Mercruiser 120 engine stumble

[sidebojj]

Ok here goes, I have a 77 mercruiser 120 in a 18' Marquis that has a bad stumble / hesitation when you get on the throttle. I know this has been discussed on here before but I think I have done everything that was suggested. Since getting the boat a few weeks ago I have replaced the following:

Points
Dist Cap
Plugs
Plug Wires
Fuel pump
Set the point dwell to 32 deg
Set the timing to 6deg BTDC
Rebuilt the carb last night
Checked compression (between 115-125 on all four)

Nothing seems to have helped the stumble, when I set the dwell and timing the engine seemed to run smoother at idle and once I got past the stumble it climbed on plane better but the stumble seems to be worse. Like most people if I ease on the throttle it will run fine but any quick accel and it will fall on its face and stall if I don't back out of it. The problem is not RPM dependent, it will happen anytime the throttle is pushed. It feels like it is a lean bog. I have tried adjusting the idle speed and mixture screws with no sucess. The bog seemed to be almost gone before I put on the plug wires, cap, carb rebuild, and set the timing / dwell, but I was getting detonation at WOT (the timing was way advanced and the point gap was too big). It will stumble the same with load or no load, in gear and out of gear. Just to be clear on the repairs done;

bought boat-changed fuel pump, points,got it running, had miss and stumble.
Cleaned carb, changed plugs, set point gap - no more miss but still stumbled
At this point it ran decent, had detonation at WOT, stumble was there but not as bad.
Changed cap, wires, set dwell and timing, runs smoother, not more det at WOT, stumble is worse
Then I Rebuilt carb, and had no change on the stumble

Rebuild kit did not come with a power valve, could this be my problem? Seems to be operating right but could be the spring is shot on it. Any other ideas? I am struggling here. I read that some people set the float differently then the factory specs? What is the correct starting point for the idle speed and mixture screws?

 

[sidebojj]

Re: Mercruiser 120 engine stumble

Can you get the powervalve for this carb? I have been calling around everyone says its not available. Anyone know where I can find a part number?

 

[Limited-Time]

Re: Mercruiser 120 engine stumble

A carb rebuild is a 50/50 crap shoot at best. That said, it sounds as if your accelerator pump is bad. There should be a healthy squirt of gas visible when you pump the throttle. It's there to eliminate the off idle lean stumble you are describing.

 

[thrasher]

Re: Mercruiser 120 engine stumble

I would also suggest checking the accelerator pump. As you say the stumble occurs at any RPM, it sounds like a fuel starvation issue under acceleration. Check the accelerator pump.

Gary

 

[sidebojj]

Re: Mercruiser 120 engine stumble

Thanks for the reply. I had forgot to check the accel pump before I took the carb off last night, but I did test it when the carb was off and the bowl was still full and I did have 2 healthy streams of fuel when the throttle was opened.

Just talked to the Mercruiser dealer he said the carb kit they sell has a power valve in it and that is the only way to buy one. Looks like if I want to change it out I will have to buy the whole kit..again. Oh well 25-30 bucks would be more then worth it if I can get rid of this stumble. Any other ideas? I am going to check for vacuum leaks around the carb, try raising the float, and some more tuning of the idle speed and mixture.

Another question. When I was setting up my carb and trying to get the pump shaft to the correct number (1 1/8") from top of housing, by bending the rod I seemed to be maxxed out at 1". Could that 1/8" cause the stumble? By maxxed out I mean as I bent the rod the number would stay at 1" but my idle screw would be further from the cam.

 

[jamyers]

Re: Mercruiser 120 engine stumble

Thanks for the reply. I had forgot to check the accel pump before I took the carb off last night, but I did test it when the carb was off and the bowl was still full and I did have 2 healthy streams of fuel when the throttle was opened.

Just talked to the Mercruiser dealer he said the carb kit they sell has a power valve in it and that is the only way to buy one. Looks like if I want to change it out I will have to buy the whole kit..again. Oh well 25-30 bucks would be more then worth it if I can get rid of this stumble. Any other ideas? I am going to check for vacuum leaks around the carb, try raising the float, and some more tuning of the idle speed and mixture.

Another question. When I was setting up my carb and trying to get the pump shaft to the correct number (1 1/8") from top of housing, by bending the rod I seemed to be maxxed out at 1". Could that 1/8" cause the stumble? By maxxed out I mean as I bent the rod the number would stay at 1" but my idle screw would be further from the cam.

When you had the carb apart, did you make *sure* all of the internal passages were *clean*? Might double-check the tiny air bleed holes on top of the venturi cluster (the 3-screw part that goes into the tops of the throttle bores), make sure they're clear.

If everything is *clean*, then my money is on the accelerator pump. If you're straightening the link and the throttle is opening, the link isn't mounted correctly or there's something else wrong.

 

[sidebojj]

Re: Mercruiser 120 engine stumble

All passages are clear and free, soaked the venturi in carb cleaner (twice now) and used compressed air. Everything looked to be free when it was put back together.

