Gun-shot backfire and Holley carburetor

I'll bet this has been covered in the forums before, and I've learned a lot of good information in the threads posted on this site, but I still can't solve the problem. For background, my recently acquired 1967 CJ5 has a V6 engine and it starts and runs great. I had some issues with exhaust "popping", but after cleaning up some crankcase breather hose leaks and replacing the PCValve and tuning her up with new plugs, new plug wires, new rotor, new dist cap, and installing a pertronix ignitor and coil the "popping" has stopped. Now my problem is that on deceleration I get a periodic LOUD exhaust backfire that sounds like a gunshot. It's really embarrassing. Like walking down the sidewalk and, just a the attractive woman coming the other direction looks at you and smiles, you unexpectedly let loose with a deafening trousers thunderclap. Also, living in a high fire danger area in tinder-dry southern California, it might even be a hazard. I've tried the trick of hooking up a shop-vac in reverse to the exhaust pipes and looking for leaks with soapy water, but no bubbles to be seen, so I don't think it's an exhaust leak. The jeep has headers and dual exhaust, which may contribute to the problem. The timing with the vacuum advance tube blocked and at 700 RPM idle is set at 5* btc. All cylinders compression are between 112-116. The manifold pressure holds steady at 16lb at my 2,200 foot elevation. The mechanical advance in the distributor moves freely and the rotor springs back after I rotate it by hand. The vacuum advance can seems to be working OK. I can get the plate to move by pulling the hose from the carburetor and sucking on the tube. So I know the vacuum advance diaphragm isn't broken. Now I'm thinking maybe I need to do something with the carburetor. It is not an original. It is a Holley. I'm not sure of the model because I couldn't see any numbers on it that looked like a model number. I read a good thread on this site about tuning a Holley carburetor, but it didn't look exactly like the one I have. Can any of you identify the model from this picture? I would like to be able to look it up on the Holley site. I imagine the adjustments are similar. Any other advice on how to stop the exhaust backfire?

Thanks,

 

That looks like a Holly 350 cfm 2bbl #7448
Look on the front of the air horn ( the part that sticks up inside the air cleaner)
The # should be stamped there. Backfiring in the exhaust is a sign it's running rich- you may need to re jet.

 

Thank you both. I will search again for exhaust leaks and try smaller jets. I haven't tried adjusting the idle mixture screws yet. Is that a good place to start, and can that by itself cure a "too rich" condition? The Holley site says that the factory sets the carbs for 70º at sea level and that you should use a jet that is one size smaller for every 2,000 feet of increased elevation. I live in the mountains above Santa Barbara at 2,200 feet, but drive to sea level every day. Does it make a difference? It backfires the same on the mountain as it does down by the shore. I don't think the Holley carb has a dashpot. James, was your leak at a gasket or actually in the header tubing?
Thanks,
Craig

 

The Holly getting advice is good, as far as it goes, but also remember that you're dealing with an engine that's only 225 cubic inches.
I'm not sure what the factory jetting is set up for, but I'm sure it's a much bigger engine.
The idle mixture screws only affect the idle circut.

 

2x But don't run it too lean. That makes for exhaust valves with a very short life and isn't much better on pistons and rings. Personally, if you get the Jeep into the rocks, any Holley center pivot bowl version would not be on my desired card list. They REALLY don't do well at steep angles. That said, it could be converted to the side hung bowl.

 

​You may want to double check that timing. It may be a bit retarded.

 

Thanks, Doug. I just checked and the jets that are in there now are size 56. Do you have an opinion as to how much smaller I should go? I could get a few different sizes and try them, but what would be "too lean" as John warned about?

 

Just offhand that sounds pretty small, but I can't offer an opinion on that without knowing for sure what size jets came in it stock. Normally, you can only go around 8-10 jet sizes before you're out of the calibration range of the carb.
Two things you need to do-
1. Get the carb # & look up the factory jetting
2. Look at your plugs & see if they show indications of running rich

 

Thanks.
It looks like the factory jet is 61. I will try to check the plugs tomorrow.

 

mehhh
mineruns great but sounds like a ups truck when decelerating ,,,,I like it people look and get out of the way

 

If the plugs are near white and the exhaust white to light tan, it is probably too lean. 56 jets are not to likely far off what you need but you could drop a couple sizes and just watch what happens to the plugs and exhaust. It's been a long time since I had the 450 cfm Holley 4bbl apart on my 225, but I think those are 48 or 49's in both sides. But the primary side of that carb is a lot smaller than yours.

 

I too had the loud back fire on decel many years ago. Sometimes it would be right after I turned the key off. Pretty much ruined the mufflers. This was with the stock R 2G carb. Never did figure out the problem but it went away.

 

OK. I looked at the plugs today. The three on the driver's side are clean and white, the ones on the passenger side are dirty and dark. Here's some pics.



Those pictures were taken after it had been idling for about 15-20 min. I tried to adjust the idle metering screws after watching a video via the Holley website. Didn't seem to change anything. I also re-checked the timing, which appears to be per specs. Checked the compression and cylinders 1-6 read 115-112-112-115-116-116.

Here's a little video so you can see what it sounds like. The exhaust on the passenger side sounds different than on the driver side. So, any thoughts on what my next step should be given that things seem to be running rich on the passenger side, but the compression all looks good?



Thanks,
Craig

 

You may want to read a bit on the proper distributor instalation in an oddfire V6. Several members here have run them for years out of time. As we say, do a search, your plugs are telling me that your distributor isn't installed right. That and the backfire. Then again, if your dwell is off, the passenger side plugs aren't going to fire correctly.
I believe many of the members have been working on these OF beasts for over 50 years, my first was a 1962 Fireball.
Don't be afraid to ask questions.

Telling us your location would also help, there may be a member close that could help you.

 

Will say that there are probably more than a few 225 o.f. running even fire distributors unknowingly. If your jeep is spinning it's wheels and/or breaking your T90/T-86/T-14 then you have a properly tuned 225v6 O.F. :twisted:

 

when i got my jeep the distributor was exactly the problem. the po had installed the correct cap, but it was the pickup coil that was the give away. i purchased a new odd fire pickup coil on rock auto. very cheap, and worked well. now i have a range of adjustment that the jeep never had before. although, i still have initial time at 5 degrees btdc. i am still having some carb issues with the mc2100 swap, but overall, it's much better than the 2g in my rig.


part number D1995C

http://www.rockauto.com/catalog/moreinfo.php?pk=266829&cc=1023174

 

Thanks. I think that's where the problem lies! Awhile back I was concerned that the plug wire going to the number one cylinder was attached to the distributor at the post with the number 3 molded into the cap. I set the No. 1 cylinder to TDC on the compression stroke and the rotor lined up with the No. 3 contact on the distributor cap so I left it alone. Now I have reinstalled the distributor so that the rotor points to the No. 1 contact when No. 1 cylinder is at TDC. Put all of the plug wires in the proper firing order and re-set the timing. It starts right up and idles smooth and the plugs on the passenger side are now looking clean with good color. But I still get a loud BANG if I increase the RPMs and then let off the throttle.

I will do some more research here as you suggested. Is there an easy way to tell if I have the right distributor and cap? The new cap that I put on is the same kind as the old one that I replaced.

This is a 1967 CJ5 and I am in Santa Barbara, CA

Thanks,
Craig