1998 bf 90 carb sync

I encountered the symptoms of engines run with ethanol fuel ( rough idle, poor performance, etc.) and went down the path to correct the problems. Changed fuel lines, filters, cleaned tank, installed fresh fuel with stabil, cleaned carbs, replaced idle tubes, pilot screws, needle valves for float bowls, spark plugs. I gave my boat to a dealership to have carbs synced and ran last season with the same issues but not quite as bad. Things did seem to improve the more I ran the engine. Feeling the carbs were still the culprit I have just gone through them again and things have gone well so far. The engine started right up and with a little adjustment is idling at 950 rpm. I started to check the carb sync. and my vacuum gauge needles are bouncing ( all four ) wildly. The good news they all seem to be behaving the same. The bad news I can't understand why? I did a compression check when I had the manifold and carbs removed and the compression was a little low ( 175 to 180 ) but balanced. The motor has around 1200 hrs on it. Any suggestions?

I am not sure what kind of vacuum gauges that you have, but the ones Honda sells have a small valve on each gauge, which you can use to cut down the swing on the needles.

It sounds like if your balance is probably good, there may be some passages that you may have not completely cleaned or the idle mixture screws are not set correctly. Should be 2 1/4 turns out. You may also have some hose issues (coming apart inside) if you used the ethanol fuel very long.

I highly recommend the Honda Carburetor Manual to step you through the carb cleaning. It has great pictures of the various fuel flow paths in each type of Honda carb, especially the 90 HP (which has a few more transitional paths than the others), and shows how to check to be sure that they are clean. It is $40, but well worth the money. http://www.helminc.com/helm/product...=&from=result&Style=helm&Sku=TM044&itemtype=N

The low compression could be due to valve clearances out of adjustment. You might try shooting a little oil in one of the cylinders to see if it brings your compression up. If it does, it points to the rings....but they may just be carboned up. A good dose of Sea Foam in a 3 gallon tank should help decarbonize the cylinders if you thing there is an issue there. You might try the Sea Foam treatment before you take the carbs apart again. It might clear your passages.

One last thing....use a fuel treatment....not just Stabil. Stabil Marine, Pri-g, etc are stabilizers and fuel treatments...which clean passages, decarbonize, fight the effects of ethanol....in other words....they sound like "Snake Oil"...but do help.

In general, I can not remember a lot of 90's that came in with a compression of over 180 or 190. So, I think your issues are really in the carb passages.

Mike

Thanks for the reply Mike. I have ordered the carb manual, and your hunch was right. I manifolded my own vacuum gauges together. What thru me off was one of the gauges was designed to check auto vacuum and fuel pump pressure. Is the oscillation due to the isolation of the individual cylinders? I paid $600.00 dollars to my local honda dealer last year and was very disappointed with the results. I am old enough to realize education is never free, but that was a rough one. I also wonder if I have my dashpot adjusted correctly. Does the carburation manual cover this? From my CLYMER manual I have a .008-.032" dimension from the middle of the throttle cam. Does this sound correct to you? The motor wants to spit back through the carbs, but I'm hoping this is an out of balance condition. when I cleaned I used Berkible 2+2 and lacquer thinner. I hope these old school chemicals weren't too rough. I don't submerse anything but the metal body and bowl. check float levels and blow every passage out with compressed air. I removed my thermostat and boiled it in a pan to make sure it was operating. Trophy season for stripers opens Saturday so I need to hit the ground running.

I hope the dealer did more than just balance the carbs for $600. I just finished one and with all the gaskets, o rings, slow jets, spark plugs, fuel filter, carb balance and test under load at the water for almost $1000. A good carb job takes time. This one was pretty bad and I had to use heat and vise grips to remove the primary jet in two of the carbs. So those had to be replaced.



The oscillation is due to the fact that each cylinder is "sucking" on and off. That in itself varies the vacuum that the gauge is seeing. So you are seeing an on and off with the center of the swing probably being the right number.

Carb balancing is going to be noticed more at idle than at higher speeds.

The carb manual shows in great detail the dashpot cam adjustment. I have found that if you can just see light between the cam and where it contacts, you are good to go. It also shows how to check to be sure that the accelerator/deaccelerator system is working right. I doubt if most anyone ever checks it....we all just assume that it works. It only takes a few minutes to run through the test. It certainly should be checked if there is an acceleration/deceleration issue.

