GS850 need help with jetting

[Guest]

Ok, this bike is driving me nuts and i'm almost out of riding time. Can anyone help me dial this thing in?

79 GS850 - 80 cylinder head, CV carbs. BS32s, cleaned, new boots all around.
Stock airbox, K&N drop in filter. Stock 4-2 pipes. Idles at 1000, carbs synced.
New exhaust gaskets, tight bolts.

Currently 115 mains, 40 pilots, 180 pilot air, mixture screws about 3.5-4 turns out.

Symptoms -
Lots of popping, surging at about 50-65mph steady state, popping at idle, and on revving.
Full throttle, 4th gear, stops accelerating at about 7,000 rpm. Won't go any higher.
Haven't done plug chops, very hard to do that, I live on a busy hilly road, and have a long gravel driveway, and no good way to carry tools to do it on side of road. It's not running good enough yet to go far away to a parking lot or something. I can wing the throttle in neutral and get it up to redline, but not in gear moving. When I rev it up, lots of popping / backfire. When I shut the bike off after warming up or running it on the road, I get a bit of a late backfire/pop.
I know these things were a little lean new, did I just go too far with the K&N?

Had an inline filter in it, I yanked it and no change. Still popping.
My thoughts were to go up a size or two on mains, shim needles and move to a 42.5 pilot to get some adjust-ability back on idle. Mixture screws don't do much. I've never gotten idle to change much with the 40's. Z1 appears to be out of 117.5's, would 120's be too far?

[TheCafeKid]

Did you change out the head on this? You sure your cam timing and ignition timing are correct? And are your sure your points and condensors are still good (79 didn't have electronic ignition like the others) and if so, have your verified your ignition advancer is working properly.

Aside from me thinking your pilot mix screws are too far open, your jetting doesn't, from reading it, sound to be that far off. With the stock pipes and an inbox filter you shouldn't need to up-jet far, if at all. What's stock? 112.5 or 115 on that bike?
I wouldn't buy any larger jets til you're sure your spark timing and cam timing are right. Also make sure your airbox is sealed tight, and you've replaced the Orings behind the engine side intake boots. An air leak there or both will cause you problems.

Just my .02

[86turbodsl]

Yes, I changed the head. The 79 head was shot and the 80 was available. Cam timing is good, i've had a timing light on it and it advances.

I did valve timing, new orings, etc. All the usual stuff. New points and condensers, timing set with timing light.

Mixture screws i started at 2.5 and it ran terrible. More turns out helps it.

Would the jetting cause it to not be able to hit high rpms?

I've seen some threads on K&N's being overoiled, how to tell that? The oil makes it red, should it be not red?

Stock jets are 115's.

The airbox was pulled apart and new strips of foam were put in both side covers. The filter carrier had foam strips added and i was careful when I slid it in. It's an 83 airbox. It looks sealed. There's still a snorkel on it.

[TheCafeKid]

What do your plugs look like? I know you said you can't do plug chops but just taking a look at them can give you an idea of an overall read. White isn't good obviously.
You could try larger jets. Or shimming the needle or both. An easier, surer way of doing it would be a stage one dynojet kit. Even in stock form, the DJ kits help.
Are the 4-2 pipes stock or aftermarket?

[86turbodsl]

Pipes are stock. Haven't had the plugs out lately. Last time they were lean. I'll check em again and report back.

Edit: All 4 plugs look about the same, white electrode, white / clean insulator, slightly dark / metal outer thread area.

If the air filter was over oiled, wouldn't it be rich / sooty? So does this mean the stock jetting is insufficient on this bike?

[Steve]

Stock pilot fuel jets are 42.5, that's probably why it feels lean.

With those jets in, a stock bike (like two of mine here) will use about 2.25 turns out on the mixture screws. Your K&N filter might need just a bit more.

.

[86turbodsl]

Shouldn't the main jets be lean then too? I ordered some 120's just now from Z1. 117's are out of stock.

[Steve]

I have 120s in my son's '81. It has a K&N insert in the stock airbox and a 4-into-1 pipe.

