Backfire in the Carb

[PaulQ]

Here goes,

 

94 fullsize Blazer 350 with Vortec heads and Vortec intake(Existing TBI), in the process I put on new roller rockers, pushrods, and lifters along with a new crane cam(Duration 254/ 264, Lift .401/ .423). Also put on new headers.

 

I finally got some codes and here's what they said or at least how I interpereted them(EGR, MAP Sensor, fuel pump relay, and electric spark timing or ignition module).

 

The EGR is brand new so I replaced the MAP Sensor, haven't gotten around to the fuel relay( don't think it's related to this problem, although I could be wrong, and the EST/Ignition mod. I believe is from setting the timing{Not sure about this})

 

In park, neutral, and idle everything is perfect I can rev and hold at any RPM with no backfire. Under load it's a different story. I don't notice the backfire at lower speeds or higher speeds, just seems to happen when I get on it good around third gear I would guess. The idle is still fluctuating just not as bad. Currently have it timed at 5* advanced. That seems to be the best happy medium at the moment.

 

Also blowing water or moisture out the exhaust at startup( which would lead me to believe the valves) just not sure how much more I could adjust them. Adjust when running( back off rocker until it clatters tighten 1/4-1/2 after clatter stops. I've tried both 1/4 and 1/2 turns and neither seems to make a difference.

 

Sorry to keep throwing up these long posts just trying to get resolved. Thanks to everyone past and present for taking the time to assist me in my time of need.

 

Hope I didn't leave out any important info.

 

Paul

[Likarok]

My best guess is that you are one gear notch out on your distributor timing or you have your plug wires out by a couple of cyclinders?????????

 

Check Top Dead Centre on #1 cylinder and check order of plug wires????????

[PaulQ]

New plug wires, and order is good...d**n I knew I forgot something.

[Likarok]

Does it start easy? If you have to advance the distributor to get it running than I think you are out one gear tooth on your distributor placement.......my best guess???????

[PaulQ]

Man here's the hard part for me..It starts better than ever and sounds real good and even drives great except for the occasional backfire in the carb...It just kills me that I can't get on it and see what it can really do now..

[Likarok]

Well Paul you got me..............what carb are you running? Could you just have it set way to rich......Did you get the speed shop you bought your cam at to match your spark plug heat range to your cam????? Backfireing though is generally a timing problem I have always thought.

 

Sounds like time for professional help. Sounds like you had fun putting it together. The ultimate gearhead job. Congrats.

[PaulQ]

Stock TBI Carb, as far as I know I didn't change anything to do with the mixture ratio. As far as the plugs go what do you mean( you talking about the gap or what).

 

Thanks for your help,

Paul

[Likarok]

Well spark plugs have a heat range they operate at. Because you changed the cam you probably need a plug with a hotter heat range. This simply means that the electrode dealy on the bottom of the plug would be longer for a hotter plug or shorter for a colder plug...........I think, could be the other way around. You should take it to a garage you realy trust and hook it to a scope, or at least review your problem with a mechanic that is into perpormance stuff.

 

If you have driven the truck for a bit after the build..........pull a plug out. If it is wet looking I would think you need a hotterplug.......since you didn't change the TB. Normally you want your plugs to look kind of whitish brown on the electrode. I would think though that the guys selling the cam should be able to figure the right heat range out for you.

 

Years ago when I use to do this stuff it would take sometimes a month to get it running to peak. When you got there you didn't touch a thing. About that time you would uncap the headers and go looking for trouble. ahahahaha.

[Likarok]

After thinking about this you could also have your cam timing out too. Does it have timing chain on it or gears. Are you sure you timed your cam to "top dead centre"?????????

 

I still think it is a timing thing!