I was trying to shorten the link to get to 1-1/8" from 1" since the power valve end of the link was maxxed out, the other end has to move which rotates the throttle lever clockwise, away from the cam.
pg 5B-7 and 5B-8
http://www.boatfix.com/merc/Servmanl/13/13B5R2.PDFend

 

[sidebojj]

Re: Mercruiser 120 engine stumble

Update:

Well I confirmed that it is a lean stumble (by using a spray bottle while I opened the throttle, stumble went away) and I found that the Accel pump is not performing properly. Just as you guys suspected. When I pulled the carb off to do the rebuild I forgot to check the pump operation until the carb was removed, and at that time it was working. Is there anything about the function of the pump that would allow it to work uninstalled and not pump when it is installed? Well, its not working now. Hopefully someone here has intimate experiance with these carbs and can help me.

The pump is installed correctly I confirmed and re confirmed, even tried flipping it around on the bracket. The problem I am having is that the pump is not pumping. With the top of the carb off I can get it to pump with the accel pump (in my hand simulating the motion of the carb linkage)only at the very top of its stroke. I have tried the new pump, old pump, and a combition of new and old parts on a third pump. If I put the pump in its housing and make a quick downword motion to simulate cracking the throttle it will squirt normally then fizzle out. When I re assembly the top of the carb the pump is no longer in the stroke range that causes the pump to work at all. When I crack the throttle it just drips a little gas instead of a steady squirt. The pump seems to bind on itself as the throttle rotates the shaft which pushes the pump up and down. It almost seems like the pump is just compressing on its spring rather then moving down the bore and pushing fluid into the carb.

Does anyone have any suggestions other then taking it to a carb expert? Something definatly seems wrong about the way the pump is setup. With the top of the carb off when the throttle is completly closed, the pump is hitting the bracket it mounts to such that it can not hang straight up and down. In order to get it straight up and down (to ride in the bore) The pump compresses its spring and sits off centered up against the bracket. The Bind I feel in the throttle linkage is most likely the spring coming unbound and straightening out to move down the bore. This probably makes no sence but I am trying to explain it the best I can.

I raised the float level in the carb, because it was on the low side and the pump did not want to do anything at all with it at that level. Blowing with compressed air showed no blockage in the passage from the pump to the venturi.

 

[tonka_shores]

Re: Mercruiser 120 engine stumble

What kind of gas are you using? If you are using 89 octane with a 10% ethanol blend, that's your problem. I had the same issues with my '72; hesitation, backfire, stalling. I was using gas from the local gas station. I tried the same fixes, carb re-build, tune up, timing. Didn't help.

I now use 92 octane non-oxygenated (no ethanol blend) mixed with some 100 octane racing gas. Without additional mechanical changes, the boat runs like a cat with it's tail on fire. So much new HP, I spun a prop hub (old prop)

One other thing, I don't know if your 77 needs leaded gas, but I have to used a lead substitute. CD2 makes a concentrate that treats a lot of fill-ups

 

[capthook]

Re: Mercruiser 120 engine stumble

Is this a Rochester or a Mercarb? If it is a Rochester take this into consideration. When I bought my rebuild kit it did not have the correct spring for the accel pump. My spring that I got in the kit would sit on top of a ridge inside of the accel pump walls and not down on top of the ball on the bottom of the carb in the accel pump wall. What would happen is that ball would shoot out the first time I tried accelerating. I took the top of the carb off and saw the ball rolling around at the bottom of the carb. I then noticed that the springs was not sitting on the ball like it should. Just my .02 cents.

 

[sidebojj]

Re: Mercruiser 120 engine stumble

The carb is a Rochester. The link posted above for the MerCarb was just for reference. If you look at the attached exploded view which "ball" are you talking about? Maybe I am missing something. The only ball I have in my carb is the one under the spring under the venturi. I belive it is called the pump discharge ball item 44 http://www.tocmp.com/manuals/Carbs/Rochester/2-Jet/Manual/Part2/MCarbRoch73802GPart202.jpg

 

[capthook]

Re: Mercruiser 120 engine stumble

I have never posted a pic like this so bare with me if it does not work. The ball in question is number 36 it is located right under the acceleration pump and is held down by a spring. Mine was corroded in there and I had to dig it out.

 

[capthook]

Re: Mercruiser 120 engine stumble

Try this go down half on the page and you will see an exploded veiw of my carb. I am not seeing the ball in your diagram so we may have different carbs.

http://forums.iboats.com/showthread.php?t=57418&page=2

 

[sidebojj]

Re: Mercruiser 120 engine stumble

That is interesting. Can anyone confirm weather my Rochester 2g Carb should have a ball under the accel pump? My kit did come with two different size balls but I just replaced the Pump discharge ball with the same size from the kit.

 

[jamyers]

Re: Mercruiser 120 engine stumble

That is interesting. Can anyone confirm weather my Rochester 2g Carb should have a ball under the accel pump? My kit did come with two different size balls but I just replaced the Pump discharge ball with the same size from the kit.

