Exhaust Backfire problem

Hi. I'm new here. I've been getting lots of valuable information off these forums and I'm very impressed with the knowledge. So here is my first question (sorry, its a little long):

I bought my 1970 CJ5 about 2 months ago. It has the 225 odd fire in it. Acording to the guy I bought it from, the engine had been rebuilt by the guy he bought it from and only had about 3000 miles on it. He didn't have any information as to what was done in the rebuild though.

Anyways, about a week after buying it, it started backfiring out the passenger side on deceleration. Pretty much anytime I'm driving and I take my foot off the gas, it starts backfiring out the exhaust (Headman headers into Flowmaster exhaust). There's also a slight stumble on the same side at idle. Other than that, it seems to run strong and I have no other problems. Since this started occuring I have:

1. Put on a new MC 2100 carb
2. New HEI from CRT Performance
3. New plugs and wires
4. Replaced all gaskets from the head up (heads, exhaust, intake, carb...)

None of the above has changed the backfire in any way. My vacuum is at about 15 steady. The compression is at about 145-150 on all cylinders except #6 which is at about 135. I'm thinking Its time to rebuild the heads, but I can't really spend the $400+ to do it right now. I was just wondering if there was anything else I can try before I go there.
Thanks in advance.

 

Check timing? First thing that popped in my head...

 

Carburetor needle adjustment maybe? Mine started doing the same thing (except on drivers side) the last time I tried to adjust my carburetor mix leaner. I just haven't messed with it since. (lazy)

 

The 3 things I would check for this problem are, exhaust header leak, the cold air will cause it to do this. This could be caused from a gasket or a crack or small hole in the header.
The carburetor mixture needle, running to rich possibly,
timing is off.

 

What should the timing be at? The book I have says 5 BTDC. CRT says that with the HEI I should be around 6-8 BTDC. I've seen on the forum where guys are around 10-12.

I tweak with the timing and the carb some more and see what I can do.

 

X2

 

x2

 

Ok, I've tweaked with the timing and mixture and I can't find any leaks on the header, and it's still backfiring. It seems to run ok at idle (other than an occasional stumble on the same side). I can rev it up and I can't get it to backfire while its sitting there. But as soon as I start driving it and take my foot off the accelerator, it starts backfiring. If I'm going downhill and use the gears to keep my speed down, it seems to get even louder. As soon as I push the clutch in, it stops backfiring. Any other ideas. I need to get this thing driving ASAP.

 

On an MC2100, the power valve is the usual suspect. If it's blown, you'll get a very rich condition all the time. These valves are often blown out by a backfire. Later Holleys have a check valve to prevent this, but the MC2100 does not. If you take the cover off and there's gas in there, then there's a leak in the power valve diaphragm.

<edit> You say a new carburetor since this started... How did you adjust the idle mixture? Typically you'd set it to "best lean idle." Seat them all the way, then turn them out 1.5 or 2 turns. Adjust them together in the same amount until the idle speed starts to drop, then out till the idle smooths out.

Exhaust leaks too - put a few drops of ATF or brake fluid down the carburetor and look for smoke coming out between the exhaust flange and the head.

 

Do this while its running or off?

 

Running.

 

Ok. That's what I figured, i just wanted to make sure.
No smoke anywhere. The mixtures were set pretty much like you were saying, but I used a vacuum guage to fine tune it. I have the timing set at around 6 BTDC, does that sound about right? Which powervalve should I have for this engine?

Thanks for your help.

 

Re the power valve, there may be a number on the side of the valve that indicates at which vacuum it will open. Holley says a 6.5 will work for most applications
http://www.holley.com/data/TechService/Technical/power_valves.pdf

 

obviously getting raw fuel out the exhaust valves and igniting in the hot exhaust......carb problem. I would set timing as far advanved as I could without starting to ping.

JB

 

I had a holley carb on it and then switched to the MC2100 (brand new) and it didn't fix/change anything as far as the backfire goes. Any ideas?

 

Do you have a manual choke? If so, try pulling it out a bit and see if the backfiring goes away. Not a fix, but may help in troubleshooting.

 

What octane gas are you using?

 

A normal vacuum will be 18"-22"Hg with the needle remaining quite steady. I might consider a steady 15"Hg as late ign timing, but it could also be a intake manifold leak/vacuum leak. Also late valve timing is evidenced by a low, steady reading, but lower than late ign timing 12"Hg or so.
Joe

 

I'm using 87 octane and pulling the choke didn't seem to make any difference as far as the backfiring goes.

However, I had the carb off today. On the stock intake manifold, there are two ports that come up to where the carb mounts. I've been told these are exhaust ports that come up to heat the base of the carb (an early attempt at emmisions??). Anways, I noticed that there is a small gap on both sides of my carb adapter plate where these are open into the carb/intake passage. I'm assuming that with the stock carb, these were totally blocked off. Could this be causing my vacuum leak and possibly be causing fuel to get into the exhaust??? Can I completely block/plug these? If so, how would you recommend doing it? Thanks everybody for all your help.

 

im sorry, but i have had the same EXACT problem with a leaky header several times. either its a warped head, warped header braket or a leak in the actual tubing where it all welds together (or maybe a hole in the tube further down the header itself)...or maybe at the flange where it bolts to the muffler. can be VERY small and cause this problem. upon decelleration, you draw a bigger vacuum in the headers and it sucks air in ANY crack. any unused gas will fire when the exhaust valve opens and pop. i would recheck the welds (i had a pair of headers break at the weld on the passenger side and it popped like crazy). when you put it together, us that exhaust gasket sealer (real sticky). that should help seal the gasket just incase. but i think its a crack. grab a magnifying glass is needed. remove and do a vacuum test on it if available.