Optimal RPM at idle?

[lizmcl527]

Not real happy with my gas mileage (11-12 mpg) these days in my '41 Plodge, so I've been thinking about what might be going on. I thought maybe I might be idling too fast so I hooked up a tachometer and was getting about 750 rpm. Checked the vacuum, and I'm getting a wobbly reading of about 15-16. Tried adjusting the idle screw on the carb, but that's as good as it gets.

 

Engine is a Canadian long-block 218, with about 5000 miles on a full rebuild. What sort of readings *should* I be getting under ideal circs?

[jeffsunzeri]

You should be seeing 17-22 on the vacuum gauge with a steady needle.

[desoto1939]

Do you also have it set to the proper timeing mark.   That can also affect the gas mileage Also check that the road that comes across the top of the engine might need to be backed off could be  pushing the carb to run rich.

 

Rich Hartung

Desoto1939@aol.com

[48ply1stcar]

Huh?

[Don Coatney]

Double huh???? Rich please explain further as I do not understand your comments? What road (possibly throttle rod?) and how would that make the engine run richer?

[Don Coatney]

To the original poster, is your vacuum advance working correctly?

[55 Fargo]

Perhaps the notion of a bent throttle "road" is causing a faster idle, causing a rich mixture of fuel, not sure if that is what Rich intended to say.

I would think it would only cause idle RPM issues, not fuel mixture issues.....Rich were you running "lean" on your post......

[lizmcl527]

I'm suspicious of the vacuum advance -- it wasn't replaced at the time of the rebuild, and I have an NOS replacement on the way that I'm planning to try.

 

Timing is still where it was when the engine was rebuilt.

 

When I pull the hose from the wipers off to hook up the vacuum gague, the RPM increases -- which would be consistent with a vacuum leak somewhere. right?

[meadowbrook]

I think he meant the choke rod between the Sisson device and the carb.

[Don Coatney]

New old stock is not the way to go with a vacuum advance. The internal rubber diaphragm has aged on the shelf and it is not compatible with the gasoline/ethanol blends sold today. Terrell machine in Texas can rebuild (with new rubber) your original unit. 

[Plymouthy Adams]

I'll readily agree with the old aged rubber but as only vacuum is present on the diaphragm the fuel of today should play no ill effect...I have never seen raw fuel in a vacuum line to the advance ever..if feel gets here you have other problem of greater concern

[greg g]

Vacuum is low should be closer to 20 inches for a fresh rebuild, needles should be steady. Idle rpm should be 500/600 rpm. Is the idle smooth or rough? Is your throttle butterfly closing all the way? If not no amount of fiddling with the idle mixture screw will have any effect. Is your choke fully opened at operating temp? What do your spark plugs look like? Is the a strong gas smell at the exhaust and or black smoke present when you rev the engine?

I would suspect the one or more of the following to be in play

Loose/leaking intake manifold or carb to manifold connection.

Improperly adjusted throttle linkage, keeping the butterfly open at full release.

Choke not opening all the way.

Improper timing.

Weak or non existent vacuum signal to the step up valve inside the car but at or.

Leak in the vacuum line to the wiper motor.

And finally valve lash to tight allowing valves to not fully seal when engine is at proper op temp.

Go to the second chance garage website and look at the page that deals with how to read and interoperate vacuum gauge readings. Hope your problem is external. Good luck.

Edited September 12, 2014 by greg g

[TodFitch]

Original question was about what the idle RPM should be. I can't find a reference for what it should be in any of my maintenance books published in that era. There are references to how high the oil pressure should be at "idle speed" or what the manifold vacuum should be at "idle speed" but I don't see anything that says exactly what "idle speed".

 

750 sounds high to me, I believe I have mine set around 450 or 500 but I imagine it would need to be higher for a car with fluid drive.

[lizmcl527]

Idle is rough until the engine warms up, with an occasional miss. Plugs look fine -- no fouling that I can see. No dark exhaust or heavy gas smell, but it spits a little condensation out the tailpipe until the engine is warm.

 

Occasionally I'll get a backfire out the tailpipe when accelerating from a stop -- not a bang, more of a "chuff."

