Solved - 1F w/SD40 carb stumbling at steady light throttle

[oregon fj]

So far today - original(?) coil mounted terminals down, during checks found some oil leaking out the HT terminal. Replaced coil with new one, mounted terminals up. No change.

While using the hand throttle to make it misbehave, manually changed the ignition timing, tried both advance and retard, no help either direction.

Pulled and looked at all 6 plugs, nothing of note. All 6 look clean and nice.

Used timing light to check consistency of spark while it does its thing. Spark appears to be consistent, no blips or skips evident.

Took a 30 second video in hopes that others might be able to actually hear it. Results are so-so. The skips and stumbles sound like pops in the video. Not sure this will be of any value.

 

[Pin_Head]

Sounds like a lean misfire. The popping comes from the mixture that is too lean to ignite by spark lighting when the exhaust valve opens and then the uncompressed mix igniting the mixture in the intake runner when the intake valve opens. The RPM sounds too high to be a transition fuel problem. Probably some dirt banging around in the main circuit.

I think it is time to take it apart and see if some dirt got in there somewhere. The choke may not help because the main nozzle and or the emulsion tube and main jet are obstructed and choking depends on pulling fuel from the main nozzle.

 

[oregon fj]

Well, I'll dig into it and see what I find. Odd that if the main circuit is partly blocked that it pulls hard and smooth at large throttle openings.

 

[Pin_Head]

Yes, it is peculiar. Maybe something gets plugged at low vacuum and opens up with higher vacuum. I’m just guessing. I have never seen a lean miss with no load on the engine.

 

[oregon fj]

I have never seen a lean miss with no load on the engine.

Leave it to me to find something new.

 

[oregon fj]

Any suggestions on where I should look for foreign matter in the system?

@65swb45

 

[thebigredrocker]

Are vacuum numbers good when it mis-behaves? Steady needle? Sounds like it’s stumbling a bit on my phone.

I’d be curious what happens if you manually get it in the burble position and mist some fuel with a squirt bottle.

 

[oregon fj]

Are vacuum numbers good when it mis-behaves? Steady needle? Sounds like it’s stumbling a bit on my phone.

I’d be curious what happens if you manually get it in the burble position and mist some fuel with a squirt bottle.

Don't know the numbers, I was using a borrowed gauge to set it up, will have to retrieve it to test.

Yes, it stumbles. Not great to drive that way.

Adding some fuel manually is a good idea. Ill try that tomorrow.

 

[oregon fj]

So...

Let us assume—purely for sake of argument—that despite starting to work on trail bikes in 1969, and doing my first valve job on a Ford Y-block V8 in 1973, and working on cars, trucks and motorbikes ever since, let us assume that I may well be an idiot.

If I were indeed such an idiot, I might well be embarrassed about situations which point out said idiocy. In such a case, it would be easy to let a thread like this one slowly fall off the first pages of the forum and die so that I would not have to allow people to know about my state.

That would not help any other up and coming idiots, however. So let us continue the discussion.

Removed the carb today. Stripped it completely, blew out every jet, orifice and passage. Reassembled and installed it.

There was no change at all.



As you might expect, this situation gave me reason to pause, and think.

At this point I was pretty comfortable with the thought that the carb was good, and not the issue. And if it wasn't fuel delivery, it had to be ignition. But I had ruled out most of that. Timing changes had no affect, both vacuum and mechanical advance was working, and now that I replaced the coil, everything was new.

So for lack of anything else to do, trying to feel like I was doing SOMETHING, I once again pulled the spark plug from #1. Looked much like I expected. A fresh, nearly new Autolite (only short reach plug I could get locally when I needed them). And as I looked at it, the gap looked quite small to me. Even just by eye, it looked wrong.

Factory spec (with points) is 0.8-0.9mm gap (.032-.036"). That plug was about .026". Corrected that and proceeded to check all the rest. That first plug was the widest gap of all 6. Two more were about the same, two were at .024", the last was at .022".

Started it up. No more stumble or pops. Took it for a drive, was totally acceptable, considering it is ancient tech.

I will edit the title of this thread to try to make it useful for others and searches. I hope this all might help someone else in the future.

 

[Pin_Head]

That is pretty crazy, but it had to be a crazy explanation because everything else didn't make much sense.

What else doesn't make sense is that it was running fine under hard acceleration. Electric misfires get worse the higher the air/fuel charge density and at higher RPM where the coil has less time to saturate.

You can go back to liking carbs now because it wasn't the carb's fault.

 

[oregon fj]

You can go back to liking carbs now because it wasn't the carb's fault.

No, it wasn't the carb's fault. Still don't have to like carbs, though. That's why I pay other people to rebuild them for me.

 

[oregon fj]

What else doesn't make sense is that it was running fine under hard acceleration. Electric misfires get worse the higher the air/fuel charge density and at higher RPM where the coil has less time to saturate.

Here's my guess. (Yes, it is only a guess.) I suspect that it might really be just a slight touch on the lean side at those conditions where it was misfiring. Weak spark from narrow gap + weak (lean) mixture = stumbling. Under harder acceleration the mixture must be & have been closer to correct and easier to light off even with the weak spark. Now that I have better spark, it runs OK throughout.

 

[wngrog]

Great job on the resolution!