Curing Distributor Timing Woes With A New LST Camshaft From COMP

Sometimes fate presents you with the opportunity to improve the performance of your engine — whether you wanted to or not. At some point, it seems that everybody will experience a weird set of circumstances that often goes unexplained. This was our time.

For more than 10 years, we’ve had a Chevy H.O. 350 crate engine in our ’65 El Camino. Through a set of circumstances, the engine has not gathered more than 4,000 miles, yet it’s already had two cams in place of the original. We also replaced the original carburetor with an aftermarket fuel injection system. While initially the engine ran fine, after a couple of years of short trips and car shows, the engine was now running poorly and would not respond to tuning, despite multiple attempts to find the problem.

We finally decided to revert back to a carburetor in the expectation that this might help isolate the source of the trouble. It did, but not in the way that we anticipated. Many enthusiasts (and sometimes this author) blame EFI when the source of the issue is completely unrelated to the EFI system. In our case, the engine seemed to initially run better with the carburetor but it quickly fell back into its old bad habits of an off-idle hesitation, poor throttle response, and reduced power.

There’s an old saying in the carburetion business that 90-percent of carburetor problems are ignition related. While rechecking timing we discovered that it was highly erratic at idle. Worse yet, when partial throttle was applied, the timing would retard badly — perhaps 15 to 20 degrees — and then immediately jump ahead to 10 to 15 degrees advanced. This indicated a problem with backlash between the distributor gear and the camshaft drive gear.

cam gear wear

Here is what we found when we removed the cam from the engine. Note the excessive wear on the distributor drive gear. Oddly, the gear on the distributor looked normal.

Finding The Culprit

We yanked the new HEI distributor, but the gear looked fine. This demanded pulling the engine from the car. After removing the cam, the distributor drive gear was found to be badly damaged and was clearly the cause of the problem. However, the cause of the gear failure was still unclear.

When attempting to install the new cam, we noticed that Number 3 cam bearing was very tight with a burr or lip on the trailing edge of the bearing that wanted to force the cam forward. This was a one-piece rear main seal small-block that was configured for a roller cam. Since the original cam had been a flat tappet, we were using a cam button on the front of the engine that may have allowed the cam to move forward more than desired.

Our new cam used the stepped nose to accommodate the factory limiter plate to prevent the cam from moving forward. The stepped nose also requires a different cam timing set because of its smaller bolt circle. (Left) COMP’s newest hydraulic-roller cam design uses a stronger 5150 steel core but has an iron distributor gear installed onto the shaft. Note the roll pin that locates the gear and the rear journal (right). We will use a melonized distributor gear on this assembly just to make sure, as the melonized gear is compatible with all cam gear materials.

In order to change the cam bearings, we felt the best way to ensure the new bearings were installed correctly was to completely disassemble the engine. With the engine completely disassembled, this allowed us to inspect for internal engine damage. Thankfully the rod and main bearings all looked excellent.

Once the new cam bearings were in place, and after we thoroughly washed the engine with hot soapy detergent and then flushed it clean, we reassembled the engine. This time we used a factory cam limiter plate, as this crate engine was based on the one-piece rear main seal style small-block that will accommodate a stepped nose hydraulic roller camshaft.

COMP also applied its micro-surface enhancement (MSE) process to our camshaft. The process dramatically improves the surface finish of the lobes yet is much more than mere polishing. MSE reduces waviness in the lobe surface while increasing its load-carrying characteristics.

Fixing The Shaft

A possible reason that excess distributor drive wear can occur is if the distributor shaft bottoms out against the oil pump drive shaft. To measure this, install the distributor with the cap and rotor removed. Pull up on the center cam portion of the distributor shaft. If the shaft does not move, then the shaft is binding on the oil pump drive. This will require specific nylon shims under the distributor collar to increase the distance to the oil pump shaft. It did not appear that our distributor was bottoming out.

