Gasoline in my TH400 shift modulator hose?

[Mrturner1]

Well it's been one little thing after another recently and I have another mystery to solve, however, these little kinks have been getting worked out and in the meantime I've been daily driving.....yes you read that correctly lol daily driving my old truck finally. And I gotta tell ya, it is a truly gratifying feeling. AND! I'm happy to report that these last few little hiccups are NOT a product of my "fixing" things that ain't broke they are just the last of the wrinkles getting ironed out.

So this time I'm totally stumped, but hopefully you guys have some idea of what's going on here. For those that don't know the truck, it's a 1970 C10 LWB, 10.3:1 SBC and has a TH400, as well as a Holley 750 double pumper, manual choke carb. The trans shifting is run by a vacuum operated shift modulator, and that has a fairly large vacuum hose running from the passenger side of the transmission, all the way up to the carburetor, and on my particular application I plugged it into the back of the carb because that's the biggest vacuum port the Holley carb has.

I came across this issue because I got a shiney new trans dipstick, and when I removed the old one I knocked the vacuum hose off of the shift modulator, and a little bit of gasoline came out of it. This is my first Holley carb so I'm still learning the ins and outs, but I don't know what to make of this. The vacuum hose plugs into the carb, but why would gasoline be passing through down to the transmission? I have noticed that when idling, it's very very rich smelling and seems to be bad enough that it pisses people off at stop lights because eyes are burning every which way, and folks don't like that

So far I have only changed the jets in the carb, and adjusted the idle air/fuel mixture screws. Other than that it's all factory. Jets are 73/80, squirters are the factory .031" I believe (or .31" can't remember), the pump cam hasn't been touched, I'm not sure exactly what that changes, nor do I know what the different colored springs do. The power valve is the factory 6.5, and again I'm not sure what changing that would do. My buddy at the speed shop says to measure vacuum with a gauge, and divide that number by 2 and that's your power valve but I haven't tried yet for lack of a vacuum gauge and motivation. I picked up a 4.5 power valve just in case, but I'm not sure if a smaller number (like 4.5 compared to 6.5) makes the valve come in sooner or later. I think the power valve is one of the first things to address because of I'm understanding my reading correctly, it can affect the idle circuit which seems to be my problem regarding watering eyes at stop lights.

So what do you make of the gas in the shift modulator vac hose? It was about 2 teaspoons worth of that makes a difference. Also, the truck feels strong and runs great all the time so I figured it can't be a blown PV or bad carb gasket.

Matt

[NoNeck]

I plugged it into the back of the carb because that's the biggest vacuum port the Holley carb has.

Hey Matt -

The vacuum modulator hose should be plugged into the port on the INTAKE MANIFOLD, and not the carb (see photo). If you are experiencing an overly rich condition, the modulator valve probably pulled in a small amount of gas over time.

The big port on the carb is used for the Power Brake Booster if you have one. If not, it needs to be plugged off.

One more thing, unless you have that Holley 750 jetted correctly for your small block you are just cramming raw fuel out the exhaust. I know that bigger sounds better, but a well-tuned Holley 1850 with 600 to 650 cfm would give you better gas mileage and drive ability.

Just my opinion. Good luck.

POP

[Mrturner1]

Originally Posted by NoNeck

Hey Matt -

The vacuum modulator hose should be plugged into the port on the INTAKE MANIFOLD, and not the carb (see photo). If you are experiencing an overly rich condition, the modulator valve probably pulled in a small amount of gas over time.

The big port on the carb is used for the Power Brake Booster if you have one. If not, it needs to be plugged off.

One more thing, unless you have that Holley 750 jetted correctly for your small block you are just cramming raw fuel out the exhaust. I know that bigger sounds better, but a well-tuned Holley 1850 with 600 to 650 cfm would give you better gas mileage and drive ability.

Just my opinion. Good luck.

POP

Wow thanks for that info, I have been having a really rich idle and this probably explains it. I used to have it plugged in to the back of the manifold but I swapped from an Edelbrock to a Holley recently and the back of the carb is in the way of the vacuum port. I never even considered the possibility off it pulling fuel through. I guess I'll have to find some kind of 90* peice that I can thread in that will be low enough to clear the carb.

[dflarsen77]

The vac mod is supposed to be a 1/8 line. A rubber hose comes off the modulator, and then there's supposed to be a hard line that runs up to a port where NoNeck mentions. But the hard line is supposed to have a high loop. I don't have a pic of it, but the hard line goes about 4 or 5" above the port at the top, and then a short 1/8" runs from the hard line to the port. Anyways, the high loop is supposed to stop it from getting gas in there. Whatever you do, make a high loop!