'75 Mercury 1500 stalls on acceleration

[sbryant]

Hello to all! I have two questions:<br />I just bought a 19' Sabrecraft with a Mercury 1500 outboard motor. My questions concern the motor:<br />1. I think the fuel/oil ratio is supposed to be 50:1, but I was told when I bought it to use 1 quart of oil to a full tank of gas (16.5 gallons) Isn't that 66:1?<br />2. I'm having a problem with the throttle. When I first got the boat, the tank was full of a gas mix that was 1-3 years old, and had approximately 2 quarts of oil in it. When out of the water, the motor ran fine, revved fine, etc. Under load however, it died when going past 1500 rpm or so. I drained the tank today, put in some marine engine cleaner stuff, a quart of oil (2 cycle marine), and filled it with the 16.5 gallons of gas. It ran great... for a while. Then the same problem started cropping up. I discoved that most of the time, I could get it to throttle up to 4000 rpm if I gunned the engine a few times first. Is this because I still haven't worked all the old crap out of the system? Do I need to clean the carbs? Help! <br />Any comments or suggestions would be GREATLY appreciated.<br /><br />Thanks for your time!<br /><br />-Steve in Idaho

 

[sbryant]

Re: '75 Mercury 1500 stalls on acceleration

2 updates to this question:<br />1. I've talked to Mercury and a Mercury mechanic, and found out definitively that 1 quart to 16 gallons isn't going to cut it. <br /><br />2. I'm still trying to figure out the throttle problem, but from reading other posts here, it sounds like it may be the engine sucking air when it's trying to get fuel. How could I test this for sure?<br /><br />Thanks in advance,<br /><br />Steve in Idaho

 

[sbryant]

Re: '75 Mercury 1500 stalls on acceleration

Yet another update: On Saturday, I went through the entire fuel line from tank to motor, replacing and sealing all fittings, and replaced the primer bulb. The problem is still there. In fact, it's getting worse. The motor now dies randomly, and I have to let it sit for about 5-10 minutes before I can restart it. I have discovered however that if I tap the choke when the motor starts to die that it will pull out of it, and sometimes even finish throttling up. This seems to me to indicate a problem in the fuel delivery system, possibly a clogged filter or something. Am I on the right track? Please, any comment, even one telling me I'm way off base would be appreciated at this point.

 

[kev_alaska]

Re: '75 Mercury 1500 stalls on acceleration

You will most certainly need a carb rebuild after setting with old gas for that long. With your choke response the fuel pump may need some attention as well. A good manual can show you the ins and outs of carb rebuilds...<br /><br />kev

 

[ronnie33]

Re: '75 Mercury 1500 stalls on acceleration

tapping on the choke,wich is on the carb,may very well be freeing up the needle and seat,letting in a little extra gas to tempororily solve your problem.anytime a fuel system sets for very long it would be wise to go through the whole system.

 

[RandyJ]

Re: '75 Mercury 1500 stalls on acceleration

I agree with these guys on the idea of carb clean up. As for 50:1 mixture... hm let's do a little math... 8 oz to a cup, 16 oz/pt, 32 oz/qt, 128 oz/gallon... 16.5 x 128 oz = 2112 oz.<br />2112 divided by 51 (50 parts gas to 1 part oil) = 41.4 oz (divide by 32 oz/qt = 1.29 qt or about 1 1/3 qt) I'd put a qt and somewhere between a cup & a pint of oil. It's close enough that you'll get a good lube job and worst case is maybe a little extra carbon on the plugs now and then.

 

[what200]

Re: '75 Mercury 1500 stalls on acceleration

The way I figure my mixing is 1 pint or 16oz to 5 gallons for 40:1 or 6 gallons for 50:1. I wouldn't run the motor anymore until you have cleaned the carbs.<br />FYI- the inline guru I know runs his inlines at 32:1

 

[sbryant]

Re: '75 Mercury 1500 stalls on acceleration

What would everyone suggest I use to clean the carbs? I've been waiting for the service manual to arrive before I do any work on anything connected to the motor, but it should arrive today. <br />Thanks for your help!

 

[sbryant]

Re: '75 Mercury 1500 stalls on acceleration

New update:<br />The service manual arrived Friday. Unfortunately, it's for the wrong years. (ARG!) It's only off by one year, so I consulted it concerning what could be causing the problem. Found and cleaned the wire screen filter on the fuel pump, there was alot of grit and various other crap. Hoping against hope, took the boat out yesterday, so that I could do a couple more tests. It ran ALOT better than it has been, but after a while, same problem occurred again, trying to stall out when passing 1500 RPM, and it did die at WOT once. I discovered that if I have my wife pump the bulb while throttling up, there is no problem, and it runs like a champ. When the motor died at WOT, we sat around for 15 minutes to half an hour, trying periodically to start the motor. No go. It finally started when I sprayed WD40 into all three carbs and let them sit for about a minute. I'm thinking everyone's right about the carbs needing cleaned. (Not that I ever doubted you guys.) I'm wondering, can regular automotive carb cleaner be used on marine carbs, or would WD40 be a better choice? What should I use?<br /><br />Thanks in advance,<br />Steve

 

[icwebpeople]

Re: '75 Mercury 1500 stalls on acceleration

take the carbs off and rebuild them. it will save you a lot of agrivation on the water later.if you go cheap or half assed you will just be standed on the water sooner or later.

