Topic: Weak spot just off of idle on my CB550

[titan joe]

I'm new here and new to SOHC 4s. I've worked on many 80's bikes, but I'm like most people... still learning a lot despite all I've learned.

So I got a 1976 CB550 about a month ago that's a bit of a franken-bike. It's got a 400F tank and seat, a 4 into 1 exhaust and a couple of mixed accessories.  The running gear is all CB550. In about 3 weeks I fixed up about everything (rebuilt brakes, properly mounted the tank, seat and rear fender and re-wired the bike. I also pulled the engine and freshened up the top end.

Here's what that included:
I pulled the head and jugs from the block, threw away the base gasket for about 37 thou of squish near the edge of the piston. I polished the tops of the pistons and the roofs of the combustion chamber. I cleaned up the intake and exhaust runner just by smoothing out the rough cast and matching the intake runners. There was a lot of carbon build up, so I dispensed with that. The valves lapped in very quickly, I checked for clearance with some clay (timing is slightly retarded), replaced a stuck ring I foolishly broke (and gapped it). The goal was to refine the engine a bit but not get very extreme.

The result:
The bike runs great. After break-in I've synced the carbs and have been paying attention to how it feels. It feels great overall...

There are a few minor things that I'd like to dial in. When the bike is warmed up, there is a small dead zone at around 1/8 throttle or less. It's not noticeable when cold. I'm thinking that it's a rich mixture. I've tried turning the idle mixture screw out (I assume that they are the air passage type, not fuel) and they seem to make no difference. I've only gone about a turn or two. The weak spot can be pushed through with more throttle.

The last small issue is only when warm as well. Occasionally the idle will race a bit. But not all the time. I have plenty of free play in the throttle cables, so I doubt it's mechanical problem with the controls. I have new o-rings on the intake runners. The boots look solid.

So anyone have any ideas? The points are new. I checked them a few days after the first start and the ignition timing seemed dead on still. I may give those a look again.

[Duke McDukiedook]

Up the carb needles one notch?

[BomberMann650]

The mixture screw needs to go IN to richen the mix.
If you have a timing strobe, use that to verify the advancer mechanism is returning from full advance to the F mark at idle.  Very common that it doesn't.  Old springs.
If it's not the advancer sticking.  Double check the big idle screw.  If none of that changes things.  You may have leaned the mixture too much.
Get some new plugs, set the mix screws to stock position.  Perform an idle plug chop and go from there.

[HondaMan]

Very common.
There are 2 things you should do to start:
1. Replace the O-rings in the carb castings where they bolt to the head. They don't seal anymore.
2. Cut off at least 1/2 turn of one of the springs on the spark advancer (see the "Thoughts of Hondaman" posts for more on this) to slow down the over-active advancer, caused now by sacked-out, heat-annealed springs. It advances much too soon. Delay the total advancer to 2500 RPM or even 3000 RPM to stop the overlap burnback into the intake tract, and you will be much happier with takeoff torque.

[BomberMann650]

Hondaman, have you written any deep thoughts regarding timing for "performance  enhanced" cb motors?  I would be interested in learning to tune a bit more oomph from a slightly modified motor.

[titan joe]

Great feedback!

I'm at 4500' too. Lowering the needle I'll do after the spark advancer spring adjustment. I remember reading "Thoughts of Hondaman", and thought about that, then dismissed it, oh well. I tried something like this on my ST90 years ago. Also, the intake runner seals are new, so the advancer sounds like a good place to start.

Thanks guys. Let's see what happens...

[Stev-o]

Hondaman, have you written any deep thoughts regarding timing for "performance  enhanced" cb motors? 

Hey Moo...go grab a beer, here is your reading assignment....


http://forums.sohc4.net/index.php/topic,7401.0.html

[BomberMann650]

Hondaman, have you written any deep thoughts regarding timing for "performance  enhanced" cb motors? 

