Topic: Help diagnosing CB550 cylinder 4 firing

[Deltarider]

Yep, turn the crank about 90^o further than the 'F' (static) mark and you will have the widest opening. It will look like this (pic).

[carnivorous chicken]

The point gap needs to be set at the point with the greatest opening; sorry, but I don't recall where that is compared to the timing marks..

This just wouldn't work for me. I just timed my bike, and if the point gap was at .016 there simply wouldn't be enough adjustment in the plate to get the timing anywhere near where it needed to be on idle. I am nearer to .013. These are ND points. Happy to hear if I'm doing something wrong.

[joegeis]

Okay, update time!

I reeeeeally took my time with points gap and static timing. I checked, adjusted, checked, adjusted, multiple times, getting it to a tight 0.012. I got static timing set correctly, and on the "F" marks (shown in the attached image). I got the bike fired up, and even got it to idle at ~1400 rpm! I check #4 dynamic timing and got a healthy, consistent strobe, just a little advanced - directly over the letter "F" (slightly ahead of the F mark). I checked #3 and got the same result - just above the letter "F." I did not try to adjust the dynamic timing further at this point, because I noticed a return of my cold cylinder 4 issue. I could tell the #4 exhaust was blowing cold air. Measuring temps of the exhaust manifold, #1-3 were ~160-170 degrees, but #4 was still a cool 95 degrees. Does this mean #4 is not firing?

The bike does not like to idle, the idle creeps up and doesn't like to come back down, and I've been unable to sync the carbs. There is definitely still some kind of gremlin in the system, but I'm at a loss.

1 - Is my assumption that cold exhaust manifold = no combustion correct?
2 - If so, how else can I investigate and troubleshoot?

[carnivorous chicken]

Did you try switching plugs and/or wires with #1?

[grcamna2]

How's the plug cap? Sometimes they get weak and don't fire.

[jonda500]

The point gap needs to be set at the point with the greatest opening; sorry, but I don't recall where that is compared to the timing marks..

The cam has a little dot punched in it indicating the thickest part.
John

[Deltarider]

[...]
The cam has a little dot punched in it indicating the thickest part.

Please clarify. I don't know what you mean.

[joegeis]

plug and plug caps are brand new. I swapped 1 & 4 spark plugs first, and it ran the same. Then I swapped coil wires between 1 & 4 and it ran the same. Then I pulled the cap off the #4 plug while it was running just to see if it would have any effect. The bike actually did start to idle a little worse. I replaced the plug and it picked back up to it's usual idle. Then I pulled the plug cap off #1 and the idle got quite a lot worse. #4 still is cold, hovering around 90 degrees. Here is a video of the whole ordeal. Right after this video, I opened the bowl drain on #4 carb, and it had fuel in it. What the heck could be up with cylinder #4 if it has good compression, is getting fuel, air and spark??

https://photos.app.goo.gl/Ldjazyt7PUY1iND57

[jonda500]

[...]
The cam has a little dot punched in it indicating the thickest part.

Please clarify. I don't know what you mean.

In the picture you posted above, the dot is clearly visible on the side of the points cam.
John

[Deltarider]

Yes, but that's for assembling the advancer correctly after dissassembly, to avoid assembling 180 degrees out. It does not indicate the widest opening. The position of the advancer in that photo does, some 90 rotation degrees further than the T or F mark.

[joegeis]

Today I took the carbs out, disassembled, cleaned and polished them. I found the o-ring in the #4 idle jet screw was torn, so I replaced that. I ensured I could get flow on carb cleaner and compressed air through the air tubes on the bell. Nothing seems blocked. I don't see any reason why the fuel wouldn't be getting emulsified and fed into the cylinder.

I got it all put back together and fired it up. It was able to idle at 1400 rpms without dying, but the idle still wants to hang when revved, and the #4 exhaust never got warm. I think I'm officially out of ideas. What else could be the problem here??

[scottly]

Before when you pulled the #4 plug wire, the idle did get a little worse and then got better when you replaced the wire, so the cylinder is firing, although it's not adding as much to the team as #1. This might be just a carb sync issue, where the #4 slide is set lower than the others. How does the bike run going down the road, where the sync isn't as important? Is the problem only when idling?

[joegeis]

Before cleaning the carbs, I could pull #4 plug wire and the bike would idle slightly worse but not die. Last night after cleaning, when I pull the #4 plug wire, the bike died immediately. I'm not sure if this is an improvement or not.

Scottly, good question. I have not actually taken the bike for a ride through this whole ordeal. I'll rip around the neighborhood and get it good and hot and then take some temp measurements of the exhaust header again. I'll try a vacuum sync again as well. Previously adjusting #4 had zero impact on vacuum.

[grcamna2]

Before cleaning the carbs, I could pull #4 plug wire and the bike would idle slightly worse but not die. Last night after cleaning, when I pull the #4 plug wire, the bike died immediately. I'm not sure if this is an improvement or not.

Scottly, good question. I have not actually taken the bike for a ride through this whole ordeal. I'll rip around the neighborhood and get it good and hot and then take some temp measurements of the exhaust header again. I'll try a vacuum sync again as well. Previously adjusting #4 had zero impact on vacuum.

Make sure to adjust all carbs(Synch)to match your 'base carb setting',sometimes they can 'all' be far out of adjustment and require you to bench-adjust/synch and match all the 'synchronizing screw's thread length' i.e. equal adjustment on all 4 synch screws, and confirm it visually(on the bench,by looking at/measuring each slide's height and the length of each synch screw's threads)using the idle setting thumb screw;raise & lower the thumbscrew while you are Bench synching & Matching all the synch adjusters evenly.
You may need to take a good amount of time doing this,before re-installing the carbs.

[joegeis]

I took it out for a test ride. Bike never died on me, but it wanted to idle all the way up at 3k rpm, even with the idle adjustment knob turned out all the way. When I got back to the garage, I sprayed starting fluid around the carb boots, looking for air leaks, and I found that the #4 boots between the carbs and the engine are leaking. The engine would rev up quickly when spraying there. I don't think its from the aluminum manifold, I think its the boots and clamps. I am using original clamps, but I replaced the dried original boots with ones from 4into1. While trying to tighten the clamp further, the clamp started to spin around the boot slightly. The clamp was tightened all the way and couldn't go any further, so even warm, I would think it should be tight enough not to allow it to spin at all.

I'm going to search for some better quality boots, or maybe even throw the original ones back on. Maybe I can find some way to rejuvenate them first. I'm not sure this is my smoking gun, but it definitely isn't correct!

[joegeis]

I finally got my new OEM boots and clamps in the mail. I installed those, cleaned up my bench sync (it was close, but had to lower the slides slightly). I also added the in tank fuel filter and removed the inline filter and replaced last year's gas with fresh. The bike is running so much better now! It started right up, and after running with choke for 20 seconds, it came down to a nice and steady 1100 rpm idle. All cylinders were reading the same exhaust temp, and the throttle was responsive. I sprayed starting fluid around the boots and there was no response, so my leak is gone! I'm going to ride it to the office tomorrow and see how it does. I'll give the carbs a real vacuum sync in the next few days as well.

Otherwise, it seems like I'm back in business and have no more major issues. Thanks for all the help getting this figured out!

[scottly]

Well done! If it's idling steady at 1100 RPM, I'd say your sync is good enough and leave it alone.