Topic: 1975 CB400F Carb Synch and Jetting issue

[r6100mph]

Bike has aftermarket "Uni" type pod filters, the cheaper foam type not the K&N type and some soft of after market silencer.  Still has the 4 into 1 header pipe.  Valves within spec, and timing is good.

Cleaned the carbs thoroughly, pulled jets, valve seat and valve, all are good and clean.  Float height is correct or at least within a mm or two

Air screws 1 1/2 turns out, seems to be optimal, any more or any less, performance drops

slow jet is #40 and main jet is #90.

First the plugs looked pretty lean, light grey to almost white,  which is my first sign of running lean.

In denver so higher altitude, 5280 ft plus or minus.

Get the carbs back on and try to sync them with a morgan carbtune, which I have used delightfully before on cb350, 360, cl360 etc, etc.

I cannot get vacuum on the carbs.  So i start adjusting the slides down, and only get vacuum on the #1 and #3 cylinder.  #2 and #4, I run out of adjustment.

If I manually push the adjusters down, I can get a reading, but I have run out of adjustment.  Cannot screw them down any more.

I can get it to idle by bench synching the carbs, but then there is no vacuum reading.  

But if I adjust #1 and #3 cylinders to get a reading, the idle is completely shot.

My thoughts and question is:

Since it's running with foam pods, and after market pipe, it will be intaking more air, and expelling more air, thus needing more fuel to keep up.

So I'm wondering if the pilot/ idle jet is way too small.

In theory, the slides need to be almost closed to create vacuum, but since the idle jet is too small, it's not taking in enough fuel, and not running correct.  

I can open the slides enough to get it to idle, but then when I rev it up the rpms stay at about 3-4K.  Hence the slides being too open, and not shutting all the way.  

Does that sound about right?

When I rev it up, sounds great, so I think the main jet maybe correct or near correct size.  

I can also go the other direction, and adjust the throttle stop screw, so the rpms dont stay reved at 3-4k, but then it wont idle.

Sprayed around carb intake boots for leakage, and no change in performance.

Throttle cable is smooth, with acceptable slack, and over-run

Any thoughts would be much appreciated, because I'm stumped!  Maybe I keep trying to adjust something, that wont work and I need larger idle/ pilot jets.

Thanks!
Sorry for the lengthy post, but just wanted to include all the details

[Bodi]

Normally these run with 38 pilot jets when using less restrictive filters and exhaust. I don't know why you have low idle vacuum though, very strange. This seems backwards I know, but true.
The adjusters work in conjunction with the idle setting, which is also affected by the choke cam clearance (the choke cam will stop the throttle shaft from turning to a lower idle setting once the clearance is taken up). So if you've run out of adjustment on the individual linkages you must undo the idle screw a few turns and reset the choke cam clearance, then you will have adjustment available at the linkages.
The idle will be pretty odd if you don't get them synchronized.
Check for vacuum leaks at the carb insulators too, the O-rings get fossilized by the head heat.
90 is a pretty big main jet if you have 408cc pistons. Once you get the carbs synced do a plug chop at WOT to make sure it isn't overly rich.

[r6100mph]

Just so I'm clear on your explaination, your speaking of the throttle stop screw correct?  Unscrew that a few turns, which I have done,  and reset the choke cam clearance. 

Choke cam clearance? 

Are you speaking of the butterflies on the intake side?

So jetting numbers go....lower the number the larger diameter hole, hence larger jet size?

so a 150 would be smaller than a 90?

Also what do you mean by a plug chop?  does that mean at WOT, cut the ignition?

Thank you for the reply!  I was thinking it more something manual like resetting the choke cam clearance, and not necessiarily the jets

[r6100mph]

the choke cam is located right above the actual choke lever right?  It has a stop and nut on it?  When the choke is off, the cam is open and when the choke is on, the cam is closed.  By open and close, I mean touching the stop.

If that is it, how do I adjust it?

