1976 Johnson 25 - idle tune help

Having trouble tuning the low speed on this motor (1976 25hp rope start). All new ignition components, carb cleaned (soaked in solvent and blown clean) with all new gaskets. 7/16" spark gap on both plugs now. I have a compression tester on the way but don't have measurements yet. Seems to run awesome one throttle plate opens (linkage lined up to begin opening just at second mark on cam) and really hums up to WOT.

However, I have to have the tiller handle set on "FAST" to start and it won't idle down below "START." I know that the tiler handle markings are suggestions but it seems to be running too fast. I read somewhere here that I should be able to idle down to where I can see the flywheel teeth but I'm nowhere near that. My main concern is shifting at too high RPM and causing extra wear/damage to gears.

Here's the procedure I went through last night:

Before starting, I turn low speed needle valve in until it seats and then back out 1-1/2 turns. In gear on lake, I lower speed (using tiller handle throttle) to where it barely stays alive. Motor vibrates a lot at this stage. Turning needle valve in does not seem to affect idle speed at all, but after 1/2 turn the motor will begin to cough and sneeze (too lean?). Turning needle valve out does not seem to affect idle speed either but after 1/2 out it seems to bog down and run rougher. Either way, I don't get to work throttle down any more than it is.

I have the throttle cam and link set per the manual to just begin to open the butterfly as it reaches the second mark on the cam. When I have it running about as slow as I can get it, the cam is still in about this position. Any past that it wants to die. Could it be that the low speed valve is plugged and not letting any/enough gas though, so that it only wants to run with the throttle butterfly valve open or about to open?

Is it worth pulling the carb apart and cleaning again? Maybe I didn't do a good enough job the first time, especially around the low speed valve. I did pop the core plug and thought it was all clean. My other thought is to decarb, as it's probably never been done and I can see the fouling in the cylinders through the plug holes. Not sure if this can improve idle, however. Also I have thought about going back to the J6C plugs instead of the J4C. It had J6Cs in it when I got it but I replaced them with the manual-recommended J4Cs.

Any insight or ideas would be greatly appreciated. Thanks!

kimcrwbr1 said:

Did you put a new carb mount gasket? Sounds like you may have a vacuum leak? Pull the carb and lightly surface the flange on the carb with 200 grit on a glass table. Just pull the carb in one direction only until it all shines the same. Make or get a new gasket and do not over tighten the carb down. Nice and snug is all that is necessary. Good luck on finding the J6Cs the J4Cs should work just fine.

Click to expand...

Thanks! I did use a new gasket and surfaced it when I did the first rebuild. However I will check it again -- I have another rebuild kit on the way.

Thanks for the vote of confidence for the J4Cs. Thought maybe I was getting bogged down with extra fouling from the "cooler" plugs with all the idling I'm doing trying to sort this thing out.

A follow up to this: how does gas get up to the low speed jet? I can't quite figure it just from looking. Does it come up through the brass tube that sticks up through the float, or though a passage along one side or the other? I can't see any holes inside the needle valve hole other than the hole into the opening where the three small holes are (under the core plug).

Update after last night's tinkering: I soaked and sprayed with cleaner all components again, and the passages for low speed jet are now definitely clear. I was mistaken earlier that I had not resurfaced the carb/manifold interface surfaces. In fact the carb face was the original rough casting with a raised ring around the throat. I lightly surfaced this and touched up the manifold face as well, CLEANED everything very well and reinstalled carb using new gasket and a very light coat of Permatex aviation gasket sealer on both surfaces. Did the same to the carb body/bowl interface surfaces as well.

I did find that the low speed needle valve BUSHING was, I think, damaged upon previous installation. It is a tight friction fit into its hole, and I read on another thread that it's okay to use the needle itself to seat it as you screw the needle in. However, when comparing the bushing that I removed from the carb to the new one from the kit (both Sierra parts, BTW) I found that the one I installed previously would slip over the second (steel) shoulder on the needle while the new one would not. If, in the process of forcing it into its home with the needle the last time, it had deformed and slid over this shoulder and become stuck somewhere before fully seated, I could see that causing any number of weird problems. I seated the new one with a different tool.

As a side note, I find it very difficult to "lightly" seat the needle (before backing out 1-1/2 turns) given high amount of force required to turn it through that rubber bushing. I tried it without the bushing and you can really feel when it first touches that tiny nylon bearing; with the bushing in place it becomes much hard to feel it "lightly" seat.

One other thing -- cold compression test reads 125/123-ish.