Motor dies when I increase speed.

froggy

Recruit
Joined
Aug 2, 2007
Messages
1
Hi All

My first post and also new to boating .

Background on the motor. 1979/80 Mariner 70hp.Its attached to a 16 foot fiberglass open bow Sea sprite. In 2005 the powerhead was rebuilt. Compression ranges from 135 to 140 psi on the three cylinders.
I am pretty sure it hasn't ran much since the rebuild as the tag on the boat expired in 2005. Previous owner could not get it started replaced the CDI unit and played with the carb adjustments. (poke n hope)

I bought the boat figured out it wasn't getting any spark and took it to a pro to get it running and a tuneup. ( seemed the prudent thing to do).month and 1/2 later I get it back with a new stator , rectifier and water pump.

Take it to the lake..Starts right up..I,mma happy:). I putt around a lil ..life is good :D, Then I decide to open her up.I gradualy increase the speed ..goin faster then when I get to around half throttle it dies.:eek:.starts right back up..get er goin dies again. So next time I start out slow then slam the throttle fwd and she spits n sputters but takes off goes pretty good but I think should go faster :confused:.

Since then.. I have bought a spark checker, gets spark at each cylinder.
I have started it up with just one cylinder at a time by unpluggin the spark plug wires. it starts on each cylinder but #1 starts harder and runs rattier then the other two.

bout the only things left on the ignition sys. is the trigger and coils.

The bulb , hoses, vent seem to be workin ok.

with the motor in a tank ( garbage can) I can slowly increase the speed. Its only under load that there is a problem.

Any suggestions would be appreciated. Waitin on a timing light/tach to get here..hard to find them in auto parts stores anymore.

bought some sea foam, haven't tried it yet..thinkin maybe fuel pump maybe carbs..maybe trigger...also thinkin I just paid $600.00 and shouldn't be havin this problem. $68.00 an hour plus tack on a premium for parts. shucha deal :mad:.

Sum it up...

rebuilt powerhead,
new stator, rectifier, CDI, waterpump, plugs
spark on all three cylinders...runs on each cylinder
sat for a couple years but was just tuned by a pro..so he says :rolleyes:.
Idles and goes slow fine,can only go fast if I slam the throttle fwd fast when under load..what coulds it B??

Thanks
Froggy
 

Barnacle_Bill

Admiral
Joined
Feb 8, 2004
Messages
6,469
Re: Motor dies when I increase speed.

Since it sat for a few years your high speed jets in the carbs could be gummed up.
 

renny1

Petty Officer 1st Class
Joined
Jul 10, 2007
Messages
219
Re: Motor dies when I increase speed.

i had exactly the same problerm and i adjusted mixture screws on carbs and now runs beautifull
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,175
Re: Motor dies when I increase speed.

Froggy, First I would have a friend pump the primer bulb as you try to accelerate. if this fixes it, rebuild the fuel pump. If not, remove the brass hex bolt at the base of each carb and give it a blast of carb cleaner. This may clear out the main jet. Try it again. If still no go, open the top carb idle mixture screw 1/8 turn and try it again. If that faile open the bottom idle mixture screw 1/8 turn and try once more. If still no go, you need to pull and clean the carbs.
 
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