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75 Mercury issuescanbt startfuel running out of carb

J

jstluise

Member
"Hello everyone,

I recently

"Hello everyone,

I recently purchased a 7.5 Mercury outboard in excellent condition. I was amazed to find out it is a 1975 year from its serial...it's in that prime of condition. The individual I bought it from had it stored indoors for the last 5 years. He couldn't get it started and just wanted to get rid of it. Sounded to me like an issue with the carb so I jumped at the offer.

He said he could get it to turn over, but only at full throttle and only for a little bit until it choked out.

First off, when the fuel bulb was pumped, gas would come out the carb. This was an issue with the float. I pulled off the carb and took a look. Didn't see anything wrong but cleaned it up and put it back together. That issue was resolved so the float is working properly.

After that, still gas would come out the carb when trying to start. Not a lot but I dont think it is normal. Still, we couldn't get it to fire at all. Someone suggested it could be the fuel pump diaphram. I bought a gasket kit for the fuel pump and changed it out. Still nothing, there is still a little fuel that comes out the carb but not as much.

Even if you flood the engine are you ever suppose to see any fuel come out the carb? I wouldn't think so....

Any suggestions? I have spark. One thing I found odd was that after trying to start it for a while and pulling the plugs, the inside of the cylinders seems relatively dry....I would suspect quite a bit of fuel to be inside after seeing how much would come out the carb.

Any help would be great.


Thanks"
 
"Have you checked the reeds? m

"Have you checked the reeds? might be plugged solid or stuck? one of the few things I can think of that would prevent fuel from getting into the cylinder (and would explain it coming out of the carb because it would have nowhere else to go). Unfortuanately, with your model you have to pull the power head to check them. So you may want to wait for a second opinion."
 
"Thanks for the response

We

"Thanks for the response

Well this is what I was thinking...that there is something between the carb and the cylinders that is preventing fuel from getting into the cylinders. I did not know about the 'reeds' that you speak of.

To me, if I flood it I should find a substantial amount of fuel on the plugs and in the cylinders...but nothing really at all...there is a trace amount of something on the plugs but nothing like there is too much fuel.

I've only got a back fire once or twice when trying to start it. Also, tried some starter fluid once and after a while of pulling it actually started and ran for a second until the starter fluid was burned up. However, I like to stay away from that starter fluid.

It makes me feel better to know that there is actually something between the carb and the cylinder...I just wasn't aware of it.

I am in the process of getting a manual for the motor and then I can take a look. Hopefully someone else will chime in with some advise.

Thanks again"
 
Reeds act as the valves with 2

Reeds act as the valves with 2 stroke engines - they open/close to let air/gas/exhaust in/out of the cylinders. On your 7.5 they are located around the drive shift which is why the power head must be removed to access them.
 
"So I looked around a little o

"So I looked around a little online about reeds because I didn't know about them. From what I understand they are pretty much one way valves that control the fuel/air mixture that goes into the cylinders. That does make sense that if those valves were plugged up, only a very little amount of fuel will be able to get into the cylinders.

That explains why the guy said he could only get it to run on full throttle for a short amount of time.

If anyone could shed some light on this issue that would be great. To me, it makes sense the reeds are causing the problem.



Also, I see that Boyeson power reeds are pretty popular for outboards. If the reeds are the problem should I replace them with Boyeson reeds or just clean up the originals?"
 
Great! I wasn't sure if I

Great! I wasn't sure if I would be able to see them by taking the carb off. I will definitely try that.

Thanks
 
"sounds to me as if a reed is

"sounds to me as if a reed is stuck open or is missing, you won't see them all with the flashlight"
 
How many reeds are there? I&#

How many reeds are there? I'll figure it all out when I pick up the manual. But this sounds like exactly what you guys are saying.

I appreciate all the help and I'll let you guys know the progress.


What do you think about Boyeson reeds if i have to end up replacing them?
 
"its only a 7.5hp, boyeson ree

"its only a 7.5hp, boyeson reeds or merc will do fine. those reeds are on a cage that wraps around the crank, looks like 8 of them from the link I gave you"
 
"If I am remembering correctly

"If I am remembering correctly you should have 10 total - there is two semi-circular blocks with 5 in each. The two separate blocks are bolted together around the crank. When you get the bolts out you can give them a bit of a "whack" to get them to separate. But it has been a few years since I tore down a 7.5/9.8
"
 
"I had a similar problem with

"I had a similar problem with my 1968 Merc 200. It turned out that on my engine, the rubber oil seals on both ends of the crankshaft had deteriorated and were no longer sealing the lower crankcase. Without a good seal, there was not enough vacuum to open the reeds. If the reeds look OK on your engine, you may need to replace these oil seals. They don't cost much, but on my engine, it involved breaking open the crankcase."
 
