JoeCool66

I’m not a mechanic… I have a general idea of what I’m doing and I need advice…

I have a performance problem that has gotten worse and I’m not sure what it is. I’ve got a ’66 327/300; mostly stock except for a Mallory Unilite (mechanical adv) and a Holley 4160. It was running okay, had fair but slow acceleration and had some shutter at about 5000 RPM (yellow is at 5000 and redline is 5300). Thinking I could improve things by playing with the advance curve, I first replaced the cap, rotor and new plugs and even added a Mallory power filter/fuse for good measure (without changing the timing). Although the car idled smoother, I immediately noticed the car developed more shutter under hard acceleration at about 4000 rpm. I also noticed the water temp was a bit warmer than normal (only in slow traffic). I checked the timing and it was 18 initial, full advance of 42 @ about 3000RPM. Thinking I may have disturbed something changing the cap/rotor, I went and adjusted the timing by replacing springs and lowering the max advance to 38. I also checked the advance movement and tabs and all looks okay.

I have left it with the purple/grey springs, initial set to 18, full adv of 38 in by 2200. It idles smoothly, revs solid and has better acceleration but has a bad shutter at 4000-4500. One more strange thing… out of 4 test accelerations, one time it ran perfectly (smooth, powerful acceleration up to read line). Once more clue… I noticed that the timing mark is fairly smooth during the entire advance curve but once it maxes out, it fluctuates between full advance 42 and 38 (it does not stay steady at full advance) – is this my problem?

What gives here? My only guess is that I have a distri problem that I made worse by messing with it replacing the cap/rotor but I don’t know what to check. Help!!

Frankenvette 63

Joe, doesn't sound like it was a component you changed if you had the problem before. The modules usually either work or not, but there is a test procedure outlined on the Mallory Troubleshooting site:

http://www.centuryperformance.com/ma...oubleshoot.asp

Check their suggestions, plug wires could be arching if the connectors aren't solidly connected. Is there any backfiring? Also, you can email tech support, or better yet, call them for an immediate answer.

427-390

All the documentation that I have read here on the CF tells me you should be at 36* @ 2700 - 3000 RPM for optimum performance, not 2200 RPM as you have indicated. Others with more knowledge then I can probably narrow down this issue for you.

427-390

Joe,

Here is a link regarding timing that is in the CF C3 section. Hope this helps. Good luck.

http://forums.corvetteforum.com/show....php?t=1430339

Coves4me

Joes66, Joe, if I were you, I would look into selling the distributor you have and put one in that has a vacuum advance unit. Your low RPM acceleration will improve, your high end won't be affected and your gas mileage will definitely increase. This advice is worth what it cost ya'.

JoeCool66

Thanks for the input… I’ll play with this a bit more during the weekend. Also looks like I have more reading to do…

I talked to the Mallory tech and he suggested I check the ballast resistor. I noticed mine is cracked (hard to see since the prior owner painted everything black!!! ). Maybe I’m not getting enough voltage for the coil to build up its charge at hi RPM.

Question: is it normal for the timing to oscillate at full advance? As I described, it is fairly smooth as it advances but at full advance it moves up and down over 6*.

Thanks… I’m telling myself you learn more when things don’t work the first time…

Donald #31176

The jumpiness of the timing at hi RPM is probably excessive end gear play or a loose module plate. Suggest you follow Coves4me advice and get a VAC for your distributor as it will also result in lower engine temperature.

PS Good idea if you have the distributor out is to recurve the ignition map to optimum.

Frankenvette 63

I recently abandoned a Mallory mechanical dizzy for a MSD vacuum advance dizzy. The best thing I can say about the MSD over the Mallory is the ease of setting the curve, very simple with the MSD. As for better low end performance, little to none. The only place I see any difference is taking off from a dead stop I don't have to feather the pedal to get it going (3.08 gears and an M21). WOT, no real change, which makes sense, vacuum falls off! As for temperature change, I think this is a wives tale for the older engines we run, didn't make a bit of differnce in my temp! As for better gas mileage, it makes sense, but I wouldn't change a dizzy for mileage in an old Vette! One thing the vacuum advance did that I hate, it mellowed out the lope of the cam at idle!

Joe, you need to get a copy of Lars Timing Tips (check BarryK's web site). He makes timing and curving very simple. Your Mallory is a little more difficult to curve than most others, harder to change the springs. You have to get the maximum advance set first, and others are right, 36* at 2500 to 3000 is a good range. Once you have maximum advance set, you'll need to change the springs to get the curve you desire.

The bounce at high rpm's could your timing light, don't read that much into it. If it's an old motor, it may have some timing chain stretch, could be the dizzy has some tolerance issues. But you'll drive yourself nuts chasing that crap and you'll not see that much difference in performance.

Check the plug wires, and the connections on the cap and plugs. Did you get the module back in place correctly? Those things are a pain to push down while tightening the lock nuts. Best of luck!

SPLITRAY

Another thought...take a sharpie and mark your harmonic balancer accross the face just to make sure it's not moving around on you.

DansYellow66

It's not usually as common in GM cars (but this isn't a GM dist) but your distributor bushings might be shot. Pull the cap and see how much up and down, and how much lateral movement you have. It could be excessive end play or bearing slop.

JoeCool66

Didn’t work on the car as much as I wanted (Christmas shopping, parties etc.. … where are my priorities??). But I have come to some conclusions…

First, FrankNVette was absolutely right.. Adjusting the curve on the Mallory is definitely a pain in the a**. I also broke down and got a dial-back timing light to make my life easier.

