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Discussion Starter · #1 · (Edited)
Edit: SOLVED - Cracked insulation on the original ignition coil leading to weak/no spark.

1970 CL350, stock everything. I have been dealing with some backfiring through the right carb and the tailpipe after a top end rebuild. At first I thought it was a fuel mixture issue so I here's what I've done:

- I sourced a replacement carburetor which I rebuilt using all the original brass, soaked in cleaner, new felt seals on the throttle shafts, oiled, etc.
- New Honda insulator + new gasket to ward off air leaks
- Adjusted valves, re-adjusted valves, re-re-adjusted valves.

And yet still it backfires, through the carb and the tailpipe. Here's the weird rub: it only seems to happen when under load (i.e. in gear). In neutral, the bike starts cold with practically a just a glance at the starter button, it idles without any dips/variation in speed, I can rev it in the driveway with no hanging revs and returns direct back to idle. And yet when I hop on, kick it into first gear, and start to pull forward, I almost immediately get a pop in the right carb. It will continue to pop while I switch up through second gear, and then seems to mostly be fine at speed (I can't bring myself to go on an extensive ride until I solve this problem, so I have no data for cruising at 40-50mph or anywhere but around the block).

Timing-wise I've got a Pamco installed and I've never felt like I've had it totally dialed in. I can get it to hit the LF and F marks exactly at idle, but then it won't advance all the way to the hash marks... Or I can set it so that both cylinders end up fully advanced but can't get it to idle slowly enough to come all the way down to the LF/F marks. A local mechanic warned me that I'd see this problem with the Pamco and that I'd have to settle one way or the other... Right now it's set at the former so that the idle is "perfect." My first plan of attack tomorrow is to advance the plate a little so that the latter is the case, and then take it for a spin.

Before diving into the top end to fix a leaky gasket I had put 500 error-free miles on the bike after rebuilding the whole engine. I just want to get back to that sweet zone. Any thoughts on helping with this diagnosis/Pamco advice? Thanks y'all!

Edit: I should also add that the left cylinder reads the same as the right using a timing light but seems to be fine; dry spark plug with a nice brown color, no backfiring... the right side is black and sometimes wet with gas, which is why I initially thought it was a carb issue. I actually haven't checked the right side plug since I put on the different carb and new insulator, I will do that tomorrow morning as well.
 

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"the right side is black and sometimes wet with gas"
Try swapping the plug lead/coil from side to side before you buy any more parts. I always set the timing to hit the marks at full advance, that's where it runs 95% of the time.
 

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Im going to stick my neck out here But I thought the only way a motor could backfire thru the carb it had to fire with the valve open. Timing problem?

Bill
I had the same thought, Bill... if he puts the points back in it and gets the timing right at full advance with a strobe, it would rule out that possibility
 

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^^+1^^^ start with the ignition.. get your points set.. from there I'm a little curious about it "under load"..
If I read the OP correct, the top of this motor has had work (gasket). I had a similar issue (no power under load) on a different TWIN motor. Started, idoled fine.. take it out.. you had to SLOWLY bring in the throttle.. otherwise it felt like something was "rubbing" not allowing the motor to increase RPMs...
This particular motor's rotor is a pressed sleeve to the magnets and I remember at one point it was "loose". I Thought maybe it has REALY come loose and it rubs on the stator under load... NOPE, wasn't it.. now what?
Went through the gammot, timing, valves, compression, leakdown test, carb leaks, no luck.
This was my first motorcycle "rebuild" (holed a piston, long story).. And it has a copper head gasket, and I knew nothing about anneiling. Cut to the chase, I pulled the head, inspected the the gasket, looked like it was leaking into each other..
Cleaned the surfaces, ANNEILED the gasket (heated), and installed....
BINGO!! That was several years ago, no issues since..
Just my $ .02..
 

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Discussion Starter · #7 · (Edited)
OK, update after today's fiddling:

First off I do not have a set of serviceable points or a condenser, so a quick swap to check is out of the question for now. When I got the bike and restored it I went straight to a Pamco and never replaced those parts.

