[Shiny]

My takeaway: If a carb goes dry in 2 or 3 hours, it's not typical, but by itself, I don't know that it matters if it starts and runs well when the bowl is filled again.

If it goes dry in 2 days, that's probably in the range of normal for a vented carb with an effective anti-drainback valve. Going dry in 5 days is well within the range of credible evaporation rates. Here's one example:

https://forums.maxperformanceinc.com...0&postcount=20

Way too many variables to guess at the underlying causes for the difference between 2 hrs and 2 days. Temperature and leaks are just two possibilities. Other factors include fuel formulation, vent size, bowl volume, and I'm sure there are more.

All these things are hard to measure and diagnose. For me, I think adding an electric pump fixes all of the possibilities if I decide I don't like cranking.

Mike

[Shiny]

Originally Posted by Shiny

Thanks, Kenth

I'll take a look. It appears in line with my thinking.

Thanks again Kenth. These CUBE pumps look like high quality solenoid pumps that would work well for priming. The only concern is the manufacturer's recommendation to put a filter before the pump. They do say NAPA is a supplier, which is a good thing for me.

With or without a "pre-filter", it's not clear if such a pump would restrict flow if put in "series" with a mechanical pump (flow-through?). Probably safest to plumb it "in parallel" but that's a lot of extra plumbing.

I'll have to wade through all the variations in part number and configuration, but seems they have several that are rated to deliver between 3 and 4.5 psi, which sounds good for priming. Maybe some of the PNs include a pre-filter...

But if these solenoid pumps don't save significant cost, I suppose a rotary vane pump that can deliver higher flow is an option.

I kind of like the idea of a noisy solenoid pump that can "signal" when the carb is full (assumes the noise would drop when the flow stops).


Anyone else use a solenoid pump in line with their mechanical pump to prime their carb? If so, what pump has worked for you?


Mike

[Kenth]

The "pre-filter" would be more like a non-restrictive "screen" much like the marine Quadrajet filters.

https://www.pegasusautoracing.com/gr...BxungTkO9q0ksP

[Kenth]

Originally Posted by Formulajones

It's not the fuels fault. It doesn't just disappear in thin air unless you're excessively heating it or leaking or?????

You wouldn't think that the increased heat from the engine a while after the engine is turned off would cause the fuel to evaporate?

[HWYSTR455]

Yeah, heat is a big factor, and the amount of fuel too. A QJ bowl is fairly small.



.

[Formulajones]

Originally Posted by Kenth

You wouldn't think that the increased heat from the engine a while after the engine is turned off would cause the fuel to evaporate?

Yes but the problems start and worsen from these guys wanting to excessively heat these carbs in the first place, running the engines hotter than they really need to be and various other factors and then wonder why heat soak is so bad and why their fuel is boiling away. Instead they want to blame the fuel.

I'm running the same crap fuel, actually considered worse out west here than many other states, yet it's working fine for me on the Vega, the firebird, the Camaro and chevelle. I don't have extra pusher pumps on any of them. Bowls aren't drying up and even after sitting for 4-5 days and even a couple weeks in some cases I don't have to excessively crank the car to start it. The nomad will sit a month at a time and it still cranks and starts within 5-8 seconds and I don't even use the choke. I find that perfectly acceptable and no different than when these cars were new.

[Kenth]

Engines get hot from burning gasoline, there's no getting around it.
And heat is one of the three factors in converting the liquid fuel into a combustible gas. (the other two are velocity and amount of fuel).
The fact that the temperature of the engine rises slightly after the engine is turned off is nothing new.
The problem of fuel evaporation has always existed.
Luckily, the engineers who designed the Quadrajet didn't make the float housing any larger than necessary so that fuel loss could be kept as low as possible.
Then the fact that the oil pressure builds up during the time it takes for the fuel pump to fill the carburetor before the engine starts was certainly part of the design.
There are treatises written by smarter people on "The Principles of the Internal Combustion Engine" and "Car Engines and Motor Fuels" to read if you are really interested in the facts on the subject.

