brainsoft

I don't know anything about my engine internals. I know it has not been rebuilt (94,000km), but I don't know if it has had the cams changed at some point. I'm trying to time it by Lars guide, but my initial timing seems way out. I am getting steady 15" Hg at idle at the Distributor which is fed from a port on the front of the Holley. When I connect the gauge to the manifold directly the gauge jumps between 15" and 20" constantly (edit: this was not infact the proper carb manifold reading, but the reading new the transmission/headlight/climate control T on the top of my intake... see picture below) , but I assume that's due to the lack of any buffer because the gauge was cut in so close to the main manifold port.

I forgot to take any vacuum reading at part or full throttle, but I know the distributor is currently getting 15" at idle, and it idles like a jerk from the timing. Just need to check that my distributor vac can is properly selected, but not sure what the vac levels were originally. I'll have another go and see if I can change the springs to get the initial timing up without the centrifugal advance taking it past 36. If I set idle timing in the 8-12deg range it's running up to 42deg+ with the vac disconnected and part throttle, which would put the overal timing up to... 54? Too high at any rate. I have left it with the crappy idle and 36 deg initial+mechanical advance. Didn't actually check the vac advance, but it runs great now on the road. Better response while driving, just the rough idle and a carb that won't tune

edit: can't believe I timed it to a smudge on the wheel and got it running as well as I did!

LudemJo

I would be concerned about this first. I just ran a vacuum check on my 74, and was seeing 18" at idle and the needle was just perceptably vibrating. When I would increase the rpm, the vacuum would increase to 22" and the needle was rock steady.

Everything I have read indicates that there is a problem if the manifold vacuum is fluctuating as much as you are seeing. Might be time for a compression check to ensure there is not a valve problem.

This may not be a timing problem...

John

scottyp99

http://www.secondchancegarage.com/public/186.cfm

Shows some very interesting things you can diagnose with a vacuum gauge.

Different springs in the distributor advance mechanism will not affect how much advance it gives, just how fast it comes in.

I am always amazed when people post on this forum that they "are using Lars's paper to set the timing", but are still doing it all wrong. Study the paper intensely, everything you need to know is in there.

How long have you had the car? Have you ever re-built the carb? With Holleys, a dis-assembly and a good cleaning, followed by re-assembly with fresh gaskets, can sometimes work wonders on driveability.

Scott

brainsoft

Sorry I am still learning about the car it is my first older car so i am experimenting with the tools and learning the components as I go. I haven't opened up the distibutor to find the springs even. I am a licensed tradesman and a smart guy i am just slowly working my way through troubles i know about. Lars' paper is written for more knowledgable people so I am working up the learning curve as I go. Any advice on the fluctuating vacuum would be appreciated.

Carb was rebuilt after I got it. It is a crappy carb, I will work with it once I get the ignition settled

LudemJo

Not a problem at all, if you look at the scenarios shown in the link in Scottyp99's post it shows you what the vacuum gauge will do under different circumstances. Based on your description, it sounds like a valve problem. Take a look at the link and watch each of the scenarios closely to determine which most closely resembles what you are seeing on the vacuum gauge hooked up to manifold vacuum.

The link is one of the best for helping you understand what the engine's manifold vacuum is trying to tell you about how your engine is operating. Post your findings, and people will help point you in the right direction.

You may want to pick up a compression tester, I think you are going to need it. That will also give you a chance to pull your plugs and see how they look. Take your time and enjoy the journey, I spend more time working on mine than driving it.

John

Snoopysvet

That link is AWESOME!

scottyp99

It is awesome. I had no idea you could diagnose this much stuff with a vacuum gauge until about a year ago. I put that link in my favorites folder.

Scott

brainsoft

I have a lot of scenario 3 vibrations. the jumping needle I was talking about is on the #8 intake runner, so it may be jumping because of that. the Reading at the PCV valve is around 18 and vibrates. I guess that may mean I have a performance cam of some sort in there.

Opened the distributor up a bit more and see the springs and weights now. The springs in there look like a lot less coils than the ones in the 928g kit so I think they are stiffer. Cleaned everything up, but going to keep doing research through the day tomorrow.

How can I make my timing mark more visible? I cannot do anything, the mark is so damn hard to track. I think the damn light skips blinks sometimes too. My timing light is the cheapest you can get and I can barely see the mark. I don't know if a better gun will make it easier to see. I tried painting the balancer with flat black and then painting the mark with white but it didn't help. What do you guys do to increase visibility of the timing mark?

LudemJo

Where are you reading your vacuum from? Are you using the manifold vacuum line at the base of the carb? I don't understand how you know it is on the #8 intake runner. Even with a perfomance cam, the vacuum should not fluctuate as much as your initial post indicated. I use a "T" connector, and hook the vacuum gauge between the manifold vacuum port at the base of the carb and the vacuum advance can on the distributor. You can also use the PVC port at the carb, but I wouldn't measure through the PVC valve itself.

As far as timing marks go, I bought some timing tape from Jeg's that sticks on the harmonic balancer and gives markings all the way out to 80 degrees. You need to know the diameter of your balancer to pick the one with the right scale. Alternatively, you can make another mark on your balancer as discussed in Lar's timing paper. I didn't have too much luck making a new mark that was visible with the timing light, that is why I bought the tape from Jeg's.

Good Luck!

John

brainsoft

The readings i took in the first post were at the tee on the manifold itself that feeds the headlights and tranny and climate controls. The tee is on the #8 runner on my old Edlebrock C3BX dual plain intake

I took better readings from the main manifold line that comes from the bottom of the carb and leads to the pcv valve. I removed the valve and connected the gauge there on the main line. The needle vibrates at 18" at 1200rpm. No wild swinging or anything like that. Not sure why it was jumping at the other location though.

I need to grab a larger Tee to run manifold to the distributor, it is running on ported vac, but even that is 12". Vac can on distributor operates at 2-10".

brainsoft

This is where the wonky readings were taken.


Now that I have the new cap/rotor and wires things are running much smoother and I will take new readings on the weekend after I change the harmonic balancer and can actually tune it up a bit.

Here are the new marks I made on the harmonic balancer. Very visible when I can see the marks on other wires. Not visible on #1, hence the harmonic balancer swap.