Can't find my rpm's ('75 Merc 1500)

[scottswierenga]

Some background:

obtain boat Aug 2011
motor '75 Mercury 1500
hull '74 Checkmate MX-16

maiden voyage sept '11
indicated speed mid 40's
indicated rpm's 4800-ish

(fast foward to two weeks ago)

second voyage may '12
incicated speed 34
indicated rpm's 3200

third voyage
indicated speed 34
indicated rpm's 3200
later that afternoon: little throttle response, wot got to only 1000 rpm

Solution??
Ideas?!

Where is a specific question though.
Presuming: If we think of my controls as a clock and neutral is 12, forward is between 9-12, and reverse between 12-3...
1) where on the clock should my gears be kicking in?
2) where on the clock should my throttle be picking up?

 

[Will30]

The 1500 Merc on I have on my Trimate II runs closer to 5800 Rpm with a 21 p alum Prop ... Shifter Soon as a move mine forward mine goes right in gear . Mine runs about 55-56 mph with the 21 p prop and my Checkmate is 16.3 ft ...

Have you ever done a Compression test ???

 

[ckmate225]

you should be in forward gear by about between 10-11 o clock, but it sounds like you are running out of ignition, have you checked for spark and charging? if your not charging the boat will run off the battery for a while but once that gets weak the motor will start to loose power and ignition,Does the boat run better and a fully charged battery then start to loose performance as the battery gets weaker?
Thats where I would start looking. Also you can start to do damage to other components with a bad charging system so you should at a minimun check the voltage at the battery with the engine running at idle you should be around 14.5 v
What style ignititon system to have in that motor?
Good luck!!

 

[scottswierenga]

I did a cold compression test, last fall when I was having a No-Fire issue, since I couldn't get it to fire and get to Operating Temp. At that time I found that I have two cylinders running a little lower compression than the others. I think most were running about 118-ish psi cold while cyl. 2 & 3 was were in the 98psi & 95psi respectively.
I have not done a new compression test now that I have gotten it back and running.

My F/R Gears engage about the same point, the 11 o'clock position. My throttle doesn't seem to pick up until I'm at about the 9.5 o'clock position and continutes down the arch until about the 8 o'clock position where the throttle lever runs out of travel.

Spark should still be good but I have not checked the charging system. I would be curious to find out if the electrical output to the battery is good though.

To answer your question about what type of ignition I have, the engine says Thunderbolt Ignition and there is a distributor with spark advance...that's all I know.

 

[Bill Gohr]

It will always go into forward sooner than reverse because the c dog is spring loaded into forward.

Those had iffy fuel pumps, make sur that is good, I'm assuming you already did the carbs, and I doubt the ignition is fading away, the one thing on those motors that most people miss is the water jacket in the exhaust, if you get a leaky gasket it will blow water directly into the cylinders nad it is usually the top 3 and can be any of them

 

[scottswierenga]

Last Friday I dug into the cable system to see if I might be able to tighten things up and learned that the control uses a cam style system which means the shift and throttle pick-up points are set and non adjustable. Next I disconnected the throttle cable from the engine to check the linkage and found nothing strange. I reassembled everything, started the engine and threw the idle lever to see what I got...th engine jumped straight up in rpm. I shut it down and the wife and I headed to the lake for a sunset cruise.

While we were out we pulled about 5200rpm and got to 42mph according to gps and experienced NO issues the whole evening. These are my best numbers yet with the boat and I'm really suprised because nothing was adjusted, cleaned, or replaced...all I did was remove, disassemble, inspect, and reinstall. In essence, I never found a cause to the symptom, but now the symptom has vanished...hey...I'll take it!!!

 

[scottswierenga]

Do you think that the two cylinders making low compression could be the culprit of the engine not coming up to speed? Should I be thinking of replacing the rings on the pistons? (presuming the piston heads and pistons aren't too far gone...)

 

[wired]

you should be in forward gear by about between 10-11 o clock, but it sounds like you are running out of ignition, have you checked for spark and charging? if your not charging the boat will run off the battery for a while but once that gets weak the motor will start to loose power and ignition,Does the boat run better and a fully charged battery then start to loose performance as the battery gets weaker?
Thats where I would start looking. Also you can start to do damage to other components with a bad charging system so you should at a minimun check the voltage at the battery with the engine running at idle you should be around 14.5 v
What style ignititon system to have in that motor?
Good luck!!

I dont know that there is a 9 amp charging system inline in existence that will kick out 14.5 Volts at idle. Under way over 2500 or so sure if you don't have many accesories but idle is going to be more like just over 12 volts.

Personally I'd be looking at the carbs, timing, their adjustment and float level.

 

[Bill Gohr]

Overcharging was a common problem back in those days, if there is no load on the battery and it is continually charged, no matter how smal the charge the battery voltage climbs.

As far as your lower RPM, I think we're just avoiding the obvious, first of all the 118 numbers are on the low side, and the ones under 100 are just not acceptable.

I just did a comp test on a 1500 powerhead my friend has for sale, the lowest was 120 and a high of a little over 130.

 

[scottswierenga]

I agree that the compression isn't were I want/need it to be. Even though these are cold engine compression readings, I don't hold out much hope of numbers rising much with a warm engine.

My brain cells feel fried from work at the moment so I have to have to smack myself for asking...but what is the cause of low compression?

(I can't hardly wait for this -4,328 degrees outside B.S. winter to stop so I can get to work on the boat. The real question is should I tackle the engine or the interior rebuild first...or do 'em simultaniously... so much to do!)

