popping through carb
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Freshman User
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From: Magnolia Ms.
popping through carb
A little background. 83 F150 4wd 302 carb'd. Original motor threw a rod, but was running fine before. Found an 86 302 out of a Mustang. Swapped motors, used our intake and dizzy because it didn't have one.
Put #1 at TDC on Compression stroke (piston up, timing mark on 0) Correct? Started motor, found some minor vac leaks, and son broke ears off carb where you mount it, changed carb from my other truck that we just drove. Set timing at 9 degrees for now, idles like a new kitten. Firing order is 1-5-4-2-6-3-7-8 checked 3 times. Wires came off other engine and were good.
Now to the problem. Idles good, but when you give it a good kick of the throttle it pops through the carb, also when you let out the clutch and start to go it pops. Not a pop-pop-pop-pop, but a single pop everytime you give it gas. You can slowly give it gas by hand out of gear and it revs fine. Do I still have a vac leak or do you think the cap or wires messed up from the time it took to swap them between motors ( a week )or do I have the dizzy in wrong? I was assured the motor was good, they are reliable and I do believe them.
Sorry for the long post but I wanted to give all the info I could.
Put #1 at TDC on Compression stroke (piston up, timing mark on 0) Correct? Started motor, found some minor vac leaks, and son broke ears off carb where you mount it, changed carb from my other truck that we just drove. Set timing at 9 degrees for now, idles like a new kitten. Firing order is 1-5-4-2-6-3-7-8 checked 3 times. Wires came off other engine and were good.
Now to the problem. Idles good, but when you give it a good kick of the throttle it pops through the carb, also when you let out the clutch and start to go it pops. Not a pop-pop-pop-pop, but a single pop everytime you give it gas. You can slowly give it gas by hand out of gear and it revs fine. Do I still have a vac leak or do you think the cap or wires messed up from the time it took to swap them between motors ( a week )or do I have the dizzy in wrong? I was assured the motor was good, they are reliable and I do believe them.
Sorry for the long post but I wanted to give all the info I could.
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From: houston
Popping through the carb is a sign of a lean mixture. Most likely you need to increase the amount of fuel squirted by the accelleration pump. But... you could still have a vacuum leak. Hook up a vacuum gauge and see what it reads at idle. It should pull a steady reading around 18", give or take. Another possibility could be a leaking exhaust valve. It could be burned, cracked, sticking or improperly adjusted. This would show up on your vacuum reading as low vacuum or a bouncing needle on the gauge. Good luck.
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From: Magnolia Ms.
Came off a 400 and was running fine no more than 15 minutes earlier, vac gauge reads 15 max with a 1-2 constant flux. I can slowly rev it and it goes smooth and sounds good. It is only when you hit the throttle fast or try to lat off the clutch and put it under a load. May still have to check the intake gasket. What's the easiest way to check the valves without a compression gauge? Pull off the valve cover and watch the rockers move maybe?
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From: Magnolia Ms.
We also pulled the plug wires off each one at a time and popping didn't stop, swapped wires and cap off other truck same results. i think we may have a vac leak at the manifold, sprayed a whole can of carb cleaner around intake with no change, Did make the motor bog down by spraying at back of carb, I thought a vac leak when sprayed would make it idle higher?
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From: houston
Since you got a change in the idle speed when you sprayed the back of the carb, there is some sort of leak back there. Once that has been resolved, check your vacuum again. 15" is a little low but it could be a leak. How many miles are on the engine? I would still try and run a compression check. If the compression is even and you still get the fluxuation, it's probably one or more worn valve guides. A worn guide causes the valve to close unevenly, causing a vacuum leak for a fraction of a second before the valve seats completely. All things considered, I believe increasing the amount of gas delivered by the accelerator pump will fix your backfire problem. Good luck.
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From: Magnolia Ms.
We still have this pop when the throttle is rev'd. Like I said it idles good and you can rev it slowly. 1. Checked compression, 172 on the highest and 164 on the lowest cylinder.
2. Pulled off intake and changed gaskets, pulled off carb and changed gaskets.
3. Vac advance works and we have 36 degrees total advance.
4. Carb worked good on other truck ( drove truck and switched carb same day)
5. New wires, rotor, cap and plugs.
6. Plugged all vac lines except for the brake booster. Brakes seem to work fine, no constant hissing when you hold them.
7. Changed fuel pump and took off fuel filter.
8. Put linkage to accel. pump in every available spot.
9. Pulled hair out and cursed truck. Threatened to burn truck to the ground.
10. Shopped e-bay for large stick of dynamite.
11. Swapped firing order and truck wouldn't even start.
I know I am forgetting something we did. It is only 1 big pop through carb when throttle is hit fast. Help !!!!!!!
