Limp mode with no alarms 90A

[Herman Melville]

I am in limp mode off and on, surges, back to 2000rpm, 5 mins full rpm for 5 min, back to 2000 rpm…

there are no alarms

no over temp alarm

no low oil alarm

I have checked that the temp switch alarm works and that the low oil alarm works

so they are defo not triggering

so how can I be in limp mode ?

any help much appreciated

[ausnoelm]

First you need to determine if it is indeed "limp mode" pretty rare to not have an alarm too! Could the rev in neutral switch be broken/loose/out of adjustment?

[ausnoelm]

Hang on, is this a two stroke?

[ausnoelm]

If it's a two stroke (that it seems it is now) test spark when it's playing up, test all cylinders, the 3 cylinder has a rather unique ignition system.

[Herman Melville]

Thanks for help.

Yes it's a 2 stroke. I think in USA it would be 90TLRC - engine code is 6H1. Anyway it's a 1994 3 cylinder 2 stroke with autolube and power tilt.

I am pretty sure it is in limp mode. It will not go over 2100 rpm and runs like a dog (like it's on 2 or something) but no I don't know that for sure. It would be useful.

It pops in and out of this status - goes from dog to sweet and back again. When it's in 'dog' mode it sometimes even blips like it's trying to go back good again....

The reason I think it is in limp mode is that it is fixed on this 2000 rpm level max - never 1800 ,never 2300 - really fixed on it.

I DID get an alarm about 2 weeks ago - when it was stone cold - one continuous beep. for about 15 sec - then it stopped.

I have since tested both alarm systems in case the buzzer was broken and it was silently alarming at me. Temp - pull off the wires and short it out - alarm comes on straight away. Oil sensor is easy - I just pulled the sensor up out of the oil so the float dropped and the alarm came on straight away. So I think both sensors are working and that the alarm circuit is working....

But I do not know what the original alarm was 2 weeks ago. It is a big red flag though of course.

I am trying to be methodical about this. So as far as I can understand the only way it should go into limp mode are the 2 sensors.

It would be really useful to be able to know for sure one way or the other if I was in limp mode.

I presume that limp mode can only be caused by 12v on the pink wire going into the CDI unit? I wonder if I could probe the voltage there? or even disconnect pink there and see what happens?

The other thing I am going to try is run her out for 10 mins with both temp and oil sensors disconnected - that might give a huge clue.

Finally yes I could clear out the carbs and look at the ignition circuit ht leads etc... maybe a coil or an ht lead is intermittent...... I tested the coils and the primary resistance is out of spec on all three but they are aftermarket I think.... the secondary resistance is correct on every coil to the end of the ht lead. The resistances of the plug caps which should be 5kohms are not in spec and I changed the worst one... but they are able to run the thing well when it suits them... so can't be all that bad...

Bottom line is if it wasn't fixed so firmly to 2000 rpm I would look more closely outside the limp mode thing.

ANY useful suggestions much appreciated - especially with how to tell if I really am in limp...

[Herman Melville]

I am starting to wonder if it might be the CDI unit

[99yam40]

as was said check to see if spark or timing is changing when the problem shows up so you have some idea what to chase.
and inductive timing light that you clip around the plug wire will show spark and what the timing is doing easily
Move light to each plug wire to see what they are doing