FWIW

[Formulajones]

There's engines get hot from combustion and then there are those that choose to make things hotter for no real reason. And when you do that you accelerate the issue.

Pick your poison.

But I think the point got across that fuel just doesn't disappear into thin air all by itself in a half hour that others state. It's accelerated by other means, and that can for sure be manipulated to ones benefit if you so choose.

[dataway]

Is the problem of evaporation any worse with the later carbs with the larger vent tube than the early ones with the smaller tube?

[Shiny]

Originally Posted by dataway

Is the problem of evaporation any worse with the later carbs with the larger vent tube than the early ones with the smaller tube?

If you think of the carb as a jar with a hole in the lid, then yes, all other things being equal, the bigger hole will allow a higher rate of vapor loss.

But that "all other things being equal" thing makes it pretty complicated. Gas evaporation happens. How fast it happens depends on both the "drivers" (temperature, composition/vapor pressure, surface area, concentration of vapor in the air outside the vent, etc.) and the "limiters" (concentration gradient, geometry, etc.).

So my guess is GM knew the answer to this question, but IMO other differences in the carb designs, engine designs, operating temperatures, etc. would make it VERY difficult to confidently say it's practically worse with the later carbs.

Just for grins, my car hasn't been started in a few months so I'm going to measure the time it takes to start. I expect the bowl to be empty from evaporation. I normally just pour some gas down the vent but I really don't know how long it will take to fill the carb just by cranking.

I do know it takes so long I want a primer pump. My tolerance may be different than others, though. 5 seconds cranking may seem OK to some, may feel like forever to others...

I plan to let it warm up, then shut it off and measure the carb temperature as it cools. I'll try to monitor the fuel loss inside the carb, but not yet sure how to do this.... any ideas (I won't open it up for this)? Maybe a dip stick down the vent?

Mike

[78w72]

Originally Posted by Shiny

I plan to let it warm up, then shut it off and measure the carb temperature as it cools. I'll try to monitor the fuel loss inside the carb, but not yet sure how to do this.... any ideas (I won't open it up for this)? Maybe a dip stick down the vent?

Mike

Should be able to use the acc pump and watch for fuel squirting out while you operate it by hand, might not be perfect but will show when the fuel level in the bowl is so low the pump cant pick any up which will be basically when the bowl is dry or very close to it.

5 seconds of cranking isnt all that much but could be longer than some like... if thats the case just break it up some, do 2 or 3 2-3 second cranks, thats what I do when mine goes dry after about 4-5 days of sitting in hot 80-90 degree summer temps, takes longer to go dry in cooler temps. Thats for my 2 cars with 1978 q-jets... my 81 with a CCC q-jet can sit for over a week if not 2-3 & it will start up in about 3-4 seconds of cranking, not sure why but the 81 is noticeable different.

Im considering a full intank pump set up like tanks inc, but until then & for the last ~10 years Ive just dealt with it by doing a couple 2-3 second cranks when needed, not really a big deal.

[78w72]

Just saw the fuel in a jar thing... wouldnt it be more accurate to put just a very small amount like 1oz of fuel into a small container the size of a q-jet fuel bowl, then put it on top a heat source closer to the temps of a running engine? Seems that would resemble the environment the fuel is in better than a big jar of fuel at room temp?

[Shiny]

Originally Posted by 78w72

Should be able to use the acc pump and watch for fuel squirting out while you operate it by hand, might not be perfect but will show when the fuel level in the bowl is so low the pump cant pick any up which will be basically when the bowl is dry or very close to it.

Thanks for this. Yes, I can do this. It will reduce some volume in the bowl, but probably the simplest way to know if the pump has re-filled the carb.

5 seconds of cranking isnt all that much but could be longer than some like... if thats the case just break it up some, do 2 or 3 2-3 second cranks, thats what I do when mine goes dry after about 4-5 days of sitting in hot 80-90 degree summer temps, takes longer to go dry in cooler temps. Thats for my 2 cars with 1978 q-jets... my 81 with a CCC q-jet can sit for over a week if not 2-3 & it will start up in about 3-4 seconds of cranking, not sure why but the 81 is noticeable different.

