Plugs fouling

[ericcarr]

Okay as some of you may know I just got done putting a new carburetor on my 1990 7.4 454 motor. I know most of you are going to give me flack for not having a marine carburetor on here but I couldn’t pass a good deal down so I picked this one up to throw on here and see what it would do. Why in the world with my spark plugs be turning this black within 10 minutes of running time? I just installed all eight brand new spark plugs with a gap of 0.35….fuel pressure is right around 8 psi..

 

[ScottinAZ]

plugs that black are likely running rich. You may need to jet down a size or two. IMHO if those are new plugs there isnt enough time for the "fluffy" matte black deposits to really develop which is indicative of a rich condition. Oily black is usually (of course) burning oil, but you would likely be getting blue smoke from that. Are you getting any BLACK smoke out the exhaust, which would be indicative of a rich condition?

 

[ericcarr]

Well…I did notice in the driveway after running the motor for a bit that there was black film on the driveway….it didn’t smell like oil but it did just smell like exhaust gasses.

 

[ScottinAZ]

with the new carb, I would bet that its running rich. Likely you will need to rejet, chances of it having the "perfect" tune on it out of the box is slim to none. Its gonna be a bit of trial and error, but you will get it there.

Rejet, run it, check plugs, rinse, repeat until the engine is happy and the plugs look good. This is the joy of carburetors.....

 

[ericcarr]

The motor is pretty hard to start and just today when I had it running it stalled out a couple times. I didn’t notice any blue smoke but I think it was a tad bit black..

 

[ericcarr]

with the new carb, I would bet that its running rich. Likely you will need to rejet, chances of it having the "perfect" tune on it out of the box is slim to none. Its gonna be a bit of trial and error, but you will get it there.

Rejet, run it, check plugs, rinse, repeat until the engine is happy and the plugs look good. This is the joy of carburetors.....

Ok so I guess I should pull the jets, where exactly are they located at in holleys?

 

[Scott06]

Its running rich, was this just idling ?

first verify choke is opening properly, suspect it is closed too long.

did you adjust the idle mixture? Double pumpers may have idle mixture adjustment on all four bores. jetting is also probably needed but im assuming you weren’t running it fast enough to really get into the primaries just on the hose

 

[ScottinAZ]

Ok so I guess I should pull the jets, where exactly are they located at in holleys?

jets are inside the float bowls attached to the metering blocks. They have reusable float bowl gaskets available for these carbs. This is the real benefit of a Holley, you dont have to disassemble the whole carb to change jets.

Of course start with the mixture screws, but I have a feeling that you are pretty far off in the tune, hence why I stated jets.

 

[ericcarr]

Its running rich, was this just idling ?

first verify choke is opening properly, suspect it is closed too long.

did you adjust the idle mixture? Double pumpers may have idle mixture adjustment on all four bores. jetting is also probably needed but im assuming you weren’t running it fast enough to really get into the primaries just on the hose

I’ve neglected to add that the guy that I bought this carburetor off of had it on his boat for 10 hours which was a 454 also, Not sure if that matters or not

 

[ericcarr]

with the new carb, I would bet that its running rich. Likely you will need to rejet, chances of it having the "perfect" tune on it out of the box is slim to none. Its gonna be a bit of trial and error, but you will get it there.

Rejet, run it, check plugs, rinse, repeat until the engine is happy and the plugs look good. This is the joy of carburetors.....

I neglected to add that I did not buy this carburetor brand-new, I bought it off of a guy that put approximately 10 hours of run time on it which he had it on a 454 also

 

[Scott06]

I’ve neglected to add that the guy that I bought this carburetor off of had it on his boat for 10 hours which was a 454 also, Not sure if that matters or not

verify the bowl isnt overflowing fuel down bores at idle due to high fuel pressure or bad needle seat and float.
make sure choke is fully opening, try tweaking idle mix screws

ay have been on a 454 but not sure what was done to it In terms of jetting power valve etc. likely was on a car with typical hop up mods? may have been done to work with low vacum and big camshaft. If you just dropped it on At this point would get the it set back to factory jetting and settings and start from there as most performance carbs will work ok out of the box.

by the time you strip this down and undo any mods PO did, you are better off just rebuilding your Q jet correctly.

