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140 mercruiser top speed question

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farmerrick53

Contributing Member
I have a 18' strarcraft with a sterndrive (alpha one or just a few serial numbers pre-alpha, so far all parts I have needed were spec for alpha one) 15 pitch prop, NEW 3 blade aluminum mercarb 2 barrel points ignition. I operate a 7000-7500' above sea level. I am running 91 octane ethanol free, have the timing static at 6btdc and full advance of 30* all in by 4000 rpm. engine seems to check out healthy I did re ring it and re do the cylinder head about 100 hours ago. My top speed with 2 people and a lot of fishing gear is only about 30-34 mph gps depending on the temperature that day. I even lose usually about 200 rpm and about 2 mph from early morning when it is cool to afternoon when it warms up. does this seem normal or should i be looking for a problem? top rpm at WOT trim down is 4100-4200 and trimmed up is 4200-4400. engine runs smooth, starts easily and all plugs look good in color. valves are adjusted to 1/2 turn in (tight) from clickety clack. The load is 2 down riggers, extra balls, electric motor, honda kicker motor, 2 batteries, cooler, and all the stuff we need to troll. fuel capacity is 24 gallon- 2 12 gallon tanks, and if i leave all full or one totally empty seems to not make a difference. plains out in seconds comes up in rpm very smooth.

I did have some carb issues I mostly resolved this year, it was needing leaned down some a smaller power valve and a slight jet change seem to have helped it a lot. I definitely do not want to go too lean though, so better safe than sorry and make small changes. I have recently started blending the fuel 50/50 100 low lead av gas and 91 pump premium since lead additive is getting hard to find around here and all 3 heads I have for this engine are soft and call for leaded gasoline, the av gas pure runs pretty rich but blend gives me the satbilizer and the lead additive and runs normal (for it anyway) I can't detect any problem nor any abnormality, it definitely smoothed out when I added another degree of timing, but that is about max spec for the engine by the book, and again, better safe than sorry. I have a 13 and a 17 prop I can try. The outdrive gear ratio is 1.89 or 1.98:1 if i remember correctly, I can verify if that sounds bogus.

Carb starts runs and opens and closes properly, choke closes when cold on startup, gradually opens, throttle plates open all the way then come open slightly past center, I haven't really worried about this adjustment since I just run it usually at 4000 rpm cruising and don't really crank it up to wot very often and if I do i just pull back slightly on the lever to get all in throttle. When I went to lake powell I richened up the jets about 2 sizes and it went 35-36 I think and revved to 4500, even with their heat, the lower elevation it really liked, I did not change the prop for powell, just didn't want to lean out on top since i had a lot longer runs than local water. thanks and happy boating.
 
Sounds like the timing advance is later than it needs to be...without knowin the details (version of the 140) its a bit of a guessing game.

at 7Kft, your jetting may still be a bit fat...hard to say since the details aren't very clear....

Also, sounds like the prop size is right on the edge. I'd be inclined to finalize the prop by setting the WOT RPM to the upper end of the range in the worse case conditions...you're engine will never object to a lower pitch prop as long as you don't exceed maxRPM.

cooler inlet air temps will enable the engine to increase its output...without the delta T being quantified, its only a guess...but I'd say span you cited is reasonable.
 
ok got it thanks. I leaned to the next smaller power valve and it actually went too lean, so my jetting is possibly able to lean it down slightly on the main, but definitely not able to go a LOT leaner. Timing is mostly pulled in by about 3000 rpm and a little bit more about 2 more degrees or so between 3000 and 4000 rpm, much closer to 3200 than 4000 rpm, I just was saying that ALL timing IS IN by 4000 rpm. maybe a 13 prop is in order and try it. do these speed/boat sizes/rpm range/condition seem OK or is it low for the boat? a lot of mention of 19 foot boats running 40 mph with a 3.0l and I am 18' and barely maintain 30.... but elevation is a big factor.
 
farmerrick53 said:
ok so 20% and that means 140 becomes closer to 110 then. maybe that is all its got at elevation
Thanks.
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I would think so at that elevation. If your plugs look ok in terms of rich/lean you may be able advance the ignition some. I thought there was a merc document on high altitude where it said you could advance your timing 2 degrees or so. You can google it if you want to try it but like anything else if you look at two different pages you get three different answers...
 
exactly... there is a recommendation and I am 5* past maximum timing which is not unacceptable for my elevation in my humble opinion, I just don't want detonation from too much. I can go one more degree probably safely.
 
the tsb says 2* for 5000' in elevation, so I figure my timing from the 3* that my book calls for is now in the 5-7* range. I have been playing with it a bit.
 
I ran across an interesting equation for ESTIMATING the top speed of a boat recently... For my 23 Ft ChrisCraft Lancer deep V with 5.7 @ 260 HP its spot on. Top speed = (Square root) of (Boat gross weight/ HP) <times> Hull factor. Hull factor ranges from 250 for a flat bottom boat to 150 for a deep V. I had reengined my boat from its original 283 CID @185 HP engine to a 5.7L @ 260 HP and was just about spot on what my GPS told me. You really need to be careful to account for ALL the stuff in your boat weightwise... FWIW, top speed went from 30 to 35 mph exchanging a Mercruser 5.7MIE with FWC for the original RWC 185FLV Chriscraft. An additional 75Hp netted 5 mph.
 