 

[Will30]

I agree that the compression isn't were I want/need it to be. Even though these are cold engine compression readings, I don't hold out much hope of numbers rising much with a warm engine.

My brain cells feel fried from work at the moment so I have to have to smack myself for asking...but what is the cause of low compression?

(I can't hardly wait for this -4,328 degrees outside B.S. winter to stop so I can get to work on the boat. The real question is should I tackle the engine or the interior rebuild first...or do 'em simultaniously... so much to do!)

I have two 1500 inlines . The 1975 has 130 psi warm and my 1976 that I had rebuilt runs at 145 psi warm . The rebuilt motor fires like its fuel injected. What a difference it makes when the reeds are new . The guy that rebuilt mine is in south carolina and deals with inlines 95% of the time and does great work at a great price . His name is Wayne Canino . I can send you the info if you want it ..... Will

 

[paulsboats]

compression

Are the two cylinders with low compression next to each other?

 

[Will30]

mercury power tune

You can always try and can of Mercury power tune and see if it will help out any . It is really good stuff ....

 

[scottswierenga]

Are the two cylinders with low compression next to each other?

Yes, cylinders 2 & 3 (it has been so long since I've been inside the engine that I do not remember if the count ascends or decends from the top cylinder)

 

[scottswierenga]

I have two 1500 inlines . The 1975 has 130 psi warm and my 1976 that I had rebuilt runs at 145 psi warm . The rebuilt motor fires like its fuel injected. What a difference it makes when the reeds are new . The guy that rebuilt mine is in south carolina and deals with inlines 95% of the time and does great work at a great price . His name is Wayne Canino . I can send you the info if you want it ..... Will

I'd be happy to take down his info, it certainly couldn't hurt to see what his charge would be to do an overhaul.

 

[Will30]

C.C. Marine

Mercury Inline Specialists

________________________

330 Glenn Ferry Rd

Fair Play S.C. 29643

Tel. 864-287-2638

clams_canino@earthlink.net


Invoice Date:*
6/05/13 Will Switzer / Merc 1500 - PH)

Parts: (1)
774-9137A9 x 2 (used pistons) …….…………………………… $127.67
39-64715A12 (complete ringset)……….………………………. $ 81.30
26-41953 x 3 (3 crankseals @ $14.20) ….…………………….. $ 42.60*
4038-16 x 6 (crankpin rod bearings @ $15 per set)…….…… $ 90.00 (2)
27-64941 Intake (transfer) Cover Gasket…………………………..$ 8.13

21-42866 (upper check valve $47.51)………………………… $ 23.75*
21-42657A3 (lower check valve $ 3.72)………………………….$ 3.72 (4)
32-9938779 (bleed Hose 22”)…………………………………….. $ 9.74


Total Parts……………………….…………………………..…………..$ 386.91

Labor: (flat - 10 hours) (3) …………………………………………….$500.00*
Total………………………………………………………………………$886.91


Return Shipping:………………………………………………………...$110.00 (5)


Total Due…………………………………………………………………$996.91*


Notes:

(1)*Used part charge is 50% off current Mercury retail Price.
New parts are at current Mercury retail price.

(2)*2038-16 is aftermarket bearing set – Mercury wants over $80 per set!

(3)*No charge for time spent packing, cleaning, shipping etc….

(4)*No idea why price**different for upper/lower check valves. Used upper took the sting out a bit

 

[DropTop5.0]

That's such a good deal. One can have a great running inline for less than $2k.

 

[Bill Gohr]

So was

C.C. Marine

Mercury Inline Specialists

________________________

330 Glenn Ferry Rd

Fair Play S.C. 29643

Tel. 864-287-2638

clams_canino@earthlink.net


Invoice Date:*
6/05/13 Will Switzer / Merc 1500 - PH)

Parts: (1)
774-9137A9 x 2 (used pistons) …….…………………………… $127.67
39-64715A12 (complete ringset)……….………………………. $ 81.30
26-41953 x 3 (3 crankseals @ $14.20) ….…………………….. $ 42.60*
4038-16 x 6 (crankpin rod bearings @ $15 per set)…….…… $ 90.00 (2)
27-64941 Intake (transfer) Cover Gasket…………………………..$ 8.13

21-42866 (upper check valve $47.51)………………………… $ 23.75*
21-42657A3 (lower check valve $ 3.72)………………………….$ 3.72 (4)
32-9938779 (bleed Hose 22”)…………………………………….. $ 9.74


Total Parts……………………….…………………………..…………..$ 386.91

Labor: (flat - 10 hours) (3) …………………………………………….$500.00*
Total………………………………………………………………………$886.91


Return Shipping:………………………………………………………...$110.00 (5)


Total Due…………………………………………………………………$996.91*


Notes:

(1)*Used part charge is 50% off current Mercury retail Price.
New parts are at current Mercury retail price.

(2)*2038-16 is aftermarket bearing set – Mercury wants over $80 per set!

(3)*No charge for time spent packing, cleaning, shipping etc….

(4)*No idea why price**different for upper/lower check valves. Used upper took the sting out a bit

So was this rebuild for just a shortblock? Appears to be alot of parts for a common rebuild missing

 

[wired]

"Fair Play, SC" . I love it.

 

[Will30]

Thos was just for my block I had rebuilt . I stripped it down and sent it down to Wayne and I had it back in a week .