Pinging

[FJRdoc]

I need some help from my FABO friends regarding fine tuning the Duster 340. I decided to try optimize the tune on my car today. The 340 is a stock rebuild with a Lunati Voodoo cam, AVS carb, factory intake and exhaust manifolds. Ignition is a Prestolite dual point converted to a Pertronix.
I unplugged the vacuum advance and got the RPM's up 3500 and set total advance to 35. I then adjusted curb idle to approx. 900 RPM with the idle solenoid stop as per the shop manual. Timing at idle was 5 DBTDC.
Carb mixture screws were adjusted using a vacuum gauge to maximize idle vacuum. Distributor clamp was tightened and vacuum line attached. The car sounds great at idle but not so good going around the block. Pings a lot with part throttle and up hill. I thought that I should be able to adjust the advance preload on the Prestolite, but Allen wrenches don't seem to engage anything when I attempted to rotate counterclockwise.
Any suggestions?

 

[crackedback]

Throw the factory tune up specs out the window. It should idle easily at 750-800 and 700ish in gear if set up at idle correctly.

You will have to limit the advance in the distributor and increase initial/idle timing. My guess it will want at least 12* at idle if you have the smaller voodoo cam. The springs in your distributor may also need to be changed.

Might take a bit to get right.

 

[dano]

Set vacuum advance last, so disconnect and plug it for now. How did you get the advance to 35* at 3500 rpms?

Start with the basics:

1. Set initial timing with timing light and vacuum gauge. Advance to max vacuum on gauge while dropping the idle to ~700ish then see where you are on the light. Sometime its a balance as over advance makes it hard to start.

2. One you know your initial timing you can calculate total. (Initial* + distributor* = Total*). Example: 12* initial + 22* distributor advance = 34* So you need a good initial setting that works.

3. Now you know what you need in the dist you'll probably have to weld the slots and file or get a FABO plate. You set the distributor to 1/2 of Distributor advance so from above 22*/2=11*

4. Timing curve i.e when total advance is all in at RPM wise. This is a play with spring weights and bend tabs deal.

5. Adjust vacuum advance is your canister has the option. Other wise buy one that will or make a limiter.

I'm sure other will chime in with some finer points on the process.

 

[TrailBeast]

I'd betcha the vacuum advance at part throttle is causing your problem.
Part throttle throws a lot of advance in. (like cuising around the block.)
Try what the others suggested and disconnect your vacuum advance first thing to stop the pinging.
Then deal with the details as the guys described.

FYI, vacuum advance adjusting doesn't change how much advance comes in, but only when it comes in in relation to the throttle openings.

 

[Mattax]

I'm going to slightly disagree with Rob on this one because

Ignition is a Prestolite dual point

So for the moment that's what you've got to work with.
(The only thing the pertronix will do is steal a couple degrees at high rpm. 3500 is not high rpm in this context.) With the new cam, he's right that some adjustment to the curve (changing springs or tension on them) ought to be done. Then you can get a better idle.

The distributor has the factory curve in it, so you got to work with that. What year and is it A/T or manual transmission?

I unplugged the vacuum advance and got the RPM's up 3500 and set total advance to 35.

Minor oops and a major oops.
Minor oops. Plug the vacuum hose. Golf tee works great.
Major flaw in this method is the factory advance curve was not meant to be set this way. That's a method that can be, sometimes has to be, used when the idle isn't stable or idle is of less concern than mid to high rpm timing.

I then adjusted curb idle to approx. 900 RPM with the idle solenoid stop as per the shop manual. Timing at idle was 5 DBTDC.

I think you're halfway there but the timing and rpm may be mixed up.
With these, the sequence can be important. And the cam change may be playing into it. But knowing the year or seeing the specifics we won't be guessing on this side of the screen.

 

[Slantsix64]

Whats your compression ratio? Take some timing out of the dizzy with a limiter plate, i tuned my v-can last by using a E-clip to limit Vac Advance. On my 318 im running 9:5 CR with the cam advanced 4 degrees, once i took timing out of the dizzy bumped initial up that when she woke up.

 

[FJRdoc]

Thanks for the help guys. The car is a 1970 with auto tranny. I will try my hand at tuning the car tomorrow with your suggestions. I'll keep you posted. Compression ratio is 9.75 to 1.

 

[crackedback]

5 BTDC is not going to run optimum no matter what he does to get it to stop pinging. Start at the foundation (idle) and work your way up is my approach. Limit mechanical and get the idle in the proper spot. 900 RPM is very high for the 01-02 voodoo cam IMO.

Total timing method is good for race cars, not street drivers.

The OEM specs were a mish mash of garbage to get emission under control and are NOT the best set up when you change things like the camshaft. They weren't even good with the stock camshaft in 1970. The 69 was 10 BTDC IIRC which was a bunch closer with the same OEM cam in 1970... HMMMM what changed??? Nothing of note other than govt regulations.

