Throttle Question?

[duckhunter55]

Should the back butterfly valve on a 1978 25 hp johnsons carb be wide open when throttle handle is turned to full throttle? Im not talkin about the choke valve but i guess the throttle valve, behind it. Reason i ask is cause when i turn my throttle all the way open the throttle valve only opens a quarter or less. By pressing down on the throttle linkage ican get it to go wide open, but not with throttle handle. Also on throttle handle there is play during the turn which i guess is whats causing the looseness in the linkage. Also after starting motor foe first time then cutting it off, when you go back to start it will take at least 10-12 hard pulls, and then pulls get lighter and it starts. Nothing in powerhead has been messed with, and the motor was running fine back during summer.

 

[commonarcwelder]

Re: Throttle Question?

I set mine horizontal at WOT. There should be an adjustment on the throttle linkage to timing plate right under the flywheel. Put motor in forward and turn handle to the wide open position. Loosen the set screw on the adjustment and open the butterfly horizontally and tighten your set screw. You'll be able to see how it functions.

 

[F_R]

Re: Throttle Question?

Closely related to that is the point at which it starts opening. Thre is a mark on the cam that opens it. The butterfly should just barely start to open when the roller is aligned with the mark. I believe the cam can be moved in or out to adjust on that model.

All this affects starting, and running and it is no wonder you are having problems.

 

[JB]

Re: Throttle Question?

Get a Manual and do a link and synch, Hunter. Clearly the throttle handle and the throttle butterfly are not synchronized.

 

[Silvertip]

Re: Throttle Question?

Yes -- the butterfly needs to be perfectly horizontal in the carb bore at wide open throttle. The link & sync procedure is the process that synchronizes throttle opening with ignition timing. That is a relatively detailed process that must be followed to the letter and far too detailed to go into here so the recommendation of obtaining a service manual specific to your engine is a good one.