Agree 2 or 3 seconds is not a big deal. I had it stuck in my head that continuous cranking is hard on the starter so have also used "bursts". My choke and fast idle are close to working so I will press to set and crank 3 seconds. Then repeat. I figure the starting sequence is designed to squirt some raw gas into the manifold so pressing before each crank makes sense to me.

Im considering a full intank pump set up like tanks inc, but until then & for the last ~10 years Ive just dealt with it by doing a couple 2-3 second cranks when needed, not really a big deal.

I just replaced my tank and thought about it, but not worth the money and effort to me either. My car still isn't on the road so it sits a long time between starts. If I drove it every few days, the carb might never dry out. Hope to find out!

Mike

[78w72]

Excessive cranking is hard on the starter, but usually heat is related to that like for longer cranking closer to 10 seconds or more... for a classic car that isnt a daily driver I dont think doing shorter bursts of cranking as needed would be an issue. I have a stock reman'd starter on one of my cars that is at least 30 years old, was in a mild 455 TA daily driver back in the 90's, then was pulled with the motor when the car was junked and saved for future use, been in a 400 TA for about 15 years now & still working great.. knock on wood.

[Formulajones]

Originally Posted by 78w72

Just saw the fuel in a jar thing... wouldnt it be more accurate to put just a very small amount like 1oz of fuel into a small container the size of a q-jet fuel bowl, then put it on top a heat source closer to the temps of a running engine? Seems that would resemble the environment the fuel is in better than a big jar of fuel at room temp?

I'm sure you could come up with all kinds of ways to test. I did it this way for a couple reasons. 1, I'm not spending a bunch of my time on a silly subject that honestly is simply just a non issue. 2, that was meant to show the few in here that fuel simply does not disappear into thin air by itself very easily like some eluded to without some sort of excessive heat source, or a leak somewhere. Notice I didn't just give it a tiny vent hole either, I left the lid off of a jar LOL

You guys can go on thinking 4-5 seconds is excessive cranking after a car has sat for weeks and put your pusher pumps on if you wish, I'm not here to tell you otherwise, but there is nothing abnormal about that. These cars were the same way when new. EFI has spoiled the masses. In fact some EFI tunes actually crank for several seconds before they hit just in case you weren't aware. That's the starters job and they are cheap and easy to replace. If you're burning starters up you got other problems.

[HWYSTR455]

Some new and aftermarket EFI ECUs require 3 rotations to reference TDC/trigger, that's why they crank a few.

Carb cars when properly setup can start on a 1/4-1/2 a rotation.

The 461 I had in the LeMans with a holley, 2 pedal pumps and touch the key when cold, hot required no pumps. If it went 2 or 3 rotations I would stop cranking and try again.

That's not after sitting for a while though, but it would work the same if it only sat for a day or two.


.

[78w72]

Originally Posted by Formulajones

I'm sure you could come up with all kinds of ways to test. I did it this way for a couple reasons. 1, I'm not spending a bunch of my time on a silly subject that honestly is simply just a non issue. 2, that was meant to show the few in here that fuel simply does not disappear into thin air by itself very easily like some eluded to without some sort of excessive heat source, or a leak somewhere. Notice I didn't just give it a tiny vent hole either, I left the lid off of a jar LOL

You guys can go on thinking 4-5 seconds is excessive cranking after a car has sat for weeks and put your pusher pumps on if you wish, I'm not here to tell you otherwise, but there is nothing abnormal about that. These cars were the same way when new. EFI has spoiled the masses. In fact some EFI tunes actually crank for several seconds before they hit just in case you weren't aware. That's the starters job and they are cheap and easy to replace. If you're burning starters up you got other problems.

