Mercruiser 188 backfiring through the carb ONLY going forward

[siavoshdana]

I have a strange problem here on a late 70s Mercruiser 188 with a Ford 302 on it.

- I have recently replaced the distributor to a HEI distributor. New plugs, new wires.

- She starts right up, idles smoothly, revs up to 5000 no load

- When I put in gear GOING FORWARD, she has no problems, no stalling, no backfiring. Slowly give gas up to ~1500rpm and she has no issues. As soon as I try to go above 1500, she starts backfiring into the carb and as soon as I go back to 1500rpm she calms down.

- I put in gear GOING BACKWARDS, she has no issues and revs up to 5000.

here are my humble thoughts:

- If I had a timing issue, it would be there load or no load, forward or backward
- If fuel mixture was too lean, i should get the same issue going backwards.

I'd really appreciate it if you could let me know your thoughts.

 

[Scott Danforth]

welcome aboard.

hope the distributor is a marine unit

you have a fuel issue

not the same loading forward as reverse.

 

[siavoshdana]

Hey Scott,

Thank you! I'm very new to this world. I'm waiting on a marine distributor, but currently it's an automotive dist.

What kind of fuel issue are we talking about? Carb needs to be cleaned? Fuel filter needs to be changed?

I am very worried that I might have a camshaft problem on one of my cylinders that causes the intake valve not to seal properly and therefore the backfire backwards through the manifold.

I am going to go out and hook it up to a fresh pit of gas instead of the tanks. what do you think?

Mind you that the boat has been sitting for about 4 years before I got it.

Really appreciate your response.

Cheers,

 

[Bondo]

- If fuel mixture was too lean, i should get the same issue going backwards.

Ayuh,...... Welcome Aboard,...... Yer thinkin' is off,... The load is much greater in forward,.....

It's goin' Lean,.... Probably the carb, but the entire fuel delivery systems is suspect,.....

 

[Scott Danforth]

. I'm waiting on a marine distributor, but currently it's an automotive dist.

Thats an explosion waiting to happen

What kind of fuel issue are we talking about? Carb needs to be cleaned? Fuel filter needs to be changed?

your running lean, start with the filters and progress to a carb rebuild

I am very worried that I might have a camshaft problem on one of my cylinders that causes the intake valve not to seal properly and therefore the backfire backwards through the manifold.

valves not opening is a camshaft or rocker issue, valves not closing is a valve or spring issue. compression test will tell you where to start looking, low compression on a hole then you look to a leak-down test to see the kind of issue.

two most common reasons of backfiring are running lean and improper timing

 

[Rick Stephens]

Steps to take:

1) Install marine distributor and set timing

2) Compression check

3) Rebuild carburetor - be especially diligent to get every passageway perfectly clean and open. Run wire or fishing line through the passages. Be particularly anal about float height.

 

[alldodge]

With the boat in the water and clear of anything, disconnect the shift interrupter and try to accelerate

Reconnect after test, it needs to be there to be able to shift into neutral

 

[siavoshdana]

Update:

I had my firing order set on 302 firing order: 1-5-4-2-6-3-7-8 and as I said, it was idling and revving with no load.

I read on another forum that all marine 302s have 351w firing order which is 1-3-7-2-5-6-4-8. Put that order in and she sounded different.

Took her out and problem was solved. Was able to plane to 30mph.

Ran, out of gas and had to switch my tanks. The moment I switched and started, the engine overheated. (i had removed the thermostat to make sure the impeller was working fine)

When running, the water intake hose is cold, the top of the thermostat housing is cold too but the exhaust manifolds are extremely hot as well as the risers.

I dont wanna blow the engine so I stopped running it, but I cant make sense of why it happened after I switched my gas tanks.

One more thing that I recall is that I was planing around 130 degrees and I hit the power trim switch to lower my driver a little bit and as soon as I touched the switch my temp reading jumped to 160, then I switched tanks, started and tried revving it and it sneezed in the carb, and with that one sneeze the gauge jumped to 200 (literally jumping).

please help a newbie recruit out

 

[Scott Danforth]

you need the thermostat in the motor to control the flow of water thru the block. without it, the water simply goes out the exhaust and the water in the block doesnt get pushed out by incoming water.

if you didnt replace the impeller then do so.

 

[alldodge]

You have something that is clogged up. The Ford open cooling routing is kind of weird IMO but does work. Look at the gasket between elbow and manifold