[Bodi]

The choke cam opens the throttle plates when you raise the choke lever to close the butterflies. The cam is between carbs 1 and 2, on the opposite end of the shaft the choke lever and linkage are on.
The choke cam adjustment is pretty hard to get to, it is under the lever from the main throttle rod to the 1/2 carb linkages. The 8mm nut locking the adjuster screw is accessible, but the screwdriver slot on the end of the adjuster screw is impossible to get at with the #2 linkage on.
You should set the idle screw so the face of the bracket and the adjacent face of the levr arm are parallel for a decent starting position that will give a bit of adjustment "lower" and lots "higher", setting each slide to just close with its linkage screw. (the two throttle rod levers are identical castings - the 3/4 side has a steel plug where the idle screw hits it and the 1/2 side one is threaded for the choke cam clearance adjuster)
Keihin jets are sized in 1/10 mm - a 110 jet has a 1.10mm hole. You need a smaller pilot jet when you use pods and a free breathing exhaust.
Yes, a WOT plug chop is just running wide open as long as you dare in top gear, pull in the clutch as you hit the kill switch. You can also do plug chops at any throttle position to diagnose mixture over the full throttle range. Use good used plugs, new ones will not show the colour very well. Champion plugs show better than NGK (but foul a lot easier as well, NGK rules for actual riding if you go with normal plugs). There are a bunch of tuning posts here, but basically you start at WOT to get the main jets correct, then work on idle to 1/4 throttle to get the pilot jet right, then look at mid-throttle to get the needle height and profile correct. You can't easily do much with the carb slide cutout but some adjustment against it is possible by modifying the main emulsion tubes.

[r6100mph]

Thank you so much!  I really appreciate the in depth explainations!  I'll give it a go tomorrow and report back on the results.

Thanks again!

[r6100mph]

Got a little more adjustment out of the slide adjusters, but cannot get vacuum still!

If I manually push down on the adjusters, i can get a reading or if I rev up the engine, i can get a reading.

If I push down on them while idling, the engine want's to die, because there is no air.

Should I turn the air screws out, thus allowing more air when idling?

I have them turned out about 3/4 of a full turn, which provides a pretty smooth transition from idle to quarter throttle. 

If I turn the out any more the bike wants to die from idle to 1/4 throttle.

I'm still thinking the carbs need a larger pilot/ idle jet, but maybe i'm way off.

Any thoughts.....please!

[tsflstb]

I'm pretty sure stock is 2 turns out on the air screws, so you have them pinched down pretty good.  If you can turn them out a bit and see what that does for your readings.

I'm about 1 3/4 turns out with a stock air filter in a slightly opened up airbox.  I just did a synch with the Morgan and had vacuum readings in line with the manual (16-24 mmHg or something like that).  I have to kind of shake or tap mine sometimes to get a reading when the steel bars stick, but you've used it enough to know that...

Kind of a tough one.  I'd triple check the float height, maybe set them to raise the fuel level a bit higher.  Try taping off part of the pods to limit the airflow just to get them synched.  I think you're on the right track with larger pilot jets.

If it's not the pods leaning you out the other thing would be poor compression across all cylinders.  Have you done a compression test on the engine?

[1timduke]

How old is your sync gauge?   Is it possible you've got a leak in the rubber hoses somewhere?

-Tim

[r6100mph]

Problem with turning the air screws back out around 1-1/2 or 2 is that is runs worse, the more I turn them out.

Turning air screws out gives more air, hence leaning out the mixture....is that right?  Or vice versa?

I have not checked the compression on the cylinders yet.  But I am going to tomorrow. 
This should give me a clear idea if the valve seals or seats are bad.  There has got to be a vacuum leak somewhere!

My sync gauge is a Morgan Carbtune which is less than a year old.  Only used 3-4 times.  Yes I did make sure the restrictors are near the engine and not the carbtune.  Hoses look good, even blew compressed air through each one, and they are clear.

So question on jets..... right now I have 90 main jet and 40 idle jet.  Stock I believe is 75 main and 40 idle. 

What are your thoughts about me starting with new jets that are stock size? 

Correct me if I'm wrong but isn't a 90 main larger than a 75 main?  If so, then maybe the 90 will be best for the pods and pipe.

Also clip position on jet needle is 2nd from top, isn't stock in the middle position?

I would like to start from stock settings and see where that goes.  Do you think that is a good idea?

I will also check the float levels again.

MAIN PROBLEM IS THAT I CAN FIGURE OUT IF IT NEEDS MORE FUEL OR AIR AT IDLE

WHEN I TURN THE SLIDE ADJUSTERS DOWN AND GET THE SLIGHTEST VACUUM READING THE REVS STAY UP BECAUSE I HAVE THE THROTTLE STOP SCREW IN TOO FAR JUST TO MAINTAIN IDLE. 

