Backfire on startup (1 Viewer)

[CruiserTrash]

1983 FJ60, stock smogged 2F, daily driver, runs well every day, no history of backfires, not ever. This morning I ran an errand for 30 minutes, then let the truck sit for two hours. It’s now 90*F outside with 31% humidity. I’m at 5300’ elevation and the truck

I went back out after the two hours to go somewhere else and got one small backfire at startup, followed by a couple misfires while I was letting the truck warm up. No smoke from the tailpipe so I’m guessing it was in the carb. No raw gas smell. I just drove the truck for 20 minutes and it ran as it always does, no more misfires when I arrived at the destination and let it idle. Cabin gauges: 17inHg, 12.5 A:F, 190*F coolant temp. About once per month I get a misfire or three when letting the truck warm up.

Thoughts?

 

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[CruiserTrash]

View attachment 3059804

I know better than to post and not check the FSM first. Shame on me haha!

 

[Spike Strip]

Where was the backfire? In exhaust or through carb/intake?

Before you do your next cold-start (on a warm day), check the sight-glass on the front of the carb to see if the float bowl has emptied.

 

[CruiserTrash]

Where was the backfire? In exhaust or through carb/intake?

Before you do your next cold-start (on a warm day), check the sight-glass on the front of the carb to see if the float bowl has emptied.

In other words the fuel might have leaked into the intake manifold and ignited?

 

[Spike Strip]

In other words the fuel might have leaked into the intake manifold and ignited?

Yes. Backfire is unburned fuel igniting either in exhaust or ignited fuel vapor escaping past the intake valves because of late timing.

A too lean mix can also cause backfire in some cases, also. A lean burn is slow, and that can lead to unburned fuel out the exhaust.

Try cranking the engine WITHOUT the choke on for a five seconds, then start normally and see what happens.

Have you checked your timing. And timing on cold starts ?