While all this work was going on, we contacted COMP Cams about a replacement cam. They told us that they were in the midst of changing from their ductile iron-based camshaft cores to a stronger 5150 alloy steel cam core configuration. It was during this discussion with COMP’s Valvetrain Engineering Group Manager Billy Godbold that we decided to try a new line of lobes based on the company’s very successful Low Shock Technology (LST) design.

We installed the new cam after placing Driven assembly lube on the distributor drive gear and all the lobes and journals (left). Production small-block Chevys starting in 1986 changed to a one-piece rear main seal and were machined to accept a factory-style hydraulic roller cam with a stepped nose and this limiter plate.

We previously tested an LST lobe on a stroker iron-block 6.0L LS engine displacing 404 cubic inches. During that test, we witnessed a 17 horsepower improvement at 6,000 rpm despite the fact that the cam timing numbers were essentially the same. The improvement came from revised acceleration rates and additional lift on the closing side of the lobe.

If you elect to run a mechanical fuel pump while using one of these new 5150 steel core camshafts, this will require using a bronze-tipped mechanical fuel pump pushrod. A stock fuel pump pushrod cannot be used because it will gall on the steel cam and damage the camshaft.

Godbold suggested a pair of Low Shock Technology intake and exhaust lobes that would work for our mild 350 small-block which could be easily transferred to a small-block Chevy stepped-nose hydraulic-roller cam core. While this is a custom cam, COMP will produce this type of camshaft for anyone who wants one. We’ve included the grind numbers for this particular version, but COMP literally offers dozens of lobes in their custom lobe catalog that could be used for a more aggressive small-block Chevy, Ford, Mopar, or most other engines where these lobes can be adapted.

New LST Camshaft Specs

Advertised Duration  Duration @ .050 Lift Lobe Details
Intake 276 degrees 218 degrees 0.553 in. 33292S, MGZ
Exhaust 295 degrees 229 degrees 0.525 in. 33698, MBX
Intake Centerline 106 degrees
Lobe Separation 110 Degrees

Note that the LST cam offers the same intake duration at 0.050 inch as the traditional Extreme Energy grind but delivers 0.058-inch more valve lift. On the exhaust side, the 0.050-inch tappet lift duration is slightly longer by 5 degrees with 0.023 inch more valve lift. With more modern and better flowing cylinder heads for the small-block, this added lift and higher RPM stability, the LST offers more performance as we saw in the LS engine LST cam comparison test.

With the new parts in hand, we slid the custom COMP cam in place along with the factory limiter plate and a new timing set. We then verified the position of the cam with our degree wheel just to make sure it was installed properly. We also liberally lubed the distributor drive gear on the cam along with all the lobes and journals. With the engine re-assembled, we dropped it back into place and re-fired it using a crankcase full of Driven SAE30 break-in oil to ensure the cam would be happy during break-in. Within a day we had the engine running.

The full range of new LST lobes are not in the current printed COMP Cams catalog but can be found on COMP’s website. The hydraulic-roller families are the MGZ, MGH, MBZ, MBX, MGJ, MGD, MGR, LPM, LSD, XLD, XLP, XIX, and LSO lobes. Here, we’ve copied a section of the MBZ lobe listing. Note that this lobe family offers a tremendously wide range of duration and lift specs. The full MBZ listing spans all the way to 274 degrees of duration at 0.050 (332 degrees at 0.006-inch tappet lift). These lobes are compatible even with rocker ratios of 1.8:1 so there is a wide spread of opportunities. These master lobes can also be applied to a broad variety of engines including small- and big-block Chevys and many others.

With our deadline approaching, we haven’t had time to put many miles on the new combination, but we can assume a mild increase in peak power and also that our drivability will be much better now that we’ve eliminated all that distributor gear backlash. Sometimes it just takes a little more effort to get all the components working as intended. Hopefully, this has concluded our jaunt into the world of weird failures.

With the new camshaft and distributor timing gear installed, the engine is running and much happier now that the timing fluctuations have been eliminated.

Article Sources

More Sources

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
Read My Articles