Misfire? Honda cb750 1972 cafe racer

[dmythology]

Hi All,

I've been slowly restoring a Honda Cb750 1972. It's been a long 3 years of learning everything soup to nuts. I finally have the bike running but now can't for the life of me figure out why it misfires - very subtly - at all RPM ranges.

Here's a run down, Engine has been rebuilt - was rebored (I think to 800cc is that correct?). I have Dyna 2000 performance coils and electronic ignition. Carbs have been cleaned, and cleanned and cleaned some more. Tank is aftermarket.  Jets are a larger size then stock. What else do you need to know?

I've hooked up timing gun and can see that with every spark plug, the light skips a beat ever so often (and can hear the pop as a result).

I've checked the plugs (these are brand new iridium NKGs) and going against online example pics of normal plug coloring, they appear good although at the very base of the plug, the coloring is a bit darker than then more tannish look in the example pics.

When on the road and running up at 4500-5500 rpm, sounds great, very smooth, but who can say if it's misfiring at this range or not? When I slow down (come off the highway) and rapidly bring throttle down to idle, there is a lot of popping. Is this normal?

I'm not a mechanic- just self-taught repair guy - which is why I might not be describing this well. However, I'm pretty sure the misfiring now isn't fatal, but its annoying and one of the bugs that I'd like to troubleshoot.

Any help would be appreciated! Many times!

Thank you,
Dmythology

[TOOLS1]

Nice looking bike.
I am kind of confused, is it missing, or backfiring? Or both?
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[dmythology]

It's misfiring. Definitely

[TOOLS1]

Since you have been checking for spark with the timing light, can you tell if it is doing it on all 4 cylinders, just two, or only one. The answer to this question will really help to narrow down the problem.
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[shinyribs]

It's possible the pods are blocking the air bleeds on the carbs. But, since you can see it with your timing light, it's probably ignition rather than carburetion.

Did you check the rotor-to-pickup clearance on your dyna ignition?

[Re-run]

I would put the 8dea plugs back in, only for a bit. Tune the bike with those in instead of messing up the iridium plugs. Iridium plugs are fragile and don't take kindly to cleaning. you don't have to do this, I only say it to save you some cash since you can buy 4 copper plugs for the price of 1 iridium at most places.

Anyways, I would assume the coils, if they are dyna, have removable wires. Check these to make sure they are installed correctly and tightly. I would also check the plug caps. Make sure the ohms are not over 10000, they should be lower but we will address that later if needed.

All connections and grounds should be clean.

As shiny said, check the gap for the pickup. To far apart will cause a misfire.