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BBC wont idle down below 1500 then stumbles backfires

D

ddabal

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"I just put a rebuilt 454 gen

"I just put a rebuilt 454 gen v motor in my formula.
I bought it used, it came with all the paperwork only 1 season on the motor. it has a dui ignition. the problem is I cant get to motor to idle down below 1500 or so rpms. turning the idle adjustment screw out anymore and the motor stumbles then stops, backfiring once or twice through the carb before coming to a complete stop. wires are not crossed. cant get the idle down enough to do a proper timing but it should be close enough as is.
has me stumped?"
 
"Ayuh,...
Sounds like the Idl

"Ayuh,...
Sounds like the Idle Circuit in the carb is Plugged....
Rebuild or replace the Carb..."
 
"Hi
guys thanks for the input

"Hi
guys thanks for the input, I dont think its the carb as different carbs yielded the same results.
the timing I'm not sre about because I cant get the idle low enough to check it. I guess there could be a leak somwhere in the manifold. as the boat is 3 hrs away I didnt have time to check it. I'm bringing a vacum gauge there this weekend perhpas that will shed some light. (I hope)any other ideas would be appreciated.
Dennis"
 
Recheck your distributor to be

Recheck your distributor to be sure you're not off a tooth. This would definitely cause the symptoms you have. How easy or hard does it start?
 
"the distributer being a tooth

"the distributer being a tooth off would not cause this problem with with your DUI (davis unified ingnition) do boats have egr valves ? valve stuck open, (another source for a vacumn leak).

what sort of intake/carb combo are you running? i once had a BBF that did something simular, i had the wrong carb base gasket on it (wrong "edelbrock gasket",edelbrock carb/edelbrock intake."
 
""the distributer being a

""the distributer being a tooth off would not cause this problem with with your DUI (davis unified ingnition) do boats have egr valves ? valve stuck open, (another source for a vacumn leak)."

You can tell him without seeing it or even knowing what year or model it is what kind of ignition system he has, yet you don't know if boats have EGRs?

People with half the information are more dangerous than those with none.

Recheck your distributor please."
 
"rick,

his post clearly sa

"rick,

his post clearly says he has a DUI ing.
with that setup (GM HEI) or most other ingnition systems, you in fact can drop the distributer in, in ANY position,however you must wire it acordingly. with the direction of rotation (dist.) and firing order (18436572 for a BBC). then of corse ingnition timing would need to be set (around 10 degrees BTDC)

if his timing is on the money, then his dist. position is fine.

my boat does not have a EGR (73 LM318), i suspect his does not either,i'm not sure if older boats had EGR's or not, but i assume the later engines do.

a stuck partially open EGR is simply a vacumn leak."
 
"My first suspicion would be v

"My first suspicion would be vacuum and I missed the DUI line completely, but he can't check his timing and only thinks it is close. You're right, if his timing is on the money, the distributor is right, but we don't know that it is. My point was to be sure it is right. This means wires to rotor position, not necessarily physical location of the distributor housing. And no, he does not have an EGR.

I recently installed a Mercruiser reman 454. It gave me fits, wouldn't idle down, backfired upon acceleration, etc. Troubleshooting was complicated as it was MPI and nothing checked out faulty, either mechanically, or electronically.

Bottom line, Mercruiser had the cam off one tooth. What a pain."
 
"rick,

i agree it seems li

"rick,

i agree it seems like a pretty strong vacumn leak, if he swaped intake manifolds-that would also be worth a look (manifold to cylinder seal)it could be leaking on the underside where you wouldnt be able to see it.

even with the timing off the engine should respond to backing off the idle speed.

(rick)
on your mpi, i think those are the same as a vortec system (sfi), you can check your dist. postion easily with a scan tool. the specific parameter heading fails me, but it should be 0 +/- 2 degrees,this is the differance in position in the crankshaft position sensor and the camshaft position sensor.(cam sensor is in the distributer housing)this will usually set a code if far off (say 8-10 degress)

i think i kinda missed your point, a cam being off a tooth (cam to crank) would surley be a problem-esp with a modern engine management system. did mercruiser cover the costs to rectify this? i like Jasper's warranty (parts and labor) "
 
"Yes, they did cover the event

"Yes, they did cover the eventual repair but I never recovered all the lost time in troubleshooting, or the two 3 1/3 hr. round trips I made to correct it."
 
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