Slight misfire after cleaning enriching carbs - 78 750f

[Rockwork]

Hey guys, what would cause a slight misfire by a low-level carb cleaning and enriching the mixture (2-1/2 turns)?

Beforehand the engine was running smooth, but I cleaned the jets and ports while I had the carbs off to adjust the mixture to try to get it to run cooler. I adjusted the cam chain, and the timing is spot on. Before I take the carbs back off and re-clean them in case dirt got lodged somewhere while cleaning, I am wondering if there is something else I'm missing.

Thank you for any input.

[TOOLS1]

Exactly what is it doing? Do you still have the points ignition? What spark plugs are you running?
TOOLS

[Re-run]

Well, what happens if you adjust the mix screws back? So you pulled the carbs off and pulled the mains or did you pull the mains and idle jets? Before turning the fuel screws further out, did you count the turns to seat them? Did you pull the slides out at all? Are you sure the float needles are seated properly and the float height is still correct?

I would pull your plugs and see how sooty they are. Likely you made the mix too rich. The backfiring can be caused by fuel not burned properly that then turns into hot carbon. This hot carbon can cause the next a/f charge to burn at the wrong time. Especially if you are still using points or old plugs. Points are reliable, but not particularly good at allowing coil saturation. This reduced saturation weakens your spark.

Also, never just start turning the fuel screws and hope for the best. Just as with a carb sync, you want to have your a/f tune the be balanced across the bank. Each carb is going to be slightly different. So you need a decent tach to hook to the bike and you follow the procedure in the manual. Get the bike in tune. It will be fine in tune as it was designed. If you think it should be slightly richer, then make small adjustments, 1/8 turn at a time. The fuel screw is sensitive to changes. A little goes a long ways.

[Rockwork]

TOOLS, it's just running rough. Still fires on all 4. Road 300 miles yesterday. But not as smooth as normal. Pamco ignition. Iridium plugs. The plugs have about 11K miles on them. Is it time to change?

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On Jul 26, 2016, at 8:43 AM, TOOLS1 [via Honda CB750'S] <[hidden email]> wrote:

Exactly what is it doing? Do you still have the points ignition? What spark plugs are you running?
TOOLS

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NAML

[Rockwork]

Re-run, yeah the mix might have been a little drastic. I had it at 2 turns before, but someone here suggested in another post 2-1/2 to run cooler. I can set them back. 

I pulled the mains and idles to make sure everything was clean. I pulled the slides but didn't adjust the floats this time, so I figure those are still good. The float needles might be able to be replaced. 

Makes sense about fowling the plugs. I make it less rich and see what happens. 

Thanks. 

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On Jul 26, 2016, at 9:50 AM, Re-run [via Honda CB750'S] <[hidden email]> wrote:

Well, what happens if you adjust the mix screws back? So you pulled the carbs off and pulled the mains or did you pull the mains and idle jets? Before turning the fuel screws further out, did you count the turns to seat them? Did you pull the slides out at all? Are you sure the float needles are seated properly and the float height is still correct?

I would pull your plugs and see how sooty they are. Likely you made the mix too rich. The backfiring can be caused by fuel not burned properly that then turns into hot carbon. This hot carbon can cause the next a/f charge to burn at the wrong time. Especially if you are still using points or old plugs. Points are reliable, but not particularly good at allowing coil saturation. This reduced saturation weakens your spark.

Also, never just start turning the fuel screws and hope for the best. Just as with a carb sync, you want to have your a/f tune the be balanced across the bank. Each carb is going to be slightly different. So you need a decent tach to hook to the bike and you follow the procedure in the manual. Get the bike in tune. It will be fine in tune as it was designed. If you think it should be slightly richer, then make small adjustments, 1/8 turn at a time. The fuel screw is sensitive to changes. A little goes a long ways.

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NAML

[TOOLS1]

I was thinking it might be time to change points and plugs, but that should not be the problem. Since you removed the carbs, you could have a vacuum leak. Does the idle hang when you let off the throttle? Funny thing is, you can have a vacuum leak and be too rich at the same time.
TOOLS

[Re-run]

Pamco and iridium are about as good as you can get without an optic system. 11,000 is fine for iridiums, since they go 100,000 in a car.

Since you removed the slides though, you will need to sync the carbs. I am betting if you sit at idle in neutral, there is a fair difference in noise with the clutch pulled or not.

Also, what made you think the bike was running hot? It is an air cooled bike, so it will seem hotter than a bike with water cooling. Depending on where you live, I would add an oil cooler before messing with a/f mix.

[Rockwork]

My daily riding is on the NYC streets. So we sit in a lot of stop and go traffic, with top speeds under 25 mph, when temperatures are generally greater than 85, and the urban heat island effect adds a few more degrees. Poor CB gets pretty hot and usually starts smoking, and it doesn't help when idle RPMs go from 1K to 1.5 or 1.8K. I had seen suggestions here to enrichen the mixture and use 15W- or 20W-50 oil. So I'm trying both. Today, when it was over 90 and heavy traffic CB didn't smoke and the RPMs weren't up as high. However, I will enrichen the mixture less and see what effect that has on smoothing out the firing. You are correct about the backfiring -- slight but it's there. What's an oil cooler?

BTW, my misunderstanding about the sliders. I didn't remove them. I always hate taking the carbs off and feeling like I'm runnig the risk of inviting a vapor leak like TOOLS warns.

Thanks, guys.

[Re-run]

An oil cooler is a unit that sits between the filter and engine. It has 2 connectors and uses hoses to attach to a radiator.

In hot temps, especially with stop and go, a 20-50 oil would be fine to use. A good synthetic would also work well as synthetics usually have higher temp points than regular oil does. Heavy duty oils, commonly called diesel oils, are good for this, too.

More fuel can cool things a bit, however it can be pretty stinky and make the rider stink. Since you get the backfires, Maybe turn in each screw by 1/8 and see how it goes.

[Rockwork]

I reset the mixture to where it was when it was running right (leaner) and rode it yesterday. It actually felt worse. So dirt or grime may have gotten lodged somewhere when I cleaned them. I'm taking the carbs apart and recleaning everything this weekend, and I'll replace the needle valves which I don't know if I've ever done.

[Rockwork]

Hi Guys, just want to report back. Cleaned the carbs more thoroughly. Some gook must have got lodged in one of the ports the first time. It's now running as smooth as ever, and idle is more consistent. Either a slightly leaner mixture (2-1/8 turns) or the 15W-50 oil seems to help it run cooler, and it no longer smokes from being too hot.

Thanks for your input.

[Re-run]

Well, depending on the oil you had before, some do smoke sooner than others. Partly why many of us use a heavy duty oil, commonly a diesel oil such as rotella.

Well, hard to say how turning the fuel screw would do that, so it may have just been a coincidence. Maybe time to replace the fuel filter.