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Old 12-02-2020, 09:25 AM   #1
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Default Backfiring and spluttering

Hi Guys,

First time posting after many weeks of trying to fix my boat and have decided i need help.

Motor Mercruiser Thunderbolt 5.0 Carby

Issues: Backfiring when reving it and under any load

Boat starts up perfectly idles like a dream with no miss or troubles no smoke white or black however as soon as its reved up or put under any load it starts miss and backfore out carby. however once it gets passed this point to top rpm runs fine again ����

List of items done this far.

New dizzy Cap
New Rotor
New Leads
New Plugs
New Coil
Carb Rebuilt
Set timing- checked timing is a advancing when revs increase
New Fuel Filter and Water Seperater

Not so sure where to go from here and seems like all i do is throw money at it and up up with a spare set of parts as nothing has resolved the issue so far. Thanks in advance for any help. Cheers Josh.
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Old 12-02-2020, 12:10 PM   #2
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Did you check the timing after replacing the cap, rotor, etc.?
If so, did you ground the purple lead when setting the base timing?

Which carb are you running? 2bbl Mercarb?
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Old 12-02-2020, 12:49 PM   #3
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Welcome aboard!

What ignition system do you have Thunderbolt IV or V? If V then did ground the purple lead as JP asked to set base timing? Did you check for signs of water in the fuel? Dump contents of spin on filter into a glass jar and let settle, any water will be at the bottom.
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Old 12-02-2020, 04:29 PM   #4
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I agree that your problem is likely caused by timing.
Take a look at your spark plugs. Mercruiser manual has good discussion on plug analysis on page 1C-1.
There is a chance that your carb is jetted incorrectly, or, has a bad accel pump.
I bought a rebuilt carb from a local mechanic with a good reputation. I had to return the carb twice before he got it right. Point being a rebuilt carb is not always good and don't assume it is good.
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Old 12-05-2020, 01:43 AM   #5
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Hi Guys firstly thanks for the reply's

So its a Thunderbolt 5

Am aware of earthing out white and purple wire to set base timing and thats the proceedure i have followed.

I now have 8 degrees BTDC at idle when not earthed out, idle is 650rpm and it will only rev up to about 2500 Rpm now and timing advances to approximately 15-18 degrees at that RPM.

I have replaced the ignition sensor module today and still the same problem, again starts up first hit idles perfectly, but now wont rev past 3000rpm and coughs and backfires out carb with reving previously was reving right the way to 4500 or so but with the same symptoms as described.
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Old 12-05-2020, 04:04 AM   #6
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also 2 barrel carb, and have replaced fuel filter water seperater, had quite literally maybe 5 drops of water settle to bottom when put in a clear container
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Old 12-05-2020, 12:43 PM   #7
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Take a look at the spark control graph in section 4 of the manual. At 2500 rpm your timing should at least be 25 degrees. Some manual instructions that should help: testing thunderbolt V ignition (1C-9), ignition system specifications (4B-4), Timing discussion (4B-5).
I suspect either wiring or ignition system component. ICM is suspect if all checks look good.
It's worthwhile to double check plug wires are tight and routed properly.
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Old 12-05-2020, 01:44 PM   #8
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Verify the plug wires are in the proper firing order.

Did this problem exist prior to changing the distributor cap? I have heard of bad Mercruiser caps. Has this engine ever ran good for you? Have you checked the fuel pressure?
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Old 12-05-2020, 07:42 PM   #9
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Will Check timing again today and compare with graph for advance I think i have made a mistake in my previous post timing advance was @ 15-18 degrees at 1800 Rpm I checked that against the graph and seemed right i then priceeded to rev engine to 2500 with what seemed to be worsening conditions so stopped at that stage however will tripple check again today.

Problem started after boat sat for a while in my shed unused for maybe 12 months, had never happened previously and i have owned boat for 4 years, only started changing parts as a process of elimination to resolve the problem.

will also confirm all plug wires are tight and in correct firing order, im sure i have them right as problem was there prior to any changes and run fine previously + idles with out any missing again i will conform this is correct however.

appart from that im at a complete loss where else to go I read about the following that may be a problem
Interrupt switch
Fuel Float Levels
Vacuum pressure and leaks
Fuel pressure

this will be my list of items to tick off the list next, seems im just throwing money away continuously replacing parts that dont resolve the issue, i guess on the bright side i now have spare parts for when they actually go bad !!! Also just had to do a new starter motor yesterday grrrr it hasent yet but this boat is starting to get to me !!
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Old 12-05-2020, 10:17 PM   #10
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Being that the boat sat the long the fuel may be bad or the pickup in the tank clogged. What year is the engine? 1998 or newer should have an electric fuel pump. Again I would verify pressure is 5-7 psi.
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Old 12-07-2020, 04:05 AM   #11
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Did you put a vacuum gauge on it? I’d give that a go before replacing any more parts. Could be something as simple as the carb gasket leaking vacuum at higher rpm’s. Just my 2 cents, good luck.
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Old 12-08-2020, 01:56 PM   #12
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Agreed. If all that checks out, I’d verify the float level adjustment in the carb.
OP mentioned it was rebuilt. Also verify which hole the accelerator pump rod is in.

Inspect the bowl for any water drops as well.
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Old 01-10-2021, 05:08 PM   #13
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This a long shot but it happened to me and drove me crazy until I found the problem. Just like you, my 4.3 merc would pop and backfire when under full throttle. Ran beautiful at idle or mid throttle.
The problem turned out to be a checkvalve that was located on top of the tank in the hose connector was tight and would not allow sufficient gas flow under high loads. I simply loosened the spring on the ball check and been running great ever since.
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