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| Intermittent Top Cylinder BF75 |
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Message |
    
James D. Brewton
New member
Username: hondabf75
Post Number: 1
Registered: 11-2009
| | Posted on Friday, November 20, 2009 - 09:31 am: | 
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Tough one here: 2006 Honda BF75. One way or another, I got a tank full of bad gas/water in the tank (I hate E10), needless to say the engine ran very poorly. I drained/flushed the fuel tank, new fuel filter on the engine, new water separator on the boat. Pulled the intake manifold and carbs. Rebuilt the carbs without changing any adjustments. Everything looked brand new, no obstructions, but I soaked everything in SeaFoam and dried with compressed air anyway.
Same old story right? This is where it gets interesting. After a careful reassembly, the engine ran great for the next three trips. Total run time was probably about 2 or 3 hours, but plenty of time for this to show up. On the 4th trip, it starts to run rough and I can tell that it's missing.
Troubleshooting:
I discover that on start ups it runs fine (purrs like a kitten). I blip the throttle. Every third or fourth time I do this, it starts missing. I can make it go away by blipping the throttle a few more times.
By process of elimination, I discover that the missing cylinder is #1 (Top). But again, I can make it purr or miss just by blipping the throttle.
-All cylinders have good spark (even #1 while missing).
-All spark plugs have been replaced.
-Compression test was good on all cylinders.
-All valve clearances are within limits IAW Honda shop manual.
-All throttle cables are adjusted IAW Honda shop manual.
-I have not had the carbs synchronized since the rebuild, but, I made no adjustments, and it idles perfectly upon start up. If it needed a sync, how could it go from missing to running great just from a blip of the throttle.
-I have tried to find intake manifold and carb gasket leaks by spraying carb cleaner in the area while running with no joy.
-I pulled and tested the thermostat, thinking unequal heating of the cylinders, it checked good.
-Swapped the coils, problem stayed with #1 cylinder.
-I've ran the engine with a separate fuel source to cancel that out of the equation also, no joy.
I searched this site and others with no joy and I'm close to being defeated. I don't want to go to a dealership, but I'm at that point unless you guys have any ideas. |
mike
Advanced Member
Username: hondadude
Post Number: 253
Registered: 03-2009
| | Posted on Friday, November 20, 2009 - 10:04 pm: |
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There are a few things that come to me off the top of my head...
-Remove top carb (it is really #4) and check for small particles inside the carb that could be getting sucked up and blocking one of the jets and then released when you blip the throttle.
-One other thing to check while it is off, would be to look for small cracks in the idle jet(the long tube going up through the carburator) You did remove it to clean the carb, didn't you?
Also look for nicks in the tube that may have been made if you removed it with needle nose pliers. Should use a screwdriver to gentle pry it up.
-Check for any loose hoses (fuel and vacuum) going to and from the carburator. Don't forget to check the ones behind the carburator.
- Hopefully you used a new intake manifold gasket. Make sure there are no gouges in the surface from cleaning.
- One last thing to fully isolate it to the carb is to swap the top two carburators. If it follows the carb, then you know for sure.
- If you haven't changed the order of the carbs, the balance should be close to what it was. Swapping the carbs will probably mess up the balance, but at least it helps you isolate the problem point. |
James D. Brewton
New member
Username: hondabf75
Post Number: 2
Registered: 11-2009
| | Posted on Friday, November 20, 2009 - 11:20 pm: |
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I appreciate the post Mike.
-I did remove the idle jet on all carbs, I inspected for cracks, but I definitely could have missed something or damaged on the reinstall. I have rebuilt a few engines, but this is my first with carbs.
-After I removed the idle jet, I realized that I didn't pay attention to the position of the jets. Should they be in line with the direction of air flow, or perpendicular to the direction of air flow? I'm sure it matters, doesn't it?
-Yes to the intake manifold gasket. The main reason I haven't retraced my steps on the carbs is because I spent nearly $200 on new gaskets and o-rings alone. The packs are inexpensive enough, but after you multiply everything by 4, it hurts.
-I haven't interchanged carbs or there internal parts thus far.
-I'm almost certain that I put some nicks/scratches in some of the idle jets on removal, but at the base in the bowl. No tool marks on the tube part. |
mike
Advanced Member
Username: hondadude
Post Number: 256
Registered: 03-2009
| | Posted on Saturday, November 21, 2009 - 07:36 pm: |
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The manual says nothing about the orientation of the slow speed jet. I just put them in and things seem to work.
You have probably already tried to flush the carb out...if not emove the drain screw and it will be more of a flush.
Generally if you do not use caustic cleaner, you can pull the carbs apart again and not have to put new o rings in. Try shooting some brake cleaner through the passages and blow out with compressed air. There could be a hose partical in the bowl.
The carb base gasket probably will not stick like the old one did and you can reuse it if it is not torn.
Did you remove the idle mixture screws and clean that part of the idle passage? If so, maybe a slight adjust to the idle mixture screw might fix your problem.
Whatever this turns out to be...it will probably something very simple. |
James D. Brewton
New member
Username: hondabf75
Post Number: 3
Registered: 11-2009
| | Posted on Saturday, November 21, 2009 - 11:06 pm: |
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Rookie mistake, the idle mixture screws have caps on them and I did not remove them during the rebuild. It looked as if they were designed to break if you tried to remove the little quarter turn cap. I know I can buy new ones, is that what Honda intended?
You don't have a trick for getting these carbs off without removing the intake manifold do you? |
mike
Advanced Member
Username: hondadude
Post Number: 257
Registered: 03-2009
| | Posted on Sunday, November 22, 2009 - 07:58 pm: |
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I have never tried to remove them without removing the intake manifold.
Hopefully, the gasket will be intact.
Sorry! |
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