Yamaha 90A - Am I in limp mode?

Herman Melville · 18 August 2022, 07:24

My 90A gave a continuous alarm at start up and now won't go past 2300 rpm.

Am I in limp mode?

It is hard to tell because my taco does not seem to haver an alarm symbol. it cannot have been over temp because the engine had not been fired up.

It is also intermittent - one minute full power - the next is 2300 rpm....

Only 2 sensors seem to be able to put it into limp mode - oil level and temperature. Is it possible to disconnect / bypass them to see if either is broken ?

It also runs like a dog as if on 2 cylinders - I thought a dodgy HT lead but you can pull each plug cap off while it's running and it seems to run equally badly with any of the plugs off - so inconclusive.

Itys got full oil, new stat, new fuel filter, bulb stays hard, idle screws reset to 1.25 turns etc... all seems in order and when it feels like it will go to WOT

I just think I am in limp mode but don't know how to tell that or resolve it - the manual just says take it to a dealer..

Cheers

neil.mccrirrick · 18 August 2022, 13:43

Why have the idle screws been reset?

neil.mccrirrick · 18 August 2022, 13:43

Sorry I assume you mean air screw not the idle screw they are different things

Herman Melville · 18 August 2022, 19:42

Yes in fact I meant to say mixture screws - one on each carb.

Manual says 1.25 turns +- 0.25 turns so I have set them all to that.

Anyway the idling is good now so it seems to have worked.

I am still intermittently in limp mode limited ro 2000 rpm - no alarms have sounded so I don’t know how I am in limp mode or how to get out of it.

Orwell boy · 18 August 2022, 20:22

Fuel tank breather clear ?

Herman Melville · 19 August 2022, 08:00

thanks for inputs yes it has a whacking great big breather tube that is not blocked

I reset the carbs to their 'stock' mixture settings to rule them out of the picture - they won't be perfect but they will be OK like that. In fact it made the idle better.

my issue is that sometimes / off and on my rpm drops to 2000max. exactly 2000 max (plus or minus 100) and then the engine takes off again...

I am convinced since it's so close to and always 2000 rpm that it is limp mode.

But I have no alarms going off and I have tested that the alarms work.

How can I tell for sure if I am in limp mode or not...

A1an · 19 August 2022, 08:29

will it rev past 2000rpm when its out of gear?

Herman Melville · 19 August 2022, 08:40

yes good point

it will rev way past 2000 rpm when it's in neutral

neil.mccrirrick · 19 August 2022, 09:19

Have you stated ona different thread that you are misfiring and disconnecting ht leads has little impact?

It would help if all the information was in one place, however if the above is true I think you have an electrical issue. Maybe one of the coils is shorting out. Tis will mean you can rev but it will fail under load and react as you describe

Herman Melville · 19 August 2022, 09:42

@neil.mccrirrick yes I did have a missfire issue and general bad running.

I thought it was a separate issue so put in on a different thread sorry.

It has been so precise on its 2000 rpm when its in dog mode that I really thought I had limp mode plus the carbs needed sorting etc - 2 probs at once.

Anyway in detail then - when it is in dog mode I was convinced for a while it was running on 2

I first pulled off the plug caps one by one to try and see which cylinder made no difference - this test was inconclusive. it ran crap on 3, or any of the other 2 - not much difference.

I took off all the coils. The spec is 0.18 to 0.24 ohms on the primary. Each coil was exactly 0.5 ohms. All out of spec. All aftermarket. All worked. so I assume that the extra 0.3 ohms is not enough to stop sufficient energy going through.

The spec on the secondary is 3.26 to 4.88 kohms - each one of the three was in spec.

Then I measured the resistance of the plug cap - it's the screw type that has the 5kohm resistor in it. All were over. 2 of them were 16k, one was 180k. I got a new OEM plug cap (which measures 5kohms) and put it on where the bad cap was. No difference.

I agree that it could be some kind of intermittent coil / spark. Which one it the question.

I have checked that the earth terminals on each coil look good btw.

I could get an OEM coil and go out and swap it from cylinder to cylinder to try.

What has stopped me is that they are £130 and the 2k rpm thing is just screaming out limp mode. I mean it's always and every time bang on 2000 rpm. Never less never more, every single time.

more info for you....

When I wind the mixture screws in and out each cylinder does not behave the same.

The bottom 2 when mixture is wound all the way in just gradually die as I would expect.

The top cylinder pops and misfires as the mixture screw is wound in - so something different happening there.

