GPz1100A2 afterfire

Hi all,

My 83 GPz1100 unitrak is popping and banging through the exhaust once it warms up. I'm running the standard DFI.

It starts first time on the starter and runs fine for the first few minutes. Then once its warmed up it starts to pop and bang on tickover and when revved.

It starts with a few pops every few seconds then gets progressively worse as it warms through, until it will let out an almighty bang every 20 seconds or so and lots of popping.

So far I've...
Replaced the coils with dynas (it now runs exactly as badly as with the old OEM coils)
Ignition leads are new as well as caps
New plugs
The pulsing coils check out to spec on my multimeter.
Throttle bodies have been synchronised with a vacuum guage
It seems to be pulling fuel thru strongly
Doesn't seem to be any air leaks on the inlet rubbers.
No codes flashing on the control box

Only thing I can think of is valve clearances are out (although these were set within specs a few months ago and the bike has done hardly any miles)

Attached is a pic of the plugs. They don't look a consistent colour so it's not happy about something. 

 

Looks like your check valve mechanism is clogged by rust and need a good clean. Fuel pressure will get to high, bike is runnig on fat air/fuel ratio. Its a common problem on the EFI bikes.

kzrider.com/forum/2-engine/615786-fuel-t...asket-sealant#847300

Thanks Scirocco. replaced that last month with a brand new one as the old one was rusted, so I don't think it will be that.

I too am leaning towards something mixture related but not sure.

Possible misfiring?
No spark loads exhaust with raw fuel and boom.
I'm not familiar with the fuel injection and whether you can disable individual fuel injectors.
Essentially, you would be doing a cylinder balance test with fuel instead of spark.
Sounds like the system is ok in open loop or warm up.
You could also get 4 inline spark testers to see if a particular plug(s) is misfiring.
An IR temperature gun aimed at exhaust pipes may also give clues.

wrote:

Thanks Scirocco. replaced that last month with a brand new one as the old one was rusted, so I don't think it will be that.

I too am leaning towards something mixture related but not sure.

It's too rich, especially #3 and #4.  That can cause a backfire.  Did you install the tank check valve before or after
replacing the spark plugs?  New spark plugs are needed. 

What's the air filter look like?  How many miles are on the bike?

Can you post a pic of the vacuum gauge set-up you are using?  I only use mercury synchronizers.  Most of the vacuum gauges
I've seen are unbranded, imported junk and I would never use them.

Has this bike been sitting for a long time?  How old is the gas in it?  If the gas condition (age, octane, etc.), is not known, I would 
drain the tank and run a fuel injection cleaner through it.  I use Liqui-Moly brand, and it will clean the injectors very quickly.  I
also use a product called Star Tron.  I think dirty / clogged injectors are likely and causing it to run rich but it's easy to fix with
chemical technology.  HEET dry gas (red bottle) is also a good idea, to remove water.

These bikes will never reach closed-loop, as they don't have an 02 sensor.  They are in Alpha-N mode (TPS and RPM mapping).

Check for air leaks by using soapy water in a spray bottle.  

Is the bike on the original fuel pump and fuel pump regulator?  You should verify fuel pressure.  I believe it should be around 33PSI,
at idle, but verify that first via a factory service manual.



  


 

Just for your interrest. Check valve internal parts.

 
 
 

Thanks guys, some great advice there.

Bike has been fired up every weekend while I've been trying to find some issues but hasn't really been on a run for ages.

Petrol was fresh about a month ago but it wouldn't hurt to replace.

The vacuum gauges are the dial type so I guess could be inaccurate.

Air filter was replaced a while ago but hasn't run significantly - looks clean

Injectors were all removed and I checked they were not blocked by running a small AA battery charge thru and spraying solvent thru. They all seem to be opening when the bikes running using a bulb tester.

One thing i did notice yesterday when I ran it again was plugs 3 and 4 were significantly hotter than 1 and 2 when I pulled them out. i.e I couldn't touch them, but 1 and 2 were not quite as hot.

1. I'm going to re check the check valve to see if its sticking
2. Change the plugs
3. Run some injector cleaner thru with fresh petrol

Whats the best way to check the fuel pressure. Is there a pressure tester I can buy?

Thanks again guys for all your advice!

 

Your plugs should be too hot to hold after the bike has run.
The insulator nose hovers between
500°C (932°F) to 800°C (1472°F).
I'd be curious why 1 and 2 are not hot.

Yes, me too. If it was coil related you'd think it would be 1 and 4 or 2 and 3.

I'm thinking its something to do with fuel delivery. I'm going to pick up an infra red gun tomorrow and see what temps the exhausts are running once it warms up.

Hard to know. I've given up on guessing because a lot of times I have been wrong.
But those cylinders seem a bit cold.
Let us know what you find with the IR gun.

If it's the original fuel pressure regulator, you should just change it.  Here's the one I bought for my 1982 GPZ1100:
Change epay to ebay in the link.

www.epay.com/itm/381918378218

The fuel pressure regulator must maintain 33PSI at idle. 

Remember, this is a primitive DFI system.  There is no air/fuel correction.  It's not a closed-loop system, so It
simply will not run correctly if the fuel pressure is off. 

Also, make sure the vacuum hoses to the fuel pressure regulator and air intake boots are in good condition.

The backfire is occurring in cylinders #3 & #4.  Excess fuel is being burned in the exhaust pipes of #3 & #4.
The heat transfers to cylinders #3 & #4.  So, those spark plugs got hotter.   

Awesome, thanks for the link. You're right, it is the original regulator, so it won't do any harm to change it out.

Will get one ordered.