Topic: Weak spot just off of idle on my CB550

[TwoTired]

Without and exhaust gas sniffer stuffed up the exhaust (or the wide band sensor), your best window into combustion conditions is the deposits on the spark plug tips.

I prefer starting the process with setting the main jet.  That means wot run up to top speed.  Kill switch, clutch, coast to stop and pull the plugs.  Before that, get the bike to op. temp and install brand new, or plugs with no deposits.
Read plugs, adjust main and repeat test until the plug show desired deposit patterns.

Now do 1/2 throttle setting under load runs.  Same process, read plugs, raise or lower needle to get plug deposit color as desired.   

After getting the needle in proper clip position, do runs at 3/4 throttle (you did mark your throttle grip, right)?   The color should be OK, if not you need to find a different taper needle, larger or smaller diameter for the mixture at the throttle position selected.  1/3 to 1/4 throttle position test runs follow with similar reading and needle taper adjustments.

Finally the pilot screw is set.  Ride in top gear at idle RPM.  Whack the throttle to the 1/2 position.  IF stumble, increase the idle fuel mixture to more rich.  Repeat test and adjust, until you can whack the throttle and the engine will reliably pick up RPM reliably.  (Won't be fast in top gear, though.)

If you can whack the throttle more than 3/4, to get smooth RPM increase, your idle mixture is already too rich.  Lean it out, or risk carbon fouling plugs under longer idle times.

Your done, go ride!

Easy as that! 

Cheers,

[titan joe]

Cool.

Lots of pulling the plugs. Maybe I should build a sniffer.

[titan joe]

I built a sniffer setup for my wideband, then it died. I'm not too happy with PLX. I bought a new sensor on a black Friday day for about $60, and compared.  The heater circuit is broken so the sensor is trash after less than 2000 miles! That's beside the point.

Now I welded in a bung right into the collector of my exhaust and set up the wideband sensor again.  The results were rich. The idle is dead on but the whole range is rich overall. However it is most rich at wide open throttle. That's really where it gets bad. To put some numbers to it, I'm running a lot around and AFR or 11.5 to 12. At higher engine speeds 3/4 throttle yields an AFR of almost 14, so that's not bad. Moving the needle should trim that. At wide open the AFR shoots to 10 (the bottom of the scale) and is super rich. That's also where I get misfires.

I'm not surprised since I added a lid for the airbox and got much worse running (way too rich). So, I have a simple clean up port job, no airbox lid and stock jets. I figured that the light porting would require a slightly larger main jet, but now I'm thinking that the less turbulent airflow downstream of that carburetor helps increase the vacuum in the venturis. But I think that the biggest factor is the aftermarket exhaust. It's got two chambers and a fairly small tubes connecting them. I'll pull the baffles out and see how that effects the mixture. It's pretty loud like that, but it will be informative to see the effect. Then I will likely make a new perforated tube and fiberglass packing muffler core to replace the baffles and give it a very free flowing system. Then I'll go down in jet sizes to get an AFR around 12 at wide open throttle (unless the free flow muffler make it lean out, but I doubt it).

[crazypj]

Where are float levels set?
In my experience, 'modern' fuels need at least 1mm more than factory spec and sometimes +2mm (depends on bike and carb.)
Even unleaded is usually 'engineered' for fuel injected engines with multiple O2/gas sensors. I haven't tried 'aircraft or marine unleaded they may possibly be different?

[PeWe]

Hondaman has desribed this in an old thread. Search Thoughts of Hondaman

[crazypj]

Not sure if I saw it and maybe commented?
I'm not as 'specialised' as Hondaman but we get along OK.

[HondaMan]

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.

The spark advancer's springs are now heat-annelaed and too soft. Try cutting off first 1/2 turn from one of them and try it to see the improvement: it is not unusual to have to cut 1/2 turn from the other then, and even another 1/2 turn from one of them (1 full turn) to improve this situation. It is caused by too-much ignition of the air-fuel charge during overlap cycle, by the waste spark, which burns away part of the next cycle's charge in each cylinder until the air moves fast enough (usually 1800 RPM-ish) to start getting an unburnt charge into the cylinders.

(I have 'fixed' this issue on probably a hundred 500/550 bikes, or more, and the 750 suffers it, too...)

[crazypj]

Cutting the advancer springs was a 'performance' modification' from Cycle World magazine in the 70's so don't ave any concerns about doing it. I've used Suzuki and Yamaha advancer springs but they are a bit 'stronger' than Honda ones so change advance curve.

[titan joe]

Hmmmm, I guess I should have started a new thread. I just figured why start a new one if I'm dealing with tuning this bike. I have done the spring cutting. It helped a lot! It still has a little hiccuping there, but I didn't know that sometimes you should cut twice. That's something new to try. At this point, the bike is a lot of fun to drive and I'm looking at dialing in the top end, but I'll probably cut the springs again. I was going to try that on my XL175 because it acts similarly.

I set my floats to the factory position and raising them would make things richer as far as I understand. I need to go a little leaner overall and especially at the top end. I may need to go down. I'm also running non-ethanol gas. One my cafe I saw it mess with the AFR a little bit. Maybe I'll see what a tank of that does to the mixture.

My last post is concerned only with tuning the top end and mid range. Maybe I should have started a new thread for that. Regardless, I find just about every post on here informative. I've read The Thoughts of Hondaman several times now and still glean more from it each time through. Time to read more.

[crazypj]

If you raise float level you lower fuel level as fuel shuts off earlier

[titan joe]

Haha! That true. I'm thinking right side up on the bike, but the way it's measured is essentially up side down. Now that makes sense.