Helping a GS newcomer - backfiring

[Guest]

Hey folks.

Met a guy last year at a bike event. He asked about my bike because he'd just bought a '79 GS1000 project (running). He asked if he could call for advice once he got working on it, I said "of course" and handed him my card. Couple of months ago he emailed me - I couldn't even remember what he looked like, not even vaguely, but hey, he bought a GS, I'm sure he's not a psycho.

So he's not a psycho. After I went over there to help him get it running and discovered that his carb connector tubes leaked like crazy, I talked him through the process of fixing the intake leaks (new orings) and rebuilding the carbs (that was a tough sell), while he was waiting for new connectors from Z1. He got it running OK, and called me to see about bringing over the carb tools.

It's a '79 GS1000 with an unbranded (as far as I can tell) 4-into-1 shorty. He's running pods. Both came with the bike.

We adjusted the idle air screws per the VM rebuild tut and synced today. It was way smoother at idle, and he said it pulled way better, but he was still getting significant popping and backfiring through the exhaust above half throttle.

There's a lot going on, and I'd like some help figuring out where to start. I suggested that he do some plug chops, in the hope that a fuel adjustment will be the simple answer. At the very least, it'll tell him if the problem is in more than one cylinder or which.

I also know he's got an exhaust leak at the head (missing bolt), but I'm out of my depth with the 4-into-1 and the pods in the first place.

What would you guys check first? Thanks!

[Dave8338]

You are looking for a Stage #3 kit from Z1. Pods and an exhaust of most any kind, NEED a stage #3, to even get it one the road. I have a good friend who has nearly EVERY size jet available for the mikuni carbs, and is still working on getting the mix "just right"...

[Roostabunny]

Thanks, Dave!

Hey, help me out, though. I'm confident that if he's still running the stock jets it'll be $100 well-spent, but a couple of obvious questions come to mind that'll help me talk him down out the "bike-night-only bike" tree (in other words, it looks cool but I won't ride it more than 15 miles or so). I'm asking out of my own ignorance...

1.) How do the pods and aftermarket pipe contribute to the problem we're seeing? I'm confused because I see flames with the backfire and assumed that meant too much gas/too rich. Meanwhile, I feel like less restrictive pipes and air filter would tend to lean things out.

2.) Assuming he can't any info from the PO, how to we determine if the PO already changed the jets when they installed the pipe & pods? Seems like the stage 3 comes with 138 and 142. I've never evaluated mine, are they marked on the jet? What's the stock size?


For those reading this thread months later, here's the link for the '79 GS1000 kit Dave's talking about - http://www.z1enterprises.com/Dynojet...7879-4057.aspx

[spyug]

He has two issues as I see it.
1) Air infiltration on the exhaust side causing the backfiring.
2)Unrestrictive pods causing leaness in the carbs ( as alreeady identified)

It may also be that the exhaust pipe/muffler may not offer any or at least minimum backpressure which will compound the problem.

I would start by fixing the exhaust leak. Replace the missing stud and ensure the gaskets are good. I'd next have a look at the exhaust system and insure there are no holes in it and the joint at the muffler to pipe is tight and leak free. I'd be checking the muffler too for some kind of baffle.

As far as the air induction side of things, what type of pods are being run? If they are the cheap Emgo type, they offer no air restriction and or air filtering and are really just cosmetic. Change them for K&Ns or get some Uni foam ( sold in sheets)and cut some to fit the inside of the pods. The foam does need to be oiled too to work. You may not need to change the jets at that point so try it first.

Plug chops will tell you where you stand so do try that too before slapping in new jets but if you do think they are necessary take a look at what's in there and then perhaps go up 2 sizes at a time and repeat the chops until you get it wher you want it.

Time consuming for sure but there is no quick and dirty way of dialing things in as each setup is going to be different.

Good luck with it and let us know how it goes.

Cheers,
spyug