1974 - Mercury 1150 - RPM drop @ WOT

**Big_Totoro** · 07-31-2015, 12:16 PM

Hello!

I'm new to boating, and have bought my very first boat. It's a 1971 Sidewinder. It's powered by a 1974 Mercury 1150.

Engine starts fine, even when cold. From just in gear (barely moving) I can punch to WOT (instant throttle response) and the engine goes up to 5000 rpm. It's all I can do to hold on sometimes. Get up onto plane easy and very quick.

Once I'm just up onto plane the RPMs drop to 3600 rpm. I on;y stay up at 5000 rpm for a second or 3 maybe. I'm running a 17 pitch prop. I've had someone prime the fuel bulb while at WOT and there is no change (good or bad).

It almost seems like it's flooding itself? I ease off the throttle all the way back down to barely moving, but then quickly goto WOT again and it then will bog and slowly climb onto plane and up to a max of 3600 rpm.

I'd appreciate any help. I don't know alot about boats or engines, but I'm not afraid to get in and start pulling things apart if I have too.

thanks!

**Mark75H** · 07-31-2015, 12:32 PM

How old are the fuel pumps? Does the primer bulb stay hard?

**Big_Totoro** · 07-31-2015, 12:39 PM

Originally Posted by Mark75H

How old are the fuel pumps? Does the primer bulb stay hard?

Bulb stays hard. Fuel pump I have no idea. This was the reason I had someone pump the bulb while I was @ WOT. It didn't make any difference.

**bearclaw** · 07-31-2015, 02:38 PM

What I want to see are pics... for nostalgia's sake. What a cool find! As to your problem: I've seen them act like that when they're pulling old crap up again the intake in the gas tank.
With the age, that would make me wonder about that... Best to figure it out before you go much farther - it could be leaning out, and those motors really don't like that.

**Big_Totoro** · 07-31-2015, 02:58 PM

Originally Posted by bearclaw

What I want to see are pics... for nostalgia's sake. What a cool find! As to your problem: I've seen them act like that when they're pulling old crap up again the intake in the gas tank.
With the age, that would make me wonder about that... Best to figure it out before you go much farther - it could be leaning out, and those motors really don't like that.

I'll try and get some pictures soon

I don't think it's the fuel tank becuase I'm using a brand new fuel tank. There has been no difference switching between the old red tank and the brand new one. Both tanks have the vents open and there isn't any vacuum in the tanks from not getting enough venting.

**Robby321** · 07-31-2015, 03:47 PM

OK, just a thought here as had a similar problem my (new then) 74 XS1500. In think 1978, it started and went WOT like the beast it was, but would drop R's down a grand or so fast. Back to idle, nail it and here we go again. I simple could not figure the prob. Called my Merc guru and he said he got a service bulletin to remove the 3 small filters in the gas lines where they went into the carbs. I can't remember just where the were put but pull a line off at a carb and see if theres any tiny screen in there. If ya find one, out and take the other 2 out too. My middle carb had a gunked up filter. Took them out and "back to the beast" again. Don't hurt to look. If that aint it, rebuild the fuel pumps. And make sure the diaphragms you replace are the SAME, as variations. Good luck!

**Robby321** · 07-31-2015, 03:58 PM

OK, a little more. Like I said as I don't remember, but these screens were small and end the gas line. As in the diagram, I don't "think" it was #14 filter, but check them anyway to make sure clean. Also what I said about the pump diaphragms. Hope this helps.

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**Robby321** · 07-31-2015, 06:31 PM

Originally Posted by bearclaw

What I want to see are pics... for nostalgia's sake. What a cool find! As to your problem: I've seen them act like that when they're pulling old crap up again the intake in the gas tank.
With the age, that would make me wonder about that... Best to figure it out before you go much farther - it could be leaning out, and those motors really don't like that.

Yep as agree as its a fuel, NOT "flooding" but LEAN OUT issue. Easy to check any way, just yank a few plugs. Black carboned up is rich, lean will show clean plugs. And on the old Mercs" SURE DO NOT LIKE leaned out, as I did TWICE a "hole in one", #3 both times running to high advance, but that was running (best gas) at damn near 26 degrees, normal 23 Then. BUT! The gas now has "went south" and my Merc Guru here the famous "Dr Frankenmerc", will NOT let any stack he rebuilds out the door unless at now 21 degrees. And run premium gas, 40-1 oil. And thinking even if ya got the carbs with the #14 screens in there? I'd still yank'em and toss a filter inline instead (nothing special, simple automotive plastic see through filter, less than $5) Got one in bought my outboards, after the primer bulb, before motor and ZERO problem.

And DAMN RIGHT "we need pix!" Love the old "sidewinders", and of course .."Merc Stacks!"

**Robby321** · 07-31-2015, 06:36 PM

Gonna add a bit more about old Merc gas hose (Motor wise), and age. I'll get to it later as in the shop now, pix house puter. You also could have a deteriorated rubber inner on one or all. I'll post more on this later. Don't worry, we can figure out "the beast". Just don't rush into anything without asking here first. Wish the best!

**Dave Strong** · 07-31-2015, 07:42 PM

Robby you talking #14 in the diagram? For the filters.

Dave

**Robby321** · 07-31-2015, 07:52 PM

Originally Posted by Dave Strong

Robby you talking #14 in the diagram? For the filters.

Dave

Yes, but the ones I pulled were out the fuel lines IN there, do NOT think that, as the XS probably had diff carbs. Thats the only pic I have. Wish I could remember more but 4 decades ago. But STILL worth checking IF the OP has same type carbs. And STILL would pull them anyway, and run a inline filter instead. My Merc is covered up so I could take a look, but to DAMN HOT OUT! More about this later...

**Dave Strong** · 07-31-2015, 08:18 PM

That diag you showed had those goofy hoses that were crimped to the fittings, the hoses likes to come apart on the inside. Think I know the filters your talking about, tapered screens that are like fuel line Id.?

Dave

**Big_Totoro** · 07-31-2015, 08:28 PM

Thanks for all the knowledge guys!

I'm going to try and take a good look at the things you mentioned this weekend. I'll take some photos too.

The 70's Era boats and engines bring back my childhood memories! Lots of good times out on the water with my gramps!

**Dave Strong** · 07-31-2015, 08:44 PM

Funny worked on a couple L-6's today, one got a stator the other older one got the rain forest cleaned out of the cap. 2 happy customers.

Dave

**Robby321** · 07-31-2015, 08:52 PM

Originally Posted by Dave Strong

That diag you showed had those goofy hoses that were crimped to the fittings, the hoses likes to come apart on the inside. Think I know the filters your talking about, tapered screens that are like fuel line Id.?

Dave

Yes SIR! TAPERED INLINE! But managed to do a do a oil change the van, but ITS ONE HOT DAY an d back the house now. Hey OP poster! Take the wrap off, and take a pic the carbs, and look for any markings on the carbs. That will tell us more. Should be a (think) a WK # Let me find sometime...