78 merc 900 RPM

[terry90]

Hey all I would like some info on WOT RPM on this power head. The 900 is rated at 45 to 5 k wot, the 115 is 5 to 55 k wot ,same power head bore stoke pistons crank etc. All I see is carb jets and rpm to get more hp. So hears my deal can I harm my 900 by turing it 5 to 55 wot ? With my 23 in SS prop it will spin 5k all day with 2 on board,and 54 to 56 k when I'm alone. Runs sweet good hole shot , but it will turn the 23 easely. And on a 2nd note Whats the best way to start this merc. cold ? when warm it fires before you can let go the key. cold I don't know if I'm flooding it or should I stay on the choke. Some times it fires in 45 secs. some times its a 2 min fight . got good fuel the ball pumps up tight just not sure on it.
Thanks for any helpon this
Kevin
78 terry m4.7 bass boat 78 merc. 900 2nd owner

 

[ondarvr]

Re: 78 merc 900 RPM

Carb jets don't create power, if they are different, then there are other differences between the two.

 

[terry90]

Re: 78 merc 900 RPM

I will go back to the book on the specs. again , seems I read same bore and stroke same power head. Was just wondering on the wot rpm 45 to 5 on the 90 and 5 to 55 on the 115. on the 125 and 150 hp bore and stroke changes and wot goes to 6k. So can I turn my 90 to 55 k safely ?
Kevin

 

[Chris1956]

Re: 78 merc 900 RPM

Kevin, The two motors share a lot of parts like cranks and rods. They have the same Bore as well, so they have the same displacement. Check out the part numbers for the blocks. The port timing is likely different, creating the power. The 115HP also may have power ported pistons.

 

[Georgesalmon]

Re: 78 merc 900 RPM

Choke the heck out of it cold. I don't think you can flood them.

 

[Chris1956]

Re: 78 merc 900 RPM

What kind of speed do you see at 5000RPM, with a 23" pitch prop?

 

[terry90]

Re: 78 merc 900 RPM

Hey guys just got out of the shop , long a## day. Yes pistons or reed valve and the higher wot rpm should make more power, makes sence. Do you see any prob turning the 900 to 55k , Was thinking could it run lean at 55 and be ok at 5 k. Was cold as hell on the res. last week and the wife stayed in the camper. That crazy 900 wanted to run at 6k on glass so I cut it back to 5. On the speed part boat speedos for what they are 5 k is mid 40's on a tri hull , I think I would have been close to 48 poss 50 but backed off when I trimed it and was seen 58 to 6 k I dont think the 900 is geared for high speed so I was very happy

 

[terry90]

Re: 78 merc 900 RPM

Thanks on the cold start tip. I've had people tell me to choke it till gass is running out lol. Going out fri am I will stay on the choke till it fires. Should I use the fast idle too or wait till it fires? I had a 03 90 hp merc. 3 cyl. and it was like starting your car. Then the obama care took my job and my boat , so I'm just happy to be on the water again.I'm new to old mercs.
Thanks for your help
Kevin

 

[Georgesalmon]

Re: 78 merc 900 RPM

I put the fast idle about 1/2 way up but every motor seems to be a little different. When it fires the fast idle lever comes down quick. Then I may have to push the choke a couple of times to keep her running. After about 30 - 45 sec of running I can usually stop choking and return the fast idle to idle. The rest of the day I don't need the choke at all unless i spend 5 or 6 hours trolling on the kicker then I may have to do my cold start again. Depends on the weather.

 

[Chris1956]

Re: 78 merc 900 RPM

Terry, I am surprized you can turn that 23" pitch prop on a tri-hull. My Merc 1500 had difficulty turning a 23" pitch prop on my 16' speed boat. Total weight was 1000# plus my body. I doubt I ever turned more than 5500RPM, but that is close to 60MPH.

 

[terry90]

Re: 78 merc 900 RPM

I put the fast idle about 1/2 way up but every motor seems to be a little different. When it fires the fast idle lever comes down quick. Then I may have to push the choke a couple of times to keep her running. After about 30 - 45 sec of running I can usually stop choking and return the fast idle to idle. The rest of the day I don't need the choke at all unless i spend 5 or 6 hours trolling on the kicker then I may have to do my cold start again. Depends on the weather.

Thanks for the info. I will give it a shot on fri. AM should be bout 35 deg. out and water at low 60"s it will be a good test. I just keep thinking I'm flooding it so I get off the choke and go full on fast idle. You know the old car deal hold the pedel to the floor.
Thanks Kevin

 

[terry90]

Re: 78 merc 900 RPM

Terry, I am surprized you can turn that 23" pitch prop on a tri-hull. My Merc 1500 had difficulty turning a 23" pitch prop on my 16' speed boat. Total weight was 1000# plus my body. I doubt I ever turned more than 5500RPM, but that is close to 60MPH.

