'78 GS1000 Loss of Power / increased fuel consumption

[crookedspoon]

Over the last 1000km, I noticed loss of power and increased fuel consumption. They go hand in hand obviously.
In may, I installed a DynaS ignition, together with new DynaS coils and was amazed to notice a 15-20% better fuel economy.
For a few trips, I felt wonderful finally reaching 230km on a tank without having to switch to reserve.

Then I put in new tyres, checked the valve-clearance (didn't have to change any shims) and put in new rear-brake pads. After doing so, my range went back to pre-electronic ignition times ? or even worse.
Warmed up and at idle, the engine revs about 300rpm less than it used to - which causes frequent stalling at traffic lights.

Initially, I assumed the fresh brake pads were dragging, which they might have. But now, when on the center-stand, the rear wheel runs freely and the disc stays cold after driving a bit without applying the rear brake.
A bit later, I noticed one of the 2 bolts of the exhaust bracket at cylinder 2 was missing. That easily could've influenced back pressure hence influenced engine efficiency. I put it a new bolt though and that didn't change anything.

Tyres are new and pressure checked.
Ignition and coils are new ? when installed, the coils received proper voltage.
Chain and sprockets are from beginning previous season.
Carburator has been cleaned (ultra-sonic) and set 3 seasons ago. Spark plugs are quite black at the moment, might lean-up the mixture now but I can't imagine that running rich causes a 20% increase in fuel consumption... I didn't adjust any of the settings lately.

Any ideas where I can start looking for potential problems? This performance loss can't just 'happen' can it?

happy to provide more details if needed!
'78 GS1000

 

[geol]

Frequently, plugs can soot up if the spark isn't good. Since the valves have proper clearance, you could check compression to verify that compression hasn't dropped significantly but the problem more likely is spark. You might check your battery voltage while the bike is not running. If it has less than 12.5 VDC (roughly) then either the battery is dying or not charging. Trouble shooting is fairly easy but if the battery has low voltage, the bike will run poorly.

Even with NEW coils/Dyna S, if voltage at the coils is poor, the spark will be poor and hence plug soot up and reduced mileage. The wire feeding your coils is O/W and it sits in a connector for each coil with the wire coming from your Dyna S ignition. With the engine not running and one of the two connectors that go to the coils unplugged, check battery voltage on the O/W with key on. If you don't see close to the voltage you had at the battery, the coils are getting poorly powered while the bike is running. Easiest way to fix this is to run the coils off a relay with power coming from the battery (well, the hot lug on the starter solenoid connected to the battery).

 

[tom203]

Do posplayr's quick test to make sure the dyna is happy


http://www.thegsresources.com/_foru...System-QUICK-TEST/page2&p=1272192#post1272192

 

[Big T]

A sudden loss of power typically results from something coming apart

Pull your spark plugs and post up some pictures of what they look like.

It may be your petcock has gone bad and is dumping fuel into #3 cylinder, choking it off

 

[crookedspoon]

Seems like it was a combination of a bad spark on cylinder 4 and maybe some gunk in the carburator.
I put a bottle of carb-cleaner through the carbs & replace the plug & caps on cyl 4, that made things right again!
Will have to fine-tune the carbs as well, seem to be running a little rich!

which leads me to the next question: http://www.thegsresources.com/_forum/showthread.php?240936

thanks