TOPIC: Mercury 1250

Hello, I am looking for advice on prop selection and power. I have a 1977 Glastron Gt150 the use to have a Chrysler 105hp on it. That motor finally went to outboard heaven and I replaced it with a 1969 Mercury 1250 125Hp. The Mercury is suppose to run between 4800-5300 at WOT. When I put the motor on the boat I transferred the tach that came with it . It had a 17p prop and was revving over well 6000 rpm at 38 mph. I bought a new 19 and 21P prop. Here is where it gets weird. The 19p goes 42mph at 6100 Rpm and the 21p goes 40mph at 6000rpm , slower than the 19p?. My old Chrysler 105 would touch 50mph at 5800 rpm with a 21p and 45mph with a 19p at 6000 rpm. I have fooled with motor angle and motor depth. The Merc sounds great but doesn't have the same or better speed with the same props??? Prop diameter is about the same, the lower leg on the Mercury is visually smaller. I confirmed the tach RPM using a model airplane tach and making a felt pen line in the flywheel, the tach is reading about 200rpm fast. Any thoughts??Im confused..Brad

This is only very general information from a novice. I have run a 150hp 1974 in-line 6 and a 100hp 1965 in-line 6. I was doing prop testing this past weekend as well on the 1000. Are you comparing both aluminum props or both bronze props (and not mixed comparisons?). are the blade shapes the same or just similar. Are diameters exact? Are both cupped or both un-cupped? We had a larger diameter 19" pitch that would tach lower than a 23" pitch and a bit slower with about 1" more diameter and a bit more round ear surface area than the bigger pitches that had the smaller diameter and a bit less of a round ear...this was an 18' checkmate open bow v bottom...it was several hundred less. This weekend I tried 2 completely unrelated props. 3 blade bronze 17' pitch conventional ear merc prop with no cup on the 100hp on a 550 pound Powercat against a 19" 2 blade with cupped blades with less of the round ear and more cut of blade and more rake...These are not normal increment testing, but all I had...it lowered 600rpm under 25mph, and then 800 less above 25...at about 35mph, the hub spun out of the old prop as I quickly shut her down (not after pegging 7,000 for a second...scary because I just finished breaking that motor in after re-build. I have always been amazed at how every rig and every prop and rig combo can be so sensitive to each factor. A cupped prop may give a bit of bow lift just enough to reduce friction better than a no cupped for same trim level. I twice had different props excessively cupped (the 2 separate prop shops called it double cupping) to help me with a blade when I was stuck between 2 props that just did not do what I needed. Those were light bots with little hull in the water. I am sure there are a few Merc experts here that know you motor and have likely run and propped one before, so my stuff is very general and you may already know way more than I do. Super nice rig and good luck getting more specific info.

First thing I would do is check the gear ratio of the lower unit. Your engine speeds vs prop pitch are way out of line for what this motor would pull.

It's possible that someone may have replaced the original lower unit with one from an 800 or 850 4-cyl. Those have a 2.3:1 gear ratio and would require a prop with a lot more pitch to get the rpm down into the recommended operating range. BTW around 5500 is probably good for this motor, although it's likely plenty happy to spin up to 6000.

You can check gear ratio by turning the flywheel by hand (15/16" socket required) a number of turns, then checking prop turns. The (3) ratios that were used on Inlines are 2:1 (the most common), 2.3:1 (also used on later-model 650-4's), and 1.78:1 which IIRC was used on early 1150's and maybe even 1350's.

I'd bet money that you have 2.3:1 gears; I just don't see how those engine speeds are possible with the prop pitch you're using, and not have boat speeds in the mid-40's to 50mph.

HTH & let us know what you find.........ed

Hello Ed, I greatly appreciate your input, I would have never known or thought of that in a million years. I'm going to check that first thing when I get to the lake again in a week from now. If it is a 2.3:1 ratio can the problem be fixed with prop?
Thanks , Brad.

Meteor wrote: If it is a 2.3:1 ratio can the problem be fixed with prop?

Hi, Brad, you should be able to go to a prop with higher pitch, although it's not optimal to run that gearbox with a higher-horsepower motor because, as I recall the explanation, the pinion gear is weaker because it has less teeth than say, a 2:1 gearbox. More stress running those gears with lots of horsepower.

But in the real world I'd expect it would hold up OK. I ran an old Merc 1000 with the 2.3:1 gearbox and it worked well, never had any durability problems.

That being said, I'd probably think twice about running a 2.3:1 on a 1500!! You'd need to swing quite a large prop to make up the difference.

