Help, 12HT gurus

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Jul 3, 2004
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Anchorage, AK
I have a surging problem.
85 HJ75 ute with a 12HT swap
rebuilt 12HT (supposedly for more power)
30% bigger injectors
max fuel screw basically all the way out. the other screw on the firewall side is set such that there is minimal smoke while accelerating.
steady driving at 65mph/10-12psi boost. surging comes and goes, but is definitely happening intermittently at 65mph, generally boost sits at 10psi. boost fluctuates between 10-12psi under steady power, or increasing power to maintain 65 on inclines.
No noticeable surging at 55mph, or 60mph, or while accelerating.
I originally had the fuel return plumbed to the fuel in line to the engine thinking this might be the cause. I re routed the return to the rear tank and no change.
Any thoughts?
 
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It came as a fresh rebuild. What would make it surge up (increase power) vs surge down (decrease power) and only at a specific constant speed and boost level?
 
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It came as a fresh rebuild. What would make it surge up (increase power) vs surge down (decrease power) and only at a specific constant speed and boost level?

Its probably not timing.. its just that timing issues do cause a hunting symptom at idle. Is the pattern of surging entirely predictable and the 'cadence' the same every time?
 
Joined
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Its probably not timing.. its just that timing issues do cause a hunting symptom at idle. Is the pattern of surging entirely predictable and the 'cadence' the same every time?
All regimes except conditions states, no noticeable surging. In the right conditions it is predictable in that it will do it randomly for periods of time. Would the “rack” in the pump be susceptible to a week spring which could cause this surging? And the to me really weird part is that the surging is upward on the boost gauge instead of down. If sitting at 65mph steady boost at 10psi it will go up to 12psi and back down to 10psi and back up 12psi and repeat until it somehow decides it had enough.
 
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All regimes except conditions states, no noticeable surging. In the right conditions it is predictable in that it will do it randomly for periods of time. Would the “rack” in the pump be susceptible to a week spring which could cause this surging? And the to me really weird part is that the surging is upward on the boost gauge instead of down. If sitting at 65mph steady boost at 10psi it will go up to 12psi and back down to 10psi and back up 12psi and repeat until it somehow decides it had enough.

I am way out of my depth when turbos are involved.. sorry.

How is the boost compensator achieved on the 12HT pump? I think I'd start there, but I'm just spitballing now with no real experience to draw upon.
 

AussieHJCruza

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Got a picture of this piece?
I don't have a 12HT near me and can't seem to find anything online that shows it properly, but from memory, it hangs off the governor under the intake manifold.

There should be a boost line running from the crossover to the boost compensator. I haven't investigated but assume they work the same way as a boost comp on a rotary pump and defuels in the absence of boost rather than increasing fuel under boost if that makes sense.
 
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One of these?

F91843CB-84DB-4FB5-BA33-5A421CF4DC0B.jpeg


2BD18A51-2D83-4825-8952-32CE521BA69F.jpeg
 

Nz Nath

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That's a throttle damper. I think it only works at low throttle or when you close the throttle, but I'm not certain of that. I've read about people adjusting it, maybe try searching old posts.
 

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