Mercruiser 4 cyl carb issue

[13crazyhorse]

I have a 79 mercruiser 470 with a Rochester 2 bbl carburetor. The boat has been running perfectly all summer up to this point. My son went to use it the other day and it wouldn't start. He tried again about a week later still nothing. Turned over well just wouldn't start. So I went up this weekend to check it out. After trying to start it with no luck I could hear gas running into the carb after I tried to crank it over. I removed the carb and the intake manifold had about an inch of gas in it. I dried everything out with towels, removed the plugs let it air out and reinstalled the carb. The engine started right up and ran for about 30 seconds then quit. Same thing gas running in the carb after it stalled, manifold filling up again. I removed the carb, got a rebuild kit and went to my camper and rebuilt the carb as best I could. I'm a long ways from home so I couldn't blow everything out or clean good with solvent. I replaced everything except the pump assembly because the kit was missing the clip to hold it on, not quite like the old one. I took the carb back to the boat reinstalled and exactly the same thing happens. I also tried adjusting the float with no change. Anybody have any suggestions on what I'm doing wrong or missed? The gas is running in towards the bottom of the carb below the jet area and continues to run in for a couple of minutes after turning the engine over. I'm home now so can go through it again and also install the pump assembly and blow everything out. Thanks for any advice.

 

[Bt Doctur]

Re: Mercruiser 4 cyl carb issue

sunken float, needle and seat damaged

 

[13crazyhorse]

Re: Mercruiser 4 cyl carb issue

sunken float, needle and seat damaged

I took out the float and held it under water doesn't seem to have a hole. I replaced the need, seat and gasket? That was my first thought too.

 

[Tail_Gunner]

Re: Mercruiser 4 cyl carb issue

What you can do is let the carb soak in white vinegar for 24 hours followed by a baking soda bath to neutralize everyting. You must disassemble the whole carb power valve jets evertthing..If you have access to a garden hose blast out the circuts under high pressure water..and of course season to taste...

Actually if you do that the vinegar will clean it extremely well...watch it bubble up after a few hours it sounds corny but it work's

http://www.youtube.com/watch?v=wKiXLE6z58c

Little side note Do Not Try This In Your Shop.. Raise's to many eyebrow's

 

[13crazyhorse]

Re: Mercruiser 4 cyl carb issue

Interesting. I'll give it a try, like I said I didn't get a chance to clean it good up there or blow anything out. I can't image how it could be anything else. Hopefully that will do the trick cause it's about 300 miles round trip to the boat and we want to do some fishing!

 

[stonyloam]

Re: Mercruiser 4 cyl carb issue

Well looking at your symptoms: the carb floods! Well the only way that gas can get into the carb is past the needle and seat, right? Under normal fuel pressure the float, needle and seat act together to keep the fuel at the proper level in the bowl. So I believe that is where you need to look for the problem. New needle and seat from kit, so chances are that is OK. float drop set, so should be OK. That leaves float weight (if gas inside will sit too low), should be 9 grams, might have a leak. Just because you see no bubbles does not mean that it does not have a leak. Shake the float to see if there is fluid inside. Last thing to check is fuel pressure, if too high can force the gas past the needle. Could be a fuel pump problem. That is all I can think of, good luck.

 

[13crazyhorse]

Re: Mercruiser 4 cyl carb issue

Do you think the fuel pressure would suddenly increase? The fuel pump on the motor may be the original, if not it's the one that I've had on their for alot of years with no problem. I've tried a couple of different float levels with no different results. There is no fluid inside the float at all. I can blow into the gas intake and pick up on the float and the air flow stops, not alot of pressure of course. I totally disassembled the carb again and there are some dirty parts, maybe a good cleaning will do it. I don't have any way to check the fuel pump pressure I guess I could buy a gauge?

 

[stonyloam]

Re: Mercruiser 4 cyl carb issue

Is there fuel in the sight tube? If you have a ruptured diaphragm in the pump it will dump fuel into the carb through the clear tube from the pump. You could probably borrow a fuel pressure gauge from Autozone tool loan. Is the needle assembled correctly (rod, spring, needle body)?

 

[stonyloam]

Re: Mercruiser 4 cyl carb issue

OK just re read your original post. Worked fine, then floods for no apparent reason, rebuilt the carb, no change. I am beginning to suspect that it is not the carb, and you should take a real close look at the fuel pump.

 

[13crazyhorse]

Re: Mercruiser 4 cyl carb issue

I would tend to agree with you that maybe it's not the carb. I suspected the needle and seat but when I replaced with new was quite suprised there was no change. Even if I had a fuel pressure tester there is no way to hook it up to this application. I hate to spend money that is not going to fix the problem and a fuel pump for this engine is almost 200 bucks. I wouldn't mind if I knew that was going to fix it but I though the carb kit would do it too. There is a repair shop at the marina where my boat is maybe I'll call them and see if they have any way to test the fuel pressure. I'm kinda running out of options with this and am going to have to take time off from work just to do more tests.