Years ago I had a 15' Sidewinder SS with a 1350 Merc on it, and a Stiletto 3-bladed hi-rake prop worked really well. Had good hole shot and the raked blades gave some lift which helped "air out" the hull. If you use a prop with high rake you'll need to either cut your trim tab to clear the prop blades, or use one of the special tabs that are designed to be used with that type of prop.

BTW if the 1250 is running 6000+ it'd be really really screaming. That sound is unmistakable. Maybe take a short video of it at WOT the next time you're out.

Cheers.......ed

Thanks for your kind words and advice, between your post and one about gear ration I have lots to check and try...Brad

Funny thing you should say that about the prop rake, I had to modify my trim tab to accommodate the 2 new props I bought. You know your stuff !! Brad

That prop already has a lot of rake! I learned the hard way, too, when I bought that stainless Stiletto prop and much to my surprise the prop hit the stock trim tab. Called the school of hard knocks!

The Mercury trim tab is a nice piece if you want to do an upgrade from your modified trim tab.

I found my numbers from my 1965 merc 100hp with 2:1 gear ratio (my prop is an original merc bronze 3 blade 17" (sorry but do not presently have the diameter...but my prop is not cupped and does not have your rake as mine easily clears the stock torque tab). Running in second pin (so very flat and full wet hull on a 550 pound Powercat 15T with triple hull), I only got 5100rpm at 37 mph at WOT. I ran out of time testing 3 props to go back and re-test at a higher pin setting to get some air under the hull. Great info on the gear ratio issue. I think I will look up that prop formula and run my numbers to see where efficiencies (or lack there of) exists.

I'm looking at the zoomed-in pic of the prop, and it looks like there's quite a bit of gap between the L/U housing and the prop itself. There are 2 steps at the fwd end of the prop, the 1st is quite visible then the one fwd of that has a slightly smaller i.d.

Does the smaller i.d. extend any distance into the L/U housing? If the wrong combination of thrust hubs is used, the prop can stick too far out and then excessive exhaust gasses flow over the hub, causing uncontrolled ventilation.

I would imagine that the new prop has an integral huib at the front. Your propshaft should have a beveled edge where the thrust washer rides, and the thrust washer should be the thin, flat one, P/N 13191A 1.

Note that the older style props didn't have integral hubs and used a longer thrust hub at the fwd end. If this old-style hub were to be used with the new-style prop, the prop would stick out more and this could cause ventilation problems. Another reason for high rpm and low speed, lots of slippage!

Just a thought; if there's minimal clearance between the o.d. of the fwd machined portion of the prop body and the labyrinth seal at the rear i.d. of the gearcase, you shouldn't get much if any exhaust going past there.

Wow, another good point. The prop I used comes with a hub kit for that motor ...And... a nylon ring that can be snapped on to the back of the prop to take up excess clearance. My motor was not listed as one that needs the nylon ring but worth looking at.. lots of things to check now and allot more understanding from my end. Thanks!!! Brad

Meteor wrote: Wow, another good point. The prop I used comes with a hub kit for that motor ...And... a nylon ring that can be snapped on to the back of the prop to take up excess clearance. My motor was not listed as one that needs the nylon ring but worth looking at.. lots of things to check now and allot more understanding from my end. Thanks!!! Brad

Yeah, that might be what it needs. Just as an example, years ago Merc, OMC and others put out bulletins regarding drilling "ventilation holes" in the prop body, in order to let the prop spin up when you're taking off. Exhaust gasses flow out over the blades and cause a "controlled" ventilation or cavitation as it were.

These holes were typically 3/16" to 5/16" although some prop mfr's have slots or holes on their high-performance props that are even bigger. Usually used on a Hot Rod boat with a very large-pitched prop, gives plenty of slippage for the hole shot. So if even a small set of holes can cause a useful ventilation, you can imagine what exhaust gasses pouring uncontrolled from out the gearcase and around the prop blades would do!

Back in the Day we were running different motors and I drilled a prop for my brother's old Merc 1000. Put 3/8" holes in it. It had a great hole shot, but instead of gaining speed and having the exhaust sucked out the back instead of still pouring out the holes, it just kept coming out the ventilation holes. It slipped like a bad clutch in a car, or the rubber hub of a Big Twin Jonhrude. It wasn't the greatest prop, so no big loss. But we sure found out how it worked (or didn't)!

