85 johnson j150tlcos idling issues

[keefallan]

Here's the deal. I bought a frankenstein motor from a man in Savannah Ga. He told me it was an 90. There are only a few parts that are that year(trim relays, lower unit, maybe more). Coils, power packs(yes 2 not one) electronics, no not a 1990. The powerhead is a '85. I obviously didn't know all this at first. But now its time to make lemonade. The engine runs. It ran better when I first bought it. It would idle then. Now it won't stay running in idle. It will still rev up no problem. I changed the coils(all brand new good resistance readings), new wires(bad readings on old wires; 380 ohms on new ones), and new power packs(old ones were old and spliced up). The motor didn't change any. I haven't touched the timing or stator or timer base or rectifier yet. I have ordered all new for them. I am going to basically treat this motor as if it is an 85. Also I'm going to put on a brand new 85 engine harness. The other guy spliced up or butchered up a 1989 harness to get this to run. What I forgot to say is this: the engine is brand new(bored, rings etc.). He did put the powerhead together right. Compression is fine. The lowest cylinder is 85 the best is 90. It does have low compression heads. So now for the questions. What will make the engine die at idle(it is coughing alot) but rev up and seem to run fine when I throttle up?Can I shave the heads to bring up compression just a touch(175 specs?) I am open to any suggestions to help make this motor somewhat normal(so I can understand exactly what make this thing tick). I guess I should set timing to 85 specs?
Lastly, I forgot to mention, the engine has never left my stand. I've only run it for only four times or so and I'm still running double oil. Plugs do seem to be wet when I pulled them after running rough. I have only run it for 5 minutes at a time. Help?

 

[emdsapmgr]

Re: 85 johnson j150tlcos idling issues

When the engine idles, the butterflys are closed. The fuel passes through the idle passages in the carb body. If these get any dried fuel or crud in them, the engine won't idle. A "lean sneeze" is a typical symptom. As soon as you open the butterflys, the carbs will draw through the other jets and may run smoother. You may need to pull the carbs apart and clean these passages with an aerosol carb cleaner. Your engine may have the low compression bathtub heads. A set of higher compression heads will improve the performance. Old 235 crossflow and some of the later GT/XP heads will make at least a 10-20 lbs compression improvement. After changing the heads, I'd check the plugs to make sure the powerhead is getting enough fuel at top end.