Hey Moo...go grab a beer, here is your reading assignment....
http://forums.sohc4.net/index.php/topic,7401.0.html

Thanks, found what I was lookin for.

[riverfever]

I fought a bad stumble at 1/8 throttle for months on my 76 550. It was a carb issue in the end (tuning).

[titan joe]

I finally got around to modifying the advancer and then re-timed the ignition. It was firing late. I'm guessing that there may have been some initial break-in for the points. After getting the timing dead on again it seemed to run pretty well.  I was looking for a nice ride today, but the bike was hard to start and ran mostly on 1 & 4. Ugh! So something is wrong with the ignition. It improved slightly when warm, but this popped up after 2 days of sitting. I cleaned the points, but it did not go away. So I've been looking at ignition stuff since it looks like 2 & 3 are the culprit. They do fire sometimes. I'm going to go through the low voltage side and check for spark leakage on the high voltage side. The wires on the coils aren't confidence inspiring. I've read about the Hondaman ignition. This is something I've always thought should be developed. Still, I need to fix the current issue.

[TwoTired]

Swap the condensers and see if the problem follows.

If not pull the spark plugs and report what the deposit pattern is/ are.

Cheers,

[titan joe]

I'll step you through what I've done today so far...

I fired the bike up. It's missing on two most of the time. Shut it down. 2 & 3 are barely heating up, 1 & 4 are hot.

I swapped condensers, replaced the stripped out mounting screws and let the bike cool a bit.  Fire it up, 2 & 3 are hot, 1 & 4 are cold.

Sounds like a bad condenser...

So I swap them back to confirm and it runs on all 4.

I go down the the block and it start to break up.  I come back to my house and it's running good, so I do a short drive.

The problem today came an went several times but mostly was not present. The temp outside was about 50.

This is the first time I have seen the problem mostly go away.

So I'm left wondering if it was a bad condenser or if that was just circumstantial.

I'm going to look around for the old ones, but knowing my luck I never throw anything away except for the one time I shouldn't have.

[TwoTired]

Could you tell us what brand condensers you have?  Some brands appear to have QA issues.  It's possible for them to be temperature sensitive, and/or intermittent.

Personally, I think I'd pull the plugs to look at deposit patterns.  Sooty plugs, can self clean with higher running temps and stop being intermittent.

Such spark plug events can divert attention to a completely different cause.

Cheers,

[titan joe]

I regularly don't do short runs because I like to fully heat cycle the engine. I'll pull plugs next. I bought these condensers from 4into1, but they have no label, so probably bottom of the barrel Chinese. I did just check my battery and it read 12 even, so that is dead. It's on the charger now. I should have checked that earlier. I'll be putting my voltage monitor back on this bike. It never did like charging at idle, but it would get up the 13.5 on the street. What do you guys like to modernize the voltage regulator?

[TwoTired]

Wait.  You have a charging issue and immediately jump on the Vreg?  Well, maybe it would help.  But, I much prefer knowing a part is bad before spending money to replace it, perhaps needlessly.  I'm not much of a gambler.  All my bikes still have a perfectly functional stock Vreg in them.  They usually work fine, as no one had twiddled with adjustments they don't understand.
I HAVE had switch and connection issues, rectifier failures, even a bad field coil.  There's been no need to "modernize", only repair what's broke or deteriorated.

I also haven't taxed the charging system with higher wattage headlights, ignition systems, etc., to draw away power from battery charging.

Have you seen a diagnostic process, to isolate just what is wrong with your charging system?   Want to?

Cheers,

[titan joe]

Here be the plugs:





They look good to me, but I'd be interested in what more experience people think.

Wait.  You have a charging issue and immediately jump on the Vreg?  Well, maybe it would help.  But, I much prefer knowing a part is bad before spending money to replace it, perhaps needlessly.  I'm not much of a gambler.

Don't worry, I hate throwing parts things too.

