Idle timing increase when engine warms up

[1950split]

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

[clonebug]

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]

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.

[Teeroy]

To eliminate cable causing it, unhook it and see what it does
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[mikedjames]

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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1974 VW T2 : Devon Eurovette camper with 1641 DP T1 engine, Progressive carb, full flow oil cooler, EDIS crank timed ignition.
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[Millionmph]

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

[1950split]

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

[Zed999]

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.

[Millionmph]

If you got heavier spings toss those in, maybe your advance springs are too light

[heimlich]

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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[VW_Jimbo]

[1950split]

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.

[heimlich]

[Glenn]

[heimlich]

[Glenn]

[rayjay]

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.

[1950split]

If memory serves me right I have a blue and a grey spring. I will check which bushing is fitted.

Could a pinhole in the casting of the heads be the cause? I’ve had my heads checked upon rebuild and the guy found, performing a vacuum check that some air was circulating through a pinhole between a inlet valve guide and the corresponding intake runner in the head. He told me he had fixed it with some grinding and filling with some kind of epoxy or Liquid Metal in the intake runner on the side of the piston.

I am now wondering if this repair has not gone (and hope it has not damaged the combustion chamber/piston.

A leak down test should be able to tell if this is the case?

As this high idle appeared suddenly and I have checked the distributor, swapped carbs, changed the inlet manifold gasket plus the fact that adjusting idle speed by reducing ignition advance does not work (it deed always in the past on all my engines), this hypothesis seems possible. What do you think?

[1950split]

Hi,

Back from abroad I have decided to adjust my idle mixture.

I ended up with 2 idle screws open 1/2 turns and 2 open by 2 turns.

As I thought that this difference is odd, I opened all 2 turns.

Then the AFR was at 11 and the engine stalled, impossible to have it run.

I am completely lost now. I have never had issues like this with my engines over more than 10 years in adjusting carbs..

I now have a doubt that my battery voltage is too low and that my Mallory Hyfire 6 al is not sparking enough. Or could it be a sparkplug issue?

The only good news is that all 4 idle screws are responsive..

[VW_Jimbo]

Check for a vacum leak. Take a drag off of a cigarette or whatever you like. Then blow straight into the carb (engine OFF) while holding the throttle open. Did you see any smoke shoot out anywhere? You might want to try a few times, or use a smoke machine.
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