MGB

Hello all. Have a '78 and have been chasing the same "no start after sitting overnight" for a while. Just thought of something else, after the car sits overnight, I pumped the accelerator linkage by hand and observed the fuel spray going into the venturis in the carb. , only a couple of full sprays and then it stopps spraying completely, not enough to start the engine. I disconnected the inlet fuel line, the paper filter and end of fuel line were completely DRY. Reinstalled the fuel line and cranked the car for 20-30 seconds with a strong battery turning the engine over fast and pumping the accelerator like a mad man with no better results. Shouldn't that be enough to pump more gas into the carb and the engine start??? Going now to disconnect the fuel line again and crank the engine over to see if getting fuel while disconnected. As other folks have indicated, the engine starts great with a few shots on starting fluid before-hand. Maybe the fuel pump needs to be turning a lot faster as when the engine is actuall running to work properly. The engine runs and starts all day long after the initial start in the morning. I know this is long but maybe it'll help others too.

John 65

Thanks for posting, in my case being that I swapped carbs yesterday, and then no problems this morning starting, for me thats telling me its something in the carb itself. I drove it before and just for the heck of it, after shutting the motor, knowing I had a full bowl of fuel, I looked down the carb and worked the linkage and it only dribbled out, just like when its cold in the morning. No spray like when the motors running. Acc pump ? would it matter if the motor is running or off to make any difference in the spray ? I know the carb was full before, I checked the spray moments after shutting the motor.Thanks all.

MGB

Hey, we may have different problems after all. I just cranked my engine hard and fast for 25 seconds with the fuel line disconnected and NO fuel came out of the line. I'm thinking my fuel pump is weak. Like I said, after my initial morning start, mine starts all day like is should. But then if everything worked perfect on these old things what would we have to work on??? good luck and keep us posted and I'll do the same.

...Roger...

If the carb is not leaking internally the bowl can be sucked dry because of a bad check valve in the fuel pump. This is very common. If inside the carb you have a needle and seat with a window in the seat the problem will be worse than a seat without a window.
Lars mentioned at one time to remove the little wire keeper on the needle and see if that helped-it did on my car. But that was a band-aid fix because I knew my fuel pump had a leaky check valve.

The fuel pump relies on vacuum to pull fuel up and out of the fuel tank and forward,if the rubber hose at the tank is not tight the fuel pump make suck air instead of fuel.
Ever tried to siphon a water bed ?

highschool67

I just rebuilt 10 Q jets I had laying around just this past week. Each kit averaged about $22. I was getting tired of grabbing a carb off the shelf that I pulled from a donor car or was given by a friend that said "it worked fine when we pulled it off". You should get a good spray from your primary jets when you pump the pedal. It can only be the main accelerator pump. Even with a low float adjustment you will have enough fuel in the bowl for at least 3 or 4 good pumps of the pedal. I have found that a lot of hard starting are mainly...
1. A improperly adjusted choke. Qjets like the choke fully closed on cold starts. They tend to run a little lean on start-ups.
2. A bad accelerator pump or clogged orifices that lead from the pump to the main venturi jets. You can take compressed air an blow through these areas within the carb and usually blow out any obstructions. This goes as well for the inlet from the paper inlet filter to bowl passage. I've seen alot of carbs that did not ever have the filters replaced or just had the filter removed. Without another inline filter you will get funk in that passage. I've seen a lot of the paper filters so clogged it was impossible to get gas by. Pull the filter and do a visual then blow through it. You should get a pretty unobstructed flow of air.

Make sure your choke is fully closed on cold starts. Have someone hold it closed while you pump the pedal. See if that helps.

Since you have the carb apart blow compressed air through the bowl passages.

Put a new filter in or check the one you have in it.

I salvage every electric choke from Qjets that I find. I replace the manifold vacuum bimetal spring with an electric choke. I have not rebuilt a Qjet, that I have replaced the thermal/mechanical bimetal spring with an electric choke that I can not reach in and start the car without even giving it 1 pump with the pedal. Even the divorced choked carbs set up correctly only require 1 to 2 pumps if that.

