1990 Johnson 88 stumbles at low rpm

[Bonstrosity]

As stated in the title I am having some issues at low rpm. The motor starts idles and runs fine however when going to throttle up on the water there is a slight miss around1500- 2000rpm. If I throttle past it, the motor revs higher and the issue goes away.

To give a history this motor recently had the powerhead replaced. The carbs have been rebuilt, they were missing the roller that went on the throttle plate which was replaced. There were also new plugs with the rebuild. I have new coils on the way as the back epoxy area looks cracked on a couple of them. This was an issue with the old powerhead too I assumed it was bc of the missing piston ring and low compression in one of the cylinders.

 

[emdsapmgr]

Couple of things to check. 1. That roller should have a clear plastic "sleeve" over it. Many of these sleeves deteriorate with gas/oil on them. That plastic "skin" over the roller has just enough clearance, that if it's missing, that the engine won't run right. If you just put a new roller on, you should be ok. 2. Possible you may have some debris in the mid-range jets inside the carb. You need to disassemble the carbs and remove those mid-jets in order to do a visual inspection on them.

 

[Bonstrosity]

I should have been clearer, I replaced the roller because the sleeve was broken off like you said. I'm also confident the carbs are clean, as part of the rebuild they were soaked with all of the passage ways blown out. This is why I'm starting to move towards ignition. However if the coils don't seem to help the problem I may redo the carbs again as I've seen that happen before.

 

[racerone]

Report the results of a compression test here.

 

[Bonstrosity]

125 across all 4 cylinders.

 

[emdsapmgr]

I do not subscribe to just "blowing out" the jets. During any overhaul, they must be removed completely, so that you can perform a visual inspection on each.

 

[Bonstrosity]

The carbs were completely disassembled soaked in a bucket of carb dip and all jets were inspected and then rebuilt with new parts.

 

[Bonstrosity]

The carbs were taken off and cleaned by a local shop and reinstalled. They ran the motor in a tank and could not reproduce the issue. When I took it out on the water I experienced the same problem. It appears to be right when the butterflies open on the 2 carbs if that helps.

There is a very minor amount of play between the carbs as one will open slightly ahead of the other I'm considering replacing the plastic retainers at the end of the linkage to tighten the play between the carbs.

Could this be a timing advance adjustment due to it being a heavier boat?

 

[jakedaawg]

It's a linkage adjustment. The butterflies must all open at the same time.

 

[emdsapmgr]

The linkage adjustment is a process called "sync and link." This process syncs the carb butterfly opening with the spark advance. Critical to good running. The procedure is detailed in the factory service manual. Was this procedure followed during reassembly?

 

[Bonstrosity]

I have read quite a bit about the link and sync. I was under the impression the shop had done this. I had asked for it specifically but they must not have done it correctly. Looks like I'll be going back to the shop or looking at another one.

 

[Bonstrosity]

Just to give an update, went to the library and got the manual I needed. Took it to the lake and followed the link and sync procedure. I took the play out of the carb link so they opened up at the same time. However what seemed to help the most was adjusting the cam pickup timing and getting that dialed in. Thanks for the help and advice from everyone.