I'd bet you'd be happier if you can find a 2.4:1 drive unit to replace the 1.98:1 you have now...

You may give up some top end speed but the engine should run forever and tolerate the 'other variables' much easier....
 
Just verified by my seloc manual. 6* btdc for a 140 4cyl 181cid. I am exactly there so I likely need to move it up to 10 ish and I bet that will give it a little bit new characteristics.
 
2.4?? never heard of it. must be much newer, or something my seloc manual only specs down to 1.98:1. I have several outdrives, maybe i can look and see if i have a deeper one.
 
30-35 mph is pretty good with a 15 pitch prop.
A 21 pitch may give you a few more mph and possibly break 40 but you will lose max rpm and it will be harder to plain off.
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Advancing the timing will not give you any more speed as the prop determines rpm at wide open throttle. Assuming you are getting full advance ((base (6*) + advance (~24-26* ) = 30-32* total)) which if you were not you would most likely know it.
Running points with advance weights and springs works well but with a 4 cylinder you can only get so much.

Based on elevation you have what you have...
 
according to the parts manual it looks like the alpha one gen2 interchange parts for the upper unit. maybe i can order the gear set and bearings and rebuild it to a deeper ratio. thanks
 
I have not tried to change prop at all. maybe I will put the 19 on and see if it planes good and still revs. I was afraid that more prop would pull top rpm down more and lose top end, might be worth trying I have 13 15 17 19 and 21 pitch props, might as well try it.
 
i can prop it to a 19 i'm just afraid thatmy rpm may drop into the 3800 range and i don't want to work that engine that hard. might be worth a try because the drag of the prop is always so constant. I need to just make a few changes and go to the lake its getting better all the time with a little jetting a little timing a little tuning and it gets better all the time.
 
farmerrick53 said:
the tsb says 2* for 5000' in elevation, so I figure my timing from the 3* that my book calls for is now in the 5-7* range. I have been playing with it a bit.
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As long as you knew that , sounds like you are hitting the hp wall for that engine at that altitude
 
A prop pitch change 15 to 17 will lower top rpm by 200-300 rpm and the same for 17-19 and so on.

So if you go all fhe way ffom 15 to 19 that will be about 500 rpm less
 
i mean the timing can give it some more rpm. not to argue but if the engine needs timing to perform and burn the charge, incorrect/late timing will kill power and rpm, essentially killing speed. I played with this one day on step at WOT with a friend captaining the vessel and a 5* change say from 30* to 35* at 2* increments with a digital timing light on the same day same fuel same lake same test run, it lost 200-250 rpm with late timing and ran all blubbery and barely idles then at 35-36* it woke right back up and picked up rpm and speed all with a small timing change. Now, I should have left it there but for some reason my memory was incorrect in remembering 3* instead of 6* so i set it back to 6* thinking that was 3* plus the 3* for elevation. My own mis calculation/incorrect memory. I verified in the seloc manual and calling for 6* at sea level then 2* more per 5000' means I need 8*-10* static. This will likely give me 100-200 rpm, maybe enough to now pull a 17 prop. need to check and see. back to the muffs. thanks for the help.
 
The 2.4:1 gear set I was looking at (in the MCM service bulletin) actually changes the gears in the lower housing....if you want a winter project, I'd suggest getting a used lower and go from there...given that you only had 140 ponies best case back when everything was new (at sea level), I'd think you'll see the biggest improvement with the additional 0.5 in gear reduction....I think it will also help mellow down the temperature sensitivities....

BTW, you'll find the factory manuals have a lot more detail that the SELOC stuff...they aren't perfect but are well worth the investment...
 
I just recently rebuilt and resealed my lower unit has about 50 hrs on it and works flawless. it would suck to start over and be back to leaks and worn parts :-( all on a wim
 
farmerrick53 said:
I have been very happy with the seloc manual. maybe i would like the factory even more, but the seloc is very adequate for me so far
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The seloc is better than nothing but when you get the OEM manual its is really helpful with details and procedures. If you do a lot of your own maintenance well worth it in my opinion. I found used ones on ebay that were like new and not expensive
 
thanks guys. I am looking forward to bringing the timing up some more and trying it again, but sounds like the elevation and 140 hp is the deal breaker here. Maybe i need to super charge it or add a shot of nos!!
 
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i dont need to go faster, i was just asking. simple question, i don't need the fuel bill of a v8 or the winterization headaches of a v8 or the added weight. no thanks. mine is exactly what I want. I know i can go faster, i choose not to.
 
thanks for the help, i know i can give it more timing and contrary to common belief, timing does affect rpm power and tuning, if it is out too far.
 
Contrary to what you think you know,..

You are working with a 4 cyl 140 hp at best in high elevation.

You will not realize any real gains asjusting the timing beyound the specification.

What ever gains, may be offest by possible damage to engine.

Chasing speed with your set up and enviroment is a waist of time.

Just my opinion
 
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