Topic: Misfire when warm - cb550f

[percolate]

Us northerners are pretty happy that spring is finally showing up! Unfortunately my 76 550f has been acting up. About 3 mile into a ride it seems to start miss firing, slightly at first and progressively gets worse until the engine just won't go. Last ride out I pushed it several blocks and only after letting it sit for a bit was able to limp it home on almost full choke. I had to keep the throttle pretty wound up and it would barely go. Occasionally I would get little bursts of power when all cylinders would fire. These bursts were for half a second or so.

Typical last few rides: It cold starts just fine on full choke, choke is totally off in 10 - 15 sec? It purrs at idle and after a 5 - 7 minute warm up I hop on and go. I ride it pretty conservatively for a bit to let everything fully warm up. It runs super smoothly, and, well it's just great. It seems to have a little stumble if I'm cruising at about 3100 rpm and roll on the throttle a little bit. Whatever. But then it will start to stumble at higher rpms and pretty soon it's stumbling at any rpm and it gets hard to keep it going. If it does kill, restart is difficult and gets worse. The two days before this started happening I went through about 1-1/2 tanks of gas and everything worked perfectly.

Fuel delivery seems good? I haven't pulled the petcock and checked the screen but I have clear inline fuel filters and there seems to be ample fuel in them. (I know some of you swear againse these, maybe I'll be one of you after this.    ) I recently POR15'd the tank and everything looks spic and span inside so the problem seems to lie elsewhere.

I pulled all the plugs and they all look the same and look really good. Plugs have about 1400 miles on them, the caps are new and I clipped the wires when I installed them. I installed a HM ignition 200+ miles ago and it was running fine. Timing was set last fall just prior to winterization, I used a dwell meter as I don't have the stroby kind. Carbs were last cleaned last spring and I only run no-ox gas.

I really don't know where to look for a problem at this point and would love some input. Thanks!

[percolate]

More thoughts:

Maybe I'm lean as suggested in grumpy's similar thread?

http://forums.sohc4.net/index.php?topic=121473.0

I looked at the plugs as they were after my last unsuccessful ride so not a real broad indicator. None of the plugs were oily or wet or sooty or the like so that tells a little something. As for jetting and such, I have tintop's air filter set up, 42 slow jets as is stock and 105 mains with the needle in the middle. Maybe richen by one needle notch and see what happens? I don't want to dive into the carbs simply on a hunch of mine.

Thanks.

[percolate]

Ok, so once again I post a question and THEN do my due diligence to sort things out myself. Here's another fine example. Pulled the petcock and found a little bit of fibrous material on the screen. Clean the screen and reinstall. Next pull a float bowl to see if there's any crud in there. Yup. There is. So pulled the carb rack and will be sending it in for ultrasonic treatment this week.

Maybe my hunches are starting to be worth a little something.

Cheers.

[lucky]

Us northerners are pretty happy that spring is finally showing up! Unfortunately my 76 550f has been acting up. About 3 mile into a ride it seems to start miss firing, slightly at first and progressively gets worse until the engine just won't go. Last ride out I pushed it several blocks and only after letting it sit for a bit was able to limp it home on almost full choke. I had to keep the throttle pretty wound up and it would barely go. Occasionally I would get little bursts of power when all cylinders would fire. These bursts were for half a second or so.

Typical last few rides: It cold starts just fine on full choke, choke is totally off in 10 - 15 sec? It purrs at idle and after a 5 - 7 minute warm up I hop on and go. I ride it pretty conservatively for a bit to let everything fully warm up. It runs super smoothly, and, well it's just great. It seems to have a little stumble if I'm cruising at about 3100 rpm and roll on the throttle a little bit. Whatever. But then it will start to stumble at higher rpms and pretty soon it's stumbling at any rpm and it gets hard to keep it going. If it does kill, restart is difficult and gets worse. The two days before this started happening I went through about 1-1/2 tanks of gas and everything worked perfectly.

