Mercury 4cyl stumbles at hole shot and underway

[redgoat]

OK. I have a 54 year old Mercury I/O 120 hp, 153 cu.in L4, 2.5L Last year, it developed an issue with hesitation and stumble at increasing throttle--especially at hole shot. It starts quickly, idles just fine and has a perfect point dwell and perfect timing. I suspected old ethanol gas and also discovered a bad fuel pump. Replaced fuel pump, replaced all gas with fresh, ethanol-free 91 octane gas with a stabilizer and a lead substitute added. Also replaced my fuel filter and did a complete carburetor rebuild. Not too long ago, I replaced the ignition wires and spark plugs along with the points, condenser and coil. I should note that the distributor is the original from the factory. I am at a loss as to what to replace to correct the problem. Could it be something as simple as a bad condenser? Replace my plug wires and spark plugs again? Distributor cap? Distributor? I feel that the issue is ignition related and likely something simple but I have not figured out what it is. Also, I am the original owner of this '67 Sea Ray. Any suggestions?

 

[m1dn]

I have same engine and same issue - diagnosed down to Accelerator pump in carb, replacing next week.

You give it a throttle, it bogs 2/3 times, kick throttle back and forward again, repeat until it goes - basically fuel not pumping adequately by accelerator pump in carb.
I took mine apart, it's old inside and the spring+rubber sticks, didn't have parts to replace yet

 

[Bondo]

OK. I have a 54 year old Mercury I/O 120 hp, 153 cu.in L4, 2.5L Last year, it developed an issue with hesitation and stumble at increasing throttle--especially at hole shot. It starts quickly, idles just fine and has a perfect point dwell and perfect timing. I suspected old ethanol gas and also discovered a bad fuel pump. Replaced fuel pump, replaced all gas with fresh, ethanol-free 91 octane gas with a stabilizer and a lead substitute added. Also replaced my fuel filter and did a complete carburetor rebuild. Not too long ago, I replaced the ignition wires and spark plugs along with the points, condenser and coil. I should note that the distributor is the original from the factory. I am at a loss as to what to replace to correct the problem. Could it be something as simple as a bad condenser? Replace my plug wires and spark plugs again? Distributor cap? Distributor? I feel that the issue is ignition related and likely something simple but I have not figured out what it is. Also, I am the original owner of this '67 Sea Ray. Any suggestions?

Ayuh,..... Rebuild yer carb,.....

 

[Rick Stephens]

Ayuh,..... Rebuild yer carb,.....

What he said.

 

[redgoat]

I have same engine and same issue - diagnosed down to Accelerator pump in carb, replacing next week.

You give it a throttle, it bogs 2/3 times, kick throttle back and forward again, repeat until it goes - basically fuel not pumping adequately by accelerator pump in carb.
I took mine apart, it's old inside and the spring+rubber sticks, didn't have parts to replace yet

I like your suggestion. Where did you order your accelerator pump? I'd hate to buy another complete tune-up kit just to get the accelerator pump & spring.

 

[Eric 3.7LX Alpha 1]

I like your suggestion. Where did you order your accelerator pump? I'd hate to buy another complete tune-up kit just to get the accelerator pump & spring.

You mentioned you had the same issue last year, you changed the fuel pump, filter and rebuilt the carb. Did the issue go away after changing/fixing those things or are you still chasing this problem continuously since then?

 

[1960 Starflite]

I like your suggestion. Where did you order your accelerator pump? I'd hate to buy another complete tune-up kit just to get the accelerator pump & spring.

If you bought a complete kit, the pump should have been in it. You may have to adjust the cam plunger for stroke. Setting should be on instructions in box. Of course I haven't bought a kit for years so they might not sell a complete one now.

 

[redgoat]

You mentioned you had the same issue last year, you changed the fuel pump, filter and rebuilt the carb. Did the issue go away after changing/fixing those things or are you still chasing this problem continuously since then?

Still having the same issue. When I rebuilt the carb, I used a new, old stock quicksilver rebuild kit. Have the rubber on the accelerator pump changed over the years to last longer or could the one I used to replace have been too old to use. Also, can ethanol in gas affect the accelerator plunger? Also, I do not know of any adjustment for replacing the pump.

 

[redgoat]

Still having the same issue. When I rebuilt the carb, I used a new, old stock quicksilver rebuild kit. Have the rubber on the accelerator pump changed over the years to last longer or could the one I used to replace have been too old to use. Also, can ethanol in gas affect the accelerator plunger? Also, I do not know of any adjustment for replacing the pump.

Also, this might be significant. I have a Filt-O-Reg fuel pressure regulator device installed in front of the carb inlet that has been there for perhaps 20 years. Could the internal workings of this device be destroyed to the point of affecting my correct fuel delivery to the carb bowl and thus cause my hesitation?

 

[Eric 3.7LX Alpha 1]

Yes, it's possible. There are two tests in Mercruiser Service Manual #1 you can try. Pump volume and pump pressure test. (I have the page attached). The manual doesn't mention anything about an inline pressure regulator. What was the story behind it? Some pressure regulators have a tapping on it to accept a pressure gauge, but from what I can see from photos online that one doesn't.

 

[Scott06]

Still having the same issue. When I rebuilt the carb, I used a new, old stock quicksilver rebuild kit. Have the rubber on the accelerator pump changed over the years to last longer or could the one I used to replace have been too old to use. Also, can ethanol in gas affect the accelerator plunger? Also, I do not know of any adjustment for replacing the pump.

Yes ethanol can affect carb parts on really old items. If yhe kit was made in the last 20 years it will be made of ethanol tolerant materials. Mikes carb parts is a good source for individual parts like an accelerator pump

 

[redgoat]

Problem solved. It wasn't carburetor or ignition causing my hesitation. It was simply a blocked fuel tank vent hose. After replacement with a new vent system, the boat performs perfectly. I'm surprised no one suggested that the fuel tank may not be properly vented. With a 54 year old boat that has never had any service to the fuel tank and components it's no wonder this issue occurred in the first place. But thanks to those who responded who offered suggestions.