I don't think the insert alone would require much in the way of a main jet change, but your bike might be different.

.

[86turbodsl]

Ok, I actually got to do a full throttle plug chop. I dumped the tools out by the road and went about a 1/4 mile down the road, checked nobody was coming, and gave it heck up to my driveway. Cut ignition and coasted into the driveway. Yanked 1 and 4 right out by the road. Both white electrodes and clean insulators. I'm betting it's very lean. It's got 115s in it now, and I'm getting 120's delivered tomorrow.

Another thing that may make my bike wierd: It's a 79 engine with an 80 head and CV carbs, but it still has the 79 exhaust with no crossovers. Maybe since the 79 pipes are different from the 80s, there's a flow difference? Dunno, but i'll report back after i swap to 120 mains, and 42.5 pilots. I figure it needs it since the screws are out at 4 turns now.

I'll say one thing though, it's sure popping a lot now. Anytime I open the throttle, it's a popping maniac...

[chef1366]

Put the 42.5 pilot jet in and keep the main at 115.
Then see how it runs.
Where did you get the carbs? You may have 750 jetting in there with the needle jet and jet needle.

[Big T]

Raising the needle should have more effect than bigger jets, at least to start

Do what Chef says and then raise the needles.

You sure there's no air Leaks?

[86turbodsl]

Chef,

It's got 850 parts in it. You should know. You sold them to me.

New boots on both in/out sides, airbox rebuilt with new foam strips both ends.

My understanding was that full throttle was on the jets. The plug chop was very lean. I've got a K&N in the airbox and it's the new ethanol gas, and 80's were jetted lean from the factory. Why would I think that it wouldn't be lean when the plugs look lean? Am I missing something?

[850 Combat]

Are you certain all the pilot passages are clear? In particular the pilot air from the bell mouth to the progression holes? I had a stubborn clog there once. Took me a while to figure it out. You are certain all the pasages are clear and the rubber plugs are installed in the pilots too?

[86turbodsl]

I really soaked those carbs good. I made the little matchstick tool and cleared all the passages, then sprayed with carb cleaner for good measure. Reinstalled all parts and rubber plugs too. You're the second one to ask about the pilot circuit. Is there some interaction I'm not understanding? Seems to me the airflow requirements at idle are pretty low, so the pilot fuel circuit would be equally small. The main jet and needle circuit are much larger and meant for high rpm / throttle usage. I have bad popping at speed, surging, lean looking plugs, and a 7000 rpm wall I can't get past. On the other hand, the bike does idle ok with the mixture screws out about 4 turns. So it seems to me that the bike's pilot circuit is ok, if a little small at #40. Please explain why I need to be focusing on the pilot circuit. Help me understand.

[86turbodsl]

Ok, went out and pulled off the airbox in prep for jet swaps tomorrow. While I had it off, decided to take the end caps back off and inspect. First one looked great, 4 pcs of foam with a nice imprint of the airbox all the way around. Pulled the second one and all 4 pcs gone. Floating around inside the airbox. Like they were never stuck to the lid. So I sand the lid down and cut 4 new pcs of foam, install them and both covers on the airbox. Reinstall on the bike, and now I can get over 8000 rpm, but it's still doing lots of popping and exhaust backfiring, and plugs were still reading lean. Not as lean as before, but still lean. throttle revving seems lazy. Wing it and it takes a second to take off.

I got one word for airboxes - PITA.

I'll pull the carbs in the morning and get some different jets in there and see if I can get the popping to stop.

Edit: Pulled the airbox this morning to check on the new foam seals and I can see now that the distance is greater on that side, so I added another layer of foam seal, enough that I had to shove and hold the end cap down on that side, so it should be a good seal now. Put it back on the bike and went for another WOT plug chop. The center electrode is still white, and the insulator is clean, and it's still popping and backfiring so I think it's still lean.
Jets are supposed to be here today. I think the plan is 120's, raise needle and 42.5 pilots still. If it seems rich, I'll back it out from there.