Prop QUestion for a Mercury Thunderbolt 800 80 HP

[Faztbullet]

You need power trim..it hydraulic. As you run and raise the trim the boat will pick up speed and nose will rise,then you can get thru the porpoise. This will increase the RPM and speed.

 

[krizm200]

Hi everyone,

Jimmbo, I have a question on linked and synced. I noticed when I was first rebuilding the engine that there are 2 screws locked by a bolt on a plate in front of the distributor. I thought the idle adjusting screw was literally to adjust the idle of the engine. Now I know that is wrong. So I was thinking about it last night. That screw is to adjust the sync of the distributor (spark) to the BTDC of the piston. I mention it because the idle screw was at least 1/4 inch off the distributor at idle on the throttle. It wasn't doing anything. So my engine is not synced at least for idle, which would make the engine harder to start (and it is). The mechanic at a marina set it this way. If I purchase a timing light, I can readjust any setting that is incorrect, the idle and the WOT setting. Makes me concerned about the WOT setting as well.

Does this makes sense?

 

[jimmbo]

On most non ECM 2 stroke outboards, including yours, you set the timing for max advance, and check/adjust the linkage so the carbs begin to open at a certain timing point. Idle speed is determined by retarding the timing. It will vary based on the pitch of the prop, No adjustment, except just limiting the retardation so the engine doesn't stall.

 

[krizm200]

OK, so the idle adjust should be fine. I'm getting the timing light this evening (see if I can find one locally) and check the max advance. I'm going to check the carbs as well, just to be sure they are opening completely. This boat has been quite the adventure.

 

[jimmbo]

I'm not going to say your motor is adjusted correctly, as I can't see what is happening with the linkage, or how its adjusted. If you follow the procedure for timing and sync. The idle stop screw is the one that sets the idle speed(remote cables unhooked, then the cable length needs adjusting so as to put a bit of preload against the stop). The other screws are for max timing and pick up timing.

 

[krizm200]

Hi everyone,

So I have discovered that I am an idiot. I took the engine apart again last night. TDC for piston, the timing plate, and the ignition are all synced, I checked the butterfly in the carbs, and found they are only opening 2/3 to 3/4 of full open. I can open it more with my finger on the carb shaft, but not with the throttle cable. I checked reverse, and the throttle will open completely with the throttle cable!

So I'm guessing that is why I am not getting at least 5000 rpms at WOT. So now the question, how do I adjust the throttle cable so the carbs are wide open? Is it the cable itself, or do I need to adjust the throttle handle? The handle seems to stop at one point going forward, but the carbs can still open another 3/8 inch or so.

Thanks everyone!

 

[Chris1956]

MerControls have plenty of travel, so if you have one of those, you should be good. Your motor has a throttle stop screw above the distributor. Loosen it and see if you can open the throttle plates more with the control. Don't adjust anything else.

 

[krizm200]

Hi Chris,

I checked the Throttle Stop screw. The distributor does not connect to the TSS when pushing the throttle arm to full open. But, if I go full throttle in reverse, it does. So either the cable is not adjusted correctly, or the throttle arm is not traveling far enough in forward position.

It is a Mercury throttle system. I am planning on experimenting with the throttle arm setting and and the cable adjustment at the engine tomorrow morning.

 

[Chris1956]

The plate above your distributor should have 3 set screws. They are "Idle Stop", Throttle Stop" and "Max Spark Advance".

The throttle stop screw should stop the throttle cable from travelling any further. The distributor would have stopped sooner, as it is limited in travel by the max spark advance set screw.

If the TSS is loose and the throttle cannot be pushed enough to open the carbs fully, obviously, something else is wrong.

Some of the earlier distributor models had an adjustment on the lever between the throttle arm and the distributor. That might fix your issue by making it longer. I never adjusted the one on my '73 Merc 850, so I cannot say for sure.