"I thought of that also, but h

"I thought of that also, but he said it was spitting fuel out the carb, which means back pressure. Good point though. Either way he'll have to crack the case open.
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"Hey guys thanks for all the i

"Hey guys thanks for all the ideas. I want to tear into it as soon as possible, but I am still waiting to finish up the last couple weeks of school.

When I tear it down will i be able to see pretty easily if the reeds are the problem? I mean, will they be gummed up to where I can see they are not working? I am not even sure what they look like so I guess I will wait and see until I have it apart.

Thanks again"
 
"Spoke to a mechanic at the ma

"Spoke to a mechanic at the marine engine place and he said he has never heard of or seen the reeds sticking on these motors....what do you guys think? It seems as though it is possible if you guys are suggesting it.

He said that the suction created by the engine is what pulls the gas in from the carb...so if the reeds were fouled there would be no suction, and thus no gas. Is this reasonable to think?

How exactly is the gas pulled in through the carb? Is it by the suction?

I just really want to make sure what it is...at least pretty sure. The problem with the reeds sounded like the problem so I was going to tear into the engine shortly but after talking to that guy I had to stop and think.

Thanks for all the help"
 
"Some number of posts back you

"Some number of posts back you noted that when you pulled the plugs, the cylinders were dry. That indicated that gas was not making it into them at all, hence the diagnosis of stuck reeds. But basically, the fuel pump (diaphram) and the bulb on the fuel line (initially when you pump it to prime the carb) gets gas to the bowl in the carb, then suction (or vaccum) created by the cylinder sucks the air/fuel mixture in through the reeds which open on the induction/compression stroke...."
 
"Thanks for the response.

[

"Thanks for the response.

"But basically, the fuel pump (diaphram) and the bulb on the fuel line (initially when you pump it to prime the carb) gets gas to the bowl in the carb, then suction (or vaccum) created by the cylinder sucks the air/fuel mixture in through the reeds which open on the induction/compression stroke...."

Okay, see this was my understanding on how it worked. But now, I guess my questions is how does the gas from the bowl in the carb into the mouth of the carb? The mechanic said that the vacuum created by the cylinders is what does this....so he is saying that if I have gas coming out the of the carb, there is vacuum because the vacuum is pulling the gas out of the carb. Therefore, if the reeds were stuck, there would be no vacuum and thus no sign of gas.

Yes, when I pulled the plugs after a number of pulls with full choke the cylinders looked dry. The plugs had a little bit of something on them but not as much as I would have thought if the cylinders were flooded....on this same not I have gotten it to fire a couple times (not a backfire) and also it ran a little bit when starter fluid was used.

If only some of the reeds were fouled would this creat enough vacuum to pull the gas out of the carb but just not enough vacuum to get gas/air into the cylinder?


Sorry for all the questions...I will call around to a couple marine engine places to see what they think...I appreciate all your help though.}"
 
"Here is my understanding of t

"Here is my understanding of the fuel pump on your particular model. You have the 1st generation carb that included the fuel pump as part of the carb on the small Mercs (at least that's what you should have based on your model year/hp).

The diaphram that pulls gas in from the gas tank and then pushes it into the carb works on vaccum from the cylinder. If you removed your carb from power head, you would notice 2 very small holes (one on each side) below the "throat" of the carb (and there would be corresponding ones on the carb itself. These are the vaccum ports. When the cylinder goes up it creates a "sucking-in" vaccum which travels through these ports and "pulls" the diapham so that it suck gas into the "fuel pump" part. Then on the down stroke, the pressure reverses and squeezes the diaphram pressing the gas out, into the carb. So although they are "co-located" the carb/fuel pump actually work independantly. So the reeds would have no affect on the fuel pump operation. At least this is my understanding of it (after giving my dad a call (small engine mechanic since the 40's)...."
 
"I cannot thank you enough for

"I cannot thank you enough for the help you have provided me in trying to diagnos this problem.

At first with all the gas coming out of the carb a friend suggested that the fuel diaphram was shot and that is why so much fuel was getting into carb. I had the carb apart and changed the fuel diaphram and made sure the float was working correctly.

Okay, so that makes good sense...that the fuel supply is independent of the reed operation.

