Trojan Boat Forum

Looking for suggestions here or folks that may have had issues in the past.

I have twin 255 Mercs (350ci GM) in my 1975 Tri Cabin. I've been chasing a problem with the port engine not coming to WOT RPM. She only spins up to 3100-3200 RPM tops. Here are the details and where I set with the issue.

1. Starboard engine runs to 4000 with no issues. Both props are balanced and stock, 20x22.

2. Shafts are new and balanced.

3. Both are V-Drive with Velvet Trans

4. Distributor was replaced, still same problem with brand new distributor. Timimg is set for 6 degreed BTDC as Merc manual suggests.

5. Carborators rebuilt.

6. Fuel Filters Replaced. There is a shut off valve connecting the two fuel tanks. I opened it thinking maybe an issue with the pick-ups in the tank. Nothing changed.

The engine in question is new, less than 100 hours on it, long block replacment. All new, however all external items were put on from the old engine. Like distributor which had some minor rust on the weights and springs so I replaced it.

Now i'm down to fuel. It has to be. I've checked the throttle linkage and its wide open when at WOT. I have not checked it while underway with the flame arrester off to see if the vacume secondary is open. But the carb has been professionally rebuilt. I have doulbe diaphram fuel pumps and I have no fuel in the glass bowl but i wonder if its delivering the fuel. My merc manual says to do some testing on volume and pressure which i will figure out how to do without making a mess.

Says it should pump 1pt of fuel in 30-45 seconds, to disconnect at the carb, start it and run the fuel into a cup and check it. To check pressure, disconnect at the pump, insert pressure gauge and check, should be 5 1/4 to 6 1/2 PSI at idle to 1000 rpm. I'm just unsure about doing this part of it and I'm thinking of hitting up a pro.

Its frustrating that a virtually new engine is having issues turning up to RPM. I know its not a TAC issue, i have an engine sync and they are truely in sync at 3100ish, starboard engine has an other 1/4 throttle to go and it takes WOT throttle of the port to get 3100.

Thoughts? Ideas? Think its fuel? Anyone seen anything like this? It runs like a top, no spitting or ruffness at all, just ends at 3100.

Marine distributor installed? Only 6 deg BTDC? Did you check total advance, does it fall within spec? Just setting base timing is not enough, you need to ensure total advance is being achieved so you can eliminate ignition as a probable cause. Manual should have a TA curve you can reffer to.

compare the advance with an Advance timing light"

Could be a prop ding?

If you were in salt water I'd flost the idea of growth in one of the exhausts also.

Take the throttle cables off and see if the carb opens farther.

RWS

Yes replacement new from mallory, marine distributor. There is a power curve in the manual but no marks beyond 4 atdc. Says it should advance 24 degrees. I had someone suggest timing at WOT. Advance to max RPM and then back it off a bit. I've never done this.

The curve should state 24 deg at a given RPM. You need to ensure it's achieving that. Use a gun with an Advance feature. If you don't have one, measure the diameter of your balancer, and go to a good auto parts store and ask for a degree tape to fit that diameter. Tape it around the balancer and you have your markings for checking TA with a standard timing light. Actually better doing it this way as there is no chance of electronic error that you may get from a gun with an advance feature.

When you confirm you're achieving TA, then you can move on to something else as a possibility. TA is something you should confirm on a new distributor install anyway.

did you replace the distributor weights, and springs, with the same weight/tension replacements?

Can you do this advanced timing test at the dock or do you need to perform it under load conditions? The engine will rev go 4k in neutral without problem it's only under load does it fail to come up to rpm.

On the weights and springs. It's a whole new Mallory distributor, weight, springs, and points were already setup and installed. I dropped it in, wired it and timed it with no change in performance.

http://catalog.mallorymarine.com/Produc ... fault.aspx

i have just installed 2 new mallory ylm579av units in my f32 and my wot underload is basically the same. i did know to expect this as i have debated on the purchase and spending the money for them however summit had good enough prices for me. anyway i have chrysler 318's in mine. i had read many folks had to change the springs because they were not getting full advance after installation so that was what i figured i would be doing also. after my installation i set mine at the 5degrees btdc and checked my advance i am only gettting about 19 at 3000rpm. i have already purchased the spring kit and will change them just havent done it yet, the spring kit i have came with instructions on setting up the distributor with the correct springs for any application. sure it would have been great for the distributors to come ready to drop in but after reading so much on the internet about them not, i was okay with it.
check the advance at the dock at around 3000 rpm's see what it is. if its not what the specs say then you will know that the dist is not advancing enough

Personally, I would not wind her up that much in neutral. I don't like going any higher than 2200 - 2500 RPM in neutral. Not good to rev them that high with no load. Best to take her out to go that high with RPM to check total advance...IMO

I would be careful about using advance kits , to much advance and you could cause a spark knock ( you may not hear it ) and cause piston damage .

The specs of total advance maybe a lot different from marine to car/truck .( they use EGR that reduces spark knock and may allow more advance )
where are you getting the specs ?

Mercruiser engine manual. 22 to 26 degrees at WOT it says. 24 would be in the center of the range.

I will check this out this weekend and report back. If its off, i will have to order a weight and spring kit. I sure hope this is it.

Well I thought i would post a message with updates. I'm about at the end of my "trial by error" and going to call a mechanic.

I've tested the advance with timing tape, all is well, at 3k she advances 24 degrees as it should based on the engine manual. I even power timed it at WOT, slight advance, then retard getting the engine at MAX RPM, still nothing changed. At idle after power tuning it was at 8BTDC, 6 is the stock setting based on the manual.

I've replaced all wires and plugs, they all appear to be burning OK. I did a pressure test while changing plugs, letting it turn over and compress 4 times per cylander. Pressure should be 150psi new, well the lower end is new but the heads are not. I found most at 130psi, and 2 lower, one at 105 and one at 110, both on the left bank (center two cylandars).

So thoughts are maybe the heads were damages or valves are not seating properly. The engine seems to run rich at WOT, i figure due to not being able to get RPM. Just has me puzzled. In the long run it is a cruiser and i can still cruise around.

I would say props right, but I know based on having them both redone last winter they are stock 20x21 RH/LH and the one engine has zero issues. so they are right.

Thats all i have, done messing with it for now, when I'm ready i will have an engine expert come out and check it out.

Thanks everyone for your suggestions.

You have checked the tachs with a photo tach right ?

1) timing is set right
2) you are getting the right advance on the distributor
3) without load you are hitting desired RPM, so throttle cable is probably okay
4) you said that your carb is wide open at WOT

Fuel issues? You should have seen some symptoms of that with the old engine.

So... brand new engine? My money says whoever did the rebuild (or build) f'd up, and the block or components they used (possibly not marine) just doesn't have the balls it should have. I'd be telling the manufacturer/rebuilder it is a warranty issue and they need to either get it working right or refund my money. Just because it's new doesn't mean it was built right. Even if your low compression on a couple of cylinders is related to heads and unrelated to new block, I don't think low compression should account for 25% reduction in overall output.

prowlersfish wrote:You have checked the tachs with a photo tach right ?

Well no, but i have an engine sync and I know both by sound and the sync light its in sync with the other engine. Port is WOT, starboard is 3/4 throttle.