I have been fighting this hard starting, when hot issue since I got the car. ( ‘71 350/270 4 speed, stock rebuild ) The OE correct Q-jet was recently rebuilt and performs perfectly. This occurs when the car is at operating temp, shut off then trying to restart. It could be after only a few minutes, but usually after it’s sat for awhile ( 1-3 hours ) and it always requires at least 4+ seconds of cranking and pumping the gas (twice) sometimes 3 times and holding the pedal to the floor. When this happens the only way it’ll start is to give it a shot of starting fluid.

I’ve added a phenolic spacer. Nothing. Once it starts, it runs perfectly. No hesitation, stumbling or bogging. Several people have told me that they think the fuel in the bowl is evaporating causing this hard starting, excessive cranking problem. They all suggested an electric fuel pump. I think I have what I need for the Electric pump, ( see pic ) Is there anything I’m missing?

I hate to give up on this but I’m tired of never knowing if it will start or not. I’ve been reading all the posts I could find regarding this. I know after I remove the fuel pump mounting plate I’ll be able to remove the fuel pump pushrod. I must block off the fuel return line, as I won’t need it. It should mount at the rear somewhere near the tank. ( Any pics of the best mounting location? )There’s also a third line that connects to the mechanical pump that I don’t know where that goes as I haven’t followed it yet. The Holley pump mixtures is the one the Holley tech suggested. He said he’s used it and it will support up to 400hp. He said I wouldn’t need a fuel regulator either.

If there’s anything I’m saying or doing wrong, or I’m missing please LMK. Thanks, Steve

 

Who rebuilt the Q-Jet, and does the fuel filter have a check valve? Putting in the correct filter (per @lars ' suggestion) halved the time it took for a cold start in my 79 (proper timing helped, too). But that was a cold engine, after sitting for days.

What is your timing set to? If your timing is retarded, you may be cooking the fuel out of the carb as it sits for a few hours.

But, if you do get a new pump, why not an in-tank fuel pump and a regulator? You can dial-in the exact pressure you want for the carb, and be halfway to an EFI conversion when you give up on carbs.

 

Before spending any money on fuel pump's you might consider fixing the issue that is causing the heat. I have had both headers and standard exhaust manifolds both over-cook my starter motor. I tried heat deflectors and all sorts of silly tricks before trying the Gear drive starter motors. My unit has a permanent magnet motor which is a real benefit over the stock. The close proximity between the headers and the starter motor made it difficult to keep the starter from getting heat soaked.

I tried beefing up the windings inside the factory starter, and finally replaced the starter with a new gear drive permanent magnet motor starter. This little gear drive is rated for up to 18-1 compression ratio equipped engines. These motors are tiny and they frequently can be rotated on their bracket to get the best clearance. My engine is 12.25-1 and it spins this engine of mine up. It draws less current as well which is a nice plus, I bought mine 25+ years ago and it was ~$200.

Now I have no issue starting my car when it is warm as it will crank when I ask it to. The new starter is nowhere close to the same size of the old unit, it is about 2/3 the size, Just be sure to get the right unit based on your Corvette's Flywheel size for your Corvette.

This should help those out there who have had or those will have a heat soak issue. The starter is a fairly easy solution.

You can also get the electric cooling fans to run "on" for 10 minutes after shutdown to slow heat soaking. I have a system that will allow it to run the fans for as long as it takes but I also use a Deep Cycle Battery in my C3.
Trying to run a L88 style engine lets you deal with H-E-A-T. Then because my C3 is a 1968 it has the smaller radiator even though it is a big block equipped car. I would love to put a 1969 on up BB Radiator in my 1968 chassis. There is the room but am not so sure about cutting an fairly original early Corvette just to get a substantially larger Radiator in between the wheel wells.

 

A electric fuel pump will almost instantly refill the fuel bowl at key on. But your fixing the symptom. Not the cause.
inline pumps are notoriously unreliable. And noisy.
If you really want a electric pump, consider that electric fuel pumps are pushers, not pullers. A intank pump will likely be far more reliable and of course less noisy.
Oh, and I don't see a pressure regulator on your list.

