Good morning all. I've been working on a 292 with a 3x2 intake for several years now. I've gotten to a point where I can drive the vehicle for short trips, but I'm intermittently getting a backfire under acceleration once the engine warms up. Its also cut off on me twice at a stop. I think it might have been vapor lock, because it restarted pretty quickly. Ive sent the carbs out to be professionally rebuilt. I called the cam manufacturer and my initial timing is where it should be. (10⁰ btdc) I've been trying to do some research about what else might be causing my concern. I'm wondering if I'm getting an issue from the heat riser ports running under my center carb. I'm using the wide open gaskets, not the truck ones with the smaller opening. Could this be my issue? I'm trying to make the power tour this year and I'm running out of time. Any help would be appreciated. Cam is an isky 301333, Weiand intake. 292 bored .090 over. Red's headers that have been ceramic coated to lower under hood temps, gasket-match porting on a set of G heads.

Have you jetted up for the ethanol in the fuel?  I would install a 1/4" phenolic spacer under each carb, remove the metal from the intake gasket heat riser hole, block the heat riser with a thin piece of metal, and set the initial timing at 12-14* and try it again.  You can get the spacers from Speedway in various thicknesses.  Backfiring is typically caused by lean mixture of fuel.  Joe-JDC




JDC

I have not touched the jetting at all. I sent them away to Charlie Price at Vintage speed. Like I said, there is no metal in these intake gaskets at all. Ive set up an air fuel gauge with an oxygen sensor to monitor the mixture. Its at about 13.5 to 1 at idle. I am intentionally leaving it rich to avoid the lean condition. I have hard lines bent for my fuel already. If I have to go back and make those again due to a spacer, I'll pull the entire set up off and sell it. 
  

2 other causes of misfire/backfire is a leak in the intake gasket or leak somewhere in the exhaust most likely closer to the header.


1936 Ford 3W Coupe : 56 T-Bird 312, 47 Packard 3 speed, 40 juice brakes.
1968 Merc Cyclone FB GT 390, Getting a better front clip!
1977 Ford F-250 Supercab RWD Explorer Long box.
1976 Chev Camaro RS LT
Future rebuild : 1949 Ford F-1

The first thing I would do after seeing your setup is to wrap the copper lines is to insulate them with a wrap or rubber hose to keep the heat from radiating on them and causing vapor lock.  Copper is an excellent heat sink, and that may solve your problem.  I cut 3/8" fuel hose to length, and split it to slide over the metal fuel lines.  Works just fine.  Joe-JDC

JDC

Interesting. I'll try that out. I'll keep everyone posted on that. Of course it's raining today, so I won't know today. I'm also trying to sort out an aod kickdown cable, but that's an issue for another day.

Still backfired on me a few times while accelerating. I don't have a way to get the actual temp, but all the linkages in that area are hot to the touch. Engine also stalled out on me at a stop. Transmission shifted sporadically even though I made no adjustments to the kickdown linkage.

Joe is hitting the right angle I think... check out this post on my blog concerning ethanol in the fuel and what it can do in high temp situations. The boiling point of gasoline is very low when ethanol is included in the mix...
https://www.hotrodreverend.com/post/2019/07/13/1955-ford-part-81-running-with-the-devil




Daniel Jessup

Loveland, Ohio

aka "The Hot Rod Reverend"
check out the 1955 Ford Fairlane build at www.hotrodreverend.com

13.5 is not rich  enough with the ethanol fuel in my opinion.  I would feel safer with 12.65-12.85 at idle and under acceleration.  Float levels may be off even though you got the carbs rebuilt due to shipping and handling.  I still think you need a phenolic spacer, and 1/4" should be able to be fitted under the base of the carbs with the fuel line block moved up to match.  The lines could stay the same.  Joe-JDC

JDC

Ditto on the comments about ethanol laden fuel running leaner.  I typically have to add a couple of jet numbers to the jetting to compensate for 5-10% ethanol fuel.

I have had a couple of Ford Y 3X2 setups recently in the shop with the Ford/Holley 2V carbs that had been ‘professionally’ set up elsewhere.  Both setups initially ran extremely lean when dyno testing them as delivered.  Upon disassembling the carbs, I found that the end carburetor power valves had been removed but the end carb jetting had also been further reduced from the stock numbers.  On both of those 3X2 set ups, the primary jetting was #55 and the secondary (end carbs) was #53.  In the grand scheme of things, when blocking the power valves, jetting should be increased, not decreased. 

On the last one I ran, I bumped the jetting up to 59P and 60S and immediately saw a plus thirty horsepower improvement along with the lean popping during load being reduced so I knew I was on the right track.  I ended up with 59P and 67S jetting and the engine both idled and throttled up well.  This particular engine had an Isky E4 camshaft so nothing wild in that department.

Lorena, Texas (South of Waco)