1984 90hp johnson troubleshoot challenge

[logger8614]

I have waited along time to generate this post due to aol. ( filter problems) Thanks to the moderators I am now able , after months of just being able to read pertinent posts, ask the knowledgeable board members a question: My Johnson , model number J90TLCTE, has a drop or a misfire at rpms over 4000. This problem first presented itself as a major drop in rpms and speed followed by an abrupt increase in speed and rpms followed by a decrease ad nauseum. Shop number one could not duplicate in test and did rough justice by replacing power packs. 400$ and no change. Shop #2 replaced the stator I believe( blue ring underflywheel attached to throttle advance) and no change. Shop # 3 replaced one secondary ignition coil and while the significant drop and increase in rpms has stopped there is still a misfire or drop above roughly 4000 rpms. Here is what I believe to be an interesting piece of the puzzle. Every trip, starting to WOT it is fine. Whole shot is strong, rpms are at 5300-5400 and top speed is 45-48mph. After about 25-30 minutes at WOT or after a couple of stops to fly fish for stripers, it starts with the intermittent hiccup. Real quick. So fast the tach does not even register the drop but I can hear it and feel it.
Being that I have limited funds, have been dissatisfied by three shops and am about to get laid off from my teaching job in California, I would like to try to tackle the problem myself. I am armed with a shop manual and a voltage meter and that's about it. After reading the many posts on iboats about these engines I am convinced its electrical in nature. I pulled the fuel tank and replaced fuel lines. the bulb is always firm. None of these shops have mentioned bad compression or weak spark as a problem.
I have two theories based on what I have read and what I think I can tackle myself without making matters worse.
The secondary ignition coils(old three) look possibly original. The plastic housing is starting to break down and the drop changed in characteristic after the last shop replaced the coil on the lower left cylinder on the side of the fuel pump. So I could replace the other three although I assume he would have checked them when he found one to be faulty. The other idea is to rebuild fuel pump as it is long over due and because the pulse of cylinder #4 initates the pump it may be fouling the cylinder or plug. The spark plug on the lower left cylinder (fuel pump side) has a little bit of fuel mixture getting by or maybe because it is the cylinder that has the problem was bucking the plug loose and allowing seepage.
If I am forgetting any important information please remind me and I welcome any and all opinions including the fact that it is a 25 year old motor and the fact that it still runs moderately good should be enough and to stop chasing perfection.
Eagerly awaiting the boards input,
Joe

 

[Faztbullet]

Re: 1984 90hp johnson troubleshoot challenge

I had a customers 200 looper doing almost identical to what you describe. After customer had to 2 certified shops he called me to get a idea of what it could be, I told him to go fishing and when it broke to call me. I met him about dark and removed cowling to install timing light and started engine. There to my suprise was 2 spark plugs with light blue arcing running down sides of plugs!!! I pulled wire and inspected and nothing looking bad,took zip tie and tighted boot to plugs and arcing went away!!! Reinstalled cowl and customer test ran, after about 20 min (i was starting to get worried) he came up to dock and said miss was gone and picked up 300rpms on top end and idled better. I replaced his wires ,installed ties too and sent him along. Sorry for the long details but you might give it a try one evening

 

[ondarvr]

Re: 1984 90hp johnson troubleshoot challenge

Coil failure is very common on an older motor and is the first thing to check and/or replace and since you said they look to be in bad shape just replace them. A loose plug can cause odd problems also, not much expense in that fix.

 

[emdsapmgr]

Re: 1984 90hp johnson troubleshoot challenge

Try taking a friend and an inductive timing light with you on your next outing. When it starts to act up (electrical problems act up once the engine reaches normal operating temps) put the timing light lead on each of the 4 plug wires at 4000 rpm in gear. Check for strong, constant flashes from each wire. You will be able to see intermittents or weak flashes on specific wires. If it read your note correctly, mechanic 2 replaced the timer base, as that is the part that is link-connected to the throttle advance arm. The stator generates the power to fire the ignition and you need to verify that it puts out the proper voltage. You will need a peak-reading voltmeter which is not a common device and the original service manual. (you can get an adaptor for a common household voltmeter.) Low stator output voltage can cause an occasional random misfire on all coils.

 

[logger8614]

Re: 1984 90hp johnson troubleshoot challenge

Thanks for the replies. I have ignition coils coming and when I install I will twist tie down plug wires. I have one other question. I also had a part replaced that was a thick metal/plastic circle about the diameter of a mayo jar with electrical leads coming off of it. Was that the stator

 

[emdsapmgr]

Re: 1984 90hp johnson troubleshoot challenge

The timer base is about that size. It has a metal arm that protrudes from one side and a single wire with two lead ends on it. It's base is made fit in a groove that lets it move back and forth. The stator is larger and fits around the timer base. It looks like a 6" coil and has two electrical leads that come off 180 degrees apart. It has four drilled holes in it for solid mounting to the top of the block.