Popping, f*rting, missing -- ??

[Guest]

G'day all, I need your collective wisdom and musings on this fine Australian summer day.

I've got a 1980 GS1000S, you can read about it here.

Now then, over the last few months it's started mucking me around. Once *fully* warmed up, the old girl is fine. But it takes a loooooong time to thoroughly warm up (all parallels with the fairer gender have occured to me, don't worry) and so even after 10 minutes of idling, as soon as you take off and put even a mild load on the engine, it sputters and f*arts, missing and popping out the muffler and doing a sort of "pfffftt" bacfire into the carbie(s). A few miles later it's fine. It will be fine for the rest of the day, even after it's been parked for a few hours. But the next day, we have to go through all the warming-up rigmarole all over again.

I have not been able to figure out which carb/ cylinder is doing it -- if any particular one is the culprit.

OK, time for a bit of history. The carbs were given a full clean out and sync. not that long ago. Top-end engine rebuild less than a year ago. Valve clearances all fine, set at the wider end of the recommended range.

My question is, how likely is this to be an electrical/ ignition problem? I'm wondering if it's taking time for some dodgy component, eg. a coil, ignitor box, or whatever, to get warmed up and start performing?

The bike has the original coils, and the leads & spark plug caps are the usual dodgy-looking affairs... well, consistent with being 28 years old, anyhow.

Another question is, What's the go with the resistors that allegedly reside in the spark plug caps? Do these ever play up?

So, that's the problems and my thoughts so far. All comments, prods, prompts and questions welcome. I'm sure we'll get this sorted...

Cheers,
Mike.

[DanTheMan]

Hey, Mike,
H'llo, and how'yre doin??? Sorry I can't really help you here, my 850 exhibits some of the same symptoms, I think they may disappear when I fix the boot leaks and shim the valves....This seems to predominately be the reason for all of those problems.
Haven't been to Perth since '93, is it still as much fun for us Navy blokes???:-D

[Steve]

Sounds like you might still be running a bit lean. I see from your build page that you have VM carbs, unlike the US versions that had BS (CV) carbs in '80. Try turning the lower fuel adjustment screw out another 1/4 turn to see if that helps.

Have you checked the color of the plugs? That will tell you a LOT about how the engine is running.


.

[tfb]

Thanks Steve! The standard screw settings for my 1980 GS1000S are, according to my Suzuki workshop manual, 5/8 turns out for the fuel screws and 1 3/4 for the air screws.

But I have taken your advice, and put the fuel screws out to 3/4 to see what difference that makes. It seems a lot better, but starting it from stone cold tomorrow morning will be the final test!

DanTheMan, thanks for your ideas there; I went and checked all my intake manifold clamps just in case. When I did the top-end rebuild, I put in new O-rings at the manifold/head interface, and the manifolds were still very supple with no cracks visible anywhere. (So I think they must have been replaced at some stage... original manifolds would be shot to bits by now.) But in any case, couldn't see any evidence of any leaks.

Once in a while we get boat-load of US Navy sailors turning up down the road at the Fremantle port. I think Perth and Freo are much more fun these days than 15 years ago, for sure! Better pubs, better food. And if you're a techno-freak, they've got free Wi-Fi all along the Freo cappucino strip. So you can sit there pretending to do work on your laptop ;-)

[chef1366]

I've experienced these problems with loose fitting spark plug caps or water in the fuel tank.

[tfb]

OK Gentlemen, here's the score.

I took Steve's suggestion and put the fuel screw on each carby out to 3/4 (from the 5/8 suggested in the manual). Air screws have been left at the standard 1 3/4 turns out.

Hopped on the bike this morning and not a single pop, f*rt, misfire, or anything like that... so whaddayaknow.

My only reservation is that I have tried a slightly richer mixture in the past (but achieved by turning the airscrews in slightly), and all I ended up with was plugs that fouled over the course of a couple of weeks.

So I will just see how things go. Thanks again, and if there's any further installments I'll post them here.

Cheers,
Mike.

[Dave8338]

If everything else is set correctly and in tune, 1/4 turn out should not make the thing foul the plugs. Only because I have not followed the complete project...have you gone through the "plug chop" game yet?

[tfb]

Depends what you mean by "plug chop". If you mean taking the old girl for a flat-out blast down a deserted back road somewhere, then pulling the clutch, killing the engine, and then whipping out the plugs for a peek... no, I haven't done that.

But I have taken a gander at the plugs after the 'real world' conditions of running around town, etc. and for a few weeks they looked no better or no worse than the plugs on any of my other bikes.

But then it'd start missing upon startup, and so with a quick feel of the header pipes I would pull the plug on the misfiring cylinder, and sure enough... carbon fouled. Not caused by oil-burning because it was a dry sooty look.

So that was when I leaned the mixture off a touch, using the air screws. Seems it was a little too much, even though it was only 1/8 of a turn, which I wouldn't have thought could throw things lean enough to produce the leanness it apparently did produce.

So we will see how the bike performs over the next few weeks, and when we get some cooler weather, too.

For the record, pretty-much everything on the mechanical side of things has been done... top-end rebuild (using original pistons but fresh rings, etc.), clutch basket rebuild, new plugs, clean air filter, cleaned out the carbies, new O-rings for the inlet manifolds, reconditioned 4-1 exhaust system. In fact the only thing that's not original on the engine side of things is the exhaust system.

Thanks for the input, guys. This bike is turning out to be the most challenging one I've ever had in terms of getting the carbies tuned, that's for sure. But then having air screws AND fuel screws on each carbie is bound to cause a few headaches!