high idle in neutral, low when in drive in 1976 automatic

[Mark92131]

 

2 hours ago, rufurt said:

I could always go Pertronix. 

 

Spend a little more and buy the 123 Ignition Bluetooth distributor.  Then you don't need to plot your advance curve, you just build one to match the mods you have made to your motor.  You can also build a MAP table to add or subtract advance from your curve over RPM and vacuum conditions to mimic the functionality of your pod.  You will still need a timing light to verify TDC at the flywheel, but once that is done, all the advance and MAP tables are entered on your phone or tablet.  All the cool kids have one, and we almost have Conserv on board.

 

Mark92131

[Dudeland]

FWIW I would look at the brake booster since your foot would be on the brake when the car is in the drive. Maybe it is drawing too much vacuum and messing with the baseline. Maybe the check valve isn't working.


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[TobyB]

Rufurt, if you don't have the throttle- up pod on your carb, there's your problem.

 

The 38 is barely open at idle, doesn't have the right progression drillings for feeding an M10, blah de blah...

...so it doesn't deal well with the extra load of the transmission.  I don't think you're going to have much

luck getting a good idle in both neutral and drive without some sort of idle- up...

 

t

 

Edited November 14, 2018 by TobyB

[rufurt]

Toby, Ahh the progression drillings. I have read some of you other posts about them. So if the throttle plate was thinned near the holes, would that allow more air to feed the idle and deal with the extra load?

 

Has anyone use a throttle-up pod on a 38? Can it be done? It makes sense that it would help. 

 

Thanks for the advice guys. 

[Simeon]

4 hours ago, '76mintgrün'02 said:

am a believer in using a ported vacuum signal for the pod, since manifold vacuum will bring in all of the advance at idle; whereas the ported signal will not add any.  

 

Isn’t this likely to be a big part of the problem?  Vac advance is all in at idle and perhaps even included in the initial advance setting?

 

OP, did you disconnect and plug the vac advance line when setting the timing?

 

when the load changes on the engine, the vacuum falls, this removes the advance and causes a stall inducing condition. Running ported, the vacuum may not be in play at idle so it wouldn’t be affected. 

[rufurt]

Simeon, yes, vac advance line was unplugged from distrubutor and the line plugged when I set timing. Right now the vac line is connected to the manifold.

 

When I had the idle set lower (set to about 1000 when in neutral), the engine would eventually stall when coming to a stop, but not right when I let off the gas. I should test those conditions again and see if it dies while coasting (foot of gas) or only when at a full stop. The load on engine might be different while coasting versus stopped while in drive.

[TobyB]

Quote

Isn’t this likely to be a big part of the problem?  Vac advance is all in at idle and perhaps even included in the initial advance setting?

 

Nope- best idle seems to be in the 8-12 degree range, but best POWER comes from getting the timing well into the 20's just after throttle tip- in.   Or maybe Yup- I'm starting to get lost here... I blame the drugs...

 

Anyway, using ported vacuum leaves vacuum advance out of the idle equation, but kicks it in at low- speed cruise, transition, etc.

I learned this the hard way...

 

So, Rufurt, try taking your dizzy pod (since it's advance?   I lost track...)

signal off the ported signal at the base of the carb.  It might help...

 

Quote

The load on engine might be different while coasting versus stopped while in drive.

 

Yuss, totally- when the transmission wheel speed drops below (engine idle speed x trans drive ratio) the torque converter will start to load

the engine.  Try coasting- I bet you'll idle pretty well down to about 15 mph or so, and then you'll see/hear/feel

the transmission start to  load the engine.

 

As to the vacuum pod, I don't know.  I've never messed much with auto transmissions.  The one car I had for a while with one

(1965 Volvo 122) had a pretty high idle in neutral, and engaged drive with a solid 'thump' through its BW 35...  no vacuum pod, 

and very worn 1.75" SU's.

 

t

 

Edited November 15, 2018 by TobyB

[rufurt]

Warmed up the car last night (took a while as it was a cold evening) and then switched the vacuum line that goes to the ditributor from the manifold to the carb's ported line. 

 

Having it hooked the carb port dropped the neutral idle down from around 1900rpm to about 900rpm. At first blush this seems to work. I readjusted the idle speed to about 1100 rpm.

 

Wasn't able to drive it around last night, and I might be done driving for the season as our first snowfall is tonight and they are salting the roads. As you well know, our cars will melt up here in the rust belt if driven in the winter.

 

I guess this gives me a chance to work on the interior. Not that I can affford $450 for a single seat pad. Maybe I'll buy some new carpet. Though if I don't buy carpet I can buy a 123 dizzy, heh, who needs carpet.

 

If I get time, I will take out the dizzy, check it, rebuild if necessary, and plot an advance curve. Thanks for the help. If I get a chance to test drive I'll let you know how it goes.

[Roland]

19 hours ago, rufurt said:

if the throttle plate was thinned near the holes, would that allow more air to feed the idle and deal with the extra load?

 

For various fuel delivery/progression issues, strategically placed bleed holes have been employed rather than altering the shape or size of throttle plates.  Fortunately, ill placed bleed holes can be corrected with solder or epoxy filler.  That said, achieving the correct air fuel balance by using bleed holes is somewhat of a black art, since it involves not only the size and number of orifices, but placement of the holes in relation to progression ports.  Jetting, emulsion tubes, camshaft design, operating speeds, compression and yes, ignition timing all factor in to the mix, too.

 

 

 

The idle speed issue encountered with an automatic transmission equipped M10 engine was also a problem with air conditioned equipped vehicles, for perhaps obvious reasons.  This had been a common problem with many small displacement engines of the 70s and 80s.  Even large displacement engines were equipped with devices to adjust idle plates as part of emission controls. 

 

 

 

 

Common idle solenoid use on GM engines of the late 70s.

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