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Author Topic:   Best Prop. for Montauk 17 w/ Merc. 100 2s ?
KDW posted 10-15-2003 07:16 PM ET (US)   Profile for KDW   Send Email to KDW  
I didn't receive any response to my question under the "repairs and mods" section, so I thought I would try here.

I have a '98 Montauk 17 with a Merc. 100HP 2s. I had a few problems getting the motor to run smoothly, but most issues are now fixed - except for a little roughness running to/from 2 to 4 cylinders. The motor has a Mecury Laser II 20P prop installed. When running with the motor set in the lowest hole (but the inside edge at the top engine bracket is still 1" above the top edge of the transom), I could only run a maximum of 4400 - 4600 RPM (surging). Based on a lot of discussions on this site, I raised the motor up one hole position(now about 2" above top edge of the transom).

This move definitely helped. It seems to take less power to reach/hold plane and now I can acheive about 4800 RPM at WOT, but no more even working with the trim. Actually, the motor surges a bit at WOT from 4600 to 4800 RPM, which I don't understand since I shouldn't even be close to the rev. limiter set point? Also, I checked/cleaned the carbs and the high speed jets were clean.

I can only guess that I need to change props in order to come closer to maximum rated RPM of 5250? I would really be satisfied if I could just get it to a smooth 5000 RPM.

Any advice would certainly be appreciated.


Clark Roberts posted 10-16-2003 06:16 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
KDW, you should be able to get 5250 rpm with the 20" prop! Something is not right or set-up correctly IMHO. If carbs are clean and adjusted correctly (check that you are getting full opening on all four carbs at wide open throttle (WOT). A "link and sync" exercise will ensure that timing is also full advanced at WOT. The surging indicates fuel delivery problem... check inline fuel filters/clean/replace/eliminate... check all fuel line connections (especially the clip-on at engine) and check fuel tank pick up (inside tank). I would use a different tank (known to be clean) and see what happens.... good luck... Clark... Spruce Creek Navy
Bigshot posted 10-16-2003 01:48 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Tach accurate?
ward1 posted 10-16-2003 02:13 PM ET (US)     Profile for ward1  Send Email to ward1     
I have a similar setup Montauk + 115 Merc/Mariner. The surging does sound like the rev limiter as I had the same situation turning a 17" pitch prop. Are you sure the tach is accurate? What kind of speed are you getting? I would think with a 20" prop you should be real close to 5250 as I turn 5100+ with a 21" rapture.
Richard Quinlivan posted 10-16-2003 02:26 PM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     
I have a 1995 125 Merc 4 cylinder classic two stroke motor. It has the 2 cylinder to 4 cylinder transition at ~ 1800 rpm or so and the 5250 rpm max. I believe it is similar to your motor. This motor does not have aany sort of rev limiter. I have had my rpm to~5600 at times although I don't do it often since this motor has some known internal balance problems at high rpm.

You surging is either fuel or ignition related. I have never seen any surging in my motor (fortunately).

Richard Quinlivan posted 10-16-2003 02:30 PM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     
I have a 1995 125 Merc two stroke classic. This motor is similar to your 100 hp I believe. These 2 strokes are a classic design and have no electronics except the ignition modules. They do not have rev limiters. I have had mine to 5600 rpm with no problem.


Richard Quinlivan posted 10-16-2003 02:32 PM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     
Sorry for the double post. It didn't seem to except the first one.


lhg posted 10-16-2003 02:37 PM ET (US)     Profile for lhg    
For a Laser II prop, I would recommend installing the engine in the 3rd (middle) set of the 5 hole pattern. Most of Mercury's performance line of props pick up 2" of EFFECTIVE pitch when run fully submerged. They need to run surface piercing to correctly use the stated pitch. You should be running the medium sized vent hole plugs also. With the 20" prop, you should be getting about 45mph top speed.

From everything I have seen in Mercury's prop literature, the 4 cylinder 100 is no more powerful than the 3 cylinder 90. I think that's why it's no longer made. Don't ask me why this is the case. I have suspected for some time that the 90 is under-rated.

