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Results 1 to 12 of 12
  1. #1
    Join Date
    Oct 2010
    Location
    East Contra Costa County, CA
    Posts
    7

    Default 1996 Mercury 9.9HP 4-stroke Carb question

    Hi all,

    I have the subject motor and I just cannot get the carb set right. At one time I was able to get the motor started, but then after a minute the rpms would start increasing until they got up to somewhere around 1500-1800. Then I would lower the idle speed and it would run. But if I stopped it and let it set for more that one minute it would not start again until I backed out the idle speed adjustment screw again. Then I would have to go through the whole thing again.

    Now, I am at the point where I can get the motor to start every time (almost) and the rpms remain stable, but I think the mixture is way too rich. And, if the engine does not start on the very first pull on the rope I could be there for a while. When it does start I can take a clean white paper towel and place it near the exhaust hole just under the top cover and in just a second or two there will be a black area on the towel. It doesn't matter if I do this immediately after starting the engine, or after it has been running for several minutes, the result is the same.

    So my question is, how do I lean the mixture on the carb? To make things more difficult for you, this is the "Bodensee" engine (or "Bondensee" if you prefer). There are only two adjustments that I have found and I just cannot seem to get the correct setup. No matter what I do, the mixture seems to be too rich.

    Thanks in advance,

    Wayne

    1996 Mercury 9.9HP 9.9MLA4ST serial 0H008826

  2. #2
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    9,033

    Default Re: 1996 Mercury 9.9HP 4-stroke Carb question

    If fuel is left in the carburetor between use it will plug up the jets.-------------These small jets are VERY SMALL. and often hard to see.----------------I think your best bet is to clean the carburetor by taking it apart and examining all jets ( use a magnifying glass if needed)--------------------Should not be difficult.

  3. #3
    Join Date
    Nov 2009
    Location
    temecula, ca., usa
    Posts
    1,474

    Default Re: 1996 Mercury 9.9HP 4-stroke Carb question

    Hi,

    First let me say that I am in NO WAY an expert on this. I don't know this engine and I don't know this carburetor. I ran across your post while researching a basket case motor I'm considering buying.
    But, I've been a mechanic for many (too many) years and am an accomplished Honda tech so I thought I'd throw a couple of ideas at you.

    I looked up the carb parts breakdown and see that it uses a heater coil type fuel enrichment system for cold start and an accelerator pump circuit. Refer to the link below. Item 3 is your heater. Item 15 is the pump rod and item 17 is called a "choke shaft".

    Honda uses the heater type choke system and it is my experience that the heater units are very reliable but, then again, that's Honda. The system is designed to be in the FULL RICH mode when the engine is cold and then gradually lean out as the charge coil sends current to the heater. That's how you test the heater. You take it off and apply 12v to the leads and watch to see the pintle or needle extend within about a minute or so. Probably need a Merc manual to verify this though so, agian, I'm just giving you some food for thought.

    http://www.boats.net/parts/search/Me...TOR/parts.html

    One thing that caught my eye on the parts list though was the fact that the "accelerator pump rod" (item 15) had threads and what appears to be an adjustment nut (item 16) associated with the "choke shaft" (item 17). It made me think that you might be able to adjust that "choke shaft" on that rod and change your mixture.

    I can tell you that I do many Honda carbs and the ones that have accelerator pump circuits are pretty finicky and just will not run well if that circuit is clogged or malfunctioning. So, racerone makes a good point in that the carb, all carbs, need to be drained religously and be kept clean internally to function properly.

    These little carburetors are usually pretty simple and, if you take your time and study each nook and cranny and figure out where all the passages lead they can be cleaned with WD-40 and carb spray. Digital photography at teardown will help you get it back together properly. I wouldn't start though without some better info than I've offered so a repair manual is always good and don't reuse the old O rings or seals.
    Good luck.

  4. #4
    Join Date
    Oct 2010
    Location
    East Contra Costa County, CA
    Posts
    7

    Default Re: 1996 Mercury 9.9HP 4-stroke Carb question

    Thanks for the replies.

    Regarding the cleanliness of the carb, the motor had been sitting for a while before I got my hands on it. One of the things I did was to remove the carb, completely disassemble it, and dunk it in some carb cleaner for a couple of days. It wasn't too dirty to start with, and after a good scrubbing, soaking, and blowing it out with compressed air I am reasonably certain it is spankin' clean. The one thing I am worried about though, is that I had to re-use the old gaskets. They ones I took out did not LOOK bad as I did not damage them while disassembling the carb. At the time, I did not have a source for a kit, so I will probably re-visit that.

    Regarding the heater coil for enrichment, I removed that from the carb and performed the test outlined in the Mercury service manual. It passed. Sounds like the same test as the Honda uses.

