700sci low on power
 

i have twin 700s in my boat and the one side is lagging in rpm at same throttle position by about 3-400. Leak down test is all good. No codes or warnings being thrown. per vessel view that side is burning about 2-3 gallons less fuel per hour than the other side at 4k rpm. Props were just reconditioned about 5 hours ago and show no damage.

Any advise on how to diagnose what could be happening. Could it just be coils, spark plugs and wires?

I'd say you are down a cylinder on that side. Check wires/plugs. Could also have an injector not firing.

i assume starting with just replacing wires/plugs and see if that fixes is best way to find out? Will injector not firing throw a code?

Have you hooked a scanner to it or just reading V-V

Edit: also check fuel pressure

All i know right now is it is it wont run at same rpms as other engine throughout the rpm range. Will scanner tell injector or ignition issues? I assumed V-V would tell me anything that a scanner would tell me.

I had 700's and had the same issue. Although it was not readily apparent other that I noticed a slight change in the exhaust sound. The coils will not show on the computer as not firing. The computer shows a signal sent to fire but does not know if it did. Mine ended up a bad engine wiring harness after testing everything. We also swapped coils around. It took lots of time to troubleshoot that one......

Was it an igntion harness? Did you also see the rpm difference between the engines?

I think with a scanner you can invoke a cylinder miss fire test with the 555 controller to see if all is well or may point you to a particular cyld.......... have you tested the fuel pressure ?

Years ago, I had an issue with my pair of 496HOs where one engine was always lagging by 300-400 RPM. It turned out to be a bad Crankshaft Position Sensor. The CPS can start to go as such can start go bad as such before throwing out error codes.
That would be something I would replace just for good measure. They are fragile, so not sure I would try swapping them.

Blower belt slipping??

If you infrared the cylinders to see if there is a temp difference on the exhaust manifolds that could tell you if you have a cylinder issue?

Have your local Mercury dealer use G3 and do a drop cylinder test. EFI engines are incredibly good at seemingly running OK on 7 cylinders. We had a client with a 38ZR w 700s who had no idea one coil had failed. Needs to be done in the water (even if still on the trailer) in gear around 1500rpm.

I plan to do that exact test in the next few days. Doesn’t matter whether it’s under load or not? It seems to idle fine, but I was thinking about at least put it in gear for a minute or two under some load to see if that if I can tell a difference between the temperatures on the runners.

I have a good boost, equal on both side on both engines

When was the last time you replaced your in-line fuel filters? They are small and flow a lot. Easy to get clogged. How about Fuel/water filters? Mercury had a recent bad batch of these.

I think i may have found the problem. looks like somewhere along the way the throttles got misadjusted and one doesnt engage the throttle body until about 2 inches after the other. Will adjust this week and see if that does it.

Sometimes its the simplest thing....

Thanks to all for the great suggestions as always!

FYI, adjusted the throttle linkages and now both motors run consistent rpm and have similar boost profiles. Thank to all for the great suggestions, sorry to waste your time on something so simple, but i am VERY happy that this was all that was wrong!