KDW posted 10-17-2003 10:03 AM ET (US)     Profile for KDW  Send Email to KDW     
Thanks to all for the good advice and help.

I don't have GPS or any way right now to check the actual boat speed at WOT. I can only assume the tach is right.

It is good to hear from others that have, or have had similar motors. If surging isn't normal and it can't be the rev. limiter, then I think it will be wise to start with Clark Robert's recommendations.

When I first checked, the #2 carb. from the top was barely cracked open when the other 3 were fully closed at low throttle. At WOT, the #2 carb. butterfly would open 100%, but the other 3 would only go to about 90%. I had surging and lower than expected RPMs during this time as well. I saw this as a potential problem, so I set the carb. linkages where all the butterflys come fully closed together at the lower end of the throttle stroke. Now the butterflys move together in the same amount, but don't open 100% and only open maybe 90%. The only other way that I can see to set the linkage so all butterflys will open to 100%, will "crack" the carb. butterflys just a bit off of the fully closed position, and I was concerned this would cause problems at lower RPMs?

Also, it's interesting about the Mercury II props and that I may be able to raise the motor one more hole up. If I do this, how will I know if I've gone too far?

Thanks again,


KDW posted 10-18-2003 09:11 PM ET (US)     Profile for KDW  Send Email to KDW     
Well, I spent a few bucks and some time on the Merc. 100. I followed the Merc. Service Manual instructions for synchronizing/adjusting the carbs, oil pump linkage, and accelerator pump. I changed both fuel filters on the motor and rebuilt the fuel pump with new parts. I installed a new fuel hose with primer bulb. I didn't mess with the timing because the max. adjustment screw still had what appeared to be factory paint on the thread and nut.

I was really hoping the fuel pump was the key because the rubber check valves looked a little warped and were replaced by new plastic discs, so I thought there might have been a flaw in this design. I was really disappointed to test run today with no noticeable improvements. I still have surging at WOT and the tach only reaches 4800 RPM at full trim.

The last thing I know to try is to change fuel tanks. I noticed the primer bulb will not firm-up completely when squeezed and I can hear "bulbing" in the fuel tank when the primer is full and I keep squeezing. I'm not sure if this is normal, but I was always able to squeeze the primer bulb for my old Johnson 125 with steel tanks until the bulb couldn't be squeezed in any more. I suppose one other thing I can try is to install clear tubing after the fuel pump and look for air bubbles.

Any other advice would certainly be appreciated.

My motor makes a "clacking" noise from 3000 to 3500 RPM. It almost sounds like a piston rod knocking. Is this normal? Do you hear this on your 125HP?



KDW posted 10-21-2003 02:12 AM ET (US)     Profile for KDW  Send Email to KDW     
Guess I'm at my wits end now. I tried changing to a fuel tank that is known to be good and still can't get more than 4800 RPMs out of her at WOT.

I'm really starting to think the Merc. 100 won't sling the SS Laser II 20P any faster. Maybe I should try an aluminum 18 or 19 pitch?? I know its a stretch, but would high octane gas help any??

Starting to feel a lot like I'm talking to myself lately. The scary part is that I'm starting to carry on a pretty confusing conversation.

Anybody out there got any other ideas about what I should try??

Clark Roberts, Bigshot, and Sal, if you're out there I would certainly appreciate your opinions.

Sleepless in Anderson.....

Bigshot posted 10-21-2003 03:09 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Tach is inaccurate....oh I said that already....have you checked it? Try an 18 or 22" and see if you still get 4800. Saw a weird one last week. Guys older 150GT Johnson was running like snot. Checked al the spark and did comp test, all good. Ripped down carbs and clean as a whistle. Decided to try the stator. Removed the flywheel and the kwyway had broken....but jammed so he was off 2 cyls. Therefore it was hitting all 6 but in the wrong firing order. Installed new keyway and she purred like a kitten.....weird one. If it aint carbs or fuel, then must be electrical. Stator or trigger could be off or bad. I would focus on tach being right first. They might not have a rev limiter pre say but they will surge etc if running above redline. What was your compression? If you have not done a comp test yet, why are you futsing with the other stuff. Being at 80lbs in 1 cyl would answer all questions.
KDW posted 10-22-2003 10:54 AM ET (US)     Profile for KDW  Send Email to KDW     

Thanks for the reply. Got you on the tach. and I've been planning to check. I've got a hand-held that I plan to try and compare to this weekend to see if the one on the boat is off. I also plan to borrow a 19" pitch aluminum prop and give it a try (can't find anything else locally right now).

