1986 supra 351W BAD miss under load after warm up

[bonomosupra]

so ive pretty much had this problem for a year now. Once the boat warms up if i put a heavy load it starts to miss really bad but it will kinda clear up after i cruise under little load. yesterday i adjusted the timing and did like 20 out the hole digs and it ran great and only had a very small slight miss so i thought i had it. Here are the things ive done
New cap and rotor
new spark plugs
chaeck timing around 6-8 degs
new water separator filteri have a mallory electric ignition and distributor

also what are the best spark plug wires

thanks

[Jetlink]

Check your ignition coil, mine went bad and I spent a significant amount of time chasing all sorts of issues that resolved with the new coil.

[michael hunter]

Are you still running points?

[bonomosupra]

No it’s all electronic

[secondsupra2004]

Check your ignition coil, mine went bad and I spent a significant amount of time chasing all sorts of issues that resolved with the new coil.

Good place to start. Also, has the carburetor ever been rebuilt?

[bonomosupra]

Carb is new just picked up a coil and a fuel pressure gauge bout to slap that on there. What about plug wires what are best?

[secondsupra2004]

Carb is new just picked up a coil and a fuel pressure gauge bout to slap that on there. What about plug wires what are best?

I used the standard PCM wires all the years I had my 89'. Never had an issue. On my current boat, I bought a set of Standard Blue streak ignition wires from Rock Auto. These were a step up from the stock Indmar wires.

[bonomosupra]

Can you get them from a auto parts store? If so do you have a vehicle to reference for them?

[secondsupra2004]

Can you get them from a auto parts store? If so do you have a vehicle to reference for them?
I don’t have the info anymore on my 89’. My 2004 takes different wires.

[michael hunter]

What electronic ignition brand are you running ? Some of the electronic conversion system were problematic.

[bonomosupra]

I have a Mallory distributor. Update took it out after the new coil ran good at my house as soon as i hit the water ran like shit fuel pressure was down . Had to pump the gas just to get it rolling. So i replaced the pump and a little better pressure but still same thing now it just runs rough at idle! Any suggestions?

[Jetlink]

Can you have a friend come with you and get some video to post on YouTube and link here? I would also include video of it running on the hose at home, basically document the whole thing of what you are experiencing and post the links here for us to check out.

[bonomosupra]

yeah i probably can .
so update . i replaced the solenoid that controls the starter and had a ignition stud that powers the coil because i volt at the coil are around 6v and it goes up with the rev it. when i had it idling i removed each wire and found 1 3 and 7 didnt change the idle as if there were not firing. im assuming maybe the distributor need to be replaced?

[michael hunter]

That i term is usually not used with electronic ignition . I am not familiar with the mallory set up . All of my boats are running DUI . If you have 3 dead cylinders you should start with a compression test .

[covertcombatant]

I would put the D.U.I distributor in and call it a day. I fought my electronic Mallory ignition for a little while. The coils kept overheating and having problems. The boat would miss when the coil heated up and if you were idling waiting for a skier the boat would die. It wouldn't start until I soaked a rag in water and through it on the coil or sat and let it cool down.
Threw the D.U.I distributor in and fires up on the first key turn and can run and idle all day long.
Spend the money and enjoy the boat.

[Supra_Comp]

I would put the D.U.I distributor in and call it a day. I fought my electronic Mallory ignition for a little while. The coils kept overheating and having problems. The boat would miss when the coil heated up and if you were idling waiting for a skier the boat would die. It wouldn't start until I soaked a rag in water and through it on the coil or sat and let it cool down.
Threw the D.U.I distributor in and fires up on the first key turn and can run and idle all day long.
Spend the money and enjoy the boat.

100% Agree. I had a similar problem with my 85 Comp for years and could never figure it out. Turns out it was a bad coil, ballast resistor, and bad wiring.

Another thing to check is to see what voltage you are getting at the positive side of the coil. They typically like to see 9-9.5V; if you are seeing over 10V, I would look at how it is wired (this also happened on my boat...full 12V was being sent to the coil which caused overheating and after time terrible spark".

[codyb209]

Anyone know the wiring for the ballast resistor? I have a feeling mine is wrong from previous owner

[Supra_Comp]

Essentially you want is to make sure you have 12V going to one side of the ballast resistor, then on the other side of the resistor you should have around 9V so you can take a wire from that point on the resistor and feed the positive side of the Ign Coil. Those coils only want 9-10V; if they are getting full power during operation they could be seeing 13 to 14 V because the system is charging which causes the coil to overheat and give a weak spark.

Another key part is that you want to have an "externally resisted coil" and a points based distributor. You can search YouTube on how to test what coil you have.

https://teamtalk.mastercraft.com//attachment.php?attachmentid=43269&stc=1&d=1230475845

Also watch this video series from lively (a member here) made a while ago. This really helps explain it in detail.

https://www.youtube.com/watch?v=7LyJtyJPSEM When he had it wired wrong.

https://www.youtube.com/watch?v=9-N2OgZGpzo When he changed the wiring so the positive coil was only receiving power through the resistor.

[Sixtydriver]

So I had this problem on my 1989 Supra Sunsport...has electronic distributor changed ignition coil, fuel pump, spark plug wires, checked timing and still ran with a miss or a backfire...changed plugs to a hotter heat range and still a miss...then checked bolts on the Holley Carb...they were loose...tightened the carb bolts and it has run great ever since? I think it was both improper plugs AND air getting into carb from loose bolts!