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Idle timing increase when engine warms up
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1950split
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PostPosted: Sat Mar 23, 2024 11:06 am    Post subject: Idle timing increase when engine warms up Reply with quote

Hi Everyone!

I have a strange issue.

My 2276 engine with a msd distributor and Mallory Hyfire 6 AL2 is doing strange things
When I start my engine, idle is at 10 degrees btdc at 1000 RPM. But soon after I start driving, idle increases at 1400 rpm, ignition increases to 15 degrees btdc.

I then readjust timing to 10 btdc/1000 rpm, and after a few blips on the gas pedal, I am back to 1400 rpm/15 degrees btdc.

I have torn the distributor apart, cleaned and lubed it. Then I have switched from 48 IDA’s to a set of DRLA’s (as I thought it might be carb related), but nothing changed. I have the same issue whether with a CSP or a Berg linkage.

I also changed my intake gaskets.

What do you think is the issue? I don’t want to drive with such a high idle/15 degrees btdc.

Could a sticky gas cable be the issue? But the Berg has strong return springs so I have some doubt that this is the problem.

Thanks!

Phil


Last edited by 1950split on Sat Mar 23, 2024 1:21 pm; edited 4 times in total
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clonebug
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PostPosted: Sat Mar 23, 2024 11:10 am    Post subject: Re: Idle timing increase when engine warms up Reply with quote

Check your throttle linkage. Once the engine warms up it might expand and pull the throttle plates open which will then increase rpm and bump the timing up due to rpm increase.
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1950split
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PostPosted: Sat Mar 23, 2024 11:13 am    Post subject: Re: Idle timing increase when engine warms up Reply with quote

Hi,

Yes I have set up my linkage (rod lengths) with the engine warm and running in order to monitor if rpm increase when I tighten the Gene Bers rods.
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PostPosted: Sat Mar 23, 2024 4:36 pm    Post subject: Re: Idle timing increase when engine warms up Reply with quote

To eliminate cable causing it, unhook it and see what it does
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PostPosted: Sun Mar 24, 2024 10:41 am    Post subject: Re: Idle timing increase when engine warms up Reply with quote

Yes, your engine runs more freely when it is warmed up .

So it climbs up the advance curve on the distributor as they all do, increasing timing and RPM on the way.

A stock bus would set timing at 800-850 rpm , you may do better aiming for a lower idle speed, with your 10 degrees timing advance. In a stock distributor, 1000 rpm is about where idle timing starts to climb from the plateau below 1000 rpm.




I can get my EDIS based ignition on my 1641 to idle at 600 rpm with 10 degrees advance, I have programmed in a timing "valley" around 850 rpm, with timing advancing both sides of the valley. I prefer it at 850 rpm though.
In fact I have watched it pick up and idle from 500 rpm..
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Millionmph
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PostPosted: Sun Mar 24, 2024 4:00 pm    Post subject: Re: Idle timing increase when engine warms up Reply with quote

Read the title, and nobody mentioned the obvious yet, vaccuum leak when warmed up....
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1950split
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PostPosted: Mon Mar 25, 2024 9:24 am    Post subject: Re: Idle timing increase when engine warms up Reply with quote

When I have been adjusting my carbs in the past, I have been able to adjust idle speed by rotating the distributor.

In the present situation it has no impact, I rotate the distributor but almost instantly idle goes from 1000rpm to 1500rpm.

Vacuum leak, not at the manifold gasket level.

I’ll recheck my valve play , I guess that perhaps a play that is too tight with a warm engine could cause this.

Ultimately I’ll do a leak down test. That will at least help me to find which cylinder is guilty
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PostPosted: Mon Mar 25, 2024 9:46 am    Post subject: Re: Idle timing increase when engine warms up Reply with quote

I think you have your timing set just on the point the advance kicks in. When it does, the engine revs higher so the advance is maintained, which keeps the revs up.
If your distributor is adjustable in a way that delays the advance kicking in I'd try that. Otherwise back off the idle timing a degree or two.
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PostPosted: Mon Mar 25, 2024 12:23 pm    Post subject: Re: Idle timing increase when engine warms up Reply with quote

If you got heavier spings toss those in, maybe your advance springs are too light
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PostPosted: Mon Mar 25, 2024 5:21 pm    Post subject: Re: Idle timing increase when engine warms up Reply with quote

RPM and advance are directly correlated.