 

[krizm200]

I found it!!! Why the throttle was not opening up to full! The throttle connection was upside down, so it was hitting the shift linkage bolt below it. Movement was restricted to 3/4 throttle. I'm about to take the boat out and check for WOT, rpms, and mph. I bet I get the 5000 or so I am supposed to!

 

[krizm200]

So I have to say going above 37 mph on my little Mako 15 is way too fast! I pulled back on the throttle at 38 mph, rpm's and mph was still rising above 4300, a little scary on an engine from 1972. So the throttle linkage as upside down was the issue. Never would have though to look without everyone's help! Thank you all for the suggestions! It was the adjusting of the throttle cable that gave it away. As I moved the throttle nut closer in, the travel kept getting less, until the shift would no longer go into forward or reverse. Figured it had to be something jamming somewhere!

I may still try the 13.25x17 prop. Though I have to admit, the boat cruses really well at 30 mph.

 

[jimmbo]

What RPM did the motor attain at full power?

 

[krizm200]

Max rpm I reached before cutting back on the throttle was between 4300 to 4400. At that point I was above 37 mph with my 8 year old son and me in the boat. The tach didn't show the engine getting any higher rpms, but the GPS speedometer showed the speed was increasing slowly. Above 37 mph seemed too fast for my 15 foot Mako. I would think the 13.25x17 would put the rpms right where they should be. I would rather have less stress on the engine anyway since it is a 1972, but that is just my thought.

 

[jimmbo]

How does above 37 seem too fast?
Your numbers for rpm and speed with a 19 inch prop don't line up. 4400 rpm with a 2.3:1 gear ratio, a 19 inch prop, and allowing 12% slippage, gives a speed of 30mph. I suspect your tach is not accurate

 

[krizm200]

Hi Jimmbo,

I think you may be right on the tach. I trust the GPS speedometer, ( i suppose I could clock myself across the lake and check time to verify my speed). I did install the tach, and set the pulses for a 4 cylinder 2 stroke, I think it was 12 pulses.

37 seems too fast because my former 14 ft aluminum with a Tohatsu 9.8 hp never went above 20 mph. I'm not used to going anywhere near that speed on the water. Even my father in laws pontoon boat would never have gone above 20 mph, if it even hit that. I like cruising around 25 mph on the fresh water lake. Fast enough for me.

 

[jimmbo]

I believe your engine has a 12 pole alt, so the tach should be set to 6. Until you have an accurate WOT rpm reading, there is no way to determine if you are running the correct prop.
You might like cruising at 25, but as soon as the kids get hooked on waterskiing, especially slalom, they are going to want more speed. You boat can and will give them that

 

[krizm200]

I bet you are right on the water skiing comment! My wife won't let me go over 20 mph, she thinks that is fast enough! I'll set the tach for 6, I know that is not the setting currently. I thought I had red on some forum it should be set at 12, but I'm willing to try 6. I'll test it again this weekend and see if that setting makes more sense.

Thanks!

 

[jimmbo]

20 is too slow for water skiing, the boat's barely planing, 23-24 would be minimum, and 30 minimum for decent Slaloming. As for the wife, leave her ashore.

 

[krizm200]

Now there is a comment! I don't think I will leave Susan ashore. I might have her watch our daughter while water skiing, and not tell her how fast we are going.... She would be busy taking pictures and waving the red flag anyway.

I was going to try around 25 mph first, with Jenna on 2 skis first anyway. That is still a few months away.

 

[krizm200]

Hey Jimmbo, question on the Faria Euro tach for outboard motors. I checked what my tach is set for, it is on position 5. According to my Faria Tach manual, this is for a 12 pole ignition system. Basically, it shows setting 1 is 4 poles, 2 is 6 poles, 3 is 8 poles, 4 is 10 poles, and 5 is 12 poles. There is a setting 6, but no mention on what the poles are. I am assuming position 5 is the right selection.

There was also mention that if the tach seems to stick, the mount may be too tight. I'm going to loosen the mount in case the tach is sticking around 4200.

So I think the tach is set right. Wonder what others think.