70hp Yamaha Power Drop/Surge

[mdegood]

The motor is a 1987 Yamaha 70hp. It will start and idle fine. When you take off, it powers up (@5200-5500rpm) and planes out great. After @ 10-15 secsonds power drops to @ 4000rpm (WOT) and will stay there for @ 20 secs and then it will start surging. The first time this happened the primmer bulb did'nt feel like it has getting firm when you pumped it. So, changed it, still no joy. But this time when the power drops off you can squeeze the boulb (@10-15 times) and power will jump back up to WOT, and stay there as long as you keep pumping. So I say cool, and change the fuel pump. Same problem, no change!<br /> Now, my question for all you guys of knowledge. Do I go downstream of primer bulb towards tank and start looking for problem or head towards the carbs? The carbs were rebulit @ 7-10 hours ago. Water sep/filter on motor is clean and in line filter is also clean. Any and all help is appreciated, fishing season is here and I sure don't want to miss it.<br /><br />Thanks!!

 

[mdegood]

Re: 70hp Yamaha Power Drop/Surge

Oh yeah, one more thing. Choking it made it want to die all together.

 

[jim dozier]

Re: 70hp Yamaha Power Drop/Surge

It still sounds like you have a fuel restriction and/or air leak in the fuel line tankside of the fuel pump. Check the tank pickup, anti-siphon valve, hose connections, fuel bulb, fuel-line disconnect at the enginge, and hoses for cracks and leaks and blockages.<br /><br />Did the problem start before or at or after the carb rebuild?<br /><br />I read in another post once a similar problem was caused by a too small inside-diameter replacement fuel line. I don't know what is appropriate for your engine but make sure its big enough.

 

[mdegood]

Re: 70hp Yamaha Power Drop/Surge

It happened after the carb rebuild, I think we got 4-6 good, no problem trips out of it. I have repalced the fuel line from the bulkhead to the primer bulb (3/8 id) and the hose from the bulb to the quick disconnect looks to be factory style hose, maybe 5/16" id. I wouldn't think that it would be the quick disconnect, as that is rarely ever removed, but will check again. Guess it is now time to drain and remove tank and replace all lines. Thanks, will let everyone know what I find.

 

[willxuout]

Re: 70hp Yamaha Power Drop/Surge

Hey,<br /><br />I had the same problem with a 1996 evinrude 225hp v6. the problem was that when I had a tiny crack in fuel intake tube in the fuel tank were it screws into the fuel tank fitting. it was sucking air into the fuel line and causing the motor to miss and lose lots of power. Make SURE you have spark on all cylinders (Stator may be going bad).

 

[willxuout]

Re: 70hp Yamaha Power Drop/Surge

Hey,<br /><br />I had the same problem with a 1996 evinrude 225hp v6. the problem was that when I had a tiny crack in fuel intake tube in the fuel tank were it screws into the fuel tank fitting. it was sucking air into the fuel line and causing the motor to miss and lose lots of power. Make SURE you have spark on all cylinders (Stator may be going bad).

 

[papasage]

Re: 70hp Yamaha Power Drop/Surge

borrow a portable tank frome someone see if it helps.

 

[mdegood]

Re: 70hp Yamaha Power Drop/Surge

Ok, I put a 6 gallon tank on and here is what I found.<br />Warmed up engine, motored out and went to WOT. Engine came to life and hit a max of 4000 rpm. But did not miss a beat. Everything ran fine, but I am concerned that it only was able to obtain 4000rpm. Normally it will max @ 5200-5500rpm. I also noted that going into a turn ment losing @ another 800-1000 rpm. The only thing I can figure is that:<br />1) The fuel line dia was smaller on the portable.<br />2) It had premix in it, so I got a double dose of oil.<br />After trailering the boat, drained the tank. Found only a little bit of water (@ 2 droplets penny size). But more importantly fond that the pickup tube turned fairly easily on the fitting. Actually I was able to pull it of the barb w/ factory crimp still on it. Also the bulkhead fitting sure came off rather easy. So new fittings and hose today and try again in the morning. <br />Will post results.<br />Thanks for all the input!<br />Mark

 

[Jammu]

Re: 70hp Yamaha Power Drop/Surge

Look at the fuel filter O-ring. Is it there ?

 

[mdegood]

Re: 70hp Yamaha Power Drop/Surge

Ok, now I really need everyones help! Heres the deal. Drained, removed tank (no debris found), removed pickup tube/check valve (replaced), replaced all fuel line from tank to engine, including quick disconnect, replace in filter, replaced fuel bulb, replaced all fuel lines in side cowl (from QD to carbs), removed and cleaned carbs, also checked floats.New fuel pump (not rebuild). Checked fuel filter/water sep at engine, oring in tacked and there. Replaced spark plugs, checked compression (120 psi across the board).<br />Here is what I get when I run the boat. Starts and idles fine, can motor out off marina with no notable problem. Bring throttle up to @ 4000 rpm and boat planes out fine, increase throttle to @ 5200-5500 rpm, runs GREAT!!! for @ 20 seconds then loses power to @3800-4000 rpm. With the throttle still WOT if you pump the bulb power will increase, but you can not keep pumping or it falls on face again. What you have to do is one slow squeeze and power maintains untill bulb is empty. While running with this problem, removed cowl and young son said clear fuel filter/ seperator was loosing fuel level until bulb was pumped.<br />So I talked wife into running WOT, I could not see fuel level dropping! Also removed air box and could not see any problem with the fuel in the carbs (no drip no spitting ec..) I really need some ideas on what to check next. Boat shop 3-5 week lead time to even look at! (must be nice to have that much work!) Thanks for any help.. Speckled trout time is here!