I did a compression check early on in this process and get 130 to 135 psi on all 4 cylinders.

Interesting about the stator and flywheel key. I will check this if nothing else pans out.

I probably can get someone to drive while I squeeze the primer at WOT and/or hit the choke at WOT as a last check to assure I'm not starving the motor for fuel.

If nothing else raises it ugly head, then I suppose I need to confirm max timing setting as well and I haven't done that yet.

Thanks again for the help.


Bigshot posted 10-22-2003 11:20 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
AGAIN....DON'T mess with the timing. Try a squirt bottle with gas in it and while your buddy drives, squirt gas in each carb. If it bogs then you have plenty of fuel, if it picks up or smoothes out then you have a problem with that carb.
KDW posted 10-27-2003 05:20 PM ET (US)     Profile for KDW  Send Email to KDW     
I checked a few things this weekend:

1. The boat tach. is accurate based on a check against a handheld unit.

2. Changed prop. to 17" S.S. Solas and got 5,100 RPM max. with trim (still surging).

3. Changed prop to 19" Aluminum Solas and got 5,100 RPM max. with trim (still surging).

4. Applied choke during WOT surging - engine bogged.

5. Squeezed primer bulb during WOT surging - no effect.

I think I've got a good combo with the 19" alum. prop., just can't get rid of the surging problem. Also, I hear a "clacking" noise from 3,000 to 3,500 RPMs. I can also feel a slight vibration when I hear this noise. It doesn't sound normal. I wonder if this related to the surging? Can the reeds make this sound in a 2 cycle? If I pull/push on the flywheel hard enough I can hear a "knocking" sound. I really can't see any movement (play), but hear it. Is this normal? Could a loose/bad cam shaft bearing be causing the rattle I hear? I'm worried a bit about the noise as it may be a BIG problem. However, this seems unusual for such a new ('98) motor with less than 200 Hrs.

Any advice is certainly appreciated.


lhg posted 10-27-2003 05:54 PM ET (US)     Profile for lhg    
Sounds like it may be time to take it to a dealer. These situations are difficult to "armchair" diagnose.
Sal DiMercurio posted 10-27-2003 10:13 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
I second IHGs suggestion.
You can trouble shoot the whole engine & end up still not knowing.
I kind of have a hunch Bigshot might be on the right tract with the keyway slipping out.
It really makes no sense at all to even fool with props, as your getting 5,100 rpms with a 17 & a 19 & there "HAS" to be 400 rpms difference between them no matter how you cut the cake.
I think it's under the flywheel at the very other end of the engine from the prop.
Bigshot posted 10-29-2003 11:12 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Ps....there are no camshafts in 2 smokes.
KDW posted 10-30-2003 01:14 PM ET (US)     Profile for KDW  Send Email to KDW     
Thanks to all for the advice. I took the boat to the shop yesterday.

Guess I need to proof read my writing better. I mean't CRANKSHAFT and not "cam shaft" in my previous post.

Thanks Bigshot for correcting my mistake.


KDW posted 11-04-2003 04:23 PM ET (US)     Profile for KDW  Send Email to KDW     

Here's a new one on me. I just spoke to the mechanic that worked on my Mercury 100 2s. He said that the carbs were in the correct location, but the high speed jets for #2 and #4 carbs were swapped.

After he changed the jets back around, both the knocking noise between 3000 - 3500 RPM and the high speed surging were fixed. WOT is 5250 RPM with a 19" prop, which is perfect.

The shop that originally cleaned my carbs had to have made this mistake. I doubt they will cover the repair cost, but I plan to try anyway.

A difficult, but valuable lesson learned.

Thanks to all for the help throughout this troubleshooting process. What a relief!!


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