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1998 bf90 missing on launch

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brad durden

New member
Hello from a newbie that needs guidance. Just got done cleaning and rebuilding my carbs on bf90. Yesterday synced carbs and took it for a test run. The boat would stall a bit on launch then jump on plane so headed for shore to check it out. Messed around with the spark plugs I took the boot off one at a time causing stall. Till I took off the bottom one causing no change in idle. So I pulled plug to check for spark thinking that was my problem but it had spark. So what should I check next ... Thanks for any help, Brad
 
OK, finally got to test compression on my motor. Here are the numbers. #1-118 psi,#2-117 psi, #3-113 psi and #4-111 psi. Are these numbers trouble?

Thanks, Brad
 
Compression should be 213 plus/minus 14.

I suspect that you did not take compression with the throttle wide open.

Try it again at full throttle and I am sure the compression will come up. If it get fairly close to 200 you should be good.

Your bottom carburetor needs to be cleaned again. Pay attention to the idle and midrange circuits. Once you remove the jet set (long skinney tube going up through the carburetor) take some brake clean with the straw that comes with it and stick the straw up the same path into the top of the carburetor. Shoot brake clean through it and you should see good spray coming out of the pin sized holes at the rear top of the barrel of the carburetor. If it does not, then you need to back probe with small wires, compressed air and more brake clean.

Mike
 
There's also a cool little test you can do to check the working integrity of each cylinder. It's for guys that want to know the whole story and don't mind doing a little work to get it. You sound like one of those guys.
It's called a "running compression test". You perform it much like the cranking compression test except you don't remove all the plugs and you don't hold the throttle wide open. AND you take your readings with the engine running just up off of idle. About 1200 rpm.
And, don't worry, this doesn't damage your tester gauge. Because, a normal reading for a running compression test should be approximtely 1/2 of the pressure read during a PROPERLY performed cranking test. An example might be:
You get 180psi on a given cylinder doing your cranking test. Your running test should result in a 85 to 95 psi range for that cylinder.
A "normal" result doing this test informs you that the cylinder sealing and valve spring response is adequate for proper combustion under dynamic operating conditions. You may only choose to perform this test in the event that your problem cylinder still misses but you have eliminated other reasons. YES, a cylinder that has good cranking compression can FAIL this test. You also need to understand that fuel delivery and spark should be disabled for the cylinder being tested.
I throw this out there in case you go through that carb again and still have a problem.
Good luck
 
Wish I would have seen your post earlier jgmo. Went to lake to do a compression test earlier and came back with these numbers 1-175, 2-152, 3-170, and 4-152. This does not look good to me, but I don't know. I'm kinda learning as I go. This time tested all plugs out no spark or fuel open throttle. I appreciate you guys impute. Brad..
 
Well, with those readings, I don't imagine a running compression test is necessary. You have an 18% discrepancy between the highest and lowest cylinders so that is a "fail".

But, all is not lost. The next step is to find where the compression is going in 2 and 4 and the answer could be as simple as valve lash is too tight. That's what I would check next and hope that you just need a little adjustment.

Another possibility is that you could have a timing issue. Notice how the pressures are somewhat symmetrical. 1 and 3 are similar to each other and 2 and 4 are similar to each other only lower. I'm wondering if this could be from a sloppy belt or a "bouncy" tensioner skewing the valve sequencing just a tad?? Speculation on my part but worth checking out. Because, IN MY OPINION, if you could bring 2 and 4 up to within 10% (or less) of 1 and 3, I think you would have a serviceable (albeit not perfect) engine without doing any extensive internal work to the powerhead.

If the compression can't be improved through valve adjustment, or belt/tensioner replacement then you may want to do what is called a "WET" compression test. This is performed just like the "DRY" test you just completed with the exception that you put a small quantity of oil on the piston of each cylinder being tested. If the compression for a cylinder goes up substantially with oil on the piston, that would be an indication that the initial pressure loss is due to ring wear. Doing this test requires that you make every effort to get the oil to spread out on the CROWN of the piston. With an outboard, this means propping the engine so that the cylinders go from horizontal to vertical. Depending on where you are and what you have to work with, this may not be very easy.

Other than that, a cylinder leakdown test, which requires a special tool as well as shop air, might tell you if you have a valve seating problem or are losing pressure past the rings. Most shops forgo the wet test for this one these days. But, if you don't have access to the tool and/or air, the wet test is a viable alternative for diagnosing what you are up against.

Think POSITIVE! Think VALVE ADJUSTMENT!

Good luck.

Good luck.
 
Everything Jimmy said......

But after you check timing and before you adjust valve clearances, you might try to decarbonize the engine.

For your engine, I like to use an Evinrude/Johnson or CRC Engine Tuner with a straw. I think Sea Foam makes a can with a straw too. I have found a real easy way to do it. Hook your vacuum balancer lines up again like you are balancing the carbs. Get the engine to operating temperature and increase the throttle a little above idle. Then remove the hose to one of the cylinders and put your finger over the end of the hose going to the engine...to keep it runnning. Take the engine tuner and spray a generous dose into the hose that your finger was on. Quickly, do that going to each cylinder a few times....back and forth. You may want to do it a couple of extra times on the troubled cylinders.

The engine will smoke like mad. Do all the above without a lot of time in between and once finished quickly shut the engine off. Let it sit for a while. Then start it up and run the engine at fairly good speed to blow it out.
It seems to work better if the engine is working under load but doing it on a hose should work ok.

Check your compression and seem if there is any improvement. If so, another treatment might help a little more.

You will probably have to change your oil after you finish, since it will be contaminated.

Even though you bring the compression up, you still may have an issue with the #4 carburetor.

Mike
 
OK, feel kind of silly now. Thought we had ran all the old gas out of tank but we didn't. So on the way to the lake we emptied the old gas and replaced with new. The difference was crazy, a little Rough at first then it started running great. The boat would launch great from a stop and idle smoothed out. Now the only problem is it idles a little rough when cold. When run on plane for a few seconds it idles great.
 
Interesting conclusion to your problem. I wonder why only #4 was not firing....maybe that is where the bad gas settled?

I am glad you have things worked out...but I assume your compression has not changed. I still recommend a decarbination as I described to help that out. It should also help your idle...although, it does take the 90 a little time to warm up and idle correctly.



Mike
 
Thank you Mike and Jimmy ... was taking boat to lake to do a decarb when it started running like this. Still going to do the decarb, but it was great to see it run like this again. I was starting to get a bit discouraged. Thanks again guys!!! Brad.
 
To answer your question Mike....I just go back to his compression findings. He's got a weak cylinder and it reared it's head with the less than volatile gas. Hopefully the decarb you suggested does a number on that and gets it back up on par with the others.
Good luck brad. Let us know how the decarb goes.
 
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