 

[ScottinAZ]

I neglected to add that I did not buy this carburetor brand-new, I bought it off of a guy that put approximately 10 hours of run time on it which he had it on a 454 also

just because it was on a similar engine does not mean its tuned for YOUR application. Carbs can be VERY finicky about seemingly minor things. It should be fairly close, or it wouldnt run at all, but its gonna take a bit of tinkering to get it dialed in right for YOUR engine.

 

[Lou C]

A few years back I thought my QJet was too old to be rebuilt properly so I tried replacing it with a Holley 4160 that was jetted for the 4.3. Well had the same problem as the OP here. Rich running fouled plugs & calls to Holley tech support I tried everything they suggested. Finally in disgust I pulled the QJet off a shelf in the garage & carefully rebuilt it, to my surprise it was 100% better! I tweaked it a bit more with a better quality accel pump from Cliffs High Performance and it’s great now. I did take that Holley completely apart and later found the likely culprit.
Do not run an auto carb on a marine engine!

 

[Rick Stephens]

As I said before before, don't run an auto carb on a boat. A road vehicle operates totally different. Loads on a car or truck are infinitely variable and most of the time run at far less than full torque output. A boat runs full torque 100% of the time. The design and calibration priorities are completely different.

To calibrate any carburetor you need that carburetor's manual and enough study to understand it and apply the information to your motor. Then methodically start with idle circuits and work way through transfer ports and on up to main jets, needles, power valve and what have you. All depending on model and manufacturer of carb and application.

 

[JASinIL2006]

As I said before before, don't run an auto carb on a boat. A road vehicle operates totally different. Loads on a car or truck are infinitely variable and most of the time run at far less than full torque output. A boat runs full torque 100% of the time. The design and calibration priorities are completely different.

To calibrate any carburetor you need that carburetor's manual and enough study to understand it and apply the information to your motor. Then methodically start with idle circuits and work way through transfer ports and on up to main jets, needles, power valve and what have you. All depending on model and manufacturer of carb and application.

Your post brought to mind this old thread that discussed the problems with running an auto carb in a boat:

whats the diff between marine carb and auto carb?

i was just wondering if there is much difence in an marine and auto carb . im wanting to get a new one . or if anyone knows a good place to get one at a cheap price the one i have now is a rochester 2bbl # 173 7028188 thanks for the help

forums.iboats.com

 

[ericcarr]

Okay quick question, I just got done pulling the bowls off of the carb and I noticed that on the front bowl they had jets 65 stamped on them and on the rear bowl them two jets had 71 stamped on them. Is it uncommon to have two different size jets in a carburetor?

 

[Scott06]

Okay quick question, I just got done pulling the bowls off of the carb and I noticed that on the front bowl they had jets 65 stamped on them and on the rear bowl them two jets had 71 stamped on them. Is it uncommon to have two different size jets in a carburetor?

Not uncommon at all, its typical primaries and secondaries have different jetting.

 

[ericcarr]

Am I correct to assume that the primaries are for the idle and secondaries are for throttle?

 

[Lou C]

Remember the front bowl is the primary system & the rear bowl is the secondary system. In order to answer this question you need to know the model # and the build # stamped on the air horn. Then look up the specs on Holley.com

 

[Scott06]

Am I correct to assume that the primaries are for the idle and secondaries are for throttle?

No the idle circut is for idle Through about 1800 rpm.
Primary and secondary are for when throttle is open primaries open first then as you continue on to full throttle secondaries open.
Do your self a big favor and get the technical manual for this carb read up on how it works. Or buy a holley carb tuning book.

idle mix screws control the mixture at idle i think they are on sides of metering block. Start there before adjusting jetting. You haven’t even had the engine running fast enough to touch the primaries let alone secondaries

also make sure transition circut holes are not uncovered at idle that will dump fuel in

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