low RPM's Low Power

[lennygilbert]

78 chrys-75hp.<br />running 17p Prop<br />Compression-150,151,150Psi.<br />Fuel mix-50:1<br />boat is 16ft loaded with 2 batt,trolling mtr,9gal gas,3 people at around 360lbs total.<br /><br />At wot i am getting only 2800rpms and running at 28mph(according to the gps) on board spedo only shows 20mph but i found a crack in the pito tube.<br />I pulled the plugs and the top and bottom are black and the center is clean.However the previous owner did say he replaced the plugs within the last two months and it hasnt been used that much.Prev owner also stated that he mixed the fule with more oil than what it called for just to be on the safe side..<br />Here are my thoughts on why the rpms and speed isnt there. <br />1. since the owner ran to uch oil,maybe the high speed jets are clogged??<br />2.the tach is off<br />3.needs to be decarboned<br /><br />Right now the plugs are NGK HBUW(i think).They are the flat bottom type.<br />What are the correct plugs to use?<br />This boat will mainly see medium to high speed,pulling tubes during the summer,I'll be useing the trolling mtr for fishing,<br />Thanks:

 

[eurolarva]

Re: low RPM's Low Power

Test the center plug for spark. When motor is running you can try diconnecting the wire from it and see if it has any effect on the motors performance. All three plugs should look identical. Sounds like number two is not firing which would explain the reduced power you are experiencing. Maybe try swapping the clean plug with a dirty one and vise versa. Did you actually test compression or were those numbers given to you from the previous owner? Clean plug can be a blown head gasket even though it has been replaced. If you swap the plugs and the number two plug gets clean I would retest compression. That new gasket may be blown or not installed correctly.

 

[lennygilbert]

Re: low RPM's Low Power

Before i bought it the guy did a compression test using one of the Rubber nipple type compression testers and got 120-125 on all cyl.I used my tester which is the screw in type and got 150-151 on all cyl.I did 4 tests in a row and they were all identical.<br />I did swap the top plug with the middle plug.The top plug was black.So im going back out wed and will see how the plug turns out.

 

[lennygilbert]

Re: low RPM's Low Power

As stated before i did a compression check and found 150-151 on all cyl.I pulled the plugs and found the middle to be clean as new and the to and bottom to be normal black color.I put the clean plug into the top cyl and the dirty plug into the middle cyl and took it out.Had the same low rpm/speed issue.pulled the plugs and now the middle was clean again.Like it had been steam cleaned.I did another compression check and got the same 150-151psi in all 3 cyl's.I am assuming that the head gasket has gone bad.<br />Does anybody know the part number to ask for at the mercury dealer??Hopefully this will solve my issue.The boat starts with no hesitation at all.9x's out of 10 it will start before i even have a chance to let go of the key.<br />forgot to mention that i am getting a spark at all three cyl.I had the grounded to the block and watched them while someone was cranking the motor over for the compression check.

 

[eurolarva]

Re: low RPM's Low Power

27-820438 is the part number and it cost $39.80. This part number superceeds the old merc number F369529.

 

[b.gagnon]

Re: low RPM's Low Power

The head gasket could be a problem, or you could be running too lean on that cyl(either one is not good).<br />Are you sure about your rpm's?? I have a 1979 70hp Chrysler with a similar setup, and with a 17p prop turn about 4100rpm at 28mph with a max of 35 @5100rpm. I don't think my boat will plane @ 2800rpm. I think you need to check your tach. I also had a problem with the middle cyl being too clean, and a carb adjustment fixed it.

 

[lennygilbert]

Re: low RPM's Low Power

Originally posted by b.gagnon:<br /> The head gasket could be a problem, or you could be running too lean on that cyl(either one is not good).<br />Are you sure about your rpm's?? I have a 1979 70hp Chrysler with a similar setup, and with a 17p prop turn about 4100rpm at 28mph with a max of 35 @5100rpm. I don't think my boat will plane @ 2800rpm. I think you need to check your tach. I also had a problem with the middle cyl being too clean, and a carb adjustment fixed it.

I thought about maybe that cyc running to lean,but the fact that the plugs looks brand spanking new,and then i put in a dirty plug that came from one of the other cylinders and it cleaned it too.It does sound odd that i have excellant compression on that cyl and could still have a gasket issue.How can i adjust the carb??<br />According to my gps the fastest i can get it is 27.4MPH after trimming it up.I cant even get anywhere near 35mph.It does have one of those fins on the lower unit to help plane faster.I have to pretty much keep it at WOT to stay on plane.I have tried both my 15 and 17pitch props with pretty much the same results.At idle it runs pretty smooth with the occasional miss.I would pull the carbs off and do a good cleaning but I cant seem to find a manual anywhere local.I was trying to get it done by this weekend.I dont need the book for the actual carb cleaning but for the sync procedure after pulling the carbs off.

