The Stovebolt Site Forums The Shop Area Engine Shop Hard starting

Without using starting fluid it is very hard to start. This is on my 50 3100 216 chevy. I have noticed a vacuum when removing the gas cap. I tried removing the cap and it still only started w starting fluid.

I have heard that mud dauvers may build a nest and clog the vent for the gas tank. Is it possible to see this vent without removing the bench seat? Where is it located?

What else should I check in regards to the starting problem?

Thanks. Your responses are always helpful.

Are you running a 6 volt electrical system? If so, check the voltage that's available at the + side of the coil while the engine is cranking. If it's less than around 4.5 volts, find and fix the dirty connection somewhere between the ammeter and the coil, including the ignition switch contacts. 6V systems are VERY unforgiving of loose or dirty connections, including grounds. Drill a small hole (1/8" for instance) inside the gas cap to be sure it's venting.
Jerry

James - It sounds like your gas cap vent is clogged, or if you recently replaced the cap with a new one, it’s possibly not the correct vented type.

Alternatively, a new supposedly vented cap may be bad out of the box. It’s been known to happen.

On your tank vent question, you can see the upper opening by removing the gas cap. Small hole on the upper side of the fill neck. But to check the entire vent you need to pull the seat back to get to it.

Aside from these things, there are other possibilities that could involve your fuel pump, carb and even ignition.

I’d start with checking/resolving the tank venting first.

Make sure that your choke is closing all the way.

1950 didn't use a vent line. Only an overflow pipe. They had a vented cap. Make sure you have a vented cap. When emissions laws went into effect in 1971, vented gas caps were no longer used. Most parts houses don't have anyone that knows that GM used vented gas caps before then.

when it starts, does it run fine?

How long does it set before starting? Fuel evaporation is a real problem now.

Does your gas cap look like this one?

If so, they all seem to be crap from the get go. Ol' Roy had the same problem you describe. When removing the cap,a huge rush of air was sucked into the tank. The fuel pump was creating a vacuum in the tank because fresh air couldn't get into the tank to replace the gasoline that was withdrawn.

Anyway, I disassembled the cap and removed the little valve and spring.

Bend those two metal ears holding the plastic parts in place. Everything comes out and you can get the valve out and put it back together. That's been a couple years now and the cap works as it should.

The exact same problem occurred last week with a smaller vented cap of the same design on a '64 Fairlane I was working on. The cap was brand new.

I think The Filling Station or Jim Carter or one of the restoration houses sells a replacement vented cap that's all metal without the spring and valve that's more like the original and works properly.

Sorry just now reconnecting. I changed my 3100 to 12 volts years ago because of starting issues.

I installed an inline electric fuel pump .which i run after the truck isn't started for some time. The hard start is recent. Starts quickly w starting fluid and runs great.

Could be a few things if its a no start when the engine is HOT,

One, to much fuel after it is shut off. The float can't hold the pressure of fuel heating up, fuel then overflows into the intake creating a over rich mixture. After ten minutes the fuel has evaporated, the truck starts like normal. Mine has a five or six minute window where it will fire right back up depending on how hot the motor is and how hot it is outside, if I miss that window of time, in ten minutes it will fire right off. In between these times, it cranks for awhile to clear it out.

Two, not enough fuel. The tank slowly builds a vacuum due to poor venting or wrong cap, or the fuel pump heats up and can't pump due to leaking valves. Theres no fuel in the carburetor, so no start. The tank will bleed off the vacuum over time, or the pump cools off and it starts ups. If it's the pump, your electric should be able to blow through the mechanical and the truck should start, but the electric may not be able pull fuel if the tank is in a vacuum.

Three, the ignition condenser or coil is going out from heat. Once either one cools off, the truck starts as normal.

The fact that it starts with starting fluid would lead me down the fuel road, starting fluid has no effect on ignition. You need to look down the carburetor before trying to start it to see if it has fuel or not.

OK- - - -it's time for me to get ignored- - - -again! Low voltage to the coil primary winding can create a weak spark, one than can just barely ignite starting fluid and totally fails to make gasoline burn. Since diagnosing that condition requires a little bit of mental gymnastics and the ability to use a voltmeter, it's seldom, if ever considered. On a 6V system, there needs to be a minimum of 4.5 volts available at the upstream side of the coil. For 12V systems the primary voltage available at the coil needs to be 9.5 volts or so. Sources of low voltage can be loose or dirty connections at the ignition switch, excessive starter current draw, and/or something as simple as dirty battery cable connections. The KISS principle needs to be applied before more exotic theories get considered.
Jerry