Merc 800 80HP stalls

[frabcabert]

My merc 800 stalls when i put it in gear. Warm or cold, reverse or forward, makes no difference. I have low compression on my top cylinder, could this be my only problem? I had the engine checked a few weeks ago, rebuild carbs, new fuel lines, etc. I have to put it in gear and throttle it right away to get it going, once its going, then I can drop it right down and it putters fine. I feel that upping the Idle speed will bring it to high as i think I am closer to 1000 rpm at idle right now. now i do have a 19 pitch prop on it, do you think that could cause something like this (on a week engine)?
Thanks for the help.

 

[Chris1956]

Re: Merc 800 80HP stalls

It is likely the carb idle mixture adjustment is off. Try richening is slightly.
BTW - A 19" pitch is only suitable for a fairly light 16' runabout or smaller boat.

 

[frabcabert]

Re: Merc 800 80HP stalls

Thanks Chris, I'll try that. I figured that the pitch was to great, so i have a new 17 pitch now, gona have to go out and try fixing that. My boat is a 16' Trihull (fiberglass) I would say its fairly heavy.

 

[Gomer50]

Re: Merc 800 80HP stalls

I am not sure if this will help very much but I own the exact same motor with the exact same problems number 1 cylinder low compression 115psi and wont Idle correctly and will rebuild soon.I used a 17 Pitch prop on a 15 foot trihull weighing empty 1200 lbs.with the original lower unit and could not produce more than 4800 rpm which is at the bottom of the suggested rpm rating for this motor.I had to drop to a 15 pitch prop in order to produce 5100 rpm at WOT and cruise comfortably at 4800 rpm.Please take into consideration this is for my boat this might not apply to yours.But I do know if we are not producing the proper rpm this will shorten the life of these motors,it causes pre-detonation and carbon build up, burnt pistons etc.If you have any questions to what I have done/repaired to try and get better results feel free to ask.

 

[frabcabert]

Re: Merc 800 80HP stalls

The thing is, mine idles fine. It just stalls when I put it into gear, then once i get it going, i can idle in gear all day if i want. It's just the gear shifting. As though the resistance that creats stops the engine. It shifts smoothly though. Thanks though, I will see how the 17 pitch works.

 

[frabcabert]

Re: Merc 800 80HP stalls

Well, Finally took the boat out today, 17" pitch prob, Huge difference. Gets on plane quick. Trolls better. Happy!