Re: Flat Spot @ 1/2 throttle Merc 3.0 Alpha 1 drive
I have posted on this a few times, this question seems to come up every couple months. With this new bulletin board hopefully you'll be able to search and find all the posts on the topic.
For a search, try looking up my username (ron7000). you can also try including "3.0 mercarb bog hesitation" in the search and scroll thru the relevant results.
provided you don't have any other engine trouble, the cause is most likely a lean-out condition in the carburetor when it goes into the transition circuit.
First and foremost, the carb internals must be clean. No scale, water, or dirt. If you have that or varnish and it's clogging a passageway then it's a problem. Fix that first. Also no gasket leaks or vacuum leaks. This should be obvious.
The transition circuit is the circuit between the idle circuit and the main circuit. The main circuit is what runs off the main metering jets, the idle circuit off the little idle mixture screw. Idle circuit is < ~1000 rpm. Main circuit is > ~2500 rpm. With the mercarb, and rochester, if one circuit is way outta wack it will affect another circuit. So if the idle circuit isn't set properly then adjusting it may fix a large part of the problem. Set the idle mix screw 1.25 turns from closed. Then with engine warm at ~700 rpm in gear turn idle screw slowly clocwise (lean) till engine sound/smoothness/rpm changes and note screw position. Then turn other way past the 1.25 open position till sound/smoothness/rpm changes and note position. Set screw to position midway between lean and rich positions, if anything a little on the richer side is better. You may also use a vacuum gauge for this procedure, but for a 4 cylinder vacuum isn't steady at idle so it won't be of much use like it would on a healthy v-8. The best way is to just set the screw midway between too rich and too lean.
Next will be power valve- if you get wide open throttle rpm which is > 4200 rpm and your idle is more or less smooth then your power valve is ok. If not then replace the power valve. If you have any suspicions toward it, replace it, it's only $10.
If all's well so far, then figure out if the flat spot is there all the time. If you accelerate real easy and it sorta sucks air around that magic rpm and bogs out for a second then catches and takes off, read below. If not, and the flat spot is only there when you jam the throttle open quickly then look at the accelerator pump. I've heard Mercarbs had a tech service bulletin for an inadequately sized accel. pump for certain models or years. You may need to upgrade that, otherwise always accelerate easy and don't jam the throttle open. Some carbs have a 3 position accelerator pump lever, you can try moving it to the other position which gives a full stroke (more fuel squirt) when the throttle is opened quickly. Most likely the solution here is you need a bigger squirt of fuel, because the motor is leaning out when the throttle plates open so quickly. However, it is a 4 cylinder not a v-8... if you're jamming the throttle wide open super fast from a dead idle then understand there's only so much the carb can do on a 4-cyl and that condition may not be fixable especially if the engine isn't in top condition to begin with.
For the part you've been waiting for. If you're this far and the flat spot is there all the time including easy acceleration then the solution lies in the transition circuit of the carb. I've been told Mercarb's are an emissions (emission-friendly) carb which basically means they run lean. Going by exactly what the service manual lists for specifications may not fix the flat spot. The solution lies in a combination of the following in priority: (1) raising your float level so there's more fuel in the bowl so the mixture is richer, (2) increase main jet size 1 or 2 sizes, (3) richen idle mixture as much as possible via idle mix screw but most likely sacrifice idle quality, (4) change power valve to a higher opening vacuum number if available.
(4) -> probably not possible, I know on my carb merc only offers one power valve so I'm stuck with that. If you can find replacement power valves, they are worth a shot.
(3) -> it's been said the California model carbs have a tamper proof idle mix screw. The guy here who goes by "DonS" has posted at length on it. If that's what you have, you may need to get around it but don't be too quick to think that's the biggest cause of the problem. There's other things you can do to make up for it.
(2) -> because the carb runs lean to begin with, stepping up main jet size will richen the Air-Fuel ratio and will get you more power including in the transition rpm range where the flat spot is.