Fuel delivery seems good? I haven't pulled the petcock and checked the screen but I have clear inline fuel filters and there seems to be ample fuel in them. (I know some of you swear againse these, maybe I'll be one of you after this.    ) I recently POR15'd the tank and everything looks spic and span inside so the problem seems to lie elsewhere.

I pulled all the plugs and they all look the same and look really good. Plugs have about 1400 miles on them, the caps are new and I clipped the wires when I installed them. I installed a HM ignition 200+ miles ago and it was running fine. Timing was set last fall just prior to winterization, I used a dwell meter as I don't have the stroby kind. Carbs were last cleaned last spring and I only run no-ox gas.

I really don't know where to look for a problem at this point and would love some input. Thanks!

You need to tell us what kind of exhaust and what kind of intake and if it still has stock jets and stock needle position.

If you used a dwell to set points that is fine, you do not really need the strobe unless you think the advance unit is rusted solid.

[percolate]

There was gummy crud in float bowl #1 so I pulled the carbs and have opened them all up. All 4 bowls had gummy floaters in them and even a little sludge in the bottoms. Clearly a fuel delivery problem and time for a carb clean. I also removed my inline filters and routed the fuel lines up to eliminate any low spot.

Everything's apart and ready for ultrasound tomorrow (can't wait to see its gender!!). And the brass parts are in vinegar for an overnight soak. Hope to have things back together by tomorrow night.

Another lesson in checking the things you should check.   

[percolate]

Argh! I've officially crossed the frustration line with this bike.    Now on to chasing the cause for erratic spark. I pulled the carbs and tore them down for an ultrasonic (AKA magical) cleaning. They came out great as expected, did a good vinegar soak of all the brass parts - jets and such, and put everything back together. Adjusted floats as well, they were all over the place, now they're to spec.

Fired up the bike and it ran pretty smoothly until it was well warmed up, then back to the hesitation game. Blip the throttle and as it returns to idle speed it stumbles and wants to kill. Now warm, it really doesn't want to start. I let it sit for a while and started it again, this time stumbly from the beginning. I noticed pipes 2 and 3 getting nice and warm, 1 and 4 not so much. I pulled spark plug #4 and grounded it to the engine and pushed the starter button. SUPER erratic spark - no spark, faint spark, robust spark, etc etc all over the place.

My plan is to open the points cover and confirm correct timing and happy points and assuming that's all good, I guess start suspecting the coils? I have to head out for most of the evening so might not get to this until tomorrow, but am I at least heading in the right direction to diagnose this malady? How do you confidently bench test coils?

To lucky's point of information, here's what I'm running:

- tintop air cleaner contraption
- 38 slow (stock), 105 main (up from 100 stock per tintop recommendation)
- needle in middle position
- stock 4-1 header with Mac Megaphone muffler
- valves set to stock spec - 0.02" intake, 0.03" exhaust
- HM ignition module (installed about 200 miles before these problems arose)
- stock coils, new caps with wires clipped 1/4" prior to installation
- de7 plugs (or whatever that Bosch lettering is, it's the 7 kind) with about 1400 miles on them

Any pointers would be great, even the super obvious kind. It amazes me how many opportunities there are to screw things up. 

Thanks!

[Vinhead1957]

So you still have points!  The blue wire set (1 and 4). Check the points surface and try a different condenser.  They aren't that expensive and worth a shot.
It may be the HM ignition but you can eliminate it and test it that way also!

[Mooshie]

+1

Unhook HM ignition (very simple to do-though I do not think that would cause what you are saying but easy enough to do).  Clean/file points of all crud.  Gap and time.  Are the points new?  If so they will definitely need to be filed and do not hook up HM ignition until all the gunk they put on the points is burned off (about 500+ miles).  Coils rarely fail. Look at condenser. 

[percolate]

Mooshie & Vinhead - ahhhhhh. I feel a wash of calm and an easing of frustration from your words.  This was the exactly the guidance I was hoping for. I will dig into points & condenser land tonight and let you know what I learn.

Thanks!