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33ss 8.1 crusader engines fluctuating rpms HELP

GAS engine, transmission and generator repair and maintenance discussion forum.
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mrterry72@gmail.com
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Re: 33ss 8.1 crusader engines fluctuating rpms HELP

Postby mrterry72@gmail.com » August 30th, 2021, 12:19 pm

Well I have given up will call in the mechanic to see whats up .. off topic what is a good cruise power setting when the engines were running strong i was running about 3200 rpm but it seemed to be guzzling the gas $$$ ..

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Re: 33ss 8.1 crusader engines fluctuating rpms HELP

Postby buster53 » August 30th, 2021, 12:54 pm

mrterry72@gmail.com wrote:Well I have given up will call in the mechanic to see whats up .. off topic what is a good cruise power setting when the engines were running strong i was running about 3200 rpm but it seemed to be guzzling the gas $$$ ..


Someone else can probably give you a more accurate answer, but I’m guessing you are burning 25-30 gallons an hour for both engines at that speed. If you really want to run in economy mode, keep it at hull speed or under, around 7 - 7-1/2 knots. Do NOT run between hull speed and full plane. A lot of people think they are saving gas running around 12-15 mph. Wrong. You will get more miles per gallon running 18-20 mph than you will at 13.
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Re: 33ss 8.1 crusader engines fluctuating rpms HELP

Postby cpoint » October 21st, 2022, 3:54 pm

Glad I read this subject, so I will piggy back to this, unless I should start a new subject. Been gone for some time, back now, and did not use the 356 for last 2 years much. In any case, always had issues with injectors, they were replaced twice. started to have same problem this year. Called my mechanic, he is very knowledgeable with mercruiser and engines in general. Trust him fully, as he is the one who finally got me going after spending a fortune in the beginning with bad fuel related issues. In any case, Last month called him over, he said he did not think it was injectors, we took it out and he hooked up the computer and tested as we cruised. and found that after 2k rpm it was not getting enough fuel. Bypassed port tank with his portable, no issues, we were flying. So turns out there is some blockage at high flow starves engine for fuel. Either at fittings, line or pickup line. Told me I can use small compressor to blow back if in the pickup line filter, or replace the fitting where hose goes to tank, or empty the lines from engine side to tank side into a bucket do see if there is stuff in the line or where the on/off/aux switch. Makes sense. Did not have time to check that yet as time is almost winterization. So most likely I will do it next season. But regarding above chat on 2 micron filter, he also told me other than replacing port tank, which I know has issues, had it cleaned and emptied twice, still gunk in it. He said add 2 micron raycor by the switch before the built in 25 micron original filter, at least that will keep the injectors clean. As that is an expensive hobby replacing or getting them cleaned. So I purchased the 2 micron raycor, but have not installed. First I need to clear the mass in the line, if all good, than install the 2 micron and test again. :clap:
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Re: 33ss 8.1 crusader engines fluctuating rpms HELP

Postby bud37 » October 21st, 2022, 5:24 pm

cpoint wrote:Source of the post First I need to clear the mass in the line, if all good, than install the 2 micron and test again.


2 microns ??? seems a bit small to me....... before you try this, check the fuel flow ( in Gph) thru that filter......remember what you will need to run on plane and for the hp output at the speed.

Just good to check first.......good luck...
The above is strictly my opinion always based on years of doing...remember to support local business , it pays back.
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Re: 33ss 8.1 crusader engines fluctuating rpms HELP

Postby km1125 » October 21st, 2022, 5:39 pm

cpoint wrote:Source of the post Glad I read this subject, so I will piggy back to this, unless I should start a new subject. Been gone for some time, back now, and did not use the 356 for last 2 years much. In any case, always had issues with injectors, they were replaced twice. started to have same problem this year. Called my mechanic, he is very knowledgeable with mercruiser and engines in general. Trust him fully, as he is the one who finally got me going after spending a fortune in the beginning with bad fuel related issues. In any case, Last month called him over, he said he did not think it was injectors, we took it out and he hooked up the computer and tested as we cruised. and found that after 2k rpm it was not getting enough fuel. Bypassed port tank with his portable, no issues, we were flying. So turns out there is some blockage at high flow starves engine for fuel. Either at fittings, line or pickup line. Told me I can use small compressor to blow back if in the pickup line filter, or replace the fitting where hose goes to tank, or empty the lines from engine side to tank side into a bucket do see if there is stuff in the line or where the on/off/aux switch. Makes sense. Did not have time to check that yet as time is almost winterization. So most likely I will do it next season. But regarding above chat on 2 micron filter, he also told me other than replacing port tank, which I know has issues, had it cleaned and emptied twice, still gunk in it. He said add 2 micron raycor by the switch before the built in 25 micron original filter, at least that will keep the injectors clean. As that is an expensive hobby replacing or getting them cleaned. So I purchased the 2 micron raycor, but have not installed. First I need to clear the mass in the line, if all good, than install the 2 micron and test again. :clap:


You might want to check out the pickup tube in the tank. 356 Carver in your era had an issue with the supplier of the tank where they had a copper pickup tube in the aluminum tank. They actually had a recall program to send out a plastic pickup tube that could be used instead. The copper tube would start to disintegrate and cause very fine particles to flow downstream and clog filters. I would imagine at some point the pickup tube would become porous so you'd could draw air in as well.

