Cali EV power loss when hot

I have an intermittent problem which is becoming very irritating.
After about 20 miles or so, depending on conditions, I start to get power loss. It comes on suddenly and sometimes goes away for a while if I rev hard.
I’ve tested compression - good on both cylinders hot or cold.
Good spark even when fault occurring.
No error messages from ECU (now) even while fault occurring.
Tappet gaps checked and corrected, very little adjustment needed…
Fuel pressure consistently good.
Inlet manifold vacuum testing proved interesting; Readings initially were good but when the fault kicked in the vacuum in both inlets dropped simultaneously and by the same amount (halved).
This suggests to me that wherever the problem it is something that is common to both cylinders. I.e. not coils, valves, injectors or vacuum leaks.
I’m currently looking at the cam chain and tensioner.
Any suggestions gratefully received.

Perhaps a blocked fuel filter
Stop and some of the gunk drops off leaving a better path then it builds up again over time
This happened to my EV. it would bog down on a hill.

Simply a matter of flushing the filter backward, you should have seen the rust that came out/

Check that the fuel filler moat drain is not blocked.

I’ll have a look at that.

Main fuel filter was changed very recently. Old one not showing any debris.
Removed fuel tap and associated filter from tank. Some gunk and rust all on the outside of the filter. Inside looked clear. Flushed it out anyway and while at it flushed tank as well. Removed fuel cap holder and cleaned everything, again nothing in particular to report.

Cam chain and tensioner a little slack but not enough to cause the problem. While I’m in there, might as well change them anyway.

Any more suggestions?

What exhaust crossover do you have? I only ask because my Suzuki GN250 exhibited similar symtoms once and it turned out to be the muffler packing breaking down and blocking the exhaust.

As far as I can tell it is just a simple expansion chamber.

  • on closer inspection it appears to be just a cross pipe.

The power loss, are you sure it’s not missing on one cylinder at times.
My EV was fitted with resistor caps and also had the carbon core HT cable which was open at one end.

A simple check you can do is pull both plug caps and measure the Ohms from cap to ground, it should be identical on both sides.
From cap to ground gives you cap + lead + coil = about 8 k from memory.

DO you have an electric petcock, is it possible it’s closing?

Another thought - tank suck if the breather is blocked - just crack the filler cap next time you lose power to see if it improves

Did you check the moat drain with some light oil while you had it apart both of my EV tanks were plugged solid with rust.

Power loss from one cylinder would show as uneven inlet manifold vacuums. Vacuum readings match throughout all tests. I tried the resistance check but got an open circuit from both sides.
Petcock appears to be working as pump maintains pressure throughout.

About to take the bike for a run to see what effect removing the rust from the tank had.

So after a good run, the intermittent fault seems to be cured. As removing the rust is the only real change made, points have to go to Kiwi-roy - thanks!

However, the intermittent fault was disguising another fault. Now fairly consistent. Lack of power below 3000 rpm (varies). At higher revs the power usually comes back in - often with some gusto. Can be quite alarming.
Also occasional backfiring on deceleration or when trying to start the motor with the throttle held open. This is not popping but the kind of bang that brings the neighbours out to investigate the explosion.

Next I’m going to have a look at the TPS again and I’ll remove the exhaust and give it a shake to see what falls out.

No tank suck. However, the moat drain is rusted solid. Unable to clear it so it still is.

The moat drain blockage is what causes the tank to rust, the rain water (it does rain over there doesn’t it?) collects in around the filler and has no where to go but in the tank where it turns to rust. You don’t see this happen because it’s all gone before you get the cap out.
The drain is curly like a pigtail. To free it up use a short piece of inner cable in your electric drill. I take the tank off so you can attack it from both ends.
Perhaps use some light oil on the wire, eventually you will get it then once year or so a couple of drops of oil to stop it rusting again.

It does rain over here, sometimes it even stops for a while.
I’ll follow your advice. Thanks.

Do you have the large P8 ECU?
It has a brass screw visible beside the connector that adjusts the CO, I turned mine about 1/4 turn CCW from memory and it cured the annoying backfire by making the idle just a little richer The screw is connected to a 3 turn pot, it will click if you turn it too far.

Yes it is the large P8. Will be checking the CO screw when I do the TPS and Throttle body. Thanks for the PM.

Sorry I’ve been away for a while.

Tried turning CO screw 1/4 turn CCW, which seemed to make the backfiring worse but possibly improved low end torque. So returned it to original and then 1/4 turn CW which didn’t seem to make any difference to the 1/4 turn CCW.

Removed the the exhaust, pipes and silencers. No obvious problems there.
Ran the engine without exhausts (briefly, I’m trying to maintain a vestige of neighbourly relations), no real difference.

Obviously EFI, what sensors has it got? Have you checked them or tried replacing?

How old is the bike? Things like temperature sensors can pack up just from old age. Sounds to me like ECU is getting wrong temperature information. Also temp sensor must have good thermal conductive path from cylinder head, presume that’s where it would be located. Just guessing as don’t know this bike intimately.

It’s a 1999 model (with the older P8 ECU)

Sensors connected to the ECU - RPM, TDC (both replaced), Oil temperature, Air temperature, Barometric pressure and Throttle Position Sensor. However, the ECU is showing no error messages for any of them.

Yes,the oil temperature sensor is located in the cylinder head. I’ll do a resistance check on it at various temperatures and report back.

Oil temperature sensor has tested good.

So 18 years old. As are the sensors unless been replaced in the interim.

Not surprised there’s no DTC’s, but then temp changing is not an error. However the sensor(s) could be telling lies to ECU.

With the exception of the pressure sensor, all sensors have now been checked and/or exchanged.

I am going to do a couple of quick experiments on with the pressure sensor then I shall return to the fuel supply.

Fuel was the first thing I tested and ruled out as pressure appeared to remain constant throughout when measured just after the pump. Will see if I can connect a gauge between filter and injectors.

My trouble was front sensor small crack used fiat one niw working fine it expanded putting the ecu into confusion mode & not firing right but fine on stand still & good spark