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Slow Idle

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R

RonR

Jedi Trainee
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Folks,

The slow run valve on my 3000's HD6 carbs has never seemed to be able to get the car to idle correctly.

The service manual states to start with turning it out 2-1/4 turns, then turn it in if the idle is too fast.
The idle has never been too fast. In fact, backing out the screw 6 turns will still not get the carbs to idle properly.

The engine has a higher performance camshaft, and a 10:1 compression ratio.

Any suggestions on what I might be doing wrong, and what might help my situation, will be highly appreciated.

Thanks.
Ron
 
I have the same issue with my DWR8 performance cam, HD8 SUs and higher compression. It won't idle at all using the slow run screws. I have to use the fast idle screws to set the idle speed, and close the slow idle screws.

I think the issue is caused by the overlap in the cam.
 
John,

I am currently using both the slow run valve and the fast idle screws to get it to idle.
I wonder if closing them or having them part way open (out) makes any difference.
 
RonR said:
Folks,

The slow run valve on my 3000's HD6 carbs has never seemed to be able to get the car to idle correctly.

The service manual states to start with turning it out 2-1/4 turns, then turn it in if the idle is too fast.
The idle has never been too fast. In fact, backing out the screw 6 turns will still not get the carbs to idle properly.

The engine has a higher performance camshaft, and a 10:1 compression ratio.

Any suggestions on what I might be doing wrong, and what might help my situation, will be highly appreciated.

Thanks.
Ron
Click to expand...
I would check the butterflies for excessive play. The teflon seals when new, still leaked. Be sure to check the seal on the slow run valves adjustment. Delrin bushings will probably take care of that. There’s a guy who modifies and installs them.
 
high idle speed on warm engine is often caused by a vacuum leak at the carbs and/or intake manifold
 
Patrick,

The butterflies seem fine.
The parts book does not show any type of seal on the slow run screw. I see no way to add any bushings either. Apparently, the screw threads are all that seal the screw, assuming it needs sealed.

NutmegtCT, the issue is not a high idle, it is the opposite: the inability to get the engine to idle using the slow-run screw.
 
RonR said:
Patrick,
...

NutmegtCT, the issue is not a high idle, it is the opposite: the inability to get the engine to idle using the slow-run screw.
Click to expand...
Thanks - my mistake!

SU has some adjustment steps which might help. Single:

HD Carburetter Tuning – Single Carb Adjustment Guide

Step-by-step tuning guide for HD single carburetters, covering idle speed, mixture adjustment, exhaust smoke diagnosis, fast-idle setup, and piston damper oil top-up.
sucarb.co.uk sucarb.co.uk

Dual:

HD Carburetter Tuning – Multi-Carb Adjustment Guide

Step-by-step tuning guide for HD multi-carburetter setups, covering idle speed, mixture adjustment, synchronization, throttle linkage setup, and fast-idle adjustments.
sucarb.co.uk sucarb.co.uk

Tom M.
 
Last edited:
RonR said:
John,

I am currently using both the slow run valve and the fast idle screws to get it to idle.
I wonder if closing them or having them part way open (out) makes any difference.
Click to expand...
I have the slow idle screws closed all the way. It didn't seem to make much difference if they were open or closed.
 
RonR said:
Patrick,

The butterflies seem fine.
The parts book does not show any type of seal on the slow run screw. I see no way to add any bushings either. Apparently, the screw threads are all that seal the screw, assuming it needs sealed.

NutmegtCT, the issue is not a high idle, it is the opposite: the inability to get the engine to idle using the slow-run screw.
Click to expand...
You have seals for the slow run valves. Go to a MOSS catalog and look at the exploded view to see all the parts. I keep spares in my parts supply.

You will also see the throttle shaft bushings too. They are Teflon type strips.
 
If the butterflies are not 100% centred in the throat of the carb then the car will not idle as the butterflies will always be partially open . Crack the screws on the butterflies loose a bit close them fully make sure they are centred properly and 100% closed and retighten the screws .
 
I'm a little late to the party but I'll throw my 2 cents in. I have DWR8 cam and 3 HD6 and no difficulty with the idle speed (normally around 700-800). I have always had the throttles completely closed on HD carbs (I used to run twin HD8s). In mine, if I want to, I can crank the idle speed up to 1000-1200 or more. I do not think the cam grind/overlap has anything to do with the idle difficulties some of you are having. I suggest its's the carbs themselves, butterflies not centered, shafts needing re-bushing/sealing, or the bypass passages partially blocked. I think checking/cleaning the passages may help. Something like oxyacetylene torch tip cleaners would work. And fresh o-ring seals on the slow run needles to make sure there are no frayed bits of o-ring partially disrupting the airflow.
 
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