[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.

[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