 

[AutoTechman]

Re: '75 Mercury 1500 stalls on acceleration

My experience with my 90HP....I had the same problem, it turned out that the #4 Cyl was low on compression and the fuel pump drives off of the #4&5 cylinder pulses. I also had a very tiny air leak in the fuel delivery system at the quick connect on the the engine...after repairing the air leak the engine would run up to about 7/8 throttle....then go slightly lean....I cheated and added an electric fuel pump. This was no cure for the #4 cyl, but at least the boat ran at 100% throttle and ran rather well I might add.

 

[VMAX]

Re: '75 Mercury 1500 stalls on acceleration

I just had the same problem with my 140 horse, 6 cyl mercury. Turned out to be a weak fuel pump. I found out like this:<br />I put the muffs on in the driveway and primed the carbs with the bulb until hard. Then I disconnected the hose to the carbs and installed a 1 foot hose to the pump outlet and ran it to a plastic bottle.<br />I then started the engine and let it run on the fuel in the float bowls at about 1000 rpm (idle speed) until it died (about 20 to 30 seconds). When the engine died, I measured the fuel in the bottle at about 3 ounces, which probably would not be enough to fill the three float bowls up again. I then reasoned that the engine burned fuel faster then the pump could replace it and I disassembled the fuel pump. I checked both check valves and they were OK, but I found that the pump is built with a sandwich of two identical gaskets, with the diaphram in between them. One of the gaskets was installed upside down, which caused a vacuum leak to one of the two chambers in the pump. After reassembling the pump properly with new gaskets and diaphram, I performed the same fill-the-carbs and run-the-fuel-out test, and this time I had about a quart of fuel. Way more than I thought it would take to run the engine at higher speed. The interesting thing about my running problem, was that the motor pumped enough fuel to idle, and if I could coax it into the upper RPM's it would pump OK, but it couldn't pump enough in the 1500 to 3000 range to accelerate normally. I hope this helps you in some way. John

 

[sbryant]

Re: '75 Mercury 1500 stalls on acceleration

VMAX: how did you tell one of the gaskets was upside down? Did all the curves not match up? I rebuilt my fuel pump last night, based on what I've read here, and what my mechanic told me, and it'd be pretty hard to put the gaskets in wrong and still get it to look/assemble right.<br /><br />Steve

 

[what200]

Re: '75 Mercury 1500 stalls on acceleration

If you got the 66 to 74 manual that's OK. The 73-74 1500 is pretty much the same as yours.

 

[sbryant]

Re: '75 Mercury 1500 stalls on acceleration

What200: They're fairly similar in some ways, such as the fuel pump, but there's no way in heck I'm going to attempt a carb rebuild without the correct manual. The 66-74 manual doesn't have any carb diagrams that match what I've got. There's one kinda similar, but when dealing with tiny parts that are vital to performance, I prefer an exact match. =-) Heh. It's going to be hard enough as it is. Luckily, I talked to Mercury, and they are sending the correct manual, and even letting me keep the 66-74. Not too shabby...<br /><br />Steve

 

[VMAX]

Re: '75 Mercury 1500 stalls on acceleration

Yes, the curves on the gaskets were mis-matched.<br />Both of the gaskets on my pump are the same part, and they are not quite symmetrical. There is a small hump on one side of the gaskets which must match the hump on one side of the pump for a vacuum passage. One was just flipped over causing a leak.

 

[sbryant]

Re: '75 Mercury 1500 stalls on acceleration

It's been a while since my last update... I rebuilt the carbs this last summer, and that took care of 90% of the problem. The float bowl of the bottom carb was 1/4 full of sand and dirt. *sheesh* The throttle and sudden death on the water problems cropped up a couple times after the rebuild, but in both instances, more crap from the fuel tank had clogged the filter. I plan on putting a clear in-line filter in before the bulb this spring, before I take the boat out again. I also discovered what I think was another contributing factor to the problems while I was winterizing the motor. I was using fogging spray on each of the plugs and in the cylinders, and when I tried to put the wire for number 4 back on, it wouldn't snap. I looked into the boot, and the snap ring was crushed into wire end of the boot itself, leaving a gap between it and the sparkplug post. If 4 was firing at all when we had the boat out, I'd be suprised. Well, more fun for me when I prep the boat for this year's time on the water! =-) <br />I want to thank everyone who helped me with these problems, your advise was invaluable!<br /><br />-Steve in Idaho

 

[Davidandkerrie69]

Re: '75 Mercury 1500 stalls on acceleration

Steve, I had exactly the prob you describe, bogging off idle, runs great at WOT, use prime to get past the bog. I had a complete carb rebuild, fuel tank flush out and fresh fuel to fix the problem. Ended up costing $600 AUD to fix. Don't use old fuel or fuel from 'cheap' independant stations. I learnt my lesson the expensive way and gladly pay a few cents more buying fuel from brand name stations.<br /><br />Good luck! David

 

[what200]

Re: '75 Mercury 1500 stalls on acceleration

Also,<br />the older inlines need at least 92 octane gas and keep the timing below 21* BTDC or 20 if you want to be safe. The #3 cylinder often doesn't get along too well with cheap gas.

 

[Gold Bear]

Re: '75 Mercury 1500 stalls on acceleration

Steve in Idaho <br /><br />Stumble during acceleration is typical of a lean idle mixture screw setting.<br /><br />Count the number of turns that it takes to lightly seat the idle mixture screws on all three carbs. I'll bet you will find the screws are set at two turns or less.<br /><br />That is not open enough on these engines. From that point turn open the screws to 2-1/2 turns and see how she performs. If there is still some stumble open them 1/4 turn each until the stumble goes away. <br /><br />My 1971 inline six operates best with the idle mixture screws three turns open.<br /><br />Good Luck,<br /><br />Gold Bear