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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Originally Posted by Irish

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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Yeah, I think that was the case.

I uploaded a couple of videos on below thread,showing the operation of this Nikki carby primary and secondary valve operation.

http://www.nissanpatrol.com.au/forum...l=1#post706761

RB30 & TB42 Nikki carbies are essentially the same.
They are either tuned differently or jetted slightly differently to suit each engine.
The replacement Nikki carby for sale on ebay is listed as suitable for both engines.
A good indication that it's the same carby external and internally.

There is one maybe that could be my problem as the second one was blocked

The linkages were sort of seized at first until I got some lube onto them but they seem ok now but I'll go back and make sure everything's in order but I can open the secondary using the linkage that directly operates it but pulling the throttle cable only opens it slightly

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Originally Posted by CarbyLife

The linkages were sort of seized at first until I got some lube onto them but they seem ok now but I'll go back and make sure everything's in order but I can open the secondary using the linkage that directly operates it but pulling the throttle cable only opens it slightly

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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 you can get the slight opening of the secondary throttle valve by wide opening throttle when the engine's running, then it's good.
You should also see petrol spraying out of secondary venturi.

However, if you carby secondary throttle valve opens a little bit when the primary throttle is wide open(when the engine's NOT running), then
something's not right with your carby, it's been mechanically modified or it's not the original Nikki carby, but something else.

In mine it doesn't and the way the primary and secondary throttle linkages are attached, it's impossible to mechanically open the secondary
throttle by wide opening the primary throttle. They are not mechanically connected via throttle linkage, but rather connected via vacuum lines.
I guess the way it works is that the vacuum diaphragm moves from extra vacuum when the throttle is wide open & the connected arm pushed on the linkage connected to the secondary throttle valve.

Move the throttle shaft side wise and see if there's any play. If there's any play, it's possible throttle valves(both primary and secondary) get stuck. If you can manipulate the throttle shaft that way and move the throttle valves and get them to stick to the walls, then throttle body is not good, 'cos it's too worn out, which means replacing the throttle body with a used one. In that case, it's better to replace the whole carby with a used one or a brand new one.

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Originally Posted by CarbyLife

There is one maybe that could be my problem as the second one was blocked

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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?