I took my 72 GS for a rip yesterday after changing the fuel pump to a new stage 1 style. Once I turned it over a few times and the pump primed, the car starts with no issues. At cruise, everything runs fine. While merging on the interstate, I went wide open in second gear (2004R) and the rpms climbed appropriately. After shifting to third, about mid way through the power band with the throttle to the floor, the car began bucking and surging really hard as if it was running out of gas. I let off and hit 4th gear at cruise throttle and the surging stopped. While in 4th, I floored it again and the same thing happened. If i drove gingerly, no issues. Floor it, surge. Can it be im emptying the small fuel bowl on the rochester qjet that fast? Fuel tank is 1/2 full. Please advise on where to begin troubleshooting. Thanks guys!
One other thing, when I returned home and got parked in the garage, I popped the hood and could hear what i thought was the fuel boiling inside the carb. Sure the engine was hot, but never overheated. I have an SP1 and a 800cfm qjet.
You need to rig up a fuel pressure gauge and see what's happening. It could be the Stage1 pump isn't keeping up, or the Q-jet is not set up correctly. You may need a bigger needle/seat or different float setting.
Check or replace the rubber sections of the fuel line. They delaminate on the inside and will collapse/suck closed under heavy throttle, at cruise/light throttle they're fine.
I'll check all of these things. The stage 1 pump I just installed is NOS and replaced a worn out non stage 1 unit with the smaller bowl. That is when these issues began. I bet it is the old fuel rubber hoses collapsing under the suction of the stronger pump. Imy certain these are still the original pieces so that will more than likely be the culprit. I had Mark rebuild and tune my carb and I'm pretty sure I have the high flow needle and seat combo. I feel comfortable ruling that out. I will double check my float level. I was tracking 9/32 as the proper height. I may raise it just a bit and see if that helps. Question:what is this black plastic piece inside the bowl area of the carb? I'm sure there is a reason for it, but it seems like it takes up some of the much needed bowl real estate.
Also, how do I address the boiling of the fuel in the carb. That doesn't seem right. I don't have the hood clearance for any spacers. More gasket material?
This is common when you have winter blend fuel in warmer temps. I would put 4-5 gallons on race gas in it and see if the problem goes away. If it does, you may just need to be patient and wait for your area to switch to summer blend.
Not sure about this Larry. I have original 70 heads on this engine and an SP1 intake. Is there a way to check? I was never tracking a need for this modification.
If you are not sure, then we can assume that they aren't blocked. Hot exhaust is heating the intake on each side. Aluminum transfers heat faster. That may be why the carburetor is getting really hot.
Engine Temps are 175-195. I have a 180deg thermostat installed. No issues with overheating at cruise or at idle that I've seen. Any ideas about this plastic piece next to the fuel bowl of the carb?
Are you using the fuel return line off the pump, the smaller one that goes back to the tank? Could be vapour locking (which as mentioned would be pretty easy with winter blends of fuel). The only reason i say that is once upon a time I was told if you block that line the pump makes more pressure (that was good I suppose!!LOL), and it worked fine but on warmer days same thing, about the upper third of 3rd gear thing would fall flat on its face like it ran out of fuel...put line back on and hey it all works good again. If the car ran fine before, you change pump and are having issues now first thing to check is the pump as Larry already mentioned. Methods are in the service manual or put the old pump back and take it for a run, if the problem goes away you have your answer. NOS just means old in some cases, if might be pooched inside.
The black plastic piece is what holds the float in place. You can modify it to get a little more fuel in the bowl. Maybe Mark did that already? Those other items mentioned should be checked first.
That's good engine coolant temperatures. What I am talking about is the exhaust crossover passages. They are in the middle of the head between each set of intake ports. The stock intake has a crossover passage that runs underneath the carburetor. The exhaust gasses crossover from one head to the other heating the bottom of the intake. This helps with fuel vaporization in cold weather. It also heats the choke thermostat. Ever see engine paint peeling in the middle on each side of the carburetor? That is exhaust heat. The sides of the intake are exposed to exhaust heat. Bottom line, it makes the intake manifold get hotter than it would without the exhaust heat. You could see if the problem resolves itself with summer blend fuel. If it doesn't, I would remove the intake and block the crossover passages in each head. Most guys use cup plugs.
Lots of ignition advance will cause surging.I just fixed a similair problem and the advance plastic stop was missing broken in distributor housing .At 2000 rpm there was 50 degrees total and rpms went up and down 500 at a steady cruise .Idle it ran beautifully.
Absolutely, this can easily happen when you couple a stock vacuum advance with light springs that bring the mechanical advance in very quickly. The engine will surge during cruise conditions. At WOT though, there will be no vacuum advance.
I have an accel hei distributor in mine. I power timed the motor a year ago per Larry's thread. I'm not running the vacuum advance currently and have the total timing at 32° all in at 2500rpm. I haven't noticed any hot start issues or sporadic idle problems. I'll add this to the list of things to rehash. I got under the car today to inspect the fuel lines. I didn't see any tears or dry rotting but it seems like cheap insurance to change out the rubber sections. I didn't realize there were so many rubber hose pieces, only assumed it was hard line the whole way from the tank to just before the fuel pump. Seems like it will be easier to drop the tank to get to it all. I am running an in line filter just before the inlet of the qjet and the internal filter is still installed in the carb as well. Do you guys recommend removing the small internal filter for the qjet if running an external one? I'll be replacing it as well since I'm in there.
