Spark plugs on a 1987 johnson 150 gt

It currently has champion UL77V in it but a parts source shows champion QL77JC4 as plug for it.Only reason I am even looking at plugs is it idles and runs rough at idle speeds but jumps on plane and runs out like it should in fact it screams for it's age.Im suspecting that the Ql77JC4 with proper gap may fix idle and slow running roughness.The motor just is not happy when idling around.Any opinions from the guys that have experience with these old motors?

one thing is resistor and non-resistor, ql is resistor, the ul's could be ring fires/surface gap, my experience with surface gap was they fouled easy, i use the ql's on my older 3 and 4 cyl john/rudes, i'm not familiar with the 6 cyl's but mine seem to like them better than the surface gap plugs, you might even try a little hotter plug, might try some seafoam in the fuel, check the fuel filter and pump screen, a compression test while you have the plugs out wouldn't hurt

yeah the UL's are surface gap thats why I think the gap plugs will help.It has been decarbed and all that has water sep.Really runs good all around but idling around which I do alot of scoping electronics.

that's what i would start with then

Sounds like you might have carburetor idle circuit problems. At idle pull sparkplug wires on each cylinder to see if all are hitting at idle. If you have one or more not hitting, clean that carb good.

I bet those correct plugs may help. But also make sure your ignition system is sending good juice to them. Slap a DVA on a volt meter and make sure you are getting 150V+ on your orange leads down to the coils. If the ignition checks out, then go to the fuel side. Like already said, start with the carb idle circuits.

Might have better luck using NGK plugs. Here is the conversion part numbers.

Champion QL77JC4

= = = = = = = =

NGK BZ7HS-10

I will check all and get back with results.

Had a bad battery and now it idles better but tach is going crazy now reading more rpms than it was before at wot.went all the way to 7000 rpm when before reading would be 5500 to 6000 rpms.

Interesting, I’ve always been told that the battery is just to start it. After it’s started you can throw the battery over the side of the boat and nothing will change, at least on the ignition like you have on that GT150. I’d be interested to hear opinions from others on this.

On back of tach. is a round cylinder # rheostat switch. Rotate it multiple times and then set it to the correct number of cylinders your motor has. Have seen this cure many erratic tach. readings. Think switch contacts get some build up on them over time.

recheck the battery ground cable, and check voltage at the battery while running to make sure your getting around 14v, i'm leaning towards the rectifier now

Hey do you mind posting a pic of the front of that tach? From the looks of it i have the same minus the sticker and was wondering why it never responds to the engine rpm properly. Thanks!

I use to think that the battery had no effect on the ignition but was proven wrong. 200 Merc had no power and found after a lot of work that the owner had connected the on-board charger up to the cranking battery backwards. Corrected the chargers connections and the motor ran great.