Engine Idle Drops A Lot in D

[lomchivok]

I'm assuming the block hasn't been decked to zero and the heads haven't been shaved cause that would bring the compression up more than 9.1. the reason I say that is by doing the a fore mentioned can cause misalignment of the intake to the heads causing a vacuum leak. It does sound like a vacuum leak to me. My 318 was doing the same thing... decked and shaved...it was a vacuum leak. Squirt liberally some carb cleaner around the intake bolt flanges and around the carb base and listen for a stumble or rpm drop. I did that and it actually stalled the engine out. after I machined the intake flanges and set up the Holly T port sync it straightened right out. timing is 18* initial and 40 all in at 3500. I also got the lowest actuating adjustable vacuum canister I could find and set it to give me 24* at idle to help stop stalling at in gear idle. I use a pertronix distributor and used their limiting bars to set the all in mechanical advance. At freeway speeds it pulls in about 48* total timing.

 

[74Dart318]

I just hooked up the vacuum gauge to the manifold vacuum port on the driver's side of the Eddy carb.

At 750rpm with about 18° timing the vacuum gauge is only reading 10 in.

I am guessing I should be searching for a vacuum leak.

 

[T56MaxTorq]

Mopar is right. And then when you get it to idle low and steady, with the vacuum gauge on the highest point, slowly turn the mixture screw(s) inward til the idle stumbles and back off 1/4 turn. Making sure to simultaneously tune speed.

You can stil be leaking air and can easily check to see if it is by spraying single connections carefully with carb cleaner and listing for an idle stumble.

 

[lomchivok]

I just hooked up the vacuum gauge to the manifold vacuum port on the driver's side of the Eddy carb.

At 750rpm with about 18° timing the vacuum gauge is only reading 10 in.

I am guessing I should be searching for a vacuum leak.

Don't know much about the eddy carbs but yes, 10 inches seems low. Mine with a holly is 14 inches at 800 and about 10 in gear
Edit: Mine has the same cam and heads as yours.

 

[AJ/FormS]

I just hooked up the vacuum gauge to the manifold vacuum port on the driver's side of the Eddy carb.

At 750rpm with about 18° timing the vacuum gauge is only reading 10 in.

I am guessing I should be searching for a vacuum leak.

or valves not sealing,or in-the-basement compression,or a very retarded cam.

My 360 engine idles at 10/11 with a 276* cam at 750rpm and 14* of idle-timing.. But I can pull it down to 550 with the clutch,and retard the timing to 7*, and it will pull the car around the parking lot with a starter gear of 10.97.
But this engine has valves that seal and over 175psi cranking compression, and the cam may be by now, slightly retarded; (cam installed in 2004).

At 10" of vacuum it wouldn't take much less for the PV's to start dumping fuel. If they did that at idle, the engine would stall.

Car: 1974 Dodge Dart Swinger
Engine: 318 Bored .030 with KB pistons
~9:1 compression
Indy LAX heads
TTI shortys, full exhaust
Eddy air gap, Eddy 650 with elec choke
Comp Cams XE268H (Duration @ 0.006": 268° / 280°; Max Lift w/ 1.5RR: .477" / .480"; Lobe Separation: 110°; Intake Centerline: 106°)
Trans: Rebuilt 727, shift kit, stock converter
Rear-end: 8.25, SG, 3.21 Gears

 

[69_340_GTS]

Why use carb cleaner? It removes paint, dissolves rubber, and makes a mess in general. Use a propane torch, unlit. The vacuum will suck the propane in and produce the same results as the carb cleaner.

 

[nm9stheham]

I used this PCV valve
https://www.summitracing.com/parts/frm-fv163dp

This PCV looks like a standard Mopar PCV. As such, it will have a transition vacuum level from the low idle flow to the high cruise flow at about 12-14 inches. If you have an idle vacuum of around 10" in D, then this PCV is wide open to cruise flow level, and that is essentially a moderate vacuum leak. If the RPM's go up when in P, then the vacuum level will go up and the PCV will transition to the lower idle flow, and the RPM's will rise even more; it feeds on itself to push the PCV past the transition point. Your idle symptoms are spot on with this issue.

This is a common issue with the stock Mopar PCV's: it's the wrong PCV valve for even a moderate cam. Pinch off or block the PCV hose leading to the carb base as a test and I bet you will see the idle RPM's in D jump up 100-200 RPM. That is a sure test of this being an issue.

Regardless, of what other things are going on, you need to change the PCV; it is part of your problems here. A Fram FV 191 has a lower transition vacuum level that is appropriate for this cam, but is not a direct physical fit. Do this before you make other changes, or you will have to go back over everything again once you do fix the PCV.

 

[74Dart318]

This PCV looks like a standard Mopar PCV. As such, it will have a transition vacuum level from the low idle flow to the high cruise flow at about 12-14 inches. If you have an idle vacuum of around 10" in D, then this PCV is wide open to cruise flow level, and that is essentially a moderate vacuum leak. If the RPM's go up when in P, then the vacuum level will go up and the PCV will transition to the lower idle flow, and the RPM's will rise even more; it feeds on itself to push the PCV past the transition point. Your idle symptoms are spot on with this issue.

This is a common issue with the stock Mopar PCV's: it's the wrong PCV valve for even a moderate cam. Pinch off or block the PCV hose leading to the carb base as a test and I bet you will see the idle RPM's in D jump up 100-200 RPM. That is a sure test of this being an issue.

Regardless, of what other things are going on, you need to change the PCV; it is part of your problems here. A Fram FV 191 has a lower transition vacuum level that is appropriate for this cam, but is not a direct physical fit. Do this before you make other changes, or you will have to go back over everything again once you do fix the PCV.

