1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

[Br1dgemann]

I have a 1999 Johnson 250 v-6, carbed outboard that will run fine from idle up to 2000 RPM and from about 3600 RPM to 5600 RPM. Between 2000 RPM and 3500-3600 RPM, the engine feels like it is not running on all 6 cylinders, it is very rough, sounds like it is missing and if the wind is right, I can smell raw gas. The engine does not set any alarms while this is occurring. The system check horn sounds when I initially turn the ignition key on, and the system check gauge illuminates all the warning lights, so I know that it is working.

I have checked the plugs and they do not look fouled. I use Champion QL78 YC. The power pack is good ? I had a brand new one that I carry as a spare which I installed and the problem is still occurring. The stator was replaced about 2 months ago when one of the magnets came off the flywheel and wasted the original one. I replaced the magnet and checked that all the other ones were properly attached and located. I have also checked for spark ? I have good spark on all cylinders with a 7/16? gap. I re-checked the connections between the power pack and stator, timing base, and out to the ignition coils. All those connections are clean and good and in the proper locations, with no damaged wire insulation. I tested the shift circuit switch per the OMC manual and it is good. I also checked the blocking diode for the S.L.O.W. system and it checked out satisfactory per the manual. The connections at the battery and the starter are clean and tight.

I have new fuel line from the tank to a Racor filter and from the Racor filter to the fuel pump. The primer bulb is genuine BRP, not an aftermarket cheapie. The hose clamps are genuine BRP plastic ones, not the worm gear type. I replaced the fuel lines a while back because they looked weathered in some spots. I also sprayed carb cleaner into each idle air-bleed orifice and each mid-range orifice and saw a nice stream exit at the back of the carb throat, so I believe the passages are clear. I also have carb rebuild kits, but I have not torn them down, because I am still not sure it is fuel related.

Lastly, performed a link and synch with the exception of WOT throttle timing.

Any suggestions with respect to what else I can check would be appreciated!

 

[Big flop]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

Spraying cleaner into the jets is only going thru the air circuit
Part of the carb.
I would go ahead with the carb cleaning.
Pay special attention to the small brass tubes going
into the float bowls. They are the idle & off idle fuel pickups.
Take side covers off & spray tuner up them and watch
for consistence flow in all of them.
A small tag wire will help move any crud in them.

 

[Br1dgemann]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

Thank you for the input, Big flop. I will do that later this week. My work schedule for the next three days is not going to be conducive to working on the motor in the evening.

Just to be clear, You are suggesting I remove the carb bodies and take them fully apart, cleaning all passages along the way, paying attention to the small brass tubes going into the float bowl, and then reassembling, reinstalling and adjusting them.

Okay, that will be my next move. I will re-post when it is completed and let you know how it runs.

Thanks, again.

 

[jonesg]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

It requires first ordering carb kits that come with new hi speed jet and all gaskets.
Read the top secret file. Follow the directions.
http://forums.iboats.com/showthread.php?t=244013

 

[Br1dgemann]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

jonesg,

I already have rebuild kits for the carbs. FWIW, I do not remember seeing a new high speed jet in the kit, and the shop manual states to re-move and re-use the existing, but the kits did come with new floats and the gaskets. I will have to double check the contents, though... I could be mistaken.

The secret file is a good piece of information...thank you for referencing it.

 

[Big flop]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

Yes take the carbs off as a complete unit & take all the way apart.
OMC (BRP) engine tuner is a great carb cleaner,
Little hose for shooting all the little passages.

No main jets in carb kit.

 

[Br1dgemann]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

So, I had some time on Saturday and was going to start to remove the carbs, when I saw the hose going from the vacuum switch to the fuel line "T" below the fuel pump was off. Essentially, I was sucking air into the fuel pump. I re-assembled it and took it to the ramp for a test. Engine runs better now in that RPM range, but now "surges". I suspect this is still fuel delivery problems. Next step - carb rebuild??

 

[jeffersondodson561]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

At what rpm is it surging? If you have a portable tank, hook up the tank directly to the carbs (have to pinch off fuel line that was going to the carbs) and have someone constatly prime the bulb while operating in that rpm range. This will tell you whether it is carbs or fuel delivery/fuel pump issue.

