timing question

[john davis]

my 75 vette l-48 with edelbrock performer heads 64cc comp cams 268h edlebrock performer rpm manifold headers into duals no cats and 700r4 with 3.70 gears started running like it didnt have the power it use to have I took it to a guy and he said the timing was retarted real bad cause the hold down had got loose I dont know how much it was off or what it is set at now but it has a slight miss in the engine at low rpm above 2000 rpm the miss goes away the car now runs very strong also if you lug the engine there is a small amount of engine knock not much I have been runnign 87 octane so I put in some 93 the knock is gone but the miss is still there. Do you think the timming is advanced to much? I am not much of a mechanic but I am going to buy a timing light and try and check it my self any tips also which way do you turn the distributer to retard timing? Do you turn it with the engine running if so wont the thing shock your hand? I have a quarter mile marked off on a country road cars hit between 95 and 97 mph does that sound about right with my set up. also does any one have a rough idea how much torque and horsepower I have got thanks jd

 

[ssvett]

JD, I have a couple of ideas,and they are only guesses. But, I do believe that you are a little advanced, due to the pinging, unless you are over 9 to 1 compression 87 octane should be fine. My therory about the miss is, you may have fouled a plug when the engine was out of time,or toasted one. Pull your plugs and check and regap(If I was going to take them out I would replace them with good AC Delco plug),just my oppinion. I would say that 95-97 is pretty good for a 75 Corvette with stock compresion and auto. That would make me happy with the ol' 74.........Hope this helps......Steve

 

[Jack]

timing

John: I agree with Steve...check the plugs first. I’m assuming you have a GM HEI distributor. Invest in a high quality timing light e.g. MSD pn 8990. Jegs at www.jegs.com has them for $95...jegs pn 121-8990. Yes, you set timing with engine running. There is high voltage but low amperage present. If wires/cap are leaking it’ll sting heck outa you but usually not life threatening (unless you have a pacemaker...then have someone elso do it!!!).View dist from Top. To Advance you turn dist CounterClockWise. To Retard turn ClockWise. Loosen clamp just enough to permit rotation...careful, a little bit goes a long way. Here goes: hook up timing light, check all wires are clear of pulleys-belts-hot exhaust, start engine, set RPM per GM tuneup specs, check/adjust timing by turning dist, tighten clamp, recheck timing aften tightning clamp. You might also check that your Vacuum Advance Can (conical metal can w/rubber hose) on side of dist is ok. With age, they often stick or leak...causing improper timing. They’re cheap to replace...get one at dealer or local parts house. If you still have misses after you’ve properly set timing and vac unit’s checked out...then you might have a compression check performed. John, running prolonged & badly retarded timing often can/does burn pistons/valves...causing cylinder pressure leaks...and misses. Hope this helps. Let us know how things turn out.
Jack
'71 CPE

 

[Steve Hartom]

timing ?

John,
Couple of things I ran into with my '73 when I started bolting things together. If you haven't already done it. Get an inexpensive Volts- Ohms Meter(VOM) and check the resistance of all of your (new?) ignition wires. They should be fairly high resistance but all will be reasonably similar in readings, the longer ones will have slightly more resistance than the shorter ones. If one or two have no resistance 0-5 ohms or infinite ohms (OPEN) throw 'em out. I treat wires like first dates. It doesn't matter if they are A C Delco, Accel or what. Brand new in the box does not mean they are right.
The other thing was when the new intake went on so did the Holley carb. I had a problem with detonation in the wrong place it sounded like it was coming from the exhaust manifold at near idle. This turned out to be a LEAN mix in the primaries of the carb not igniting in the engine but when it was in the exhaust. Jetting up the primaries took care of it.
An after thought, G Tech sells a unit that is fairly accurate for those back road 1/4" mile ET Test and Tune session. (95 mph = 15.00 sec in the Quarter Mile)
Good Luck
Steve

 

[Jack]

recurve distributor

John: I forgot to mention this in previous post, but after you've checked everything ignition (SH is right about VOM-wire resistance) and other possibilities (eg cylinder leakage)...and you still have misses...you might have the distributor professionally setup and recurved. There is probably someone in your area equipped to do this for small money. If not, there's a fellow in Michigan named Mike Delahanty ( Mdelahanty@aol.com ) with SUN distributor machine...he can help with this. Just remember the KISS principle...check the obvious first...recurve later. Also, Total Timing is important too...that is maximum advance. This number is debatable but...your motor will probably do well with about 38-40 degrees of Total Timing when both mech & vac units are fully advanced. This occurs at an RPM (also debatable and peculiar to your mods) when motor is at or about cruising (maybe 2200 - 2600 rpm)...typically just below detonation point. The timing marks on your stock harmonic balancer probably do not go that high...get timing tape (a scale that permanently adheres to hb) from somewhere like jegs or summit or maybe even local parts house. Before purchasing Ttape, you'll need to know hb's diameter (likely 6.75" or 8").
Jack
'71 CPE

 

[Tom Bryant]

timing

John,

I didn't see it mentioned in the other posts but when you check your timing on a vacuum advance distributor you need to disconnect the vacuum hose to the advance unit and plug the hose with a small punch or something so you don't have a vacuum leak. When you are setting timing you are settin initial timing and don't want the vacuum advance working.

Tom

 

[68mule]

This may sound dumb, but check your wires too. My engine shakes them out of their holders periodically, and they get burned on the exhaust manifold. I have gone through a few sets of wires before I figured out a way to keep them stationed. The fact that your power problem is off-and-on indicates all the stuff the other guys have said, but it's worth a check anyways.