THEN ON THE OTHER HAND, IF I TURN THE THROTTLE STOP SCREW OUT, THE REVS DONT STAY UP BUT THEN THE BIKE WONT IDLE. 

I really appreciate everyones help!!!!!!  It's so great to bounce ideas of people especially if they have gone through a similar experience.

I'll keep you posted!

[tsflstb]

At your altitude it's hard to imagine jetting would be that far off with pods.  On the air screws in=less air=richer, out=more air=leaner.  A fraction of a turn makes a big difference in off idle performance there.

Stock needle position is in the middle notch.  Your current setup is one notch leaner, but that still shouldn't affect idle.  I'm not sure what the stock main jet size is, but I also have 40 slow jets. 

So the bike runs fine except for the idle issue?  Are the pods the type you oil?  It's easy to over do it and pretty much block all the airflow to the carbs also.

[r6100mph]

I would say it runs pretty good at and above 3K rpm. 

But I do not have the carbs sync'd yet.  I bench sync'd them so they are fairly close, but who knows how each cylinder is pulling.  Carbs maybe the same, but don't know if each cly is pulling the same, or creating the same vacuum.

I think that's when things switch into the main stage of the carb and use the main jet

The pods are a type of foam, no real oil, not like the K&N, I think they are basically generic UNI's. 

Not even my bike, just tuning it up for a friend.

A pic of the filters, and aftermarket mystery pipe.

Also, note, the over flow tubes are not hooked up.....would that have anything to do with it?

And I blew out the hoses that vent to outside air, so those are good.

Tomorrow I'm going to re-check valves, timing, float heights, etc, etc.

If that doesn't help, I'm going to get #75 main jets, and reset the jet needle clip to middle notch.  Hopefully that will get me started.

Thanks again everyone!

I'll be updating.

[Bodi]

With K&N pods, Kerker header/supertrapp muffler, megacycle street cam, and Yoshimura 466 kit I have 88 mains and 38 pilots.
90 sounds very large for a 408cc engine with stock cam.
Did you check the emulsion tubes for clogged air holes?

[r6100mph]

Finally got it!

I went through and double checked the valve clearances, and timing, they needed attention.

I should have checked this to begin with but I took my buddies word that they were good.

A majority of valves were to tight on clearance, thus not allowing the valves to close, and hence the vacuum leak.  

Timing was slightly off as well as point gaps.  

Took a while to sync the carbs because they all were way off, but after closing the slide adjusters, and at the same time turning air screws out to compensate for closing slides, I finally got vacuum readings on all 4 cylinders.  All be it, they are in the low range, around 12 Hgcm.  Air scews are turned out 1 1/4 turns.

After some test riding and final adjustments on the air screws, it's running very strong.  

The 90 main jet seems to run great, but have yet to pull the plugs, and examine.  I need to get it out on the open road and do a plug chop to see if they are too lean, as they looked when I first got the bike.  But that will be another day

Now i'm having some other electrical issues, which I'll probably start another thread on.  

Basically, the battery isn't charging.  I checked connections from the alternator/ strator, (which ever term is correct, please let me know), and they all read around 6 volts, except for one wire which is orange and white going to the rectifier.  that wire is reading 12 volts plus or minus.  

The battery is probably toast, but he said he has gone through 3 batteries in the past few months.  Could there be something in the electrical system that fries the battery?

Anyway, thank you everyone for the great help!  It was frustrating, but so rewarding to see vacuum readings, and pulling strong from idle.

Keep an eye out for the electrical issue, first I have to search for similar threads or get yelled at!

[Bodi]

