360 timing and vacuum questions

[Scotty Dive]

So a good friend of mine swapped my 258 to a 360 in my CJ7 that I removed from a J10. It had been rebuilt, ran well and had good compression across all cylinders. We believe it had a mild cam installed during that rebuild. But dont have any other details on the rebuild by the guy I purchased it from. It had 70K on it. It has a 2150 carb and a CRT HEI. Other than those items its stock.

Engine runs really nice, but at idle the engine rpms go up and down. We were wondering if that was because of the cam, but I started to throw a timing light on it and a vacuum gauge and noticed some odd things. Initially the timing was set at about 12 BTDC but that brought only 7 inches of vacuum. When I adjust the HEI to get highest vacuum (17) the engine settles down and RPMs and vacuum is steady, but I cant even see the timing mark. I have an adjustable timing light and I rolled the dial until I saw the timing mark at the 10 BTDC and the dial on the timing light was at 32. Does this mean that to get the highest vacuum my initial timing is 22 degrees? That seems not right. Bad for the engine? Is that what the cam did???

I was taught to time an engine by highest vacuum. Is this different in the AMC V8s?

[MysticRob]

...Engine runs really nice, but at idle the engine rpms go up and down. We were wondering if that was because of the cam, but I started to throw a timing light on it and a vacuum gauge and noticed some odd things. Initially the timing was set at about 12 BTDC but that brought only 7 inches of vacuum. When I adjust the HEI to get highest vacuum (17) the engine settles down and RPMs and vacuum is steady, but I cant even see the timing mark. I have an adjustable timing light and I rolled the dial until I saw the timing mark at the 10 BTDC and the dial on the timing light was at 32. Does this mean that to get the highest vacuum my initial timing is 22 degrees? That seems not right. Bad for the engine? Is that what the cam did???

I was taught to time an engine by highest vacuum. Is this different in the AMC V8s?

Hunting idle sounds like vacuum leak to me.
Are you checking timing with the vacuum hose off the distributor and plugged?
Typically you should warm the vehicle up first, then set the timing either at factory specs or a couple degrees above for added performance, then adjust the carb idle circuit for hightest vacuum pulled at the correct idle RPM (that is if all other carb circuits are tuned correctly and are NOT affecting the idle circuit).

[Ristow]

thats exactly correct. with the vacuum advance plugged in at idle the mark will be gone,way past the timing marks on the cover. yeas,22 degrees and even more at idle with vacuum plugged in is normal.

[Scotty Dive]

Thanks Guys. so Ristow.....

I do have thethe hose that ran from the carb disconnected, capped off at the carb and I have the hose to the HEI plugged with a Golf T. I just want to make sure you know what my exact situation is here. So, it is still correct to adjust the timing to full manifold vacuum manifold port plugged and golf t in hose, adjust the timing until I reach the most vacuum? Which would be ~22 degree BTDC?

Wont that bring overall timing at 2500 to 3000 rpms way over 29-34 total degrees of advance?

[Ristow]

i have never timed my engines to max vacuum at idle. i use a gun and dial them in by the timing marks. i thought you have the vacuum advance hooked up. i think you are too advanced if you dont have it connected and are at 22 degrees. or your timing marks are off.

[MysticRob]

Thanks Guys. so Ristow.....

I do have thethe hose that ran from the carb disconnected, capped off at the carb and I have the hose to the HEI plugged with a Golf T. I just want to make sure you know what my exact situation is here. So, it is still correct to adjust the timing to full manifold vacuum manifold port plugged and golf t in hose, adjust the timing until I reach the most vacuum? Which would be ~22 degree BTDC?

Wont that bring overall timing at 2500 to 3000 rpms way over 29-34 total degrees of advance?

Like I posted earlier, base timing is adjusted with all vacuum removed from the dizzy and any leaks plugged. Vacuum leaks will not allow you to adjust base timing properly.
Once base timing is adjusted then you simply adjust idle circuit (and fix any other maladjusted circuits that will affect the idle circuit) to desired RPM and highest vacuum setting. Do not use the dizzy to adjust vacuum, only the carb.