I had a $25.00 Merc 700E Dockbuster and we threw that on my brother's 13.5' Sea-King runabout. I drilled some ventilation holes in the prop and it was pulling slalom skiers out of the water, no sweat. My good buddy did a step-start one time, I had gassed it really good to show off to some guys that were laughing at our old boat & motor. It took off so fast he said his head was swimming! Those were the days, cheap motors and cheaper gas. Not so much nowadaze!

I think we're that much closer to figuring out what the deal is........ed

I agree, it has turned from a mystery to me to some possible problems or remedies for me to check into. Next weekend I am back out at the lake and will have some answers I hope. The last problem to fix with the motor then will be the center carb, it drips fuel out the front slowly at idle causing an erratic idle and sometimes stalling at idle . I am hoping I can take the carb a part and fix it with cleaning as I cant seem to find carb kits for this motor . My only source for parts has been Ebay and there is nothing listed. Is cleaning it typically going to do the job with those carbs? This motor sat for about 6 years not used when I got it this summer.

The drip might be caused by the wrong float level, or even a fuel-soaked float. Replacement floats are ethanol-resistant. Here's a carb breakdown with pics at marineengine.com:

www.marineengine.com/parts/mercury-outboard-parts/1250/2314956-thru-2839368/carburetor-assembly

Carb gasket/packing kit (which is usually all you need for carb work) on eBay for $9.79 shipped:

www.ebay .com/itm/Carburetor-Gasket-Kit-Mercury-3-9-135-HP-18-7006-1-Side-Bowl-Carb-1399-1717-/382045560573?epid=1223523448&hash=item58f3b09efd:g:Q6YAAOSwzqFY9G4J&vxp=mtr

This site parses eBay links, so copy, paste into browser, remove spaces and it'll work.

Also possible the needle/seat is leaking, you can get replacements for that too. See the carb parts list and then look up the part on eBay, it might have a cheaper shipped price. $7.95 flat-rate shipping at marineengine.com.

Some of the prices at marineengine.com are actually cheaper than what you'd find on eBay, and if you end up ordering a lot of parts you may wind up with a better deal paying the flat rate shipping charge. I find for just a single part, it's usually cheaper to order off ebay.

Thanks for the info, so glad I posted my problems here as I received great info I would have never found or thought of. Next week end I am back to the lake and can check out some of these ideas..

Any thoughts of how much clearance to the gear case housing the prop is suppose to have ? Should one drain the carbs for winter or is using fuel stabilizer OK?

If you're using non-ethanol stabilized fuel, you're probably OK to just run the carbs out of fuel then tip the motor up to utilize the "self-draining" feature. Store the motor in its normal, tilted-down position. If you're running ethanol-laced fuel, might be a good idea to pull the main jet brass plug out of each carb and let them drain fully. Take care not to lose the fiber washer under each brass plug. The plugs take a 7/16" wrench/socket.

The gap between prop body and gearcase shouldn't be so tight that it rubs. But actually the way it's supposed to work is that the turned-down portion of the prop body extends into the gearcase maybe 1/4" or even a bit more. You'll note that the inside of the gearcase has parallel grooves cast/machined into the i.d.

Those grooves and the close proximity of the hub to the grooves, form what's called a labyrinth seal. This effectively seals exhaust gasses from coming out of that area, without ever touching. The reed blocks are machined in a similar manner on their i.d.

en.wikipedia.org/wiki/Labyrinth_seal

Very common to see that type of seal on multi-cylinder 2-stroke engines, as it seals effectively and lasts a ton longer than a rubber crankshaft seal. You have to have some sort of sealing between cylinders, as the crankcase vacuum generated from one cylinder would otherwise effect the other.

Carl Kiekhaefer's engineers ingeniously designed their multi-cylinder engines to not only use the seal between cylinders, but to incorporate that into a reed block wrapped around the crankcase. A very novel design!

Anyway, so much for the labyrinth seal lesson! Let us know what you find down there...........ed

I will be checking it all out this weekend, I will post next week what I found.. Thanks for all the thoughts and advice...I am hoping the problem is the prop gap as that might also explane why the 21p prop was actually a bit slower than the 19p, the props were both made by the same company and in the same series but the 21p is cupped a bit on the edges..Brad. [/b

Dude, that's a HUGE ventilation hole I see in one of the pics, did the prop come with those?

What is that, about 3/8"? If so, WAY too big for an inline and likely your problem (along with prop fitment at the gearcase).

Did some rubber plugs come with the prop? Because I would plug those holes then see how it runs. As I was relating in previous post, when I drilled huge holes in the prop on my brother's Merc 1000, there was so much slippage it would never gain speed. Barely got on plane. Note that the Merc TSB recommends starting out with a 3/16" hole with a max of 5/16".

