Port engine WOT only getting to 3500 rpms

charlg

charlg

New Member
Jan 7, 2007
745
Newton, MA
Boat Info
2000 280 Sun Sport
/ Raymarine C80 w/ Radar / Sirius weather
/ SH GX5000S VHF
Engines
Twin 350 Mags / B1's
/ Corsa exhaust
The port engine reaches WOT at 3500 rpms and won't go over . This is the second season with this issue. This spring, while in the driveway, my mechanic noticed that the port outdrive was not going all of the down. He fixed the pistons and hoped the problem was solved. Since it didn't, he put a computer to it that said the throttle position sensor was bad. I replaced it, but it did not change anything. Any other ideas?

The starboard engine at 3500 rpms is only about 3/4 WOT. Top speed (port WOT / starboard 3/4) is only 38-39MPH with only me on the boat. It used to be high 40s to 50mph.

Other than that the boat runs like a top.
 
I don't think he did. Is that something that can get out of adjustment?
 
Had a similar issue. Turned out to be a fouled spark plug. Changed all eight and back to normal.
 
The plugs were changed after the problem started last year and nothing changed.

I am noticing that the throttle does not change from 3/4 to WOT. It gets to about 3400rpms at 3/4 throttle and stays the same to WOT. From start to 3/4 everything seems normal. Again, this is only happening on the port engine.
 
So when did this problem start? I see by reading your post it started a couple of years ago, that's too long to be dealing with an issue like this. Why hasn't your mechanic fixed the problem? If your self diagnosing and only letting the mechanic fix it a trial by error approach this will take forever. Diagnosing this problem involves checking fuel pressure and fuel quality then moving on to ignition and then if you have to compression checking with a leak down test. If you have a real mechanic working on the boat he should have already suggested these items. Did the mechanic plug a computer into the harness? If you want to resolve this issue tell your mechanic to fix it or go find another one.
Rob
 
The problem started at the very end of last season. At spring start up , the mechanic focused on the outdrive. Since splashing, he has hooked up a computer and it showed the throttle position sensor was bad. I replaced it, but it has not helped. The trouble is it's costing me $$ every time we try something, and it takes a couple of weeks to get him back. I was hoping for any answers that I could try myself rather than paying him to keep coming out.
 
charlg said:
I don't think he did. Is that something that can get out of adjustment?
Click to expand...

push the throttle lever at the helm all the way down and disconnect the linkage at the carb...if the throttle on the carb can be opened any further than was opened with the linkage the linkage is not set properly....the linkage should be set so that the throttle on the carb is all the way open when the throttle handle at the helm is pushed all the way down....

cliff
 
charlg said:
The problem started at the very end of last season. At spring start up , the mechanic focused on the outdrive. Since splashing, he has hooked up a computer and it showed the throttle position sensor was bad. I replaced it, but it has not helped. The trouble is it's costing me $$ every time we try something, and it takes a couple of weeks to get him back. I was hoping for any answers that I could try myself rather than paying him to keep coming out.
Click to expand...

I had a very similar problem last year. The distributer was bad. Try swapping with the other engine and see if the problem goes away.
 
Ok so it's time to start eliminating things you've already ruled out. The possibility of a bad distributor, can be proven and swapping is the sure fire quickest way but it still is trial and error. Why was the mechanic focusing on the drive? Anyway last year there were some defective distributor caps floating around and I've been the unfortunate recipient of bad spark plugs right out of the box so don't rule them out. Are your props in good condition? If your handy take the distributor cap off and look inside if it's corroded replace it. I'd double check the plugs wire firing order, sometimes even pros mess up. You have to start ruling things out go for the things you can do yourself.
Rob
Also pull the coil wire out completely and check both ends for corrosion, I've also been there done that.
 
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Did a conmplete tune-up at the end of the season....new plugs, rotors, coils, etc. Didn't change anything. Motors run smooth.

We noticed while on the trailer that the port outdrive was not going all of the way down. There was about a 6 inch difference than the starboard, and we were hopeful that was it.

Prop looks OK. No dings.

Keep the ideas coming. Thanks.
 
charlg said:
Did a conmplete tune-up at the end of the season....new plugs, rotors, coils, etc. Didn't change anything. Motors run smooth.

We noticed while on the trailer that the port outdrive was not going all of the way down. There was about a 6 inch difference than the starboard, and we were hopeful that was it.

Prop looks OK. No dings.

Keep the ideas coming. Thanks.
Click to expand...


I did a full tune up, including plugs, wires, cap, rotors and the boat ran fine for about 2 hrs. After that I couldn't get one of the engines over 3 grand. Screwed around with it for about a month before I read about the bad caps. Once I replaced the cap she ran like a champ. You might have got one of the defective caps.
 
if your props are good then the problem is fuel or ignition. Since both motors run from the same tank then your problem is probably with ignition. You are fortunate to have two motors so you can test theories without paying for unneeded parts. Start by swapping out distributors as suggested. Check all spark plug cables and make sure all are properly seated (both ends) and not crossed. remove the fuel filter for good measure and empty into a clear jar. Replace if necessary. What props are you running with?
 
You didn't mention ignition wires. 12 year old wires are suspect. Compression test is a must at this point.
 
first address the lack of RPMs. I would do as suggested and start easy. Distributors, seated uncrossed wires, fuel filter. Compression or even better leak down test only after you exhausted the first few items. I had an issue where RPMs would fall off one of the engines after I pegged the throttle full and tried to go WOT. We ran the gambit of tests. Nothing conclusive. It was the end of the season so I did my usual winterization, spring recommission (plugs, caps, filters etc.) and I swapped out my props. Issue went away so it could have been any one of those things I changed. Those lazer II props you are running are costing you thrust by the way. Fast prop for a single, light boat, not enough surface area for your twin 350's and the weight you are pushing. Mercruiser Mirage or Quicksilver Thunderbolts (same prop as mirage without added price) would be a much better choice. I love the difference they made with mine.
 
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SteveRuss said:
Those lazer II props you are running are costing you thrust by the way. Fast prop for a single, light boat, not enough surface area for your twin 350's and the weight you are pushing. Mercruiser Mirage or Quicksilver Thunderbolts (same prop as mirage without added price) would be a much better choice. I love the difference they made with mine.
Click to expand...

What size/pitch props did you go with? How did it change things? Do you get on plane faster? Top speed? Did the rpm to mph change?
 
Went with the same pitch but went up in diameter by an inch. Original props were 14.5X17 Vengeance props. New ones are 15.5X17 Thunderbolts. Better holes shot and better all around handling. Mid-range on a calm day I cruise at about 38mph at 3500. Didn't see much of a top end difference but you can feel the props bite better especially in turns.
 
CliffA said:
push the throttle lever at the helm all the way down and disconnect the linkage at the carb...if the throttle on the carb can be opened any further than was opened with the linkage the linkage is not set properly....the linkage should be set so that the throttle on the carb is all the way open when the throttle handle at the helm is pushed all the way down....

cliff
Click to expand...

:thumbsup: First things first... You can also check linkage by having someone push the throttle at the helm while you watch the throttle body (or carb?) butterfly open. It should continue to open between 3/4 and full throttle. If not, there's your problem. If it is, then remove the cable and see if you can move the linkage more by hand...
 
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