Mercury inline problems please help

[ss von scalliwag]

Hi there to all,
This is my first posting however I visit the forum fairly often as I find it a goldmine of information from people who know their outboards!!!
Anyway,
I have a 1985 mercury 75 hp inline 4 CYL. brown band.
It has plenty of power top end and idles just fine, but the problem I have with it is that when I shift into gear it doesn't seem to have any torque bottom end and dies when in gear. I have to adjust the neutral idle real high to 1200 - 1400 rpm just to be able to troll in gear at 750 rpm. It seems to lose at least 450rpm when put in gear.
I have cleaned and rebuilt carbs, idle adjustmust screw is pefect on both carbs. Fuel pump serviced etc. Starts easily and last time I checked had good comp.
Basicly it just loses 400 rpm as soon as I shift into gear.

Please can anyone help me as its driving me nuts, all I want to do it go out and do some fishing.
Any suggestions would be much apperciated and I welcome all possible solutions to my problem.
Best regards,
Mark.

 

[Laddies]

Re: Mercury inline problems please help

First thought is it's a little lean on the idle mix or it's on a small boat with a high pitched prop, or a prop with a lot of diameter

 

[ss von scalliwag]

Re: Mercury inline problems please help

Laddies, none of thoose reasons are responsible for my problem. Thanks for the reply but all the mentioned solutions are checked and double checked.


Any other ideas anyone?!!

Many thanks in advance

Best regards mark.

 

[ss von scalliwag]

Re: Mercury inline problems please help

Just for more details the prop is quicksilver 17 inch pitch and 13 1/4 diameter. Which Im pretty such it about right for this engine. I'm getting 5250 rpm at WOT.

 

[Laddies]

Re: Mercury inline problems please help

Your RPM is about right and if everything else is OK including the synchronization, your engine is probably suffering from a problem that plagues engines with high dome boost port pistons, excessive taper and out of round bores. Not what you want to hear but a very good possibility, the engines will show good compression but the pistons and bore will be to loose to develope low end torque.

 

[turbinedoctor]

Re: Mercury inline problems please help

How long have you had this motor and how long has it been doing this, just started all of a sudden or been coming on and getting worse over time?

Have you checked the timing?
Made sure it's firing on all cylinders?

 

[Faztbullet]

Re: Mercury inline problems please help

Laddie's gets the prize!!!!! Also worn reed block labyrinth grooves will cause this symptom.

 

[ss von scalliwag]

Re: Mercury inline problems please help

Hi Guys,

Thanks for all the reply's you have posted. Your suggests and diagnoistics are appreciated.

Just in response to a couple of possiblities, I'm pretty sure I'm not losing the spark on one cyclinder because I had it on idle and pulled one plug off at a time to see the engine's reaction and it had a big impact on how it would run i.e when I removed either one of the 4 plugs she would spluter and stall or at least drop rpms.

Ive only been out 3 times on the boat. First time it was fine and ticked over o.k. Then the next two times it seemed to have got progesively worse.

As for timing, again I don't think its out because as soon as its above 900 rpm in gear its absolutly fine. It pulls well and has very smooth acceleration all the way to WOT. And it never spluters or coughs or stalls along the way.

However I don't have the facilities to test timing so I'm gona bring it to my local marine workshop this morning. See what they can do. I HATE this option because its bloody expensive and a bit of a kick in the groin to my mechanical abilities as this was my project and I have come a long way since I first got her. Nothing ran or worked. All it had was good comp. So I took a gamble and bought it.

Anyway a BIG thank you to all thoose that have replied to my posting and all your input is highly valued. Any other suggestions would be great (hopefully a quick fix one LOL!)

Will let you all know how I get on. Once again thanks to all for the replies.
Best regards,
Mark.