No luck with rotating the timing plate. Still getting little pops here and there from the carb and the tailpipe, and still experiencing the most prominent of symptoms when under load (+ slight bogging when coming off idle in 1st gear).

Let me float this idea: a couple of weeks ago when fiddling with this same issue, I had the right side (problem side) high tension wire unplugged from the spark plug and I started the bike on just the left cylinder. While it was running, I saw that spark was continually arcing from the ignition coil where the lead is glued to the coil... to the bracket that the coils are mounted to. My current theory: this is not normal... would this arcing indicate a crack in the wire there and potentially cause one in every 1/5/10/20 sparks to not make it all the way to the plug when it IS plugged in? This may explain the rich condition indicated by the plug as not every cycle is being ignited... Hm...

I could test this by flipping the coils and seeing if the left side starts to act funky, but I've had the gas tank on and off again already today to triple check that the intake valve was not too tight (it was not) and I don't have the energy to do it again until tomorrow.
 

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You're getting an arc from where the spark plug wire is glued/mashed/alien gummed into the coil? I would call that a bad coil. No idea what downstream effects you'd be having, but I'd certainly call that not-desirable behavior :D
 

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OK, update after today's fiddling:

First off I do not have a set of serviceable points or a condenser, so a quick swap to check is out of the question for now. When I got the bike and restored it I went straight to a Pamco and never replaced those parts.

No luck with rotating the timing plate. Still getting little pops here and there from the carb and the tailpipe, and still experiencing the most prominent of symptoms when under load (+ slight bogging when coming off idle in 1st gear).

Let me float this idea: a couple of weeks ago when fiddling with this same issue, I had the right side (problem side) high tension wire unplugged from the spark plug and I started the bike on just the left cylinder. While it was running, I saw that spark was continually arcing from the ignition coil where the lead is glued to the coil... to the bracket that the coils are mounted to. My current theory: this is not normal... would this arcing indicate a crack in the wire there and potentially cause one in every 1/5/10/20 sparks to not make it all the way to the plug when it IS plugged in? This may explain the rich condition indicated by the plug as not every cycle is being ignited... Hm...

I could test this by flipping the coils and seeing if the left side starts to act funky, but I've had the gas tank on and off again already today to triple check that the intake valve was not too tight (it was not) and I don't have the energy to do it again until tomorrow.
I was thinking bad ignition, or sticky intake valve until I read this.:eek: However I would still go back to points if it were my bike. I do not like walking.:rolleyes:
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I will vote to get yourself back on getting the bike running well on points first before jumping on the Pamco wagon.
Otherwise you will be chasing your tail on this one.


Pamco is not going to fix a poorly tuned and poorly running engine and will only make troubleshooting it all that much more difficult.
 

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Discussion Starter · #12 · (Edited)
The culprit:



It's an overcast day here, but take my word that you can you see the wire through the insulation at that crack. That's where it was arcing to the frame when I had the boot off the plug. Replacement coils came in the mail today and I swapped them out and voila. Other than having to readjust the right side carb that was way out of adjustment since it was adjusted to the bad coil, the problem is solved.

To sum it up: cracked insulation on the ignition coil led to weak spark, led to overly rich mixture (fouled plug) + unburnt gas that backfired back through the carb and out the tailpipe.

Edit: just for fun, here's a picture of the recovering patient, cleaned up and ready for riding season here in TX. Next project is to redo the seat. My pan is all rusted out and the foam is non-existent. Someone on here messaged me that they'd sell their rear-hinge seat pan to me, check your notifications and get back to me!

 

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The culprit:

It's an overcast day here, but take my word that you can you see the wire through the insulation at that crack. That's where it was arcing to the frame when I had the boot off the plug. Replacement coils came in the mail today and I swapped them out and voila. Other than having to readjust the right side carb that was way out of adjustment since it was adjusted to the bad coil, the problem is solved.

To sum it up: cracked insulation on the ignition coil led to weak spark, led to overly rich mixture (fouled plug) + unburnt gas that backfired back through the carb and out the tailpipe.
and there it is... good detective work!
 
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