Tach readings

[Crowbar]

305 gas engines upper helm reads 3000rpm on both engines. Lower helm reads 2000 rpm on both engines at the same time. At idle, all are the same. Lower helm tachs were replaced recently. Any ideas?

Thanks for your help. Both tachs were set for six instead of eight cylinders. On to the next fix.

[bud37]

Check the back of the tachs to see they are set for right pulses for 8 cylinder.Do you know if the upper tachs are right or the lower when they get to 3000 rpm, I would see if I could check that against a tune up meter/timing light.Something that is known good.
If the boat is new to you check the setting on the back of the upper tachs as well........

[mjk1040]

Interesting they are 1,000 rpm's off!

[Viper]

Did this just start or has it been this way since you got the boat? If it just started, find out first which station is reflecting the wrong reading. You must confirm this at the engines. Given that the lower tachs are new, one could assume that they are correct but I wouldn't rule them out completely, hence the need to confirm which reading is correct. As mentioned above, confirm that the selector switch on the back of the tachs are set properly for the number of cylinders. If they are, note the position of the switch. These switches tend to corrode internally and result in erratic/incorrect readings. Gently turn the switch back and forth several times, then return it to its original position. Don't force the switch, if it resists turning, try pushing it in slightly then turn. This is normal for some brands. This helps clean the internal contacts and may correct the readings for a short time or years, hard to say, but it depends on the condition of the switch and environmental conditions. If this doesn't correct the issue at least temporarily, then you'll need to do further diagnosis as it can be anything from a tach replacement to wiring, corroded harness plugs, poor/corroded connects, etc.

Keep us posted.