help diagnosing intermittent stumble - t3

[yackee]

Hi all,

Hi guys, I'd greatly appreciate any advice on where to start looking to help fix this problem:

I just got back from a great 1,000 mile trip on the back roads of Wisconsin & Minnesota (Hwy 12 from Madison to the border is particularly nice). My T3 ran great, until about mile 700, when I noticed a very short "stumble" (loss of power) when cruising at a steady 60-65 MPH up a hill. It felt like it was running out of gas, or lost spark. About 20 minutes later, another stumble, lasting not more than a second. The rest of the trip it would periodically stumble, almost always as I was holding a steady throttle at 60-65 MPH, 4,000 or so RPM. Sometimes it would go 5 minutes before stumbling again, sometimes 15 minutes. By the last leg of my trip, it seemed to be doing it more frequently, and sometimes it would stumble for 3 or 4 seconds at a time; I would just ride it out and it would then resume running smoothly. I can't "make" it stumble, it just happens unpredictably, except that it didn't seem to be doing it as much at low speeds when I was in town, mostly just when I was cruising at highway speeds. The bike continued to accelerate without problem, though the idle was pretty low at stop lights (perhaps unrelated). The problem is also unrelated to going over bumps (i figured if it was a loose wire, a bump would make it more likely to happen, but it never seemed to "cut out" when I hit a bump).

It's not "bad gas" as it happened across three different fill-ups; All of the ignition components (points, condensers, coils, diode board) have been replaced in the past couple of years (the alternator and regulator are new and the bike charges fine, though I suppose that is irrelevant). I thought maybe it was related to the high heat yesterday, but it was doing it again this morning when temps were in the 70s.

I would be very appreciative of any theories as to what might be going on. The riding season is short here in Wisconsin, and I would like to get it sorted out so I can enjoy the next couple of months!

Cheers,

Jason

[Groover]

I would suspect a failing coil. They act up when they get hot whereas they may work at cooler temps if they are starting to fail. I don't know of a good way to test a coil, maybe someone else can pitch in. Do you have the original oil-filled style or do you have the epoxy insulated type? Not that it matters, just curious.

[fotoguzzi]

you don't still have those metal covered plug caps do you? get new NGK caps and how are the plug wires? check that they are seated well in the coils.

[acogoff]

     The first check I would make is the female end of the wires that go between the points and coil at the coil. They get corroded and brittle with age. Even if they look good, I would crimp on some new ends there just for grins.

[yackee]

Thanks for the replies so far guys.

I have the oil-filled coils, they were replacements bought from MGCycle; the plug wires are new-ish and the caps are NGKs.

If it is the coils, is there a modern alternative, like a Dyna coil, that would work with points and that are recommended over the stock ones?

[Groover]

If it's the red cap coils from MG, they're actually wax filled.  Nothing wrong with that, but one could have failed. I don't know if going with the epoxy style will be any better as far as reliability goes. Someone else can pitch in. Check the current ones you have for leaks. If they've leaked, you'll find wax around the front of the engine.

[Chuck in Indiana]

Ok, this is going to sound off the wall, but drain your float bowls. If there is a little water in one, it'll do that.

[normzone]

Ok, this is going to sound off the wall, but drain your float bowls. If there is a little water in one, it'll do that.

He's correct - and here's something else you may find in your float bowls that has done that to me.

Very fine rust particulates coming out of your tank. Fine enough to get through the filters.

They collect in the bottom of the bowl - appearing like iron filings. Every so often some get sucked through a jet.

I wound up creme coating my tank.

[Antietam Classic Cycle]

Petcock screens and round filter screens in carbs free of debris? If inline filters are fitted, it might be time to change (or remove) them. Could be partially plugged gas cap vent holes not allowing adequate fuel flow.

[normzone]

I added in line filters - you could actually watch the stuff appear in them over time, and eventually the finest pieces would make their way through the filters and into the float bowl.

[Antietam Classic Cycle]

I added in line filters - you could actually watch the stuff appear in them over time, and eventually the finest pieces would make their way through the filters and into the float bowl.

I've used them in the past, but sometimes you can have too much of a "good thing". With petcock strainers, inline filters and strainer screens in the carbs, there can be too much restriction for fuel to (gravity) flow sometimes.

Without inline filters, the amount of sediment collecting in the float bowls was unchanged.

[Chuck in Indiana]

Doesn't matter what kind of filters you have with water. It'll sit there in the float bowl and cause that occasional miss. There's a reason that you are supposed to drain the float bowl on an airplane at the annual or 100 hour inspection.. 

[yackee]

Just to (hopefully) close this out, I checked out everything that people suggested. Thanks for the tips! It looks for now like the problem was that the point for the RHS cylinder had opened up; I think the max recommended gap is something like 0.43 mm; it was at about .060. The RHS spark plug was black with soot, so I am guessing maybe the extra gap was making weak spark and letting the plug intermittently foul.

Anyway I re-set the point gap, changed both plug, and a test ride indicated no problem.

Jason

[Groover]

Good deal, thanks for the follow up. We all knew that answer, but we were just testing you to see if you could find the simple solution  - and you did, good job to look there!