Vacuum Leak at the distributor

[sk8er32283]

I was under the truck messing with the throttle position return springs and heard a vacuum leak that sounded like it was coming from the distributor. The engine is from a 1978 truck and has a "regular" distributor with vacuum advance timing. If I had to describe the sound it would be a hissing sound that got louder as the engine rpm's rise. I am mostly a diesel guy so forgive me if my lingo about distributors is off a bit. Please correct me if I am not calling something by its normal name. Here is a picture of the distributor. I have always felt that the truck was not performing up to snuff and maybe this has a little to do with that issue. Or it could be that the engine is from a 78 when they had all the emissions stuff designed into the engine. Please let me know if there is any other info you need from me to help out with the trouble shooting.

[tcrist]

To isolate that or any noise you can get a mechanics stethoscope and pinpoint where the noise is actually coming from.

I would just check all vacuum lines. These rubber lines will get hard and crack.

The only vacuum in the distributor is the vacuum advance. It could have a bad diaphragm and if it is leaking it could be what you are hearing and why the truck does not run as good as you think that it should.


That should get you a start at least.

[mechanicalman]

Originally Posted by sk8er32283

I was under the truck messing with the throttle position return springs and heard a vacuum leak that sounded like it was coming from the distributor. The engine is from a 1978 truck and has a "regular" distributor with vacuum advance timing. If I had to describe the sound it would be a hissing sound that got louder as the engine rpm's rise. I am mostly a diesel guy so forgive me if my lingo about distributors is off a bit. Please correct me if I am not calling something by its normal name. Here is a picture of the distributor. I have always felt that the truck was not performing up to snuff and maybe this has a little to do with that issue. Or it could be that the engine is from a 78 when they had all the emissions stuff designed into the engine. Please let me know if there is any other info you need from me to help out with the trouble shooting.
Attachment 1374760

Use a hand held vacuum pump and connect it via hose to the vacuum advance pod and see if it holds vacuum. If you don't have a pump, you can remove the dist cap and with your fingers or a pair of pliers, hold the vacuum advance all the way advanced, put your finger over the hole where the vacuum hose goes, then release the arm and see if it holds vacuum.

If it don't hold vacuum, replace it.
http://www.summitracing.com/parts/sm...del/c10-pickup

That distributor is off the old 71 engine, right? It's not a 78, it's a "points style" dist.

Next time you need to change your plug wires, you should consider changing over to an HEI. Or maybe even now if it suits you.
Something like this:
http://www.ebay.com/itm/SBC-CHEVY-35...822f6f&vxp=mtr

And this as you will have to run a new wire from the fusebox to replace the reisitor wire that now goes to your coil. This one has a built in coil and this pigtail plugs into it then you splice same or slightly larger wire to the fuse box to "Ign unfused" post.
http://www.summitracing.com/parts/sum-g5210/overview/

[mechanicalman]

Originally Posted by sk8er32283

I was under the truck messing with the throttle position return springs and heard a vacuum leak that sounded like it was coming from the distributor. The engine is from a 1978 truck and has a "regular" distributor with vacuum advance timing. If I had to describe the sound it would be a hissing sound that got louder as the engine rpm's rise. I am mostly a diesel guy so forgive me if my lingo about distributors is off a bit. Please correct me if I am not calling something by its normal name. Here is a picture of the distributor. I have always felt that the truck was not performing up to snuff and maybe this has a little to do with that issue. Or it could be that the engine is from a 78 when they had all the emissions stuff designed into the engine. Please let me know if there is any other info you need from me to help out with the trouble shooting.
Attachment 1374760

First, remove that hose at the source, plug it, and see if the noise goes away.

[hamjet]

Pull the dist. vac hose off the carb end and suck on it. (or blow). if you can continuosly get air through with no restriction, the vac. diaphrams shot...

[sk8er32283]

Thanks for th quick responses. I will trouble shoot a little more tonight as suggested. I am thinking that I will just upgrade the system if I need to buy parts anyways.

[sk8er32283]

OK I got a chance to look for the vacuum leak. I found one 9th en the front of the carb. I capped it off. The vacuum advance didn't seem to be the culprit. I did pull the line off plug it and rev the engine. I still heard the hissing sound. I think it is just the sound of the air moving through the normal path in the carb. I am just hearing it through the filter. Let me know if I am off or not.

[1972BlueC20]

Your problem is those Autolite plug wires.....LOL

[sk8er32283]

I know the spark at the engine is not the best but it runs for now. I just got some more money and going to start looking into a new ignition system. I read a little on the hei upgrade from the late 70s gas or maybe just getting a whole new system. Including wires. Any suggestions?

[1972BlueC20]

Originally Posted by sk8er32283

I know the spark at the engine is not the best but it runs for now. I just got some more money and going to start looking into a new ignition system. I read a little on the hei upgrade from the late 70s gas or maybe just getting a whole new system. Including wires. Any suggestions?

I was just messing with ya! Autolite is a Ford brand, like AC Delco is GM, ect. Shouldn't make a huge difference on wires, some say that you should always use what the designers of an engine used when designing the engine. Others will say it doesn't really matter. I personally always use Chevrolet stuff on my truck if possible or like MSD, ACCEL ect.

As for the ignition system, if your plugs and wires are really old it's always a good idea to replace them. As for HEI, that's what I have, but just make sure you replace your stock ignition resistor wire with a new 12g wire that gets a constant 12 volts with the ignition on. Your current setup only gets 12v at start up then drops down in voltage. You can search the forum for information on this. Also Pertronix is a great option for getting rid of the old points style ignition. I've used it before and liked it. Good luck!!