Topic: Cant seem to pin down misfire

[HondaOwnerLOL]

I recently picked up a 1977 CB550k. PD46A carburetors.

After getting it home, I started to diagnose issues, namely some primary chain noise and a poor idle. I took the carburetor out to clean it, and while doing so I recognized that the number 3 carb slide was farther open than the others. I did a bench sync and that was no help, in fact, after the bench sync the bike started misfiring on that cylinder. After recognizing that one of the carb jets was not allowing as much carb cleaner through as the rest of them, I decided to do a full rebuild. After rebuilding the jets all seemed to be working as intended and I couldn't find any more issues with the carbs. Maybe due to my inexperience. The idle improved after the rebuild, but the apparent chain noise was still there and the misfire isn't gone.

I checked for spark, all 4 are good. I checked timing, It was good.

At first, I checked for compression with the throttle closed. I got 90 in all cylinders EXCEPT number 3 where I was seeing 60.

I checked compression again with the throttle open at the request of a user on the HondaCB subreddit who insisted the compression test needed to be done with throttle wide open. I got 130 across all 4 cylinders, so I guess he was right about that.

I'm pretty unsure of what to do at this point. I cant imagine what could possibly be wrong. Does any of this sound familiar? Can anyone help me out?

[HondaMan]

It sounds much like a leaking valve. They often get 'crusty' from sitting a long time, which makes them leak for a time. Interestingly enough, this issue is sort of self-correcting in all of the SOHC4 engines (least so in the CB650 series) with some miles. The fastest recovery from it comes from adding some oil to the gasoline (I use 2-stroke oil) and then running it out on the hiway for a while, like 100-mile trips or so. The 750 recovers the fastest from this, though: it takes a little longer to try this on the 500/550 (and longer yet on the 350F/400F), but I'd recommend trying that before pulling the head to go in.

[HondaOwnerLOL]

Wouldn't a leaking valve cause the compression to drop below the others? (Thanks for the quick reply btw)

[dave500]

first thing is that all four compressions read the same with the throttle open thats a good thing,the other first thing is to go over the entire ignition,check the plug caps resistance aswell,be sure the ignition is squared away first,set the tappets and cam chain,then last is the vacuum sync,going by carb slide gap gets you in the ball park,the vacuum sync really gets it set correctly.

[bryanj]

PD carbs are terrible for the pilot jets and pathways blocking, thats the pressed in ones that do need to come out

[dave500]

yeah i wasnt going to mention those PD carbs,they wont be the miss fire though.

[BenelliSEI]

I read you did your “bench sync”. Have you done running carb sync with a proper set of gauges?

If so; did you connect all four instruments to one running cylinder and synchronize them first?

Properly sync’d carbs will make the chain and clutch noises disappear.

[Deltarider]

If so; did you connect all four instruments to one running cylinder and synchronize them first?

With many gauge drum type underpressure meters this cannot be done with all four meters simultaneously. One after the other is OK.

[chiefum]

If so; did you connect all four instruments to one running cylinder and synchronize them first?

With many gauge drum type underpressure meters this cannot be done with all four meters simultaneously. One after the other is OK.

Why not use plastic tees? I do that before I sync the carbs.


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[BenelliSEI]

If so; did you connect all four instruments to one running cylinder and synchronize them first?

With many gauge drum type underpressure meters this cannot be done with all four meters simultaneously. One after the other is OK.

Why not use plastic tees? I do that before I sync the carbs.


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My Motion Pro rig actually came with a multi barb piece for doing the gauge sync. I do it first, every time

[HondaOwnerLOL]

I read you did your “bench sync”. Have you done running carb sync with a proper set of gauges?

If so; did you connect all four instruments to one running cylinder and synchronize them first?

Properly sync’d carbs will make the chain and clutch noises disappear.

Yes the carbs are properly synced. My current issue is the misfire in cylinder 3.

[HondaOwnerLOL]

PD carbs are terrible for the pilot jets and pathways blocking, thats the pressed in ones that do need to come out

I soaked the carb bodies in pine sol and shot carb cleaner through AND shot compressed air through, which cleaned up the other carbs just fine. Maybe I need to redo it? The jets are all brand new.

[dave500]

i have an old v8 powered car with a 4 barrell carb,i just hook the guages all tee piped up together and use the carbs solid and steady vacuum advance port to check the guages,i can unwind the dampers and no shaky!

[Deltarider]

I'd like to see a pic of that vacuum gauge set. Once when I had all 4 on one port, I experienced interference between the chambers (drums?) inside them, causing the dials to show different pressures. When I later tested my set with a stabil, not pulsating vacuum pump, they all 4 proved perfectly calibrated. Hence my advice not to test them simultaneously, not dynamical, on one port, but one after the other. Or joined on one stabil, not pulsating vacuum pump.

[dave500]

a single port from a single cylinder will pump and pulsate unless you increase rpm and operate the damper valves somewhat?,hooking it up to a steady smooth vacuum source like on my V8 car its stable even at idle,mine are dedicated vacuum only,unlike some cheaper types these dont measure pressure as well,they also read left to right,or counter clockwise,some might think that sucks?(PUN INTENDED)

[chiefum]

I'd like to see a pic of that vacuum gauge set. Once when I had all 4 on one exit, I experienced interference between the drums inside causing the gauges to show different pressures. When I later tested my set with a stabil, not pulsating vacuum pump, they all 4 proved perfectly calibrated. Hence my advice not to test them simultaneously on one exit, but one after the other.

It's an old pic, bit I think this is it.

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[dave500]

Yay real Honda guages,with real Honda factory fitted vacuum in them,that dont suck!

[Deltarider]

Am I the only one having experienced that unwanted interference with all connected to one and the same port simultaneously? Mind you, the 4 meters proved perfectly balanced when tested afterwards.

[newday777]

Am I the only one having experienced that unwanted interference with all connected to one and the same port simultaneously? Mind you, the 4 meters proved perfectly balanced when tested afterwards.

I never tried to do the 4in1 ports. Always off of 1 carb or use a Mighty Vac.

[BenelliSEI]

Am I the only one having experienced that unwanted interference with all connected to one and the same port simultaneously? Mind you, the 4 meters proved perfectly balanced when tested afterwards.

No issues for me. I use an older Motion Prro unit, 4 X glass pipes / blue liquid style.

Back to the poster’s question. He replaced “all the jets”. What brand did you use? Aftermarket brass is notoriously poor quality and could be a problem.....