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Rough idle 1150RT

David Sharpe

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David Sharpe


- rough idle at around 1,000 RPM

- some surging at 5 to 10 MPH steady speed through

parking lot at work

- bike has traveled 68,000 miles

- gas mileage has dropped from 42mpg to 39mpg

What I did

- full tune up, replaced plugs, oil, checked valves, balanced carbs

- checked cables to make sure they were seated after accidentally

pulling on the right throttle cable out from the junction box because

it looked to tight, pushed it back.


What I found

- loose spark plugs before tuneup


But the rough idle persisted


What I did

- replaced the Oxygen sensor.

- verified spark at each spark plug.

- rebalanced the throttle bodies


What I found

- balance was way off

- rebalancing turns out perfect


But the rough idle persists



- with fast idle on bike fires up and idles at 1500 RPM

- after about 3 to 4 minutes with fast idle still on RPMs

drop to 900 to 1000 and bike idles like a Harley.

- doesn't die at idle


What should I do next?


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How did you check for spark?

The symptoms you describe suggest a stick-coil as favourite to me - these can still give a spark outside the engine but as the spark-gap resistance increases with pressure can internally arc causing a miss-fire under cylinder pressures.


If you can borrow known good coils, try that as the best diagnostic. Otherwise, disconnect the lower plugs one at a time - if the engine stalls or shakes like a 1930's John Deere the stick coil on that side is dead - worth checking both sides even if you find one bad coil.



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David Sharpe

I checked for spark by removing the sparkplug wire, inserting an old spark plug and grounding it on the cylinder head. I got it to cause the idle to become worse just once but it didn't do it again so I thought the coils were ok. Should I just pull the plug wire or do I need to ground it against the cylinder?

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Afternoon David


With all you have done a primary (stick) coil could be the issue. Maybe try taking an old (known good) spark plug and opening the electrode gap to 3/16” , then use that (special) spark plug to repeat your spark test. You need to see a snappy bluish to bluish white spark across that 3/16” gap every time.

Also look CLOSELY at the coils for signs of carbon tracking or arcing to the inside of the valve cover hole.


Otherwise maybe a problem with your TPS—Check the TPS voltage at closed throttle (no choke)- it should be around .365-.385volts (every time the throttle returns to closed throttle)


If you can get hold of a GS-911 check the engine temp that the Motronic is seeing. A failing oil temp sensor can cause over or under fueling and a rough erratic idle.


Then check your fuel pressure at idle—A plugged fuel return hose or partially disconnected quick disconnect can force high fuel pressure therefore over fueling at idle.


If nothing found in the above try running it with the 02 sensor (completely) disconnected. A problem with cross talk in the 02 circuit, or the 02 wires running too close to a spark plug wire. Or someone mistakenly soldering the 02 wires. Or other 02 circuit issues can definitely cause a poor and erratic idle.


Otherwise look for a worn cam lobe (low valve lift), or collapsed push rod, or tight valve, etc.


What is your cranking compression on each side?


If still nothing found maybe an injector problem or possibly a plugged or malfunctioning barometric sensor inside the Motronic.


If you can find a temperature sensing gun see if the exhaust pipes are the same temp at idle (might at least point to a side for you)


Get the stick coils verified as FULLY functional first then move forward from there.


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I replaced the stick coils and after balancing the TP, it ended ALL surge, hesitation erratic idle etc. I've done a ride this afternoon at 7 C and i'm still amazed to find how good is the throttle response now. The TB balance was ok after adjusting the right TB stop screw. With just the right BBS adjustment, it was unstable. If both BBS are near the same adjustment to get good balance, the right stop screw adjustment is ok.

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David Sharpe

I checked the spark of the primary coils with a spark plugged gaped to almost 3/16". I saw a continuous blue, purple to blue white spark. Not a spectacular spark. I seemed to get a brighter spark coming off of the spark plug threads than through the gap at the tip of the spark plug.

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  • 2 months later...

Once again stick coils cause a problem. It needs one of us brave souls to do a retrofit conversion and see if the motronics will drive a set of conventional coils and then fit pre twin spark HT leads.

Who's first?


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I replaced both stick coils in the spring with the newer model (third ?)


If one of both fails later, I'll replace them with a conventional coil.

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