Topic: Contradictory fuel mix signs?

[wildcatmahone]

Hi wonderin if any of you can shed some light on this for me. 74 CB750 surging at low rpm-plugs are black and sooty? Both are signs of a lean AND rich condition and dont know where to begin....

Stock carbs
Runnin K and N Pods.
4 into 1 RC header and megaphone.
New Petcock inline filters fresh gas (mid)
Float bowls recently set to spec.
Carbs are clean and bench synched.
Valves points cam chain just tuned last weekend.
Air bleed screws 1 1/2 turns out.
Idling at about 1500 rpm 25 psi reading on the oil gauge.
Getting plenty of gas in both lines and throttle is very responsive.
Runs WORSE with any choke.

Anything I should try to sort this? Diagnosis?

[kslrr]

At what rpm does the surging stop?

Have you done a plug chop at different throttle position and loads?

[wildcatmahone]

If I bring it up to 2 grand with the throttle stop screw it'll stop surging no plug chops yet. By the way runnin 120 mains and 40 slow jets.

[BobbyR]

You ared way too rich at idle. With Pods you would be lean. When you said the surging went away at 2K that is a tip off since you are off the idle jets and into the mains at that point. Also you said it runs badly with any choke. Someone may have accidently(like I did) or intenionallty punched out the idle jets to compensate for the carbs. Sooty plugs = rich.

[TwoTired]

There are three fuel metering devices/circuits in the carbs.  The slow or pilot circuit.  The slide needle jet circuit, and the Main jet, which also feeds the slide needle jet.

They each have dominance at certain throttle positions.  The main is dominant at 3/4 to WOT,  the slide needle is dominant at 1/4 to 3/4. And, the slow circuit is dominant below 1/4 throttle position.

They all have some bleed effects at other throttle positions when not dominant.

The slow circuit is so sensitive that is has it's own fine tune adjustment, that being the Idle Air Bleed Screws.  The factory setting in the book assumes the stock air filter and exhaust.  Change those and the factory setting is nearly meaningless.  You will have to modify this in accordance with the other modifications.

The carb must run a bit rich at idle to compensate for vacuum loss when the throttle is opened and still have enough fuel for engine pick up.  But, it should NOT be so rich as to soot up the center insulator on the spark plugs.

A restrictive air filter can make all throttle positions run richer.  Are your pods clean?

A "Bench synch" is NOT good enough to fine tune the jetting on carbs.  You MUST do a vacuum sync.  This alone may stop your "surging"  But, it won't cure sooty plugs.  And, choke will only make them more sooty.
You must also learn or relate what position your slide needles are set.

I'll bet your fuel mileage sucks.

You can check here to see what Honda thought the correct jetting was with the stock components:
http://www.motorcycleproject.com/motorcycle/text/specs.html
You'll have to identify your carb casting number, though.  (JN = Jet Needle position.)

You will likely have to start with "plug chops" at WOT to determine the main jet size.  Then continue the plug chop tests at 1/2 throttle position.  Using clean plugs.  And then finding the correct Idle Air Bleed Screw setting based on throttle response under power from low RPM.  When you can do up to 1/2 total throttle position changes and get brisk acceleration, you have it right.  Do not expect to snap the throttle open from idle and have the engine pick up.  If you can, then expect the spark plugs to soot up while idling.

Lastly, the slide needle and slide needle jet can rub on each other, causing wear.  The resulting change in dimension on either part can allow more fuel delivery and rich conditions.  When you have the carbs apart, check for this.

Cheers,

[wildcatmahone]

THx Bobby and TT for the info, I know this topic has been kicked to death on this forum. Filters are clean. I'll try to get some plug chops done and invest in the motion pro vacuum gauge with some pics as well. My mileage sucks real bad TT. Not easy in the city with all the traffic...can I go WOT in first couple of gears and still get an accurate plug chop? Where should I look for the casting mark on the carbs so I know where to take it from there. Thx for the suggestions.

[BobbyR]

As a fellow NY boy myself I am glad to help. Somebody at some point made a change. Some guys do some whacky sh1t to make pods work. Check the needle clip poistion etc. If your plugs are that sooty now, a plug chop will only tell ya what you already know. Go over everything point by point, figure out what is grossly wrong, then fine tune with your plug chops.











 

[wildcatmahone]

Cheers Bobby first think is to tear off the carbs and check the needle positions...are you comin to the vintage bike show on Sunday I posted the details up on the open forum and calender.

[TwoTired]

Filters are clean.