There are probably several hundred relevant posts.
In a nutshell, the alternator is no bigger than it needs to be. What I mean is that on a brand new bike it will keep the battery charged if you do mostly open road riding. City driving with lots of stops will discharge the battery without exception. The alternator does not - can not - produce as much power as the ignition and lights use below about 3000rpm.
With a 35 year old bike the situation is usually bad. The wiring harness connectors and switch contacts have developed a bit of corrosion and electrical resistance. An owner has wanted a brighter headlight and installed a 55W H4 unit to upgrade from the original 35W sealed beam. Someone decided to try 3 ohm coils to improve the spark.... etc. These things need more power than the original parts and there's not nearly enough extra power available. Every extra watt of load you add moves the break-even rpm higher and closer to where there is no break-even point at all and the battery is discharging all the time regardless of engine rpm.
The regulator and restifier do occasionally fail, but you can spend a pile of cash replacing and upgrading these with no improvement if you're trying to get more power from the alternator than it can produce.
- Don't try and use a 55W low beam headlight. You can get 35W H4 lamps (35/35 or 35/65 if you rarely use high beam). Te 35W H4 bulb is much brighter on the road than the stock feeb-o-light.
- 3 ohm coils are not recommended for the 350/400 fours.
- Forget about heated grips, electric vests, driving lights, and fog lights.
- Renovate the wiring harness connectors, especially on the black wires. There are many posts about this procedure.
- Check the harness connections on the alternator yellow (stator coil) wires, these go through the engine plug, the rectifier plug, and also through bullets under the left engine cover. Probably the ones under the cover are toast, almost all the ones I have checked have the clear cover sleeve burned coal black from overheating caused by corrosion; the high temperature under the cover accelerates the corrosion. The best solution is to cut these bullets off, then solder the connections before adding a layer or two of heat shrink sleeve over the connection. You can still remove the terminals from the engine plug shell if you someday have a need to take the alternator stator off the bike. (note there is no special order for connecting the yellow wires, just connect all three to the rectifier input wires)The engine plug and rectifier plug terminals - male and female - should be cleaned, females tightened, and protected with dielectric grease.
The white and orange (? - not sure of the colour) wires are for the alternator field coil and should also have the bullets renovated or cutpff/soldered.
The rectifier is fairly easy to test with a multimeter.
The regulator is not easy to test or adjust, but is tractor simple and rarely fails. Oregonnelectric has a solid state replacement, Electrex has a replacement regulator/rectifier (you don't use the old rectifier), and there are a few posts amd websites that will tell you how to use common (and much cheaper) automotive electronic regulator modules to replace the original electromechanical one.

The usual cause of poor charging on this model is the use of an H4 55/65W headlamp.
Just about tied, or even ahead, is the harness connector problem.
Occasionally I see blown rectifiers - usually somebody jumped the battery from a car with the cables reversed.
The regulator case sometimes rusts through, letting water inside and ruining the guts. Still, failure is quite rare. The thing really is reliable - it may not do as good a job as a solid state one but it works OK.
You can measure the voltage between the battery + terminal and the black wire terminal on the regulator as an indicator of the harmess's health. There should bo less than 2 volts. More means a lot of loss in the harness, and a reduction in alternator power because the field coil can only get the voltage available at the regulator terminal... and alternator output is proportional to field coil power input... less voltage at the regulator equals less field power equals less output current equals battery discharge equals lower battery voltage equals lower voltage at the regulator... and so on.

[r6100mph]

Again, great stuff!  THANK YOU!

I see your point about the bike, just being able to keep up and charge the battery with open road riding.  For the past week, I have only been idling while working on carbs, etc.  and only a couple of short trips up and down the road.

I did find two yellow wires coming from the alternator to the rectifier, that had the insulation melted and wires were touching. 

All other connections have been cleaned, but without dielectric grease.

Does dielectric grease keep condensation and moisture out?  Is that it's main purpose?  Or does it also provide a conductive base for the actual connectors?

One thing the bike does have are LED turn signals.  But I thought LED's are more efficient in energy usage than typical incandecent bulbs?

I'm going to replace the connections where the yellow wires melted together, and heat shrink the connections. 

Then I will charge the battery, so I'm starting fresh.
Check the voltage with ignition off to see if there is any current drawing from the battery.
Then start the engine, and rev it up to 3-4K and see if there is a slight increase in voltage.

If there is an increase then it should be working correctly

Does that sound like a decent plan of attack?

Again, I really appreciate all the help!  I've learned so much in the past couple of days!

[r6100mph]

Final update:

WHen the bike was running, I could see a major consistent drop in voltage, which I was thinking it was the LED turn signals, front and rear, so I unplugged them, and voltage drop wasn't as bad as it was.

Replaced some melted together wires, and connection going to the rectifier.

I get voltage increasing with higher RPM's, maxed out volts around 13 or so.  So the charging system is working.

[Industrial Rat400f Killer]

Just in case you decide you need reg/rec I bought mine from this guy and, while I mixed up some wires when installing them and had a heck-of-a-time finding the problem, once they were in and the bike was wired right again they seem to be doing a very good job, even better than stock maybe.

Only about $120 for both.
http://www.oregonmotorcycleparts.com/index.html