Fuel getting into modulator vacuum line

[Clem Z.]

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

you want to drill the.015 hole in the valve body not the diaphragm.

[Ed P.]

Re: Fuel getting into modulator vacuum line

Roger that. Makes sence. I have ran the engine with both fuel pump vacuum lines off and no fuel is leaking. Remember this is an inline six which originaly had the vacuum line coming at a 90 angle off of the top of manifold. I changed line to a straight fitting with about a two inch rise above manifold. Ed

[Ed P.]

Re: Fuel getting into modulator vacuum line

Drove car today around the block with the lower fuel pump lines off and capped. Also set all three float levels. Fuel still pours out modulator line after shutdown. I then tried just running car in garage and there was no fuel after shutdown. This problem started the after transmission rebuild. I have no idea what trans rebuild would have to do with the problem enless old modulator diaphragm had a small hole in it allowing the vacuum to purge line. I think my next step is to rebuild all three carbs. Any advice as to where to purchase the best kits. I have found a few suppliers but would like the most complete kit. Ed

[Roy B.]

Re: Fuel getting into modulator vacuum line

Look under Corvair sights having a Turbo engine carb kit , it's the best kit to use. You know the Modulator Function is to smooth out up and down shifting while in drive, Try to de-connect the vacuum at the intake wrap a piece of cloth around the fitting , drive the Corvette ( it don't hurt nothing ) then stop to see if the cloth is wet with GAS , that would tell the story (engine or trans problem. )

[Ed P.]

Re: Fuel getting into modulator vacuum line

Update
OK rebuilt all three carbs. Installed new coil and tuned up engine. Still gets fuel in modulator. Disconnected modulator line and installed vacuum gauge that I ran inside car. Went for the routine drive around the block. Vacuum 15hg at idle never went below 5hg during drive. Removed vacuum test hose and out runs the fuel. I am at a loss as to what to do next. I did a compression check and all pretty close with average of 130psi. I have been thinking possible intake manifold crack or blocked exhaust system at manifold or downstream causing back pressure. One guy told me he has seen one side of the exhaust welded shut to get more heat many times. Sounds a little far fetched but I am game for anything right now. Ed

[Troy P.]

Re: Fuel getting into modulator vacuum line

Still stumped.

If I understand a comment Roy once made, the 55 Powerglides does not have vacuum modulator. So I wonder how your car would drive with it disconnected and the vacuum line plugged?

[Roy B.]

Re: Fuel getting into modulator vacuum line

OK now you know it's not the trans pulling vacuum ,Are you dead sure that only ONE line from the intake to the trans ?? NO other connection to that ONE line. (like the fuel pump or wiper motor?) Is the T brass fitting at the intake the correct hole size to the steel line..

[Joe R.]

Re: Fuel getting into modulator vacuum line

Maybe your modulator diafragm is full of gas from a prior problem, and the gas is getting sucked up the vacuum line to the manifold?

Joe

[Ed P.]

Re: Fuel getting into modulator vacuum line

Funny you should mention that as I have already done that and the trans seems to shift better without the modulator. I will have to research to make sure I wont damage trans with prolonged driving. Ed

[Roy B.]

Re: Fuel getting into modulator vacuum line

It don't damage the trans , the governor over rides it .As I said it just makes the trans shift smoother on up and down shifting and shift quicker if disconnected .
But I still dont know how gas is getting into the vacuum line if only ONE line from intake to trans with out some other line being connected to it.

[Ed P.]

Re: Fuel getting into modulator vacuum line

Well here is final update. I made up a line from intake to modulator with a 5" loop at the intake and fuel still gets in line. The next thing I did was plug original vacuum port and ran line up to fwd port on intake. No fuel in line, yea ha. I guess I will never know the real cause but suspect something wrong with intake. Thanks for everyones help. Ed