1976 1500 Mercury 90 hp compression,vacuum and fuel ??

[randyd12n12]

1976 1500 Mercury 90 hp compression,vacuum and fuel ??

I have a 1976 1500 mercury that does well while in idle, but stalls under a load. I have taken it to two different mechanics who have given me two different points of view. Go figure. I am told that the compression for this engine should be at about 140 -150 but what they got was about 90 - 100. if the higher number is true how do I increase my compression to get it up to specs ? Then one mechanic said my engine may be losing vacuum ???? Sounded weird to me. Is this possible? I was also told that the older engines run weird due to the difference in fuel from them days and that I should change my oil to gas ratio to double. Needless to say I was not happy with the band aid solution or the different opinions. What I do know is that the engine does start fine but when I put it in gear she stalls any suggestions would be greatly appreciated. <br /><br />Randy

 

[2cycle]

Re: 1976 1500 Mercury 90 hp compression,vacuum and fuel ??

Re: 1976 1500 Mercury 90 hp compression,vacuum and fuel ??

First off, I would check to make sure the idle air mixtures are dialed in on each carburetor. The idle needles on these usually need to be at least 2 turns out CCW from a lightly seated position to accelerate under load. I would assume that your mechanics checked this, but who knows? If you are running too lean the engine will likely idle in the water, but stumble/stall under a load. Re: the compression issues, you may have wait for Clams and others' more expert opinions to jump in, but I'm thinking that 120-130 PSI is more in the ballpark for the 1500. It should still run around 100 provided there's not a large variance (more than 10%) between cylinders. <br /><br />You may also want to run some Sea Foam or Mercury Power Tune through it to see if those numbers will come up. Do a search for "Decarbing" and you will find a lot of reading and suggestions. Also, check out your fuel pump diaphragm (a cheap replacement), as well as making sure your fuel tank is venting properly. These could also account for the "vacuum loss" that the mechanics are describing IMO. I'm suggesting all of these things because I have the same engine and had the same symptoms. That is a good motor when set up right as others will attest. Good luck and keep posting!

 

[2cycle]

Re: 1976 1500 Mercury 90 hp compression,vacuum and fuel ??

Re: 1976 1500 Mercury 90 hp compression,vacuum and fuel ??

Originally posted by randyd12n12:<br /> I was also told that the older engines run weird due to the difference in fuel from them days and that I should change my oil to gas ratio to double. <br />Randy

Run at least mid grade 89 and most importantly make sure that the max advance on the timing is no more than 21 degrees BTDC. No need to double the oil ratio IMO if you are using a quality TCW-3 oil. The differences in modern fuel are why the timing was reduced to 21 degrees instead of the original 27 or 23. You may want to run the oil at 40/1 for peace of mind. Make sure the engine is propped to make at least 5500 RPMs also at Wide open throttle. These motors have been out of production for 17 years now and many mechanics may not have a lot of experience with their nuances. I think Clams Canino said something to the effect "that the mechanic usually needs to be in his 40s to have a chance."

 

[randyd12n12]

Re: 1976 1500 Mercury 90 hp compression,vacuum and fuel ??

Re: 1976 1500 Mercury 90 hp compression,vacuum and fuel ??

Thanks for the suggestions, I do not have much experience with outboards, so this is all a learning experience for me. <br /><br />Randy

 

[2cycle]

Re: 1976 1500 Mercury 90 hp compression,vacuum and fuel ??

Re: 1976 1500 Mercury 90 hp compression,vacuum and fuel ??

Randy,<br /><br />You can do most maintenance items on the motor with the help of a good Clymer or factory manual and basic mechanical aptitude/willingness. Getting a manual is by far the best $25-30 you can invest in the motor. Also, you can find out volumes just by using the Search function to read old posts/topics. The manual will allow you to learn the motor inside and out so to speak and will pay dividends as long as you own the motor. Keep us updated! <br /><br /> JF