470 4 barrel conversion, one cylinder missing

[HorizonblueDK]

I have converted my 470 into a 4 barrel carb, since I had a donor motor with all the parts needed. I rebuild the carb, took all the time in the world to make sure everything was right, cleaned and blew out every passage inside, installed bushings on the primary shaft. But something is wrong, I'm not sure it's the carb...On the first test run on the ocean, in fired up and ran well. I waited until the temp was ok, before I started to accelerate. It went on the plane and I ran at 3000 rpm. After maybe 5 seconds, the engine started bogging-sounds like it was running on 3 cylinders, so I backed down. Trying to accelerate again and it did so, without backfiring, but still only 3 cylinders. So I humped back to the marina, on 3 cylinder most of the time, sometimes 4. It stayed alive, even when docking, on 3 cylinders. So after a break, I started it and it ran fine, took a little trip, but same story again. Before the test run, I replaced the spark plugs, since there was two different types in it.... I had some NGK BR6FS laying around, not new, but the looked ok, so I re-gapped them to 35 thousands, maybe a little more. Last year the engine ran fine, with the two barrel carb (and different spark plugs). Compression is around 120-140 psi on all cylinders. I suspect that I have an ignition problem, but I would like to hear opinions.

 

[Eric 3.7LX Alpha 1]

Can you put a temporary fuel pressure gauge inline just before the inlet to the carb to verify fuel isn't the issue? I actually had one for a while installed permanently using a 3/8" inverted flare tee and adapted the middle of the tee to a gauge thread, but the weight of the tee fitting and vibration of the engine caused fuel line leaks. Had to remove it. It was nice to see the pressure on the inlet of the carb if there's engine trouble, you can quickly eliminate a clogged fuel water separator or pump issue. Maybe a tee fitting and fuel pressure gauge inline between the fuel pump and carb inlet on some coast guard approved Type A fuel hose.

 

[nola mike]

Can you put a temporary fuel pressure gauge inline just before the inlet to the carb to verify fuel isn't the issue? I actually had one for a while installed permanently using a 3/8" inverted flare tee and adapted the middle of the tee to a gauge thread, but the weight of the tee fitting and vibration of the engine caused fuel line leaks. Had to remove it. It was nice to see the pressure on the inlet of the carb if there's engine trouble, you can quickly eliminate a clogged fuel water separator or pump issue. Maybe a tee fitting and fuel pressure gauge inline between the fuel pump and carb inlet on some coast guard approved Type A fuel hose.

I think it's usually easier to rule out ignition problems first, even though they're less likely. When it's misbehaving throw a timing light on each lead. Not 100% but it will point you in the right direction.

Also, hi @Eric 3.7LX Alpha 1! Haven't seen you around in a while!

 

[dubs283]

eric and mike have you pointed in the right direction, i would add that along with fuel pump pressure at ALL rpm you should check fuel system vacuum as well, no more than 2 in/hg.

a dropped cylinder is easy to detect with a cylinder drop test, pull ignition leads one at a time and check rpm drop is consistent with each cylinder.

replacing old plugs with used plugs is pointless, get new plugs. in fact it's in your best interest to perform a full tune up

 

[Scott Danforth]

here are the culprits for a carbureted engine missing on one cylinder

• bad spark plug - agree with above, get new
• bad spark plug wire - get new
• improperly installed spark plug wire - properly install
• bad distributor cap - either bad contacts or carbon tracing. replace cap, dont try to clean it other than for a quick test.
• bend distributor shaft - very rare, however can happen. this may pull the rotor away from the cap enough
• really low compression on the one cylinder - this could be a hole in a piston, bad rings, bad cam lobe, broken valve spring, damaged lifter, damaged push rod, bad valve, bad valve seat. to find this, you need to start with a compression test.

Notice, not a single one of those is fuel system related. if your looking at the fuel system, your barking up the wrong tree.

 

[nola mike]

Notice, not a single one of those is fuel system related. if your looking at the fuel system, your barking up the wrong tree.

Unless it isn't actually running on 3 cylinders (or misfiring on the same cylinder). A fuel related problem can make the engine run like crap and sound like it's missing.

 

[Scott Danforth]

Unless it isn't actually running on 3 cylinders (or misfiring on the same cylinder). A fuel related problem can make the engine run like crap and sound like it's missing.

missing on many cylinders can be poor fuel quality or improperly maintained points.

 

[shiloh87]

From what I learned while rebuilding my 1984 470, and from a local 470 guru, he advised me against doing the 4 hole conversion.
The high hp 470`s, 485 and 488 ran 4bbls, also had different head set up, ie push rods stronger springs and a lumpier cam, all to get 15-20 more pony`s and eat way more gas. The cams are next to impossible to find.
Not that this helps any just thought I`d mention it.

 

[achris]

From what I learned while rebuilding my 1984 470, and from a local 470 guru, he advised me against doing the 4 hole conversion.
The high hp 470`s, 485 and 488 ran 4bbls, also had different head set up, ie push rods stronger springs and a lumpier cam, all to get 15-20 more pony`s and eat way more gas. The cams are next to impossible to find.
Not that this helps any just thought I`d mention it.

You've been misinformed. The ONLY difference between a 470 and the 485 and 488 is the exhaust manifold and the carb. Everything else (head, valves, valve springs, camshaft, pistons, crankshaft, rods, flywheel, electrical system), all the same. Just go to any of the websites that list parts and check it out...

 

[nola mike]

missing on many cylinders can be poor fuel quality or improperly maintained points.

