Carburetor Leakdown??

electricaldoctor

electricaldoctor

Well-Known Member
TECHNICAL Contributor
May 14, 2008
3,538
Thousand Islands, Rockport, Ontario.
Boat Info
1988 390 EC
Engines
454 Mercruiser Gassers
I am having difficulty starting my older 7.4 l. gas engines this year. I have to pump the throttles several times and crank the engines over for quite a while before they start. After that, they start right up as soon as I turn the keys. Are my 30 year old fuel pumps just getting tired or do I just need some sort of anti-syphoning valve installed to keep the fuel in the carbs?
 
Welcome to the wonderful world of Rochester Quadrajets!
The mechanic next door warned me of exactly what you are describing when I rebuilt my Qjets.
Apparently there are two plugs in the bottom of the carb that are press fit or glued in some manner and over time develop slow leaks that drain the float bowls. So your first 'x' seconds of cranking is only trying to refill the float bowls and not putting gas in to the engine.

Google Quadrajet leak down and there is a ton of info you will find on the cause and cure.
 
The plugs you refer to are called Well Plugs. When overhauling a Q Jet they can be sealed with epoxy like JB Weld. If I were the OP I'd replace the original 30 YO fuel pumps as part of preventive maintenance. By a good quality MARINE fuel pump such as Carter. Should cost about $100 plus labor. When I did mine I replaced the thermal choke with an electric choke.
 
electricaldoctor said:
I am having difficulty starting my older 7.4 l. gas engines this year. I have to pump the throttles several times and crank the engines over for quite a while before they start. After that, they start right up as soon as I turn the keys. Are my 30 year old fuel pumps just getting tired or do I just need some sort of anti-syphoning valve installed to keep the fuel in the carbs?
Click to expand...

Mine do that if I don’t use the boat for a two weeks or so. I think the bowls dry out in the heat of summer and the mechanical pumps just take more time to refil them. I debated putting in electric pumps but for the minor difference who cares.
 
yep. every time i rebuild a Q-jet that hasn't had the wells sealed i seal them with JB Weld. use a file to clean up the bottom of the wells so the JB Weld adheres to it. don't 'glob' it on though because those wells fit down inside pockets in the base. i have had to re-shape a couple i have done because i put it on too thick. no big deal, just an extra 10 minutes with the file and a couple test fits.

all-in-all Q-Jets are great carbs and i wouldn't replace one unless i had too. i love them
 
Creekwood said:
Mine do that if I don’t use the boat for a two weeks or so. I think the bowls dry out in the heat of summer and the mechanical pumps just take more time to refil them. I debated putting in electric pumps but for the minor difference who cares.
Click to expand...

For a quick and easy diagnosis, I am going to see if I can source out am inline fuel check valve at NAPA. I know that there is a fuel filter on the carburetor where the fuel line is connected. Hopefully, I can find something that will fit in there. I willl try that before tearing the carbs apart. If the gas stays in the float bowl, then I know that it is being syphoned back down through the fuel pump. If it doesn't, then I know that the well plugs are leaking and I will have some work ahead of me.
 
electricaldoctor said:
For a quick and easy diagnosis, I am going to see if I can source out am inline fuel check valve at NAPA. I know that there is a fuel filter on the carburetor where the fuel line is connected. Hopefully, I can find something that will fit in there. I willl try that before tearing the carbs apart. If the gas stays in the float bowl, then I know that it is being syphoned back down through the fuel pump. If it doesn't, then I know that the well plugs are leaking and I will have some work ahead of me.
Click to expand...

I believe your mechanical fuel pump have a check valve built into them. Most likely you are taking an unnecessary step.
 
electricaldoctor said:
For a quick and easy diagnosis, I am going to see if I can source out am inline fuel check valve at NAPA. I know that there is a fuel filter on the carburetor where the fuel line is connected. Hopefully, I can find something that will fit in there. I willl try that before tearing the carbs apart. If the gas stays in the float bowl, then I know that it is being syphoned back down through the fuel pump. If it doesn't, then I know that the well plugs are leaking and I will have some work ahead of me.
Click to expand...
Don't see that working. The fuel inlet is above the fuel level in the bowl. Probably will cause a fuel restriction as well. Why not just fix it or replace the carbs ?
 
Most carbs will evaporate the fuel out of the bowels if they sit for a while, my q-jet does it if it sits for more than 3 or 4 days and both webers on my 4.3 do to it too, just the nature of a carb!
 
My 7.4 has the same issue not a problem it is just some priming before turning the key . All my old Chevy trucks were like this too 3 or four pumps of the pedal and then 1/4 throttle would fire them right up. You won't get a carbeurated engine that will cold start after dsys of sitting without manipulating the throttle.
 
My 2jet carbs on my twin 470's dry up if I don't start them once a week in this Las Vegas heat. If I don't I then have to prime them with a little fuel from a small bottle, only takes a very litlle amount to start it up to fill the carb bowl. FYI put the flash filter back on before starting. I rebuilt one of the carbs and it was missing the accelerator pump check valve and that caused drain back into the bowl so thus no pump and no start. An electric fuel pump would solve all that problem but I don't want to put out the money or time right now.
 
There are a bunch of them on Ebay Just type in Mercruiser 97399 and it will com up.
 
Mercruiser # 97399 crosses over to sierra # 18-7288-1 which list for $171.85 and its supposed to be a stocked item at my dealer
 
Handsome Transom said:
My 7.4 has the same issue not a problem it is just some priming before turning the key . All my old Chevy trucks were like this too 3 or four pumps of the pedal and then 1/4 throttle would fire them right up. You won't get a carbeurated engine that will cold start after dsys of sitting without manipulating the throttle.
Click to expand...

The advantage of having to "crank" the engine several revolution as it starts is the oil pressure has time to build up before starting. For that reason I have no problem with having to crank the engine a bit prior to starting.
 
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