OK, but just to make sure you aren't fooling yourself, take the filters off, put in clean spark plugs and give it a short drive.  Check for effects of choke when cold.  If it needs more choke for cold warm up and your plugs don't soot up without the filters, your filters only look clean.  Anyway, you are going to tune the carb to whatever filters you have, and it would be unfortunate to find out afterward that the filters were a root cause of prior problems.
Your call...

I'll try to get some plug chops done and invest in the motion pro vacuum gauge with some pics as well. My mileage sucks real bad TT. Not easy in the city with all the traffic...can I go WOT in first couple of gears and still get an accurate plug chop?

The more time spent at full power output, the more likely the plug deposits will reflect combustion conditions.  I only know two ways to get this info, either a test track (or reasonable facsimile) or a Dyno.  Those are the tools for the job, a hammer, or bumper jack just won't do it, and isn't a reasonable substitute.  The alternative is to keep trying different settings/adjustments until you can tolerate the behavior. (I'd think that would take a lot longer to get right, though.)

Where should I look for the casting mark on the carbs so I know where to take it from there. Thx for the suggestions.

I don't know exactly.  But, they should have stampings to match the numbers found on the chart I referenced.

Cheers,

[wildcatmahone]

Ok I just checked my carbs definitely clean filters are only couple months old as well. Tore into the carbs to check what position my needle clip is in but cant undo the two phillips screws inside the valve....are these suppose to come out. If not how exactly do you get to the needle clip?


Had a good look down into the valve it appears that the needle clip is in the third slot down from the top. Would that be a richer or leaner setting?

[TwoTired]

Had a good look down into the valve it appears that the needle clip is in the third slot down from the top. Would that be a richer or leaner setting?

Compared to what?
If we knew what your carb stamping was, we could tell.

[wildcatmahone]

Had a good look down into the valve it appears that the needle clip is in the third slot down from the top. Would that be a richer or leaner setting?

Compared to what?
If we knew what your carb stamping was, we could tell.

Ha had to go back and check stamped 657B.

[TwoTired]

So, the chart says that they were in the 4th from the top clip position when it left the factory and had 105 mains.

Clip in third position leaned the midrange throttle position and the 120s fattened the 3/4 to WOT throttle positions.

Just a guess.  But, maybe the 120s made it so rich that the needles were lowered to keep from fouling the plug after the first mile.   Maybe...

Pretty odd though.  Usually pods require the jets/adjustments to get bigger/larger.  I'd check the emulsion tubes for clean and the Mains air jet for clear feed to the emulsion tubes.  Also, wear on the needle jet and slide needle.

Cheers,

[BobbyR]

So, the chart says that they were in the 4th from the top clip position when it left the factory and had 105 mains.

Clip in third position leaned the midrange throttle position and the 120s fattened the 3/4 to WOT throttle positions.

Just a guess.  But, maybe the 120s made it so rich that the needles were lowered to keep from fouling the plug after the first mile.   Maybe...

Pretty odd though.  Usually pods require the jets/adjustments to get bigger/larger.  I'd check the emulsion tubes for clean and the Mains air jet for clear feed to the emulsion tubes.  Also, wear on the needle jet and slide needle.

Cheers,

+2 on all TT said. It is a point by point procedure to sort carb problems out. No shortcuts, as Bob W says "carbs suck"

I did not know about the Vintage bike event, where and when?

[wildcatmahone]

So what would you reckon my next move should be apart from checking the tubes and mains...change needle clip position, go down in main jet size I got a set from 105 to 120. It is surging below 1/8th throttle forgot to mention a little bit of popping through the muffler as well thanks for all your help Bobby and TT.

So, the chart says that they were in the 4th from the top clip position when it left the factory and had 105 mains.

Clip in third position leaned the midrange throttle position and the 120s fattened the 3/4 to WOT throttle positions.

Just a guess.  But, maybe the 120s made it so rich that the needles were lowered to keep from fouling the plug after the first mile.   Maybe...

Pretty odd though.  Usually pods require the jets/adjustments to get bigger/larger.  I'd check the emulsion tubes for clean and the Mains air jet for clear feed to the emulsion tubes.  Also, wear on the needle jet and slide needle.

Cheers,

[TwoTired]

Once you know the carbs are totally clean and vacuum synched.  Your can either keep trying different settings/adjustments until you can tolerate the behavior.  Or, plug chop (or dyno test) to learn where your mixture problems are and make changes in the direction indicated.

Or, you can put on the stock air box and filter, go with original exhaust and use the factory settings to make the bike run as intended.

No one has a bike in your configuration.  So, to make it run right, you will have to determine its needs and adjust accordingly.

Cheers,