It's good to hear that the regulators are very reliable and pretty much never need work.

I initially made a rectifier for the bike, but replaced it with a junk yard one that tested good. Doesn't mean it hasn't failed.

I got a Chilton with some test stuff, but if you guys have a guide for testing the charging system that you prefer I'll use that.

As a note, I looked up the stock headlight and it's listed as a 50/40W unit. I'm using an H4 which is 60/55W, so there is a greater draw there. I did that because the low beam filament was burned out, but I've considered putting an H4 LED in it's place just to lessen the electrical load and may still do that.

Now to my most recent test ride. The battery charged up and showed 12.9V on the meter before turning the key. Key one is dropped to 12.6V and revving it would get to 13V, but it seems pretty weak. After the ride revving wouldn't really take it all the way to 13V. Not good. It started like how I'm used to it starting, one kick, even while cold. The ride started good, but after just a few minutes, it broke up a bit, but then fixed itself and never came back. So I'm going to troubleshoot the charging system. This ain't over yet.

One another note, when it's not misfiring it runs the best it ever has. The timing mod does wonders for low-end torque!

[TwoTired]

I don't like the shiny deposits on the third plug over.  Plug got too hot, possibly a lean condition.  If it is flash deposited metal it could be conductive, interfering with spark, and adding to your issues.  I had one like that once that was intermittent.  Got another to replace it with?  I would, at least while you sort running issues.


Here is something I've posted a few times on the forum:

Charging system methodical verification checks, CB750, CB550, CB500, CB400, and CB350.

Begin with problem verification and characterization with recorded data.
A -- Fully charge a known good battery.  Let it rest for 2 hours, off the charger, and measure the battery voltage.  (Target is 12.6-12.8V.)
B -- Start the bike and measure the battery voltage at idle, 2000, 3000, 4000, and 5000 rpm.
C -- Repeat the measurements of B with lighting off.

The above tests identify charging system success, failure, or degree of "faulty".  The success voltages are listed in the Shop manual.

D -- Assuming the above indicates faulty, do check the RECTIFIER diodes with a diode tester or ohmmeter capable of testing diodes and uses more than .7 volts to make the test(s).
Of the twelve tests made in D, six must read low ohms and six must read very high ohms.

E- assuming no faults were found in D,  Measure the white and green wires disconnected from the REGULATOR.  CB750s should 6.8 ohms - ish,  CB550s/350s/ and 400s should read 4.9 ohms- ish.

F- If there are no bullet holes or road rash/divots on the alternator case, the stator is probably good.  But, you can check for yellow to yellow continuity (.35 ohms) among all the wires, and that no yellow wire has continuity to the engine case.

G- Assuming no faults found in D, E, and F, measure the disconnected terminals of the REGULATOR.  The black and white terminals should measure zero ohms (subtract meter error if there is any).  Higher than Zero ohms, indicates internal contact contamination needing cleaning and attention per shop manual.

H - Assuming D, E, F, and G have not found faults. We can verify all the of the charging system minus the regulator is functioning correctly, by using a temporary jumper to connect the disconnected white wire (normally attached to the REGULATOR) and connecting the White directly to the the battery POS terminal.  Repeat the B and C tests.  However, if at any time the battery voltage rises above 15V, stop the test.  Such an indication would prove the charging system capable of maintaining a known good battery.  If this test never achieves 15V, then there is a wire/connector issue in either the ground path leading back to the battery NEG terminal, a wiring/ connector issue withe the rectifier RED path to the battery POS terminal, or you made a mistake in D through G.

I - (not used, can be confused with L)

J -  The only parts that remain to prove or expose are the REGULATOR (in active mode) and the electrical path between the battery POS terminal and the black wire that connects to the REGULATOR.

