Yellow Bullet Forums banner
1 - 17 of 17 Posts

·
Registered
Joined
·
8,147 Posts
Discussion Starter · #1 ·
A friend of mine with a 347 Ford has a backfiring problem. When he gets on it and then lets off at the top of a gear, it backfires once through the exhaust. We datalogged the A/F, and it has an off-scale lean spike right at the time of the backfire. We just got it running a few weeks ago, and we put Proform metering blocks and a Proform main body in the carb (not my idea). It doesn't do it with a brand new 750 Demon on it, so it must be the carb or the tune in it. I have my own theory as to the cause of the problem, but I'd like to hear your ideas first.
 

·
Registered
Joined
·
8,147 Posts
Discussion Starter · #3 ·
My theory is that the baseplate is worn and the throttle shaft bores need to be bushed.
 

·
Registered
Joined
·
753 Posts
Usually when you have an afterfire out of the exhaust it is due to being too rich, not to lean. Too lean usually comes back through the carburetor. Now you can get a rich enough exhaust where the fuel lights off in the exhaust showing what appears to be a lean A/F.

What do the spark plugs & exhaust ports look like, and how does the engine idle?
 

·
Registered
Joined
·
8,147 Posts
Discussion Starter · #5 ·
We don't know what the exhaust ports look like, and the last time we ran the car it had the Demon on it. It idled fine with the mixmaster carb on it... we'll put it back on, run it a little, and check the plugs.

Can you read .log files from an Innovate DL-32?
 

·
Registered
Joined
·
753 Posts
We don't know what the exhaust ports look like, and the last time we ran the car it had the Demon on it. It idled fine with the mixmaster carb on it... we'll put it back on, run it a little, and check the plugs.

Can you read .log files from an Innovate DL-32?
Yes I can, but if you're getting false information it won't be worth anything. For as much time as it takes to remove the headers and look inside that would be my first step.
 

·
Registered
Joined
·
8,147 Posts
Discussion Starter · #7 ·
Just an update for anyone who searches in the future: the issue turned out to be that this motor had Hooker Super Comp headers with a couple slip tube primaries. What solved the problem was clamping header wrap around the slip joints so the headers couldn't suck in air to combust the fuel in the pipes. We should have re-tuned the car with the wrap on the headers, but the guy who owned the car moved to the other side of the state...
 

·
Butch
Joined
·
8,209 Posts
A friend of mine with a 347 Ford has a backfiring problem. When he gets on it and then lets off at the top of a gear, it backfires once through the exhaust. We datalogged the A/F, and it has an off-scale lean spike right at the time of the backfire. We just got it running a few weeks ago, and we put Proform metering blocks and a Proform main body in the carb (not my idea). It doesn't do it with a brand new 750 Demon on it, so it must be the carb or the tune in it. I have my own theory as to the cause of the problem, but I'd like to hear your ideas first.
All you have to remember is bad sounds such as popping, back firing etc means the following,,,

Out of the exhaust, its rich

From the carburetor, its lean
 

·
Registered
Joined
·
8,147 Posts
Discussion Starter · #9 ·
Very good advice, trmnatr. I'm sure there are exceptions to the rule, like a lean miss or something like that, but by and large, I think you've laid out a very good rule of thumb.
 

·
Butch
Joined
·
8,209 Posts
Very good advice, trmnatr. I'm sure there are exceptions to the rule, like a lean miss or something like that, but by and large, I think you've laid out a very good rule of thumb.
After seeing many flat lobes, you see a trend

I told a friend with a hydraulic roller he had his valves too tight,,, when done running had to back them off 3 turns before anything was felt in the rocker :rolleyes:

If I couldn't hear, I couldn't tune. Before we went with a new converter I told Nate it was trashed, by how it sounded and he was like "WTF" but has seen how I have done by ear thus far, sent it into ATI and it had a bad sprag :p:p:p:p
 

·
Registered
Joined
·
8,147 Posts
Discussion Starter · #11 ·
I told a friend with a hydraulic roller he had his valves too tight,,, when done running had to back them off 3 turns before anything was felt in the rocker :rolleyes:
:smt119 Holy balls, man! You definitely have to pay attention when you're setting the lifter preload. I've set it with the engine running and with the pushrod twisting method, and it never ran better than when I used the pushrod twisting method. Some people have the feel, and some don't, I guess. :smt102
 

·
Butch
Joined
·
8,209 Posts
:smt119 Holy balls, man! You definitely have to pay attention when you're setting the lifter preload. I've set it with the engine running and with the pushrod twisting method, and it never ran better than when I used the pushrod twisting method. Some people have the feel, and some don't, I guess. :smt102
I lay three fingers on the side of the rocker, you loosen it until you feel a slight bumping, or hear it, then adjust it just till it goes away, then listen as you pre-load it, usually go from 12 o'clock to 2 o'clock but go slow and listen to the engine, as you go too much it gets a little rougher

This engine here in the video on my buddies car was done like this, with 222/230 @.050" on a 108 lobe center, with 10:1 in a 355 with a single plane intake. It has 220 pounds cranking compression and 17" vacuum. I did the engine also though. Also has a 750 double pumper I modified, and this video is with the car dead cold

 

·
Registered
Joined
·
8,147 Posts
Discussion Starter · #13 ·
I can't remember exactly how I did it with the engine running. All I remember was tightening the adjuster nut until I didn't hear lifter clatter, and then I preloaded it probably the same as when I adjusted it with the pushrod twisting method. What a mess with all the oil dripping onto the headers and down the side of the block!
 

·
Butch
Joined
·
8,209 Posts
I can't remember exactly how I did it with the engine running. All I remember was tightening the adjuster nut until I didn't hear lifter clatter, and then I preloaded it probably the same as when I adjusted it with the pushrod twisting method. What a mess with all the oil dripping onto the headers and down the side of the block!
Take an old pair of steel valve covers, glue the gasket to the valve cover, and cut the center out, this way you can adjust roller rockers with no oil splash

On stock rockers you can use the clips, but roller rockers you can't so I cut two valve covers
 

·
Registered
Joined
·
8,147 Posts
Discussion Starter · #15 ·
I would've done that, but I never had any stock stamped steel small block valve covers. Now I have a big block, and although I have a spare set of stamped valve covers, I have a solid cam and I don't like to trash my feeler gauges by adjusting valve lash with the engine running. It's just a personal preference thing, IMHO.
 

·
Butch
Joined
·
8,209 Posts
I would've done that, but I never had any stock stamped steel small block valve covers. Now I have a big block, and although I have a spare set of stamped valve covers, I have a solid cam and I don't like to trash my feeler gauges by adjusting valve lash with the engine running. It's just a personal preference thing, IMHO.
I don't see how you would adjust a solid lifter engine running,,,

Only ones you adjust running are hydraulic or hydraulic roller
 

·
Registered
Joined
·
8,147 Posts
Discussion Starter · #17 ·
It seems like I've heard of people adjusting solid lifter cams with feeler gauges with the engine running, but that doesn't quite sit right with me. Plus, I don't regard my feeler gauge set as a consumable.
 
1 - 17 of 17 Posts
Top