It sounds like your motor is running lean. Again, I am assuming that the idle mixture screws are adjusted properly. Honda does not like to have their carbs dipped in anything but water (using an ultrasonic cleaner). There is a coating on the outside of the carbs that dipping can remove.

Blowing out passages does not allow you to see that they are actually allowing fuel to flow through them properly. The manual, shows you how to spray solvent through each path and what to watch for to be sure they are clear. It they are not, it also makes some suggestions as to how to clear them. There are a lot of colored pictures of the carbs in various states...idle, transition, full throttle. It shows the air paths and fuel paths. There are lots of pictures of every procedure. There is probably a lot more than you really want to know....after all...you just want to go fishing.

I think you will be surprised at all the things that need to be considered.

The 90 has three sets of pin sized holes at the rear top of the inside of the carburetor. There is a single hole for idle and two sets of 4 or 5 (can't remember the exact number) for transition. If any of these do not have a good strong stream through them, you will have issues. You may have to back probe or back spray through these holes to help clear them out. Of course compressed air helps too. I must repeat, if there are not good strong streams coming out these ports, the motor will not run correctly.

I generally use inexpensive brake clean with a straw type sprayer to shoot fluid through. It is not too caustic and does have some cleaning properties. Since I use an ultrasonic cleaner, I almost never have to use carb cleaner, although you might have to in order to clear some of the stubborn passages.

I am not sure if this is going to help you get on the water by Saturday.

You might just want to try to back your idle mixture screws out 1/8th to 1/4 turn each to see if it helps the sneezing. That just dumps more fuel out of the idle port. It may help in transition but it will probably not help your idle and could possibly foul your plugs if you idle for a long time. At higher speeds, it will not matter. That might at least get you through the weekend and give you some time to fix the real problem.

One other thing, that should not take too long is to check the valve clearances. That can also be a cause for a lean condition if clearances were not correct.

Sorry for the rambling....

Mike

Mike;
I received the recommended carb book and sync tools and went after the problem today. I started with a compression test, 180,170,170,165. i began to check valve clearance and noted what may be an issue. When aligning "T" on the start pulley to "T" on cam pulley to "T" om engine timing cover, I ther three marks were out of alingment. When I brought cam pulley mark and timing cover mark to alingment on compression stroke the start pulley "T" mark looked to be exactly 60 degrees out of rotation or one bolt on the 6 bolt pulley hole pattern. Because I knew the motor would run I decided to go ahead with my valve clearance check, and carb. sync. The motor started and idled. Through the vacuum sync I noted the engine only pulled 9" vacuum at idle but was stable. The gauges showed the motor to have late ignition timing, and it does spit through the carbs on number two. I did hit something a while back and am wondering if the timing belt didn't jump, but have a hard time believing it woul run at all if the timing were off as much as i suspect.
By the way the carb manual is the best i've seen, and well worth the cost. I wanted to check out the forum before moving the belt. What do you think?

I think you may be looking at the wrong marks.

There is a timing mark on the engine for the cam pulley and a different one for the starter pulley.

If you are at the back of the engine. There should be a T and an arrow on the lower timing cover at the 12:00 position next to the cam pulley, that the T and arrow on the cam pulley should align with. Once the arrrows are aligned, check for the T on the starter pulley (should also have an arrow or a line). That should align with with an arrow (looks like a triangle) on the lower timing belt cover at about the 5:00 position.

If it does not align, rotate the starter pulley one more revolution. Hopefully, they will align.

If these still do not align, I recommend checking the key on the crankshaft to be sure that it is not sheared before you go changing the timing belt.

If every aligns, then you can try backing the idle mixture screw on #2 a little to see if it helps the sneezing. You may have to pull it off again and make sure all the flows are good, especially through the idle port...the single hole in the top of the carburetor.

Mike

Thanks Mike. I did not see the triangle mark on the lower timing belt cover, but I will check. I believe its okay. I was thinking all the marks aligned in a straight line down the center of the motor. With your help it is running much better. I feel like it needs a little refinement to be right. Thanks again, rusty

Good deal!

Mike