2. Pulled off intake and changed gaskets, pulled off carb and changed gaskets.
3. Vac advance works and we have 36 degrees total advance.
4. Carb worked good on other truck ( drove truck and switched carb same day)
5. New wires, rotor, cap and plugs.
6. Plugged all vac lines except for the brake booster. Brakes seem to work fine, no constant hissing when you hold them.
7. Changed fuel pump and took off fuel filter.
8. Put linkage to accel. pump in every available spot.
9. Pulled hair out and cursed truck. Threatened to burn truck to the ground.
10. Shopped e-bay for large stick of dynamite.
11. Swapped firing order and truck wouldn't even start.
I know I am forgetting something we did. It is only 1 big pop through carb when throttle is hit fast. Help !!!!!!!
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Elder User
Joined: Feb 2004
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From: Fort Rucker, AL
How's your timing? Is it a stock cam? My 302 did that real bad and I increased the timing and it only does it every now and then. It's like a big fart through the carb. Caught on fire the other night, that was interesting. Anyway, just a thought.
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Freshman User
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From: Magnolia Ms.
Set timing at 4,6,8,10 degrees still pops. stock cam. Changed firing order to 1-3-7-2-6-5-4-8 and it barely runs, so firing order is correct. Will check for timing chain slack today. Any way I can check the cam easily? One more thing, we took off the EGR spacer under the carb and replaced it with a phenolic spacer, could this be our whole problem?
Elder User
Joined: Feb 2004
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From: Fort Rucker, AL
I'm not trying to insult your intelligence, okay? but make sure your timing is on the correct side of Top Dead Center. I had mine way off by having her at 10 after TDC for the longest time until lightning struck me one day I actually looked at more than the numbers on the flywheel.
Points or electronic ignition?
Points or electronic ignition?
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Joined: Jan 2004
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From: houston
If you do not have a functioning EGR system and the carb is designed to function with one, you will experience many of the same problems that you would with a lean running condition. By introducing exhaust gases into the combustion chamber it changes the A/F ratio to rich condition. Since burned gases have little or no oxygen, that raises the percentage of fuel compared to that of oxygen. You are either going to have to richen up the carb or get the EGR functioning again. Good luck.
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From: Magnolia Ms.
debestuss, I didn't take as an insult, sorry if it sounded that way.
I turned motor by hand until the #1 ( passenger side front ) was at the top and the balancer was at 0 degrees. That's right I think. It has the original electronic ignition. I'm thinking now that I should check to make sure it is at the top on the compression stroke. the piston comes up twice in 360 degrees I believe. we may have it 180 out but it seems to run too good for that. I'll also put the egr spacer back on as suggested by holland501
found out after 40 years of this stuff that it is the simple things you miss that gives you the most headaches. I can seem to remember to put all the nuts and bolts back on and then forget to put gas in it and can't figure out why the thing won't start.
I turned motor by hand until the #1 ( passenger side front ) was at the top and the balancer was at 0 degrees. That's right I think. It has the original electronic ignition. I'm thinking now that I should check to make sure it is at the top on the compression stroke. the piston comes up twice in 360 degrees I believe. we may have it 180 out but it seems to run too good for that. I'll also put the egr spacer back on as suggested by holland501
found out after 40 years of this stuff that it is the simple things you miss that gives you the most headaches. I can seem to remember to put all the nuts and bolts back on and then forget to put gas in it and can't figure out why the thing won't start.
Last edited by TigerDan; Apr 18, 2006 at 08:23 AM.
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Freshman User
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From: Magnolia Ms.
Couldn't edit post for some reason. Once the #1 was at the top, 0 degrees on the balancer and the rotor was also pointing at the #1 on the dizzy cap. We never actually felt for compression at the #1 cylinder.
Hotshot
Joined: May 2004
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From: The hills of No. Calif.
If it's 180º out it'll pop back through the carb alright, and thats about all it'll do. It won't run at all.
Welcome to the thrills of advancing age and terminal CRS...(now where did I leave my pants...?)
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Welcome to the thrills of advancing age and terminal CRS...(now where did I leave my pants...?)
(We're currently experiencing technical difficulties. Please don't adjust your set...)
Last edited by TigerDan; Apr 18, 2006 at 08:25 AM.