Port engine fuel usage.

aweimer · Post by **aweimer** » Thu Aug 12, 2010 11:06 pm

I thought I would post this here for comment.

First a couple details. She has twin 255hp mercs, 350 chevys. The port engine has had a long block replacement the year I purchased her. On the initial trip down to lake Erie Ohio, about a 80 mile trip the port side used almost 1/4 tank more gas than the other. This season I have had some issues with the port side stalling at idle while in gear. When starting after the stall you can tell she stalled from running too rich from the cloud of unburned fuel. This last week I tracked the fuel usage on the 46 mile round trip to put in bay, the Port engine used about 19 more gallons of fuel. Also the port side struggles to come up to full rpm at wot, only about 3400.

This year the props and shafts were replaced due go hitting submerged log in the maumee river. So i don't see props or shafts causing RPM issue. I suspect the carburetor, I adjusted it and still have issue, however the adjustments are only for idle really as I understand it. This weekend I removed the carburetor and have it off getting soaked and rebuilt.

What are you thoughts? Could a warn out carburetor really cause that bad of fuel usage? The starboard engine runs like a top, little lower oil pressure being 35 years old but still a solid 40psi at idle and under power. She also spins right up to 4000 wot, however I have never kept it there for more that a few seconds because the port side cannot keep up. Also to note the port side doesnt run ruff or anything under power. I have had to increase the idle a bit to keep her running at idle while in gear.

I welcome your input.

aweimer · Post by **aweimer** » Thu Aug 12, 2010 11:43 pm

I feel like the carborator is the root of the issue, however i have had some folks tell me that poor fuel economy could be a bad valve too, which could also impact the WOT RPM.

I guess i will see when i put the rebuilt carb back on.

rooferdave · Post by **rooferdave** » Fri Aug 13, 2010 8:53 am

I would have suggested a compression test before the carb rebuild, it still is not a bad idea and it is a good idea to change your plugs when you do this as they are out already

turtlem1969 · Post by **turtlem1969** » Fri Aug 13, 2010 9:42 am

Aweimer- what type of performance do you get with them 350's in your tri-cabin, other then the one is messing up right now, was thinking of putting them in mine when it came time.

aweimer · Post by **aweimer** » Fri Aug 13, 2010 12:05 pm

rooferdave wrote:I would have suggested a compression test before the carb rebuild, it still is not a bad idea and it is a good idea to change your plugs when you do this as they are out already

I have not done this however I do have the invoice from the long block replacement spring 2009. So the whole lower end, pistons and such are all new.

aweimer · Post by **aweimer** » Fri Aug 13, 2010 12:09 pm

turtlem1969 wrote:Aweimer- what type of performance do you get with them 350's in your tri-cabin, other then the one is messing up right now, was thinking of putting them in mine when it came time.

Runs fine. According to the data I received with my owners manual in 1975 the 255hp mercs were an option replacing the 318s. With slight trim she rides on top easily at 2400 rmp and 16mph. Since the one side will not come up to wot rpm she does 22mph at 3200 rpm. This is with standard bronze 20x21 props.

k9th · Post by **k9th** » Fri Aug 13, 2010 2:49 pm

My Tri-cabin has the same twin Merc 255s and I cruise at 18-19kts at 2900 RPM and burn about 24GPH total.

turtlem1969 · Post by **turtlem1969** » Fri Aug 13, 2010 5:20 pm

So that explains the 240 gals worth of fuel tanks in em.

captainmaniac · Post by **captainmaniac** » Fri Aug 13, 2010 6:23 pm

It makes sense for there to be some difference in fuel consumption for each engine, but not that much... I burn about 10% more on the port than I do on the starboard.

Big D · Post by **Big D** » Fri Aug 13, 2010 8:14 pm

Check the simple stuff first. Make sure the port and starb tanks are isolated; if you have a "crossover" valve in the fuel line, make sure it's in the closed position. This will give you a true picture of an engine's fuel consumption over the other. The unburned fuel could also be an ignition issue; it would also explain not being able to achieve rated WOT.

jimbo36 · Post by **jimbo36** » Fri Aug 13, 2010 9:07 pm

I have the same problem with my starboard engine, 400 chrysler. lags behind and won't go over 3000. I live with it because I don't really need more speed. It just bugs me

and sometime makes it hard to syncronize the engines. My carbs are fine but I am suspicious about the distributor calibration/advance. Can this be checked at an autoelectric shop? Anyone? Jimbo36.

sehord · Post by **sehord** » Fri Aug 13, 2010 9:47 pm

Several things. First, you already have the carb out so replace it and see what happens. If no improvement, Pull a single spark plug and take a look. If your running that rich it should be soot black (pull a plug from the good engine and compare if your not sure). Second, pull the rotor cap and look inside for any scratches, arc marks or moisture that may be trapped inside. Next I would buy an innexpesive timing gun and check your timing. one of those should get you running ok, with the notes tha the compression should be new.....

Big D · Post by **Big D** » Fri Aug 13, 2010 9:58 pm

Jimbo, if you have a timing light with an advance feature, you can check the distributor yourself. You'll need to know what the TA (total advance) for your engine is. Get the advance curve for your engine. It will tell you what your advance should be at a given RPM. This is more important than base timing which is what most people seem happy enough to settle for. Not only will improper TA reduce performance, it can also damage your engine.

RWS · Post by **RWS** » Sat Aug 14, 2010 8:47 am

I had a similar problem with 454's.

Confirmed the problem with flow scans. One engine burning 5GPH more than the other at certain RPM's.

Swapped the carbs from one engnie to the other, the problem followed the carb. (quadrajets)

Found the issue - one carb has THE WRONG METERING RODS.

Swapped the metering rosd for the correct ones - PROBLEM GONE.

Swap your carbs first and see what happens.

RWS

g36 · Post by **g36** » Sat Aug 14, 2010 10:39 am

my chrysler 318 stbd side was not reaching above 3000 rpms either i pulled the top of the distributor and found the advance was frozen solid lots of rust and after freeing this there were no springs left. definetly a problem. i have the original distributor and there are reportedly no springs for it so i went to autozone and bought a spring kit and played around with the different weights and now the motor gets apx 4000 rpms and the throttles are almost equal now at cruise where before they really werent close. i would be thinking about iginition as a possibility as others have stated.