TB42S Straight Petrol Won't Rev Past 3000RPM - HELP

Hi fellas,
I have this TB42s that will not rev past 3000RPM, It starts, Idles and drives fine but will not exceed 3k revs. It sounds more like it's being limited instead of struggling to rev higher. What could it be? I don't think it's a fuel issue, Although when I open the throttle by pulling on the cable whilst looking down the carby the secondaries do not open? Does the motor have to be running to open up? I think my problem may be the 2 springs on the rotor button but I have not yet checked. I'm after advice on what it could be, It recently had a problem where it would only run for short periods of time before dying but I found out it was a blocked fuel filter and after removing it she runs sweet now apart from the problems listed above. It used to be on gas but was told by the owner the carby was rebuilt and tuned for straight petrol. Any help is appreciated! It also has Pertronix ignition. Thanks!

try looking at the vacum advance,if it is ceased the timing will be out once it's going into the higher rpm range..

Try manipulating the second stage linkages at the back of the carby to if it is stuck.

The secondary butterfly only opens a very small amount when I open the throttle wide but I feel like it should be opening a lot more than it does, It just cracks open slightly..

The butterfly should move easily to all positions when manipulated. My carby RB30 is basically the same and was not letting the engine rev. I had to take the carby off to free the second stage by smoothing the bore and butterfly with some fine wet and dry sand paper. I then cleaned the carby thoroughly. Make sure you release the fuel pressure before undoing the fuel hoses.

Take off your aircleaner and look down into the carby. There's a little scoop looking piece of metal that sits under the secondary butterfly thats supposed to direct air/fuel more efficciently into the manifold.

The part can wear causing it to slide forwards and either block the butterfly from opening properly or jam it wide open(much more dangerous).

If you cant see it, pop your carby off(4 bolts) and check it out. If it is loose just epoxy that mofo in place.

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Originally Posted by Irish

Take off your aircleaner and look down into the carby. There's a little scoop looking piece of metal that sits under the secondary butterfly thats supposed to direct air/fuel more efficciently into the manifold.

The part can wear causing it to slide forwards and either block the butterfly from opening properly or jam it wide open(much more dangerous).

If you cant see it, pop your carby off(4 bolts) and check it out. If it is loose just epoxy that mofo in place.

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You mean the scoop thing that goes on the inlet manifold, right?
In RB30, it rests on the carby base plate.(picture below).
In my case, it had broken into pieces by some weird reason.
I documented it in below thread.

http://www.nissanpatrol.com.au/forum...d-rattle-noise

There may also be another fuel filter near the tank.

>>>tappin from tassie

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Originally Posted by CarbyLife

Hi fellas,
I have this TB42s that will not rev past 3000RPM, It starts, Idles and drives fine but will not exceed 3k revs. It sounds more like it's being limited instead of struggling to rev higher. What could it be? I don't think it's a fuel issue, Although when I open the throttle by pulling on the cable whilst looking down the carby the secondaries do not open? Does the motor have to be running to open up?

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As far as I can tell, yes. Secondary throttle valve is "vacuum" operated rather than mechanically triggered by the throttle shaft/linkage(in this particular Nikki carby). There's a large vacuum diaphragm attached to the secondary valve linkage to trigger it. The best way to see the operation of secondary throttle valve is to look down the secondary barrel while revving the motor high. You should be able to see opening a little bit and secondary venturi spraying a bit of petrol at it. It's a bad idea to look down into the carby without wearing eye goggles 'cos a strong enough backfire can instantly cook your eyes to go blind "naturally".

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I think my problem may be the 2 springs on the rotor button but I have not yet checked. I'm after advice on what it could be, It recently had a problem where it would only run for short periods of time before dying but I found out it was a blocked fuel filter and after removing it she runs sweet now apart from the problems listed above. It used to be on gas but was told by the owner the carby was rebuilt and tuned for straight petrol. Any help is appreciated! It also has Pertronix ignition. Thanks!

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Does it still have the LPG mixer on top of the carby or has it been removed for good?
Did you remove the fuel filter or did you replace it with a new one?

If the secondary throttle is working as it should and fuel get sprayed by secondary venturis on higher revs,
I suggest you spray some carby cleaner into the carby throttle valves while revving high. Make sure to warm up the engine for few minutes before you do that.
Not being able to rev high is a classic sign of fuel starvation I believe.
I guess it's a matter of finding out where it happens.
If it was converted to LPG previously, then it's possible the original mechanical fuel pump was replaced with an intank or external electric fuel pump. If that's the case, checking fuel pressure and fuel pressure regulator is a very good idea.

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Originally Posted by dom14

As far as I can tell, yes. Secondary throttle valve is "vacuum" operated rather than mechanically triggered by the throttle shaft/linkage(in this particular Nikki carby). There's a large vacuum diaphragm attached to the secondary valve linkage to trigger it. The best way to see the operation of secondary throttle valve is to look down the secondary barrel while revving the motor high. You should be able to see opening a little bit and secondary venturi spraying a bit of petrol at it. It's a bad idea to look down into the carby without wearing eye goggles 'cos a strong enough backfire can instantly cook your eyes to go blind "naturally".

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I took from his post that he didn't have the engine running when looking into the carb and manipulating the throttle, from this:

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when I open the throttle by pulling on the cable whilst looking down the carby the secondaries do not open? Does the motor have to be running to open up?

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