Twin cylinder trim unit dropping - Prestolite unit - Help!

[Fletch_n]

Okay people

I have this trim unit on my 55hp Johnson, it started dropping from full tilt down when left up

I have put a seal kit through both cylinders and I have also replaced the check valves in the pump.

I have bleed the crap out of it to remove air in the system

Its still dropping, and slowly getting worse, from tilt all the way through the trim.

The trim still has plenty of guts, Ive had some one stand on the cav plate and it lifts up no problems at all (through the trim range)

I'm at a loss now as Ive pulled apart the cylinders and pump once again each and no difference.

Any suggestions please?
Cheers
Fletch

 

[Will Bark]

Re: Twin cylinder trim unit dropping - Prestolite unit - Help!

Is your manual release valve closed tight?

 

[Fletch_n]

Re: Twin cylinder trim unit dropping - Prestolite unit - Help!

Thanks, yup she is tight.

 

[RRitt]

Re: Twin cylinder trim unit dropping - Prestolite unit - Help!

carefully do following.

block engine in place so that no weight is resting on trim.

remove circlip and unscrew manual release. Be careful not to twist off or wallow the screwdriver fitting. A lot of people twist off the screwdriver fitting and ruin the mrv. Combined with the oxidation of age this can cause bleed down but usually is more inconvenience than anything else. You may need to gently grab end with small vice grip and pull for final extraction.

examine end of manual release. it should have a rubber tip. touch the tip to a flat smooth surface and make sure that the rubber tip
makes contact all the way around. If the rubber is worn then that could be the source of your leak. You only need less than 1/64" of rubber to make the seal but it must be a full 360 degree. I don't really know what to advise for worn out MRV. I have a lathe and gently cut out the old rubber. If the metal is damaged then i can cut a new seat or even a whole new valve. a dremel blue polishing bell (600ish grit) will fix up the hole and smooth the sealing seat. I can guess (but it's just a guess) that the briggs and stratton carb kit might fit the tip.

if the manual release is okay then it is probably your check valves. OMC no longer sells them and DIY has a very bad track record of success. You can search the Chrysler Force forums for posts about using Briggs & Stratton carb kits to rebuild prestolite check valves. Your VB uses the exact same check valves and the conversation is 100% transferable. You can also sent it out for repairs.
Flat Rate Rebuild Evinrude Johnson OMC Trim Tilt Pump Valve Body 1974 1989 | eBay

 

[Fletch_n]

Re: Twin cylinder trim unit dropping - Prestolite unit - Help!

Thanks RRit

Ill check the MRV tomorrow and see what its condition is like, I found a NOS check valve kit and that did nothing when I replaced them.

Thanks heaps for the reply

 

[RRitt]

Re: Twin cylinder trim unit dropping - Prestolite unit - Help!

NOS doesn't mean it works. They have not manufactured those things since 1992.

If I had a rubber tire that sat on a shelf for 20 years, would you expect it to work? It helps to be out of sun and wrapped in wax paper with a thin coating of oil ... but the seals in your NOS part still won't last forever. I'd still give it 50/50 MRV versus check valve.

 

[Fletch_n]

Re: Twin cylinder trim unit dropping - Prestolite unit - Help!

Thanks again RRit

1992! I didn't know that, Thanks for your suggestions, ill re-check the seals on the check valves but the main thing I will look at today will be that MRV, not sure what ill do if its not making contact all the way around as I don't have a lathe to turn it down on. I work in the stainless industry so I was thinking of getting a small piece of tube to fit the MRV in so only the very end of the seal is showing, then heating a bit of glass up and lightly putting it on that to try and re level it? would love your thought on that idea?

Failing that like you said its more an inconvenience so it might just be one of those things I have to live with for a while.

Cheers
Fletch

 

[RRitt]

Re: Twin cylinder trim unit dropping - Prestolite unit - Help!

heat and buna rubber don't mix well. it will become extremely difficult to completely remove all the rubber specks. you are better off letting the part soak overnight in lacquer thinner to remove old rubber.

try the briggs carburetor kit. I can't think of any other cheap fix that might work without risking damage to the main body.

 

[Fletch_n]

Re: Twin cylinder trim unit dropping - Prestolite unit - Help!

Thanks RRit,

Once I got the MRV out it wasnt what I expected, so I figured my method wouldn't work!

I thought about what you said I pulled out the now old relief valves and cleaned and soaked the old ones, got some new o ring seals from the local hydraulics shop, put them back in (at which time I noticed the old ones where slightly different lengths - about 2mm which I hadn't noticed before) the new ones were both the same length?, but you hit the nail on the head, the NOS seals while they looked great when I put then in they were actually to hard, got squared off and looked like crap when I took them back out, I didn't even think of that possibility.

With the old ones back in and new seals on them, it is 98% better so far.

Thanks heaps for you help RRit - appreciated
Cheers Fletch

 

[Fletch_n]

Re: Twin cylinder trim unit dropping - Prestolite unit - Help!

Short lived, today its back to dropping, but in a rookie mistake I took the pump plate apart to blow it all out in case there was any junk in there, however now im not sure where the ball bearings and springs go under the gear plate (not sure what its called).
Would any one have a diagram of this and how they all go back together?

Cheers

 

[RRitt]

Re: Twin cylinder trim unit dropping - Prestolite unit - Help!

aw jeez. just guessing. details can vary greatly from one model year to another.

you assemble all the springs and balls onto the gearplate. hold the gearplate upside down and raise it up into the aluminum body. carefully hold everything in place with one hand. use a nutdriver to install two bolts until they begin to compress lockwasher. flip over VB, install other two bolts, tighten all four.

Two balls, approx 3/16" go into the two holes on opposite side of gears. small spring goes into recess with a small ball bearing on top (5/32"?). large spring goes into tower with large ball on top. ball bearings are inch size machine grade high precision balls. i think the accuracy to roundness is something .00005 or some other ridiculous number. You should buy them from industrial supply and avoid the local ace hardware. They are still cheap even when high precision. reuse existing springs unless they show rust or deformation.

the orings are inch size. if you use metric size orings it will be difficult to get proper operation.