Initial timing question

[PhilB]

I have a timing question on my 351W with stock Duraspark II distributor. The engine will barely run at 14 deg of initial timing. At around 25 deg initial (set at 1000 rpm with vacuum advance disconnected), it runs fine with no pinging or hard starting. The distributor was curved for 10 deg mechanical advance (20 deg at the crank) at about 2500 rpm. Crank/timing mark correlation was verified with a dial indicator and is dead-on. Using manifold vacuum, it does bog under WOT at 30-40 mph but under steady and heavy acceleration from stop it’s fine. I think using ported vacuum and further carb tweeking might solve the bog problem. What I can’t explain is why initial advance has to be so great. I haven’t pulled the timing chain cover to verify cam timing but I have no reason to believe it’s off. Cam is 270 advertised, 224/224 at .050” (Comp Cams Magnum). I’ve used two different timing lights with the same result so I don’t believe I’m getting a double reading. Any ideas?

[RobMcQuarie]

Definitley use ported vacuum, you don't want any vacuum advance at idle, only after the throttles are cracked open. Second, check the rotor phasing. I have found re-man Durasparks that had a point distributor vacuum advance unit in them. The arm that connects to the plate is the wrong length, and moves the plate out of phase, causing the engine to need excessive timing. Had it happen just recently with a customer's engine, we just replaced the vacuum diaphram with a performance one off the shelf and timing fell back into place.

[PhilB]

You are correct about the vacuum advance canister. When the distributor was re-curved, they found that a point-type canister had been installed during the original rebuild and replaced it with the proper breakerless-type. Unfortunately, it didn't correct the large initial advance problem. I appreciate your help.

[concobra]

There are a lot of possible problems so some questions need to be answered first.

1. What is the cranking cold compression reading?
2. How much vacuum is the motor pulling at the 14degree setting and the 20 degree setting?
3. What intake are you using?
4. What type and size of carb?
5. What is the running temp?
6. Has this problem existed since the motor was new?

[PhilB]

I've never measured compression but the engine is new (700 miles). Compression ratio is 9:1. The few times I've measured vacuum, it has always been around 16-17 when the engine is running properly. I'm reasonably sure vacuum would be low if I could get it to run at 14 deg initial. Intake is Edelbrock Performer with Edelbrock 600 cfm w/ electric choke. Engine runs 180-190 F. The problem has always existed. Hopes this sparks some ideas. Thanks.

[mikiec]

Could you have installed the distributor 1 tooth off?

Mike

[CobraEd]

You are sure you have a good timing lite and you are hooking it to #1 cylinder? Is the mechanical advance working and not stuck?

Ed

[PhilB]

First thing I checked was distributor installation and it was correct. If it were installed a tooth off, rotating the body (assumed there is enough clearance for tha vacuum canister) would still allow the timing to be set correctly. I've got a good Sun timing light and have double checked it with another one. I've connected to #1 plug and verified TDC and timing indicator 0 deg with a dial indicator. Mechanical advance works properly; timing advances smoothly as rpm increases.

[CobraEd]

OK, I have reflected on this some more and have the following thoughts:

First, 25 degrees initial advance and 10 degrees centrifical advance are only 35 total. This is normal and maybe even somewhat low for a performance engine. So your total advance is OK, no issue there. Also if you have a hot (hugh duration) cam, you will not build a lot of cylinder presure at lower rpm. These reasons are why the engine runs well and does not ping.

Second, the Duraspark ignition retards the timing during the starting cycle (for easier starting) and explains why you have no hard starting problems.

The only question it why won't is run when set at 14 degrees initial advance. It should start and run fine until about 3,000 rpm where it only gets another 10 degrees of advance from the centrifical advance for a total of 25 where it should then be way down on power.

Maybe your Duraspark modual is broken and stuck in start (retard) mode. The physical timing would be right, but the electrical signal going thru the Duraspark box would retard the actual spark to the plugs. This might explain it. Try getting a new Duraspark module.


Ed

[concobra]

Thanks for the response. Let's look at the information you have supplied and see what we come up with.

1. 9:1 compression. Given that this is an accurate measurement. The cam you have selected will yield around 150lbs. or more of cranking compression. If installed straight up. Try to verify this. If you find that it is below 120lbs. your cam could be retarded and therefore your are not developing enough cylinder pressure to create adequate vacuum at 14 degrees. Remember that you need to run this test with the engine cold, carb. open all the way, & allow the motor to turn over at least 5 revolutions.

