Question on ignition timing - GM 350Ci on a full size '77 Chevy Blazer 4x4. Holley 750 cfm double pumper carb, Weiand manifold, factory Hei ignition. I'm trying to squeeze a little more juice out of this engine until I can get a new cam. I'm not sure if it's a factory cam right now, but with the other mods that were done to this truck before I bought it I suspect it might not be a stock cam. - It's also got a Holley hp mech fuel pump, Flowtech headers, 2-1/2 in dual exhaust - no cats, h-pipe, twin flowmaster 40 series muffs.

I went to check the timing on it, and  damn if the timing light I haven't used for a few years is dead as a ditch-digger's canary. I run over to the local parts store and get a new Actron digital.  The readings I get are a bit confusing: After marking the timing notch on the pulley, and capping off the vacuum line at the dist., It looks like the timing mark shows up at somewhere around 14-degrees at idle (after getting the engine up to operating temp). Idle rpm shows to be about 750 rpm. I backed down the light to measure the initial timing as indicated and it reads 16.5 when I back the light down to the 0-degree mark ! The engine was not running rough and I didn't think there was that much of a power loss at that setting. I also didn't experience any detonation or other serious indications before I made any changes.  Once I started to loosen the dist base clamp bolt to adjust the timing I noticed that the dist was pretty loose and may have turned on it's own over time. I got it as close to between 9-10 as I could after several attempts. The timing light reading came out to 11.1 once I backed the light down to the 0 degree mark. On the pulley indicator the mark is pretty close to the 10-degree mark.  I did get a noticable increase in power and throttle response but it has a little hesitation now unless I pull the manual choke out a bit. I'll have to go back and re-check the rpm at idle. Rpm seemed to come down on it's own at that setting- I ran out of daylight, and didn't want to PO the neighbors any more than they already are with all the noise I was makin.

My question is: Am I doing something stupid here and not getting the right readings ? ( entirely possible about the stupid part ). Could the engine have been running alright at 16.5 degrees of timing advance ? I took the measurements several times to be sure - or is this an indication of a better than stock cam in this engine. Is there any way of telling without ripping it out ? I really don't want to buy a high end cam just to pull the old one out and find out I already had something comparable to it. I'm working at getting this thing built to a high perf monster, slowly but surely.

I appreciate any advice anyone could provide, and thanks for you patience with my stupid questions.

Ya know, I just don't like timing lights with gizmos. Its hard enough to get a light that's fast enough not to show the marks a bit retarded (that of course is something you can't see or know unless you've got the specks for the gun or can put it on an osscilloscope to measure the time between signal and flash) and then they have to put gizmos on the light that require you to guess even more as to where the timing really is. I use an old light I bought off the MAC tool truck a billion years ago and it looks it. But it's straight up, fast, and simple which makes my life less complicated.

Your problem with a 77 and the timing light from someone looking in but not there appears to be how do you sort out the fancy light's issues from mechanical issues in the engine.

If you live in a larger town or city there may be a shop that can test the gun's reaction time, look in the Yellow pages under electronic test or metrology labs.

The thing to test the goodness, or lack there-of, in the cam drive hence the distributor's coordination to the crankshaft is to set the base timing, then rev the engine up a few thousand RPM and drop the throttle closed. What you should see is the timing marks move in the advance direction as the revs come up and then drop back to the base setting as the engine falls to idle speed. If, when the throttle is closed, the timing marks move from advanced to base to advanced or perhaps even retarded in a bouncing fashion the engine is in need of repair if not overhaul. The bouncing timing marks indicate the timing gears and chain are shot and the cam is rattling back and forth against the slack in the drive system.

For the engine's reaction to what your seeing as the advance at 16 degrees, this is typical of over-advance. The engine will pick up torque in the mid range and begin to loose power or even surge up on the top end. What this tells you, is that it wants the advance to come in at a faster rate but it doesn't want more total advance.

