishing the slides.... they drop like a safe from the empire state building now. No joy - made no difference... I am more convinced that it is fuel flow now. Opening wide it gets lean and slows the bike down, opening gently I can go all the way to 9000 rpm. Theory - bad petcock or an obstruction in the fuel feed lines to each carb? Once again carbs off i suppose - any other ideas....?

 

No WAY would I have my carbs off that often. I cry just thinking about taking them off once (cause they probably need to be cleaned) let alone 4-5 times that you have now!! Im impressed.

 

Be sure that the Fuel lines from the Petcock to the Inlet on the Carbs are not pinched or kinked when you have the tank down and secured.

A bent fuel line will restrict the gravity feed and starve the bike for fuel.
If you think that the Fuel Line is kinking ... stop by Radio Shack and get some tight fitting spiral Cable Wrap ... and wrap the Fuel Line with it.

The Cable Wrap will resist kinking and keep the Fuel Flowing easier.

 

My bike bogs when you hit it, when the fuel is low.
Running it up gradaully works OK, but you can hear the change in the motor from normal runnig, it gets deeper, more throaty.
I have less then a 1/2 gallon in the tank when this happens.
Switching to resesrve doesn't help much.
I put the inline filter on the bike this year.
My thinking is that with gravity feed, when fuel gets low there is not enough pressure to supply the carbs through the after market filter.
Add a gallon of gas and it clears up almost immediately.
Maybe this helped.

 

It's possible that the filter IS to blame.

I just had a filter which wasn't at all free-flowing. The material was a paper that didn't pass fuel rapidly enough to keep the supply in the Fuel Bowls adequate for the Engine's demand.

That I couldn't "Blow through" the filter was my clue that the filer was to blame.
But, also having an Air Bubble in the filter is another cause for alarm.
The bubble wants to be filled with Fuel ... but there is nowhere for the air in the filter to escape.
The consequence is akin to "Vapor Lock" and the Fuel won't feed through the filter because the filter isn't primed ... or, as I alluded to before ... not porous enough to keep-up with the demand of the Carbs.

 

Thanks Rick,
I will check the filter it is the stone type and when the bikes not running there is air in the chamber after the filter.
I figured the gas just ran out into the bowls with none coming in behind it.

 

Update - So once again I had the carbs off - I hate having this kind of unresolved problem...... And this time rather than just spraying the various little brass bits and pieces with cleaner I went at things in a more physical way. Removing each emulsion tube I then drove a piece of small gauge wire through every single little hole in the tubes, until I could spin the emulsion tube on the wire.... Put the whole rack back together again and BLAM all of a sudden things got better , but not perfect. On the side streets I am good, pulls through all the way now, but on the highway doing 100kmh and more, it struggles again, like not enough fuel.

So I think I would like to understand from the experts (Very grateful for your input guys) - exactly how the induction process works, so attached is a very amateur drawing of the cross section of the emulsion tube. My questions are , if the small holes in the tube needed cleaning with some force, then the small hole labeled A in the valve in the float chamber needs to be really scrubbed out as well right? its a pretty small hole that I would think is vital to be the correct aperture size to ensure good fuel flow? or perhaps given a good scratch out with a tiny drill? Also how does fuel flow happen here ? When you pull back on the throttle the needle rises and pulls fuel directly from chamber C, so whats the point of having the chambers B (orange) and little holes if the fuel is not supplied through there, or maybe its drawn through their and refilled through A ?? its an odd design and requires some understanding of liquid flows and physics I am sure. There does not seem to be a route from the float chamber to the outside of the emulsion tubes (orange area in picture) other than going out and then back in through the holes... The fact that the top set of holes is bigger than the bottom 3 is a clue right? is fuel delivered only through those holes?

 

Fuel is NOT delivered through those holes.
AIR is!
The outside diameter of the Emulsion tube is linked via an Air Passage to the Main AIR Jet.
As the Needle is withdrawn ... More AIR is allowed to pass through the little holes surrounding the Tube.

