I have a Rochester Quadrajet on my boat and have been attempting to tune it. It seems that I'm getting too much fuel in with my mixture as I'm blowing black smoke. If I close the mixture screws entirely, the black smoke disappears (although I see some very faint smoke), but even 1 turn out the black smoke returns.

As I understand it, these carbs should be anywhere from 3 - 5 turns out from bottomed as a typical mixture. I can't seem to do even 1 turn without creating a rich condition.

The carb is newly installed and rebuilt by a company rebuilds them and sells to the public.

Now, as a backstory, I was having issues with boat starting prior to what I mentioned above. I removed the spark plugs to find that they were covered in black carbon (I had recently replaced all of these 2 weeks prior but have had a series of various issues since that involved me changing mixture, timing having backfires, etc. that must have lead to the fouling of the plugs). I replaced all the plugs yet again and the engine started first try.

Now, after my above mentioned tuning attempt, I checked one of my brand new spark plugs I installed last night and it too has black carbon on it after just < 10 minutes of running since purchasing. That said, the plugs still fire fine.

So, I'm not sure if I'm just burning off fuel / carbon from my prior issues still or if this is due to the carburetor somehow dumping too much fuel into the intake despite the fact that I've closed the idle mixture screws.

Additional question: Where is the fuel coming from if the mixture screws are closed entirely? I checked all 4 barrels and I do not see any fuel coming from the jets. As far as I know, this only leaves the mixture screws as a potential source of fuel.

Any ideas?

  • Did you check the main jet?
    – Solar Mike
    Sep 13, 2020 at 23:07
  • @SolarMike, not sure what I'd be checking for. The carb is rebuilt / re-manufactured. I just installed it. It should be adjusted correctly, have new parts, etc. Sep 13, 2020 at 23:25
  • It will have new parts but you have to make the correct adjustments to tune the carb to the engine. Some carbs have a main jet initial setting of 1.5 turns from bottom or 9 flats based on the hex “nut”. You have to check.
    – Solar Mike
    Sep 13, 2020 at 23:32
  • @Moab, that is my own question. This is a different question (although very similar and related). This question is specifically asking why would I be getting black smoke even at 1 turn out from bottomed? And secondarily is asking where is fuel coming from if I'm completely bottomed out with the mixture screws. Sep 14, 2020 at 2:19
  • I answered that in your other question.
    – Moab
    Sep 14, 2020 at 2:30

1 Answer 1


When I worked carbs , the mixture was determined by jet size ( not adjustable ,replace to change) and float level , and in Carters by metering rods moving in the jets ( not a Rochester design). The idle jet adjustment screws let air into the idle mix. By closing them ,air is reduced to the idle mix. Only the two primary barrels had them .The Rochester 4 MV had an electric controlled meter valve that adjusted with the same dwellmeter that is used to adjust ignition points. I mean you need something like a shop manual to adjust a carburetor.

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