Is this Dry Bowl or an alternative problem?

[marc]

Friends, after sitting overnight, my 84 J10 has begun requiring 4-5 attempts to start before it fires up. Once started and warmed up, it runs and restarts all day. The doors in the air intake are functioning. Have done a bit of work on the BBD carb, new vacuum pull off, new electric choke element, new choke shaft (operator error), all the tsm adjustments are done, the choke no longer sticks, corrected the timing. It had numerous vacuum leaks and was idling at 1200 rpm when I got it. No obvious gas leaks from the carb. Next step is to review the tsm on starting issues, nothing of use in the BBD manual. I don't think the BBD has a power valve that is cited as sometimes a source of the dry bowl issue but have I missed something?.

[marc]

One other thing, could the electric choke be a source of the problem? Will check but I believe it is set to specs, but should it be adjusted to the symptoms?

[tgreese]

What do you mean by 4 or 5 attempts? Cranking for how long?

I would let it sit overnight, then add some gasoline to the carburetor throat. Hold the choke flap open and add maybe a teaspoon full of gasoline directly into the carburetor. The engine should then fire immediately, with no extra cranking. This would be evidence that the bowl is empty. If it starts immediately, I would repeat the experiment the next night, and pull the top of the carburetor and look inside at the bowl level.

The BBD does not have a power valve like the Motorcraft. Instead it has tapered needles in the main jets that move upward out of the jets to richen the mixture at low vacuum. At high vacuum, the needles are pulled down into the jets to give a leaner mixture.

Mmm. I kinda think the TSM tells you a lot about the function of the carburetor. There is an '82 TSM free to read and download at the Tom Collins site. I think that '82 will be similarly equipped in terms of the idle stepper motor and such. Not an expert on these carburetors... I expect there is no specific discussion of this issue, if it is indeed a problem with the bowl draining. I don't know the anatomy of the BBD well enough to tell if some gasket failure somewhere could drain the bowl. Part of the problem could be the warmer temperatures of the season combined with the high alcohol content of modern gasoline. Possible the bowl is evaporating faster than it used to.

[marc]

There is a manual for the BDD carb online but it addresses running issues but not really starting issues. Will try the gas down the carb throat tomorrow morning. I ran into a comment on line about how BBDs have a reputation for dry bowl issues but no details. I actually have a second BBD, bought via craigslist for parts, not the model for the J10 but I have used it to investigate the workings of the carb and can look into the bowl for potential locations of leaks. Have the TSM on this tablet so I will look at that also. When I crank it over five times, it is brief cranks, 10 seconds or less, pump the pedal to the floor once and it starts. It had been taking only a few light pushes on the accelerator to set the choke and to dump gas, it would start cold.

[marc]

Test one, tsp of gas down the carb
Result, instant start
Test two(to be done tomorrow am),tonight, clamp fuel inlet hose to block any back siphon from the carb and see how it starts tomorrow am.(very unlikely scenario)
Argues for the dry bowl being the problem.

[Stuka]

My 81 J10 would suffer this issue if it sat for more than a day or so. It was either crank it a ton, or give it a squirt of starting fluid so that the engine could prime the fuel pump.

While I ultimately never fully solved it, I think it may have been caused by a plugged fuel tank vent. So a vacuum could be created inside the tank and it would suck the fuel out of fuel line causing the fuel pump to lose its prime.

[tgreese]

There's always the electric fuel pump solution. There are both fancy and very fancy ways to implement that... the most elegant is the electric pump plus an fuel pump controller that primes the carburetor when you turn the key on but still shuts off in the event of a wreck.

https://www.amazon.com/Revolution-Elect ... B00BNZ05JM

You can also use an oil pressure switch (Holley or other) and connect so the pump runs either when there is oil pressure or when cranking. A push button can be added for extra priming, if needed.

[marc]

regarding my 84 j10. Test two, clamping the fuel line into the carb overnight. Result, instant start as if I had poured gas down the carb!! To make sure that the Techron added to the gas did not do this I will consider the result tentative until I see, tomorrow morning, if the truck requires protracted cranking to start.
Possible causes, fuel being siphoned back down the fuel return or down the fuel pump line. I believe both of these options would require a malfunctioning needle valve in the carb.
More will be revealed.

[marc]

Possible solution, like the electric fuel pump, that doesn't directly addresses the carb problem. Would a check valve between the fuel filter and the carb be a solution?

[tgreese]

I believe there are reed valves in the fuel pump. Won't pump without the valves. Those lines should not drain.

Not doubting you, but I don't understand how "siphoning back" can happen. When the needle opens, the fuel goes up through the needle valve body and overflows into the bowl. There should be no connecting liquid between the fuel in the bowl and the fuel behind the needle.

You should get the same result if you clamp the hose between the frame and the pump, or the tank and the frame.

[marc]

I am going to repeat the test just to convince myself but tonight I will let the truck sit and try to start it again tomorrow without an intervention. If it does not fire right up, requiring cranking to start, I will repeat the test tomorrow night. I agree it is hard to figure but that is what happened. Marc

[marc]

About the float needle, if there is a small leak in the needle valve when it seats, vacuum pulled by fuel from the return line could pull fuel out of the bowl. It must be a very slow leak as it takes all night to drop the level to the point where the truck does not start.

[rocklaurence]

Siphoning will draw the fuel thats in the filter and hose but not the bowl. The needle and seat is closer to the top of the bowl-requiring that the fuel would have to some how get up to the seat. I may be wrong but there are Air-bleeds and vents that would prevent a low pressure in the fuel bowl that could pull fuel. IDK...

[tgreese]

About the float needle, if there is a small leak in the needle valve when it seats, vacuum pulled by fuel from the return line could pull fuel out of the bowl. It must be a very slow leak as it takes all night to drop the level to the point where the truck does not start.

Looking at the TSM, something else must be going on.



Even if you remove the needle and seat completely, there should remain a sizeable puddle of gas sitting in the float bowl. There has to be another path of escape.

I'd also suggest - don't know where the OP is located - the air temperature and overcast (if the Jeep is outside) on the day in question could matter.

[marc]

Got to be back flow out of the carb decreasing the charge within the carb to the point where it will not start without refilling the carb. Clamped the fuel intake into the p\fuel pump, tsm listed a bad check valve in the pump as a possible culprit, but it did not make much difference. clamped the fuel return line form the filter and the next morning, it started at the first turn of the key. Guess I am up for a change or rebuild of the carb. Even had the extra junk (non j10) BBD carb apart trying to figure out why this would happen.

Just struck me, if the needle valve is not acting as a check valve and preventing draining of the carb, fuel will siphon out of the filter via the return. Pump has to fill the filter and the carb bowl?????

[Stuka]

If this happens, then the pump has to fill the filter, and the fuel bowl, which will take a bit with the engine cranking.

[marc]

Stuka, that prolonged cranking seems to be the case. As an alternative to an electric fuel pump, I am wondering about a check valve between the filter and the carb? One problem that I can see, is that it must be a low pressure check valve as there would be minimal pressure to open it. Most valves offered that I have seen advertised are for fuel injection systems that, I believe, work at a higher pressure.

[marc]

Current plan to address the backflow from the carb is to put a Carter, Delhi or Precision fuel pump check valve between the carb and the fuel filter. Would be glad for feedback about this. As far as I can ascertain, they are rated for 180' F. I might have a line on valves rated to 300' F that would do the same thing but do hold a lager volume.

[rocklaurence]

IMO, its the fuel in the bowl evaporating.

[marc]

I understand the evaporation issue but clamping the fuel return should not allow it to start in the morning it if that is the case and I know all the trap doors are working.