2. Vacuum reading. Your measurement of 16-17 inches is low considering the timing you are using (25 degrees) and the engine RPM of 1000. Should be around 20. Therefore the ignition timing may not be your actual problem. (After all 25 degrees is 25 degrees.) It could be other things. More on this in the conclusion.

3. What intake and carb? The Edelbrock Performer and 600 carb. are very good street units and exhibit excellent idle and throttle response. The standard calibration of the carb. is close enough to allow your engine to run well with only minor tuning required. ( This stated based on your cam selection and assuming you are using standard porting) Keep in mind that a carburetor's calibration is relative to the CFM flow more than the CID of the engine. Probably not the problem.

4. Running temp is great. No problem here.

5. Problem existing since new. Given this truth you may wish to look at other problems that can occur during assembly that can contribute to this problem. More on this during the conclusion.

6. Distributor off a tooth or so. Seems a reasonable option but is not an issue. The motor has no idea where the distributor sets relative to exciting the plug verses piston position. To explain further. Even if the distributor was pulled and turned 180 degrees but the wires were placed correctly on the cap relative to timing order the motor would not know the difference. As you said earlier the timing is at 25 degrees. Thats when the timing light indicates the plug is sent a signal and that is exactly when the plug can ignite the cylinder fuel charge. If it's 25 degrees then it's 25 degrees. Since the motor will not run well at 14 degrees (and it should) and it only runs well at 25 degrees then the conclusion would not be an ignition timing problem. Advancing to 25 degrees is just hiding the real problem.

7. Your camshaft selection and how it effects this stiuation. I looked up your cam and if I am correct it is Comp Cam's # 35-414-3. Based on high performance as a gauge, this is a mild cam. This cam should idle easily at 750 RPM and pull 17 to 20+ inches of vacuum.

By the way a side bar thought. If the heads are not ported to flow 75% exhaust to intake which stock Ford heads never due. Then you are not running enough exhaust lift and duration to compensate for this poor exhaust flow. You might consider a dual pattern cam with more exhaust lift and 10 degrees more duration @0.050 than intake.


OK enough facts let's draw some possible answers.

1. Your cam is retarded. You can make a reasonable deduction to this by the cranking compression as listed in item #1.

2. Your hydraulic lifters are slightly holding the valves open. Don't laugh it can happen. This holds very true with Fords and the non-adjustable rocker arms on rebuilt motors. If you surfaced the heads, had the valves done, are running a cam with a different base circle or any number of variables this can happen. Did you verify the preload? This will show up during the cranking test as a low cylinder pressure.

3. An intake air flow leak. Probably manifold located. I would consider looking here first. This seems to be the most likely. All of the problems you have mentioned would exhibit themselves with this situation. Do you know how to test for this?

There are other possibilities but let's start here and good luck.

[PhilB]

Concobra – thanks for all the effort you put into analyzing my problem. I took some measurements over the weekend and have more data. I apologize for relying on my failing memory to supply the original info.

Cylinder pressures measured with the engine cold were all above 150 psi. Manifold vacuum at 900 rpm was 19 in-Hg. Initial timing was 23 deg. The engine will run at 14 deg, but is rough. I’m using ported vacuum advance. As initial timing is advanced to 23 deg, rpm increases and idle smoothes out. The engine actually runs fairly well; it accelerates nicely under heavy throttle (feels like the front tires are coming off the ground), but bogs under wide-open throttle. I’ve tweeked the carb about every way imaginable but can’t seem the fix the WOT problem. The best jetting I’ve found is about 3 stages rich (over factory settings) on both primary and secondary. I backed off to 1 stage rich, which caused partial throttle as well as WOT problems. Another bit of the puzzle is that during shifts, the exhaust pops fairly consistently on one side. I’m using Shell Valley’s less than elegant RTV-sealed header-to-side pipe joints with exhaust over wrap. I read somewhere that upstream exhaust leaks can cause this problem. I’d like to weld flanges on the headers and pipes and bolt the joint, but I like the flexibility that SV’s design gives. Hope I’m not imposing on you too much, but any further thoughts you have will be greatly appreciated.

[concobra]

I have received your response and your information is good to continue the process of determining your problem. It may be best to communicate verbally. If you wish you can call me at 541-447-1398. My name is Phil.