77 GS550 bogging

[RETROHOWLA]

Hi guys, im a newbie and have used the serch function but not been able to find a thread with a perfect match of simptoms so here goes

A friend of mine has just bought a 1977 gs550 with 35.000 miles for commuting on my advice

(I currently own a 79 and 80 gs1000e's hence i advised him on the GS route)

So he bought the bike blind on ebay, rode it home and came straight round to see me, although a couple of lighting gremlins its a good solid bike with plenty of life left in her.

One thing i noticed though was on riding it it seems to start, run and pull fine on small throttle incraments, but if you wind open the throttle in any gear it barely accelerates, back the throttle off a bit and it picks up and will rev to the red line

First thing i did was checked the points, gapped them, balanced them and timed them, rode her again and it was a slight improvement but still not good

I checked the air filter, i suspected it to be clogged so temporarily replaced the foam with free flowing foam to see if it made a difference, again a very slight improvement but still not right.

I checked the idle mixture screws and they were only 1 turn out each, according to the manual the 77 model should be 2 turns out each, when i adjusted them it made the problem loads worse so have gone back to 1 turn each as he needed to use the bike this week for work.

As far as i can see the previous owner has been trying to cure this problem as it has new points, condensers, spark plugs that look to have done no miles at all.

The battery is not that good, it starts the bike fine after a 10 min ride but not after being sat 24hrs

My next step was going to be a carb strip down but i once had a hesitation problem on the 79 Gs1000 that was the coils, replaced them and it flew again.

Would it be adviseable to change the coils first or is it unlikely to be this?

I think the idle mixture screws have been adjusted to try and compensate for a problem elsewhere but could it be just a carb clean and balance required?

[Nessism]

I'd do the carbs first so you can build a baseline toward a reliable bike. The VM's have two screws; pilot fuel screw which should be about 1 turn out and the pilot air screw which should be about 1.75. Lots of times people break off the tip of the fuel screws in the carb body so again, performing a proper rebuild will help you verify all the details.

[rustybronco]

One thing i noticed though was on riding it it seems to start, run and pull fine on small throttle incraments, but if you wind open the throttle in any gear it barely accelerates, back the throttle off a bit and it picks up and will rev to the red line

My next step was going to be a carb strip down but i once had a hesitation problem on the 79 Gs1000 that was the coils, replaced them and it flew again.

Would it be adviseable to change the coils first or is it unlikely to be this?

Would you so kindly measure the resistance of the #1-4 and 2-3 coils.
measure them from spark plug cap to spark plug cap.

[RETROHOWLA]

Ok thanks guys, i will measure the resistance tonight if hes free to bring the bike round, i wont be able to do a carb strip down just yet, although i think i may have a set of carbs down the lock-up to fir a 550, i will try and dig these out and strip and inspect them to consider a direct swap

[1979GS750E]

Bogging Down - Probably going lean, air box leaks?

This is the sign of a leaking airbox or air filter bypass: accelerates okay as you slowly turn the throttle, but after a certain point of turning the throttle, it starts stumbling. It stays in a stumbling mode until you back down the throttle. So, it runs okay only up to a certain throttle level, then bogs down (mixture leans out too much). It's counter-intuitive, but you must have a certain intake resistance to make most motorcycle carburetors run correctly. Most attempts to modify the air intake in any way, especially by reducing restriction, result in degraded performance. I know it sounds strange, but my gs750e can't even climb up my driveway with the air filter removed, but it runs great with the air filter in place. The restriction of the air filter creates the full venturi vacuum needed to suck the fuel through the jet. Your problem might be carbs that need work, but it might be as simple as getting the air box and filter tightened up.

[hampshirehog]

I bet it's the carbs! You'll have VMs on there, 1979GS750E is mainly talking about CVs fitted to the later bikes and they are much fussier. No.1 job on these bikes is to strip the carbs, clean and rebuild with new O rings as required (completely strip and clean, 'partially' doesn't work).

Could also be a dodgy fuel tap - clogged or vacuum not working properly, carb lines fitted to the wrong tubes, float height or wrong jets put in by PO.

Don't forget to check those valve clearances either - you'll know that from the 1000Es.

Oh - and by the way, welcome to the nuthouse. I'm up at the top end of the county, just south of Farnham. Bum start by your local boys - if they don't turn things around the Saints will be playing my team - Aldershot Town - next season .

If you need any bits I've got a heap of 550 parts I can rummage around in - just give me a shout.

[RETROHOWLA]

Hampshire hog thanks very much! good to know i have a fellow follower close by, i may well be tapping you up for parts if the old girl takes any major fettling, as far as football goes though its all over my head, dont have a clue what your on about! lol

rustybronco, the coil resistances are 1+4 =21.3, 2+3 19.8, what does that tell us?

Ive not had a chance to look at any of the other ideas yet, i will be pulling down the set of carbs i have down the lock up though, comparing jts and needles to the manual and giving then a sonic bathing and rebuild.

[RETROHOWLA]

And yes its running standard vm's, always prefered them to cv's personally

[rustybronco]

rustybronco, the coil resistances are 1+4 =21.3, 2+3 19.8, what does that tell us?

That all is well in that department...

if the air box is connected and properly sealed, get to working on those carbs.

[RETROHOWLA]

Nice one cheers, i will hopefully be able to get at it to check air leaks etc tomorrow and i will keep you posted.

[RETROHOWLA]

Ok guys the carbs are cleaned, i stripped them completely, gave all the components a good soaking in an ultra-sonic bath which brought them up a treat.
Ive rebuilt them with home made gaskets and sorced the seals from a comprehensive o-ring kit i have, it seems to have done the business, she rides really well now.
One thing i am stuck on though is the fuel screw setting, the manual seems to think they should not be adjusted from standard due to calibaration at the factory but mine have obv been fiddled with before so i have no true base to start at?

[Nessism]

Ok guys the carbs are cleaned, i stripped them completely, gave all the components a good soaking in an ultra-sonic bath which brought them up a treat.
Ive rebuilt them with home made gaskets and sorced the seals from a comprehensive o-ring kit i have, it seems to have done the business, she rides really well now.
One thing i am stuck on though is the fuel screw setting, the manual seems to think they should not be adjusted from standard due to calibaration at the factory but mine have obv been fiddled with before so i have no true base to start at?

Read my post above...

[RETROHOWLA]

Ah yes i have done but as there seems to be a very fragile tip on these screws and nothing to really seat against i am concerned screwing them till they stop and then backing out could cause damage, i have very gingerly screwed one in but there is no stopping point it seems it just gradually gets slightly tighter?

[hampshirehog]

The fuel screw and air screw to a degree work against each other - wind the fuel screw out = richer, wind the airscrew out = leaner. If you've got a Colortune set the fuel screws to 'about right' and adjust using the airscrews. In theory the fuel screws should be set at the factory so that the airscrews are all the same number of turns out but in practice it doesn't make a jot.

And you're right - the tips of those fuel screws snap really easily and can be a b*gger to get out.