Topic: Bogging, misfiring and running like trash (SOLVED!!)

[spmc]

Hey guys hoping you may be able to help me trouble shoot this.

1977 CB750f2, 4 into 1 exhaust with stock airbox and clean filter. Ran completely fine since I put it together about 3 years ago. On friday coming home from work about halfway of the 20 min commute (not highway) it started to kind of sputter a bit, almost like when you have to switch to the reserve tank when low on gas. Got a bit worse and there was some backfiring before getting home. It felt fairly similar to the way it runs in the rain when it bogs down and feels like it wants to quit unless you keep the throttle going.

I just had time to check out the spark plugs and 1, 2, and 4 looks pretty ideal to my eye, with a light tan at the top and white towards the bottom, and 3 looked fairly rich/black. Exhaust seems to have some grey smoke spurting out intermittently which is new. I also removed the airbox and made sure the boots were seated properly and nice and tight (don't think they were completely but I made sure they were and put it back together). Still ran the same way. Also checked the condensers and 1/4 (left side) shows little to no spark and 2/3 (right) I can see constant spark (similar to this but I don't think QUITE as bad

My guess is that I may have a blockage in the carbs somewhere and will pull them and clean them tomorrow, but wanted to see if the wisdom of those with more experience might think I'm wrong or have a better idea.

[seanbarney41]

couple possibilities I can think of...float is sticking or float valve leaking or jet/s may be loose, leaking, or possibly even fallen out into the bowl.  Check valve clearances too and might as well get the whole 3000 mile tune up maintenance out of the way at this point.

[spmc]

Interesting... Would a valve clearance issue show up like that dramatic a difference from ride into work to ride home?

[seanbarney41]

Not usually, but locknuts/adjusters are known to come loose

[754]

Its in the jets or the bowls..
 Not unknown to get a bit of water in gas, or some blasted into tank if you pressure wash..

[spmc]

Copy that. Thanks guys, will pull the carbs tomorrow morning and see if I can get it sorted, will update!

[754]

I think with the right screwdriver,  you can get the bowls off and some of the jets.
 Might be worth changing to Allen heads.

[HondaMan]

A dying coil can also act this way: it will act better when cold, worse when hot, and even worse yet in heavy, slow/still traffic. Also, check the resistance of the sparkplug caps: if they are the OEM caps they should be less than 8500 ohms, and within 500 ohms for both caps on the same coil. If they are marked on their sides with another number (like 5K), they should be within 93-107% of that number.

[spmc]

So pulled the carbs and gave them a good once over... nothing looked obviously blocked and they were all pretty clean on the inside. I did get that lovely pleasure of stripping the threads on carb #3 and having to retap it to an M5 screw so that was a fun little tangent.

Put it all back together, ran a little more consistent, but still intermittent backfiring, weak power, wanting to die at idle, and sounding way more throaty. I then swapped out the plugs on 2+3 (closest store I could get to only had two left) to see that was the issue, and no change.

So now I'm left confused. If it was running completely normal, then a sudden decrease in performance, and its not the fuel, spark, or air, does that mean it's probably an electrical problem?

I don't have a multimeter but I'll pick one up tomorrow and Ill get those spark plug cap readings and post what I get. Any other suggestions welcome!

[TwoTired]

Can you run the bike in a darkened room to observe for electrical arcing from the plug wires?

Cheers,

[spmc]

I could! I would just need clear instructions of what to look for where if you wouldn't mind outlining it...

[70CB750]

Can you run the bike in a darkened room to observe for electrical arcing from the plug wires?

Cheers,

Ha, replaced my cables once because of what they showed in the morning darkness.

[TwoTired]

I could! I would just need clear instructions of what to look for where if you wouldn't mind outlining it...

You are looking for arcing from anywhere along the High tension wire path to frame or engine component.  The voltage actually gets a little higher when the engine is under load conditions.  So, if you can manage to rev cycle the motor during the inspection, it would be a better test.
If you don't have a headlight switch, pull the lighting fuses.

Cheers,

[spmc]

A dying coil can also act this way: it will act better when cold, worse when hot, and even worse yet in heavy, slow/still traffic. Also, check the resistance of the sparkplug caps: if they are the OEM caps they should be less than 8500 ohms, and within 500 ohms for both caps on the same coil. If they are marked on their sides with another number (like 5K), they should be within 93-107% of that number.

So I checked out the coils, wires and caps tonight and unfortunately that doesn't seem to be the issue. Caps all come in at around 5k, each pair of wires were coming in at around 28.3 or so... the primary side of the coils were both around 5.3 as well...

So that seems a little higher than the 3-5 and 24-25 I've seen floating around but doesn't seem all that out of whack, but I really don't know too much about electrical at all.

Would LOVE any opinions/suggestions!

[spmc]

Was also wondering if it could be the timing is out of whack?

[spmc]

Well today bought a compression tester and got readings of 90, 80, 90 and 90, but I forgot to do it with the throttle wide open. Did the paper on the muffler test and it would blow out for like 4-5 seconds then sometimes suck in for the same, sometimes only blowing outwards for like 10 seconds. Also the exhaust is getting more noticeably grey as I'm doing these troubleshooting tests.

Does any of this info spark any ideas from you guys? Pretty exasperated and really don't want to have to tear the top end unless it's absolutely necessary.

[TwoTired]

Are you using a small engine compression tester, or one for V8s and big diesels?

Why didn't you get a leak down tester?  If as you say the exhaust is sucking instead of blowing, it's either valve overlap cam timing, A valve not closing, adjusted too tight, bent valve stem or burned face/ seat.

The Leak down tester would let you hear such a leak with air escaping to into the exhaust pipe.

Cheers,

[spmc]

I don't have a compressor or leak down tester so that would have to buy those. Gonna test again tonight with the throttle open and see if that changes my results. Also gonna check the valve clearances.

[BobR]

 Have you checked fuel delivery? Clogged screen in fuel valve or kinked hose? Bob

[spmc]

Have you checked fuel delivery? Clogged screen in fuel valve or kinked hose? Bob

All four bowls were full of gas when I pulled the carbs so I don't think that was the issue, and I can run it idle with the choke fully open although it's not purring like it did.

[spmc]

Throwing in the towel. I'm out of my depth. Compression test done properly gave about 135 on 1 and 4, and around 125 and 130 on 2 and 3. Valves were checked and made sure they were all in spec and I'm still having the same issue.

At the point Im extremely frustrated cause its my daily rider. Tell me where to pour the SeaFoam and I will.

[scottly]

Throwing in the towel. I'm out of my depth.

Stop, and take a few relaxing deep breaths....
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OK, now that you are relaxed and have cleared your mind, pull your spark plugs and post a picture of the business end of them.

[PeWe]

Borrow a complete ignition with coils, ht leads and caps from a working bike.

[scottly]

Borrow a complete ignition with coils, ht leads and caps from a working bike.

Why? The black #3 spark plug points to a carburetor problem, not ignition.

[PeWe]

I had a HT lead with intermittent function that behaved like carb problem. Borrow a complete set is done in a few hours and easy way to be 100% sure what is OK or not. If the problem cannot be found, verify what is OK.