If the hole size is an Optical Contusion and they're actually smaller than 3/8", I would still plug them and see what that does.

BTW cupping does increase effective pitch, so in going from uncupped 19" pitch to cupped 21", you've increased pitch and then added cup; it should have a drastic effect on speed, providing the motor has enough hp to swing the prop at speed.

If you for example had bought a 19" cupped prop, it should be faster than your uncupped 19" prop, all things being equal. But should still have a good hole shot, because cupping has a minimal effect on the low-end. However at higher speeds the cup does have effect. A cup will also hold better in corners, with less ventilation.

Hello , those were the vent holes that came with the props, they were both the same. They are about 5/16", that is the prop listed for my motor, I will try the snap on sleeve and see if the prop still fits with out rubbing. I guess I could Steel epoxy over the holes and drill them out smaller to 1/4" and see if that helps. I am hoping the problem is the nylon snap on ring that goes on the back of the prop. Thanks for your thoughts...Brad

Does anyone know the proper procedure for lining up the shift lever on my Mercury 1250. I must have missed by a tooth when I did the water pump. the motor starts to rev up in reverse about the same time it tries to go in gear so the gears make a horrible noise and don't engage properly. I tried adjusting it with the cable but ran out of adjustment before the problem was fixed. Is the motor suppose to be locked on the tilt pin in neutral or free? I have ordered a manual but it is a week or two away.. Thanks Brad

You should have enough adjustment on the controls to be able to push the shift lever all the way to the back of the rack. Usually that's where it needs to be in order to fully engage reverse. Not possible to be a tooth off on the shift rod/shift shaft alignment and have Fwd, Neutral, and Reverse.

You can tell for sure if you have an issue with Reverse by disconnecting the shift cable and manually shifting to Reverse. If it's still making noise and not engaging properly, with the lever all the way to the rear, you have a problem with the L/U. Either worn clutch dogs/worn reverse gear, or the gearcase cover ring is loose and the propshaft/gear carrier has backed out. You'd probably see the latter if it were happening.

Sounds like maybe you have a problem in the control box, either wear or out-of-adjustment, depending on what box you have. The old retangular Merc box with choke button on top, fast-idle lever on the right side, tach connection in the front, that box doesn't have anything inside that can be adjusted but it's typical for the gears on the shift arm or the shift/throttle drum to wear and then it won't shift properly.

Here's a link to that box at Marineengine.com:

www.marineengine.com/parts/mercury-marine-accessory-parts/remote-controls-and-components/outboard-1985-and-below/remote-control-assembly-electric-f

If that's not your control box, there are many others listed, just follow the link at the top of the page to go one-up in the directory.

BTW don't let the shift dogs grind in Reverse, that's a sure way to ruin them in short order.

Currently Neutral is at the back side of the Neutral gate, forward engages instantly going forward way before the throttle and the linkage is maxed out pushing the shift lever to the back of the slot. initially in reverse it wouldn't even try for reverse , only after I maxed out the shift linkage going backwards would it try to engage reverse. It worked fine before I changed the water pump so I think I must have something a skew.

Hope you get that sorted out with ed's advice. Mine did this when I bought it and guy said reverse was bad. Here are pictures of bad reverse guts. Chips prevented it from holding. Hope you don't have that problem. New parts work wonders.

Ya me too...Brad

Well Ed , I have checked out your suggestions and it looks like the prop clearance to the gear case housing must be the problem allowing constant air around the prop. I checked the gear ratio and it is 2:1 as it is supposed to be . The prop had allot of clearance as you noticed, I should have been smarter as when I compared the new prop to an old Merc prop it was much smaller diameter in that area. I snapped on the ring that came with the prop and problem solved even though my motor was not on their list that the ring should be used, obviously it needs it..Hopefully this pulls my RPM down and the speed up. I don't think the boat will hit the water again till next year now but thanks for your advice, knowledge and keen eye.. most likely problem solved... Brad

Well now that the prop/speed mystery has been sorted out I started on the problem of fuel dripping out of the center and top carb at idle and when pumping up fuel to the carbs. I removed the starter to get access to the carbs side float bowls and remove the tops to check on the problem. I was expecting old solid type floats possibly fuel saturated and metal tipped needles that were worn. I did find white hollow floats and needles with orange rubber tips, all in excellent shape. All I ended up doing was cleaning out the float bowls, filters and blowing out the seats. Everything looked really good yet they leaked by?? Now I can pressure it up hard with the primer pump and no dribbling.. I really didn't do much but the problem seems to be rectified???? Brad