Fuel getting into modulator vacuum line

[Joe R.]

Re: Fuel getting into modulator vacuum line

After restoring my 54 back 3 years ago, I had fuel in the modulator vaccuum line as well. After checking the vaccumm at the manifold, I found the vacuum was very low, I think about 10" Hg. I checked timing, and the VALVE LASH, and found that the engine rebuilder had adjusted the intakes way too tight, causing intake valve leaks in 4 cylinders. After I readjusting them all HOT, the fuel in the mod line went away.
Joe

[Keith L.]

Re: Fuel getting into modulator vacuum line

I had the same thing on a big block Camaro. But the carb was leaking fuel into intake after shut down.

[Clem Z.]

Re: Fuel getting into modulator vacuum line

GM has this problem with some dist vacuum advances and the fix was to put a vertical loop in the vacuum line before it went to the dist vacuum advance. if the line is down hill all the way any excess gasoline in the line will run to the modulator and vacuum can not pull the gasoline out unless air can get into the line at the modulator. fill a soda straw with liquid and put you finger over the end and you will see that you can not suck the liquid out of the straw.

[Ed P.]

Re: Fuel getting into modulator vacuum line

OK been to cold and busy to work on this problem. Would like the valve lash specs for the blueflame valve adjustment when hot. I found this .010 INTAKE and .020 Exhaust is this correct hot setting. HAPPY NEW YEAR

[Joe R.]

Re: Fuel getting into modulator vacuum line

Ed, that's correct. I always get the engine coolant to 170 and adjust the front 3 cylinder valves, then start it up and run again to get it warm to 170 again. I've found that adjusting all valves all at once allowed the rear cylinders to cool down as much as 15 degrees, causing erroneous adjustments.

Joe

[Ed P.]

Re: Fuel getting into modulator vacuum line

Here is latest update. Adjusted valves today the exhaust valves were very loose. The intakes not to bad. Also checked fuel pressure and its at 5 psi. I removed both lines from bottom of fuel pump and checked while running and no fuel leaking out. Put it all back together went around the block and put car back up on lift. Removed modulator line and out comes the fuel again. Forgot to mention I made sure line was clear before drive. Not sure were to go next except maybe rebuilding all three carbs. Ed

[Edward J.]

Re: Fuel getting into modulator vacuum line

Ed, I know there was menchen of the vaccum line that goes to the modulator did it come of the bottom of intake or the top?

[Ed P.]

Re: Fuel getting into modulator vacuum line

The vacuum line comes from the top rear half of intake. I have even put a bit of a loop in line with a straight fitting with no change. Ed

[Ray G.]

Re: Fuel getting into modulator vacuum line

The vacuum line comes from the top rear half of intake. I have even put a bit of a loop in line with a straight fitting with no change. Ed

Hello Ed;
It sounds like the fuel is getting in while the engine is running. Not considering carb. fuel leaking into the manifold after driving.
Is that correct?

Getting back to basics.
The vacuum modulator senses the manifold pressure and applies pressure to the throttle valve, which increases when the engine is under a greater load.

Trying to think of a way to get clean manifold pressure(vacuum).

Is there a center carb. stud that would lend itself to a hollow stud. In other words if a stud was removed would the threaded hole go all the way inside the manifold?

Just trying to think outside normal parameters.
Ray

[Edward J.]

Re: Fuel getting into modulator vacuum line

Ed, you said in one of your posts that the fuel pump and vaccum for the wiper lines intersect,does fuel pump act as vaccum pump for the C-1 cars with wipers?

If so maybe the fuel pump diafram is leaking interally?not knowing your year all that well and how the wipers operate, im looking for a reason for the fuel to get into the modulator line, does the vaccum line to modulator have a T fitting with something else that needs vaccum to operate?

[Ed P.]

Re: Fuel getting into modulator vacuum line

Yes you are correct. The fuel pump was the first thing I checked. There is no T fitting. Ed

[Troy P.]

Re: Fuel getting into modulator vacuum line

Beats me. What a mystery.

How about putting a temporary 2"-3" verticle extension on the manifold outlet where the modulator line connects and see if fuel could be pulled or pushed up hill that high to get in the line down to the tranny.

[Edward J.]

Re: Fuel getting into modulator vacuum line

Ed, Maybe a few pic.s of the vaccum line set-up and the fuel pump side of engine, maybe if I look at a few pic.s theres something I can see. do you have a digital camera?

[Ed P.]

Re: Fuel getting into modulator vacuum line

Troy
Yea I tried that and fuel still gets in there. The fuel must be getting in right after shutdown other wise it would run bad and it runs fine, a little rich but that may have been because of the valve lash adjustment. I did notice when I had the fuel gauge hooked up the it held the pump pressure after shutdown. I am going to check float level adjustments tommorrow. I read some where in here about drilling a .015 hole in diaphragm of fuel pump to relieve pressure in system after shutdown. Maybe that will help my problem. Is the modulator valve a diaphragm that would move enough to pull fuel into line when engine vacuum is released. I would say as diaphragm returns to relaxed state. Ed

[Troy P.]

Re: Fuel getting into modulator vacuum line

Man, I don't see any way fuel could be pulled by a vacuum straight up a temporary 3" pipe plus the vertical rise of the original bent vacuum line. So in my mind that implies it is being pumped under pressure.

The fuel pumps do hold the pressure after shut down but the manifold would have to be filled/flooded and the carbs leaking for fuel to be pushed up the vacuum line.

I'm thinking that one of the posters that was getting at a possible fuel pump diaphram leak might be on to something. In the dual diaphram pump fuel should be on the top side of the top diaphram and vacuum on the bottom side of the bottom diaphram. If there was some sort of passage between the two fuel could be pumped through the vacuum lines. So I suggest you tear down your fuel pump and see if you can learn anything there.