Mercruiser TBI Conversion

[RD Durham]

I'm in the planning stages of adding a single barrel TBI to my Mercruiser 485 four cylinder engine in my boat. It’s called a 485 to distinguish it from the earlier Mercruiser 185 that is essentially the same engine, with both engines rated at 185 HP at sea level. The engine was a project between Mercruiser and Ford and uses the head off of a Ford 460 V8 engine along with some other parts. My initial thoughts are to use a single barrel TBI off a Chevy 2.5L four cylinder engine. The Mercruiser 485 is a 224 cu in so that should work out to 3.7L. I'm not sure if there are any other TBI systems that would also work that are for larger displacement four cylinder engines than 2.5L. My plan would be to use a 7747 ECU as I am familiar with that system.

Right now the Mercruiser engine isn’t running though hopefully it will be sometime this month. I had to replace a piston head, head gasket and exhaust elbow about 9 years ago and other issues kept getting in the way of getting the boat back on the lake. The Mercruiser 485 has a single barrel carburetor and originally came with a points distributor that does not have a vacuum advance. I’ve never checked to see if it might have a mechanical advance in the distributor. The distributor was converted to a Pertronix unit and works fine. The reason to convert is better cold starts, and running at altitude. When I fish I like to hit most lakes really early in the morning and in Colorado at 8000 ft, give or take, it can be pretty cool in the morning even in the summer. In the past when first started it would run really rich until the engine warmed up. I also fish or take the kids & grandkid water skiing/boarding in lakes that are closer to 3500 ft or less so it would be nice to have the TBI so it will self adjust.

Any thoughts?

[Bill usn-1]

Dale,
Should be a fairly simple conversion except for the O2 sensor bung, but even that is doable on the merc.

I was working on sterndrives and inboards when that engine came out.
I would use the 2.8 v6 or the 4.3 v6 2 barrel TBI running the regular 1227747. Better to run the 2 barrel with the 7747.

An alternative would be the 2.5/3.1- 1 barrel TBI from the S/T 10 running the older/more common 1227165 ECM.

I am currently completing the development of the IH 4cyl kits with all 3 of these.

It may be possible that one of the adapters I am having machined might just work on yours.

The distr should be converted to a duraspark or mopar pickup. It will have mech adv but boats do not run vacuum adv since they are always under a load and have no vacuum. No around town driving with them!.
You will need to lock out the mech adv.


The O2 sensor can be installed with an adapter thru the large pipeplug in the side of the elbow.
i just looked them up a few months ago and IIRC it's just a NPT to 18mm thread adapter.

The 2 barrel TBI uses the same spark arrester size as a 4 barrel so that's easy.
ECM will need to be mounted in a sealed tupperware container or plastic box.

All EFI hose or hard lines ran.

1 barrel






2 barrel







There's probably others out there but here is one from Howell.

[RD Durham]

I will probably be talking to you at least about getting an adapter once I decide to go with a two or one barrel TBI.

I was wondering about the O2 sensor location. Pretty much everything is easy to get to and I have several exhaust plugs that I should be able to pull out and use for the O2. What I was wondering is that the raw water for the heat exchanger goes out through the exhaust and through the outdrive. Would that effect the the O2 readings too much having the exhaust gases mixed in with water?

Did you ever get your own tach filter made up?

[Bill usn-1]

The water dumps after the elbow rise so there is no water in the exhaust at the top. The elbow is designed to keep the water line below the top so it can't back up into the engine.
Best to run a 3 or 4 wire O2 so it is heated but it could still work fine with a 1 wire and open loop idle.

No tach filter. I just don't see the point in fuel only when timing control is so easy.
That project is low on my list.

[RD Durham]

I wonder how you could do timing control on a Mercruiser or if it's even worth the effort.

[Bill usn-1]

I don't understand the question.
A simple conversion to the distr to a magnetic pickup like the duraspark or mopar and just connect it to the ecm just like in the FAQ's here.

What style distr does ituse?
Post a pic of the outside and inside.

How was it converted to pertronics?
What kit did it use?
Was it specific for the merc or was it for a ford or chevy application?

[RD Durham]

I'll post some pictures in a couple of weeks. It is stored out at my brother's place in eastern Colorado and it's about a 45 minute drive and I have some family matters to take care of for the rest of this week. I won't be out to work on it until Saturday of next week.

The manufacturer of the distributor is Delco. The Pertronix unit is 1146A and is used in a number of other engine applications.

This is a link to an eBay distributor that is the same:

eBay Mercruiser 485 distributor

or this one:

eBay Mercruiser 485 distributor

Saturday of next week I'll check the internals. I just wasn't sure for TBI if any of the electronic distributor conversions would work or not for spark control. In the next few months I'm hoping to finish out the TBI on my Terra with spark control so I'll have a tach filter that I could use on the Mercruiser if needed.

[Bill usn-1]

Looks pretty straight forward with the flat plate.

If you have a tach filter to use then that may be easier to use to get it running and then decide if you want to change it.
Not a whole lot of timing curve to go from idle to WOT up on plane in a boat.

[RD Durham]

Originally Posted by Bill usn-1

Not a whole lot of timing curve to go from idle to WOT up on plane in a boat.

That's what I was thinking when I stated that I wondered if it was even worth it to do spark control on the Mercruiser.

[RD Durham]

I found the spare distributor that I have for this engine. It's for an older engine (170) but the engines are basically the same. The distributor does have fly weights for a mechanical timing advance. Would there be any benefit to adding spark control versus just adding the tach filter?