1. Home
  2. Technical
  3. ROCHESTER
  4. Rochester Quadrajet Technical
  5. Quadrajet Troubleshooting
Rough Idle

This can be caused by several things, but there is one item often missed by the rebuilder and that is the idle tube. These are very small passages and get clogged easily with ethanol residue. Use thin wire to clean out the passages completely. Blow out with compressed air. A whistle with air will indicate a clean passage. If wire doesn't do it, try heating the outside of the passage and then blow air through. There may be a small air restrictor in the passage that makes getting through with wire difficult.


Quadrajet

My Thermostat is Located in the Manifold

This is a divorced choke type.<

Carburetor is flooding.

Gas is running out of the top, or you get a strong gas smell? This is a sign of flooding. What is flooding? Simply, there is too much gas getting into the float bowl.

Here is a list of items to check when this happens:
  • Fuel pump pressure - check with your motors manual for the correct pressure. New pumps are especially suspect.
  • Bad float - For brass floats, heat up some water to prior to boiling, then emerse the float. There should be no bubbles. Nitrophyl floats have to be weighed. 6.5-7.5 grams would be great.
  • Check float level
  • Make sure float isn't binding - move it up and down by hand and see if you feel anything catching.
  • Make sure float doesn't rubbing somewhere.
  • Does the seat have a gasket?
  • Did all of the old gasket get removed?
  • Look for any cracks around the area the seat screws in.

Engine is Uneven, or Surges

  • Look for any fuel restriction - check for kinks in the fuel line.
  • Replace any fuel filters - There is usually one in the fuel inlet of the carburetor and often there will be an in-line fuel filter.
  • Vacuum leak - Use spray carburetor cleaner and spray around the base of the carburetor and any vacuum lines. If the RPM smooths out, then you found the problem. There could also be a leak at the intake manifold.
  • Check the float level.
  • The power valve may be sticking. It needs to move up and down freely.
  • Dirt in the system - take the top off the carburetor. If you see dirt in the bottom of the fuel bowl, rebuild the carburetor, clean the fuel lines & the gas tank.
  • Main metering jet may be the wrong size. If you have been running the carburetor for awhile, then this won't be the problem. Jets don't wear out. Make sure the jets are not loose, or plugged.


Missing Tubes

Sorry about lack of knowledge and poor terminology. What I was referring to pickup tubes are call the main well air bleed tubes (2 inside tubes ) and secondary accelerating well tubes( the 2 outside tubes) fig. 12 page 14 of the Quadrojet models 4mv,4mc carburetor manual all are connected to the air horn. My carb. doesn't have the secondary accelerating well tubes. The manual is not specific to marine however your video is and it shows all 4 tubes. Looking through your parts section I found pt # 17-91 Rochester carb. brass tube. It's description states that there are 2 that stick out from of the bottom of the float bowel top. I don't have these. My question is should I have the secondary accelerating well tubes and if I do is that the right part # ?

He needs to have all four tubes, 17-91 is one of those numbers.
Quadrajet Carburetor Kits

Idle Compensator is Missing

The Hot Idle Compensator, when used on 4MC and 4MV Quadrajet carburetors (View C) is located in a chamber at the
rear of the carburetor float bowl adjacent to the secondary bores. Its purpose is to offset the enriching effects caused by
excessive fuel vapors during hot engine operation.

In a perfect world the Idle Compensator should be installed on those applications that need it. Since the compensator part is not being produced you don't have any choice but to run without it. You won't have any problem unless you get into a high heat situation, or use your air conditioner.

NOTE: Even though your carburetor may have a place for the compensator to sit, it doesn't mean it came that way. It was used on vehicles with air conditioning.
Can't find what you need? Contact Us
TOP