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Ask Away! with Jeff Smith: Solving SBC Ignition Timing Problems

A melonized distributor gear at first will appear like any normal iron gear. But if you look closely, the melonized gear (left) offers a mottled appearance on the collar while a standard iron gear is smooth (right). (Image/Jeff Smith)

I have a weird thing happening with my small-block Chevy crate engine that has me scratching my head. The engine is a 290-horsepower Chevy crate engine with a set of headers, a Holley dual plane intake, a 750 Holley 0-3310C Holley carburetor, and an MSD Ready-to-Run distributor with a matching coil. I have about 10,000 miles on the engine — it’s in my ’68 Chevy pickup that I sometimes use to tow cars and a friend’s boat.

In the last month the engine has developed an off-idle stumble. I added a slightly larger squirter and the problem went away but then came back. Carb changes don’t seem to make this stumble go away. It only happens on easy acceleration away from a stoplight or if I go around a slower car on the street. Once the rpm is up at highway or cruising speed, it doesn’t hesitate.

Just checking all the different things it could be, I looked at my initial timing which changed from my original setting. I figured the distributor has slipped or something so I re-set it at 14 degrees. Here’s where it gets weird. If I lightly rev the engine, the timing retards below TDC for an instant and then jumps up to normal advance. At first I thought my timing light was bad, so I checked this with another light and it’s the same. This should not be happening. Do you have any ideas? — B.L.

Jeff Smith: I’d like to say that this is a new one, but we’ve actually run into this before. A friend purchased a very used up ’66 El Camino as a beater driver but noticed the engine ran poorly.

He brought it over to what he jokingly calls Jeff’s Chevelle Emporium and that’s when we discovered a similar problem to what you described.

His engine was running a very tired HEI distributor and the advance mechanism was nasty, rusty, and experienced problems returning back to its base initial timing. I thought that what might be happening was the timing was not fully returning due to a stuck advance and perhaps affecting the timing. But that really didn’t fully explain why timing would retard as it so clearly was. We substituted a new HEI distributor and the issue was solved.

A few years later, a friend complained of the same issues as you described. He thought the carburetor needed more accelerator shot. That didn’t solve the problem so we started to investigate the ignition.

An old carburetor tuner adage contends that 80 percent of carburetor problems are ignition related. I remembered that statement and broke out my timing light.

With the trans in Park and the dial-back light hooked up, upon light acceleration, the timing literally retarded about 10 degrees and then quickly advanced. If you revved the engine quickly, the timing would appear to advance normally although very abruptly. We also noticed that once the rpm was raised to around 3,000 (full mechanical advance), if the throttle was quickly returned to idle, the timing would at first overshoot about 10 degrees and then return to the base initial timing.

After a short discussion with the owner, I learned his engine was a hydraulic roller cammed crate engine using a steel cam. The engine had about 12,000 miles on it and originally had no drivability problems. The hesitation had occurred just recently, similarly to what you have experienced.

When we removed the distributor, the steel camshaft had worn the distributor gear but not as badly as we had assumed. However, even though the wear on the gear did not appear bad, there was sufficient clearance to affect the ignition timing. This backlash caused the timing to retard under light acceleration and also was the reason it would momentarily “overshoot” when the throttle was closed at higher engine speeds.

What my friend didn’t realize was that his new crate engine used a steel hydraulic roller cam. You cannot run a normal iron distributor gear on a steel cam core. Many think that you must run a bronze distributor gear but this is not the best solution for a street-driven engine as that soft material will quickly wear and the problem will return.

The correct solution is to use what is called a melonized gear. Melonizing is a heat treating process that allows the gear to be compatible with any camshaft gear of any material. So the melonized gear will work on the steel hydraulic roller cams in the Chevy crate engine but it will also work with a standard cast iron style camshaft or even those 8620 steel billet cams used in race engines. In this age of specialization, it’s rare for a part to be this universal, but in this case, it’s true.

While it might appear that just changing the distributor gear should fix the problem, there’s an underlying issue. Despite the camshaft steel gear’s material — it will suffer some wear. This may make it difficult for the new gear to wear properly. The proper repair would be to replace the cam as well, although in my friend’s case he elected not to change the cam and to try his luck.

We’ve made the mistake of running an iron distributor gear on a steel mechanical roller cam before and with just a short day’s worth of dyno time this was enough to place undue wear on the cam gear necessitating replacing both the distributor gear and the camshaft. If you think about it, high oil pressure will also affect gear wear since the distributor gear carries the load to spin the oil pump too. So if you are using a 20w50 oil with high oil pressure (70-80 psi) when the engine is cold, that places undue stress on the distributor gear.

More than likely the replacement melonized gear will solve your errant timing issue but there’s no guarantee this is a permanent solution. If the problem returns in a short time, you will know that you will have to buy a new camshaft. Of course, that will also mean yet another new distributor gear. Of course, if you buy a hydraulic roller cam using a ductile iron core (sometimes called Austempered Ductile Iron or ADI), those are compatible with the regular iron distributor gear so a melonized gear is not required.

One last bit of information. A stock Chevrolet distributor shaft diameter measures 0.491-inch. All MSD distributors use a larger 0.500-inch shaft diameter. So you will need to make sure that the melonized distributor gear you purchase is for the MSD distributor 0.500-inch shaft size and not for the standard Chevrolet 0.491-inch. We’ve included PN’s from several companies for both shaft diameters for the small- and big-block Chevy distributors.

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6 Comments

  1. Pingback: Ask Away! with Jeff Smith: Solving SBC Ignition Timing Problems

  2. David W Henninger says:

    Nice call. There are only a couple peolpe in the world that would have been able to answer that question correctly.
    Experience is huge in this case
    Dave from Comp Cams

  3. Possibly a small correction: my MSD StreetFire distributor had a .491” shaft diameter. I upgraded to an MSD Pro Billet HEI and the shaft diameter was .500”. My engine has a CompCams roller cam, so I needed a new melonized gear for each distributor (“$$$”)

  4. 20 years after selling my vette, I now know why…… And I wanna CRY!

  5. “CAUTION” when replacing a new gear, it can be put on wrong. Look for a “MARK” under the pin hole on the gear.
    This “MARK” should be facing the same direction that the rotor is facing. If it goes on the other way you will
    be set with a built in 2deg. retard. If you don’t find the mark on the gear look a the pin and comes out between
    two gear teeth that’s the one to face with the rotor.The pin that cones out under the tooth is not the correct one. I have a “Sun Distributor” and have more people come in with the gear on back wards saying they are having timing trouble. CHECK YOUR GEAR before replacing.

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