4.2 duel fuel petrol not idling

[Sandman82]

Hi all, ive tried to find a similar thread, can anyone direct me or have you seen this problem.
I have a problen on a carby 4.2, dual fuel. The car ran fine on both fuels and regularly was swapped between each. A collapsed muffler was causing it to struggle which i eventually replaced after multiple patch ups. Had trouble with idle on petrol after that however plenty of power. On gas it ran fine. Also lagged on take off petrol only.
I rebuilt the carby so the power piston stopped lagging.
Ran perfect on fuel, swapped to gas also perfect. Swapped back to petrol and it wont idle. Still power under load but now backfiring out the exhaust when decellerating and dies under 1000rpm.
Tested the gas valve by shutting off the tank so its not duel fuelling. Fuel pump is fine and new filter. No air leaks.
Had another issue at the same time which was the gas solenoid had cracked and was shorting and blowing a fuse (i think) which is why i checked fir duel fuelling from a leaky solenoid.
What i font understand is why it ran oerfectly for days then after running gas and back to petrol it faults.
Any idea?

[mudnut]

Check the changeover switch for dud contacts or loose wires.

[LostBenji]

So, issues only came after fixing/replacing the exhaust?

If so, you need both petrol and gas carbs re-tuned to suit the changed airflow of the motor.

[taslucas]

So, issues only came after fixing/replacing the exhaust?

If so, you need both petrol and gas carbs re-tuned to suit the changed airflow of the motor.
Both carbs?
If it's running fine on gas then it would seem that the gas system is ok.
I don't think any TB42 has ever had fine enough tolerances to require tuning a carby to an exhaust.


>>>tappin from tassie

[taslucas]

As mudnut said maybe check the changeover switch?

The Tb42 carby can be a very temperamental thing. If it ran well on petrol after the carby rebuild then it would seem that all was rebuilt correctly.
Most likely sounds like a fuel starvation problem.
How have you checked that the fuel pump is good?
I have heard that backfiring can damage the float too.

What are the points like?

The timing for gas is set quite a bit further advanced than what petrol likes and if there's a slight issue with spark then it will be more pronounced when on petrol.

>>>tappin from tassie

[mudnut]

As Lucas posted, backfiring can cause damage. Check all of the vacuum diaphragms for splits or holes. ( Choke break, vac advance and fast idle actuator).

Have a look at this thread. I know it is RB30, but there is a section that covers dual fuel carbies.

http://www.nissanpatrol.com.au/forums/showthread.php?28517-RB30-Facts-Figures-and-Helpful-Hints

[threedogs]

Do these carbies dry out or have a power valve like a Holley.
Backfiring on gas can been too lean a mixture or air leaks
as well

[taslucas]

Do these carbies dry out or have a power valve like a Holley.
Backfiring on gas can been too lean a mixture or air leaks
as well
They do dry out if only run on gas for a very long period but as the op said it was running fine on petrol previously AND it's been rebuilt then I think it's safe to assume it shouldn't be dried out. The op also said it's running fine on gas.

>>>tappin from tassie

[threedogs]

You will usually find 90% of dual fuel problems are electric.
Are your leads in good condition if dirty they may be shorting?
Its not starving for fuel is it, has it got an inline cage filter??

[Bigcol]

first thing you have to do is............

check and replace ALL plugs & wiring for the Gas system that looks dodgy, those crimped plugs have been on there for close to 20years +

when you replaced the fuel solenoid, did you also check that all were tight and the Earths were all attached and having good contact......?

has it got electronic ignition or the old points??
if electronic ignition, check the Earths for that
if not
check your Tachometric relay - it may be cactii or on its way
67112

N# 1 priority is to check and replace any of the old wiring that looks old / tatty / corroded / crappy

clean and replace (as necessary) Rotor Cap & Plug Leads (get Original Cap - works better than Aftermarket) get good quality Leads - designed for Gas Cars
have you got a "modified" rotor button - tis about 2-3mm longer than standard - gives a better spark AT the right place


as has been stated earlier, your timing for Gas should be around thew 15-20*BTDC while Petrol will be around 5-8* BTDC

if you tune it correctly for Gas only, you will get great economy & power (for Gas) and it will get hot running petrol (as your timing is out) not baddly, but it will be hotter than normal, and your fuel economy will be shyte

[dom14]

Hi all, ive tried to find a similar thread, can anyone direct me or have you seen this problem.
I have a problen on a carby 4.2, dual fuel. The car ran fine on both fuels and regularly was swapped between each. A collapsed muffler was causing it to struggle which i eventually replaced after multiple patch ups. Had trouble with idle on petrol after that however plenty of power. On gas it ran fine. Also lagged on take off petrol only.
I rebuilt the carby so the power piston stopped lagging.
Ran perfect on fuel, swapped to gas also perfect. Swapped back to petrol and it wont idle. Still power under load but now backfiring out the exhaust when decellerating and dies under 1000rpm.
Tested the gas valve by shutting off the tank so its not duel fuelling. Fuel pump is fine and new filter. No air leaks.
Had another issue at the same time which was the gas solenoid had cracked and was shorting and blowing a fuse (i think) which is why i checked fir duel fuelling from a leaky solenoid.
What i font understand is why it ran oerfectly for days then after running gas and back to petrol it faults.
Any idea?

If I understood you correctly, it's running fine on LPG now, but on petrol it backfires when decelerating and won't idle on petrol either.

How did you rebuild the carby? Professionally or did you do it yourself?

I didn't understand what you meant by "power piston stopped lagging".

Sounds like a carby issue, but it's a very good thing to rule out all possible electrical issues first, including spark, spark plugs, leads, etc.
As mudnut suggested, it may be a good idea to check to see whether the secondary throttle valve is sticking. Put on a pair of protective glasses before checking the running engine from the top of the carby as a backfire can cook your eyes.
Backfiring usually means a lean mixture.
And the main causes of lean mixture usually are, either low fuel pressure(clogged filter, faulty fuel return valve, weak fuel pump, bad contacts to petrol solenoid or fuel tank/pump, carby issues like sticking valves, plunger, etc) or vacuum/air leaks.

[Arfa Brayne]

Symptoms you've described sound like overly lean mixture on the idle/deceleration circuit.
This would most likely be an air leak into the inlet manifold (which you have ruled out) or a blockage in the idle mixture port of the venturi - possibly stirred up when you worked on the carby.
However it could also be a very rich mixture at idle cause by the engine cut solenoid on the side of the carby. A lot of people think this is a fuel cut solenoid, but on early Patrols it kills the engine by flooding the inlet with fuel to stop "running on" detonation firing when the ignition is turned off.
When the power is applied, the solenoid should "click" and you should feel it also with your finger. If there is no click, check for power supply and continuity with a multimeter