engine loss of power wot

[johndd]

First i would like to say how good it is to see so many avid q jet folks are still out there they seem pretty scarce at times. I have an on going issue i have run into several times over the years I have never solved but i am sure with the experience here the solution can be had. As the subject says i have had issues with q jets falling completely flat at some point through the gears it was an issue many years ago that went unsolved and funny enough has come back to me many years through a fellow racers son to give you an idea of how long ago.bear with me i dont have numbers in front of me but not sure they are needed as i believe this might be solvable with out but can locate if necesary. In multiple cases on multiple carbs different vehicles i have experienced q jet that were functioning stock q jets and as the modifications have increased the problem arose. Now this is the funny part it almost seems that there is a set amount of fuel available and when spent the car noses over. not rpm if i was to short shift the car it might get through another gear then nose over. In the past i have switched the carb from car to car it would show same issue at different stages almost directly proportional the amount of power made. My 13 second car would fall top of first or just into second a 14.0 car would get half way though second. Both sbc cam intake headers auto 3.0 gear  opening the exhaust making more power magnified the problem occuring sooner. The 14 second car ran that all day long for years and when the q jet was installed ran well pulled much harder through first but would then in second nose over.the most recent case is a 383 sbc mild cam stock heads intake headers 3.0 gear 5 speed od standard in a 70 pick up. q jet is a functioning recently rebuilt carb. My friends 18 years old son learning to drive a standard on the drag strip. As he became more comfortable getting through the gears and more rpm the same problem arose i took it for a pass and it left hard was ahead of a stock mustang through second and into third it just lays down completely flat. At first he could get through the full 1/4 mile but as he was more agressive  the sooner it happened like it had a fixed amount of fuel as crazy as it sounds. Thought of fuel issues back in the day as all have mechanical pumps but not thinking fuel in all these cases. have a few thoughts in my head some one might  have knowledge of this issue.Not sure what i can add to save back and forth questions. Unsure of current needle and seat size but previously had the largest one.Have heard of issues with the vacumm secondary valve when opening and closing throttle but this happens wot. Have heard talk of removing or disconnecting secondary vacume valve and just adjusting spring tension  but have never gone this way or seen it in action. Will check back soon thanks guys   

[429bbf]

i had the same issue but a way different set up.nothing fancy.1979 ford f250 4/4 460. 3 speed  c-6 auto. 17059212 800 quad. pull 5500 in first .5500 in second and then go flat . some times at the top of second other times right when i hit third . had 3 problems. first i had a .125 needle and seat. changed to .135. second i had a small kink in my fuel line between fuel tanks .(has fac. dual tanks ) third i had to put electric pump back close to the tank . problem solved.it acted exactly like your saying. the harder and faster you pushed it the sooner it went flat if you left it pinned it would pick back up after a second or two. which acted more like a minute when you were getting beat.hope this helps

[429bbf]

one more thing it was way worse when i was running off the rear tank because it feeds the valve with a 5/16 line the front tank has 3/8 thats what finally got me thinking fuel delivery insted of carb.fwiw

[Cliff Ruggles]

The falling flat on its' face/loosing power deal is not a carburetor issue, it is fuel delivery.  What's happening is that the fuel bowl is going low or even empty on hard runs.

The first course of action to correct the issue is to install a larger high flow needle/seat assembly.  All over the counter kits will have a "generic" small needle/seat assembly, which is a big contributor to this issue.

Once you have a high flow seat installed, and problems still occur, the fuel pump and lines are not up to par for the power level.

With the q-jet, fuel delivery is critical, not because they have a small fuel bowl as commonly rumored, because they only have one needle/seat assembly to get the fuel into the bowl.

Think about the issue for just a minute.  It really doesn't help to increase bowl capacity, the correct fix is to keep the bowl FULL at all times.  This really isn't all that difficult, most "stock" fuel systems are good to about 400hp, some a bit more.

Drag racing with good traction requires even more attention to this issue, as all the fuel is pinned up against the back of the tank on hard launches.  It's difficult, if not near impossible to use a front located sump, then "pull" the fuel thru 15' of fuel lines, which are usually 3/8" (5/16" inside diameter), to a mechanical pump, then on to the carburetor.

Heat is another factor here, as this new fuel boils easily, and one must pay close attention to fuel line routing as well, when setting up a fuel system for big power/fast vehicles......Cliff