Topic: Help with fuel height please. UPDATE

[Simon]

[Simon]

I need your opinion on the fuel height in the carburetor bowl for my stock 1975 400F.

I have decided for the first time to visually check the fuel height instead of relying only on the float setting at 21mm with the clear tube technique.

I have a rich mixture problem and believe me, this carb is CLEAN, everywhere, emulsion tube included. Needle is in the center groove as per spec. Ignition is good, D8EA plugs, new plug caps, new battery stable at 13 V, Charging at 14V+ at 3000 RPMs, new clean air filter.

It's got to be a fuel level problem... I hope...

What do you think? Too high?



I used for testing purposes only, rubbing alcohol (Isopropyl). Isopropyl is slightly denser than todays gasoline (785 kg/m3 vs 702 kg/m3) so that would mean that the reading should be slightly low in this pic. The float would be less buoyant in gasoline so the level would be theoretically higher if i had used gasoline. I tested with water first (998 kg/m3) and could not really say it gave a different reading. For your information, the leaded gasoline these bikes were probably designed for was 777 kg/m3. Unleaded gasoline was gone here in Canada in 1975 so I guess this particular bike never saw leaded fuel.

Go HABS Go

Simon

[w1sa]

Doesn't look high, if anything a bit low perhaps.....

Does it run rich at all road speeds or just when pottering around at low throttle?

Is it just on the one cylinder?

Does the corresponding spark plug have as good a spark as the rest?

Have you checked/adjusted camchain tensions, valve clearances and is the timing correct at idle and at advanced engine speed?

[Simon]

The bike runs great when cold. Choke is necessary and stays on for a bit, normal like, and a few minutes later, say 5ish, it starts to bog at takeoff to a point where I really have to rev up the engine before releasing the clutch if I want to keep the motor running. After 10 minutes, fully warm,  I can't rev it up no more. It just dies. Idle is fine and at higher RPMs it's fine. I would say the problem is at the first 1/8 throttle. Plugs tips are dry but real dark brown surrounded by dry black.

All 4 sparkplugs are identical.

All sparkplugs have the same spark. I would say good spark.

Every adjustment I can think of is spot on. Cam chain (pretty silent I might add), Valves are as per spec with no offending noise, Timing is right on the F at idle and in between the advance lines at 3000, done withe a timing gun, Carbs are synched with a mercury manometer every time I unmount the carbs, meaning often...

In the Clymer maintenance manual, there is an illustration depicting how to do the clear tube test. The illustration suggests that the fuel level should be lower than what I have. I have no idea if the illustration is accurate or not. You know... Clymer.... not the best reference in the world...



Simon

[CB500_k2]

I use this method to set my float levels.  Using gasoline my levels are 2 to 3 mm below the flange line on the carbs.  My CB 500, 400f and 350 run just fine at this level, confirmed with plug chops.  I was initially surprised to see the effect of liquid level on performance.  After I resurrected my 500 from death's door I found that it was necessary to run with some choke even after warm up.  As a last resort, I checked the liquid levels and found them to be ~1/2 inch down from the flange.  Rather time consuming trial and error resulted in the current setting.

[Simon]

K2

Do you mean 2 -3 mm below the top of the bowl (Red line) or from where it tapers off (Green line)? Not sure what «flange» really is.



Thanks

Simon

[fmctm1sw]

Are you sure your plug caps aren't leaking?

P.S.  your boys have a very tall order ahead of them on Friday...

[Simon]

The plugs caps are not leaking. Checked them in the dark and no light show... Caps are new. 5 ohm.

What's your opinion on the fuel height?

PS:

Yep, a tall order for sure. Winning against the Capitals is like winning a small Stanley Cup. I said it once and I,ll say it again, GO HABS GO!

