GS 550 Midrange missfire

[Guest]

Hi All. I have a 1983 Gs550e 8valve and I have a terrible misfire above 6-6,500 rpm. in fact it's so bad that I can't really get it to rev above that. (runs quite well below that) when I got the bike it had a GS850 Motad exhaust fitted and ran exactly the same. It hadn't been used much for a while so I removed the carbs and had a look inside the float bowls. It seemed pretty clean in there but I did find that one of the main jets had fallen out and was just resting in the float bowl! bingo I thought, re fitted it and reassembled expecting an improvement. I was disappointed as it ran EXACTLY the same!. I have since fitted a proper GS550 Motad exhaust and a set of carbs I found on eBay which had been re boult with all new gaskets and o rings and even stainless screws throughout and were very, very clean. I fitted these along with the exhaust and was again disappointed to find the bike ran exactly the same. when I fitted them I put the vacuum gauges on to sync them all up and the idle and low running is fairly sweet. just that over 6,000 rpm thing still there and still as bad. does anybody out there have any advice as to what to do next with the bike? and help or advice would be very gratefully recieved. thank you in advance
Pete

[dorkburger]

Welcome.

You'll need to provide more info, and do some verification of things. Is the bikes maintenance current? Valve clearance checked, carb / intake tract tight and leak free, etc? Carb jetted for the aftermarket exhaust? Electric issues can also cause running problems.

This is a very helpful site, but not one of mind readers.

Also we like introductions, and especially bike pics.

[Pete jordan]

Hi Glen, thank you for getting back to me. Here's a pic of my bike taken in my hometown of Newquay in Cornwall. I really am not sure of the valve clearances yet as I've inly just got the bike up and running. It was in quite a state when I bought it and spent the winter, and most of the summer getting it together. The intake seems leak free to me. after checking by spraying easy start around them and listening for a change in running. there is no top end noise that one might put down to wide clearances though. I had charging issues with the bike when I first got it running so I replaced the stator, reg rec and battery, so that's all good now. the spark plugs after a 50 mile run would appear to show it's a bit lean though. I have changed the engine oil and filter and the air cleaner element is correct and clean. As for the jetting I really don't know as it's all as it came to me. I was led to believe that the Motad systems were designed to run with standard jetting, but again I'm not sure what that is supposed to be or indeed, if that statement is correct. thank you again for getting back to me I really appreciate any advice that can help me get this lovely old bike running sweetly.
cheers
Pete

[dorkburger]

Nice looking cycle!
Valve clearances on these actually get smaller over time, and can eventually lead to the valves not completely closing and hard starting if left too long. Something to put on the list. Otherwise, based on your saying the plugs look lean and the bluing of the pipes, my guess is it may be lean.

Hang on. Others more knowledgeable then me will surely chime in.

[steveb922]

I'd do the valves first, and check the jets. If they are genuine Mikuni jets, the numbers should be easy enough to see. The Motad shouldn't make any difference to jetting - it certainly didn't on mine.

Once you've established a solid base, diagnosis gets easier.

If it helps, I have a few odd valve shims and jets here.

[Kiwi Canuck]

Pete, I agree with others about the valve adjustments, valve adjustment is very important on these bikes to get them running right.

Have you checked the timing advance mechanism, does it spring back when twisted, sometimes they can get seized up and not advance properly.

Pretty cool that you find a member in your area with the same bike.

David

[Pete jordan]

thank you all. will try to find the time to check my valves first then. where in Cornwall are you Steveb922? I'm in Newquay. Kiwi Canuk I will check the advance mech as it is just like hitting a rev limiter in the way it behaves.

[steveb922]

I'm maybe 30 miles east of you - just north of Liskeard. I'm no expert, but I have a workshop and a small stock of spares - you're welcome to either if they will help.

[Pete jordan]

Thank you Steveb922 that's so kind. I have a workshop here in Newquay and some spares too, never enough though! likewise if I can assist you with anything I'd be very happy to help. looks like I'm in for a biggish job now anyway as the head gasket appears to be leaking oil down the front of the engine. so it looks like it's all going to get a good looking at. have you every removed the cylinder head from one of these bikes. (any tips for me) Apart from all the hassle I've had with this one I think they are great little bikes very characterful and giving and hopefully fairly robust and reliable. My other bikes are a Honda CBRXX Blackbird and a Honda Transalp 650 I love them all and the each have their charms and foibles, the Suzuki though just reminds me of my teens and the way bikes felt then.
cheers
Pete

[2BRacing]

...... looks like I'm in for a biggish job now anyway as the head gasket appears to be leaking oil down the front of the engine.....

Before you "go big" there are two things that you should try first in case these "smaller" fixes solve the issue: (1) re-torque the head nuts to the correct torque, as these can loosen over time if not regularly checked. (2) while the valve cover is off to re-torque all the head nuts (and adjust valve clearances if not already done), check the gasket carefully for areas of possible leaks, especially around the cam chain tunnel area and along the rear of the valve cover. Oil leaking here can find its way through the openings in the head and come out in front of the head, making it look as if it is leaking at the head gasket area. Replace the gasket with an OEM one, and smear a thin layer of grease on both sides of the gasket so that it is easier to remove next time you adjust the valve clearances.

Another common area for an oil leak to occur, is from failed oil seals at the tachometer cable drive where it enters the head.

If there is still a leak, clean the engine well with degreaser and apply foot powder to the general area, and inspect closely to see where oil wets the powder when the engine is running.

[Pete jordan]

Ah thank you 2BRacing I will indeed re torque the head bolts first then for sure. good call the lea "Appears to be down the front of the block from the point in the middle where the cam chain runs. what you say makes great sense to do first though
Cheers
Pete

[gustovh]

While your checking the valve clearance, be sure that the cams are clocked correctly. Too many pins between cams will prevent the engine from spooling up all the way.

V