Again I am happy to give each carb a full service in fact it is on my winter list but the 2000 rpm thing is so repeatable it has driven me away from the carbs - if the carb was playing up then I would expect it to be much more variable.

My fault mode it much more like a perfect on off switch.

Either WOT or 2000 rpm. Nothing else.

I will not give up.

Thanks for your interest and input.

A1an · 19 August 2022, 09:58

Quote:

Originally Posted by Herman Melville

yes good point

it will rev way past 2000 rpm when it's in neutral

It’s not in limp mode

Herman Melville · 19 August 2022, 10:38

Ok then cheers if it’s defo not in limp then that’s a whole avenue I don’t have to go down

I am going to get a new OEM coil and swap it onto each spot on at time to see the effect

That’s plan A now

davej · 19 August 2022, 11:26

Just a thought, we had a big hassle with identifying a problem with our 115 yam 2S, eventually we identified the problem with the switch box, the over rev limiter function was activating after about 30 mins running. By unscrewing a particular contact on the switch box you disable that function and it no longer then runs in limp mode. I can’t remember which screw it is but when I get home after the weekend can post it. I think it’s the screw with the red paint on the bottom left when you remove the switch box cover.

By removing the wire it should run ok and then you can easily fit a replacement box.

Herman Melville · 19 August 2022, 11:36

@davej cheers thanks for the input

By switch box you mean the CDI ignition box that takes magneto inputs and sends out to the coils depending on the timing …. ?

neil.mccrirrick · 19 August 2022, 14:35

I’d start with the carbs. Check the fuel filters first and oil. When it’s stuck at 2 k pump the bulb and see if that increase revs any. If it does the fuel pump May be the issue.

Either way I’d strip the carbs down you may well have a clogged main jet on one carb and get to 2k on the pilot jets alone

Herman Melville · 19 August 2022, 18:13

Yep fuel filter changed.

Oil pump works fine snd oil bottle cleaned out so no sludge or water…

Bulb pumped when its playing up and makes no difference- in fact the bulb seems firm all the time so I thought fuel pump was good - an easy change of course.

I’ve never been into the carbs so maybe…..

neil.mccrirrick · 19 August 2022, 19:31

Main jet is the suspect on one of the carbs

davej · 20 August 2022, 22:08

Quote:

Originally Posted by Herman Melville

@davej cheers thanks for the input

By switch box you mean the CDI ignition box that takes magneto inputs and sends out to the coils depending on the timing …. ?

Yes another name for the switch box is CDI ignition unit.

On the yam v4 engines it’s on the starboard side of the engine under the rectifier.

Herman Melville · 31 August 2022, 13:47

So the motor is dead. Dead as F in fact.

Seized rigid in fact.

I am posting feed back because may be someone can learn from it.

The original symptoms were:

2k rpm running - query Limp mode.
Bad running - query carbs / ignition / fuel pump. Query running on 2.
Sluggish to turn over at start sometimes - query bad starter connections / intermittent starter motor / bad battery.

The actual cause of all of this was a "small" head gasket water leak in the top cylinder.

This caused all of it.

None of the other components or systems were bad.

Water was sometimes damping out the spark in the cylinder causing the bad running. Not the carbs / fuel / ignition etc...

A little bit of water was standing in the top cylinder after use.

It gradually rusted and seized the top cylinder. Rigid. Locked. It is not hydro-locked - water doesn't run out when you take the plug off. It's just seized over the period of a small leak developing and a couple of weeks.

There was no massive steam no big clues - it used the right amount of oil.

The only clue I can think of was that the top cylinder spark plug was slightly rusty. Not monstrously so..... but slightly and crucially different from the other two.

So I missed the clue that I should not have done and that has cost me the whole engine.

My only defence is that I have seen a LOT worse plugs than that and the plug alone would not have been a strong enough signal for me to take the head off.

Just some feedback so that others can see.

paintman · 31 August 2022, 14:58

Interesting.

Was the slightly rusty plug also unusually clean of normal deposits compared to the other cylinders? - often seen on petrol engines with hgf leaks into the cylinder which effectively steam cleans that cylinder.

I'm guessing the rings have stuck to the bore with the rust?
Remember in my teens a mate broke a Triumph Tigress scooter as the engine appeared to have seized - been standing out in all weathers for a long time.
He'd tried hitting a block of 2x2 onto the piston but as that hadn't achieved anything he dismantled the scooter.
Piece of flat steel on the top of the piston & a bit of steel pipe with a big lump hammer shifted it. Trashed the piston, but the bore was actually in quite good nick.

Worth a try?