Simon

[flybox1]

the sweet spot for my 350F is 3-4 mm's below the bowl gasket seam =  24mm's float height.
shouldnt be that much different on your 400.
21mm's, as it states in clymer is way too high

[w1sa]

Don't forget, that by raising the fuel level up to the recommended level mentioned, (without changing anything else) will most likely cause it to run even richer.  Did you also verify the jet sizes when you removed them for cleaning?

I'd also re-check the choke function,  airscrew setting and verify clean airbleed passageways. Did you try changing the mixture with the airscrew........Maybe it's slow circuit is set too rich.

[jessezm]

Isn't the float height inversely correlated with the fuel level?  As in, the higher you set the float (24mm vs. 21mm), the lower the fuel level?  In my mind's eye, the further the floats are away from the main carb bodies and into the bowl, the sooner they will cut off the fuel supply, no?  Thus, if the fuel supply is cut off sooner, wouldn't that be leaner, not richer?  Please correct me if my understanding of this is off....

When people talk about the actual fuel level in the bowls when the carbs are installed right-side-up, they're basing that on the clear-tube trick.  So when they say lower the fuel the level, you do that by raising the float height, right???

Or.....  is it different in Australia because you all are upside-down already?   ;-)

[Simon]

Thanks for the check list suggestions.

I am not measuring the float height right now, just measuring the fuel level. Very time consuming....

Jet sizes are stock Slow jet #40 / main jet #75.

Choke functions like a charm. All equal and opens/closes with no restriction.

Tested air screws at countless settings to no avail.

I am stumped.

Jessezm : that's exactly how i see it.

Simon

Simon

[CB500_k2]

Sorry to be unclear.  I measured from your red line.  What is the condition of your plugs after running?  If they are black and sooty this will confirm you are actually running too rich.

[flybox1]

OK.

remove the air filter and ride.  same issues?


i still think your fuel level is too high and you are running rich.  set ur floats to 24mm.

[jessezm]

Simon, let me direct you to this post, as I think it may help some:  http://forums.sohc4.net/index.php?topic=31376.0

If you haven't tried this yet (and forgive me if you already have and wrote it down already but I missed it...), how about new condensers???  It seemed to have something to do with the fix in the posted thread.

I'll paste the pertinent post here for convenience:

"I have now gone through all of your suggestions and the problem is so much better as to be almost but not totally cured.
1. New air cleaner:- made no noticeable difference at all to the flat spot but I now know I have the air intake just right.
2. Checked the timing and although it was OK the flash on the timing gun was slightly intermittent. So I got new points and one condenser (David Silver sent only one although I asked for 2). My  Haynes manual says that a misfire that gets worse as the engine heats up may be a faulty condenser. Fitted points and condenser and carefully re-timed and the problem seems a bit better but not gone.
3. Took out the carbs again and cleaned everything again with carb cleaner. This time I took out the main jet emulsifier tubes as recommended and they were very gummed up so cleaned them thoroughly too. By the way I tapped them out very carefully using a piece of brass modellers tube (5/32")which just fitted onto the top of the tube and did not damage the surfaces at all. Just been for a ride and the improvement is very marked.

I will replace the other condenser and give the bike a few decent runs to see if it settles down on its own. I will also check plug condition after some runs to see if this tells me anything further. Suffice to say that the flat spot is now so slight as to be barely noticeable so thanks alot for all the advice and the successful outcome."

[w1sa]

Tested air screws at countless settings to no avail

Simon

With fresh plugs in a warmed engine (choke off), you should be able to change the engine idle speed (while running), by adjusting the airscrews leaner or richer. That confirms that at least your airscrew/airbleed is actually functioning. Will the airscrew changes you make do that...?

[Simon]

Sorry to be unclear.  I measured from your red line.  What is the condition of your plugs after running?  If they are black and sooty this will confirm you are actually running too rich.

Yep black and sooty is the best way to describe them.

remove the air filter and ride.  same issues?