Are you saying that you are at idle and 10 degrees than wait 5 minutes and it goes to 15 and then you adjust it back down to 10 and 5 minutes later it goes back to 15?
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PostPosted: Mon Mar 25, 2024 10:16 pm    Post subject: Re: Idle timing increase when engine warms up Reply with quote

Millionmph wrote:
Read the title, and nobody mentioned the obvious yet, vaccuum leak when warmed up....


This was my first thought, after I read the title. Or the wrong vacum signal (manifold) the distributor (should be ported).

But really depends on how it was set up.
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PostPosted: Tue Mar 26, 2024 6:18 am    Post subject: Re: Idle timing increase when engine warms up Reply with quote

Hi Heimlich,

Yes that’s exactly what I mean.

What is weird is that it started doing this while I was on a longer drive.

At the beginning of the drive everything was fine and it suddenly started acting this way. I’lll try to back-off my idle speed screws a little but I am pretty confident that the throttle plates are nearly shut currently.
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PostPosted: Tue Mar 26, 2024 6:23 am    Post subject: Re: Idle timing increase when engine warms up Reply with quote

1950split wrote:
Hi Heimlich,

Yes that’s exactly what I mean.

What is weird is that it started doing this while I was on a longer drive.

At the beginning of the drive everything was fine and it suddenly started acting this way. I’lll try to back-off my idle speed screws a little but I am pretty confident that the throttle plates are nearly shut currently.


Sounds like the distributor is turning in the clamp. Take a marker or something similar and put a line on the shaft and put that same line on the clamp. Drive it until you see the issue and see if it moved.
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PostPosted: Tue Mar 26, 2024 6:42 am    Post subject: Re: Idle timing increase when engine warms up Reply with quote

heimlich wrote:
1950split wrote:
Hi Heimlich,

Yes that’s exactly what I mean.

What is weird is that it started doing this while I was on a longer drive.

At the beginning of the drive everything was fine and it suddenly started acting this way. I’lll try to back-off my idle speed screws a little but I am pretty confident that the throttle plates are nearly shut currently.


Sounds like the distributor is turning in the clamp. Take a marker or something similar and put a line on the shaft and put that same line on the clamp. Drive it until you see the issue and see if it moved.

Some aftermarket clamps are soft and the entire clamp twists, so the mark won't move because the engine clamp is moving.
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PostPosted: Tue Mar 26, 2024 6:46 am    Post subject: Re: Idle timing increase when engine warms up Reply with quote

Glenn wrote:
heimlich wrote:
1950split wrote:
Hi Heimlich,

Yes that’s exactly what I mean.

What is weird is that it started doing this while I was on a longer drive.

At the beginning of the drive everything was fine and it suddenly started acting this way. I’lll try to back-off my idle speed screws a little but I am pretty confident that the throttle plates are nearly shut currently.


Sounds like the distributor is turning in the clamp. Take a marker or something similar and put a line on the shaft and put that same line on the clamp. Drive it until you see the issue and see if it moved.

Some aftermarket clamps are soft and the entire clamp twists, so the mark won't move because the engine clamp is moving.


The same scribe method will work. Continue the scribe down to and include the engine case.
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PostPosted: Tue Mar 26, 2024 7:00 am    Post subject: Re: Idle timing increase when engine warms up Reply with quote

heimlich wrote:
Glenn wrote:
heimlich wrote:
1950split wrote:
Hi Heimlich,

Yes that’s exactly what I mean.

What is weird is that it started doing this while I was on a longer drive.

At the beginning of the drive everything was fine and it suddenly started acting this way. I’lll try to back-off my idle speed screws a little but I am pretty confident that the throttle plates are nearly shut currently.


Sounds like the distributor is turning in the clamp. Take a marker or something similar and put a line on the shaft and put that same line on the clamp. Drive it until you see the issue and see if it moved.

Some aftermarket clamps are soft and the entire clamp twists, so the mark won't move because the engine clamp is moving.


The same scribe method will work. Continue the scribe down to and include the engine case.

Ok, that will work.
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PostPosted: Tue Mar 26, 2024 8:12 am    Post subject: Re: Idle timing increase when engine warms up Reply with quote

What springs and advance limiting bushing are you running in the MSD distributor? When I recurved distributors for a living I always set them up so the advance didn't start to kick in for 200 to 300 rpm above idle speed. If the advance is kicking in at or slightly above idle speed you will have an inconsistent idle.
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