 

I'll check the choke and throttle linkage later today.

 

No Fluid Drive on this car -- it's a Plymouth-bodied Canadian Dodge, so regular transmission is used.

Edited September 12, 2014 by lizmcl527

[Lloyd]

Sounds like its running lean, possible vacuum leak although the occasional "chuff" when accelerating could be an ignition miss, ignition timing or the vacuum advance.

Another possibility could be a sticky valve.

 

Remember the vacuum gauge was showing low but fluctuating at the same time? Is the needle steady fluctuating in a rhythm or just drifting?

If it is steady fluctuation it could be a valve, intermittent it could be an ignition miss or a sticky valve.

If it drifts at a low reading then possibly carb adjustment or vacuum leak.

One other possibility that would show a low fluctuating needle is a leaky head gasket.

 

Check intake gaskets and carb base gaskets, I use to use a can of start fluid spraying around the intake and carb base to check for leaks but these engines have the exhaust right next to the intake - real hot - so don't think it would be a good idea to spray a flammable around there. Maybe some of these guys could give an idea how to check for intake gasket leaks on a flathead.

 

Also along with a vacuum test it usually helps to check dwell, ignition timing, and do a compression test at the same time.

Edited September 12, 2014 by Lloyd

[soth122003]

I was using a Motor's Repair manual for while on my P-15. When I was looking up the specs for Plymouth I believe the idle for all 40's Plymouths were 450-500 Rpms.

 

Joe

[lizmcl527]

Dwell was right on the button at 38 degrees and absolutely steady.

[DonaldSmith]

Idle should be higher on a fluid drive car?  

If the car has the semi-automatic transmission, the idle speed has to be low enough for the "tiptoe" upshift.  I've heard 450 rpm or so.  I can tell when it's too high.  

The fluid coupling doesn't care what the idle speed is, but if it's too high, the coupling will just try harder to move the car. 

[Lloyd]

How about a compression check?

This may show a sticking valve which would indeed cause a rough idle when cold and smoother operation once it warms up as the valve guides expand. Or a leaky head gasket.

 

Another test would be a timing check, if the mark is moving against the dampener mark you could be looking at a worn timing chain. Another way to check the timing chain is to pull the distributer cap and turn the bottom pulley by hand, just slightly both directions and watch the rotor. How much you can turn the bottom pulley and when the rotor turns will tell you how much play you have in the timing chain.

[jeffsunzeri]

 

I'm suspicious of the vacuum advance -- it wasn't replaced at the time of the rebuild, and I have an NOS replacement on the way that I'm planning to try.

 

Timing is still where it was when the engine was rebuilt.

 

When I pull the hose from the wipers off to hook up the vacuum gague, the RPM increases -- which would be consistent with a vacuum leak somewhere. right?

 

No, that shouldn't happen. When you pull the hose off, you should see a drop in RPM and rough idle (due to a big vacuum leak you just created), and it should pick up again when you plug the line or hook up the gauge.

[lizmcl527]

Speeds right up when I pull the wiper hose off, slows down when I hook up the gague. Very strange.

 

Here's one thing I just noticed -- the choke was hanging up on the screw used to connect the choke control cable. Not completely, just enough to keep it from opening all the way. I tightened this down and filed the screw head enough to give it clearance, and now the choke operates normally.

 

Vacuum has improved a bit from this -- it's now at 19 at 750 rpm, with the needle bipping a little bit but more stable than it was.

 

The idle is smoother, but when I try to slow it down to 550ish with the screw it stumbles and misses.

 

I don't have a compression tester (yet), but the last time it was checked, about 3000 miles ago, everything was normal.

[soth122003]

Sounds like the carb. The idle air mix screw. You might try lowering your idle while opening your air mix screw.

 

Joe

[Don Coatney]

I don't have a compression tester (yet), but the last time it was checked, about 3000 miles ago, everything was normal.

I suggest that when you buy a compression tester find a hand held (as pictured below) as opposed to a screw in. If the probe on a screw in tester is too long it will bend the valve as a few forum members will attest to. Plus it is much faster to do the test with the hand held.