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 Posted: 08-11-2022 08:40 pm
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Jh092
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Joined: 04-01-2021
Location: Strongsville , Ohio USA
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I’m not complaining and this probably falls under, if it ain’t broke, don’t fix it.

I recently sorted out some running issues on my JH5 (#20092) and it is running better than ever. What concerns me is it idles at about 550 and I would like to know if that’s too low and if it hurting anything. The idle is smooth and acceleration is smooth with no hesitation or dead spots thru to the main circuit.

Details:
Stroked 2.2 l, low compression 912 pistons
5 speed
Dellorto DHLA 45 E
Distributor - points. Distributor recurved by Advanced Distributor to match engine setup
Timing 15 btdc
Gas - regular

Idle should be somewhere between 850 and 950 from what I can gather. By increasing the idle, I go further into the progression holes. Currently, the throttle plate covers 1-1/2 holes. I’m concerned that if I increase the idle, I might lose my smooth transition.

Should I be concerned or just enjoy it?

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 Posted: 08-15-2022 01:30 am
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Tom Bradley
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I am very surprised that a 912 engine idles well at 550 RPM, but do not see any particular problem with it. Have you double-checked this with an external gauge? The original ones can be inaccurate after all these years.

What more concerns me is the possibility of detonation (pinging). This is especially harmful for these engines. With the relatively advanced timing (15 btdc) and "regular" gas (what octane?) this seems a possibility. The recommended fuel is at least 91 octane, which is "premium" nowadays where I live and the recommended idle advance is 12 btdc. But as long as you don't have any problem with this, I guess it will be OK.

Last edited on 08-15-2022 01:32 am by Tom Bradley

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 Posted: 08-15-2022 02:57 pm
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noomg
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Jh092,

The idle you're talking about (850 to 950) is for a stock motor, your mill is far from stock. My '67 El Camino has a modified 327 which idles nicely at 500 rpm.

The question is how it behaves at 550 rpm. From a stop if you release the clutch a little too fast will it stall or does it require extra gas to avoid lugging off the line? If it accelerates smoothly off the line without having to rev it first I wouldn't worry about it.

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 Posted: 08-29-2022 05:22 pm
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Jh092
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Location: Strongsville , Ohio USA
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Better to be safe than sorry. Although the engine came off idle smoothly without any help, there was no pinging, and plugs looked good, I decided to make a few adjustments. I decreased the timing to 12 btdc, increased the idle 850, and changed to 89 octane gas. None of these changes affected the drivability or performance but I did reduce the chance of detonation.

Thanks for the input.

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