My 351w has a bad diesling problem going on that im trying to fix. I run her at idle at 600rpms so I dont believe its an idle problem. The question would be I currently am running the timing at 10-12 degrees vaccum advance plugged then off of the manifold tree as I dont have emissions stuff on it. Should i be running it there or off of the passenger side of the edelbrock carb. Pros and cons to either vaccum? Also do you all beleive that im running too high of an advance? The rebuilder who did the stroker to me thats to give a few more ponies. So my three questions are: best place to run no emmissions 351w vaccum advance to, good initial timing, and if i have no hot spots or carb drip and its a motor under a thousand miles why am i dieseling?

 

Disregard vaccum being hooked to manifold forgot i switched it to passenger side of carb.

 

The initial advance does not sound too much [does it ping while under load?]; the idle speed is good. What temperature thermostat are you running? Have you checked for vacuum leaks? What color are your spark plugs? Are you running a hotter range spark plug? What compression does the engine have and what grade fuel are you using? Have you experimented using 89 or premium?

 

No I don't get any pings, it's runs at about 180 degrees water temp constant, plugs are not running hot, and vaccum is good while on the gauge. I usually run high octane as it doesn't do it as bad when I use that and with the engine I have to run it anyway. I'm at a loss as the carb doesn't keep pouring fuel in and it's a mechanical pump. If I turn the key off in gear instant shutoff turn it off in park and run which told me probably idle but if I go lower than 600-650 it will die when in gear.

Now with vacuum advance should I be using carb or manifold since I don't have emissions? If I recall carb is more for emissions and manifold if not but I may be mistaken.

 

Not absolutely sure I understand vacuum sources being called "carb". The problem is there are two, and sometimes 3, different sources of vacuum on a carb. One is manifold vacuum and the other is ported vacuum. The former is "on" all the time, while the latter is "on" only when the throttle is open beyond idle.

Now for the dieseling problem. A major contributor to that is too much spark advance. If you are running manifold vacuum to the advance that is at least part of the problem. I've tried both sources extensively and find that I have far, FAR better results with ported vacuum than manifold for the advance - especially if the tranny is an auto. And, btw, if your tranny is an auto leave it in gear when killing the engine.

 

Like Gary states....you need to run vacuum advance off the "Ported", or sometimes called "spark" port from the carb. This vacuum port taps off upstream (above) the throttle plates. It only starts seeing vacuum as you open the throttle plates.

Ask yourself this question....When does the engine see maximum vacuum? it is when the throttle plates are shut. Connect a vacuum gage to a manifold sourse and watch it when you push on the gas...it goes down right??

So would you want to connect the advance to a sourse of vacuum that is going to see max vacuum when your foot is out of the gas? You want the distributor to advance the timing as the engine loads up. The mechanical advance, advancing timing based only on speed. Vacuum advance works with it to advance the timing based on load, which is based on throttle position.

So as you open the throttles, the timing port will be exposed more to the manifold vacuum and therefore give you more vacuum advance.

It as nothing to do with if you have emisions or not. Has everything to do with why you advance the timing to begin with and when you want this to happen.

You see a lot of engines with the vacuum advance attached to the manifold vacuum. This leads you to be at max advance while at idle, then actually retards timing from there as you load the engine (except the mechanical helps you out too)...the opposite of what you want.
Hope this makes sense.

 

That's what I was under the assumption with as well and was my reasoning for hooking my vaccum advance to the ported side of the carb. I was just making sure that it was correct since I am running 10 to 12 initial advance as told before. Since I have no pinging I'm sure it's fine. Just didn't want to wear something out running that high on a newer engine with all that money in it.

Now that I know I'm correct there I need to figure out this dieseling. The plugs are normal color but maybe I'll try a less hot set and see if that helps and stick to high octane as it doesn't happen nearly as much running that. I'll have to start narrowing down the possibilities ie. fuel, heat, and air combustion properties. Would there be anything electrical that would contribute? I don't have a computer or anything as the electrical is out of an 80'.

 

If you are sure you are on ported vacuum, then you are ok there. But, you may have to back the initial advance down a bit to kill the dieseling. Also, drop the idle speed as low as possible, and kill the engine while in gear.

And, check to see if you have a heat riser valve in the exhaust. If so and it is stuck closed that will increase the inlet air temp and make the problem worse.

 

No don't have a heat riser valve. I'll just start trying different combinations and see where it goes. Thanks Gary and others.
Kyle