    Lastly, I will look into the accelerator pump shaft although I have to admit that looking at it I don't see how it could affect mixture. The parts list does say "shaft-choke" though so it will definitely get more of my attention.

    I had the motor running again today, and I did notice one more thing. I don't know if the engine did this before because I never rev'd the engine this high before, but I noticed some popping in the carb intake when I rev'd the engine up pretty high.

    Thanks again for your help.

    Wayne

  5. #5
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    9,033

    Default Re: 1996 Mercury 9.9HP 4-stroke Carb question

    Since you say that it is not the carburetor , then maybe look at valve clearance.

  6. #6
    Join Date
    Nov 2009
    Location
    temecula, ca., usa
    Posts
    1,474

    Default Re: 1996 Mercury 9.9HP 4-stroke Carb question

    Hi Wayne,

    Yeah, in the several years I've been doing the Honda carbs, I've never found a failed heater coil so I'm not surprised that it passed. The pure symptom for one of those being shot would be that the engine starts easily, idles up smoothly and then starts to load up badly as it warms up. Not your symptoms.

    Not knowing the composition of your carburetor, I could be "all wet" here but I have to say that it does concern me that you "dunked" the carb in cleaner. I hope you didn't use a caustic type immersion cleaner. You can't get away with that on the Honda carbs as they contain several plastic parts that are pretty difficult to remove including shaft bushings and the throttle plate.

    Popping back through the intake means that combustion is taking place while the intake valve is still open. I would check the valve lash and confirm that the timing is right on the money.

    In addition to poor timing and valve adjustment too tight, other things to look for would be weak or broken intake valve springs or excessive carbon build-up on the intake valve stems causing them to hang open.

    If the engine has been running overly rich for a while, then carbon build-up is a distinct possibility. Carbon in the combustion chamber could also account for pre-ignition by causing a "glow plug" effect but this is fairly rare and usually occurs in an engine that has been run for an extended period with the thermostat removed or stuck open.

    If you have a compression tester, you could find a hanging intake valve with that. Sounds like you are reasonably well equipped with tools and an air compressor so maybe a cylinder leak down test might be possible too.

    Going back over your original post, I see that you are concerned about the exhaust making black deposits on a towel. That doesn't necessarily mean rich although it could. It could also indicate incomplete combustion so going over the valve train and timing is warranted.

    Have you owned this motor for a while or is it a recent purchase? The motor's history, especially cooling system performance, would be good to know here.

    Lastly, replacing the carb seals and gaskets is always a good idea. Looking at that accelerator circuit and understanding how it works is absolutely CRITICAL with the Honda carbs. Those engines just will not run if that circuit is not fully open. It's as if the carb uses that circuit to "breathe". I disassemble and then force carb spray through the boost passage with the throttle plate propped fully open. If it doesn't shoot at least a 3 ft. stream out the back of the carb throat, I backflush the passage and keep cleaning until it does.

    Let us know what you find and good luck.
    Last edited by jgmo; 10-14-2010 at 08:04 AM. Reason: clarity

  7. #7
    Join Date
    Oct 2010
    Location
    East Contra Costa County, CA
    Posts
    7

    Default Re: 1996 Mercury 9.9HP 4-stroke Carb question

    Hi again,

    racerone - I didn't mean to give the impression that I don't think the problem lies in the carb. I still do. I just don't think it is clogged jets or passages. I still think it is just that I haven't found the right combination of settings yet, and with jgmo's info regarding the accelerator pump, I have a couple more things to look in to.

    But, the popping in the intake is a concern to be sure. I'll go through and check the timing, but I'm not sure I really want to get into the valve lash adjustments just yet. The power head itself, aside from the rust and corrosion that was in the front part, looks as good as the day it came off the assembly line. Not even a scratch in the paint. But a power head rebuild COULD be a winter project though. Then I would know that all of the internals and the adjustments were right.

    The carb cleaner that I used was (I think) Berryman's. The stuff you buy at the local auto parts store in a gallon can and there is a steel basket inside. That used to be some really nasty #$%@, but this last can (I haven't used it in years) was a complete waste of money as far as I'm concerned. There are a couple of plastic parts on the linkage, and I am certain that it had no affect on any them. I think I will just start using mineral spirits from now on. It did a much better job of grease removal on my hands.

    I do have a compression tester, but I don't have the adapter for the 10mm spark plug holes. I went to buy one and I can't get just the adapter. I'm not going to buy a whole new tester just to get the adapter.

    Cooling system seem to be in good working order. The water coming out of the pee hole gets a few degrees warmer than the water coming in to the motor, but it is still cool.