The AIR's mission is two-fold:
1. It passes through the ports and helps draw-up the Fuel supplied by the Main Fuel Jet.

2. The spacing of the holes "Breaks-up" the stream of Fuel and atomizes it for entering the swift stream of Intake air drawn into the engine.

The misconception that those holes meter FUEL is widespread and not understood.

Air from the Main AIR Jet is drawn through a passage which allows the Emulsion tube to be surrounded by AIR.
The Air is NEEDED to allow the Fuel to get drawn up.
The holes are uncovered as the Needle Jet is pulled-up by the Diaphragm Piston
As more holes are allowed to bleed AIT into the center of the Emulsion Tube ... more Fuel is drawn-up through the center.
As the Fuel is drawn-up ... the air holes break-up the stream so as not to sent a steady stream of gas into the head.

 

That's a great explanation Rick, but not to add too much confusion, these carbs have the flow dynamics of a jack hammer, not a garden hose. The vacuum draw is only about 25% of the time, I would suspect the fuel is sprayed after the airflow shuts down also, due to momentum. Just as the fuel flow lags behind initial airflow. Due to momentum.

 

Which is exactly why you want your Carbs to pass the "Clunk Test"

Having Diaphragms that rise and fall with the variations of Manifold Vacuum is paramount to having a Bike that will allow you to tune it up right.

I have seen Carbs so oxidized that the Pistons seem welded shut.
Welded half-way.
Frozen solid.
Needles stuck in the Emulsion Tube.
Emulsion Tubes clogged and corroded shut.
"Growth" surrounding the E-Tubes making them impenetrable.

Pushing the Emulsion Tubes out and getting them cleaned is often "Overlooked" by sellers wanting to get a bike sold and advertising the bike as "Recently Cleaned Carbs"

 

Breakthrough...... but Rick, Micarl and other experts reading this - what a blunder I have made. Rebuilding the engine I spent hours putting the Cams back on , carefully ensuring that I followed the Clymer step by step...

by eliminating all possibilities that were obvious, took well tuned and cleaned carbs off the other bike that ran superbly, then took coils, tank, TCI etc. from the good running bikes, I zeroed in on the only thing left, the top end. When I checked with the engine at TDC look what I found !\\

I have the timing off by a notch, a degree or two !!! - my question is - What damage have I done? both Cams are at the same position... about 2 degs off Next step is pull the tensioner, loosen the chain and move it back a tooth right? I am sure I checked and double checked but there is no way that the chain has stretched or shifted right?

 

You might come out of this clean.
If you were 180 - Out you would have clashed Valves and Piston Crowns.

A degree or two might not have caused any damage at all.
You would have KNOWN --> Right Away --> IF there was an Impact of the Valves to the rising Piston.

Normally, after taking the Head or Cams out ... you walk the engine through a few cycles and LOOK for the Timing Marks to be lined-up.

THAT is why you NEED a Manual and NOT a CD.
If you were following the Instructions in a Factory Manual ... you would have come to a step where they tell you to rotate the engine by hand and look for the Marks to line-up after a couple of revolutions to PREVENT being off a tooth and NOT being Timed right.

I doubt you did anything terrible.
I think you would have known (or heard and felt) any clashing.

So, ... THIS Time:
Set the Cams and Crank for their timing marks.
Softly bring the Crank around for two revolutions and see that the marks are lined-up after the engine is turned-over.

If you are ... continue buttoning it up and lets see how she runs.
If your not ... well, you don't need to go there until you find-out IF.

I hope nothing bad happened.
I think you dodged a bullet!

 

Shocked and Stunned amateur mechanic feels victorious delight in locating problem caused by amateur mechanic in the first place (me)..... WOW - what an unbelievable difference ... its a totally different bike, the engine tone is so sweet I was hyperventilating in my Shoei All along the problem was this tiny off set on the timing chain.. I think my carbs have been cleaned so many times the parts could be re-used for a cardiac implant.. Anyhow I have enjoyed 2 weeks of riding ecstacy, thanks guys for the input this was the cause all along.