I think you have a worn accelerator pump or some funk in the passages between the pump and the venturi jets. Blow it out and try it. You can also just get the pump rubber and spring separately so you don't have to buy the whole kit.

MGB

Hey again. This is where I'm at now. Just started the engine using starter fluid and let it run for awhile to make sure the fuel pump was doing it's job, and of course, while the engine is running, the spray into the venturis is a good stream of gas. Stopped the engine and disconnected the fuel line to the inlet of the carb. I'll check in the morning to see if I get several good streams of gas into venturis before anything else, enough to start the engine. Sound like a plan?

MGB

OOPS, forgot the mention that Lars rebuilt this carb a year or so ago.

John 65

Sounds like 2 different problems here MGB, thats for sure. All that pumping to come up dry ? While the carb was apart it was cleaned and blown free of any accumulations or deposits that may have formed. I read through Lars info and everytime the words "hard starting " appair, they have the words acc. pump in the same sentence with them. So its back togther and i will see tomorrow morning when cold. Also, the carb was rebuilt 3 years ago, I cant remember, but I am sure if it was starting hard when cold like this after the rebuild I would have done something about it. Then the body came off and it sat for a while. My friend mentioned that lots of the old acc pumps were not made for todays gasolines, as the umbrella breaks down. I ordered a new one. Should be here in a few days. Thanks to all for the help, will go and check those rubber fuel line fittings like Roger mentioned. Chokes working fine also 67. Maybe the acc pump is the ticket. Will know soon.

...Roger...

Another good reason to lean towards a bad fuel pump.

edit - He was having some issues with accelerator pumps.

John 65

Just tried a cold start, still the same. I ordered a new acc pump. Should be here in a few days. Maybe the old one isnt up to the fuel additives of today and it broke down ?

highschool67

Typically pump gas will not break down the plunger cup. Rochester changed their plunger cup from leather to a synthetic rubber in the 70's. The don't break down but they will shrink up over time causing the reduction in pressure. I think you are on the right track. Make sure to blow the passages out before putting in the new plunger. I always smear a little bit of oil on the dry plunger before installing. Keep us posted.

John 65

Thanks for the info. Will post next week for sure.

highschool67

Just had an epiphany moment. I haven't dealt with this for a long time but........

Do you have a inlet filter installed? If you do check to see that the paper filter has a black rubber gasket instead of a cork gasket. In 69 or 70 they changed the cork gasket paper inlet filter to a rubber gasket. It is not the gasket that we are interested in but the fact that Rochester had some cold starting problems because the original cork gasket filter had no check valve in the filter, allowing the gas to siphon back into the fuel pump and sometimes completely back to the tank! They designed the later model filters with a check valve and used the black rubber gasket as an identifier so you knew which filter you have. Auto parts stores still sell the filters without the check valve because the early fuel pumps did not put out enough pressure to overcome the check valve. Sometimes they will sell you this one by mistake. If you happen to have a cork gasket fuel filter or no inlet fuel filter your bowl may be draining back into the fuel line or the fuel pump.

John 65

Yup the correct filter is in there now. The one that came out was wrong but this one is correct. Was going to replace only the acc pump but I will get a complete rebuild kit instead and see what that does next week for sure. Thanks again

highschool67

John 65

I ordered a rebuild kit, it should be here today. Hopefully have it back togther in a day or 2. Will see what happens. Thanks.

John 65

OK carb has just been rebuilt. Putting in on soon. We put in a heavy duty ? acc pump, looks and feels like a stronger spring. I feel that tomorow morning it will start a lot eaisier, cause when I filled the bowl and worked the throttle the solvent shot out of there like if the motor was running. Never did that before. Thanks for the help, fingers crossed till the morning.

'75

Keep us posted, I got my fingers crossed.

spinadog

I was under the belief that if the acc. pump is bad, then you will get a 'cough' from the motor when you stab the accelerator...since it's not shooting a strong stream down the carb. This was the symptom with mine to which Lars responded to about 4 months ago. I'm with Roger on the check valve, though it seems odd that you replaced the carb and the problem disappeared.

jersey68l36

IMHO, looking at the pic of the epoxied fuel wells, the one in the front is leaking, draining the bowl. If it was my carb, would re-epoxy the front well.