So, now my question is have you ever seen or heard of the reeds sticking? I take it you have since you mentioned it. It was just weird because the guy from the marine shop never heard of it. (I personally think it is a possibility...and the culprit)

Thanks again I really appreciate it."
 
"Yes - it's not overly com

"Yes - it's not overly common but have seen it a few times - also seen them cracked or chipped - they are not super "fragile" but they can be easily damaged. In the case of your motor, if it wasn't properly "prepared" for 5 years of storage, there could have been gas in the system that gummed up everything - including the reeds..."
 
Is there a possibility that th

Is there a possibility that the reeds are partially fouled where the motor will run but only at WOT?

I was just curious because the guy I purchased it from said he could get it to run for a little bit at WOT before it would choke out and die.

An we have gotten it to pop here and there a few times.

Thanks
 
"I guess it is possible that a

"I guess it is possible that at WOT perhaps a little gas is making through, enough for the engine to fire - but that would be purely speculation.

The only way you are going to be able to determine for sure is pull the powerhead and split her.

At this point I think you have exhausted your other options and investigated or corrected any other possible problems..."
 
"Alright well I guess this is

"Alright well I guess this is my last resort.

Any advice for me when I tear it apart. I got a hold of a Seloc manual for it and read through the appropriate section and it seems pretty straight forward but I just wanted to make sure there isn't anything tricky I should be looking for.

Also, after getting it apart will I be able to tell pretty easily if the reeds are the culprit? I am not totally sure exactly what they look like besides....well i guess a reed.


Again...I appreciate your assistance"
 
"Yes, it is actually fairly st

"Yes, it is actually fairly straight forward. And yes, once torn down you will be able to tell if the reeds are bad or not. There is also crankshaft seals (like an o-ring) - if the reeds are ok these may be your problem (but even if it's them you still have to split it to get at them).

My advice would be, don't try and save any seals/gaskets that you have to disturb - they are still available and only cost a couple of bucks. You don't want to get it back together and find you need to tear it down again because of a bad gasket.

On this model there is kinda 3 (parts) to the reeds. There is the reed block (which kinda looks like a donut around the crankshaft). The block splits into two semi-cirles for removal. The reeds cover the (holes/ports) through this block on both halves, then there is a reed stop (kinda like a bracket) with a screw through it which holds the reeds in place and stops them from opening too far.

If you follow the manual, you should be fine
"
 
"before you tear it apart have

"before you tear it apart have you checked the flywheel key, put it up on top dead center and see where the timing marks are, just a last ditch effort before you tear into it."
 
"Yes, JB has an excellent poin

"Yes, JB has an excellent point - it may be trying to exhaust when it should be "breathing" or something in between....

If you do have to tear into though, another thing you may want to do is take a few pictures from different angles (especially easy if you have a digital camera).

I have found it helpful myself, especially if working on a model that I am not totally familiar with....

Sometimes you just can't remember for sure (and the manual isn't clear) whether or not that gasket/seal/o-ring etc went on top of or under that plate or whatever..."
 
The flywheel controls the igni

The flywheel controls the ignition timing on the motor. If it is sheared the spark will occur at the wrong time.
 
"[i]before you tear it apart h

"before you tear it apart have you checked the flywheel key, put it up on top dead center and see where the timing marks are, just a last ditch effort before you tear into it.

I will try that. It may just save me some time.

Thanks}"
 
"I had some time today to look

"I had some time today to look at the motor. My dad insisted that it is the carb because...well he is stubborn. After talking to the local marine dealer and hearing the guy say that he has never heard of the reeds fouling, my dad won't even think about that possibility.

Just to make him happy I took the carb off again and disassembled it completely (following the guidelines in the book). The needles all looked good and I couldn't see any blocked passages. This carb is clean. The only thing that seemed funny was that the book said there is a spring that goes somewhere with the valve for the fuel bowl (the fuel inlet valve that is controlled by the float). Mine just had the valve, but no spring anywhere. I think the spring just keeps the valve open (unless the float has it closed), so this shouldn't affect anything right now.

Got it all back together and tried to start her. Oh, and I got new plugs too. Same result. Pulled on it a few times and eventually fuel came out the carb.

The plugs seemed just as clean as they were when i put them in...one seems to have a little bit of something on it.

I checked the timing like you guys said and everything seems good."
 
Just saw this post... have you

Just saw this post... have you checked the compression yet? I also think it would be highly unlikely that reeds have gone bad on such a motor. Have you tried to put a few drops of fuel into the spark plug holes to see if it will then start? If it does can you keep it running by giving it some throttle or choke?
 
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