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Still havenbt figured out this problemDT85

B

bowhuntrrl

Contributing Member
"I bought a boat late last sea

"I bought a boat late last season, it had been sitting for 4-5 years. I went through the whole darn engine. I rebuilt the carbs, replaced the fuel pump diaphragm and o rings, replaced the plugs, replaced the fuel filters (engine and separator), checked the timing, cleaned the oil tank and put in new oil.

The engine starts and runs great. I can idle on the lake all day, or run at lower rpm. Once I crank it up to WOT, the boat runs about 5 minutes before it starts to die. If I pump the bulb, it takes off again. I've double checked the fuel pump and see no problems with it. I was told that the engine connector fitting can leak air, so I removed it from the system by using a double sided brass fuel barb and hose clamps. I also removed the boats in-hull fuel tank by hooking an external tank directly to the engine (bypassing the separator. It still does it. I really need to get a handle on this since the season is almost here in Massachusetts.

I know it's a fuel supply problem, I just can't figure out what it is. The only thing left is to replace the bulb. Does that seem logical or whatever other suggestion would anyone make??? This one has me baffled.Thanks."
 
Compression test is required b

Compression test is required before doing any more work--- it is always the first thing to do!
 
"I had a bulb go sick on me on

"I had a bulb go sick on me once but that one sucked flat due to the fuel line collapsing, try borrowing a known OK fuel line and running her again, if the line collapses (like mine did) it will be sucked closed and pumping the bulb will push fuel through it and open the line again.

With mine it collapsed between tank and bulb so pumping didn't work.

If the fuel line had been sitting for 5 years with fuel in it may have rotted."
 
You may want to check the pick

You may want to check the pickup inside the fuel tank. try a friends tank and fuelline and see if that works. I think you have a clogged pickup screen
 
"Same thing happened to me. E

"Same thing happened to me. Engine ran nice at idle, then I would get up on plane, then would bog down. Had someone squeeze the bulb and it ran great. My problem was when I put the fuel lines back on the carbs, a tiny, tiny piece of the fuel hose was in one of the carbs. It was the size of a tip of a needle. That cleared up my problem. 87 DT 140"
 
run a fuel pressure/vacuum tes

run a fuel pressure/vacuum test
then a spark test
u might have set the carbs too lean
 
"The engine has good compressi

"The engine has good compression, 135-138 across the board. I have the carb set according to the manual
1 1/2-1 3/4 on the idle jets. What do I need for the fuel pressure /vac test, a vacuum gauge, a fuel pressure gauge , or both ??? How a bout the spark test , what's needed for that ???

My thought was that if I pump the bulb and it takes off that it must be a fuel delivery related problem. That's why I took the original fuel tank and lines completely out of the equation. I'm going to buy a new hose and line to test that theory. Beyond that, I really don't know what to try."
 
"Hi,

Sounds to me like it

"Hi,

Sounds to me like it is drawing vacuum in the tank - is your tank breathing OK ? it might have enough ventilation to service the motor at low RPM but not enough to draw fuel at high RPM ?"
 
Getting spark on all 3 cylinde

Getting spark on all 3 cylinders
Carb bowls are spotless.
Temporarily using a portable tank as a diagnostic tool (eliminate the boat's built in tank).
 
"carb bowls spotless, means no

"carb bowls spotless, means nothing!!!
you need to pull the jets off and clean right"
 
"Just in time, I already rebui

"Just in time, I already rebuilt the carbs and still have the problem !!! The boat runs great except after 5 minutes at full throttle, at which time it starts to run out of fuel. I then pump the bulb and it takes off. The carbs were fully disassembled and cleaned, floats checked and adjusted, carbs synched, and timing and idle set. I'm getting 5300 rpm WOT and plenty of smooth power, it just seems to run out of fuel after 5 minutes at high speed.

Please go back and read the original post if anyone has questions, there's plenty of info there about what I have or haven't done."
 
either fuel pump or stuck anti

either fuel pump or stuck anti-syphon valve

maybe a restriction in the fuel line

hows the filter???
 
"i think you are getting air ,

"i think you are getting air , i would check all fuel line conections and lines between the carbs.The ones from carb to carb."
 
"Just an update. I've been

"Just an update. I've been too busy to work on it lately. My house needs work, boat takes a back seat. I did buy a fuel pressure/vacuum gauge to test it and also a new bulb/line assembly. I hope that in the next few weeks that I will be able to spend some time on it. I'll let you all know what my testing turns up. One question: Where's the anti siphon valve ?? I didn't see anything about it in the manual ???"
 
"HEY Check the valve between t

"HEY Check the valve between the upper and lower cylinder, if it faults, the fuel pump wont work optimal... it is located on the right side and is fitted with a small red hose....."
 
"What valve might that be ???

"What valve might that be ??? Is it on the starboard side, near the carbs or somewhere else?? I might finally get to work on it a bit this coming week. Unfortunately, I can't find the manual I bought for it last season."
 
"As you had the fuel pumps apa

"As you had the fuel pumps apart to change the diaphrams, check to see that you have the "match marks" (service manual term) lined up on the center housing with the "match marks" on the pump cover. There is a rib on the inside of the cover that must line up with the rib separating the 2 pump chambers of the center housing (these ribs are lined up correctly when the match marks line up). After changing the diaphragms in my pumps the mechanic failed to line up the match marks when he reassembled the pumps and it ran like yours."
 
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