popping back fire in exhaust

[bkcole2]

So I have recently done a total top end swap on my 350 in my 86 suburban. The motor is a goodwrench 350 crate motor that was installed about 3 years ago. The lower alternator bolt broke off in the head and I could not get it out with the tools I have so I used this as an excuse to upgrade the heads.

So since then I have installed afr as cast head and upgraded the cam and lifters to a comp RV style cam. I did the cam break in today and have an issue with an exhaust backfire. It lets out a repetitive pop on the drivers side. I am hoping for an overly tight rocker arm. But i wanted to see if anybody had anything else I might need to check into. The heads cam with double valve springs and I broke in the cam with only a the outers in place. The pop would go away around 2000 to 2300 rpm so that is where i did the bulk ocasional reving it up to 3000. So the pop is present above that range and at idle as well. My plan is to check over all of the basics as I reinstall the inner valve springs. Resetting the valves, setting good timing, checking firing order, replace distributor cap and wires, etc. I did not see where my timing exactly was during the break in, as my light did not work and I did Not want to spend alot of time on it while I was breaking it in.

Is their anything else I can check without getting deeper than the valve covers while I tune up this engine and install the inner valve springs? I am just praying those new heads do not have a bent valve now.

 

[Wes Harden]

5 and 7 wire crossed will give a pop and mild miss.
But since all the top end stuff is new, tight valve would my first check

 

[folkenheath]

In addition to your plans, also check your spark plugs, a good cam or heads need a better ignition system usually, and a fouled plug can cause unburned fuel to get ignited in the exhaust.

I always recommend a CDI box with most camshaft or cylinder head changes. The multiple sparks at low RPM help keep the plugs burning cleaner.

If you do have a tight rocker you need to fix it soon so you don't burn a valve. If you have a rocker that keeps loosening then that would be a bad sign of a cam lobe going flat.

 

[1-ton]

It sounds like your engine is venting compressed exhaust gas out of an exhaust valve during the compression stroke. That could be from incorrect valve adjustment, bad exhaust valve, or sticking exhaust valve.

 

[Blue85]

I did not see where my timing exactly was during the break in, as my light did not work and I did Not want to spend alot of time on it while I was breaking it in.

Since you swapped the cam, you had the distributor out and this problem could be entirely from timing or incorrect wire order.

Also, how dd you install the cam? Straight up? Did you take a picture of the timing marks? (I always do this for the sake of my own sanity should I start to question).

 

[Chickntrk]

With todays cams if it’s a flat tappet I’d say wiped lobe

 

[eodcoduto]

An intake leak can cause this too, its the bane of my existence on my Shovelheads.

 

[bkcole2]

Thanks for the comments. I finally got back into the suburban tonight and i for sire have a lean condition. The cam is dot to dot with a factory style timing gear set. But any way I have some plugs that are very lean. The first picture is 2,4,6,8 from left to right with 8 looking almost new. The second picture is 1,3,5,7 with #1 being almost like new. This to me points to an intake leak likely on the bottom part of the intake. The thing was pig rich before all the changes where made. I have reinstalled all of my inner valve springs and now need to either readjust the valves or go ahead and pull the intake and check the seals. Maybe the afr heads and edelbrock intake just dont agree with each other. What do you guys think?

 

[bkcole2]

Really 1,3,5, and 8 are too lean

 

[Wes Harden]

I'd retorque the intake, and adjust the valves. Check for any other vacuum leaks, adjust air fuel for best vacuum at idle, set the timing at @8-10° btdc. road test, check the plugs again.
New heads ? The intake/head angle could be off, ya never know. I'd kind of expect it when heads have been surfaced.

 

[bkcole2]

It is new afr enforcer heads

 

[bkcole2]

I have her running well again, thanks to your guys comments. While readjusting the the valves I gave all of the intake bolts an extra bit of tightening. I also pulled the distributor and went back through the timing process with a new cap, rotor, and plug wires. She fired right up with no lean pops and back to her normal self. I set timing at 10 degrees advanced at idle with the vacuum advance pulled. Test drive went well though she is not as peppy as I had hoped. She is definitely a little stronger!

Thanks for the advice Wes, without that I would have probably been pulling the intake. You saved me alot of time.

Next on the list for me is tunning with an afr gauge! I plan to put a build thread on the forums after I get a chance to clean her back up

 

[Wes Harden]

If there is no pinging add 2* and see how it likes it.

Glad it helped