Greetings all. I have a 1987 Dodge D150 with a 225 slant six. The truck was running well until a month ago. It started running very roughly when under load. AT under 1500 hundred rpm it runs well. Over 2000 rpm, or under load it starts missing and losing power. I have replaced the gas filter. Could it be bad gas? Or perhaps an ignition problem. Any suggestions are appreciated.

Does this truck still have the lean burn system on it?

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Thanks for the reply. I'm not sure about the lean burn. However, I researched it a bit and learned that if it has lean burn, there should not be a vac advance on the dist. I'll check it today and find out. Thanks.

Some have a lean burn module on the air cleaner inlet and some have the system in behind the battery at the DS fender. I'm pretty sure your year truck should be over at the fender.

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

There are many things that might be the cause of this, but don't discount the possibility of an ignition system problem.

A bad coil or a set of (very) bad spark plug wires can do this.

You can sometimes see leaky spark plug wires sparking all over the place if you watch the engine run in the pitch-dark.

You may have a known-good coil on another engine that is easy(-ish) to swap out to test.

Fuel (such as a clogged main jet) is another possibility. A clogged fuel filter is less likely because if it's so clogged as to affect no-load revving above 2000 RPM, under load it will probably have trouble getting out of a parking space.

In the end it's systematic elimination of possibilities.

- Eric