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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10751
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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I have a 4.3 mercruiser with thunderbolt IV ignition system.

Customer Question

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: 12 days ago.
Category: Boat
Customer: replied 12 days 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
Expert:  Tan replied 12 days ago.

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

Expert:  Tan replied 12 days 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.

Customer: replied 12 days ago.
I have checked the ignition timing and fuel. I replaced the fuel filter and even bought a new carb.
Expert:  Tan replied 12 days ago.

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

Customer: replied 12 days ago.
The cap on the distributor have two sets of number one goes clockwise the other counterclockwise.
Customer: replied 12 days ago.
No it has an electric fuel pump. You thinking pull fuel from clean fuel can?
Expert:  Tan replied 12 days 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.

Customer: replied 12 days ago.
Even though it fires up everytime and idles with no problem
Expert:  Tan replied 12 days 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..

Customer: replied 12 days ago.
Ok I will try that. What is the possibility of the ignition amplifier being bad?
Customer: replied 12 days ago.
What if the advance timing isn't advancing
Expert:  Tan replied 12 days ago.

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

Expert:  Tan replied 12 days ago.

That would mean a bad amplifier.

Customer: replied 12 days ago.
Can throttle up to check
Customer: replied 12 days ago.
Not able to throttle up
Expert:  Tan replied 12 days 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.

Customer: replied 12 days ago.
I have the amp serial
Customer: replied 12 days ago.
Here is the boat serial
Expert:  Tan replied 12 days ago.

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

Customer: replied 12 days 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
Expert:  Tan replied 12 days ago.

Ok keep me posted.

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

Customer: replied 12 days ago.
Expert:  Tan replied 12 days 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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