How to Properly Set Ignition Timing?

Hi, I have installed a new crate engine in my motorhome. It is (Everything new):
- A SBC 383 with a good RV cam.
- Edlebrock EPS intake manifold
- Edlebrock 600 cfm carb
- MSD Street Fire HEI Distributor
- Radiator
- Just rebuilt Turbo 400 w/new TQ
- No smog stuff at all
- Mechanical fuel pump

The engine company (Motor Works) recommends setting the timing at 32-34 degrees BTDC at 2500 rpm.
- I have had the timing set from 32 to 36 degrees with the same results, this is set with the vacuum adv. can connected to manifold vac.
- Manifold vac is ~15" at idle, ~20" at cruse speed, or off throttle coasting.

Here is my situation:
- The engine starts right up and idels well, high idle at ~700 rpm, low idle ~500 rpm.
- When driving the engine runs right up to 210 degrees and holds pretty well.  I have 2 good electric fans and the stock mechanical fan installed.  Driving conditions are; some idle and mostly 45-60 MPH speeds.  The thermostat is a 180 degree unit.  Once it is to 210 degree temp it will not cool back down at idle, or during driving.
- When driving is spits and sputters during acceleration, and does OK at constant speeds, flat and level roads.  It seems like it is missing, not getting back fires, but does sound like it is spitting back through the carb.
- When I shut the engine off at 210 degrees it will diesel a lot.  When warming the engine up at idle, if it is around 150 to 180 degrees, it will shut off pretty well.
- Timing at idle has been set from 15 - 20 degrees with no difference in running or shut off symptoms.  That is measured with the vacuum connected to the vac adv. can.
- I have not tried to adjust (or even check the setting)on the adjustable vac. adv. can because I don't understand the chart.
- If I disconnect the vac. adv. then the timing goes to about 8 degrees ATDC at idle.

- I checked the timing mark on the engine before I installed it and it looked to be right on.

The home page article titled "Timing Is Everything" looks like what I need, but it is incomplete on the web page, and the current news stand copy doesn't have this article in it.

Any help would be greatly appreciated.  I have a new thermostat and spark plug wires to install if needed.

Jeff  

Did you get a mechanical advance spring assortment with the distributor? If not, get them. Install the middle tension springs to make your initial test runs. Here's the quickie way to get decent ign. timing. With the vaccum disconnected and plugged, set timing at 10deg. BTDC. Warm-up the engine with the vaccum still disconnected and make a few full throttle runs from a dead stop while listening for pinging. If you have pinging, retard ign. by two deg. and try again. If pinging stops, you have the ideal curve. If still pinging, select the next heavier sping combo and repeat this sequence until pinging stops. If it did not ping on the first run, select the next lighter spring combo and test again. Continue with softer spring combo's till spark knock is heard, then use the spring combo just before it knocked. That takes care of the mechanical curve, now you can adj. the vaccum. Make shure your vac. source is manifold (vaccum present at idle). Inside the bore of the vac. canister is a 3/32 allen screw, turn it clockwise until it stops. This will allow full vaccum adv. Hook up your vac. line and go for a drive in various conditions. This will include part throttle and hill climbing. If spark knock is heard, adj the allen screw two turns at a time counter clockwise until no pinging is heard. This timing curve will provide the best economy and performance. See if the correct timing curve will allow the engine to run cooler. Post your results.  JW.  

JW - Thanks for hte detailed message!

- This distrbuter is an electronic HEI unit with no springs or weights.

- So, Here is what I did; Set the timing with the vac disonccected and plugged at 10 BTDC, with the vac can screw turned all the way in.  Vacuum is manifold constant vac.

- Test drive with vac still disconnected, ran very rough would not accelerate, but did not seem to spit back through the carb, dieseled very bad when shut off.  No pinging.

- Test drive with the vac hooked back up, ran much better but still not very well, but did not seem to spit back through the carb, it will only deisel for one hit on one cylinder when shut off.  No pinging.

- Engine coolant temp looked to be about 5 - 10 degrees cooler.

- With the vac hooked up the idle timing is at 17 BTDC.

- Would the next step be to move the vac can screw out 2 turns and try again?

Jeff  

MSD Street Fire has weights and springs. Pull the cap and rotor. They are under the rotor. Curve kit is part# 121-8428.  

Oops, I tried to look under the rotor without taking it off.  I ordered the kit, it won't be in until Tuesday.  Jeff  

JW,

At the end of your explanation you said to "See if the correct timing curve will allow the engine to run cooler."  Do you mean to then adjust the timing to 32-34 BTDC at 2500 rpm?  If so, do you adjust it with the vacuum connected or disconnected?

Thanks, Jeff  

JW,

I received the spring/weight HEI timing kit (Mr. Gasket)today and here is what I did:

- Installed the new weights with the middle springs.
- Disconnected and plugged the vacuum.
- Turned the vacuum can adjusting screw all the way in.
- Set the timing at 10 degrees BTDC at 750 RPM.
- Drove Motor Home with vacuum disconnected and plugged.
-- It did not ping at all.  It did run very rough below 40 MPH, but smoothed out after that, but still seemed down on power.

