Ignition breakdown at sustained high-ish revs R100GS

Airhead_Martin

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Hi, I'm new so be gentle. Recently acquired 1994 R100GS from well known spares place, had it delivered so didn't notice anything wrong until I had a proper ride. Whilst going along at about 60ish the bike started to cut out, thought it was a fuel problem, filled up - no joy. Had to trundle then it was ok at lower speeds / revs. Assuming fuel starvation removed inline filters and replaced tank filters - no joy. Any ideas

Cheers Martin
 
Could be the ICU(ignition control unit).
It needs heats sink paste on the mounting bracket.
This might need renewing( the paste that is).
The ICU can overheat and cause problems.
So best way is a process of substitution with known
working parts- coil, plug leads & plug caps,
ICU. Also could be the sensor in the beancan?
Welcome to Airhead World!!!!
 
Could be low float levels - take one of the bowls off carefully and the fuel should be around 1/2 an inch from the top. Replace it, fill the hoses and check the other side.

Could be blocked passages in the carb - a stiff dose of injection cleaner in the next tank of petrol can help, and blowing carb cleaner out of a can through the passages might help too - including the two passages below the air inlet , the large one should exit freely round the needle jet and the small one through a pinprick downstream of the butterfly.

But the other suggestions of the hall sensor, ICU and diaphragms are more likely, if harder to check and fix.

Sounds like you have drained the tank when you cleaned the tap filters, but if you didnt and only turned the tank on its side, drain it properly , like upside down. This was the last and most effective step for me when I recently spent a lot of time and energy getting a F650 Funduro running the way it should, and one nobody suggested.
 
As ianboydsnr said - 60mph is bang on for Carb Diaphragm. Does it just stop going any faster and then work ok once the revs come down? I thought one of my throttle cables had snapped the first time it happened to me.
 
Detail of fault

Rob Farmer said:
As ianboydsnr said - 60mph is bang on for Carb Diaphragm. Does it just stop going any faster and then work ok once the revs come down? I thought one of my throttle cables had snapped the first time it happened to me.
Click to expand...

Runs ok through the rev range until sustained high revs - motorway, after about 5 minutes starts cutting out. Have to ease off and get out of the way of super observant car drivers. Then pull off, - problem disappears until motorway speed is attempted.
Thanks for your replies, will investigate asap.
 
Doubt it is diaphragms or coils since these would be apparent earlier than after 5min at motorway speed.
Possible fuel starvation - Tank breather blocked or perished fuel hoses?
Could be heating of the ecu but again, hard to relate that to sustained high revs / power.
 
Ha, that sounds very similar to the issues I was having with Bertha. Unfortunately I don't have a true diagnosis as we ended up rebuilding the motor, replacing stator, rotor, diode board and pretty much all the wiring, but I am pretty sure that the initial issue was electrical and was a poor earth somewhere around the charging system.
It appeared fuel related, but actually was electrical.
I hope you find it easily, it gets feckin expensive when you can't.
 
Fuel starvation...check float height. ( Float should just be cutting off the fuel supply when top of plastic float is level with alloy base of the carb body - were the float bowl sits in ). Previous owner probably set it to 27mm as per ( wrong ) merkins websites.
 
27 mm deep in the float bowl works fine if his GS has 40 mm carbs, and is pretty close to my suggestion of 1/2 an inch down from the top, which works OK in 32 and 40 mm carbs.

It might or might not be the optimum, but the bike will run cleanly with it at that level and it takes seconds to check without removing the carbs.
 
Haynes show a clear neoprene tube attached to the carb drain plug. Turn the tube up alongside the carb body (makes a simple manometer) and you can see the fuel level.

Hall sensors can fail when hot but seem fine when cold. This however sounds more like a connection or electronic joint being affected by vibration.
 
Ive had the same problem with a r100r .turned out to be as the others suggested float heights ,god only knows how they moved but moved they did ,also a really silly one but this nearly got me wiped out ,traped tank breather ,just pull it off your petrol cap.and make sure you can blow through.
 
Nippy Normans supply Touratech, Wunderlich, Desert Fox and many other makes of accessories.
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