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Virtual Wrench
Virtual Wrench, Chevy Mechanic
Category: Chevy
Satisfied Customers: 14618
Experience:  35 years Master Mechanic
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Chevrolet K5 Blazer 4x4 (Full Size): I have a 1985 K5 blazer

Customer Question

I have a 1985 K5 blazer 305. on acceleration it bogs down, tries to die. and if I don't let off the pedal, backfires.
Submitted: 1 year ago.
Category: Chevy
Expert:  Virtual Wrench replied 1 year ago.
Hello,
Is this an 85 or 86 model....you listed both and the 85 has a carburetor and the 86 has Throttle body injection (looks like a carb but has injectors on top of the throttle body.
So,,,,do you know if it has a carb, or throttle body injection (TBI)
Customer: replied 1 year ago.
sorry it is an 86.
Expert:  Virtual Wrench replied 1 year ago.
Thanks,
So when you take the air cleaner off do you see 2 round injector nozzles or does it look like a regular carburetor?
Expert:  Virtual Wrench replied 1 year ago.
If you are not sure, there are some pics.
This is TBI
https://dl.dropboxusercontent.com/u/33691145/tbi.PNG
This is a carburetor
https://dl.dropboxusercontent.com/u/33691145/carb.PNG
.
Customer: replied 1 year ago.
carburetor 100%
Expert:  Virtual Wrench replied 1 year ago.
Thanks!
The problem with what you have described about the hesitation, bog down and back fire are from what we call a lean misfire and this is only caused from a couple of things. If the timing is not advanced enough it will happen, and if the accelerator pump inside the carb is worn it wont have a good pump shot stream....both of these will cause these issues.
So,,i recommend having the timing set to about 8 degrees BTDC first, and if it still has the same issue, the carb will need rebuilt.
No i am assuming there are no vacuum leaks, because if there were, it would also cause a lean misfire.
LJ
Customer: replied 1 year ago.
tried both advancing and retarding the timing. exact same result. checked the accelerator pump by warming up the truck and manually putting the level to wot with the truck off, looking down the inlet. can hear an nice steady squirt and smell the gas.
Customer: replied 1 year ago.
bog rev bog rev bog rev bog bang
Expert:  Virtual Wrench replied 1 year ago.
Hi,
When you check the pump shot you cant just throw the linkage to WOT because it will give you a false result. You have to apply the throttle slowly, just like you were driving it and make sure BOTH streams are squirting a full shot and not "spitting"
Customer: replied 1 year ago.
I don't see any squirting or spiting. just looks like its getting a little wet down there. if that makes ANY sense
Customer: replied 1 year ago.
but I also don't know exactly where to look either. can really get my head straight over it to full see down in there
Expert:  Virtual Wrench replied 1 year ago.
Yeah, it's hard to see down there. But if you shine a flashlight down there you should see 2 streams pretty easy. Even with just daylight you should be able to see some gas squirting, but if you dont, then its like i said, the accel pump is shot.
Let me see if i can find a pic of where you will see it or not....be right back.
Customer: replied 1 year ago.
I should see a steady stream at idle right?
Expert:  Virtual Wrench replied 1 year ago.
Hi,
You want to check it with the engine OFF! You dont want your face there if it backfires!
You should see 2 steady streams, one for each half of the front of the opening. Make sure the choke plate is open and look straight down while you move the throttle linkage....you will see something for sure.
I couldnt find and good pics that show the pump shot streams but here is a little trick you can try and see if it helps.
https://www.youtube.com/watch?v=GMLVVnBMQ1k
Its a 2 barrell carb but the pump is the same as your 4 barrell
Customer: replied 1 year ago.
yes I saw two steady streams. will slowly moving the throttle linkage
Expert:  Virtual Wrench replied 1 year ago.
Then the timing vacuum advance could be bad, or the timing is still set to low.
What altitude are you at? it makes a diff for the timing setting.
Customer: replied 1 year ago.
About 1000'. Here in San Antonio, TX.
Expert:  Virtual Wrench replied 1 year ago.
Thanks,
Does it have a silver vacuum advance on the side of the distributor, or a 4 wire connector (EST timing)
Customer: replied 1 year ago.
silver vacuum. pretty sure its the MSD HEI
Customer: replied 1 year ago.
after reading some articles. seemingly better way to describe the issue is Hesitating not bogging down, as the motor sounds like its about to die. then surges back to life. letting off the throttle will keep it from stalling or backfiring. Gotta be pretty quick though to avoid the embarrassment
Expert:  Virtual Wrench replied 1 year ago.
Thanks,
Hesitation is what i have been talking about the whole time :-) I knew thats what you meant...lol
You should get the vacuum advance unit tested to see if it holds vacuum. Autozone will loan you one for free and you just unplug the vacuum line form it and plug in the "mighty vac" (see below)and see if it holds vacuum when you pump the handle. If it doesnt, replace it.
Then set the timing to 8 with the vac hose disconnected from the advance, you HAVE TO USE A TIMING LIGHT, do not try to guess on this.
Expert:  Virtual Wrench replied 1 year ago.
Hi,
Here is what the vac pump looks like.
https://dl.dropboxusercontent.com/u/33691145/mighty%20vac.PNG
If you are satisfied with my service please use the required rating for me to receive credit for working with you.
You can still ask questions even after you do the required rating and there is no further charge!
LJ
Expert:  Virtual Wrench replied 1 year ago.
Hi,
Do you have any further questions bud?
Customer: replied 1 year ago.
yes, I unplugged the vacuum hose for the vacuum advance. seems to have made the problems go away.
Expert:  Virtual Wrench replied 1 year ago.
Good deal,
Just replace it and you should be good to go!
Good job.
Please remember to rate my service so i receive credit for working with you.
LJ
Customer: replied 1 year ago.
Just kidding, cut out and died on the road. didn't want to start up. finally got it back running died 100 ft later, finally got it going after retarding the timing and limped home. sigh idk what to do now
Expert:  Virtual Wrench replied 1 year ago.
Hi,
Dont be turning the dist without a timing light :-)
Now it sounds like a fuel problem. Has the fuel filter been replaced recently?
Can you test the fuel pump pressure?
Expert:  Virtual Wrench replied 1 year ago.
Hi,
Do you still need help with this?