possible exhaust restriction problem?
06-28-2004, 04:22 PM
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Join Date: Jun 2004
Location: Bolton, MA
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possible exhaust restriction problem?
Here's my story...
Have twin 1997 mercruiser 7.4l inboards, weber carbs, tbolt V ignition. The port side just doesn't have any power. It starts easily, runs smoothly, never misses or hiccups at all, but it just has no top end. By comparision, the starboard engine cruises at 3400rpm using 14-15gph and tops out around 4800rpm. The port engine cruises at 3400rpm using 17-18gph and tops out around 4100rpm.
I've swapped the carbs and ignition modules from one side to the other, no effect. Also checked (and rechecked and rechecked) timing. Initial timing is 10btdc with the purple-white wire grounded, and confirmed total timing advances to 34btdc, as it should.
The props are identical and in excellent condition.
Compression is good. Vacuum at idle is rock steady at 18".
Last weekend, I did some runs with a vacuum guage hooked up and noticed that the manifold vacuum in the port engine is about 3" less than the starboard engine when the engine is under load. Eg. at 1800rpm, starboard reads 15", port reads 12". At 3400rpm cruise, starboard reads 10 inches, port reads 7". Both engines read 18" at idle. With the normal suspects ruled out (carb, timing, compression...) I'm thinking, could this be an exhaust restriction? Anyone experience a similiar problem?
This boat has the stock 4" center risers joined to a 6" diameter collector tube that (I'm guessing) acts as a water/gas separator. There's a 1-1/2" outlet on the top of the collector that goes to an in-line silencer and dumps out a thru hull fitting above the water line, and a largish (5"?) outlet on the bottom on the collector that goes to another thru hull fitting below the waterline. I can't take this system apart to inspect anything without having the boat pulled. Anyway of confirming there is a restriction? Any other tests I can do? With this sort of setup, seems there's plenty of capacity for these stock engines.
Thanks for any info!
Have twin 1997 mercruiser 7.4l inboards, weber carbs, tbolt V ignition. The port side just doesn't have any power. It starts easily, runs smoothly, never misses or hiccups at all, but it just has no top end. By comparision, the starboard engine cruises at 3400rpm using 14-15gph and tops out around 4800rpm. The port engine cruises at 3400rpm using 17-18gph and tops out around 4100rpm.
I've swapped the carbs and ignition modules from one side to the other, no effect. Also checked (and rechecked and rechecked) timing. Initial timing is 10btdc with the purple-white wire grounded, and confirmed total timing advances to 34btdc, as it should.
The props are identical and in excellent condition.
Compression is good. Vacuum at idle is rock steady at 18".
Last weekend, I did some runs with a vacuum guage hooked up and noticed that the manifold vacuum in the port engine is about 3" less than the starboard engine when the engine is under load. Eg. at 1800rpm, starboard reads 15", port reads 12". At 3400rpm cruise, starboard reads 10 inches, port reads 7". Both engines read 18" at idle. With the normal suspects ruled out (carb, timing, compression...) I'm thinking, could this be an exhaust restriction? Anyone experience a similiar problem?
This boat has the stock 4" center risers joined to a 6" diameter collector tube that (I'm guessing) acts as a water/gas separator. There's a 1-1/2" outlet on the top of the collector that goes to an in-line silencer and dumps out a thru hull fitting above the water line, and a largish (5"?) outlet on the bottom on the collector that goes to another thru hull fitting below the waterline. I can't take this system apart to inspect anything without having the boat pulled. Anyway of confirming there is a restriction? Any other tests I can do? With this sort of setup, seems there's plenty of capacity for these stock engines.
Thanks for any info!