The issue may be too much advance too early. A sideline to getting the idle right is you don't have to crutch other areas of the tune up to cover for the likely rich condition at idle with 5 BTDC timing. A rich mixture is good for causing detonation because it requires LESS timing to start the detonation snowball downhill. Lean requires more timing to fire.

As a a test, Leave everything the way it is and disconnect your vacuum advance, is the ping gone? If so you have part of your answer which trailbeast addressed.

This is the FUN part of getting a new build running. Tailoring the systems to work in harmony and provide good performance.

 

[Mattax]

Dodge and plymouth used different timing specs so I won't swear but I've seen 5* even in '68. But its at something like 700 rpm not 900. Those years (68-69) the idle mix was leaner on all the engines, around 14:1.
Where I was going with this is that with the factory advance its very fast off idle. If the spec is 5 BTDC at 700, that will be very different than if its set 900 rpm.

But basically I agree with you.
* That engine, with that cam, should idle lower and use more intial timing. @FJRdoc its up to you if you want on timing curve at this time.
* Also I agree with the suggestion to disconnect and plug the vacuum advance as a test. If the ping is only moderate part throttle acceleration, then root of the problem is probably the mechanical timing is too advanced between 1800 and 3000 rpm to then have vacuum advance added in.

 

[crackedback]

Any significant idle drop could be from it pulling timing as well once it's dropped into gear. Whatever the timing is in P/N, you want the same timing in gear.

I'm not picking at you Mattax, just writing how I would approach it. 5* is not good for idle regardless of the carb settings or camshaft. Just not going to run well off the bottom. It's crutched because of the emissions stuff. Every SB I've had, even stock 318's, like at least 10-12 initial. Run so much cleaner and snappier off idle.

Just a guess on my part, 12-16 initial and max of about 34-35 total with pump piss that is available. The curve is going to be a pain if it advances really quick off idle. Springs will make the difference there.

Where the curve ends up all in is dependent on converter and driving loads. Vacuum advance adds a curveball here too.

 

[Mattax]

I'm not picking at you Mattax, just writing how I would approach it. 5* is not good for idle regardless of the carb settings or camshaft. Just not going to run well off the bottom. It's crutched because of the emissions stuff. Every SB I've had, even stock 318's, like at least 10-12 initial. Run so much cleaner and snappier off idle.

Definately did not take it in any negative way.

I was assuming he wasn't going inside the distributor, but you've convinced me he should. That would be best.
I completely agree with your conclusions that it would run better with initial at least 10* BTDC, and rpm and idle mix to match.

 

[Mattax]

I thought that I should be able to adjust the advance preload on the Prestolite, but Allen wrenches don't seem to engage anything when I attempted to rotate counterclockwise.

Sorry this got lost in the discussion.
The vacuum advance preload on the prestolite is probably done with washers.

I had a 1970 Plymouth manual downloaded but for some reason on the 340 distributors only have specs for the first part of the curve. However looking at all the 340 distributors from the early years can pretty safely assume the rest of the curve.
I've dashed in the maximum timing if the initial was set at 5*, and yes I see 900 rpm.
This is overlaid on 1969 timing specs which have an initial of 5* at 700 rpm (automatic transmission).

Looking at the two points you set timing by, I'm a little concerned that the heavy spring is missing or something like that.
Even so, if the timing was set at 35* at 3500 rpm, then the timing at 900 rpm should have been 10 to 11*.

Even if the springs are OK, 32 before 1800 rpm would be way too much timing to work with vacuum advance.

Measure a couple more timing points. Say 750, 1500 and 2250 rpm or something like that.
Then we can see what needs to be done.

 

[RustyRatRod]

Make sure your vacuum advance is not hooked to manifold vacuum. It should be on ported vacuum.

 

[FJRdoc]

Hi Guys. Thanks for your replies. It seems that I'm not receiving notices about posts on my threads. I'll have a chance to work on the Duster this weekend. Ill measure timing points at the various RPMS that you listed Mattax. I've never messed with distributors before (beyond basic timing). This is new territory for me.

 

[FJRdoc]

Spent the last couple days trying to get the Duster in good tune. I ordered springs and a limiter plate from FBO. I called and spoke to Don about a good starting point. He suggested the middle (silver) tension springs and using the 18 slot. Don was great to talk to and a very knowledgeable guy. Another fine Mopar vendor.
Disassembly and assembly went well. Got the idle and timing set and used an O2 sensor to dial in the carb. Took her for a spin and only experienced very slight pinging with heavy acceleration. I dialed back the timing a couple of degrees and all is well. I'm a happy man!

 

[oldkimmer]

That’s good to hear. Kim