Guys,
I need help troubleshooting a 1988 Mercruiser 350 V8 - 260 HP motor with a Rochester 4 barrel Carburetor. The engine cracked a head / blew a head gasket / burnt the valves between two cylinders. An experienced engine rebuilder repaired/replaced the heads. Once repaired he could not get the original rochester carburetor to idle properly at the correct idle speed of around 600-700 rpm. This problem existed before the engine was redone and may have caused the trouble according to the rebuiilder due to the engine running too lean. Before the engine failure it always shut off at "no wake speed" (less than 800 RPM) I was able to get around it before the engine failure by bumping the idle speed up to about 800-900 RPM. The situation also improved somewhat after tearing the carb down and rebuilding it.
I had an older Edelbrock Rochester (4903) off of a 454 automotive engine so we decided to try this one. It is a larger CFM carburetor (795 cfm) He set the idle RPM at 600-700 RPM and was able to keep it running in the shop. On the water though it was the same problem. The engine would run good at higher throttle ranges but would shut off after a minute or so at low rpm ranges.
We started to suspect an air vent issue on the fuel tank or a fuel pump issue. Vent is OK but when we pulled the output hose off the pump we found the pump was not outputting much fuel. I put a new mechanical fuel pump on. Same problem. After the engine shuts off it will easily start right away again with no throttle, but it will shut off in a minute or two.
On the water the carb acted like it has a vacumn issue sometimes because if I was in gear at say at say 1500 RPM it will run for a minute there and drop off to say 700 RPM. Sometimes it would catch back up and go back to 1500 RPM. It would always drop off again and then eventually die off. It starts again with no gas being applied, but runs for a minute and shuts down.
I decided to buy a vacumn gauge and test vacumn. At low RPM 600-1000 I cannot even get a reading, at higher RPM (1500 +) I get a steady 15 on the gauge. The gauge was connected to the lowest port on the LH front side of the edelbrock carb (Ported vacumn port?) I also monitored the timing with a timing light. Timing stays steady but drops off at the moment the engine starts to die.
I guess my next steps are to check fuel pressure and volume output to be sure everything is OK there. I would guess there is a strainer in the bottom of inlet tube in this 40 gallon (under the floor board) tank but I am not sure. I am also starting to suspect something in the Thunderbolt ignition system. Plugs, dist. cap. ignition module are all new from a year ago. Any ideas?
I need help troubleshooting a 1988 Mercruiser 350 V8 - 260 HP motor with a Rochester 4 barrel Carburetor. The engine cracked a head / blew a head gasket / burnt the valves between two cylinders. An experienced engine rebuilder repaired/replaced the heads. Once repaired he could not get the original rochester carburetor to idle properly at the correct idle speed of around 600-700 rpm. This problem existed before the engine was redone and may have caused the trouble according to the rebuiilder due to the engine running too lean. Before the engine failure it always shut off at "no wake speed" (less than 800 RPM) I was able to get around it before the engine failure by bumping the idle speed up to about 800-900 RPM. The situation also improved somewhat after tearing the carb down and rebuilding it.
I had an older Edelbrock Rochester (4903) off of a 454 automotive engine so we decided to try this one. It is a larger CFM carburetor (795 cfm) He set the idle RPM at 600-700 RPM and was able to keep it running in the shop. On the water though it was the same problem. The engine would run good at higher throttle ranges but would shut off after a minute or so at low rpm ranges.
We started to suspect an air vent issue on the fuel tank or a fuel pump issue. Vent is OK but when we pulled the output hose off the pump we found the pump was not outputting much fuel. I put a new mechanical fuel pump on. Same problem. After the engine shuts off it will easily start right away again with no throttle, but it will shut off in a minute or two.
On the water the carb acted like it has a vacumn issue sometimes because if I was in gear at say at say 1500 RPM it will run for a minute there and drop off to say 700 RPM. Sometimes it would catch back up and go back to 1500 RPM. It would always drop off again and then eventually die off. It starts again with no gas being applied, but runs for a minute and shuts down.
I decided to buy a vacumn gauge and test vacumn. At low RPM 600-1000 I cannot even get a reading, at higher RPM (1500 +) I get a steady 15 on the gauge. The gauge was connected to the lowest port on the LH front side of the edelbrock carb (Ported vacumn port?) I also monitored the timing with a timing light. Timing stays steady but drops off at the moment the engine starts to die.
I guess my next steps are to check fuel pressure and volume output to be sure everything is OK there. I would guess there is a strainer in the bottom of inlet tube in this 40 gallon (under the floor board) tank but I am not sure. I am also starting to suspect something in the Thunderbolt ignition system. Plugs, dist. cap. ignition module are all new from a year ago. Any ideas?
