dead spot on throttle cam 88 Mercury Black Max

[waveskimmer]

Hi,

135hp Black max slow hole shot. Set the link N sync by the book and notice a dead spot where nothing is moving on the carbs or the trigger advance. Just a hunch, in general, should this dead spot be where the max timing is still advancing and be fully advanced just when the carb butterflies start to open?

thanks!

[Jim Speros]

Your hunch is not correct. The timing moves first, until the linkage cam touches the roller and starts to move carb shutters. That happens at about 750 or 800ish rpms. The timing continues to move with carbs until max timing is reached. That happens about 3000 or 3500 ish rpms.

Seems you need to do link and sync again. Check and make sure all individual parts are moving freely first.

[waveskimmer]

Well the shape of the throttle cam on this engine is such that there is slight movement of the carb butterflies when the throttle control lever is moved forward, and NO more movement of the carb butterflies until the roller reaches the area on the cam which pushes forward on the roller. At about half way through the dead spot , the timing reaches max, resulting in the dead spot where there is NO movement on either the timing lever or the carb butterflies. I can redo the link and sync 100 times with the same result.

[Jim Speros]

Most cams are set with a small gap before it touches roller. Are you following a merc manual or after market selocs or something? I always used Mercury I found selocs confusing at times. Any way lets clarify what you mean by dead spot. Once top screw max timing hits stop the lower part of arm should continue forward and move throttle. Has the timing arm been removed? It is spring loaded and can be tricky to install properly. If you dis connect control cable does the arm move freely? It should move as one initially and once max timing is reached only the lower part and throttle link continue to move.

[waveskimmer]

Following the Merc manual, and occasionally the Seloc to confirm. I set it so the roller moves freely, yes

Yes the timing arm was removed to install new throttle control rod barrels. Everything moves freely

"Once top screw max timing hits stop the lower part of arm should continue forward and move throttle."

For me, once max timing hits its stop, the lower part of the arm continues to move forward, but throttles aren't moving because the cam shape is more or less under the roller and not pushing the roller forward to open the carbs. Once the roller reaches the "corner" of the cam, the carbs start to open. Thats where the cam shape changes to nearly straight up and down.

Had it out tonight and used your reference that max timing should occur at 3000-3500 rpm, and what i observed was that the rpm was at 1500-2000 when max timing was reached.
,
When I do a part# search for the cam the results show a different shaped cam, except for an ebay listing that included the entire throttle arm assembly with all the attachments.

[waveskimmer]

Ok, gonna back out of the rabbit hole a bit and assume that this problem has to do with the link rod barrels that I just replaced. The book says that the barrels need to be screwed onto the rod till they bottom out and backed out only enough to obtain correct orientation of the throttle cam. This doesn't seem to be enough to insure that the throttle cam is at the correct position on the roller when max timing is reached.

[Jim Speros]

Sounds like your on the right track now. Share your engine serial number so that I can see what you should have. The rollers and throttle link are adjustable some more then others. A pic of your linkage as is would be helpful for me too.

[waveskimmer]

Serial # B277577, I'll get some pics uploaded shortly.

Thanks!

[Jim Speros]

I see that engine serial group calls for a cam # 95791
I also see a later models has a different one.
The first like you noted pivots from bottom and is lets say like a boomarang with a bump in it.
The newer model pivots from top side and is more like a triangle with a round side.
I'll look to see what your pic shows if this doesn't explain enough for you.

[waveskimmer]

The part number on this cam is 95730 which when googled comes up as 95791.

I did lengthen the link rod via unscrewing the barrels by one turn. It helped a bit. I also adjusted the idle stop screw so the cam is further ahead in its travel and that helped even more in that at least the carbs start to open by the time max timing is reached, but now the gap between the cam and throttle stop is about 3 times what the book says is spec.

[Jim Speros]

I am not seeing any gap between cam and roller. The cam looks right to me. Can't rely too much on casting mark numbers. It looks like the idle stop screw is turned in a lot. What is the gap on your roller to cam supposed to be? Something like .050? Does the cam have both white bushings on the pivot shaft? I would try backing off idle stop screw a bit and move roller as necessary to line up line on roller to line on cam with a gap as manual described. Seems to not be off by much.
I'm gonna see if I can find a specified length for link rod.

[waveskimmer]

Yup the idle stop screw was experimentally turned in to get the cam in that position. If I set it at spec (1/8" gap between cam and throttle stop screw boss) the line on the cam is at the top of the roller.

Book says "roller just touching the cam". No gap spec

Bushings on the pivot shaft good

attached is the link N sync procedure i'm using

[Jim Speros]

I looked thru my manual and could not find any dimension for link you replaced. Did it look identical to what you replaced?
Anyway you have a geometry issue. It needs to be worked out before fine adjusting the link and sync.
I've been thinking about this off and on all evening. The idle screw needs to be turned out and adjust it to look like example in manual. Adjust link to cam for adjustment to stop as stated. Keep working on that until you can see the cam moving the carbs with out running off the end of the cam. Maybe you will need to use the short line on cam to get the range you need. Once you have the needed range of motion then do the link and sync of carb roller and primary pick up timing. The max timing doesn't change unless you move screw or stator link.

Once it is as close as you can get it run on hose. With carb shutters closed adjust primary pick up to obtain an idle about 900.
Double check max timing before you run it off the trailer.

It's something I find easy if it were right in front of me. I hope I have helped you and you get it figured out.

[OnPad]

I have a couple pics from a old 225 that I've never run. Don't know much about it, but has the same style cam. Looks like the carbs aren't open much (2nd pic) until after full timing. It does kinda have a dead spot. Third photo of cam is where max timing occurs on the cam.. Fourth photo is a newer engine with a much more linear cam at the point it reaches full timing. Might help might not?

[waveskimmer]

That's a major confirmation. Very helpful, thank you for doing that OnPad! In the third pic your cam is a bit more ahead in its travel with the roller than mine and all is dead until the roller reaches that "corner". this might have something to do with the over advanced timing symptoms i've always observed with this engine in the lower midrange.