Yes, there absolutely should be a check-ball under the accel pump, it keeps the pump from shooting gas back into the fuel bowl. The spring under the pump keeps it in place. (I just had mine apart)

The other checkball is under the venturi cluster to keep the accel pump from sucking air back into the pump passages. I *think* the smaller one goes under the pump (inlet side), your rebuild kit should tell you which one goes where - or you can see if the big one is too big for the inlet side.

If the accel pump inlet checkball is missing, the pump will still shoot fuel into the throttle bores, just not nearly enough - and you'll get a stumble or lean-out.

Also, be *SURE* that the passages leading from the fuel bowl into the accel pump are *CLEAN*. They're low spots, and so notorious for collecting dirt/varnish/water/corrosion. On mine, I had to drill out the lead casting plugs to clean the passages, then epoxy some cut-off screws back in place of the plugs (not that hard to do, just time-consuming).

 

[sidebojj]

Re: Mercruiser 120 engine stumble

Thanks for the response. I will put in the ball and see if that fixes my problem. Sounds like it will. It seems strange that every refernce I found on the Rochester carb does not show that check ball except the one that was posted here. I don't have the parts in front of me (I'm at work) but the spring in the accel pump bore is much larger in diameter then any of the check balls that came in the kit. How does the spring hold it down? Does the edge of the spring on the diamter sit over the ball in its cavity?

Thanks again for the information.

The carb rebuild kit I bought (from Napa) came with no instructions, not even a parts list, so I used the rebuild steps from a Mercarb (seems to be a very similar carb) and the specifications and adjustments for my carb. The kit did come with a chart of rod lengths, float adjustments etc. for my application.

 

[jamyers]

Re: Mercruiser 120 engine stumble

...How does the spring hold it down? Does the edge of the spring on the diamter sit over the ball in its cavity?

Yes, one side of the spring sits on top of the ball.

...The carb rebuild kit I bought (from Napa) came with no instructions, not even a parts list, so I used the rebuild steps from a Mercarb (seems to be a very similar carb) and the specifications and adjustments for my carb. The kit did come with a chart of rod lengths, float adjustments etc. for my application.

Most rebuild kits don't have step-by-step directions, except for setting the rod lengths, float height, etc. But there oughta be an exploded view of the carb...odd that there wasn't.

Let us know how it works out!

 

[capthook]

Re: Mercruiser 120 engine stumble

Thanks for the response. I will put in the ball and see if that fixes my problem. Sounds like it will. It seems strange that every refernce I found on the Rochester carb does not show that check ball except the one that was posted here. I don't have the parts in front of me (I'm at work) but the spring in the accel pump bore is much larger in diameter then any of the check balls that came in the kit. How does the spring hold it down? Does the edge of the spring on the diamter sit over the ball in its cavity?

Thanks again for the information.

The carb rebuild kit I bought (from Napa) came with no instructions, not even a parts list, so I used the rebuild steps from a Mercarb (seems to be a very similar carb) and the specifications and adjustments for my carb. The kit did come with a chart of rod lengths, float adjustments etc. for my application.

The accelerator pump spring (the one that is about and inch and a half long) should sit right on top of your check ball. This is all that is going to hold it down. Check for this ball it should be there. If it is and you can't get it out, I used a long screw and kept picking at the ball and mine finally freed up and I got it out.

 

[MtnBoater]

Re: Mercruiser 120 engine stumble

One thing I noticed when rebuilding my accel pump, was that it was nearly impossible to get the shaft from the linkage arm outside the carb, to go thru the little collar inside the carb(that attaches to the plunger) and tightened. If it's not done right, it won't pump. That might be why it works off the engine, but not after you install it. It was very, very tricky to get that shaft to line up correctly and then tighten the little set screw. With the help of an extra set of hands, and some patience, we got it to hold tight on there. Had to take the carb back off and apart to get that one little important detail corrected. Hope that made sense?

Your pump should squirt nice streams when you hit the throttle, if it isn't, then that is without a doubt the problem. Have a friend hit the throttle while you look down inside. Unfortunately I think you spent alot of money on unnecessary parts and repairs. It was worth the knowledge though right?

 

[sidebojj]

Re: Mercruiser 120 engine stumble

jamyers and capthook,

Thanks for your help with the carb. I took it apart again and dropped in the missing check ball, and my stumble is gone. Two nice solid streams of fuel when the throttle is opened. I have not had the boat back in the water yet but when running on the muffs, the stumble is completly gone. Feels good to finally get this one figured out, it had been really bothering me. Now I just wonder why the exploded views of the Rochester carb I found do not show the check ball? Maybe an old mistake that was never caught? Anyway thanks again, without your help I would have never caught the mistake.

Josh



Mtn boater,
If you read the whole post you would realize I did not throw money at unnecessary parts. When I bought the boat it had been sitting for a few years and the points and fuel pump needed to be replaced to get it running. Changing the plugs was just good maintenance. Changing the cap and wires was also a good idea as I was chasing down a slight misfire along with the stumble.