 

[Eric 3.7LX Alpha 1]

Re: Mercruiser 4 cyl carb issue

Did you try and blow thru the fuel inlet of the carb then lift and lower the float and see if the needle and seat shut positive or leak by?

It does sound like the pressure is too high. A fuel pressure gauge would be the way to go.You can get a short piece of 3/8" brake/fuel line from an auto parts store (with ends on both), 3 or 4' of 3/8" fuel hose, some hose clamps and a fuel gauge and tee-(Sold at most auto parts stores).

Cut the new fuel line a couple of inches off each end. Remove the entire section of fuel line between the pump and carb. Replace it with the new pieces and hose and gauge as a temporary setup to check the pressure.

You'll know it 5 minutes if it's within spec .between 3-6psi.

 

[13crazyhorse]

Re: Mercruiser 4 cyl carb issue

Good thinking Eric that's an easy way to check the pressure I didn't think of that. I did blow through the inlet and pick up on the float - the needle stopped the air flow when I raised it so I guess everything there is working. Since I have to travel so far to get to my boat maybe I'll just order a fuel pump and put it on as I stated before the one on there is probably the original and it wouldn't hurt to change it anyway. Then maybe I could rebuild the old one and either sell it or keep it as a spare. I was just thinking if the pump went bad it wouldn't pump enough pressure - not too much, never heard of such a thing. I'll look around and see what I can find. Thanks for all the posts so far.

 

[stonyloam]

Re: Mercruiser 4 cyl carb issue

There is a clear line going from the pump to the carb, if the diaphragm has ruptured there will be gas in that line, if it is OK the line will be dry.

 

[13crazyhorse]

Re: Mercruiser 4 cyl carb issue

Well I bought a new fuel pump and drove to my boat after work. Brought the carb home the other day, took it all apart again, cleaned everything with solvent, blew it out and reassembled. I put the new pump and carb on and cranked the motor over. After a couple of minutes it fired up and ran like a top....for about 45 seconds then stalled. I looked in the carb and gas was pouring in flooding it out. How could I possibly get exactly the same results after all this. The float and all the other parts seem to be working properly. I guess my next move is a used carb and try that. Any ideas?

 

[Eric 3.7LX Alpha 1]

Re: Mercruiser 4 cyl carb issue

The needle valve and seat prevent the flooding as long as the psi is within spec. So Im going back to that. Did you put the needle valve clip on the float bracket correctly? as in not in any of the holes?

Did you try and blow thru the carb inlet and lift and lower the float and confirm the needle was closing 100%?

Still not positive you didn't get a bum fuel pump although probably slim, the pressure test would have told us for sure.

 

[fishrdan]

Re: Mercruiser 4 cyl carb issue

Still not positive you didn't get a bum fuel pump although probably slim, the pressure test would have told us for sure.

I'd test the fuel pressure going to the carb, should be 4-7PSI. There have been several threads here with people getting bad fuel pumps putting out too much pressure, so it's possible to replace a pump that was putting out too much pressure,,, with a new pump that's putting out too much pressure.

My old fuel pump was rusted up inside and putting out too much pressure, flooding the carb.

 

[stonyloam]

Re: Mercruiser 4 cyl carb issue

Agree with Eeic, you NEED to measure the fuel pressure. Especially with a new pump. May need a spacer on new pump, read this thread: http://www.breezeworks.net/cgi-bin/...forum=1&searchSort=dateDESC&ReturnPage=Search Pay particular attention to post #7, and here http://www.breezeworks.net/cgi-bin/...forum=1&searchSort=dateDESC&ReturnPage=Search

The thick gasket (spacer) is 27-43186

 

[13crazyhorse]

Re: Mercruiser 4 cyl carb issue

This particular carb doesn't have a needle valve clip. I did notice the new fuel pump came with 3 gaskets. I wonder if using more of the gaskets will move the pump further away from the engine and have a shorter stroke on the pump thus reducing the pressure. I hate to go through the process of buying a fuel pressure gauge and rigging up all the connections to test the pressure I'm probably looking at another hundred bucks by the time I"m through with that but at least I'll have more real facts to work with. Well I can't get up to the boat until next weekend so I'll keep ya all posted on the results.

 

[Failproof]

Re: Mercruiser 4 cyl carb issue

Does the entire fuel bowl drain? If so it has nothing to do with pump psi. I have seen only 2 carbs do this, and neither mercarbs, but corrosion ate away at the bowl and would allow all the fuel to drain. Then pump would refil bowl, and it would run. It would cause hard restarting, first because of flooding, then after sitting for awhile because of empty float bowl. Eventually it leaked enough that it started flooding engine, leaking out as fast as pump would fill. These were on antique tractors but the therory holds for any carb.

 

[stonyloam]

Re: Mercruiser 4 cyl carb issue

Autozone will loan you this for free: http://www.autozone.com/autozone/ac...ostic-kit/_/N-25h2?itemIdentifier=913437_0_0_