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I have a 4.3 mercruiser with thunderbolt IV ignition system.

Customer Question
When it idles iy JA: Sometimes...
I have a 4.3 mercruiser with thunderbolt IV ignition system. When it idles iy
JA: Sometimes things that you think will be really complicated end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Tell me a bit more about what's going on so the Marine Mechanic can help you best.
Customer: It is smooth with no miss but when you put a load or try to accelerate with no load it backfires through carb or just bogs down
JA: Is there anything else important you think the Marine Mechanic should know?
Customer: It has a new motor we just put in. New cap rotor plugs wires and carb. Also replaced all the exhaust manifolds.
Submitted: 6 months ago.Category: Boat
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Customer reply replied 6 months ago
I meant to say it will idle smooth. Fire right up every time. I was wondering if the ignition amplifier is bad. I already replaced timing sensor
Answered in 1 hour by:
5/15/2017
Marine Mechanic: Tan, MerCruiser Certified Technician replied 6 months ago
Tan
Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11,091
Experience: 40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
Verified

Hello, I'm Tan. Welcome to JustAnswer. I'm reviewing your question and will reply momentarily.

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Marine Mechanic: Tan, MerCruiser Certified Technician replied 6 months ago

Have you checked the ignition timing?

Have you checked the fuel filter for water?

Timing and fuel restriction or water in the fuel will cause these symptoms.

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Customer reply replied 6 months ago
I have checked the ignition timing and fuel. I replaced the fuel filter and even bought a new carb.
Marine Mechanic: Tan, MerCruiser Certified Technician replied 6 months ago

Have you tried running the engine on a different fuel source?

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Customer reply replied 6 months ago
The cap on the distributor have two sets of number one goes clockwise the other counterclockwise.
Customer reply replied 6 months ago
No it has an electric fuel pump. You thinking pull fuel from clean fuel can?
Marine Mechanic: Tan, MerCruiser Certified Technician replied 6 months ago

If you had that wrong the engine would not start and run correctly.

We need to eliminate the boat side fuel system as a possible fuel restriction.

You could have a bad anti siphon valve, clogged pick up tube screen, collapsing fuel line or a clogged vent line to the tank.

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Customer reply replied 6 months ago
Even though it fires up everytime and idles with no problem
Marine Mechanic: Tan, MerCruiser Certified Technician replied 6 months ago

Then engine require very little fuel at idle but when you put it under a load it requires a greater volume of fuel. So if you have a fuel restriction it will start and idle just won't have any power when put under load. Backfiring is timing or fuel starvation or contaminated fuel. We have eliminated contaminated fuel filter and timing leaving fuel restriction..

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Customer reply replied 6 months ago
Ok I will try that. What is the possibility of the ignition amplifier being bad?
Customer reply replied 6 months ago
What if the advance timing isn't advancing
Marine Mechanic: Tan, MerCruiser Certified Technician replied 6 months ago

Fairly slim as the engine starts and runs. Have you checked the timing when engine is throttling up? Is the timing advancing?

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Marine Mechanic: Tan, MerCruiser Certified Technician replied 6 months ago

That would mean a bad amplifier.

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Customer reply replied 6 months ago
Can throttle up to check
Customer reply replied 6 months ago
Not able to throttle up
Marine Mechanic: Tan, MerCruiser Certified Technician replied 6 months ago

Ok it is possible the amp is bad if timing isn't advancing.

But I would check for a fuel restriction first. We need to eliminate that as a possibility before we replace the amp. Do you have an engine serial number? The amp may not be available anymore.

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Marine Mechanic: Tan, MerCruiser Certified Technician replied 6 months ago

I need the engine serial number. The number on the amp isn't any help. That is not a part number.

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Customer reply replied 6 months ago
I will have to try to get it tomorrow. Thanks for your help so far. I will try it all and see what happens
Marine Mechanic: Tan, MerCruiser Certified Technician replied 6 months ago

Ok keep me posted.

Have you verified the firing order. 1-6-5-4-3-2.

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Marine Mechanic: Tan, MerCruiser Certified Technician replied 6 months ago

Ok run the engine on an auxiliary fuel tank and let me know the engine serial number if that doesn't resolve the issue.

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Tan
Tan
Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11,091
11,091 Satisfied Customers
Experience: 40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.

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