Tern 6 hp (Terhi Valmet) - Erratic Rev's throughout range

[Fishcake42]

Hi Guys, new to the forum but you've helped me out loads over the years - you just didn't know it!

Have 2 engines, two problems (pole-opposite), other engine later.

To keep it simple, this is a single cylinder air cooled Finnish engine. After much debate with myself, I have applied logic and am convinced the fuel pump is venting gas in to the crankcase and creating a "carb by-pass" and extra rich condition, some of the fuel being helped to a vaporised state by the heat. The only seed of doubt I have is that I haven't managed to find 1 thread anywhere that references such an condition.

Spark good, Carb thoroughly cleaned, new float and needle, lever arms bent back to shape (so I could actually start it), compression good, no air leaks.

Excessive "fuel-dump" through exhaust, only managed to get it to run at full rpm by blowing compressed air high in the venturi, pumping primer cuts engine out. Any thoughts?

 

[Fishcake42]

Re: Tern 6 hp (Terhi Valmet) - Erratic Rev's throughout range

.......just to add another symptom - Idle mixture has no effect on anything, and I can completely remove the high speed jet and engine continues to run with same condition - ergo - its getting its fuel from somewhere else!

 

[nwcove]

Re: Tern 6 hp (Terhi Valmet) - Erratic Rev's throughout range

you can gravity feed directly to the carb, and trouble shoot from there. ( would love to see pics of the motor....never heard of a tern!)

 

[Fishcake42]

Re: Tern 6 hp (Terhi Valmet) - Erratic Rev's throughout range

Nor had I until I came across it at a junk sale, and since I was in the market for a kicker I thought i'd take the risk.

It's old (circa 1960), external fuel tank, and exceptionally simple - and Ive worked around and diagnosed enough engines to be a great believer in old fashioned tank style design that can be fixed with concrete and string. give me reliability over efficiency any day of the week.

The obvious answer is to change out the diaphragms in the fuel pump, but that will have to be a custom job as parts not available, but by-passing the pump (cork up the lines), and feeding carb direct makes sense to diagnose and was along the lines I was thinking. Will endeavour this weekend and update, at the very least if I am barking up the wrong (obscure) tree, then there now is a thread on this subject to help others not waste their time.

Thanks!

 

[Fishcake42]

Re: Tern 6 hp (Terhi Valmet) - Erratic Rev's throughout range

Well, defiantly not barking up the wrong tree - in fact 100% spot-on. Fuel pump diaphragm torn leaking just the right amount of gas to make it hard starting and rich running. Replaced with a make shift diaphragm (doesn't pump too well but can be compensated with the primer every 5 mins or so), she now runs like a dream. A couple of float adjustments to do is all. Happy with my bargain!

 

[nwcove]

Re: Tern 6 hp (Terhi Valmet) - Erratic Rev's throughout range

great to hear fishcake!!