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Carb jetting questions

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Old 10-11-2017, 04:37 PM
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Default Carb jetting questions

im running a new quick fuel 850 carb on my recently rebuilt 509. Promax heads, custom ground roller cam, msd ign set at 34 degrees, stainless marine exhaust.
Mechanical holley fuel pump with about 7# of psi.

im pretty sure my motor is running rich. Plugs are Black, and soot on the back of the boat

with my hp500 carb the plugs run more of a tan color.

The qf comes set up with 82 main jets and 94 secondary.

Should i jet it down, and if so how many woukd you go down with to start?

im not a carb guy and my builder and i are not on great terms (long story but lets just say i am not allowed back in his shop).

The motor does run great except for a small hicup when u put it in gear as the rpms drop. It stays running but is very close to wanting to stall. Ive played with the idle and got to where its livable but wondering if its also due to my running rich.
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Old 10-11-2017, 05:22 PM
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Have you checked vacuum and also what does the carb have for PV? You'll get it with a little patients. I never seen black plugs in my engines. Well maybe once. Rather than going into all the to do's and not to do's google BBC plug reading. Can be very helpful. People read plugs in different ways. Boating can be a little tricky cause of all the idling, cruising at 2,800-3,200 and wot. Also ck PV even though new.

once again I could go into it what I do anyway but google a reputable site and learn from it and you'll gain a lot of knowledge. If you don't have afr o2 sensors I would seriously look into getting. Btw. Make a hard 1 minute run and kill it at wot and ck a couple plugs. Starting out rich is much better than lean. I tend to tune mine a tad on the edge on a cold day and leave it. Definately consider o2 if you don't already have. Cheap insurance...
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Old 10-11-2017, 06:18 PM
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I ended up with 78 and 84 with high flow 4.5 PVs giving me about 12.5 across the range on a 502. I gained about 400 rpm once I got her dialed in.

Last edited by Baja Rooster; 10-11-2017 at 06:34 PM.
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Old 10-11-2017, 08:34 PM
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Surprised you needed to go 10 sizes lean. My 454s on the dyno were good with them stock but in the boat with wet exhaust were maybe a little rich around 11.9 to 12.1
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Old 10-11-2017, 09:00 PM
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Is everyone giving jetting #'s using the same exact carb as the OP ?Quick Fuel M-850's ?



Last edited by SB; 10-11-2017 at 09:39 PM.
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Old 10-11-2017, 09:14 PM
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Mine are qf850s
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Old 10-11-2017, 10:51 PM
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Mine is the QF850m
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Old 10-11-2017, 11:56 PM
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Quote:
Originally Posted by Mr Maine View Post
Surprised you needed to go 10 sizes lean. My 454s on the dyno were good with them stock but in the boat with wet exhaust were maybe a little rich around 11.9 to 12.1
The high flow PVs made it crazy fat over the stock two window units.
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Old 10-12-2017, 01:25 AM
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You need to figure out where it is too rich. Is it rich at idle??? If so, that has nothing to do with jetting.

You will need to make some runs at cruise rpms, then shut down and pull spark plugs to check them.
You will also need to make some WOT runs and shut down and pull spark plugs.

Do some searching on the internet about reading spark plugs. There is a lot more to it than just the porcelain's color.
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Old 10-12-2017, 09:09 AM
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Thanks everyone. I know its rich at idle. However i havnt pulled any plugs after cruise or a wot run but i suspect its rich all the way through. Ill pull the plugs after a few runs and see what i have.

This carb is untouched except adjusting choke, idle, idle air mixture screws on the 4 corners, and adjust float levels down a little bit.

Will report back after i get it back in the water. Weather is turning cold and chitty here so might be a little while.
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