(1) -> By far the biggest and easiest adjustment is raise your float level. This will provide more fuel in the transition rpm range, richening the AF mixture, and prevent the carb from leaning out. Raise your float a little and try it. If it doesn't fix it, raise it a little more and retry. You know you raised it too much when either the motor floods with gas trying to start and doesn't start, or it starts but raw fuel drips from the boosters and the motor doesn't idle smooth. Then you lower the float level till it doesn't happen. If that doesn't fix it, then the carb is incapable of metering the proper amount of fuel at that rpm range for some other reason and replacing the entire carb with a new one that you know is good would probably be the best course of action at this point.
Back to the accelerator pump... please understand that its purpose is only to supply fuel (raw fuel) when the throttle is opened quickly. Many times info is regurgitated on this topic and everybody says the flat spot (hesitation or bog) is the accelerator pump so fix or adjust that. Any book on any carburetors will explain what I'm going to explain. When the throttle blades are opened too quickly, the sudden loss of vacuum signal to the carb prevents it from metering the proper amount of fuel into the air being sucked through it. The motor would therefore stall out. That's why there's the accelerator pump. All it does is shoot a relatively correct amount of fuel into the airstream. This is to help accelerate the motor enough (increase its rpm) which then causes more air to be pulled through the carb (at a faster speed). Once the motor speeds up enough, the carb then has enough air moving though it's throat (fast enough) so that it can meter fuel properly again The accelator pump is mechanically driven off the throttle linkage, usually by a little cam, and only shoots fuel or the majority of it in a certain throttle range. This range is just above idle and a decent bit below wide open. Now depending on what the motor is being used for you may want to change out the accelerator pump cam to get the performance your looking for (think cars and dragstrip). For a boat however, this is not the case. In a boat, load on the engine is more constant and more known so you can have just one cam profile for the accelerator pump suffice for all operating conditions. The only variable then is how much fuel you want to shoot. That being said, since the accelerator pump is set to shoot its fuel in the transition range, you can cure the flat spot by adjusting the accelerator pump to shoot so much fuel that it gets the carb completely through the transition circuit where it normally leans out. You would have correct air-fuel at idle, shoot enough fuel with the accel pump to maintain a sufficient AF through transition of 1000rpm to about 3000rpm. At around 3000 rpm, the main jets have come completely online and they then maintain proper AF ratio from there on out. Most people then think problem solved, maybe it's because they always accelerate that hard and never notice. The catch 22 to this way of doing things is that this only works when you accelerate fast and hard enough to get the accelerator pump to shoot enough fuel. For some boats this may be the way you operate it 95% of the time so it's accepted. Accelerate slow and easy enough though and the flat spot will still be there, because the accelerator pump will shoot very little if any fuel at all. So... on a boat, when you accelerate "normally" the transition circuit is "supposed" to do all the work with no help from the accelerator pump. When you accelerate hard, the accelerator pump is there to make up for what the transition circuit can't handle. Adjust the carb other than that and you're running it in a way it wasn't designed for. Not saying that's good or bad, just that that's what's happening.
I slalom ski with my 2002 3.0L which is in a 19' boat. I've spent a good month figuring out this problem, which is very common by the number of posts here on this board about it. By far the biggest solution is raising the float level. That will basically cure the flat spot. If you're looking for more acceleration power then the next best thing you can do is up your main jet size. This has gained me stronger acceleration throughout the rpm range, basically 2000 rpm on up, which is a must for deep water slalom starts. My oem jets were 1.55mm. Sizes jump in 0.05mm increments. I currently run 1.65mm and found that to be the best versus 1.60 mm and 1.70mm or 1.75mm jets. And, my spark plugs are just as clean as with the 1.55 mm jets so I know it's not running too rich. Also know that by richening the AF ratio by going up 1 or 2 jet sizes will further prevent running lean at high rpm and thus prevent any pre-ignition or detonation, which is a good thing. If you increase your main jet size just 1 or 2 increments and your spark plugs don't become black or sooty, then you can be pretty sure you are running a correct, and a very healthy, air-fuel ratio.
If anyone wants a good book on carburetor theory, I highly recommend
"Carburetor Performance: How to tune&modify" from Motorbooks Powertech Series, Forbes Aird and Malcom Elston. I was able to get mine new for $12 from amazon. Unfortunately, it's scarce now and prices start at $40. But it is that good, and if you want to know how carbs work and know what your doing on any carb, and more importantly why, then this book is worth the cost.
