Backfire under load

[70GMC396]

OK, need some help - think it is just an old carb in need of some TLC. It is a 4MV on my 402 auto 70 GMC. Bogs down quickly and backfires each time I try to accellerate on the freeway about 80% to WOW - don't think it is WOW only, but could be. I let up and smooths out, but will bog down and backfire again if I try to recover. Can really only accellerate at about 50% throttle most the time reliably. It hasn't always been this way - father-in-law owned it since 1980 and confirmed that, but I resurrected the truck after it sat for a few years. I am due to replace the fuel filter at the carb and a rebuild wouldn't hurt as it heavily varnished and leaky. Any thoughts on what to check first? Also a vacuum powered valve towards and on top the left front of the intake (anti-back fire vlave or something else???) has a loose electrical connector - looks expensive to replace - could this be related? THx

[bryanw1968]

Sounds like a lean back fire. If it was a rich situation it would just bog?

[70GMC396]

Well, I'll start with the filters tomorrow. Maybe it is something simple in fuel delivery. Does seem boggy before it backfires, but i'm no expert.

[cdowns]

i'd start with the typical tune up points rotor cap sparkplugs plug wires air and oil filters pcv valve set the dwell, plug gap and timing also checking the advance also checking vaccume hose for cracks and leaks to make sure of a solid base b-4 doing any carb rebuilding

[bryanw1968]

Originally Posted by cdowns

i'd start with the typical tune up points rotor cap sparkplugs plug wires air and oil filters pcv valve set the dwell, plug gap and timing also checking the advance also checking vaccume hose for cracks and leaks to make sure of a solid base b-4 doing any carb rebuilding

Good point. Do a healthy tune up first and then see where you are.
Stuff like this can be SO hard to figure out. I just spent the last 10 days chasing down a problem on my '68 and it turned out to be a bad ignition wire.
I didn't find it before I did the following:

new cap
new rotor
rebuild carb
replace plugs
tried different coil
tried differnet ignition module
beat my head on the garage floor

ALWAYS check the simple things first!!!

Good luck
BW

[70GMC396]

Probably should have stated this in the beginning - all those things are new as I just upgraded today to HEI (the rest are recently replaced, new or in good working order) - the important part is that the engine has always done this. That makes me think carb/fuel. The fuel filters will get double checked first thing tomorrow. I replaced all the vacuum lines on the truck recently just as a precaution - some needed it, some not.

[70GMC396]

Replaced the carb fuel filter and removed the TCS vacuum thing and rerouted to the lower carb vacuum port only (full manifold vacuum - none at idle but comes on rapidly after that). Backfire gone. Hmmm...dirty filter = lean at high flow? Any thoughts? I certainly wasn't getting any vacuum assist off the high (above butterfly) port. I ran the throttle up to test the different amount of assist to the dist. but there was NONE (zero rpm change when attached) from the upper port - that seemed wierd. So didn't really have a choice but to use the lower front manifold port. It still boggs the same at high throttle though, but no longer backfires. How high is vacuum supposed to assist anyway? Not enough to effect anything towards WOW right? thx.

[PanelDeland]

I had a bad vacuum advance module in the distributor.It acted somewhat the same way.but was a 6.

[jhow66]

Your time is too low. Check it with vacumn line off dist. Plug off the rubber line.

[CrashEd]

I had that on my blazer and i found it to be my shooters in the carb was plug up a little.Look down the carb with the motor off and work the throttle and see if the 2 streams are the same. One of mine was like a flat spray i cleaned it with a torch tip cleaner and it was fine after that.One good thing though i have all new parts shouldnt have to do another tune up for a while now i replaced them all looking for the problem.

[hvychve]

by chance , have you checked engine vacuum, to see if steady ? without hearing the backfire, sorta difficult to determine if its a lean pop or a crossfire(water in cap or etc) or a valve issue, check your vacuum and verify its steady around 16-20 inches, well, if you dont have a big fat cam in that is.
raise your rpm and verify its still steady, you could possibly have a cam going flat or a lifter or spring issue , could also give you a pop, usually through intake. good luck. sounds like you got most all the other bases covered, Dwell set correctly if still points, timing? ? not too lean?

[VetteVet]

Here's a couple of easy things to check.
1. With the engine off and the air cleaner off and the choke open, open the throttle quickly and you should see a fuel spray down both venturis. If not then the accelerator pump is bad and the old rubber cup needs replaced, or the rod is out of adjustment.
2. close the choke about 1/4 and try driving the truck. If the situation improves then it's running lean and has probably got some varnish in the main jets and needs a thorough cleaning.
timing may be an issue also as was mentioned. Good luck

[70GMC396]

That makes sense jhow66 - i have it at 4 but I'll move it up to 8 - although I would think it would be much more than 4+ degrees to bog down this much. I'll give it a shot and see if that helps. thx everyone - I'l check the carb while I'm at it.

[chops333]

i have seen vaccuum advance not working do this also if you put in used hei replace the vac. advance

[70GMC396]

Backfiring went away with HEI upgrade. Now it only boggs down. Looking into carb testing and fuel supply.