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after-firing when "off-throttle"
#1
OK- I'm gonna throw a series of "potentially-related" engine symptoms at you, and a few facts, and see if anyone can help me figure my issue:

The engine generally runs and sounds pretty good (to me, but I am not and engine expert). It's certainly not "bad" for a general round-town driver. It's a 302, C4. Carb is a Holley 7508-1. PO put on this carb, and tri-y long headers with dual exhaust.

Issue: When I drive, I notice the following issues:

1) when coming off-throttle at normal driving speeds, I hear a burble-burble-pop-pop-burble-pop etc. (I want to call this backfiring, but reading a little online I think "after-firing" might be the right word...)

2) when taking off from slow speeds, I experience a good acceleration, followed by a slight hesitation, then accel/hesit... 2 maybe 3 times.

Timing is unknown now. I have not checked it.

Condition of spark-plugs. haven't looked.

I have new spark plug wires.

Vacuum is ~11.5 hot at idle (less than +-1 variation at idle), ~9 hot and in-drive (parked)

I have just rebuilt the carb (was having leaks). Runs better at idle now after carb rebuild, but same otherwise - didn't change any adjustments.

I can hear a slight hiss on right hand side of car when it idles (could it be an exhaust leak?)

When we rebuilt the carb, the instructions say that idle-needles need to be in all the way, then backed off 1+1/4 turn. idle is really rough like that, it idles best with them screwed all the way in.

So what does it all mean. Do I have a rich mixture? Wrong idle screws? Exhaust leak causing poor/wrong back-pressure...

What can I try that will prove things one way or the other?

Gonna be next weekend before I can play with it again, so be patient for responses.

Thanks for any help.

Jay

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#2
You have a few things going on.

After reading the carb is defiantly wrong as far as settings but there are other issues.

The vaccum is pretty low for a 302. I would want to pull the plugs and look at them, while the plugs are out I would want to do a compression test and see what psi the cylinders have.

My first concern is the reason for the low vacuum readings. If you had a wild cam it would make sense a mild cam should be giving you 15-18 hot.

I would want to know the idle Rpms you have as well.

The exhaust leak would be another issue that you should look in to.


The carb running better with the idle air bleeds closed is telling my your butterflies are open too far on the carb and the transition slot is exposed, so your getting too much gas at idle speed.


If you close down the idle air bleed screws all the way the motor should stall, if it stays running then the butterflies are open too far.

Close the butterfly's a little and the idle bleed screws will have more effect. 1.5 turns is just the starting point every engine is different and you could have a range between .75 turns and 2.25 turns and be in spec.

The burble when you slow down may be another sign of too much fuel at idle.

If you stop on it it goes then hesititates that indicates too much fuel. You may need to coming down 2 sizes on the accelerator pump shooter. Or if you readjust the butterflies so the idle air bleed screws work again then it may solve that issue.


The first thing I would look into is the low vacuum reading, making a compression test high on my first stop list of things to look at.
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#3
Another reason your idle screws do not affect idle is a blown power valve. If the PV is leaking it puts more fuel into the airstream.

Like 72H stated your vacuum readings are quite low, a compression test and leakdown test may be necessary. It may be a result of the "hissing" that sounds like a vacuum leak to me. Exhaust leaks have more of a puffing or ticking sound. A vacuum leak can also cause a backfire due to an overly lean condition.

I hope this helps.

Jeff T.

Low buck, touring style, '73 Convertible "rolling restoration", 351c, 2v heads with a shave and a haircut, Performer intake, Holley 650(ish), roller rockers, screw in studs, guideplates, stainless valves, Duraspark / Motorsports MSD, T-5 conversion. 1-1/8" front, 3/4" rear swaybars KYB shocks and some home brewed subframe connectors. Future plans; JGC steering box, Cobra brakes and... paint, interior, etc.

When I die I want to die like grandpa, peacefully in my sleep... not screaming, like his passenger.

[Image: 1_12_09_14_10_15_11.png]
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#4
This helps guys. Thanks alot. I have not looked for vacuum leaks, and it is quite possible - as this car is just coming out of sitting in a garage for 4 yrs without being driven, just before we got it a couple of months ago. It's coming back to life nicely, and all this help from this forum is making a real difference. Will report back when I get a chance to search for vacuum leaks. I've heard a good way to sniff them out is to spray some engine starter around locations on the intake (while running) to listen for "engine revs picking up" (indicating ingesting of the fluid). Do you guys have any other step-by-step procedures?

Thanks!
Jay

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#5
If you put your hand over the carb intake the motor should stall out due to lack of air.

If you have a vacuum leak the motor will keep going and you may hear the noise of where the leak is coming from.

If the transmission modulator hose popped off that tiny hose can keep the engine running.
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#6
BTW- I forgot to mention a fairly significant issue. PO had tri-y headers put on, and never hooked the (thermal) choke up to anything (tube was left laying loose - so the choke was essentially not doing anything). So, I left the choke assembly off when I rebuilt the carburetor. Just to add another clue to the situation.

I'm looking to get an electric replacement, so not sure how this could affect butterfly positions - they are essentially "floating" now and mostly open - so it runs crappy at idle til warm, and then seems fine.

In general, the car runs better at idle now than it did before carb rebuild.

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#7
Please double check this for your carb. It may be what you need for the choke. Chuck http://www.holley.com/45-226.asp
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#8
the issue with the choke can cause a massive vacuum leak.

the choke is attached to the butterfly linkage as well. if the butterflies are now floating around that is a major issue as well.

if this is the first time playing with a carburetor you may want to think about buying a new replacement carb with a electric choke all set to be mounted on the intake this will allow you to eliminate many issues and focus on seeing how healthy your engine is or is not.
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#9
Thx C9ZX. That is EXACTLY the link I have ben looking for - I had not been able to find it before. There is another thread of mine "which electric choke" where I was seeking this. I am going to go close that thread with this info. Thanks a lot for that.

72HCODE- It's not my first time around the block with a carb, but I should be considered beginner. I have thought of just getting a new carb. Frankly, I like the idea of figuring this one out (for my own education) but if I can't get the engine healthy on my own I will go there. By definition I have to have it sweetened up by next Feb - when my boy starts driving it. So- I'm in no rush, but I do have a deadline. This is a good project for us to learn things on, so we are looking to get our hands dirty, and diagnose some problems. It's a great way to hang out with him. The car is not getting alot of miles right now, so it should be ok. It's a project, not a means of transport right now.

My reference to "floating" was wrt to the choke linkage. Butterfly works like normal otherwise. I am going to go buy the Holley electric choke and work the vacuum leak search this weekend. More soon.

Thanks both!

Jay

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#10
OK. Got a few tidbits of data. Tonight, I checked the timing, and replaced all the obvious vacuum hoses (except for the two coming thru the plug in the firewall).

After replacing the vacuum tubes, I did the "cover the carb intake" test, and indeed the car dies quiet promptly after you really get the intake covered. I can do the same by just pushing the butterfly completely closed - car dies. Based on this, I don't believe I can have a very significant vacuum leak. In this configuration, I checked vacuum and - no change from before - 11 inches of merc.

Now on the timing, it took me a while to get the marks cleaned and highlighted so I could see them clearly with the light, but after I did, I was able to clearly determine that the pointer was pointing to 4 deg. This 4 deg is measured with the "TC" (top center) indicator being visible (rather than behind the pointer). However, I did not disconnect the vacuum advance on the dist when I made this measurement. The measurement was made at idle. I'm gonna have to go re-test.

Any thoughts thus far?

Thanks,
Jay

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