It makes me wonder at times bout the 900 and the guy I bought it from.He was the org. owner and said his wife was scared of it and it sat most of the time. The prop is a 3 blade SS no name on it but has a diamond shape then the pitch #s.It has a lot of rake and cup to it. The motor has no setback from the hull, and the cavt. plate is about 1 1/2 in. above the pad. Don't make sence to me but it just turns the hell out of it. And a fair hole shot with two on , and very good when I'm alone. Boat speedos are what they are . but the tack is on the money. Had it looked over when I replaced the triger sw.and dist belt. Do you think that power head can run at short times to 55 k wot ? The thing gets so smouth and sounds mean at that that rpm.Thanks Oh the terry is just under 16ft not sure of the weight it's a mild tri-hull a lot of lift and on plane it runs very and I mean VERY flat on the water, very little wake and not much hull in the water. It's a fun basser for the 1k I'm in to it (boat parts on motor and tires on trailer) I just dont to pop the thing but it realy wants to run .
Thanks again

 

[terry90]

Re: 78 merc 900 RPM

Sorry to rambel on but it's me again. When I bought this terry from the org. owner I did ask about the prop (same thought as you a 23 on a 90 hp?) All he said was thats how he got at the dealer and has not changed it . So what SS props were out there in 78 ? Who realy knows what happens in 35 yrs.people forget parts get changed hard to tell.

 

[steelespike]

Re: 78 merc 900 RPM

A quick prop calculation indicates 5000rpm,1.7 gear ratio,23" prop, Guess 10% slip gets 57.7 mph.
I think a tach check is a good place to start.

 

[terry90]

Re: 78 merc 900 RPM

A quick prop calculation indicates 5000rpm,1.7 gear ratio,23" prop, Guess 10% slip gets 57.7 mph.
I think a tach check is a good place to start.

The gear case on the 78 900 is 2:1 ratio. I don't know how much slip I have on this old prop , but it's clean no dings good edeges. I visted with a mech (over yrs on mercs ) he had no resaon how it can turn this 23. He said let it turn to 55k if it wants to. Had the tack looked at when we did the trigger sw. and belt. I figure it's a freek or the prop is sliping more then 10% I know it's on here somewere but what is the math to do prop slip? Both merc shops in town say it cannot turn that prop ,and my 30yrs with boats say it cant turn a 23. I'm not argueing on it I just dont buy it eather. But I run down the lake thinking whats up with this? the thing hit 58wot yestday one in boat. Got to be slip big time.
Thanks Kevin

 

[terry90]

Re: 78 merc 900 RPM

A new bunch of info on my prop !! As per my way of doing things, allwas do the easey stuff last! So I took my boat for a ride ,your boat likes to go for rides, off to a prop pro. A hour later this is what I learn. The prop started out in life as a RP 48-72760 A4 23 P .The prop has been moded to a 13 die 19 " to 21" varible pitch. Was told more rake and cup and drilled for venting,his opinon could have been done when the boat was riged, for more top end and better hole shot. As he told me in the mid 70's to early 80's terry was competing with ranger for the bass boat market. He stamped the prop with the new spec. and off I go. So it make's sence that the 900 will turn 55 wot, oh he did make my wallet easer to sit on comeing home!
Thanks all for your time and help
Kevin

 

[Chris1956]

Re: 78 merc 900 RPM

Kevin, That is very interesting. You should repost that in the Prop forum. Those guys will be quite interested. I for one would like to know exactly what variable pitch means, as I have not seen that on more modern props. I also never knew you could add rake to a prop. Any idea what the average pitch is? I would think your motor could spin a 20" pitch prop at most....

 

[terry90]

Re: 78 merc 900 RPM

Kevin, That is very interesting. You should repost that in the Prop forum. Those guys will be quite interested. I for one would like to know exactly what variable pitch means, as I have not seen that on more modern props. I also never knew you could add rake to a prop. Any idea what the average pitch is? I would think your motor could spin a 20" pitch prop at most....

I would like to move this info to the prop guys, haven't a clue how lol , I'm auto and lite truck mec by trade, and not realy up on the world of props,got the basics on die.
pitch cupping rake ect. but no clue on what can or can't be done. I pay my money the guy gives me the info. the reading I've done on this forum leads me to go with "I" boats for real info. I would like to know if I paid for his time for real info,or a line of sh#$. Can you tell how or move this to the prop guys?
Thanks Kevin

 

[Chris1956]

Re: 78 merc 900 RPM

Just highlight and copy your text using Ctrl-C. No go the the prop forum and start a new thread. Now past the text using Ctrl-V, and fix it up to focus on the prop stuff only.