This is something I had not heard of before. Frustrating because I accidentally ordered that fram PCV originally and it didn't fit the stock grommet, so I got rid of it and got the 'correct' PCV. Well I guess I will order that fram one again and try it.

 

[nm9stheham]

There may be other PCV's that have a transition vacuum level of 10" or lower, and have the right fit. I just don't know of a cheap PCV PN of the right size. I think someone had a PN in another thread..

Hey, here it is: Good quality PCV valve
Someone mentions an MP PCV PN that looks like the best choice for transition vacuum level and for fit. Mopar performance 4343581; they are all over the in-net it seems.

 

[crackedback]

JMO,

I'd isolate the intake tract. Remove everything that is vacuum actuated, plug it off and see if anything changes. If it does, you can find which vacuum item is causing problems.

Sometimes a small piece of carb linkage can interfere with an intake, causing an ever so slight sealing issue.

 

[furrystump]

Don't touch the air door until you have everything else figured out. You don't need to add variables. The air door does nothing until about 2500ish rpm.

 

[lomchivok]

I've tried using propane but just couldn't get the same results as carb cleaner.

For the PCV, I have access to a lathe and just turned out a brass plug to fit snug inside the hose. can't remember the size of hole I ended up with but was able after several attempts to get the right inside dia to evacuate crank case gasses to what I thought was adequate...is it adequate...I don't really know, but it is working.

Edit: I do run a pipe cleaner through the plug occasionally to check that it's not getting plugged up...not even close yet after 15,000 miles. The 318 is sitting on a stand in the garage now though...nothing wrong with it...I just swapped in a 408 for more loud pedal fun.

 

[74Dart318]

Worked with the timing a little more.

I think nm9stheham may have something with the PCV valve.

With the car in N, timing at 17° , 750 rpm the car runs 10-11 in of vacuum. At this setting the PCV is vibrating a lot.

When I put it in D from here the engine drops to 550 rpm and the PCV stops vibrating. Unfortunately I couldn't read the vacuum while in D and I didn't think about getting that reading.

 

[moper]

Back the mixture screws out 1/4 turn each, if the idle rises drop the speed back to 750, and see what the rpm does when you put it in drive. It's still too lean. with Carter/Edelbrocks the micture screws have a very small window of correct operation. It doesn't take much.

 

[nm9stheham]

With the car in N, timing at 17° , 750 rpm the car runs 10-11 in of vacuum. At this setting the PCV is vibrating a lot.

Interesting and probably a sign of it being on the threshold of opening up.... Did you perchance pinch the PCV hose or pull the PCV and cover the end with your finger when in D to see if the RPM's rose? The amount of suction and the noise the PCV makes is another clue; if it open to the cruise setting (N or D), the 'whooshing' noise will be pretty loud. That is what clued me in on this issue the 1st time; the PCV air sucking noise at idle was very abnormally loud.

Again, get this resolved before moving on; it can effect everything else.

 

[AJ/FormS]

Because of the low idle-vacuum, the PCV thinks the vehicle is accelerating briskly. IMO,It is doing exactly what it was designed to do. IMO fix the low idle vacuum and all your problems will go away

 

[74Dart318]

I also forgot to mention. I can get more vacuum with more timing. Just not sure if I should advance up to 25°.

Sorry for the inexperience guys. Really appreciate the help.

 

[crackedback]

Plug off the PCV for christ sake and start there. Put a dang bolt in the hose after you remove said hose from pcv.

Nothing like tuning with a bunch of unknown variables.

 

[nm9stheham]

Because of the low idle-vacuum, the PCV thinks the vwhicle is accelerating briskly. IMO,It is doing exactly what it was designed to do. IMO fix the low idle vacuum and all yor problems will go away

Part of the low idle vacuum is the PCV transitioning at the wrong vacuum level for the cam. The plain-jane stock Mopar PCV was designed for an engine setup that idles at around 20" of vacuum, not one that idles around 11-13" of vacuum.

GM (as one example) changed their PCV's as a matter of design when they put moderate cams in production cars. That is where the FV191 comes from: a 70's production GM cammed up 'hot rod' engine. Now I am curious if something like the early 340's came with a different PCV than the plain-jane ones.

 

[AJ/FormS]

I also forgot to mention. I can get more vacuum with more timing. Just not sure if I should advance up to 25°.

Sorry for the inexperience guys. Really appreciate the help.

That would be normal, and is why I never tune for highest vacuum.
That might work for a big cammed engine, I cannot say. The biggest I have tuned was a 292/108. It was happy with 14* to 18*

 

[nm9stheham]

I also forgot to mention. I can get more vacuum with more timing. Just not sure if I should advance up to 25°.

Sorry for the inexperience guys. Really appreciate the help.

Understand that what you have at idle is the initial timing (assuming the distributor mechanical advance is not coming in at idle, which it should no do.) As you increase the revs, the mechanical advance is going to start adding in. The stock mechanical advance is 22*, so if you have 25* initial, then you would have 47* total when the mechanical advance all came in, and that is usually considered certainly waaay too much under WOT conditions. A total in the 30-35 range is typically good; some have used more, but that seems to be in the case of lower compression engines. (Too much and the engine detonates.) So at 20* initial, you may already be where you need to limit the mechanical advance.

There are ways like adjustable stop plates to limit the mechanical advance if you need to run much past 15* initial. But, with your decent CR and cam, I don't think you need to go with so much initial timing. There are plenty of very knowledgeable discussions and people here on that matter.

You have the PCV and other things to correct first. So IMHO, don't further complicate your situation with too much advance. Save that as one of the last steps in the process.

As a point of interest, was your cam 'degreed' when it was installed? If so, do you have the ICL (intake centerline) angle? That info may help in understanding if your idle vacuum levels are in the right neighborhood.