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Discussion Starter #1
Hi guys! New to the Forum but looking forward to meting you all!

Unfortunately my first post has to be because I Have a problem! hopefully you guys can help me out

First details of the car
Its a 1989 GQ SWB 4.2 TB42S (carby) Running dual fuel
Gas system is Impco with a 300A Mixer

Anyway the problem is a little long but i will try to explain it clearly

Originally the car needed a general tune up when i bought it, as it was running a little rough, took me a while to get around to this but it has been done!

The problem I am having now is that whenever I am driving, it has a seemingly random misfire! As it misfires the Tacho meter fluctuates, the needle quickly drops and returns as if the power is being cut and turned back on very quickly. this causes a back fire sometimes and makes it quite hard to drive. It can happen when the engine is hot or cold, under load/no load but seems the worst when I am coasting like slowing down for an intersection, that's when it flicks constantly!

This problem happens on both Gas and Petrol but i am just trying to get it right on petrol first!

SO... I have replaced
The spark plugs (NGK BP5ES)
Plug Leads (Champion LPG Compatible)
Dizzy Cap (Bosch)
Rotor button - extended type
Points
Coil (Bosch)

And Set the point gap
Set dwell to approx 36 degrees
Timing to Approx 12 degrees
Checked the ballast resistor operation (12V on cranking 9V running)
Adjusted Tappets to .381

Also fitted brand new professionally fully reconditioned Genuine Carby

It has a great IDLE and feels good throughout the rev range except for this random miss

Have even had it on the old computer diagnostic machine at work that does a power balance etc and can't spot the problem

Me and the guys are out of ideas and I am hoping someone out the knows whats going on!

Was thinking of getting a Pertronix Ignitor but I am worried the problem is elsewhere and was hoping to diagnose the problem before chucking more parts at it hoping to fix it

Please help and look forward to hearing from you all!

Rob
 

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Discussion Starter #4
But Is this going to be the problem or are we just guessing and hoping this will fix it? surely they never came form the factory like this!
 

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Try running power straight from the battery to the coil and go for a drive as it could
be just a dodgy ignition barrel .
 

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The TB42's came from the factory with the intermitant missfire, that was a standard feature.

the only way ive seen the missfire problem resolved is by going to electic ignition.
 

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did they come from factory as dual fuel??

I had the same problem, with the same car/set up as you. Put in a Pertronix, flamethrower and extended rotor buttom, new dizzy cover and leads, new plugs. Problem gone.
 

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Discussion Starter #9
Try running power straight from the battery to the coil and go for a drive as it could
be just a dodgy ignition barrel .
Yeah have tried that, still no good! tell me the problem is between the coil and the plugs somewhere i Guess

Pertronix is looking like the only way to go :(
 

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This may not be relevant but who knows?

About a year ago I had misfiring problems and tried everything I could think of.
In the end I discovered that the petrol was contaminated. I'd filled up from a servo that had been flooded and they hadn't bothered to clean out the tank properly.

Now I buy my petrol from a servo on a hill...never gets flooded.
 

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Discussion Starter #12
Yeah i was going to get one, but its just another expense i cant really afford and im qgoing to be pretty dissapointed if it does not fix it. sounds like a couple people have had similar problems and hae fix them with the pertronix though
 

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Spark is the weakest point when running gas if theres not enuf,you will always have trouble no matter what you do....
Had to learn this from my days with rotarys......They relied on spark big time and so does gas...
You can run ****ty fuel with a weak spark(to a extent)you can with gas.
It is to do with the different temps of the fuels going in before or when they ignite.
 

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I have had the fuel cut solenoid **** itself before, but that caused stalling at low speed or idling.

I had a misfire also, fitted a petronix and never had one since !
 

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Discussion Starter #16
Spark is the weakest point when running gas if theres not enuf,you will always have trouble no matter what you do....
Had to learn this from my days with rotarys......They relied on spark big time and so does gas...
You can run ****ty fuel with a weak spark(to a extent)you can with gas.
It is to do with the different temps of the fuels going in before or when they ignite.

Still trying to get it right on petrol! Have not been using gas at
All until I get it right on petrol
 

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take your tacho wires off your coil and see what happens my tacho wire was sorting and making my coil brake down
 

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Do yourself a favour and fit a pertronix, it will fix your miss fire and tacho problems, definately worth the money, you wont be disapointed.
 

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Firstly I have a pertronix system fitted to my 89 duel/fuel and yes it is great but mine run fine prior to converting it so to the op yes I understand where you are coming from.
Firstly you have not stated that you have replaced the condenser, and for others that the pertronix solved the problem then you may have had a condenser problem.

If the condenser isn't the problem then I would still be looking at leads, Have you looked under the bonnet in the dark with motor running.

I remember years ago my brothers FC holden had a miss going around certain corners and we traced it to a lead shorting out on the master cylinder, the motor flexed and the lead touched the metal.

I know they are frustrating but good luck with it.
 

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Firstly you have not stated that you have replaced the condenser, and for others that the pertronix solved the problem then you may have had a condenser problem.


You make a good point Murraman, but in my instance when i had the miss i replaced with all new leads, plugs, dizzy cap, rotor button, and also the condensor in the end, had it tuned and timing set and i still had a miss and problem with my tacho flickering, in the end i read on here about the pertronix and i fitted that up, then my problem went away. I really think that if he just fits one up his problem will be gone, especially if he has replaced all other parts already.
 
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