The car idles fine, no backfire, no detonation. Acceleration varies depending on the springs I use (as expected). The problem is the shutter I get past 4000 RPM.

I checked all the obvious things and retighten and secured everything. Also checked the Mallory module. The erratic timing at full advance is a bit less so I’m going to assume that is not my real problem.

The play on the distributor is very little (I have no point of reference so please jump in and tell me if this is wrong). The dist play is about 1 / 4 inch CW, zero CCW and maybe 1/16 vertical (doesn’t seem too bad).

I did decide to make sure I was properly reading my timing so I set the timing mark to 0 TDC and noticed that the dist rotor was not exactly pointing at #1. It was a bit past it (about 1 /4 inch CW).

Should this be pointing directly to the #1 or am I just seeing my 18* of initial advance here?
I’m following along on another thread (Starting / Timing Question) and I’m confused…

My initial max timing (before I messed with anything) was 42*. Very high but the car ran okay. Now I’m thinking my dist is one tooth off and I don’t really know what my timing is.

Am I on the right track here? (thanks for the help /Joe)

JohnZ

Doesn't matter if it's a tooth off or not, as long as the rotor is pointing at the #1 wire tower (wherever that is) at #1 TDC on the compression stroke.

42* advance is excessive - are you sure the "shudder" isn't detonation? Can you better define the "shudder"?

JoeCool66

Thanks JohnZ… I know the concept but I honestly can’t say I know what ‘detonation’ would feel like from experience. I’m expecting it would be more violent and serious than what I’m seeing. By ‘shutter’ I mean it builds RPM smoothly and with power until it gets around 4K… then it starts to hesitate and lose power. This is currently happening with the timing set to 35* (18* initial).

It was set to 42* (18* initial) before this started. It would run okay but build RPM slowly and have a shutter or hesitation at more than 5000 (pretty close to red line). The slow RPM build was the ‘problem’ I was trying to correct. I seem to have lowered the RPM range where the problem begins when I replaced the plugs/rotor and cap ??!?!

I haven’t checked to see if #1 is actually at TDC (don’t have the right tool). I am only going by the timing mark on the balancer. It is right on 0 but the rotor it pointing just a little past #1 on the cap (about 1 /4 inch CW). Does this sound right?

DZAUTO

42deg is waaaaaaay too much!

Frankenvette 63

Joe, you can rough check the #1 TDC by removing the coil wire, then the #1 plug and holding a finger over the hole while someone bumps the engine. When the #1 reaches TDC it will blow air through the hole, you can't miss it! You should be very close with this method, but it won't be as precise as the tool.

Frankie the Fink

Your timing should be 'all in' by 4000 RPM - are you sure you are tackling the right problem ? Could it be plug wires breaking down or some thing along those lines ? Seems like a 'shudder' at 4 grand is way past the point that your distrib curve is ramping up.

65 vette dude

Joe I know you bought your car within the last 6 mos. or so. Has the shutter been there all along,and are these the same plug wires and distributor that were in your car when you bought it? I agree with Fdreano,if you haven't already,I would go ahead and change the plug wires. Even if its not the problem,at least you can eliminate them as a possible cause. Its interesting that since you retarded your timing,the shutter comes in 1000rpm earlier. Keep us posted......Gary

JoeCool66

Yep, wires and distr came with the car. The shutter was always there at 5000+ RPM but it didn’t become a problem at 4000RPM until I changed the plugs/rotor/cap. Hummmm, symptoms worsened when I messed with the plug wires. Coincedence??

Time to call Summit… The PerTronix Flame-Thrower 7mm Stock Look look pretty good. may get the TDC piston stop tool while I’m at it.

That’s the support guys... I’ll keep you posted.

JoeCool66

I found time around the holidays to continue investigating my shutter/power loss at > 4000RPM. I replaced the wires but no change. I did set the timing advance max to 36* and played with the curve and it continues to accelerate well (up to 4000).

I did spend some time confirming if the TDC mark on the cover was accurate. I found a clever test on the Internet that I wanted to pass on: I squirted a couple of shots of oil into the #1 cylinder and rotated the engine so that I was close to what should be a few degrees ahead of TDC. I then connected the hose end of my 2-piece compression tester (these things have a air valve in them that I had to remove). I extended the hose with some clear vinyl hose, added some oil to the clear hose and made a loop. The loop acted as a bubble level. As I slowly rotated the engine a few degrees before and after TDC, I could see the oil bubble move up, stop and move down. As the bubble reached it’s highest point, the mark on the balance was right at zero. I repeated this test several times to make sure.

One interesting development is that I’ve been doing my acceleration tests at WOT. If I back off just a little, the car accelerates just fine past 4000. So now I’m looking at the carburetor. I took it off the car and immediately found that every screw on the dam_ thing was loose (some could have be removed by hand! ). Also the carb base gasket was a little old and brittle. The carb is not original; it’s a 10-year old Holley 4160. I’m armed with Lar’s Holley tune-up sheet and the service manuals so we’ll see where this gets me…

Hope everyone had a nice holiday… thanks for support.

Frankie the Fink

Hmmm....I would have bet on plug wires breaking down. The carb is intriguing as I would think you are well into using the high-speed circuits significantly before you hit the 'shudder' point and so that would NOT be the problem or it would be more prevelant. But then again, I had to actually swap carbs on my '61 to confirm that was the problem on my severe richness issue.

If the shudder got worse after 'messing' with the plug wires as you posted earlier, my money is still on an ignition/distrib problem (or possibly a mechanical imbalance somewhere).

Just for giggles - are you absolutely sure your motor mounts are good ?!