Gradual loss of power at WOT

[Don Brock]

'85 Wellcraft Nova XL / 5.7L Chevy / Alpha 1.<br /><br />Did head job last Nov (due to burnt valves on two cyls with low compression) then winterized. Took it out for first time in April to finish timing it (total advance timing is set at 32 deg at 3700 RPM.) and it runs like a champ. That is until I peg it at WOT then after a couple of minutes it gradually loses power. Temp rises from about 145 to 165 deg while at WOT. RPMs are steady at 4600 at speed of 46 mph. When power dropped (I watched it slow to about 38 mph), I pulled throttle back to about 3200 RPM. Engine stabilizes and runs fine. Temp returns to 145 deg. Then if I advance back to WOT get same symptoms. Runs OK for a minute or two then gradually loses power again. Did this test four times in a row with exact same results each time. Have changed screen filter in carb (old one looked fine) and water separator with no improvement. <br /><br />Is it possible that fuel stabilizer from winterization could be contributing to this problem? I also noticed over winter that the fuel expanded and dumped through the overflow down the side of the hull. <br /><br />What's a good way to check the fuel tank vent for blockage? I've seen that mentioned on couple other posts as a possible problem. It seems to vent fuel out, but maybe its not letting air in - if that makes any sense.<br /><br />Thanks in advance - Good Vibes

 

[Bondo]

Re: Gradual loss of power at WOT

It seems to vent fuel out, but maybe its not letting air in - if that makes any sense.<br />

I Really Doubt the vent is your Problem..........<br /><br />I think you're looking at a Fuel Flow Problem....<br />It could be Anything between the Tank,+ Carb.....<br />Gas...<br />Tank...<br />Check-valve...<br />Lines...<br />Filters...<br />Fuel Pump...<br />Carb....<br /><br />You Need 5psi, At the Carb, All the Time.........

 

[SS MAYFLOAT]

Re: Gradual loss of power at WOT

Sounds to me like your going into lean conditions. This might be why you had burnt valves. A lean mixture will do this. You may have fixed what was damaged, but you didn't correct the problem of what caused the damage. You may need to change the jetting of your carb.<br /><br />Just my thought.... Good Luck

 

[MrDumb]

Re: Gradual loss of power at WOT

My engine had the same symptoms last spring when I pulled it out for the first time. The dealer said that the motor was hydro-locking b/c of bad gas. So I filled it up with premium and put stabilzer in the tank. After a couple of fill ups it went away?

 

[Wingnutt]

Re: Gradual loss of power at WOT

Gotta agree about the lean condition. Check the simple things first. The very first place that I would do would be to check the fuel filter. My Blazer had been running like crap under a load for the past month or so, and since I only drive it a couple of times a month, I let it ride until I was able to move a project out of the garage and get the Blazer in. I pulled the fuel filter and I poured out a substance that looked week old coffie...complete with the grounds. It looked even worse when I cut the inlet end off and took the filter media out. Put a new filter in and what a difference! <br /><br />If the filter doesn't do the trick, check the float level in the carb. If it's a tad low, it could cause the symptoms you are describing.<br /><br />Keep us posted.

 

[jdcook]

Re: Gradual loss of power at WOT

I had similar problem last year, float level in carb was my problem.

 

[Don Brock]

Re: Gradual loss of power at WOT

Bondo - I really like your methodical approach. I already started in the middle with changing the filters. That didnt fix it. I'm going to check fuel pressure next, then work from carb back toward the tank. <br /><br />Based on other replies, I suspect the float level may be out of adjustment, resulting in fuel starvation. The guy I bought the boat from last year said he rebuilt the carb the previous summer. Haven't done that myself in uite a while, but as I recall, float setting was usually the trickiest part.<br /><br />SS Mayfloat - Ya got my attention about fixing the symptom and not the problem. I originally thought the burnt valves were due to retarded timing, but maybe it was a lean mixture (or worse yet - maybe both). Sure don't want to burn up the new valves. A mechanic (here in northern VA) echoed your thoughts about lean mixture and burnt valves. He educated me that in years around 1985 Mercruiser was designing their engines to run leaner to make them more fuel efficient and maybe more environmentally friendly. He also suggested rejetting the carb to the next larger size - thought being that the enriched fuel mixture will cool the valves better. Sounds good in theory, but not sure....out of my league on that. Guess its easy enough to try.<br /><br />After the fuel pressure check, I'll check float level, then possibly rejetting. Anybody know where I can buy jets and metering rods for a Rochester Quadrajet (on your basic Mark 1, Mod 0, 1985 5.7L rated at 260 HP)?<br /><br />Thanks for all the great suggestions.<br />Stay tuned for the results...<br />Good Vibes

 

[SS MAYFLOAT]

Re: Gradual loss of power at WOT

The NAPA store around here carries them. Check with them in your area. Good Luck.

 

[Don Brock]

Re: Gradual loss of power at WOT

Finally getting back to ya'll. It was the carb float. Adjusted it to factory specs and the WOT power loss went away. Purrs like a lion now.<br /><br />Ran into a snag installing the fuel pressure gauge. The fuel line from the fuel pump up to the carb is steel. I cant get the line to flex enough to accommodate the T-fitting for the fuel pressure sender. I believe there are USCG regs on fuel lines. Does it have to be a steel line, or can I "legally" replace it with a copper line, which is much easier to bend to fit?<br /><br />Thanks in advace.<br />Good Vibes

 

[Bondo]

Re: Gradual loss of power at WOT

Copper is alright.... "Legally" speaking......