 

[lennygilbert]

Re: low RPM's Low Power

found that inside of intake cover had nice puddle of fuel in it.Rebuilt carbs.Now when i squeeze the bulb it gets hard as a rock and stays that way and i cant squeeze the bulb anymore,before it would get firm but i could keep squeezing the bulb.<br />Also noticed that when i looked down the carb throats at WOT that the top and middle carb butterflies were flat.The bottom butterfly was angled downward.Resynced carbs so that all the butterflies are open the same ammount at the same time.I will take it to the lake tommorrow to get results.

 

[b.gagnon]

Re: low RPM's Low Power

I took my fin off...It with it on I could only get 30-32mph! Also chech the fuel re-circ system.<br />You should get the manual from IBOATS, it will be worth every $....I would get a Seloc.

 

[lennygilbert]

Re: low RPM's Low Power

took it to the lake.When first starting it will sometimes backfire once before starting.Still only getting 2800Rpms when trimmed up and only 27.6mph acording to the GPS.I was told that it could be the plugs.It currently has the champion flat ended plugs and i was told that it needed to have the "normal" style plugs.It has fresh gas,mixed at 50:1,fuel filters are clean.I have ordered a manual but im waiting for it to arrive.Is there any way i can check that the plug wires are on the correct plug.I had a VW bug that was running the same way an i found that 2 of the wires had been reversed.<br />When at trolling speed it runs pretty ruff with more than normal smoke.And if you go to neutraul from trolling speed or wot it will usually die.<br />sometimes at idle the idle speed will go up and down.

 

[b.gagnon]

Re: low RPM's Low Power

My manual says “use J4J Champion plug with a gap of 0.030” on models equipped with a flywheel magneto or battery type ignition (points). With magnapower I or II ignition system use surface gap Champion UL18V (electronic ignition).<br />My motor had extremely similar problems. Your low top end could be due to a cylinder dropping do to fouling. That is about the same speed my boat would due before the fix. I adjusted the carb mixtures and put new plugs in, now the motor runs like new. My motor seemed to like ngk’s better! You also need to check your tach. It is a good tool for diagnosis. I bought a Tiny Tach for about $40, It is digital and takes about five minutes to install. I think your tach is a bit off…..

 

[lennygilbert]

Re: low RPM's Low Power

I just checked and i do have the NGK's,I had champion on my mind for some reason.<br />I think mine is the magneto system.It has the dist. on the side with the plug wires coming ou the bottom.I pulled the bottom off and it does have a rotor.I also noticed something on the cap itself.There are 6 "points" inside the cap.Three have a brass or copper strip that runs up the side of the cap,the other three do not.The three that do not have this strip are the ones connected to the Spark Plugs.I will try the J4J's tomorrow.I also pulled the plugs tonight which are the surface gap type and they were black as coal,with the exception of #2 it was black but not as bad as the other two.I rechecked the compression and still have 150-151 on all three cyl's.

 

[lennygilbert]

Re: low RPM's Low Power

I went to the champion plug website and they dont offer the J4J anymore,but did cross ref with the J4C,stock#825<br />If i do a lookup for my yr/hp on the champion site it shows 2 plugs to use-5931 marine plug set to .40 gap or L82C racing series plug set to .40 gap.

 

[b.gagnon]

Re: low RPM's Low Power

Is there a box to the left that says magnapower??<br />They both have a cap.

 

[lennygilbert]

Re: low RPM's Low Power

I dont see "magnapower" any where on the motor.I did notice that on the distributor cap it says "electronic"

 

[lennygilbert]

Re: low RPM's Low Power

when i checked the mixture screws i found all 3 to be 2-2 1/4 turns out.In other words it took 2-1 1/4 turns to lightly seat them closed.<br />How many turns out should they be?

 

[lennygilbert]

Re: low RPM's Low Power

bump

 

[eurolarva]

Re: low RPM's Low Power

On a clean carb that is running correctly the mixture setting is around 1 to 1 1/2 turns counter clockwise from lightly seated. Mercury site is calling out an L20V plug for that motor which is center fire type plug. Link and sync procedures are outlined in most service manuals and are critical for making the motor run correctly. If it were me I would test spark on all three cylinders. Rebuild fuel pump and carbs and go from there. This may not be a fuel related issue however it is the cheapest and easiest place to start.

 

[lennygilbert]

Re: low RPM's Low Power

spark on all 3 is bright blue,carbs have been rebuilt.Plugs are center fire type,and are very black.I counted the number of turns of the mix screw by turning clockwise untill it was seated.All 3 were set at 2-2 1/4 turns.I will turn the screws back to seated and go out to 1 turn and see how it runs then.When i go to champions website they are calling for a "normal" type plug according to the yr,hp,and make of the motor.Havent rebuilt fuel pump yet but will do this week.<br />All 3 cyls are still showing 150psi compression.

 

[b.gagnon]

Re: low RPM's Low Power

2 1/4 turns seems a bit much! I think my carbs are about one turn.