Later, he noted that reducing idle speed below 1,000 rpm caused the engine to falter badly. Once we determined that the ignition system was working as intended, the owner mentioned that when he disconnected the vacuum advance, the misfire disappeared. That’s where we started with the diagnostics on this issue. Too much timing will make the engine surge and misfire, and that can be misdiagnosed as an ignition problem. Unfortunately, this engine didn’t need the additional timing at part throttle mainly due to its higher 10.7:1 compression ratio. He mentioned that he had read some of my previous tech stories on using vacuum advance at part throttle and decided to try it. When he attempted to add vacuum advance to this timing, the engine began to misfire because there was just too much timing in the engine at part throttle at low speeds. In this case, the high initial timing was aggravated by a very quick mechanical advance curve, pumping an additional 10 degrees of timing into the engine by 3,000 rpm. Likely the reason this engine has 24 degrees of timing is that idle vacuum increased with the additional timing. Most performance engines with this much cam will be happy with 15 to 18 degrees of initial timing.

Add to this increased exhaust dilution in the intake manifold from the substantial overlap and the result is an engine that wants more initial timing. Engines with long duration camshafts also suffer from large overlap values that drastically reduce idle vacuum. Rather than keep you in suspense, the clue was when he mentioned that the engine was set with 24 degrees of initial timing.Įngines with very large camshafts (260 degrees at 0.050 is a big cam for the street) generally require more timing at idle compared to shorter duration camshafts. The clue is buried in the question and it took time to figure it out. I have to admit that I missed an important clue that he gave me with the original question that, now looking back, was the most-critical piece of information. This small-block is quite stout with compression, a big cam, serious Brodix heads, and a large, four-corner idle Holley carburetor.Īs with most issues, it wasn’t just one problem. Jeff Smith: This question required a couple of emails back and forth to finally dial in where this engine was troubled. Can you point me in the right direction so I can show them an old guy can still make more power the old school way? - W.S. All of the guys in my circle are EFI guys and I’m the only one with a carb and a distributor. It seems to happen any time the engine runs over 2,500 rpm. I have run the carb tune everywhere from 13.0 to 17.0 AFR at cruise. I’ve come to the end of my knowledge on tuning and my frustration level is rising. But from what I’ve read, it seems that vacuum advance is a good thing for part-throttle performance. When I disconnect the vacuum advance, the misfire goes away. As soon as I roll into the throttle, it smooths out as long as I’m accelerating so I assume the carb is moving to the main metering circuit. Regardless of the vacuum advance can I use, the engine has what sounds like a misfire at part throttle cruise. It idles happily at 1,000 rpm at seven inches of vacuum and makes great power. My wide-band O2 sensor says I’m running in the low 14’s to high 13’s with 12-14 inches of vacuum between 2,500-3,300 rpm on level road at a steady cruise. The initial timing is 24 degrees, with 34 total all in at 3,000 rpm using two very lightweight springs and heavy weights. The cam is a COMP 260/264 degrees duration at 0.050 with 0.645/0.630-inch lift on a 101 lobe center (cam is advanced 6 degrees). My combo is a 355 small-block Chevy, 10.7:1 compression, 23-degree Brodix 10x heads that flow 295 cfm at 0.700-inch lift on the intake and 235 cfm at 0.750-inch lift, Jesel rockers, Holley Strip Dominator intake, 750 Holley with Quick Fuel metering blocks, a large cap HEI with an ACCEL coil, and an MSD 6T box. I’ve been reading your posts/blogs/articles on tuning with a vacuum advance and it’s been very helpful. The springs merely determine the rate at which the system achieves advance.

The length of the slot determines the amount of mechanical advance. You can see the weights and springs and the arrow points to the slot. This is a photo of an HEI mechanical advance mechanism.

Advance timing sbc