1989 mercury 90hp keeps backfiring

[66tracys]

I have a 89 90hp mercury
Keeps backfiring though exhust and can't get to idle correctly. I have rebuilt carbs ,coils were bad and changed them and just replace fuel pump
And also went and bought new fuel lines . Still same thing. Compression is 125psi. New Plugs and check plug wires and there were good

 

[racerone]

Flywheel key checked ?----Serial # of motor ?------Low speed mixture needles may need to be turned out.

 

[66tracys]

I haven't checked the flywheel key
Serial# ob372404 . I have shut down the carbs (3) and turned them a little at a time really no difference
I have checked for leaks on carbs . With carb cleaner
No difference in running. Spayed into carburetors just died. Could it be bad gas it's a week old ?? It's from am/pm super unleaded

 

[ondarvr]

Put your hand over each carb intake while it's running and identify which carb is causing the problem, then turn the low speed screw more than just a little, give it a full turn or two, see what happens.

Also check to make sure each carb butterly closes completely.

If it's an electrical issue this won't have an affect on it.

 

[Faztbullet]

Backfire thru exhaust is ignition problem...pops thru carbs are fuel issues. Check key as suggested.

 

[roscoe]

Backfire thru exhaust is ignition problem...pops thru carbs are fuel issues. Check key as suggested.

This.
A sheared key will throw off timing of ignition.

 

[ondarvr]

I just went through this with mine, well it was a while ago now. it was running lean on the middle cylinder, and at idle and just above idle it would appear to backfire, it was actually a lean sneeze, but would blow exhaust out through prop sometimes and look like it was backfiring. Cleaning the carb, adjusting the linkage, then dialing in the low speed screw got it running very well.

​That's why I said if what I suggested wouldn't work if it was electrical.

 

[66tracys]

Ok tried running the test out of the 3 carbs .when I put my hand over the carbs only the middle one was pulling air in .the other two could not feel no air pulling my hand. . Any ideas

 

[skey]

Who rebuilt the carbs? may not be correct yet. set air fuel mixture screw to 2 1/2 turns out from bottoming. You said changing them didn't make a difference which is why I question if the rebuild is right.

 

[66tracys]

Ok what I did is unhook the carb from linkage and tested each one they are pulling air when iam lift the thottle. So what I did is set each one until sounds good. Or decent. Still have a miss ? But when I put it in gear and give it gas I still have a backfire though the exhust not as bad as before . Would that still be the carbs or ignition . I did the rebuild twice because I didn't think I did it right. Cleaned and left over night and then I put them back together. And just did the fuel pump to. But that doesn't mean I didnt do the carbs correctly

 

[ondarvr]

Ok tried running the test out of the 3 carbs .when I put my hand over the carbs only the middle one was pulling air in .the other two could not feel no air pulling my hand. . Any ideas

All three carbs should have equal vacuum at all times.

​Did the backfiring stop when covered one of the carbs?

 

[66tracys]

The center carb had all of the vacuum. When cover that one then it stopped running. Top and bottom had no vacuum unless I gave it gas ? Can't figure this one out .

 

[ondarvr]

The center carb had all of the vacuum. When cover that one then it stopped running. Top and bottom had no vacuum unless I gave it gas ? Can't figure this one out .

​If that's the case then the carbs aren't synched, you need to adjust it so they all close completely at idle. The middle carb is open slightly at idle, this can make it run lean.

​Then you need to reset the idle RPM

 

[66tracys]

Ok thanks for all of the advice
I'll give that try tomorrow.

 

[skey]

possibly bad reed valve or sticking reed valve