302 Backfire with Summit Carburetor/ Need Advice

[Wirenut]

I have been reading where some of you members shared positive reviews on their Summit Carb so I thought I would try one.
I purchased a 500 CFM 4BBL and installed it on my 57 Fairlane and it started up and idles great. When I go to put the car in gear it backfires through the carb and shuts off. If idle is below 800 RPM it backfires and shuts off as soon as I adjust idle screw adjustment.
I have a 86 302. 0.30 over, mild thumper cam, Performer intake with headers and Petronix ignition module (instead of points).
The heads are not the original emission heads.
I had a 600 Holley on the car for a couple of years now and the car has run fairly descent but always smelled as if burning too rich and black inside the exhaust pipes, never had a backfire issue until this new carb.
I called summit tech support today and they said to check timing so I will do that next (never had that issue before this carb).
I put a vacuum gauge on the manifold port and I have 15 lbs steady vacuum and that's the best I get after adjusting the air mixture screws on the carb at idle.
I'm not real sure where to set the timing since the 86 had ECM modules, and I have it set up with old style distributer and no points.
I have been setting the timing based on pinging under load and backing off until it doesn't ping.
Any recommendations on what to look for or what I could be missing?
I'm thinking about moving the vacuum advance to the manifold port and see if that makes a difference instead of carb port.
As a baseline timing setting, would 6degrees btdc be a good place to start or any recommendations of a general timing setting since the 86 has no degree setting that I can find, the books I have indicated to go with decal under hood.
Any advice would be appreciated..

[rmk57]

  I would start baseline timing at least 12 degrees with a Thumpr cam and it may need more. I would also leave the ported vacuum and not bother with full manifold vacuum.

[Wirenut]

Thanks Randy, I appreciate the help.

[djfordmanjack]

yes 10-12 degrees static is a great starting point. I installed a 500cfm Edelbrock on my 302 just 2 weeks ago. make sure you have ported vacc for your distributor. I don't know where that would be on the Summit carb. Also DO check your throttles @ idle.  sometimes they are open so much, that you really can't get a decent port vacc. you could try richer idle mix screw setting and setting back the throttles a little.
There is a huge discussion about ported vs manifold vacc for SBFs going on on internet forums and in general. It seems there are as many opinions as different engines. My engine is stock and has a Weiand 8011 on it (dual plane). I put timing at 10 static and ended up at ca 12-13 deg with timing light, idling on ported vaccuum. has nice idle around 750-800 and will drop to 600 in gear without stalling.
before that with the 625 cfm carb, the throttles were open so wide that the vacc advance always got manifold vacc so with same static points setting it had 25degrees running at idle of 950. below that wouldn't let you engage D without stalling.
they say the most important is max advance and that shouldn't go over 36 degrees in total. I have pretty much that on the engine at 2000 or so and it purrs like a dream and has great power in all gears and rpms. no backfiring at all.

[lalessi1]

I spent a lot of time setting up my advance curve but it is on an FE, not a 302. I have 11.2:1 CR so my car pinged like crazy at first, now it doesn't at all. I wound up with 12 degrees initial and 36 total. I had to slow down the centrifugal advance a lot and I felt like the car idled better and pinged less with direct vacuum. My distributor is a Pertronix billet type and it uses GM style advance weights and springs. I have a COMP Magnum cam. The Holley carb is set up very rich as well.

[Wirenut]

Thanks for the info, I have not been able to get back on the car yet, maybe this weekend. It sounds from your description fordman that mine is doing the same thing yours was. I have been reading about the ported verses manifold vac on the advance timing. I think I?ll stay with the ported and set the timing at 12 and see what it runs like. Since I just changed the carb it was hard to believe it could be a timing issue. The rich 600 and possibly timing off makes more sense the more I think about it. I read some good reviews of the Summitt on this site and thought I?d try one plus a 500 instead of the 600 since it seemed to run rich no matter what I did with the 600.
I appreciate the advice and experiences you guys share.

[djfordmanjack]

Looking forward to your report. I believe that 12 instead of 6 degrees static timing will give you much higher rmps in idle, so you can back down the idle set screw ( throttles). That hopefully will get you more steady vaccuum and therefore less backfiring chance and stalling in Drive. Lets see how this works, fingers crossed !

[Wirenut]

I'm embarrassed to report what the end result was. 
I had two of my spark plug wires crossed. When I changed the carburetor I had to disconnect my water hose coming off of the manifold to reroute and re flange the fuel line, so I disconnected all four of the passenger side plug wires. When I reconnected them I messed up.
Attaching them correctly cured the stalling/backfiring issues and my vacuum came up from 6 to 15.
I did reset my timing from 14 btdc to 12. I was unsure where it should be, and noticed a slight pinging on pulling hills.
I plan on taking it on a road trip in the next couple of days and will see if any pinging develops on pulling inclines.
I have always double checked the firing order and plug wire connections but this time since I had the wires routed through some spacers, I thought I put them back as they were.
I do appreciate all of you that offered suggestions and ideas.
The thing that threw me off so much, was it idled so smooth and was only an issue at placing in gear or idling to a normal idle.
Such a good forum and a great group of guys always willing to help.
I guess you can't fix stupid no matter how much you try to help..
Thank to all!
   

[Ford Blue blood]

You know if you were the only person in the world to do something like that that word (stupid) could be used....but I'm willing to bet that there probably isn't a single "motor head: that has made the same or very similar mistake while turning wrenches.  I can speak personally of this as my 351C running problems lasting so long were a direct result of forgetting the simple rule, gas + air + spark = fire!  Two months on and off of farting around with this problem turned out to be bad points.  I made the assumption the "new" points in the new distributor were good......not so much......

The most important part is you sorted it out and all is well!  Good job!

[lalessi1]

I been there and done that. I gave up and took mine to a certified mechanic to check out the engine... "well we will see what's going on after I swap these plug wires around" !