1980 GS550E not running quite right

[smilinbrad]

Bump me in the right direction please.
Work to Date: Soaked carbs, new O-rings, gaskets, fasteners, new intakes & O-rings, replacement (E-bay) coils, new plugs, new R/R (Electrolux) and battery, coil/relay mod, unified ground mod, valves adjusted (only two were tight).
I'm still getting miss fires (stumbles around 6,000 rpm) under full throttle.
Bought it this summer as non-runner w/ 6,000 miles. I'm guessing it's never run right.
Is my igniter unreliable?
It's very rideable as is but I'd like to know it's all there if I need it.
What to check next?001.jpg

[tkent02]

What does it do above 6,000 RPM? How about at almost full throttle?
How about at 9,000?
What are you using for an air filter? Which size main jets are in it? What are these Ebay coils and why were they put in?

A 550 that won't run right at 6,000 is hardly rideable.

[smilinbrad]

Stock intake box with K&N filter, stock jets (I can't read the numbers) E-bay coils from 85 GS550 (I broke the old leads trying to pull them out for replacement).
I don't think I've ever run it up to 9,000, I do most riding below 5,000 (I'm in the city)
It will go past 6,000 just not smoothly. It runs pretty rough through about 7,000 or so.
It starts up easily, warms up and idles down nicely. Runs low speed to intermediate just fine. The plugs indicate that it's running on the lean side. I did run a wire through all the jet openings so everything is clean and clear. You can tell from my work list that I'm chasing this down as well as I know how.

[hampshirehog]

My guess would be incorrect float level or a fuel tap not delivering enough petrol (clogged filter, weak diaphragm, kinked hose, perished vacuum hose etc).

[tkent02]

An inline filter not flowing enough? I agree, it's probably not getting enough due for wide open operation.

I didn't know it's possible to ride a 550 at those low RPMs. The power and the fun doesn't start until about 8,000.

[smilinbrad]

new fuel tap, new gas & vacuum lines, no inline filter

[tom203]

Check the primary resistance of these emgo coils- stock coils were 4 ohm, any lower and the ignitor might not be happy and might faulter at higher rpms.

[smilinbrad]

So I took the bike up to the local independent shop, described my problem to the owner and asked him to take it for a ride after I told him all the work I'd already done. He came back and said he's pretty sure it needs bigger jets, that it was running lean. He pulled the choke out at 6200 RPM (where it starts to miss) and he said it picked right up.
I looked around a little and my info indicates these come with 115 mains. We ordered 117 & 120 jets to try next.
Is my information accurate?

[hampshirehog]

If you're running a stock set up changing the mains will (might) only address the symptoms and not the cause.

[smilinbrad]

and that comment is helpful?

[hampshirehog]

and that comment is helpful?

What do you think is wrong with it? It's absolutely correct.

[tkent02]

What he said. If it's lean with everything stock and the correct main jet is in there, something else is wrong. Low fuel height, too much airflow, not enough fuel flow, blocked petcock, something. These bikes were lean from the factory at idle and very low power settings, but at wide open throttle they ran great, they were not too lean. A 550 is supposed to have a huge high RPM kick starting at about 7,500 - 8,000 or so. If yours doesn't you should find out why, don't just bandaid it with a fatter main jet.

Did your mechanic even check the fuel height?

[smilinbrad]

So, late last night, while battling insomnia brought on by allergy congestion, I Googled up GS550 carburetor and found some fellows method for calculating jet size requirements for modified bikes. It was his opinion that a K&N filter in a stock airbox would need main jet upsize two sizes. Hence my question: the stock size is 115?. We ordered 117.5 and 120 to start with as 115 was the only size I could find in any reference (BS32S Mikuni CV carb)

[gustovh]

Yup, you are running lean, the air filter that you are using flows more air than the stock, almost like running pods. The 117.5s should fix your problem.

V

[hampshirehog]

Why don't you take out the jets that are already in there and have a look? Somebody might have already put bigger jets in there.