vibration on tickover

[886]

any ideas, i have a mitchell 31 mk1 with a ford 120 hp engine on new flexible feet of the correct type. the problem i have is that on tickover 700rpm the vibration is bad once the revs are brought up to 850 all is well .there is no vibration when in gear and running all through thew rev range only on tickover

 

[tinkicker0]

Take a gander at the harmonic balancer. Might be worn.

 

[886]

thanks for the reply but what is the harmonic balancer

 

[PCUK]

If it happened since fitting the new feet then perhaps they are not ideal.

 

[omega2]

The balancer is an attachment on the front of the flywheel pulley, could be loose or shot. Try cracking the pipes at the pump in turn and note any difference in revs/noise, the one that makes no difference is not firing, could be tappets need adjusting. Should tick nicely at 650/700.

 

[rafiki_]

Sounds like your new engine mounts are not the right stiffnes for the harmonics of the engine at 700 revs. Easiest thing to do is raise the idle revs until you are happy with the vibration levels. Ultimately you may need different engine mounting stifnneses. Without knowing lot about the engine, these are difficult to calculate. Alternative is to source original spec mountings.

 

[PCUK]

Unless it's a total coincidence it obviously has to be the new mounts.

 

[886]

thanks for all the replies.
the problem existed befor i fitted the new mounts and was the reason i changed them .the mounts were for a 120hp 6 cylinder ford.
the problem was there when i bought the boat but was masked by a tickover speed of 1000rpm

 

[ArthurWood]

I've no knowledge of your engine, but I had a similar problem on a Mercruiser 7.4l petrol engine; it made a racket at low revs in gear, but seemed OK above 1000rpm. Turned out to be misfire on one cylinder and low compression due to damaged exhaust valve. Resulted in new engine. Sold boat!!

 

[rafiki_]

thanks for all the replies.
the problem existed befor i fitted the new mounts and was the reason i changed them .the mounts were for a 120hp 6 cylinder ford.
the problem was there when i bought the boat but was masked by a tickover speed of 1000rpm

I think you need stiffer engine mounts. You have 2 choices, try to get stiffer mounts, or accept the higher idle speed.