- Installed the lightest springs and reset the timing as above.
- Drove Motor Home with vacuum disconnected and plugged.
-- It did not ping at all.  It did run very rough below 50 MPH, and did not smooth out as much, it was still rough and still seemed down on power.

- Then I connected the vacuum and drove the Motor Home.
-- It felt about like it did without the vacuum during acceleration and ran very smooth at idle and steady speed.  However, it did not diesel at engine shut off at all.
--- I checked the timing at 2500 RPM and it is 40 degrees BTDC.

Should I go back to the middle springs and try that with the current timing setting.  Or, should I begin retarding the timing with the light springs and see how it goes?

- It is better now than it was, but I think hte middle springs felt better!

Thanks for the help - Jeff  

It should feel like it did without vaccum during acceleration, because there is little or no vaccum during full throttle pulls. Hence the need for a mechanical advance.
When you say, it runs rough below a given speed, does that mean that it is missing, blowing smoke out the exhaust or, backfiring? Really with an HEI distributor and the vaccum hooked up, it's very realistic to end up with 55+ deg adv.@ 3000rpm. Makes me think, you should experiment with more initial timing, say 15deg. BTDC and run your mechanical curve test again.  

JW,

It feels like it is missing.  No smoke.

I think you are right about needing more advance.  I tried today to back off the timing to 33 degrees BTDC with the vacuum attached @ 2500 rpm, and it ran worse.  I then went to 42 degrees BTDC and it was better.

Should the timing be set with the vacuum disconnected and plugged?  To the recommended 32-34 degrees BTDC @ 2500 rpm (or 3000 but the timing mark stops moving just before 2500 rpm).

I like the idea of doing the timing spring test again at 15 degrees.  I'll do it after work on Thursday.

Thanks again for all of the help - Jeff  

An engine of your specs can deal with an easy 36 btdc total mechanical timing figure. Dont worry about the numbers, all you need to do with your timing gun is to set the base timing without vac. hooked. Stay with the middle springs and run some more tests with just the mechanical curve in mind. Once you get it so it has crisp acceleration and no pinging, then focus on vac. adv. adjustments.  

JW454,

I started with 21 deg BTDC with the vacuum disconnected and plugged and the middle springs installed, at 750 rpm.

It ran very well but the eninge pinged just a little bit.  I then advanced the timing to 23 deg BTDC and it pinged a little more.  Then went down to 18.5 deg BTDC and it still would ping a little tiny ***

So, I switched to the light springs and set the timing to 21 deg BTDC.  It ran very well with no pinging.  Then I advanced the timing to 24 deg BTDC, and it started to ping just a little ***

Then I set the timing to 21 deg BTDC (lightest springs) and then hooked up the vacuum and drove it again.  It ran really well and did not diesel at all upon engine shut off! (Of course now the vac. adv. can is now pointed directly at the manifold vac. hard line to the tranny!)

I think this is it!  Thank you very much for all of your help!!!  I grew up with fuel injection so I didn't know anything about carbs or distributor timing.  This ended up being a great experience.  Jeff  

Hi again,

I have been able to start using the motor home to travel with my car trailer and I am having a few issues as I put miles on.  Here is a quick run down of what I am experiencing:

I have put a little over 1000 miles on the engine and transmission and I am very happy with how it is pulling up hills. I usually drive towing my 6000 pound car trailer up and down hills between 2000 feet and 5000 feet elevation.  Up 5% to 6% grades are pulling at 45 mph in second gear!

What I started experiencing is steady throttle pinging while crusing at about 55 mph (any speed really).  When I give it some gas to accelerate the pinging stops (running 92 octane fuel).  It gets worse as you go down in altitude.  This weekend it started doing it above 3500 feet elevation, and then sputtered when starting from a stop.

I'm going to check the timing again today and see where it is now that things have a few miles on them.  Engine temps are staying below 210 degrees (it was 85 degrees out this weekend), however the tranny temp kept climbing all the way home, ending at 210 degrees.

Any help would be greatly appreciated.  Engine and tranny history are listed at the beginning of the thread.

Thanks, Jeff  

Try a ported vacuum source for the vacuum advance.
I had full manifold vac for a while on my car and had nothing but issues until i switched to a ported source. I was damaging/fouling spark plugs constantly and had really bad gas mileage.  

Jeff, check out my post in the thread " Edelbrock carb vac port, which one" page three of active posts. It should shed some light.  
Jim
70 El Camino
461 solid, m20,
pics in readers rides

Thanks for the input.  That is a great article!  I had my vacuum advance hooked up to manifold vacuum, so that was not the problem.  I checked the total advance at 2500 rpm (with vacuum disconnected) and it was at 21 degrees BTDC. I raised it up to 33 degrees BTDC.  Now I need to test drive it, and perhaps adjust the vacuum advance can itself.

Jeff