I'll see if I can find the info that I found on this... but it was years ago when I was doing the research to help out a dock neighbor.
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Re: 33ss 8.1 crusader engines fluctuating rpms HELP

Postby cpoint » October 21st, 2022, 8:43 pm

KM, thanks. If you find that information that would be great. I wouldn't be lying if i said i was chicken when it comes to gas. I may end up paying some one to take care of it. I will first check the lines and fittings between tank and wall aux port switch. That i fee comfortable with.
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Re: 33ss 8.1 crusader engines fluctuating rpms HELP

Postby cpoint » October 21st, 2022, 8:45 pm

bud37 wrote:Source of the post
cpoint wrote:Source of the post First I need to clear the mass in the line, if all good, than install the 2 micron and test again.


2 microns ??? seems a bit small to me....... before you try this, check the fuel flow ( in Gph) thru that filter......remember what you will need to run on plane and for the hp output at the speed.

Just good to check first.......good luck...

Thanks, i did ask the mechanic he said it would be fine.

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Re: 33ss 8.1 crusader engines fluctuating rpms HELP

Postby Viper » October 22nd, 2022, 8:37 am

Ya you have to be careful when you start introducing more hardware into the suction side of the fuel supply circuit. That includes everything from fittings, hose, monitoring accessories, filters, etc. The last thing you want to do is introduce more restrictions which will increase vacuum and cause vapor lock issues too. I'm sure the tech is aware of that (fingers crossed), and that it's something that needs to be monitored after the install, especially on hot days that will raise the temps in the engine bay making the problem even worse. It all adds up.

Hey cp, glad to see you back man, it's been a while. Thought you sold or something without saying good-bye!
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Re: 33ss 8.1 crusader engines fluctuating rpms HELP

Postby km1125 » October 22nd, 2022, 9:54 am

If too much restriction might be a concern with very fine filters, you could always run a couple in parallel to cut the restriction in half (approximately). That also doubles the available filtering area before they get clogged.
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Re: 33ss 8.1 crusader engines fluctuating rpms HELP

Postby km1125 » October 22nd, 2022, 10:21 am

Wow, I'm kinda surprised I was able to find this. I think I actually had a one-page recall sheet from FMT too that indicated the years and models that were problems, but can't find that now. It definitely covered the "early" 356 models. However, the reason I even did the research is that my friend with a 2003 356 had the same issue and his tanks were manufactured after the April 2002 date.

Note that in this first one they talk about blowing an engine after installing "big Racor filters", which was likely due to the issues Viper is concerned about.
"Humbucker427"
I know this off track from your post, but since you asked this is what happened to me. My boat is a 2002 Carver 356 with 8.1L Horizons. In 2006, after the first fill up ever with Ethanol, I was on a 4 hour run to Long Island. As we neared the port I start getting wild fluctuations in the starboard RPM'S. I got the final port OK by running at approx 2000 RPM's and below. Dumping fuel filter/Separator I saw nothing suspicious (no water or particulate). On the return trip same issue after running 3+ hours. I got back home and the marina suggested changing the in-line filters. On another long run, the same issue occurred. The reason this only happened on long runs was that the in-line filter would clog, but once the engine was shut off, the particulate fell to the bottom of the filter and it would take two or more hours to fully clog back up on the next run. This went on for two years as my marina refused to bring in Mercruiser. When they finally did, the Mercruiser tech found the issue immediately by placing a vacuum gauge after the in-line filter. When we dumped the in-line filter and it was full of particulates of a very fine nature. It appears that my Florida Marine tanks built in January of 2002 used copper pickup tubes which appear to be disintegrating. My tanks are aluminum (I'll bet yours are too) which can react with with copper, but Underwriter's Lab say you can isolate the copper pickups with stainless adapters. FMT replaced the copper pickups in April of 2002 with plastic pickups, which they did. The fuel analysis showed aluminum, zinc, copper, iron, potassium. After installing big Racor filters to replace the in-line filters to try and mitigate the situation, the engine blew. Don't ask me the technical reasons as I'm not a mechanic, but with the engine being replaced I was able to replace the starboard FMT tank. Now the particulate situation is now happening in the port engine so I'm not sure what will happen next. Since we have the same boat, you know that the stern tank can be the issue as well affecting both engines. Not sure what I can do, as the pickups in the FMT tanks are welded in.

Still trying to analyze what going on with your boat. If I think of anything I'll write you.

Good Luck - Mike


Different person:
I had a 'Copper' problem as well. Mine was Carburated and much easier to detct when the Carb had little reddish balls of dirt stuck in the jets.
I finally found the problem when I took the whole fuel line out of the boat. At the tank, when I pulled that pickup line up out of there, I saw the end of the copper tubing all corroded and pitted. I learn everything the hard way.


Different thread:
Budmike: I do have the suspect tanks after all. I found a few copper particles in the injectors, but only a very few. About two or three in five or six injectors (out of sixteen). I spoke to Florida Marine Tanks and they told me they have had very little problems out of them, but that the problems you described have happened occasionally. They told me they feel it is a bonding to ground issue, and it shouldn't happen so long as all connections are good and water in the gas is kept to a minimum. They did offer replacement plastic pickup tubes for me, but it's darn near impossible (as you know) to get to them. I may cut a small hole in the floor on the starboard side and build a frame for the new floor piece to sit upon. That way I should be able to get to the fitting. I haven't looked at port.

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