78 mercury 900 revs up then goes back normal on its own

[Metalman423]

I have a 1978 mercury 900 that I have been arguing with (its winning btw)...jus had carbs cleaned fuel pump built and timing set. Went to the lake to test and it idles fine but revs up on its own and then throttles back down. Does it in idle or while your going then just goes back to normal
Can bad plugs or plug wires be a problem or is there something I missed? Someone please help with this I'm at my wits end!! Thanks

 

[Chris1956]

It is possible that some cylinders are firing occasionally, leading to the RPM change. Start her up and look for arcing on the spark plug wires. You should hear it snap as it arcs, and might see the blue sparks. Check the spark plugs for color differences. A real clean spark plug indicates water in the cylinder.

 

[Metalman423]

I did a compression test on all cylinders and everyone was 140psi. I'm going to buy new plugs today and replace those to see if it helps. Any other suggestions besides plugs and wires? Thanks Chris.

 

[Chris1956]

Spark plug wires are stranded stainless steel. They should have no resistance. You might measure them. Clean the distributor cap and rotor. The rotor is non-removable, is very fragile and cost $200+ to replace, if you do the work.

After you install the new plugs (NGK BUHW are recommended), run her for a while and try to duplicate the issue. If she surges, take a look at the spark plugs and see if they all look the same.....

 

[Metalman423]

Thanks chris. Put the new plugs in tested wires and cleaned the cap. I think someone took the cap off and had the brush and spring in a bind. Fixed that and she runs like new motor!!