Engine backfires with vacuum advance connected

[dholder]

1990 HO 5.0 with Edelbrock 1405 carb. and slightly lopey Comp Cam and long tube headers. Distributor from 1985 Mustang (I think). Upon initial install of this engine, it would backfire with the vacuum advance connected but not with advance disconnected, using the timed vacuum port. I left it disconnected and tuned the carb and drove it several months with no problems. Over the weekend, I revisited the issue. I adjusted initial timing several times between 6 and 12 degrees advanced and it did not seem to change the backfire issue. It runs good except upon very very slight application of throttle, where it backfires. No throttle or heavier throttle stops the backfire. The diaphragm holds pressure and does not leak. I even retuned the carb back to the standard tune which is about two steps richer and that had no effect either.
Why would vacuum advance cause popping from the exhaust?

 

[hucklburry rev2]

just due to an experience I had, I have to ask what firing order are you using?

 

[dholder]

I used the firing order for the HO 5.0. I don't remember what it actually is though.

 

[68ford]

I had the same symptom once, cleaned out carb and it went away. I had duel exhaust and it Only backfired out the pass side. Possible you have a mixture issue at light throttle and the vacuum advance just brings it to the surface.

 

[Slowleak]

You are probably getting too much advance under light throttle. That's when ported vacuum advance starts to work. I would adjust, or replace the vacuum can. This link will help http://grantorinosport.org/BubbaF250/perf/perf02c.html

 

[707Bronk]

How much does the vacuum advance your timing at idle (initial + vacuum), and how much total advance do you have when you are all in (initial + mechanical + vacuum)?

It sounds like you may have a carb tuning issue - the mixture isn't getting the fuel shot, or its getting too much under light throttle.

A picture of the carbs throttle linkage would help, and the distributor as well.

 

[williamjbonney]

I have the same flipping problem. Only after I sent my MSD back to get a new cam gear installed and they replaced the p/u and installed a new vacuum advance can on it. I hve tried everything to fix it, through cleaning the carb, adjusting the timing, checking the vac can for proper operation. Nothing has fixed it, so I will follow this looking for ideas and reading the link provided. It is been very frustrating.......

 

[Broncobowsher]

I had a couple plug wires swapped once. I swore up and down that I had them right. Finally realized they were wrong and problem solved. Don't be stuborn, just go through the plug wires one more time. Preferably just pull them all off the distributor and start over again.

 

[jeff_gates]

Ha ha,
I also had wires swapped.

--Jeff

 

[pcf_mark]

I thought the distributor turned the wrong way once so check you are going around the right way too. Since it runs good at other rpms I'm guessing your set up is otherwise good to go.

I think at idle / just off idle it is very lean in a very narrow range. When you add the vac advance the lean mixture starts burning that much sooner - quick and hot.

As an easy test open up the idle mix screws a half turn each side. If it goes away then it is a lean pop. If not just turn them back in. Changing the main jets will not have an impact on an off idle lean pop they start really working around 1,500 rpm. If it is a lean pop you could change your accel pump shot set up.

 

[broncnaz]

Check the accelerator pump. It may need adjustment you should have fuel squirting out anytime the throttle is moved. If no fuel with just a little movement then your leaning out and that may be causing the back fire. Id lean toward this as the issue since with more throttle it doesnt back fire.

 

[dholder]

You are probably getting too much advance under light throttle. That's when ported vacuum advance starts to work. I would adjust, or replace the vacuum can. This link will help http://grantorinosport.org/BubbaF250/perf/perf02c.html

I thought this also and actually tried to adjust it but it is apparently non adjustable on mine. I guess I need to get the light back out and see what my timing is actually doing. The carb is new so I dont think there are any issues with it other than tuning. I just dont know right now whether it is a carb issue or timing issue. Will tackle it again this weekend.

 

[savage]

Which cam do you have? When they rebuilt my 302,the new cam had a different firing order . it sounds like you played with the step-up Springs on the carb ,try a different Spring ,the springs work off vacuum .which port do you have the vacuum advance hook to.

 

[broncnaz]

The step up springs can make a differance but since this is mostly a off idle issue Id bet more on accelerator pump. The acclerator pump is there to cover the problem of the engine leaning out when you step on the throttle. Since its happening just of idle I would make sure its squirting as soon as the throttle is moved.