GS650G ... where to look for a solution: lose power/fuel on highway

[Simian Major]

So stock air box and filter on my 82 Gs650.
Cleaned and re-oiled filter. Carbs were 'professionally' cleaned and adjusted about 18 months ago...
Put on new fuel line (removed an older inline filter) that now gives much better throttle response this past month.
Revs easily to 9000 in garage in neutral and in first and second when on centre stand.
Can get to 7000+ on road in first and second ... and probably third too ...
BUT
get little to no response from twisting throttle when in fourth or fifth when on highway ... indeed if at at highway speed with increasing throttle to max eventually get to loosing power (and speed) and have to release throttle and coast along shoulder (urgh!) for 20-30 seconds before throttle becomes responsive again ... (problem of not getting fuel into carbs??) then can get up to speed before the cycle (i.e. lose power/throttle response due to lack of fuel?) happens again. In fact had it once stop running completely - would turn over and eventually got it to restart by using choke just as when starting from 'cold'.

My assumptions (not that they are correct) ... no real jet issue in carbs as can get high revs and speed in lower gears?
Could it be a problem with the vacuum system ? Or maybe a timing advance problem (but wouldn't that show up across all gears and when in neutral?) Or is there anything else - not been able to find a clue from searching the forums (as best as I could!).

Just after a good idea of where to start!!!!
Cheers

[Big T]

Is this a new problem, or has it always done this?
What maintenance has been done?
How long have you owned the bike?

[Simian Major]

So I have had it for about 18 months - prior to that it was my youngest's first bike; he had it just over a year.
He never mentioned any highway issues. So basically since I have had it running following when he laid it down and scrapped the case cover on the magneto side - lost all oil.
I recovered it from him and found replacement cover and gasket to get it holding oil!
Since then have replaced stuff such as brakes etc; rear gear oil and main oil change, new filter, new plugs, had coil checked - good - sealed up corrosion holes in exhaust; new clutch cable; and as mentioned new fuel line from petcock to carbs, new air filter - also used
Also seafoam in gas tank and seafoam SS14 to try and clean valves....

[Big T]

Sounds like a valve clearances adjustment is in order

[BigD_83]

There are a number of conditions that can result in symptoms you are describing, ranging from timing advance (as mentioned, and I agree, not likely), to plugged tank vent, kinked/inappropriate hoses, extra inline filters, and poor routing/lack of the carb vent hoses.

There was a recent thread with much the same problem. I thought I had subscribed, but cannot find it.

EDIT- found it:

https://www.thegsresources.com/_forum/showthread.php?258862-Bike-quit-while-running-harder-then-normal

[Gorminrider]

Yes,read BigD_83's clues and the linked thread..it seems to show petcock fuel-starvation is a fairly common issue on some 650s...

Best and simplest test of fuel starvation overall 4 carbs is turning the petcock lever to PRIME, while you're having this issue, but it'd be best to double check that "prime" does allow a good unencumbered fuel flow first.

[Simian Major]

Thanks all ... first plan to try the PRIME run ... then onto checking all vacuum tubes from tank and petcock ... plus look to order a new petcock too; probably going to need to change it out at some point!
Then look up how to check the valve clearance/gaps....cheers

[Gorminrider]

Manuals and MUCH other info at BikeCliff's library site:
BikeCliff's Website

[rphillips]

If I read this right, you oiled your stock air filter. I don't remember any stock GS air filters that were to be oiled. Maybe starving for air, not fuel. Or maybe the 82 650 stock filter does needs oil. I've not owned a GS's that did. Just say'in.

[Grimly]

If I read this right, you oiled your stock air filter. I don't remember any stock GS air filters that were to be oiled. Maybe starving for air, not fuel. Or maybe the 82 650 stock filter does needs oil. I've not owned a GS's that did. Just say'in.

GS850 stock filter was a white sponge layer - wash it in kero, dose it with engine oil and squeeze it dry.
That's why I never bought a K&N for them - the advantage of an infintely cleanable, reusable filter was there from the factory. However, in later years I've seen similar sponge factory filters start to break down far sooner than I've observed K&N ones do. I've never seen a K&N fall apart like that, but they're not the same construction and materials anyway.

[salty_monk]

Yeah the stock filters disintegrate after 40 odd years... Uni filter OEM replacement is a good bet. Cheap too

Check out the petcock, valves, air leaks (on intake side too if you've not replaced the intake boot O rings & the airbox boots). Make sure the filter is sealed to the box (there is foam that breaks down above the cage & around the doors on that airbox).

I wouldn't be surprised if you have crud in the carbs too.. presumably that inline filter was a crutch for something. Check that the filter is still in one piece on the back side of the petcock (you'll prob need a new O ring if you remove it from the tank).

[80GS1000]

Thanks all ... first plan to try the PRIME run ... then onto checking all vacuum tubes from tank and petcock ... plus look to order a new petcock too; probably going to need to change it out at some point!
Then look up how to check the valve clearance/gaps....cheers

Sounds like fuel starvation from the petcock to me...

[Simian Major]

Indeed - everything I've seen in user manual and on specs/manuals from Bikercliff site says that GS650 sponge filter is cleaned in kerosene and then re-oiled ... though I didn't use engine oil; but the specific UNI filter oil that I bought with the new UNI sponge filter....

If I read this right, you oiled your stock air filter. I don't remember any stock GS air filters that were to be oiled. Maybe starving for air, not fuel. Or maybe the 82 650 stock filter does needs oil. I've not owned a GS's that did. Just say'in.

[GSJim]

I was going to say air filter because mine disintegrated after a few years of no use, but you covered that already. Were your carbs cleaned by dipping or with an ultrasonic cleaner? Mine were dipped a couple of years ago and I wasn't happy with the performance when I roll on the throttle.

The GS650G is a really nice bike when you get it sorted out.

Jim

[Simian Major]

So inbetween showers this morning - went for a run for a few miles on the interstate with petcock turned to PRIME...

Good news ... there was no drop off in power and ran for 3 exits at 70-75 and 5000rpm ...
Not So Good News ... any attempt to get more speed or rpm by twisting on throttle produced zero effect

New petcock ordered - so replace that and vacuum lines when it arrives !!!