78 Mercury 90hp Inline 6 carb. issue

[Gomer50]

I ran my fresh Mercury rebuilt 1978 inline 6 this weekend and ran very nice except I used a lot of fuel 6 gallons in 1 1/2 hour. Throttling up and down cruising at various rpm's to break it in. I noticed that the carb. linkage that connects carb.1 to carb.2 and carb.3 are soaked with fuel. Which leads me to believe there is way to much gas in the carb throats.Carbs are not leaking anywhere else.Any Ideas on what causes this problem.

 

[Bob_VT]

Re: 78 Mercury 90hp Inline 6 carb. issue

Link and sync http://forums.iboats.com/showthread.php?t=168855 to get things balanced.

Yup the old inlines were NOT shy of drinking fuel but run great

 

[Gomer50]

Re: 78 Mercury 90hp Inline 6 carb. issue

Link and sync was done.I am pretty satisfied with the performance results, this motor sits on the back of a 1972 15 foot starcraft trihull weighing 1100 lbs, motor 300 lbs, humans 500 lbs and other junk. So I would say around 2000 lbs. 13 1/4 17 pitch SS prop turning @ WOT 5000 RPM and 36 MPH by GPS isnt too bad.

 

[Gomer50]

Re: 78 Mercury 90hp Inline 6 carb. issue

Carb. linkage that connects carb.1 to carb.2 and carb.3 are still leaking gas. Which leads me to believe there is still way to much gas in the carb throats.Carbs are not leaking anywhere else.Any Ideas on what causes this problem.

 

[Bob_VT]

Re: 78 Mercury 90hp Inline 6 carb. issue

You are not hitting WOT ...... you should be turning 5500-5800 with that motor. Are you sure the butterfly's are opening all the way, is the tach accurate? you might be better off moving to a 15" pitch prop. Is the gas leaking out of the linkage shaft ???

 

[Gomer50]

Re: 78 Mercury 90hp Inline 6 carb. issue

Yes the gas is leaking out of the linkage shaft (s). I have been speaking with a gentleman thats been dealing with Older Mercs. for a long time and he mentioned that the back draft carbs were a bad design and were frequently known as piston eaters. He suggested that I plug off the little hose outlets/inlets.It was also mentioned that when priming with the bulb the bulb can push in upwards of 16 psi of fuel pressure which will push excess gas into the carb throat and of course drible down the linkage.So all in all I think I will take these carbs back off and put on an older set without the back draft with one size larger jet. Suggested by an old Merc. guru.

 

[Bob_VT]

Re: 78 Mercury 90hp Inline 6 carb. issue

Yes a good old BB blocks that line just perfect. It was an economy thing that Merc designed in and best blocked off.

 

[Moody Blue]

Re: 78 Mercury 90hp Inline 6 carb. issue

Out of curiosity, when were the backdraft carbs introduced, and how long were they in production? Were they used on all the in-lines or specific models?

 

[Gomer50]

Re: 78 Mercury 90hp Inline 6 carb. issue

The first back draft carbs I have seen were on a 1974 85hp inline 4 and I also have 1978 90hp inline 6 that have the same ones.I have also seen these on a 1980 inline 6 115hp as well.

 

[Maxz695]

Re: 78 Mercury 90hp Inline 6 carb. issue

remind me not to ever buy one of these thanks

 

[Faztbullet]

Re: 78 Mercury 90hp Inline 6 carb. issue

You are not hitting WOT ...... you should be turning 5500-5800

RPM spec's for a 90 is 4500-5000.....

 

[Chris1956]

Re: 78 Mercury 90hp Inline 6 carb. issue

Wide open specs for that motor would be 9 gal/hour, so your fuel consumption is pretty normal. Try a V6 motor sometime. They have 3 two bbl carbs and can really suck up some fuel!

I ran wide open in my speedboat with a Merc 1500 inline six. Used 6 gal fuel in 20 min, but covered nearly 20 miles.
I recommend you check your carb float heights. Sounds like you have a small overflow, into the venturies.

BTW - Don't worry about the backdraft carbs. They will work just fine, as long an they are kept clean. Just like any other carbs.