Was just a general comment or suggestion that the test wasnt the same environment as fuel in a hot engines carb so therefore didnt represent whats happening in that situation. I understand not spending a bunch of time on something you find silly, just saying that test is nothing like what happens on a car.
2, gas does evaporate, into thin air, and can happen faster than a big jar full sitting at room temp, it doesnt require "excessive heat", 160-190° of normal engine temps with a thimble full of fuel can & will evaporate in a relatively short period of time, maybe not a couple hours but 24-48-72 hours would likely be different... add to that possible drain back and this type of hard starting is definitely an issue for many people, Ive read quite a few threads on it over the years.

I am not a "you guys", Ive been dealing with occasional fuel evap or whatever it is in 2 of my q jet cars for a long time, both carbs were built by Cliff so I doubt there is any leaky well plugs. Also for whatever reason my other car with an 81 CCC Q-jet does not do this at all, it can sit in the same garage & same temps for 2-3+ weeks & start after just one short 3-4 sec crank... but the other 2 cars for whatever reason require 2-3 short cranks to get fuel into the carb after sitting for about 4-5 days, not really an issue to do that IMO.

I can say in my experience that these cars did not do that when new or nowhere near as bad, some say the fuel was a lot better back then too so it shouldnt have been as much of a problem. Just like my 81 CCC Q-jet, Ive owned or known of other q-jet cars back in the 80s-90s that didnt do this, they would usually start in ~5 seconds of cranking the starter after sitting for a couple weeks.

Just adding my experiences owning a few Q-jet cars for 25+ years, and that an experiment test closer to the real world situation of a hot carb would have better shown what happens with a small amount of fuel... just like most other things on here, everyone has different experiences. Heres just 1 link about fuel evap that supports it does evaporate and heat effects it.

https://ranwhenparked.net/does-gasoline-evaporate/

[Formulajones]

It wasn't meant to show what happens in a car. It was just simply showing fuel won't disappear quickly without some help. Like the claim it just goes dry in a half hour or 2 days is just BS, people don't want to admit they have another issue and conveniently blame fuel quality.

If I can make these cars work fine without pusher pumps so can everyone else. But if you guys want to spend more money and monkey things up with a bunch of add ons be my guest.

[78w72]

Ok, but this situation is about fuel evaporating, in a car, when hot... totally different environment than a jar of fuel on sitting on a table.

I agree some cranking is a non issue, tried to explain it wont damage a starter doing some 4-5 second cranks.

I also have no intention of adding a pusher pump to any of my cars, but any electric pump will fix or improve the "hard" starting some have whether its from evap or drain back. If someone doesnt like dealing with cranking everytime they start their car after sitting for a few days or a week, then a pusher/elect pump is an option, lots of guys on here & out there with classic cars use them for one reason or another.

[Formulajones]

Originally Posted by 78w72

Ok, but this situation is about fuel evaporating, in a car, when hot... totally different environment than a jar of fuel on sitting on a table.

I agree some cranking is a non issue, tried to explain it wont damage a starter doing some 4-5 second cranks.

I also have no intention of adding a pusher pump to any of my cars, but any electric pump will fix or improve the "hard" starting some have whether its from evap or drain back. If someone doesnt like dealing with cranking everytime they start their car after sitting for a few days or a week, then a pusher/elect pump is an option, lots of guys on here & out there with classic cars use them for one reason or another.

You missed the point on the jar test. It was stated early on that the ethanol fuel evaporates entirely on it's own and people claiming their bowls would be completely dry in as little as a half hour, blaming the fuel of course as no one wants to admit they actually have a problem. That's entirely not true at all. This fuel on it's own has such a slow evaporation rate, as I clearly demonstrated, that there is no way the fuel just leaves the bowl from sitting for a 1/2 hour, or even several hours. There has to be some accelerant involved. Either guys are heating the carbs way too much, or you have some sort of leak somewhere. I've demonstrated with my own vehicles this fuel is a non issue.

It's easy to sit here and type on the key board and say what about this or that. Now everyone here is more than welcome to perform their own tests. I've done my part. In fact I encourage anyone to post up your results. I'm sure many here would love to see what anyone comes up with. Put up or shut up as they say