Mercruiser 200 Sterndrive Backfires, Looses Power

I took on a restoration project of a 1986 ChrisCraft Cavalier with Mercruiser 200 Powerplant and Alpha Drive. This is the GM 305 engine with the 2BBL carb on top.

The boat sat idle with just 500 hours on it for nearly 18 years after my father in law passed on. He purchased it new.

Two years ago, the restore was complete and we ran it on a couple of test outings without any major issues. It sat idle for another two years as we are in So Cal and the drought has dried up most of our opportunities of using it.

This summer, we took the boat to Lake Tahoe, and after a few altitude adjustments and a new prop, had it running just like in the days when my FIL used to take it there.

However, after about an hour of moderate running, it backfires, looses power. I pull the throttle back to idle, and everything seems fine. This progressively gets worse (happens more frequently, and at progressively lower RPM) as we attempt to navigate half the lake back to the marina. By the time we get to the marina, I can barely keep it at 1200 RPM for more than 30 seconds. It does not stall, but if I turn if off, will start immediately (fearing vapor lock).

It felt like fuel starvation, and after asking many about the problem, the consensus was replace the 2 year old fuel pump, fuel filter, water separator. This was done, and we tried again. The boat would pull a skier (me), my daughter on a tube, and have no problem getting up on plane, just like old times. By this time we are two full tanks of fresh gas through the system, always using a preservative.

This weekend, after the new pump was installed, it happened again. Same exact symptom. Parked the boat, went out the next morning and repeated the same experience.

One other note: The carb was replaced this season as the original Merc carb was leaking badly and need a rebuild. The replacement was not a Merc carb, more like a stock automotive carb. It was supplied by a local marine shop.

I'm looking for any hints as to what may be the problem, the Carb???. The Merc carb is now getting a rebuild with high altitude jetting, but it will be a couple of weeks before we return to Lake Tahoe to test it.

Everything else in the fuel system was replaced two years ago, and the tank was removed and cleaned.

Any intelligence on this matter would be appreciated!

When this issue comes up,look inside of carb for fuel as asst, advances throttle forward & back. Has fuel pressure been checked? Is your fuel tank vented properly?

johnnygjr said:

When this issue comes up,look inside of carb for fuel as asst, advances throttle forward & back. Has fuel pressure been checked? Is your fuel tank vented properly?

Click to expand...

Thanks for the reply.

The vent line is nearly a vertical run from the tank to just next to the filler tube. It was replaced when the tank was cleanwd. I do know that when I fill the tank, it burps generously through the vent. Well check it on our next visit.

After all these years, possibly a sticky anti siphon... ALSO!!! since this is a pre ethanol boat, replace all non metal fuel lines, and clean/replace the metal ones. When my 1969 boat got its first tank of ethanol laced fuel, it clean out all the old crap in the gas tank (1969 to 2007) and deposited it in my almost brand new engine's carb ( 10 hrs).

Sat for 18 years,2 years ago restored,sat again 2 years, Ethanol has been sitting in tank,lines,carb,pump, filter,fuel separator for quite a long time without use, Entire fuel system has been compromised again.Start at inside of tank at anti siphon valve/filter and flush,clean,or replace every component forward of tank all the way to carb, including carb, Pull plugs ,clean them also! Inspect ignition system cap,rotor,wires,points if it has them

johnnygjr said:

Sat for 18 years,2 years ago restored,sat again 2 years, Ethanol has been sitting in tank,lines,carb,pump, filter,fuel separator for quite a long time without use, Entire fuel system has been compromised again.Start at inside of tank at anti siphon valve/filter and flush,clean,or replace every component forward of tank all the way to carb, including carb, Pull plugs ,clean them also! Inspect ignition system cap,rotor,wires,points if it has them

Click to expand...

2 Years ago, everything in the fuel system was replaced. Fresh hoses everywhere. New fuel pump, new filters, new filler neck, vent hose. New Carb this summer. Tank was removed and cleaned (not gunked up, mostly sediment). Filters right now look pretty good. Failure happened even after a new water separator filter was installed. Boat does not surge, runs pretty well for over an hour before the problem repeats. Will start immediately if turned off.

Not sure there is an anti-siphon on this boat, but will check. Going on that theory, the next time this happens, I should be able to crack the fuel filler cap to relieve the vacuum and all should be well for another hour.... will put this on my list.

The anti siphon valve is an aluminum bodied check valve with a long barbed end for the gas line, it is where the soft gas line connects to the gas tank.

It is coast guard mandated so it must be there!!

They are cheap.............replace it!

you have to separate probable causes.

Is it fuel related?
Is it ignition related?
Is it temperature related?

Is it mechanically related?

Fuel related, use separate tank (small outboard type tank) and confirm no fuel delivery issues.
If you feel 100% confident then skip.