I only tried removing the air filter after the symptoms appeared. At that point in time, the plugs are fouled. I can't really say it did a difference apart that it would not go well at higher RPMs like it does with the filter on.

Jessezm, I read that post with much interest when searching on this forum. If only I had found dirty emulsifier tubes... They are clean as can be.

I have no condensors since I currently run Dyna S ignition. Coils are stock though.

W1sa, The idle does change when I adjust the air screws. The best setting, where the RPMs get higher and smoother is 2 turns out. Like specified by Honda.

I just finished setting the fuel level to 5 mm lower than the top of the bowl. I will try it out probably tomorrow without the carb connected to the air box (what a pain to reassemble those together) and hope for a lean result. if it is satisfactory, I will hook up the air box and filter and hope for the best.

Thanks all for your interest and please feel free to suggest an other solution.

Here is the Sphynx as it is right now.



Simon

[fmctm1sw]

Sorry to be unclear.  I measured from your red line.  What is the condition of your plugs after running?  If they are black and sooty this will confirm you are actually running too rich.

Yep black and sooty is the best way to describe them.

remove the air filter and ride.  same issues?

 would not go well at higher RPMs

Simon

Perhaps your main jets are too big?  I realize I'm the only guy I ever heard of doing this but my mains are actually smaller than stock on my 350F.  Once it warmed up it would bog at full throttle.  I run 72s in it now that I picked up from Sudco.  Put many miles on it since then...

[cookindaddy]

I'm reading this thread with interest.
When you all have fouled plugs during these performance checks (black and sooty) do you clean them before you reinstall? How?
I'm assuming that a new set of plugs have been used and that we don't have infinite budgets to just throw out the plugs and install brand new ones.

[Simon]

Since the start of the season (a few weeks), I have put in 3 new sets of D8EA. I clean them before retesting after I have changed one parameter. I first brush them off lightly with a copper wire brush and then use my trusty propane torch and heat the end of the sparkplug until there is no more black. I have done this countless times in the past on my Skidoo plugs with great success. Worked every time.

Re jetting the carb... I might just try that next....

Simon

[cookindaddy]

Thanks for the tip!

I am old enough to remember the sand blasting machines specially for cleaning spark plugs that used to be in ever corner garage.

[Simon]

I am still running rich! 

The fuel level is 5mm below the bowl gasket, maybe a little lower on #2 and I still have black plugs with the same bog, only when the motor is hot.

What can I do now?

Is there any way this problem could be electrical. I have posted before asking about a possible fault :

http://forums.sohc4.net/index.php?topic=69569.msg770795#msg770795

Did not get a straight answer about the relationship between a possible bad alternator and my problem.

Just a recap.

Stock everything except Dyna S ignition timing. Timing is good static and dynamic. Bench tested ignition coils : OK. D8ea plugs, New NGK plug caps, 5 Ohm. CLEAN carbs, stock jetting, lower than suggested fuel level meaning the float is set more than at 22mm, new clean air filter, idle screws set to specified settings and playing with them does not solve my problem, New battery, stable at 12.9V. Charging system is working like a champ. Am i forgetting something?

Bike has full power when cold, When hot, stumbles at low revs, want's to die at take off. Removing the air filter makes it go like crap, putting part choke, even worst. All the plugs are the same black and sooty, very dry.

Again, I feel I have done everything I possibly can on the carbs. Can it be electrical? Not enough spark? Bad Coils when hot? How can I test that? Bad alternator? If I get full charge at the battery, does that mean my alternator can not be the culprit?

I need help.

Thanks

Simon

[Simon]

Forgot to mention. GO HABS GO! 

[cookindaddy]

I am sure you will get good advice from the others here and I will be subscribed since I am about to come to that point in my rebuild.

I am wondering how you are checking your ignition timing?

[Simon]

Ignition timing is set with a timing strob gun. Mark aligned on the F at idle and in between the advance marks at 2500 + rpms. 1-4 and 2-3 are spot on.

Simon