Rebuilt 7.4L 454 Merc/Bravo 1 chokes out/backfires at 2200rpm

[bukowskj]

I bought my 1997 27ft Maxum with a 454 7.4L Merc, closed cooling/ oil cooling sytem boat from a friend after it overheated while idling waiting to put it up on the trailer; something went drastically wrong inside the motor, a load knocking (I was on the boat when this happened). I had the motor pulled by a local boat mechanic and brought it home for a full tear down to determine the problem. I sent the Carb to a local boat carb rebuild expert (rebuilds carbs for all the boat shops in the area, 15-30 a month. Once I had it completely tore down, I discovered that 3 main bearings had spun... at this point I had no idea why, but I figured no cooling or no oil cycling through correctly had to be the cause! I sent the block out, and the heads, to a local machinist and had everything boiled, and checked out! Nothing major wrong, so I had them replace all the main bearings, install a new crank, hone the piston walls, new piston rings, etc... whole lower rebuild! While I had it tore apart I replaced the cam, the lifters (hydraulic), rods, rockers, etc... installed a new thermostat, oil pump, plugs and starter as well. I also sent the closed cooling tank out to be checked out.. all good! I even did my best to clean any debris out of Exhaust manifolds... boiled the oil pan and all new gaskets! For the most part a complete rebuild!

I did not replace the water pump (cleaned and inspected - all good); distributor, cap, rotor (all looks good); plug wires (appeared fine); oil cooler (extensively cleaned inspected); hoses (cleaned inspected), sensors (??), etc... every thing appeared to be in good condition! I am pretty methodical and cleaned/checked EVERYTHING several times before moving on to the next piece during the re-assembly, the motor looked brand new!

I took the motor back to the boat mechanic and he was blown away how it looked after 10 days (I worked day and night on this motor hoping to get one good run in before winter hit) had him install it , put the break-in oil in, coolant and make sure it ran okay... he set the timing, and made sure everything was good... 3 weeks later he finally got to it and it cranked up perfectly!! Throttles great, very responsive, sounded perfect!! He told me I may have to adjust the timing once in the water, but it was close. Unfortunately snow hit that week, I was done for the season!! Winterized the BRAND NEW motor and had to wait to put it on the water almost 7 months!!!

At first ability to hit the lake this summer, I de-winterized it, charged the batteries and put the boat in the water!! I loaded up a few friends in the marina and headed out of the no-wake zone... just purring smoothly! Once outside the no-wake zone I started to throttle it up and once I hit 2100-2200 rpm (under a load) it began "choking out", "sputtering" and "backfiring"... I backed it off!! GGGRRRRRR!!! I tried several more times to see if it would blow through... nope!! NOT UNDER A LOAD I CAN THROTTLE IT UP TO 4000 rpm no problem...!!! Confused I tried it again under a load... same thing, 2200 rpms!!! I was upset... fortunately I had my timing gun with me, so I checked the timing and it was slightly off; I adjust the timing while on the water, under a load, since everything I had read stated the timing needs to be set at (+8 or +11, I cannot remember which one right now) at 1500 rpm... anyways when I did get it close to that, without adjusting my throttle, my rpms increased (due to advancing the timing as needed) but it never went over 1800 rpms... Tried pushing through 2200 again, same thing!! I have taken the boat out 2 more times since I had been told it may just need some "breaking in", still never blew past 2200 rpms or planed out!! I am puzzled!!!

So... I have the timing set, according to specs, idles about 600 -800 rpms, cranks up perfectly every time... oil pressure is perfect at idle and at 1500 rpms according to specs... I did not replace ANY sensors after the over heat before rebuilding the motor! I have been told to check a few things, but wanted other opinions before tearing into it further.

Been told to check:
Timing (all set - good)
Plug wires (all good - checked each one independantly, all sparky)
Fuel Filter/ water separator (brand new)
Vacuum advance (but there is not one on my distributor)
Check that secondaries are opening (not yet)
Sensors may be fried from over heat, sending a signal not allowing it to go over a certain rpms for safety, but doesn't make sense if I can full throttle not under a load??

I have read about checking fuel pressure, not sure if that's something I need to check or even how to do it?
I have read about checking compression, not sure if that's something I need to check or even how to do it?

Could the carb be adjusted wrong??

ANY ADVICE WOULD BE AWESOME!!!!
Thanks!!!
Jason

[pascavone]

i would guess, your running too lean on fuel and that always causes backfire on a carbureted engine.

So, it you can spin up to 4,500 rpm in neutral, then the next thing to do is rig up a temporary fuel tank and give that a try with a electrical fuel pump for $15 at autozone.

I would check your needle valve on the floats, and THE NEEDLE VALVE SEAT- a brass fitting that if lose will drive anyone crazy.

Lastly, If your carburetor to manifold gasket is leaking air, this will cause your engine to run lean.

Your problem is consistent, so outside of the distribute weights being stuck, and not letting the timing advance, I'd go with the fuel issues first.

[bukowskj]

Thanks pascavone for the reply!!

The carb was completely rebuilt, valves, float, even a new mechanical advance, new linkage arms, etc - everything brand new... that's all this guy does for all the boat shops in the area!!

So for the temp tank - what would doing this prove?
Do I use the same octane as normal (91)?
Can I just us a gas can outfitted with an electric fuel pump?
If I bypass the fuel pump, will gas spew all over the place as my fuel pump on the motor is pumping??

Is there a way to check if the gasket is leaking air? I put it on myself and tightened to specs? Clean surfaces, new gasket...?

[mmwjr]

Did you check to see if the choke is open when you throttle up?

[SRPATTON]

Was the carb rebuilt before or after winterizing? If it was before then I would open it back up and check it out. Make sure the spark arrestor is clean. The Mercruiser Manual also suggests check for stale fuel, look for a kinked fuel hose, bad fuel pump, etc. Don't assume all the ignition parts are in order just because it spools to 4500 with no load. It can do that with 4 cylinders.

[ss3964spd]

The advice you received for checking the timing "while under load" is, well, a load. The ignition system has absolutely no idea if the engine is under load or not. It takes very, very little gas and air to get an engine to spin to 4500 when it is not under load. Did the engine run and sound smooth until it hit 2100?

At this point it could be any number of things, most of which have already been mentioned.

You are correct that the distributor does not have a vacuum advance, and it isn't supposed to. But it should have mechanical advance so you need to ensure it is working properly. Hook up your timing light, note what the timing is at idle, then start increasing RPM's. If the mechanical is working you'll see the timing advance as RPM's rise. At some RPM the timing will no longer increase, maybe around 3000 or 3500 - but you'll have to find the specifications to be certain.

As has been said be sure the choke is fully open when the engine is up to operating temp. Double check your fuel line routing and if you haven't already replace the fuel filters. Yes, you can check fuel pressure by installing an inline gauge. I am assuming the carb on the engine is a Quadrajet - they do not like fuel pressure much above 5 or 6 PSI. How old is the gas in the fuel tank?

Does your particular 454 use roller lifters or flat faced tappets?