Topic: Random Sputtering

[Dr. Noisewater]

Hey guys been awhile since I've posted, been too busy enjoying the hell out of my recently finished build.. but I've had a slight hiccup.

It's a 75 CB550F. Since completion I've put about 1,000 miles on the bike, and it's been running like a champ the whole time. But the other day it started randomly stuttering on light acceleration around 3-4k rpm, but it's very seldom. It runs great 99% of the time.

During the build process everything was either rebuilt or replaced. Rebuilt & vacuum sync'd the carbs, replaced the entire ignition system w/ OEM bits, fuel tank cleaned, new fuel lines, plugs, etc etc. There were many headaches but everything got sorted out and it's done a solid thousand miles without any issues.

My initial guess was timing, so I checked it and it's still spot on. My next guess would be plugs. I haven't inspected them yet (can't find my plug tool), but I know it's been running a tad lean, it pops on deceleration every now and then. Factory air box, Uni filter, 4-1 open muffler, factory jet size. I've left the factory jets in because fellow members advised me it should be OK considering I'm still using the factory air box... debatable.

Would a lean condition cause the described symptoms? Could it possibly be the spark advancer?

[Dr. Noisewater]

Well I pulled the plugs today and according to the NGK plug chart they all look good. It wasn't a proper plug chop, but 99% of my riding is on the freeway.

I corrected a couple gaps that were slightly too big and popped them back in. I took it for a ride and it still stutters from time to time at low throttle, mid rpm acceleration. But performs great overall.

I guess I'll just keep riding and see what happens. I'll do a plug chop as soon as possible and update if I come up with anything.

[Dr. Noisewater]

Well the ride to work this morning was a nightmare. Falling on its face and backfiring the whole time. I'll keep searching but any input would be much appreciated.

It was about 30 degrees cooler this morning compared to last night's ride, which makes me lean back to a lean condition above idle..?

If something in the carbs was clogged wouldn't it do it all the time?

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[brandEn]

Sounds like it could be a loose connector in the ignition circuit someplace? I have had an issue like this before and it was a connector problem. Check all connections are good and wires are crimped tight.

[iron_worker]

Intermittent issues are often electrical. Generally, if a bike is lean in mid range it will always be lean in mid range (carb'ed bikes anyway)

Although the fact that the problem is slowly getting worse makes me wonder about plugged fuel feed or plugged jets. I know you said you cleaned the tank but it's another thing to check off the list.

IW

[Dr. Noisewater]

Both are good suggestions, thanks guys. The tank also got lined with redkote and received a brand new oem petcock with an intank filter. Maybe something still got through though..

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[iron_worker]

Take a flashlight and look inside the tank. Those liners do fail sometimes... and of course the failed liner ends up piled up over top of your petcock.

Not sure about the 550's but on the 750 you can pull a little bowl on the petcock and check the screen. I think the 550 maybe be inside the tank though?

Are you running an inline filter? Many a fuel problem has been attributed to clogged filters.

IW

[Dr. Noisewater]

No inline filter, just the one in the tank. I'll have to look at it  this evening. What do you mean a bowl on the petcock?

[iron_worker]

I don't think the 550 has one but the 750 has a bowl underneath that you can remove to service the petcock screen... so N/A to your instance. ha

IW

[Dr. Noisewater]

Well I peeped inside the tank and the filter still looks perfect. I'm curious about it being an electrical issue. It really is random. I rode it again at lunch and back home after work and it did better, but still had its moments. I cleaned all the connectors when resurrecting the bike but I'll go over the ignition system wiring again this weekend.

[martin99]

You sure you haven't got an airleak at the inlet rubbers? A loose clamp or split might cause it to behave like this. Just a thought.

My second thought would be failing condenser,  particularly if this is showing up more as the engine temp rises.

[Dr. Noisewater]

It seems to be more prominent when the engine is cool, but I wouldn't be surprised if it were a condenser. I remember now that the condensers are cheap aftermarkets, not Daiichi, but not OEM.

I sprayed the boots with starter fluid a few months ago after getting the carbs in and didn't get any change in rpm, but something could've happened since then. I'll try again this afternoon.

[iron_worker]

The 500/550s have o-rings that seal the carb spigots to the head which are prone to cracking and leaking due to age/many heat cycles. You could check there for vacuum leaks.

IW

[Dr. Noisewater]

A couple updates...

I was checking out the points to make sure everything was working as it should and noticed some brighter sparks on the 2.3 point while the engine was running. Every time it sparked it ran a little rough, then cleared up. I noticed the 2.3 point wasn't opening as much as the 1.4, maybe the points spring softens a tad after use, so I readjusted both and went for a ride. It seemed to actually smooth things out a tad, mainly the transition of setting off in first gear from a stop after the engine was fully warm. It used to take a little more revs after it got hot, but now it's a smooth transition. Anyway, it seemed really promising and once I started feeling like I'd fixed it... it stumbled. I just checked the points in the dark and couldn't see ANY sparks. Shouldn't there be a little spark?

After the ride I checked out the intake boots and manifold. I didn't have any starter fluid so I tried WD40, I hope it's strong enough to cause a fluctuation in rpm...  I didn't get anything that would indicate a leak of any kind.

Still haven't double checked all the connections along the ignition system. I plan to do that on Sunday if I manage to get some time away from the wifey's to-do list..

[trueblue]

Ideally the points shouldn't spark, but a little is normal.  Sounds to me like an electrical issue, you can get a connector that has opened up a little and not make a perfect connection.  It will cause an intermitent on/off, which will show up as a stutter.

[martin99]

I've seen a few examples where the points are wired to ground, with the terminals from the condenser and contact set put together rather than being separated by the insulators. Doesn't stop the bike running but can cause issues. Borrowed this pic from another thread (hope that's not bad form!)..



