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Spak Plug question...

R

Retired OG

Contributing Member
My old motor (1972 50 hp Lark) service manual lists 2 different spark plugs (both flat, surface gap type) as OEM replacement plugs....when I replaced the plugs that were in the motor, I replaced them with regular type adjustable gap plugs of the same number that I took out (they are cross referenced as suitable subs).... Ever since I've gotten the motor running, I've never been satisfied with the performance....can using regular type spark plugs affect the performance???
 
Sparkplugs merely provide a place for an electric arc to jump.----Performance ( power ) likely does not change much from one plug to another.----What is your motor doing / not doing ?---What trouble shooting steps have been done ?
 
The engine hadn't run in over 7 years....before I started working on it, I checked compression and both cyl. were right at 150 lbs. I rebuilt the carbs, fuel pump and lines, replaced the stator so it would charge, the timing sensor, rectifier, reed valve gasket (pinhole leak blowing gas out of the carbs.), Even though the CDI module appeared good, I replaced it as well and am keeping the old one as a spare....I also had to replace the shift selector switch in the control box. All timing and carb. adjustments have been made to the service manual specs.

The reason I asked this question is because last week, I spun the hub on a "Michigan" 14X13 prop that I could never get to spin at over 4500 rpm (18 MPH) without bogging down....well, the boat (1972 24' Kayot Pontoon) came with an old OMC prop with the same diameter and pitch but it will spin at 5100 before it does the same thing but MPH is only 16.5 and if I throttle back to 4500, I'm down to14.5 MPH....before I buy another prop, I need to figure things out...my first thought was to go with a 14X11 prop, but if I can get more MPH, MPG and RPMs up with a longer pitch, I'd prefer that, because it would relate to better fuel economy too....I've already experimented with motor height and trim and I'm good with where they are....Looking at "Turning Point" website, they claim their props offer larger exhaust ports and help increase HP...it's probably just sales gibberish, but I don't know...all I know, is that I'm a perplexed old guy...
 
To answer the original question, spark plugs will have absolutely no effect on the performance problem you are having. Unless, of course, one of them isn't firing. In that case it would barely get out of it's own way.
 
do you have the correct shaft length on your boat? What kind of performance are you expecting with a 50 hp on a 24' pontoon? Are you measuring speed with a gps or a speedo mounted on the boat? You need to be more realistic about your speed. Prop your motor to run 5500 rpms and it will be happy. Overpropping a motor is a good way to shorten an engines life and kill your fuel mileage. A lesser pitched prop allowing the motor to reach it's max RPM is the best way to go. A lesser pitched prop is not necessarily slower because it is spining faster. Is the link and sync correct? Do the carbs only open to 90deg at full throttle or are the over-rotating? There is no prop produced that can increase hp. If the prop is ported it can increase holeshot. Your pontoon sounds like it is a tad underpowered.
 
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flyingscott said:
do you have the correct shaft length on your boat? What kind of performance are you expecting with a 50 hp on a 24' pontoon? Are you measuring speed with a gps or a speedo mounted on the boat? You need to be more realistic about your speed. Prop your motor to run 5500 rpms and it will be happy. Your pontoon sounds like it is a tad underpowered.
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Yes, it is a tad underpowered. The speed I listed was GPS...I'd be happy with 18 mph, but why such a big disparity between props? I guess I could order a hustler 14X13 and if the RPMs aren't there, they have a "pitch exchange" program for $25....
 
Props are not an exact science. A prop with the same dimensions but different manufacturers may not act the same. It will be trial and error to find the prop that works correctly. The trick to propping is to be at the correct Rpm range. which for your motor will be 4500- 5500 rpm. Prop towards 5500 with a light load If you want more speed buy a bigger motor.
 
If you can get a cupped prop 14X11 should get you where you want to be. For best fuel economy run it at max spark advance usually around 3/4 throttle.
 
flyingscott said:
Props are not an exact science. A prop with the same dimensions but different manufacturers may not act the same. It will be trial and error to find the prop that works correctly. The trick to propping is to be at the correct Rpm range. which for your motor will be 4500- 5500 rpm. Prop towards 5500 with a light load If you want more speed buy a bigger motor.
Click to expand...

I'd LOVE to buy a bigger (newer) motor...but it's a financial impossibility. Even the thought of having to buy a new prop has me eyeballin where I'll have to punch a new hole in my belt....:rolleyes: Thanks for all the input....I'll update this thread as I go....
 
If your in a hurry leave a little earlier. At WOT you use alot more fuel, just find the right prop for around 5500 WOT rpm then watch the spark advance and note the rpm when the advance stops moving. Then just keep your throttle there you will both extend engine life and get better fuel economy.
 
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