Exactly. My point being that "running on 3 cylinders" might not be the same 1 cylinder not firing, tough to tell without actually testing

 

[Scott Danforth]

From what I learned while rebuilding my 1984 470, and from a local 470 guru, he advised me against doing the 4 hole conversion.
The high hp 470`s, 485 and 488 ran 4bbls, also had different head set up, ie push rods stronger springs and a lumpier cam, all to get 15-20 more pony`s and eat way more gas. The cams are next to impossible to find.
Not that this helps any just thought I`d mention it.

You and your local "guru" are wrong

 

[shiloh87]

"You've been misinformed. The ONLY difference between a 470 and the 485 and 488 is the exhaust manifold and the carb. Everything else (head, valves, valve springs, camshaft, pistons, crankshaft, rods, flywheel, electrical system), all the same. Just go to any of the websites that list parts and check it out..."

All the various specs are listed in the real mercury marine service manual #3
and you`re right, all online parts are the same because there are way more 470`s the the other two.
Simply slapping a 4 bbl carb on a stock 470 wont give you any hp boost, it`ll only cause issues and excessive fuel consumption.
As mentioned before, I chose not to build my 470 to 485 specs because of the lack of real original parts. Every part in my engine are original OEM parts including gaskets, not Asian crap.
I`m 60 yrs old and have been building engines since I was 15, this is my first marine engine and found it quite challenging, but I got it done and am now struggling with getting it tuned in, but I`ll get it, but thanx for letting me know I`m misinformed.

 

[Scott Danforth]

Simply slapping a 4 bbl carb on a stock 470 wont give you any hp boost, it`ll only cause issues and excessive fuel consumption.
As mentioned before, I chose not to build my 470 to 485 specs because of the lack of real original parts. Every part in my engine are original OEM parts including gaskets, not Asian crap.
I`m 60 yrs old and have been building engines since I was 15, this is my first marine engine and found it quite challenging, but I got it done and am now struggling with getting it tuned in, but I`ll get it, but thanx for letting me know I`m misinformed.

the only difference is the manifold and carb. so yes, you can slap a 4 bbl on a 470 and get a 485.

nothing special about a marine motor except the core plugs are brass or stainless, the head gasket has a stainless core and the cam has very little overlap due to the wet exhaust.

if you want to rebuild, its GM inline bearings, ford FE motor (460) head, pistons, valve train and push rods, GM flywheel and a mercruiser only front damper and block

not even Mercruiser uses all mercruiser OEM parts except the block, crank and damper. the rest were sourced from GM and ford

 

[Eric 3.7LX Alpha 1]

There is also one other change the other guys aren't mentioning, but I'm not sure if it was only for the 4 barrel carb engines only. They upgraded the heat exchanger from a 3" diameter to a 4" one, which is what I have on my 3.7 LX with 4 barrel carb. I have the same engine as you except the last version of it made in '89. I've owned this boat/engine for about 20 years now. The 4 barrel option actually does very well on fuel as long is your not riding WOT all day. These engines with the large bore from the Ford 460 piston have a lot of torque. Mercruiser basically designed their own block around the Ford big block pistons and head.

One thing you have to be careful of with these engines, they do not like to get hot. If it overheats there's a good chance of blowing your head gasket due to the aluminum block and CI head. Have a good working temp gauge on board. I run a Mr Gasket thermostat radiator cap also, so I can check the temp while back at engine, and can confirm my helm gauge.

 

[achris]

Simply slapping a 4 bbl carb on a stock 470 wont give you any hp boost, it`ll only cause issues and excessive fuel consumption.

Correct. You need to change the EXHAUST manifold too.

I`m 60 yrs old and have been building engines since I was 15,

Me too.

this is my first marine engine and found it quite challenging, but I got it done and am now struggling with getting it tuned in, but I`ll get it, but thanx for letting me know I`m misinformed.

I've built more marine engines than I can count. And not just stern drive engines, I've also worked on and rebuilt DOZENS of outboards.

 

[shiloh87]

Just read manual #3, sect 6(B), engine specifications. I`m sure someone besides me has one.

 

[nola mike]

Just read manual #3, sect 6(B), engine specifications. I`m sure someone besides me has one.

Post a screenshot

 

[shiloh87]

Here you go from mercruiser service manual 3 sec 6(B) specifications
col 1 is 470, col 2 485, col 3 488
These shows the differences in lobe lift, valve grind angles, spring heights and pressures etc.

 

[HorizonblueDK]

Here you go from mercruiser service manual 3 sec 6(B) specifications
col 1 is 470, col 2 485, col 3 488
These shows the differences in lobe lift, valve grind angles, spring heights and pressures etc.
View attachment 340853View attachment 340854View attachment 340855

Ok, so there seems to be a slight difference between the 2 and 4 barrel internals, but should that really be enough to make my engine run bad ? I would assume that it was a matter of a little power loss. I actually have the camshaft etc from the donor motor, but right now I don't have the time to swap that also. So if a 470 really can't run right, with the 4 barrel only, the 2 barrel will go back on the engine. I will do one more testrun, before deciding......

 

[nola mike]

Here you go from mercruiser service manual 3 sec 6(B) specifications
col 1 is 470, col 2 485, col 3 488
These shows the differences in lobe lift, valve grind angles, spring heights and pressures etc.
View attachment 340853View attachment 340854View attachment 340855

Interesting. The later models appear to be different. The 3.7 and 3.7lx spec the same parts. From manual 8: