Truck don't like the heat, summer carb tuning?

[texanidiot25]

What can I do to get some summer performance in my q-jet? It hates this 100 degree heat during the day, and the truck runs very weak, and even falls on it's face at WOT at the top of 2nd gear, stalling until you back off the throttle. But at night it wakes back up and does quite well.

Also, since my truck has a third gen Camaro air cleaner on it, I'll be routing some sort of setup to pull cold air from behind the headlights, so it will be getting ambient air, not under hood air.

[prostreetC-10]

They run better at night due to the cool dense air. First of all....what are you doing driving your truck in 100 degree temps? Must have A/C. Mine doesn't and it's like driving a microwave with headers and a thin floor board. Is your truck motor itself running warm? I S-canned the stock fan and went with a Flex-a-lite 10,000 rpm 18" fan. It dropped my engine temp 20 degrees.

[Stocker]

Something wrong with your carb? My q-jet has never complained about the heat. As with any vehicle, there is a slight loss of power due to lower density of hot air, as prostreetC-10 said. But I have never suffered driveability issues like you describe.

[Beefcake]

whats the choke doing? maybe your butterflies are not completely open.

[texanidiot25]

They run better at night due to the cool dense air. First of all....what are you doing driving your truck in 100 degree temps? Must have A/C. Mine doesn't and it's like driving a microwave with headers and a thin floor board. Is your truck motor itself running warm? I S-canned the stock fan and went with a Flex-a-lite 10,000 rpm 18" fan. It dropped my engine temp 20 degrees.

I live in TX! I have no choice. Plus as a pizza delivery driver... It ain't too bad with the windows down, the vents open, and the pizza in the bed... But the headers cook my floor so they can't be helping. But... It's on the to-do list

I agree they run better at night, but the difference is stunning... Even if it doesn't die (thinking this maybe the choke), it lacks HP past 3500 like crazy... But I'm used to fuel injection, so this could be normal and me not happy. Would leaning it out a tad help it deal with the thinner/hotter air? I miss my cold weather beat down from a few months ago, now I'm struggly to keep with some stock TBI trucks.

On average the truck's temp gauge stays at the 11 o'clock position, about half way between the cold marks and half way. Only time it's ever hit the half way was in traffic the other day on the freeway when I never got above 20 mph for an hour. Stock flex-fan. 4 core aluminum radiator, with a fan shroud to pull air.

Since you drag race, would pulling air from behind the driver's side head light bucket, with the head light trim off, create any ram air effect on these trucks? I chose it to pull air from since when the trim is on, it blocks just about any rain from getting in, but still gets plenty of air flow.

Originally Posted by Beefcake

whats the choke doing? maybe your butterflies are not completely open.

I did just think about that today. The choke is hooked up to nothing, but normally only shuts when the gas is pumped when the motor is off. Gotta re-do it for my choke cable, it's possible that it shuts at low engine vacuum, when the pull off lets go.

[turp mcspray]

Years ago I had the same kind of problem. When it got hot out, when I would shut off my car it would heat soak really bad. After sitting for 15 or 20 minutes It load up or flood when truing to restart and poor performance in general. I tried changing the timing, colder thermostat, adjusting the float,everything I could think of. No change. Then I saw one of those aluminum heat shields that go under the carb. Just a flat piece of aluminum. (They dissapate some of the heat before it reaches the carb) Anyway at the time they were 10bucks. I figured why not, it's only 10bucks. Problem SOLVED!! After that I run them on all of my vehicles. I've never had the same problem since. I think they are about 20bucks now. I'm not saying it's a sure thing to solve your problem but I think it's worth a shot.

[mikajo39]

You could try a carb spacer, It keeps the carb from getting hot of the intake manifold and should give you a few ponies because there is more plenum volume and the air is moving faster, Leaning out the engine with the mixture screws only effects the idle so if it's idling ok dotn change em. You could try jetting ur carb a bit leaner but if you go to lean you could do some engine damage. Don't worry my truck was kind of a turd last summer we had a heat wave of about 105 and I had my fair share of hard hot starts.

[texanidiot25]

May be swapping to a aluminum intake soon enough, guessing that may solve it too, but I'll give it a shot.

That is possible. I'll see how it acts after it's 1st start and then after a restart when I drive again.

Delivering pizzas, it's not stop starting, driving, idle, drive, shut off, restart, drive, idle, drive, shut of, and etc. Repeat that maybe 15 times in a day, plus any extra driving.

I park in the shade when I can, but it does heat soaks regularly I guess.. Though the coolant temp never rises much while sitting though, if that's any indication.

[mikajo39]

Originally Posted by texanidiot25

May be swapping to a aluminum intake soon enough, guessing that may solve it too, but I'll give it a shot.

That is possible. I'll see how it acts after it's 1st start and then after a restart when I drive again.

Delivering pizzas, it's not stop starting, driving, idle, drive, shut off, restart, drive, idle, drive, shut of, and etc. Repeat that maybe 15 times in a day, plus any extra driving.

I park in the shade when I can, but it does heat soaks regularly I guess.. Though the coolant temp never rises much while sitting though, if that's any indication.

Mine ran cool and still took forever to start up after it was hot, and an aluminum intake manifold may help, aluminum dissapates heat much better than cast iron. spacer cant hurt either lol, you can get a phenolic one (funny name for plastic?) for pretty cheap. Summit sells em for lik 11 dollars or so

[Stocker]

Originally Posted by texanidiot25

Stock flex-fan.

Flex fan is not stock. Clutch fan is, at least on A/C trucks... but you aren't overheating so it shouldn't be an issue. (Mine overheated when I temporarily had a flex fan).

Still odd though, and definitely not normal with a q-jet.

Do you have an exhaust heat riser valve? If it was stuck closed, it would force hot exhaust through the intake manifold and make the carb get extra hot -- might cause it to do strange things.... ?

[texanidiot25]

Stocker- I've never seen these trucks, stock, with anything other than a fan bolted to a water pump... Never seen them with a clutched fan stock. But it's all ways pulling air threw the rad at least.

Exhaust heat riser? The tube for the intake that runs to the manifold? If so, then nope, I have headers.

I'm gonna get motivated and rehook the choke back to my cable, and take it for a drive later on to see if it falls on it's face again. But, that still doesn't help the hot weather performance, but maybe it's something I'll have to get used to.

[Stocker]

Originally Posted by texanidiot25

Stocker- I've never seen these trucks, stock, with anything other than a fan bolted to a water pump... Never seen them with a clutched fan stock. But it's all ways pulling air threw the rad at least.

OK, you must mean fixed-blade fan, not flex fan. Yup, they pull air, but a clutch fan pulls more air when engaged, and nearly free-wheels when not needed, resulting in less parasitic drag (more power) and virtually eliminating the constant roar of air that you get with a fixed-blade fan. Dunno about 1969, but in 1972 at least, a clutch fan came standard on factory A/C trucks.

Originally Posted by texanidiot25

Exhaust heat riser? The tube for the intake that runs to the manifold? If so, then nope, I have headers.

Yup, that's it, so with headers, nope, you don't have it. Oh well, it was a thought...


Hope you can find the problem! Believe me, it is not normal to run out of power & fall on its face above 3500 rpm or at WOT in hot weather. Something is wrong for sure, and you shouldn't have to live with it.

If this thread goes nowhere, maybe try posting over in the Engine & Drivetrain forum. This forum gets more traffic though -- hopefully someone here can help you.

[texanidiot25]

The falling on face I blame the choke at the moment, as now that I think about it, it does act like it's being choked...

Long story short, learned a trick to help clear carbs out from an old mechanic friend. Choke slam, give it full throttle and when it revs high, slam the choke shut. The spike in Vacuum pressure forces out dirt. Results in the truck falling on it's face and popping out the exhaust, which is what's been happening. Gotta redrill the choke linkage for my cable and try again.


The power loss past 3500 rpm is consistent with weather though. And if floored hard, it tends to hesitate a bit before coming to life in the heat. Which is why I think it's a tuning issue, we tuned the carb back in February when it was still 60 degrees out

It's a freshly rebuilt Q-Jet, Had to swap the mounting flange with my old carb since I got this one for free of KBS when my 1st carb was giving me hell. I do recall my dad saying the float needed to be raised a hair he thought, when we put this one on the truck. But, never got around to it.

[s95930]

Mine would vapor lock. Put an electric fuel pump on and never had the problem again.

[Mordachai]

^ funny I was going to ask about your fuel delivery.

[69GMCLonghorn]

Something ain't right in Q-jet Town. Have someone take a good look at (or inside) that carb. Maybe swap it out if you have the cash. I've run a Q-jet on a BB Cadillac in 110° weather with 5 people and luggage in bumper to bumper. The car didn't overheat and no stalling form the Quadra-Puke. Good luck!

[mikajo39]

Dunno about 1969, but in 1972 at least, a clutch fan came standard on factory A/C trucks.

I have a 69 with A/c and it came a clutch fan everything was factory when I got it.

[falvarez12001]

I agree their has to be something wrong with your carb. I was driving mine last week Here in San Antonio, the weather hit about 104, and other that the burn on the back of my legs, because of the vinyl, the truck ran just fine.

['72customdeluxe]

My q-jet runs alright in our fine cool weather. It won't chirp into second anymore in the heat but it just barely would before. It does smell richer now with the weather. But the completely falling on its face can't be the weather. Mine ran like crap and actually could die at full throttle and at higher rpm's with a clogged fuel filter if you haven't checked that already. But one of these could help insulate the carb pretty good. http://summitracing.com/parts/CTR-85-254/
And it's only a quarter of an inch so it wouldn't really affect your tune.

[texanidiot25]

It's been performing decently as of late. This weekend will be redrilling the carb linkage to accept the choke cable, which will prevent if from shutting itself (it's possible at low vacuum that the linkage shuts the choke until the pull off reopens). Also got a FREE weiand intake manifold, so, with that and the adapter, the carb will stay cooler compared to the stock cast iron manifold... In theory.

[Stocker]

Sounds like you're on the right track, hope it's all good with the choke secured and the new intake in place!

[vectorit]

My burb spent it's younger days pullin a travel trailer in the high desert, and in the Sierra Nevada in 100+ degree temps. I was a kid back then, so for what it's worth...

Problems were never with the Q-jet.
I do know that the fan, and the radiator was upgraded along with an electric fuel pump.

Truck ran great, but it did get vapor lock in the higher altitudes occasionally. Which pretty much brings ya to a stop.

So, I would check the radiator. Is it a heavy duty model, and is it a healthy one at that? Meaning, could it be old and plugged up with sediment?
The fan should be a larger model, and clutch fans are great since like noted above they don't make noise when not needed.
You might also think about getting the engine water jackets flushed, depending on how old the engine is. Even if it's newer, but you run just water in it you could have caused premature sediment to settle in it.
Are you adding coolant?

[texanidiot25]

The coolant was flushed a long time ago when we dropped the motor in. Runs on GM orange antifreeze now. The rad is an all aluminum 4 core unit. The gauge rarely gets to half way, even in dead stop traffic in the houston heat. Cooling is far from an issue IMO.

when I do the intake, new filter will go in, and I'll rig something to pull air from behind the head light bucket as well. The truck has a third gen Camaro air cleaner, and I like it for the snorkal. Already pounded the opening larger to let it suck more air.