 

[Br1dgemann]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

The surging occurs between 2000 and 3000 rpms.

So, jeffersondodson561, you are suggesting that I by-pass the Oil/fuel pump entirely and connect a fuel line and portable tank post-fuel pump to the carbs. Then run pre-mix to the carbs using a primer bulb for pumping fuel and run it the problem RPM's?? Interesting. I assume if the problem remains then it would be carb-related, if the problem goes away it is either fuel pump or fuel feed related.....correct?

I need to see how I can contain the oil-side of the pump. Maybe a hose to a container to capture the oil?

 

[jeffersondodson561]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

Bingo, the fuel pump at wide open throttle only pumps 6-8 psi, easily achieved by rapid priming of the bulb. Get a portable tank with premix, open vent, make a couple of test runs. If you are having that problem at that rpms I would suspect carbs but this test will verify.

 

[Br1dgemann]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

jeffersondodson561,

I will see what I can rig up this evening and/or Friday and give it a try on Saturday.....

Since your in WPB, wanna ride shotgun?

 

[jeffersondodson561]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

Nah you can do that besides it is better that you run the motor the way you normally run it, but if it turns out to be carbs let me know, maybe we can work something out.

 

[Br1dgemann]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

Well, I ran it this weekend and still had a repeat performance - 2000 to 3000 rpm surging. The best way to describe it is that it feels like it is not firing on all cylinders all the time. Apply more throttle and eventually it will pick up and go like gang busters especially once it gets up to 4000-ish rpm.. pull the throttle back even a little bit and it falls right back into the dead zone.

I pretty much assume at this point it must be the carbs.

 

[Br1dgemann]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

Pulled the carb bodies off last evening after work. Found a nipple between the throttle bodies with a vacuum cap that was cracked/ split all to hell. Attached is a picture with my finger pointing to the location. What is this for?? and why was it (sort of) capped??

 

[Br1dgemann]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

Nobody ?!?!?

I know someone out there must have the answer. I find it difficult to believe that the experts are stumped.

 

[j_k_bisson]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

Seeing that is a a hole into the throttle body, and the throttle body is under vacuum. so replace the cap/. It needs to be there. It also would make you middle cylinder run very lean.

 

[Big flop]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

Older models had a 90 degree hose on that nipple that
that went into the air box - drain hose
If it's sucking air in think you would
Notice it more at low speed. (lean condition)

 

[j_k_bisson]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

The 90 degree hose was on the bottom nipple and the middle nipple had a cap on it.

 

[Br1dgemann]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

OKay. I will put a cap back on it.

The drain hose is present and accounted for. as is the second hose that is at the top of the air silencer.

BTW, I did not see any mention of that nipple or a hose routing for it in my shop manual.

 

[Br1dgemann]

Re: 1999 Johnson 250 has a ?dead spot? between 2000 and 3500-3600 RPM

Finished rebuilding the carbs and re-installed them on the engine. BTW, there was absolutely NOTHING wrong with the carbs - they were clean, no blockages, no debris, floats were all acceptable. Made sure the o-ring between the carb body and throttle plates did not get pinched during re-installation. No leaks at the float bowls or fuel lines.

Fired it up and it idled great. Ran the boat and it still is doing the same thing - at 2000 rpm it vibrates and losses power. Keep pushing the throttle forward and it will suddenly wake up, jump out of the hole at about 3500 RPM, get the boat up on plane and the rpms will keep climbing. Back the throttle off to try to get the engine settled into a 3500 rpm cruise and it starts vibrating, losing power, dropping rpms and the boat falls off plane. For the record, the boat used to cruise easily on plane at 3000 rpm. It is like the entire mid-range is gone.

When I got the boat home, I hooked up my CDI spark tester and all 6 cylinders are getting spark with a 7/16" gap. I placed an Ohmeter on the stator and timer base connections per the shop manual and got acceptable and even readings. I do not have a Peak reading voltmeter adapter, so I could not perform the rest of the checks requiring voltage readings.

Any ideas where to go from here? I am starting to get very frustrated with this problem.