K - Lying to and starving the regulator
The regulator can only do its job correctly if it gets a proper voltage report of true battery voltage status.  The Vreg monitors the Black wire for this status.  Measuring the voltage lost between the Battery terminals and the Vreg connections identifies problems that are not really the charging system's fault.
Two connection paths must be checked, the Battery POS terminal to the Black wire connection at the VReg, and the Battery NEG terminal to the Green wire connection to the Vreg.  A volt meter can measure these losses directly by placing a probe between the two identified points, Black path and then the green path.  The numbers are summed and the error seen by the Vreg quantified.  Anything over .5V loss is cause for concern and anything over 1V is a certain issue to be corrected.  Each connector, terminal, fuse clip, or switch in the pathway can cause voltage reporting loss.

The regulator also passes the received voltage on to to the Alternator field coil to create a magnetic field within the alternator,  The voltage level determines the strength of the magnetic field and the maximum output capability of the alternator.  Therefore, starving  the Vreg of true battery voltage leads to reduced max output capability of the alternator.

L - regulator operation/verification.
  The Vreg sends voltage to the alternator field in response to measured voltage which is battery state of charge.  Any voltage at the battery of less than 13.5V sends full black wire voltage to the alternator's white wire.  The alternator output will vary with RPM, even if "told" to produce max power by the Vreg.  If the alternator has enough RPM to overcome system load, any excess power is routed to the battery which will raise the battery voltage (slowly if depleted and rapidly if nearly full).   When the battery reaches 14.5V, the regulator reduces the voltage to the alternator, reducing output strength and preventing battery overcharge.  If the battery exceeds 14.7V, the regulator clamps the alternator field coil power to zero (0V), effective shutting off the alternator.

Because, there is electrical load from the system, an alternator that is not producing power allows the battery to deplete and the voltage falls.  The Vreg responds by turning the alternator back on in accordance with battery state/ charge level.

The Vreg state changes can be monitored/verified by observing the battery voltage state, and the White wire to the alternator field.  (Two meters are handy for this.)  The "trip" voltages can be adjusted with the adjust screw, while changing engine RPM and electrical load that the bike presents to the battery/charging system to "make" the battery reach the voltage levels need for the set trip points.  IE. with load reduced (lighting off) and the engine above 2500 RPM, a charged battery will attain 14.5 V.  Anything above that and the adjust screw needs to be backed out to keep the battery safe from harm.
The shop manual outlines bench set up mechanical adjustments that should be performed on unknown or tampered units.  These should be resolved before final trip point adjust tuning.

Note that while the system is working, the Vreg can change states rapidly before your very eyes., changing 5 times or more while you blink.  Therefore, you may have to mentally average values measured on the White wire if your selected meter doesn't do that for you.

[titan joe]

Cool. I got some stuff to do tomorrow. Hopefully I can get through this and a steering rack install.

[Deltarider]

I'll step you through what I've done today so far...

I fired the bike up. It's missing on two most of the time. Shut it down. 2 & 3 are barely heating up, 1 & 4 are hot.

I swapped condensers, replaced the stripped out mounting screws and let the bike cool a bit.  Fire it up, 2 & 3 are hot, 1 & 4 are cold.

Sounds like a bad condenser...

So I swap them back to confirm and it runs on all 4.

I go down the the block and it start to break up.  I come back to my house and it's running good, so I do a short drive.

The problem today came an went several times but mostly was not present. The temp outside was about 50.

This is the first time I have seen the problem mostly go away.

So I'm left wondering if it was a bad condenser or if that was just circumstantial.

Could it be that somewhere in the process the spade (what is the correct word?) connector of one of your condensers has accidentely contacted the baseplate giving you the idea the condenser was bad, where it actually was making ground?

[grcamna2]

subscribed 
 I agree w/ TT about that shiny center porcelain on the 3rd spark plug:I have had plugs like that fail to fire strongly.

