vibration around 2600to 2800 on acceleration and deceleration

[dolphin1]

Hey guys this one has me stumped 98 350 4barrel serial is OK 482365I added the 4BRL 4yrs ago. The boat ran fine then a weak later I notice a vibration comming up on plane andoff plane it starts around 2600 and after 3000 rpms runs great WOT 4900. 15x 17 prop new OW series drive and transome housing.So far I've changed gas, fuel filter, checked floats,new plugs, rotor, cap, plug wires,ecm,dist sensor and new prop. Made 2 runs on new plugs the odd thing is 1,2,7 and 8 which outsidE cylinders are very ligt tan almost white but 3,5,4 and 6 are a little rich. It's idling a little rough, fuel mixture when in do not stall the motor. The other odd thing is in neutral I can bring the rpms up to 1800 and secound or two later It will increase to 3100 by itself. comming back to idle it will stop at 850 for a sec or two and then to 650.

 

[300sflyer]

Re: vibration around 2600to 2800 on acceleration and deceleration

Check for a vaccume leak, perhaps an intake manifold gasket.

 

[joewithaboat]

Re: vibration around 2600to 2800 on acceleration and deceleration

Sounds like it could be carb. problem. Leaking from the welch plugs??

http://www.mre-books.com/quadrajet/carbrebuild.html

Vacuum leak makes sense as well....

 

[dolphin1]

Re: vibration around 2600to 2800 on acceleration and deceleration

Hey guys,
Thanks for the input the carb is a edlebrock 1409. Is their a way to check besides pulling the intake. I also have a 2150 ceira with a 350 doing the same thing. I already changed intake gaskets on it and it did not help.it also has a 1409 on it.Carbs are only 4 to 5 years old. I sprayed carb cleaner around the base gasket to see if their was a leak with no change in idle.

 

[300sflyer]

Re: vibration around 2600to 2800 on acceleration and deceleration

Hey guys,
Thanks for the input the carb is a edlebrock 1409. Is their a way to check besides pulling the intake. I also have a 2150 ceira with a 350 doing the same thing. I already changed intake gaskets on it and it did not help.it also has a 1409 on it.Carbs are only 4 to 5 years old. I sprayed carb cleaner around the base gasket to see if their was a leak with no change in idle.

Did you make sure both the intake and head gasket surfaces were perfectly clean, before putting it back together? Also, did you use a "marine" intake gasket and not an automotive one? If not, either of these could be the problem.

 

[John_S]

Re: vibration around 2600to 2800 on acceleration and deceleration

I'd check for vacuum leak, but also look down the throat of the carb when idling. If you are seeing gas dribbling, that is probably causing the issue. It is pooling up under the carb and those cylinders intake runners are the closest.

Dribbling could be:

- Too much fuel pressure.
- If mech fuel pump, failure and gas coming in vent line.
- Stuck or sticking inlet needle valve.
- Bad float or float level.

As far as the mid-rpm issue, that may be carb to. That range should be where you are transitioning between power/cruise on the primary side. You can remove the primary rods and springs and inspect, without a carb teardown. Should also be able to see primary jets and see if there is crud in them. Did you check the gas in the filters? Any water or other crap?