Vehicle idles fabulous in park. In drive, still idles fabulous and ready to go. Light turns green, I give it some gas and I get a brief hesitation and then fine. You can do this up until about 45mph...foot off the pedal and then when you give it gas, you feel a hesitation and then its fine. anything over 50mph and higher...there is no hesitation whatsoever. You can take your foot off the pedal at 65 mph and then step on it with no hesitation and good response. the car was fine before a 150 mile road trip. Once I got into the destination and hit the first stop light, the car was running great. Then light turns green, I step on the gas and brief hesitation before coming online and getting up and going. Vacuum is good, can't get car RPM's to change while spraying carb cleaner around gaskets/seals, rebuilt carb two weeks back with no issue, fuel filter is new, and now its out of the carb in case it was blocked. Gas is good since I refilled tank again before leaving town and hesitation is still there. Hesitation is not there when car is first started in the morning and choke is engaged(high RPM's like hiway driving where issue doesn't exist). Once choke kicks down and normal op idle...you get hesitation when stepping on pedal in park and drive. What am I looking for here? Obviously Rochester 4barrel, which is quickly going to surrender to a Edelbrock or Holley if I can't figure this out.
Sounds like the accelerator pump shot may be lacking. May be as simple as needing a replacement seal (cup) on the end of the accelerator pump. The ethanol in today's gas takes it's toll on this item unless it's made of a resistant material. http://quadrajetparts.com/rochester-quadrajet-parts-quadrajet-accelerator-pumps-c-128_22.html Devon
The accel pump cup came with the rebuild kit and it was replaced when i rebuilt the carb two weeks ago. I always here about the q-jet hesitation...is this what people are talking about? Any other ideas?
I had a similar problem with a carb years ago. It was fine until the engine got to operating temperature....then stumble off-idle. It turned out to be the accelerator pump system check-ball and seat. If fuel leaks around the check ball, it will result in a weak shot from the pump. Check the seat for any damage or debris. You may need to look at it under magnification. I actually had to tap a NEW check ball with a small hammer and punch to form a new seat on my problem carb, then replace with another new ball, but it now works flawlessly. If you try this, go easy with the hammer and punch. It does not take much persuasion to form a new seat.
The Skylark i just bought with a 401 does this exact same thing but it has a new 600cfm Edelbrock carb on it. I have not had a chance to look into it yet but i think the 600 may be small for it. I am going to try and put some miles on it and check the plugs and go from there.
I was having a issue like this too.It turned out my timing had slipped a little and the secondary flaps were opening too soon.
I had the same problem with my 2-bbl rochester from a stop, press the gas it would want to die. I think my carb needs a rebuild as well, but I changed the fuel filter and it helped a little.
Just because you got it in a rebuild kit does not mean it will stand up to today's fuel. There are plenty of kits with old parts on the shelves. It's a simple thing to open the throttle with the engine off, and observe the pump shot. And no, people complain about a bog going from primary to secondary operation with a Q-jet. The primary choke pull off and secondary air valve tension are critical adjustments for that. That is not what you are experiencing though. You are getting a tip in hesitation from stop. Biggest causes for that are accelerator pump, vacuum leak, and vacuum advance failure. BTW, they always respond great at idle in Park/Neutral with no load.
Just because you got it in a rebuild kit does not mean it will stand up to today's fuel. +2 I could write a book here on accellerator pumps, pump seals, and ethanol. Currently available/supplies over the counter pumps and pump seals are JUNK. The "blue" seals in those kits and on those pumps will not hold up in currently available pump fuel. The additive package and ethanol content kills them in short order. We've fought this deal for a couple of years now, and currently have a pump cup that will hold up in E-85 or straight alcohol. It is NOT the same blue seal that comes on the accl pumps from the manufacturers. Kind of neat story, but on occassion we buy NAPA kits when I run out of gaskets, to fill orders, etc. Their kits come with a new pump that uses a blue seal. I put those pumps in a storage bin. Some how one of those pumps got sent to a customer. The seal lasted about 2 days, they swelled up and rolled up off the pump. He sent me a pic of it, and it was the light blue "soft" seal, not the dark blue seal we supply on our pumps. We let ONE over the counter pump/seal get out of here and had to replace it in less than a week.....FWIW.....Cliff
