Carbureted head on a Fuel Injected D15A3/EW4 block
 

I've been working on an '86 CRX that I bought used a couple of months ago. It was idling weird and overheating, so I did a leakdown test about a month ago and found a very leaky cylinder 4.

I've been slowly disassembling it to take the head off and yesterday I took off the valve cover. When I did so, I noticed that this head has auxiliary valves...that's when I realized that one of the last two owners put on a D15A2 head, which is carbureted. I assume my engine is a D15A3. (The chassis says EW4 or EV4, I forget, but I can't find that engine number anywhere). In any case, I found this amusing since the carbureted head has the spark plug located in a pre-combustion chamber, which could definitely explain the weird idling. I also know from personal experience and research that the D15A2 naturally has uneven hot spots, which has caused it to crush head gaskets. That could explain the leaky cylinder.

I'll try to upload pics tonight or tomorrow. I'm just so glad that I asked the last owner for all the extra parts he had from what he did to the car. He gave me the old head (which doesn't have aux. valves), so hopefully I'll be able to swap out its bent valves and use it without too much trouble.

RE: Wrong head on a D15A3(?)
 

In the included pics, the first shows the intake manifold with the fuel injectors, showing this is a fuel-injected setup.

The second pic is of the "old" head that the last owner gave me. You can see 3 valves per cylinder.

The third pic is of the current head, where you can see a 4th smaller valve--this is the auxiliary valve used for the carbureted CVCC design.


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RE: Wrong head on a D15A3(?)
 

This is after I pulled the head off.

The first pic is of the current head--there's a small hole near the exhaust valve (the largest of the 3 valves). Normally, you'd see silver-colored threads, which would hold the spark plug. It's dark and smooth on this head, because that hole's a passageway to the precombustion chamber, which houses the spark plug. The spark ignites a rich mixture in there which propagates to the main combustion chamber and ignites a leaner mixture in the main combustion chamber. This 2-part combustion made the engine burn less fuel than many other carbureted engines at the time. The precombustion chamber should be getting a richer fuel-air mix than the main combustion chamber. In my case, the fuel injector didn't distinguish between the intake for the pre-combustion chamber and the main combustion chamber.

The second pic shows threaded holes which hold the spark plugs. This is the fuel injected design. Honda got rid of the pre-combustion chamber with fuel injected cars. The combustion chamber looks about the same, but because you don't have the precombustion chamber, the behavior combustion is very different.

I put up this thread to show why you don't want to mix a carbureted head with a fuel injected engine, even though they might bolt together. My situation shows why you can't do this for Hondas, but maybe you can for other makes. Perhaps this is why the last owner did this--maybe he did it for another make and it worked all right.

The engine ran well enough to drive the 40 miles from where I picked it up. It idled really poorly though, jumping from 1000-3000 rpm until it warmed up. It also had an overheating problem, which probably came from burning the lean mixture in the precombustion chamber.

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