Hey guys,

My '93 model with 12k miles has developed a problem. I previously had the misfire at cruise problem which I fixed with Laddies' "cig butt mod" and the bike has been running great all summer, however in the last couple of weeks she has developed a patch of very rough running that seems to correlate to a certain temperature range.
Yesterday was a typical occurrence:
So she starts and runs fine as I let here idle and warm up in the drive. Get on the road and she runs perfectly throughout the rev range as I hit the open road. As the engine comes up to operating temp (yesterday was a cool day so the temp gauge was steady at 160F/70C) still no problems for a 20 min run into the city. Now I hit a bit of off-ramp traffic and the temp gauge starts to climb...it hits 170-175 ish and the bike starts to misfire and I can smell fuel - it's really hard to keep her running - I have to constantly blip the throttle and keep her above 2000 rpm or she will stall out. Even when moving I get that awful unpredictable throttle response.
Now for the weird part: if I can keep the bike moving it comes out of the funk and starts to run smoothly again. Sometimes the rough running persists for a few minutes, sometimes less than 30 seconds. Yesterday it was pretty bad and, as it happened, I was close to my destination so I just limped in and let the bike sit while I did my business. 30 mins later I got back on and the bike runs perfectly again...until for whatever reason she hits that temp band again and it all happens again.

On warmer days it happens earlier in the riding and if the bike stays at 175 or above (never much above) I see the problem once and that's it...she runs great for the rest of the ride.

So I'm thinking it's got to be something screwy in the coolant temp sensor circuit - either the sensor itself or a bad wire or connection somewhere but maybe it's something else. I'm planning on checking her out thoroughly but I'm not sure how to go about checking for operation of the sensor - anyone seen this problem before or know how to troubleshoot the system? I'd love to hear your thoughts and get some pointers on diagnosing this one.

Cheers
Andrew

I've had a similar issue a few times...but don't have a proven quick fix for this one...(yet).

I think that at 12k miles you can discount mechanical problems.
(although you could check your exhaust manifold sealing - just in case)

You can check your temperature sensors resistance with a digital meter and immersing them in water - bringing them up from ambient to boil...or the ATS with a hair dryer.
They are just NTC resistors (resistance decreases with temperature increase)
I posted links to the temp graphs for the WTS and ATS in my post on the ciggie butt fix.
From my experience it's not likely to be the sensors; don't shell out $$$ for new ones unless it's dead.

I have a sneaking suspicion it's caused by intermittant fuel starvation by development of vapour / bubbles in the fuel lines. I'd explain this by the reduced fuel flow in heavy traffic allowing the fuel to heat up in the line longer at higher temperatures...with little cooling effect from air flow.

If the problem occurs when you can smell fuel vapour it is indicative that there is a fuel temperature sensitivity.

A quick fix might be to tweak up your idle mix pot on the ECU to very rich.
The pot only affects idle mix - so your fuel consumption won't really suffer.

Keep the tank reasonably full; the more fuel in the tank the more time it will take the 907cc heat exchanger to bring it to boiling point.
The tank has a de-gassator in it; this is the fuel return from the regulator. I don't know how well these function...and I expect that at low fuel levels it doesn't do much at all...

It may also pay to do some homework on the type / brand of fuel you're running - maybe look for something with a better resistance to vapourizing. I'm not sure on any fuel additives that might work; a dash of diesel perhaps?

G'day Laddie,

Thanks for the dope on the sensors...I'll go find those graphs and see if I can rig up a test and see how mine compare.

I had assumed the fuel I could smell was unburnt exhaust caused by the misfire - I mean she really pops and crackles! But you might be onto something because I initially thought that it was fuel related - and it does seem worse when the tank is low. But then on the day I wrote about in my last post the tank was pretty low (last 1/4) on my way out when it happened but then on my way home I juiced up and it happened practically as soon as I rolled out of the servo (with a full tank obviously). But then maybe still you are right....the servo was just off the freeway so sitting at the bowser she would just have been baking herself and all the fuel lines....

How do you reckon I could test that - wrap the fuel lines with some sort of heat shielding maybe?

For what it's worth I have been fueling the bike with Shell V-power - but after your tip I've been reading and found an article from Shell (actually about Optimax) and maybe the fuel is contributing given how primitive the EFI is on the 907? Plus, I have the warmer of the two chips that came with the bike currently installed...enriching the mix even more. Maybe I should switch back to the stock US chip and see what difference it makes?

I hesitate to post this reply since happened to my Chevy V8, BUT, almost exact same symtoms. Carburated, HEI ignition. First start in morning would be smooth and normal, two to three miles on would start running very rough, stay this way for two three miles then smooth back out and run smooth rest of the day. Figured fuel issue, went thru system, no help. Went on like this for several MONTHS, then started doing more than once a day, plus rougher with backfires thru carb. OK, gotta be ignition system. Replaced all components in Distributer one by one[ign. pickup, module, roter, cap, coil] no help. Finally two weeks ago, replaced ignition wires. FINALLY, problem solved, runs better than ever!
Probably won't help, but then again, maybe it will point you in right direction.
Carl

Hey Carl,

Do you mean you replaced the leads to the injectors, or the leads from the coil to the plugs?

It all helps, cheers.

Andrew

Edit: Whoops - just reread your post - you mean the HT leads to the plugs since she is carburetted! Doh!

No injection-carbed. Replaced plug leads, finally cured.
Carl

Got this off before seeing your edit.

I noticed that sparks were in the air if my vertical cap lead were to close to the heads.
So if your plug leads are the original as mine I guess a change to fresh ones could be worth a try.
Haven´t had any misfires while running or on the road, though.

Hi Andrew,

I have replaced HT wires for Hightech silicon ones with good quality caps and the ciggy but trick. I machined a aluminium plug with 0.7 mm of bore and a lengght of 15 mm.
first try-out was OK. no problem.
But now the temperatures are lowering at evening in Holland the problem reoccurs.
I checked a lot of connectors for corrosion and if they were affected I cleaned them.
with a mininmum of result sofar.
So, I think is a ambient temperature issue.
Any suggestions someone??

Ducbertus

Well, I am certainly none the wiser. I went out yesterday - just a beautiful day here in the Pacific Northwest - I changed nothing and my intention was just to run out the tank of V-power and see if the bike would repeat the misfire at the temp band I had seen previously. Well nothing happened! Of course I enjoyed the riding very much but now I know nothing more except that apparently the miss is intermittent. Great! Even harder to figure out. I'm heading out again this arvo - a little less juice in the tank so we'll see if that causes the problem.

Cheers

Hi Andrew,


I could give it a try to use V-Power gas. I rode today under very nice conditions over 100 km. With misfiring ofcourse, uring the warmup phase.
If the misfire disappears by using the V-Power I'll you know.

Ducbertus

Another 100 miles and used up the rest of the tank of Vpower - again no misfire anywhere. I'm at a loss. Yesterday the ride was more spirited and she got well and truly hot under the collar a couple of times. It's hard to complain when she runs so sweetly but I guess it's the frustration that comes with an aging Duc.

This coming winter I'm planning to replace all fluid hoses and hydraulic lines and give her a bit a of a spruce up under the plastics, so I guess I'll give her new HT leads and check all the ignition wiring looms at the same time.

If I experience the problem again I'll post here. Yeah, Ducbertus, let us know how you go with Vpower and this odd misfire on warm-up. I'd be particularly interested if you notice anything in the first tank or two.

Hi Andrew,
I have experienced a similar problem !.
(Not the petrol smell though)
I had the same rough running symptoms when the engine temp reached approx 129 F
I have read Laddie`s posts which gave me the Temp/Resistance charts for the Air Temp & Water Temp Sensor, which I have recently used to check the sensors on my bike.
My WTS & ATS have been replaced and my rough running has gone ! Lucky
The WTS was checked with a small hand held Fluke scope with a trend setting that allowed me to see a graphical representation of the resistance, while measuring the temp with a digital thermometer.
I was hoping to see if the WTS failed at my problem temp of 129 F, it did not, the response was good with no obvious problems across the temp range !
The ATS was more difficult to test with no means of cotrolling the air temp, but still I could find no obvious problems.
When I got my new sensors I decided to check them against laddies charts and discovered the new sensors responded much faster and with more accuracy than my originals.
I think the origanal sensors must have gone of spec with age ?
Replacing these two sensors worked for me and they were not very expensive here in UK,
I just happened to need a new thermostatic switch for the radiator fan and thought when the bike is in bits it would be a good idea to replace them at the same time !!
Hope this helps

regards

Bomber

Good info, Bomber. Thx f sharing!

Hi Andrew,

first I try the V-Power gas. If this doesn't work I go for new ATS and WTS.
See what happens than.
Ducbertus

Found another reason for the misfire-at-cruise problem in my 907 1993. I hoped to fix the problem using Laddies' "cig butt mod" but found out that the connection fitting of the presure sensor had a small crack allowing gas leakage and deviant presure figures to the ECU. I fixed the cracked connection with 2 component epoxy glue and the misfiring problem disappeared, I didn't dare to hope for such a simple solution but it worked definitely. Laddies' "cig butt mod" used to cure other 907s MAY have flatten sharply oscilating presure values originating from gas leaks somewhere between the air filter and the presure sensor. Check fittingss the tube and the air filter housing for gas leaks.

George