Intermittent loss of spark?

[Javin]

I have a 1981 gmc 3/4 350 4spd this morning I was driving in 4th gear and I let of the gas and it made a massive back fire, so then I continued on and I noticed it would drive fine but the rpm would go up hold steady, but I would let off the gas and for a split second the rpm needle will drop as if it’s losing power or spark? I replaced the distributer a year ago in just wondering if it could be something with the weights in the distributor when it loses vacuum from being on the throttle to no throttle? It idles fine and starts fine? And it doesn’t have this issue every time I get off the gas it’s more intermintent if I’m at really low rpms cruising and let off the gas it has died completely twice, but fires right up

 

[HotRodPC]

Carbed right??? Usually when you get backfire when you're moving and let off the throttle it can be a lean carb adjustment situation. Seems it would do it a bit more often. Another condition that can cause that is a burnt valve but I'd think you'd have other symptoms to go along with it if that were the case.

You might ritchen the carb the screws 1/4 turn each and see what happens and if that fixes it. Checking the timing and timing advance wouldn't be bad either for s&g. Past that, then I'd get a vac gauge and watch to see what it's doing.

 

[Javin]

Yep it’s carbed the reason why I was thinking it’s ignition it the rpms drop by 1000 for a split second and there’s no way that’s actually happening considering it’s still in gear and I’m not slamming on the brakes?

 

[1987 GMC Jimmy]

I’ve had a similar problem before, and it was related to spark advance. Of course, the cause of this was a fried ECM not doing what it was supposed to, but you should be able to vacuum test the advance canister and the guts of the distributor. A burnt valve should show up on a vacuum gauge. My money’s on a lean condition for this one. Do you have good gas and a good idle vacuum reading? A bad leak, say at the carb mounting surface, can cause stuff like this.

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[Javin]

Thanks for the info I’m defiantly saving that chart, so I took my cap and rotor off and sure enough they were burnt pretty bad on the contact points so I replaced those as well as I check the plugs and ended up replacing them, and it seems to fixed the issue completely I gapped the plugs at .35 but now reading up realize .45 would be better

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[1987 GMC Jimmy]

I'd say rust, not scorching, was your issue with that.

 

[Javin]

Yeah could be regardless was pretty happy I found the issue and she’s running good what do you think of the spark plug gaps?

 

[1987 GMC Jimmy]

I think .045 is more appropriate. HEI spark is a lot better than points spark, which needs a closer gap like that.

 

[Javin]

Yeah makes sense I’ll pull the plugs today and open them up to .45