    I can't give you too much history on the motor. My son and his best friend were looking for a kicker to be used for trolling. They had looked around for a good used 4-stroke, and you know how kids are. When I say they looked around I mean for maybe a couple of hours. Anyway they found this one on Craigslist and it was just a couple of miles from where they both work. The man that owned it said he was the second owner but he had it for a number of years but had not used it for at least a couple of years. Of course, it ran great the last time he used it. Yeah. Right. The rope start mechanism was seized as was the shift mechanism, but the motor turned and the prop turned so we bought it.

    We brought it to my house and I did some work on it. Some water got into the cover and settled in the shift linkage and it was just rusted up. All of the lubricants in the recoil starter got hard and someone had done a really lousy job of re-assembling that. So the first few days I just disassembled all that stuff, cleaned up the rust, re-lubed everything, and got it working smoothly. Then I started on the carb. But after I got the carb back on the motor, and was at least able to get it started, my sons friend got a hair up his arse and just HAD to get it mounted on his boat so it went to his place. He didn't lay a finger on it for probably six months or maybe even a little longer and then bought a Tahoe Q4, and shortly thereafter bought an electric trolling motor for it. Now he has no use at all for this motor. So it sat there a few more months.

    Then my son bought an old 1976 Silverline 22vht and has mounted the motor on it as a kicker. He used it one time and it left him stranded, which I think is kinda funny. At least it would be if it wasn't my fault. He went to Pinecrest lake to do a little fishing. After a while he just wanted to enjoy his time on the lake and did a little reading. When he got ready to come in, he gave the rope a pull and almost fell out the front of the boat (and 22 feet away was a long way to go). There was a pin in the recoil that came out and it would not turn the engine and there was no resistance on the rope. After some looking, he found the pin in the housing, put it back in and gave the rope a couple of pulls, THIS time when the pin came out . . .splash. So now he has to take the next hour or so to get his main engine started which is another long story, but he finally made it back.

    Anyway, now to me. I finally got my boat on the water again after some engine problems so I brought the motor over to my place so I could fabricate a mount. I wanted to take another shot at getting it running right before working on the mount so that is where I am. Probably a lot more than you wanted to know.

    Thanks again everyone, and sorry for the long response.

    Wayne



  8. #8
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    9,033

    Default Re: 1996 Mercury 9.9HP 4-stroke Carb question

    Have you checked that spark will jump a gap of say 3/8" on both plug wires , with a snap you can hear ?

  9. #9
    Join Date
    Oct 2010
    Location
    East Contra Costa County, CA
    Posts
    7

    Default Re: 1996 Mercury 9.9HP 4-stroke Carb question

    Well, I think I'm making some progress, but I'm just not sure that it is progress in the forward direction. I now have it so the motor will start first pull every time when cold. I can shut it down and it will immediately restart just fine. It also does not start increasing rpm anymore. But if I run it for several minutes til it's warmed up, shut it off, and let it set for more than two or three minutes it will not start. When that happens I have two options. 1. Wait for it to cool down completely which would be waiting for perhaps 30 minutes or more. 2. screw the idle speed adjusting screw IN two full turns and then it will start on the fifth or sixth pull of the rope. When it starts the rpms will go fast and then I can turn the idle speed back out right away.

    I still have not "played" around with the accelerator/choke shaft yet. And I still need to get a good rebuild kit for it and also check out some of the other suggestions, but I do think I am getting somewhere.

    Wayne

  10. #10
    Join Date
    Oct 2010
    Location
    East Contra Costa County, CA
    Posts
    7

    Default Re: 1996 Mercury 9.9HP 4-stroke Carb question

    I finally found a source for the carb kit and ordered one. The repair kit is just not available from Mercury anymore and since the engine is built by Yamaha, I spoke to a Yamaha authorized repair facility and they steered me in the right direction. The equivalent Yamaha motor for my Mercury is a model T9.9MLHU and there are still kits available for it. Almost makes me want to order two or three of them though.

    Wayne

  11. #11
    Join Date
    Oct 2010
    Location
    East Contra Costa County, CA
    Posts
    7

    Default Re: 1996 Mercury 9.9HP 4-stroke Carb question

    Just thought I'd give everyone an update. I received the carb rebuild kit (and it really is NOT worth the $25) and put it in. In doing so I found the source of my carb problems. When I reassembled it the last time, in my haste I skipped a few words in the service manual. Those words are "Align the slit in the main nozzle with the screw hole and . . ." and I just went to "Install the screw and main nozzle." So with the nozzle out of alignment, it wreaked havoc with my fuel mixture. Now that I have it installed correctly, the motor starts properly under all conditions, it runs smoother and quieter, and before it smoked a little bit and now it doesn't. Nice. Once in a while it pays to rtfm CAREFULLY.

    Wayne

  12. #12
    Join Date
    Nov 2009
    Location
    temecula, ca., usa
    Posts
    1,474

    Default Re: 1996 Mercury 9.9HP 4-stroke Carb question

    Well, all's well that ends well. Glad to hear you "got'erdone"!

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