You must check fuel pressure. 4 years ago friends boat had fuel in the bypass line they did not know what it meant.
I told them to replace fuel pump, they did. One year later boat had fuel related issues. Had them buy a second new pump.
Boat has been fine ever since..........Moral. Just because you replaced it does not mean 100% of the time it is still good, Especially fuel pumps!!
Also when it dies if you advance throttle only does it squirt gas into carb (accelerator circuit)??? If so then gas is present. That does not mean the pump is working properly at higher rpm levels but is at low rpm levels. Need a gauge set up in line with steel line feeding carb and should be 4-8 psi at all times.

If you rule out fuel then next is ignition

You have a thunderbolt IV electronic ignition.

Internal resistor coil, timing module mounted on exhaust elbow, inside dist cap is a rotor assembly and sensor.

Is coil hot? Any signs of oil leaking? connections good?

Pull cap on dist, pull rotor and sensor. If original or older sensor the circuits will be visible on underneath of sensor. If heavily corroded REPLACE IT. Will fail anyway. New ones are fully epoxied so no exposure.

Timing modules dont typically fail but usually when they do it is issues above 2200-2400 rpms......not below as it only controls timing advance.

What is your running temp? should be around 150-160*


Mechanical, well you have not mentioned any weird sounds so that is something for a later discussion.

kghost said:

The anti siphon valve is an aluminum bodied check valve with a long barbed end for the gas line, it is where the soft gas line connects to the gas tank.

It is coast guard mandated so it must be there!!

They are cheap.............replace it!

you have to separate probable causes.

Is it fuel related?
Is it ignition related?
Is it temperature related?

Is it mechanically related?

Fuel related, use separate tank (small outboard type tank) and confirm no fuel delivery issues.
If you feel 100% confident then skip.

You must check fuel pressure. 4 years ago friends boat had fuel in the bypass line they did not know what it meant.
I told them to replace fuel pump, they did. One year later boat had fuel related issues. Had them buy a second new pump.
Boat has been fine ever since..........Moral. Just because you replaced it does not mean 100% of the time it is still good, Especially fuel pumps!!
Also when it dies if you advance throttle only does it squirt gas into carb (accelerator circuit)??? If so then gas is present. That does not mean the pump is working properly at higher rpm levels but is at low rpm levels. Need a gauge set up in line with steel line feeding carb and should be 4-8 psi at all times.

If you rule out fuel then next is ignition

You have a thunderbolt IV electronic ignition.

Internal resistor coil, timing module mounted on exhaust elbow, inside dist cap is a rotor assembly and sensor.

Is coil hot? Any signs of oil leaking? connections good?

Pull cap on dist, pull rotor and sensor. If original or older sensor the circuits will be visible on underneath of sensor. If heavily corroded REPLACE IT. Will fail anyway. New ones are fully epoxied so no exposure.

Timing modules dont typically fail but usually when they do it is issues above 2200-2400 rpms......not below as it only controls timing advance.

What is your running temp? should be around 150-160*


Mechanical, well you have not mentioned any weird sounds so that is something for a later discussion.

Click to expand...

• Will look for the anti-siphon. Don't recall it, but memory fails after inhaling too much hydrocarbon.
• Ignition was totally rebuilt (new sensor, rotor, points, condenser, cap, wires, coil...). It does eventually fail at lower RPMs (less than 1500), but takes some time to get there.
• Changing the pump did not affect the symptoms. Old pump was 2-year old, new one was less than two hours TIS.
• I'll add the fuel pressure test to my list.
• Accelerator pump - problem happens during constant use (hands off the throttle).
• Temp is in the 150 range. Solid and steady. Temp sender was replaced with new during restoration (oil pressure sender too)

What it feels like, is the carb bowl slowly gets sucked dry, engine burps, pull throttle back to idle, bowl fills again, good to go for about 30-40 seconds, progressively getting slower to refill the bowl.

Full disclosure: I am the family mechanic, having built several domestic and foreign motors, particularly the older non-computer variety. I do some hobby work in Top-Fuel Drag Racing, am working on a land speed record car running at Bonneville, and have worked in General Aviation on airplane motors. None of the motorheads in these groups have been able to offer any insight also...pretty much they suggest fuel delivery issues.

You cant have a sensor and points...

1986 should be electronic ignition. Thunderbolt IV

Only issue here is you say new coil....if you have a coil for points, that is wrong if you have thunderbolt and visa versa..



Well then if you are convinced its a fuel issue you most likely will need to look at the inside of carb. Sounds like needle is sticking closed...if you are going Lean.......

kghost said:

You cant have a sensor and points...

1986 should be electronic ignition. Thunderbolt IV

Only issue here is you say new coil....if you have a coil for points, that is wrong if you have thunderbolt and visa versa..



Well then if you are convinced its a fuel issue you most likely will need to look at the inside of carb. Sounds like needle is sticking closed...if you are going Lean.......

Click to expand...

Interesting...there is an ignition coil, and an electronics module on the exhaust manifold. My notes on the rebuild show the sensor in the disty was replaced. No points (had just answered a questions about an airplane magneto!)

The anti siphon valve is an aluminum bodied check valve with a since quite a while ago barbed end for the gas line, it is the place the delicate gas line associates with the gas tank.