!977 is right in the heart of GMs cam lobe and lifter problem era. A vaccum check is pretty good at identifying and lobes or lifter that are out to lunch. You'll get a regular drop and return in vacuum every time that /those cylinder(s) come around.

The problem of determining cam timing on an engine that's installed is that it isn't easy to do. This process goes a lot better on the engine stand or work bench where you can get at the motor and see stuff. The drill is to collect a degree wheel or a timing tape for your damper, a new set of valve cover gaskets, a dial indicator with magnetic base, some wrenches, and a couple buddies.

Pull the plugs, put the degree wheel or timing tape on the crankshaft or damper, remove the left side valve cover. Have one buddy crank the engine in the direction of rotation till number 1 is at TDC compression stroke. Assemble the dial indiator onto the exhaust spring retainer. Now the gauge needs to read backward because the cam will be depressing the valve which will move the retainer away from the dial indicator. So compress the stylus about a half to six tenths of an inch. Then twist the gauge face to zero the numbers with the position of the needle. OK with buddy number 1 on the wrench and buddy number 2 reading degrees, and you reading the dial indicator have buddy number 1 start to ease the crank very slowly in the direction of rotation. As soon as you get a downward movement of the dial indicator have buddy number 2 call the degrees and write them down. Continuing to turn the engine as smoothly and slowly as possible, as soon as the needle stops going down, write down the distance it travelled from the starting point. This is lift for that lobe. Continue rotating the crank, the needle will dwell at full lift for a period and then start to move toward zero as the valve is returned to its seat. As soon as it hits the zero mark call for degrees and write them down. The data you now have is enough to calculate duration and lift. The degrees between opening and closing is duration. If you can get a reading between .050 inch of lift on the open and close sides you can get duration as its usually shown these days. You'll find there's 40-60 degrees worth of ramps between the start of movement and .050". Once done for the exhaust, the same set up and measuring techniqe is used for the intake.

Bogie

   I know what you mean about the fancy do-dads that cost a lot and are basically worthless because the detailed result can be so ambigous. I had the same experience with electrical test equipment - the Simpson 260 multi-meter the military used for years. It's a great tool, and lasts forever. Then I bought a fancy digital meter from Fluke, it was alright at first, but now it's just insane. I can't get a decent reading to save my life. I went back to the old reliable Simpson multi-meter I had stored away.

The current was a situation where I had a decent timing light that I used many times in the past without a problem, but now, as with so many other things, the fancy $100.00 one is the only one most auto parts stores carry. I needed something in a pinch so I bought it, now I got more questions than answers from it. I'll have to find a reliable light without all the bells & whistles. Probably the cost to have this one tested will be more than what a reliable one will cost. I don't live near a major metropolis. I did for 18+ years and got really sick of it. Now I live away from all the hustle & bustle. I don't miss a thing about it, either. A lot less dorks & a-holes per square mile living in the country.

I'll try the test you suggested for the bouncing timing marks, and the vacuum test as well.  I've got buddy 1 available, but not buddy 2, or 3 unfortunately, so I'll probably just go ahead and replace the cam and the timing gear set and chain at the same time with a good heavy duty one. I don't belive in saving money on this type of item so I'll go with the best my budget will allow.

One symptom I did notice is a loss of power at the high-end when the timing was set around 16 - probably due to the dist base bolt being loose and the timing setting drifting. I thought it was the fuel pump first, so I replaced that. It had been a few years since I replaced it, and I figured it couldn't hurt. The nice thing about these Holley performance mechanicals is that they give you a warning, most of the time that it's going bad. I used to tow big boats a lot over the years and they always seemed to fail under a heavy load They always gave me time to pull off the highway to a place I could replace it safely. I always carried a spare in my truck, and one in  the boat - also a 350Ci Chevy - this one with a Carter AFB carb.

Thanks again for all the great information, and your patience with my very basic questions. I'm not sure where guys like me would be without skilled folks like yourself who give of their time and experience. I really appreciate it.