Backfiring

[jasonsmaxum]

1989 Mercruiser 454 7.4l new carburetor new electronic ignition new fuel pump shes backfirng and smoking like a chimney tried adjusting carburetor?? I dont know where what to do next any ideas??

 

[jasonsmaxum]

1989 mercruiser 454 7.4 l new carburetor new electronic ignition new fuel pump. after idling around lake for awhile i go to give her gas she starts backfirng and she smokes like a chimney i adjusted carburetor right?? Idk.. Do t know where or what to do next any ideas?

 

[Lowlysubaruguy]

Was it doing this before all those parts were installed. What color is the smoke black is to much fuel blue is oil grey white is coolant or water.

cap rotor arching ignition timing way off or plug wires crossed are the simplest and not uncommon

check fuel volume pressure and condition aged fuel or water contamination another.

does it do this at idle or only under load. Lean missfire will cause backfire but not smoke.
from there you get into deeper issues

 

[jasonsmaxum]

Was it doing this before all those parts were installed. What color is the smoke black is to much fuel blue is oil grey white is coolant or water.

cap rotor arching ignition timing way off or plug wires crossed are the simplest and not uncommon

check fuel volume pressure and condition aged fuel or water contamination another.

does it do this at idle or only under load. Lean missfire will cause backfire but not smoke.
from there you get into deeper issues

Yes thats what started me getting new carburetor cause of backfiring figured a few more. Bucks just get new 1 then same issue after new 1 so installed electronic ignition still same issue so moved onto fuel pump. Was black smoke so did more adjustments on carb now grey white. went thru a couple tanks of gas since new installations set timing with light cap rotor arching ill ask my mechanic on that 1on checking fuel volume pressure ill pass that also. smokes under idle and load backfires only under loaf oh fouled out 2 sets of spark plugs also while going as slow as i can for a period of time she cough out a big slick of carbon i would call it

 

[jhande]

I would perform a compression test.

 

[alldodge]

Have to start with the basics. Lots of smoke could indicate the carb is flooding. Check your timing and fuel pressure

 

[kenny nunez]

Is the new carburetor the correct marine model for your engine?
What was wrong with the original carburetor?
Back firing through the carburetor is a sign of fouled spark plugs and as A D said from a over rich mixture.

 

[Scott Danforth]

Why did you change carbs?

 

[jasonsmaxum]

Why did you change carbs?

Backfiring and no top end figured $150 or so to get a new one having it rebuilt

 

[jasonsmaxum]

I would perform a compression test.

Results of compression test cylinders 1 100 psi 3 90psi 5 110psi. 7 105 psi cylinders 2 100psi 4 95 psi 6 95 psi 8 95 psi ok wet test results cylinders 1 100 psi 3 85psi 5 110 psi 7 110psi cyli ders 2 100psi then something happened i wasnt able to get compression i even tried going back to #1 cylinder dry test and same result no compression.. so whats your opinion on results? and why lost compression? A friend suggested battery getting low which i have a sence possible ty

 

[alldodge]

If the starter was not turning the motor over very fast, that would give low numbers. If your numbers are correct then you need a rebuild, need 100 just to run

 

[jhande]

then something happened i wasnt able to get compression i even tried going back to #1 cylinder dry test and same result no compression.. so whats your opinion on results? and why lost compression? A friend suggested battery getting low which i have a sence possible ty

The no compression second time around could be the gauge.
The results are on the low side but should be fine.
Since you had the problem with the original carb, good thing to test the rings and valves just to make sure all is good and delete those as a problem.
If your battery was getting low and the motor turning over slowly you should still get a reading.
Black exhaust smoke means burning to much fuel.
Gray exhaust smoke is caused by burning oil.
I would change the PCV valve, could be allowing oil to suck into motor.
Backfiring can be caused due to carb not properly adjust, something wrong with the ignition system and timing. Since you replaced the ignition system that can be ruled out. Check the timing.
What did the old spark plugs look like? I'll venture to say wet. But were they oil coated (black) or just wet looking (gas)?

I'm really not leaning towards a carb issue since 2 carbs equals same problem.

Personally my next steps would be: replace PCV valve, check timing, check for vacuum leaks.

 

[Bondo]

If the starter was not turning the motor over very fast, that would give low numbers. If your numbers are correct then you need a rebuild, need 100 just to run

Ayuh,...... I agree with this, but will add a long shot idea,.....
Do a leak down test to rule out seriously burnt valves as a possibility for the loss of compression,....
After all, a head job is alot less work than a full on motor rebuild,....

 

[jasonsmaxum]

The no compression second time around could be the gauge.
The results are on the low side but should be fine.
Since you had the problem with the original carb, good thing to test the rings and valves just to make sure all is good and delete those as a problem.
If your battery was getting low and the motor turning over slowly you should still get a reading.
Black exhaust smoke means burning to much fuel.
Gray exhaust smoke is caused by burning oil.
I would change the PCV valve, could be allowing oil to suck into motor.
Backfiring can be caused due to carb not properly adjust, something wrong with the ignition system and timing. Since you replaced the ignition system that can be ruled out. Check the timing.
What did the old spark plugs look like? I'll venture to say wet. But were they oil coated (black) or just wet looking (gas)?

I'm really not leaning towards a carb issue since 2 carbs equals same problem.

Personally my next steps would be: replace PCV valve, check timing, check for vacuum leaks.

Oil coated black

 

[tank1949]

Results of compression test cylinders 1 100 psi 3 90psi 5 110psi. 7 105 psi cylinders 2 100psi 4 95 psi 6 95 psi 8 95 psi ok wet test results cylinders 1 100 psi 3 85psi 5 110 psi 7 110psi cyli ders 2 100psi then something happened i wasnt able to get compression i even tried going back to #1 cylinder dry test and same result no compression.. so whats your opinion on results? and why lost compression? A friend suggested battery getting low which i have a sence possible ty

Those #s suck! But, before you pitch engine, use tester on another motor as a base line or comparison. I tried another compression tester after testing my motor's poor compression. Turns out that I had I had a loose schrader valve.

 

[jhande]

Oil coated black

Okay, compression reading aren't the best but before rebuilding the motor I would do as tank1949 suggest.

Okay, oil on spark plugs means oil is getting into the cylinders, we just don't know how much or from where yet. Could be sucked in through the PCV valve or cylinder heads (valves, valve seals or head gasket). Because the wet and dry compression test only showed a slight difference in a couple of cylinders the rings aren't total garbage yet but are worn. Getting a leak down test will tell you the condition of the valves, head and/or head gasket in the other cylinders. If you do lose pressure then a rebuild is in order.

 

[Scott Danforth]

Backfiring and no top end figured $150 or so to get a new one having it rebuilt

$150 is not a new carb. at best its a reman.