66 SB hard to start when sitting for a month or so...any ideas why

[Clem Z.]

Re: 66 SB hard to start when sitting for a month or so...any ideas why

if the choke plate does not close tight causing the only air getting into the engine while cranking is thru the float bowl vent causing it to push fuel from the float bowl into booster nozzles and into the engine for start up. i found this out when starting race cars with no choke plate you had to cover the whole top of the choke housing to get the engine to start

[Duke W.]

Re: 66 SB hard to start when sitting for a month or so...any ideas why

With the choke valve fully closed, cranking the engine causes a significant vacuum at the fuel discharge nozzles, which pulls in fuel during cranking. Without the choke valve there would be insufficient vacuum at the nozzles to pull in any fuel, and the engine won't start or will start and immediately die when the pump shot is consumed.

But once the engine starts the choke must be backed off to avoid over-rich stalling and flooding. That's what the choke vacuum break is for.

For those of you who ever operated cars with a manual chokes, you probably recall that you cranked a cold engine with the choke knob pulled all the way out, and when the engine fired you backed it off about a quarter to half of the travel to get a smooth fast idle. One technique that often worked was to pull the knob out with your index finger extended along the shaft. Then when it fired you pushed the choke in to the point where your finger touched the IP.

Each car was somewhat idiosyncratic and you had to figure out the best settings from experience.

Duke

[Joe C.]

Re: 66 SB hard to start when sitting for a month or so...any ideas why

Been following this for some time, and suspect that fuel siphons out of the fuel line and back to the tank after a long period, like mine do. You might get a couple pump shots due to residual fuel in the primary bowl when you first actuate the throttle before cranking, but that should be all. In that case, the engine should make an attempt to start, and then quickly die. After cranking for another 30 seconds or so (in 15 second segments), with intermittent pumping, it should fire and stay running.

Some carburetors beginning in the seventies used a checked fuel filter to keep the fuel line full after the fuel in the float bowls evaporates. If manually filling the float bowls using a large syringe or squeeze bottle doesn't result in an instant start, then you have ignition problems. I have started cars in sub zero temperatures with 15 year old gas, so the "volatile component" evaporation theory has little weight.

Some Pertronix installations permit use of ancient coils with high resistance, thereby permitting use of a ballast resistor. Other installations are strictly for low resistance coils and use full battery voltage during "start" and "run".

[Clem Z.]

Re: 66 SB hard to start when sitting for a month or so...any ideas why

Been following this for some time, and suspect that fuel siphons back to the tank after a long period. You might get a couple pump shots when you first try before cranking, but that should be all. In that case, the engine should make an attempt to start, and then quickly die.

Some carburetors beginning in the seventies used a checked fuel filter to keep the fuel line full after the fuel in the float bowls evaporates. If manually filling the float bowls using a large syringe or squeeze bottle doesn't result in an instant start, then you have ignition problems.

Some Pertronix installations permit use of ancient coils with high resistance, thereby permitting use of a ballast resistor. Other installations are strictly ror low resistance coils and use full battery voltage during "start" and "run".

Q jets started to use a check valve filter when GM went to fuel return line fuel pumps because the output window of the needle seat was below the fuel level and once the fuel started to evaporate the float would lower and needle seat opened and the float bowl would start to empty out thru the fuel ine.

[Timothy B.]

Re: 66 SB hard to start when sitting for a month or so...any ideas why

if you want to get away from having to fill the float bowls just install a electric in line fuel pump as it will fill the bowl for you when you turn on the ignition.

I agree that the choke valve needs to close tightly around the air horn opening for cold start as this creates the low pressure area needed for fuel to flow out the booster venturies. Without this seal the engine will suck air from the least restrictive place which is around the choke valve and the engine will be hard to start.

If installing a pusher (electric) pump, a regulator needs to be installed so pressure does not exceed 6psi +- for the carburetor. You could still retain the mechanical pump but the mechanical diaphram will stay in the upright position if the electric pump pressure is equal to or more than the spring pressure that pushes the mechanical diaphram so the pump is not really working.

One positive to the pusher pump is that the pressure in the fuel line is above atmospheric so the boiling point is raised where the suction of the mechanical pump lowers pressure in the line lowering boiling point. On our cars that's probably not an issue unless modifications like headers are used where more heat is present in the area around the fuel line.

[William F.]

Re: 66 SB hard to start when sitting for a month or so...any ideas why

Ron,
Way too much jibber jabber about your problem.Electric fuel pump way too much sugar for the nickel; you don't have a quadrajet so disregard all stuff about this, Just check voltage at coil and fill fuel bowls and let us know what happens.

[Monte M.]

Re: 66 SB hard to start when sitting for a month or so...any ideas why

I have thrown a few other ideas in before, but I just thought about a few basic things we are supposed to do with Holley carburetors.

I am assuming the engine is stock? And I am assuming you used a quality rebuild kit, or whoever built it used a quality kit. The differences in kits matters more as you get into higher performance engines.

I would check what size power valve is stock on that carburetor. Maybe one of the other guys can help you there. If you do not know what size is supposed to be in there, warm up the car and test your vacuum at idle. If it is an automatic, test it in drive without your foot on the brake if you have power brakes.

This is the mistake a lot of people make as the vacuum booster will draw some vacuum. Put your e-brake on.

Test your vacuum and cut that in half. That is a really good starting point to get the valve to open on start-up. If you have 21 psi of static vacuum at idle, put a 10.5 power valve in it. They go in really easy. If you have never done one, you can look it up on the net, or I would be happy to talk you through it.

Beast of luck,

Monte