Check your wiring against this. This is how it should look.

[Dr. Noisewater]

Thanks oldskool, I'm pretty confident everything involving my points is as it should be.

I went through some of the connections today; coil wires, wires to points, fuses. I found a black coil wire that was a tad loose so I mended that and took it for a ride. No help. I didn't try anything in the headlight bucket, would anything in there affect the ignition operation?

After some searching I found some similar ideas:
- run it without the gas cap to make sure it's not a vent issue
- check float height, although I don't know how. Total carb newb.
- maybe check timing with a strobe?
- check compression. I never have, but the power seems strong enough to indicate good compression...

I hope I can get this thing back to 100% asap. If anybody wants to add their .02 to the above ideas, or suggest something else, I'm all ears.
Thanks guys.

[HondaMan]

Here's my "checklist" for the 550 bikes for this issue:
1. Measure the ohms of the primary side(s) of the 2 coils. New, they are 4.4-4.5 ohms If using a digital meter, be sure to cross the probes first to see what it thinks "0" ohms is, and subtract that from your coil readings. If one of them is more than 0.1 ohms higher than the other, that coil is beginning to fail. They act OK when cold like this, but poor when warmed up. The hotter, the worst they get.
2. Plug caps: measure them, too. They must be within 500 ohms of each other on any one coil, 800 ohms max difference. If they are the OEM caps, they should be 7500 to 8500 ohms: if aftermarket, they should be either 5000 or 10000 ohms. If 5000, they must not be more than 5800, if 10000 they must not exceed 10800 ohms.
3. Hose clamps on the rubber carb hoses.
4. The O-rings in those casting that bolt the carb hoses to the head. They are old and hard now if not changed: this makes them leak, particularly when hot.
5. Fuses. The main fuse is 15 amps, and if the clips on the OEM fuseblock are not hot when running, I'd be surprised: the chromate on most of them is dying now from age. This makes them resistive, which heats up the fuses, often melts them in the process. It also loses as much as 4 volts to the system, making the headlight dim, the blinkers slow, and the spark weak.

[Dr. Noisewater]

Thanks HondaMan, I'm gonna go poke around and see what I find.

When you say carb hoses, does that mean the boots to the manifold?

My fusebox is actually the one you make! It's not very old, but I'll check that out too.

[RJ CB450]

Hrm.  This is a bit of a distant cousin, but going to put it out anyway cause sounds possibly similar to my 650.

Anyways, at around the 3500 to 4500, very light throttle, depending on temp both of engine and outside, my engine gets slightly sputtery too.  Not enough to notice power change, but audibly different with a little bit of roughness when holding the rpm or slowly passing through.  Usually in the lower gear.  Fourth is smooth.

If this sounds like you, is a rough spot in carbs on my end.  Just a little rich at that transition point.

[Dr. Noisewater]

Thanks RJ, but my issue seems to happen whenever it feels like it. It seems to do it more when the engine is cold, or the temperature is cooler, but it's not typical cold motor behavior. It really is totally random, at least it seems that way. It doesn't seem to care where the throttle is. If this adds anything; the first time it happened was about 10 miles after going toe to toe with an xs750. He won..

Ok just did some testing and had some interesting results.

First I checked plug caps, their values were;
1: 10.8
2: 9.57
3: 14.05
4: 11.53

I'm fairly certain these are original caps, but I just remembered that I bought these coils off eBay and I'm pretty sure the caps came with them so who knows what year they're from. They looked exactly like the ones that were on my 550, but with much nicer wires.

Then I checked the coils, first testing ohms between 1/4 and 2/3 at the ignition leads with the caps off. Both rated 15k. Looks good.

Then I checked the continuity for both sets in the same places. Zero continuity through the ends of the leads. 1/4 and 2/3. What?

Then I unplugged the blue and yellow wires going into the coils and tested continuity. BEEP. Good

Then I checked the ohms on the same wires, nothing....  0.00   What? Those are the primary's and that's the correct way to test them right?

I've read about every dummy thread on here about testing coils and I'm fairly certain I was doing it right. Are my coils bad? How would I get continuity but no resistance on the primary's and vice versa on the leads? I'm confused.

[Deltarider]

Did you select the right range for measuring Ohms? Personally I wouldn't accept plugcap resistance over 8KOhms. Better stay on the safe side and select 5Kohm resistance plug caps. Resistance will always increase over time.

[Dr. Noisewater]

Thanks Delta, I had the meter set to 20k the whole time. If it picked up the 9.57 on cap 2 it should pick up the expected 5k from the primaries, or so I figure. I'm planning to order new 5k caps.

I'm still confused about the zero continuity and zero resistance issues though.

[TwoTired]

Quick diagnostic is the replace the plugs with new.  When I tried the Denso plugs in my 77, they worked ok for a few thousand miles, then went intermittently into sputter mode, even though they looked ok.  I went back to NGK D7EA, and problem solved.

On my 74, I also had similar symptoms.  This time it wasn't the plugs, but cracked insulation at the coil HT join.  I was hard to see it without really close examination.  This allowed the spark to jump to the frame, "sometimes", and make the engine sputter, usually under load.  Finally saw it in a darkened garage.  Swapped out the coil and problem solved.

[HondaMan]

Thanks Delta, I had the meter set to 20k the whole time. If it picked up the 9.57 on cap 2 it should pick up the expected 5k from the primaries, or so I figure. I'm planning to order new 5k caps.

I'm still confused about the zero continuity and zero resistance issues though.

When measuring the coil primary (Blue to Black/White or Yeloow to Black/White tests) set the meter on x1 range. This resistance is about 4.4-4.5 ohms for the 500/550 coils, or 4.5-4.6 ohms for the 750 (they look the same, will interchange).