The new Cig I bought is running 500 efi's. The first time I took the boat out the port motor would only turn about 4500 rpm's and the starboard motor was running about 5000. It was a little over propped (30's). We thought there might be a prop problem because it did not show anything on the computer as being wrong with the motor.

We ran the boat after this a few times with 28's and they were both running about 5200 rpm's. Last weekend I had the boat at Havasu, and on Saturday the port motor would only turn about 4500 and the starboard motor was turning about 5000. ON Sunday the port motor was turning about 4800 and the starboard motor about 5200.

The shop thinks it might be a fuel problem. Does anyone have an idea what might be causing this?

Something simply make sure that the trim is the same. im sure its correct but you have to start somewere

I had a bad plug in one engine and it caused a 400-rpm decrease. The engines ran smooth but at idle I could see a twitch in the tach. I replaced the bad plug and she runs fine. Also check your fuel filters to see if you got some bad gas.
Dan

have your props balanced as a pair.
My 502's were the same. Being Niave I thought it was because the port drive is actually running in reverse and not an actual counter rotating drive Everyone says it does not matter?? But yet when I tried to turn props in for testing it took forever for the drives to catch in reverse??

But I did notice a MAJOR difference for the better once the props where matched. Especially if they are right out of the box already.

its a starting point

Does the speed also drop when one engine looses rpm? some times running both batterys in parallel can cause this condition. I had one tack read +- 400 rpm when in parallel.

Have you used a shop tach, a freind of mines tachs are 5-600 off on the tach but the same on the shop tach