Lehman 120 max rpm

I read on this board that a 1

I read on this board that a 120 should turn a max of 2500rpm My 1973 marine trader 120 lehman turns only 2150rpm out of gea with no load or in gear running down the bay. This moror runs well starts easily does not smoke and the fuel useage is in spec. Last year I installed a rebuilt injector still no change in max rpm. Why can this motor make max rpm?
Thanks
mwalters

In response to Micheal Walters

In response to Micheal Walters Nov 3/08 post.
I have a 34 DC Marine Trader with 120 Ford Lehman and I am the same. Max RPM 2200. I cruise at 1700-1800. At 2000 it starts overheating. I am quite happy at 1800. At 2000 or more I am burning one big wack of fuel.

"2650 unloaded, 2500 loaded is

"2650 unloaded, 2500 loaded is the factory spec. If you can't reach max loaded it could be overpropped, or possibly damaged gear. (but that would have other symptoms also) Sometimes boats are overpropped on purpose for fuel mileage. A symptom of overpropping is overheating when run faster.

If it won't run up unloaded it could be injectors, pipes, pump, fuel filters, fuel pump, etc. Or, it could be that someone tried to adjust the idle stop and accidentally adjusted the max speed stop screw. The max screw is directly under the idle screw on the injector pump.

K"

I think my prop is 19-23. Is t

I think my prop is 19-23. Is that overloaded?

"It all depends on your boat,

"It all depends on your boat, how it's loaded, 3 vs 4 blades, etc. For comparison, on my 40' trawler I turn a 25x18 three blade. My Lehman 120 will rev up to 2520 while pushing the boat.

So, I would doubt you're overpropped, assuming 23" diameter and 19 pitch.

K"

No it's 19 inches and 23 p

No it's 19 inches and 23 pitch. At least thats what I was told by the previous owner and I called to confirm.

"That seems like a small diame

"That seems like a small diameter for that boat, but I'm not an expert on the 34's. 23 pitch seems pretty aggressive for a 120 horse motor. We really need to have someone with a similar boat pipe up and tell us what they have. That, and have you run your engine up out of gear to see if it will turn 2650 or so.

23 inches of pitch is 20% more than what I turn. That sure seems like a lot just thinking about it. Have you gone to the propeller sites and played the numbers game there? That might give us some more insight into this.

I know that running an engine to that speed with no load seems wrong, but the governor on a diesel will keep it from over-revving like a gas engine will do. Run it up out of gear and see if the max speed screw is set too low.

K"

Ok. Not sure where you are loc

Ok. Not sure where you are located but here (Ottawa Ontario) the ice is just out of the canal but I don't launch till May 13. Will have to wait till then to run it up. Have never revved it without load so have no idea.
What sites could I check re the prop. Only have had this boat for 2 seasons. I will be working on the boat this weekend. I will measure the diameter. Is there a way a non-techie like me could measure pitch. I do have a spare prop which I think was the original and I think it is an 18 but have no idea what the pitch would be. Thanks for your help by the way.

"Pacific Northwest, Washington

"Pacific Northwest, Washington state we get to keep our boats in the water year around. Pretty nice compared to the "frozen north". ;-)

A search on propeller sizing should bring up a variety of sites that can help you narrow down a size that should work.

I don't know how you'd measure pitch without a gauge of some type. Typically the prop will be stamped with the size on the hub. Measuring the diameter will let us be sure which number to use for diameter and the other will be the pitch. Pitch can be tweaked by a prop shop an inch or two but that will give us a good starting point."

I checked it today and it has

I checked it today and it has 1928 or 19'28 or it could be 192'8 but then x 22. So I assume the pitch is 22

"And measuring the diameter it

"And measuring the diameter it's about 19 inches? So that's a lot of pitch and may be what is causing the low RPM. Now, if you only run at 1400 or 1500 RPM you should get good fuel mileage. But, But, if you try to run at 1900 or 2000 RPM you run the risk of overloading the engine and overheating.

I'd still run up to max R's out of gear to satisfy myself that the engine was set right. I'd also look around for a friendly boat owner whose boat was very similar to mine and check with him what size prop he was turning just to make sure that the assumptions I made above are valid.

K"

"Ok, thanks for all your help.

"Ok, thanks for all your help. Must be nice to be in the water all year. But then you don't have the thrill of launch day. Check my web site. http://therenback.net"

"This is the message I got bac

"This is the message I got back after sending a picture to a prop shop.

Hi Glenn: "Equi Poise" is the name of the propeller given by Federal Propeller Co. The "9 67" is the month and year the propeller was made. Very faintly is the start of "Chri" which I believe is "Chris Craft". "1923" is the part number for a Chris Craft T-36 propeller. The T-36 is a 23 x 23 x 1-3/8 x 3 blade left hand propeller. Don't know what "5981", "5844", or "X22" are. Hope this helps. Tom
----- Original Message -----
From: Glenn Buchanan
To: 'north coast prop tech'
Sent: Sunday, April 12, 2009 11:47 PM
Subject: RE: 34 Marine Trader


Would you please have a look at the attached picture and tell me what all the numbers mean. In particular does it say that it is a 19 inch prop and is the pitch 23 or 22. What do the other numbers signify and do you know what the name is. Does the 9 67 mean it was cast in Sept 1967? Anything else you can tell me would be appreciated.



--------------------------------------------------------------------------------
From: north coast prop tech [mailto:[email protected]]
Sent: Thursday, April 09, 2009 4:15 PM
To: Glenn Buchanan
Subject: Re: 34 Marine Trader


Hi Glenn: It appears as though you have too much propeller which is overloading your engine. The propeller can be re-pitched to increase the RPM's and lessen the load. The propeller size is expressed as "diameter x pitch". To verify the pitch we can measure it on our Prop Scan machine. There is no charge for this service and the analysis of your performance. We would love to be of service. Tom
----- Original Message -----
From: Glenn Buchanan
To: [email protected]
Sent: Thursday, April 09, 2009 9:49 AM
Subject: 34 Marine Trader


I have a 34 foot Marine Trader Double Cabin 1974 with a 120 Ford Lehman. I am told that the engine should max out at 2500 RPM under load and 2650 without load. I have never tried without load (and it is currently on shore) but on the water it maxes out at 2200 and if run for any time over 200 it starts to overheat. I have a bronze 19 inch with 23 pitch prop. At least that's what I am told by the previous owner. How can I ascertain for sure the pitch and is there a way to tell if the prop is overworking the boat."

"I'd be at the prop shop t

"I'd be at the prop shop to find out what I have. 1 3/8" shaft is an oddball used almost exclusively by Chris Craft. I'll bet your prop has been reworked extensively, hub opened up and pitch redone. Find out what you have and then you'll be better able to decide what to do long term.

Aren't boats fun?

K"

I have a CHB 34 DC with a 24 x

I have a CHB 34 DC with a 24 x 20 LH 1800 rpm cruise.