 

Thanks. I agree. I’d rather fix the cause. As I’ve always said, once, the car was new and someone DD the car. What? Did the dealer give the buyer a can of ether, saying oh BTW, you’ll need this to restart your car after you shut it off. I don’t think so! I just don’t know where to look anymore.

 

Having run across this several times in the past, here is my suggestion. Before trying to start the car, pull the air cleaner and look down the throat of the carb while you manually pump the gas. If you don't get a strong stream of fuel from the accelerator pump, then the carb bowls are probably dry. Cranking the engine will eventually refill the bowls. There are a couple of aluminum plugs on the base of the carb that are notorious for developing leaks and draining the bowl when the car sits for any amount of time. The old school fix was to JB weld the plugs. You can drill, tap, and plug for a more permanent fix. I just did this repair on a friend's Q-jet. Google is your friend. Good luck.

 

Thanks!

 

I am going to verify that this weekend. I’m seeing three hoses at the fuel pump. 2 go towards the back of the car. The third one i haven’t found out yet. I know the charcoal evap canister is long gone. And I don’t have a T at the valve cover. Only 2.

 

Mechanical pump, motor stops, fuel flow stops.

 

GM designed an aluminum heat shield that prevents radiant heat from getting to the carb bowl. This is not a "spacer" type of heat insulator that would prevents "conducted" heat. This is a thin aluminum plate that is a few inches wider than the carb base so it prevents heat radiating up from hitting the carb fuel bowl. GM designed one for the Holley type square bore carbs and another one for the spread bore Qjets. I added one when I started having this same issue and the problem went away.

Ebay seller thepartsplaceinc has them. I'm having trouble posting a link.

 

Thank you for that. I ordered one.

 

Be sure to put the new thin gasket on the intake and the thick regular gasket on top of the aluminum plate or it will mess with the choke linkage.

 

Copy. Thanks!

 

Just thinking after reading this ,,, Your fuel pump is fine. Your starter is fine. Fuel is leaving the carb after shut down. Also check if your engine oil is diluted with fuel. Another issue you don’t want.

 

Thank you, I will.

 

Many items have been mentioned. You may need to check each one. Modern 10% ethanol fuel is very fussy: It evaporates way easier than 70s gas, so we need to adjust the car for it:

• Accelerator pump works not all dried out. There are ethanol resistant ones.
• QJet fuel bowl plugs do not leak.
• Fuel pump gives 5-6 psi psi. Ethanol kills them.
• Once you get a good squirt after shutdown:
• Performance timing curve, not OEM. OEM makes it run hot on purpose for 70 emissions. 12* initial, 36* total, restricted 10* vac can, on manifold vac. This is your heat source
• Phenolic spacer for conductive heat.
• Aluminum reflective shield for radiant heat.
• 180* thermostat, not 195

Check it all. When done a QJet should start like efi. One small pump, maybe. If not the QJet needs Lars's touch. Even BBs run cool with the above setup.

 

How would retarded timing cook the fuel out of the carb as it sits?

 

Makes the whole engine run hotter.
Exhaust manifold temps jump up a couple hundred degrees above normal. I have seen 700* at idle on one car.
Ethanol boiling point is very low at 173*

 

"sits for a few hours"


Idles for a few hours?

 

When the engine runs with retarded timing, as was stock in the 70s, the exhaust manifolds run very hot. The heat riser heats the intake very hot.
When you park the car, you can literally hear the ethanol in new-gas boiling out of the fuel bowls on the carb. It may even squirt out of the squirters while it is sitting an heat soaking.

Gasoline is way different than it was 70 years ago.

To fix it to run on todays ethanol gas, you need to do all of this: 1) put a performance timing curve in it to cool the manifolds. 2) block the heat riser 3) add a phenolic spacer - this blocks only conductive heat 4) add a reflective aluminum heat shield - which blocks the radiated heat - the other half 5) you likely need a orifice bleed on the fuel line at the carb - this prevents the ethanol that is boiling in the fuel line from flooding the carb and making it hard to start ($)

Or you could find some 1970 gas, with a much higher vapor pressure than anything you can buy at the pump today. Cam2 race gas works. ($$$)

Or you can spend 1000s putting on an EFI system. ($$$)