73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Fuel Systems and Drivability => Topic started by: txtj79 on August 31, 2008, 10:33:29 pm
-
I have a newly rebuilt q-jet 4bbl that is installed on a 350 in a 1980 C-10. The truck runs great with the exception of bogging down when I step on the gas. It will accelerate fine if I apply the pedal like normal such as at a green light. But any time I just mash the pedal, at any speed or in neutral, it stumbles for half a sec and then takes off. Any suggestions? I'm not sure what's inside the engine. All I know is it hauls butt. It has a stock intake, heads, carb, and distributer. Even has cheap autozone plug wires. It has true dual exhaust with rusty headers and glass packs, no cat. It would barely run when I bought it.....the vacuum lines were all torn or unplugged. I fixed that and it ran fine since, except the bogging. That's why I replaced the carb. It really feels like it has a special cam in it, it has a good lope at idle. How could I diagnose the bogging problem?
Thanks,
T.J.
-
You might need more of an accelerator pump shot. Also, it could be a timing issue, what's your timing set at?
-
I'll try and explain this as clear as possible but if it doesn't make sense let me know and I'll see if I can clear up any questions.
The secondarys on a Q-jet open mechanically at the base and cause enough vacuum to make the butterflys on the air horn open, which in turn rotates a cam that makes the secondary meetering rods open up and let fuel into the secondarys. More often than not these secondary butterflys open too quickly causing stumble (bog). Rarely they open too slow. To increase the tension on these there is a screw up under the air horn, turning it in slows the opening, turn out to speed them up. All the screw does is increase/Decrease tension on the butterflys. I'll try and post a picture in the morning of this screw and relative location. Trial and error will be the only way to get it "just right" for your liking. Hope that made sense and helps.
-
Would that cause a bog even in neutral?
-
I was thinking the same thing about the accelerator pump, but then I read somewhere that the pumps in late model q-jets are supposed to be pretty good. I'm going to borrow a timing light this afternoon.
Thanks,
T.J.
-
T.J., It sure sounds like an accelerator pump problem to me also. Maybe the rubber cup is too small for the bore (somewhat common) or you nicked it in someway putting it together. If you have Two holes in the arm, the linkage should go in the One closest to the carb body. Pull the top off & have a look at the cup in the bore, it should be near onto snug, but smooth up & down.
Tex, To answer your question... No the secondaries, although at fault in many cases, won't do the bog in neutral.
Good Luck & don't give up on it, you'll figure it out! Lorne
-
Don't overlook the jets in the carb. It sounds like the fuel is too rich and it takes a little time for the air to catch up to all the fuel. Smaller jets may be the fix.
-
I had copied an info page about Quads. Here it is. Tends to support the bad accelerator pump diagnosis the guys are giving..
http://www.arkansaspontiacs.org/techstories/RockyQ-jets.htm (http://www.arkansaspontiacs.org/techstories/RockyQ-jets.htm)
-
Quick easy way to test the accelerator pump. Just depress it with the engine idling. It should stall the engine.
-
Vile, I tried your test method. The engine stumbled, but didn't quite die. Is that what you mean by stall? Should I disconnect the accelerator pump linkage from the throttle linkage when I perform this test? Obviuosly, if I push the accelrator pump all teh way, it will open the throttle......
Thanks for all the help.
T.J.
-
No, take a small screw driver and just depress the accelerator pump all the way down. Do not operate the linkage just push the plunger all the way down. It should squirt enough fuel to stall the engine. If it doesn't you know it's not pumping enough fuel.
-
No, take a small screw driver and just depress the accelerator pump all the way down. Do not operate the linkage just push the plunger all the way down. It should squirt enough fuel to stall the engine. If it doesn't you know it's not pumping enough fuel.
Hey Vile, I've read several of your responses to Accelerator Pump issues. I have a new (two month old) Holley 4160 carb...and all the sudden, when I smash it from a stop...it cuts out. I've switched out the carb temporarily with a another Holley. And it works as it should. My question is, when an accelerator pump is not working properly...what part goes bad? Is it the diaphragm, shooter, some kind of seal? <br><br>
I have not disassembled yet...as I'm not super comfortable doing so (yet). I wanted to get a little info first.. from all accounts, and looking at the accelerator pump location...it would seem simple enough to open up and switch out whatever needs to be changed.<br><br>
Let me know if you need additional information
-
Well if you've switched carbs and the problem goes away I'd check the accelerator pump and float level. If it idles and cruises fine it's probably the pump. It works just like a squirt gun. Do you have a good filter? Take it apart and see what's going on.
-
Well if you've switched carbs and the problem goes away I'd check the accelerator pump and float level. If it idles and cruises fine it's probably the pump. It works just like a squirt gun. Do you have a good filter? Take it apart and see what's going on.
thanks Vile. The truck indeed idles and cruises as it should. I'm going to investigate more. Thanks for your advice. Fuel filter is one like this:
(http://ecx.images-amazon.com/images/I/41suZCZ7t4L._SL500_AA280_.jpg)