[titan joe]

Today I spent some time making sure that the coils were getting a good voltage supply. On a fully charged battery I was only seeing about 10V at the coils. The battery would only drop to about 12.4V with the key on but the head light disconnected. So I cleaned the ignition switch and kill switch. They looked good and had no effect on the voltage drop so it must the the length and gauge of wire. So I ran a separate power supply line in 10 gauge wire from the battery to a 20 amp fuse to a relay stuffed above the coils that feeds the coils directly on the supply from the kill switch. Now I have 12.1V at the coils. But there was no change in performance. This time the bike seemed to be mostly running on 3 cylinders. 1 & 4 were running consistently, but 3 would run most of the time and 2 was mostly not running. The #2 plug was wet with fuel.

Then I took a break to weld a new U-joint onto a steering shaft for my car, but in the process of starting that I found the old condensers and spark plugs that I bought with the bike. The condensers are labeled TEC, so they are original. So I'm going to try swapping those back onto the bike and see what happens.  If that doesn't work, I'll put in a different plug, put the bike on a car battery and check my point setup and timing, Then go through the whole charging system.

[TwoTired]

They looked good and had no effect on the voltage drop so it must the the length and gauge of wire.

If it is the stock wire, it is not the cause.  Interconnections are far more suspect.


Cheers,

[titan joe]

I found the original condensers, swapped out the one I suspect was the problem and all my problems disappeared. So now I'm running both 42 year old condensers and smiling everywhere I go.

I've got a voltage meter on the coil feed harness I made so i can easily monitor battery voltage as I ride. When the engine is cold I'll see as high as 15V at speed.  After a little riding it drops down to 14.5V. So the output of the system seems strong. Before I added my sub-harness, I checked all the junctions for the black wire circuit and made sure that they were tight. Nothing seemed to change to voltage drop. It will probably run fine without the sub-harness, but it takes the coil load off the main and kill switch. I could easily move the headlight to the same circuit for a little more brightness.

Overall, the bike is much improved. The bottom end torque is way better and it is much nicer to take off from a standstill, but when fully warm below 1/8th throttle there is a power drop until you get past it. It's most noticeable at low engine speed.  I'm guessing that with the 4500 ft altitude here that it's swinging rich and I need to add some air to the idle circuit. I think that at that low throttle position that moving the needle clip will have little effect, so I'll try that if the screw fails me. If I were to guess, I'd  say that #1 is running a little richer than the rest, and I need to swap the #3 plug after I take another look at them.h

[Deltarider]

You never answered my question. Could it be that somewhere in the process the spade (what is the correct word?) connector of one of your condensers has accidentely contacted the baseplate giving you the idea the condenser was bad, where it actually was making ground? Have you checked this or not?

[titan joe]

Sorry, I thought the same thing and checked that quite often. The after market condensers had very wide fork tines on the terminal so I was double checking that.

I took the bike on a much longer ride on Thursday up a canyon road at 60+MPH. Now I got to see how the bike feels under large loads for long periods of time. It's not running perfect. The top 1/4 of throttle input will start to break up if held open too long and it does not like coming off of high throttle. It feels like a throttle cut instead of a throttle reduction. Also it feels a little choked instead of a steady power build at the top end. So the main jetting may be off. I still have a low power feeling at small throttle inputs. The bike has this 4 into 1 exhaust that I'm sure does not simplify the tuning, but I would have gone stock if I had the choice. Also, I'm running without an airbox lid since it never came with one. I might fashion one since that will likely mess with the resonance of the intake. I chose not to go with pods since my experience with tuning those is that it's not easy. I have a wideband oxygen sensor on my Cafe that has helped me really dial in that carb, so I might just weld a bung on this bike and use that to really see if I'm rich or lean and where. I've read a bit on here about tuning and you guy seem to like Keihin brass, and I kept all the original stuff and cleaned it. I basically used the carb kit I bought for the rubber. I did swap the idle richness screws because reasons I don't know, but I can put the old ones in easily enough.

Any thoughts? If I don't answer any questions feel free to threaten me with a large stick.