Have to back off a bit for max throttle?

[tfbrown1270]

I have a 1983(ish) can't find a number plate or anything to confirm a model year....Johnson 9.9...
It runs great with 105/105 psi compression and starts on the first pull...idles down to almost nothing and uses next to no gas...so I'm happy...except for one little issue....
When I give it WOT I find that I have to back off on the throttle to achieve max thrust...I can "hear" the difference when I rotate it back 1/4 inch or so, it bogs just the slightest when I rotate the throttle to the stop at WOT...
Is this a timing issue? (I set the timing this spring when I replaced an old roller) and it was perfect, the roller contacts as the slack is taken out of the throttle linkage and the throttle just begins to move)....or
something else?
I know its a small issue but its so close to perfect I feel I gotta get it right.
Oh it has new plugs, new thermostat, new water pump, new bottom end lube, I run 50:1.
This is it....

 

[tfbrown1270]

Re: Have to back off a bit for max throttle?

Timing or carburation or ???

 

[tashasdaddy]

Re: Have to back off a bit for max throttle?

did you do the link and sinc. that is timing and carboration, sounds like it's flooding at wot. too much fuel for the timing.

 

[CharlieB]

Re: Have to back off a bit for max throttle?

Actually it is a slight LACK of fuel and too much air.

Dirty main jet if it is a fixed jet, a touch lean if it is an adjustable main, a slight lack of flow from the fuel pump.

With the compression down to 105 it could be the weakened pulse to the fuel pump and there may well be nothing you can do about it except enjoy it at slightly less than WOT.

If you was to rejet slightly fatter then next year or so rering it or replace pistons then it would be too rich.

 

[Silvertip]

Re: Have to back off a bit for max throttle?

My two cents worth say the link&sync process is not quite right in that it allows the throttle plate to go past horizontal in the carb bore which begins to restrict air flow. Very simple test is to remove the air box open the throttle and see if the throttle plate is perfectly horizontal. If not, back off the throttle to where you normally found WOT and then see if it is horizontal. Link&Sync will fix that. It's probably not off by much.

 

[iwombat]

Re: Have to back off a bit for max throttle?

You can just check the roll pin on the throttle plate shaft. It's visible on the side of the carb and is perpindicular to the throttle plate. If it's vertical at max throttle position you're okay. If it's not, you need to do a link and sync.

 

[tfbrown1270]

Re: Have to back off a bit for max throttle?

Hi..the procedure I explained was a link and sync for that engine and I was very fussy about it...the throttle plate just begins to "move a billionth of an inch (slight exaggeration) as the roller gets to the index mark on the timer base.

I'll check to see that the throttle plate doesn't go beyond horizontal but how could it if the link/sync was done properly?

And about the compression I thought 105 was good...what should it be?...I did a couple of decarbs this spring so no more compression hiding in that engine I guess.

 

[iwombat]

Re: Have to back off a bit for max throttle?

I believe the last step of the link & sync is to set the cam adjuster (screw on the end of the cam) so the throttle plates are fully open at max.

 

[Scaaty]

Re: Have to back off a bit for max throttle?

Anything over 90 is good, you at 105 are in great shape. I had an 80's Johnson that went over center at WOT...check the butterfly first

 

[tfbrown1270]

Re: Have to back off a bit for max throttle?

Thanks everyone...

and I didn't know about adjusting the cam adjuster...