1973 Mustang Mach 1 backfire

Hello everyone!

A friend and I are working on our first muscle car. We have it to the point where it can start up but we are getting a "popping"/backfire from the carb when the throttle is opened quickly. If it is opened slowly the engine will spool up just fine and everything sounds great.

http://www.youtube.com/watch?v=WFqsXKlhqqs

Car is question:
1973 Mustang Mach 1
4160 600cfm Holley
351 C
67,000 miles
4 speed

Compression Check Results:
Cyl______Reading
1________150
2________180
3________150
4________145
5________160
6________180
7________150
8________150

As of this point we have replaced the solenoid/starter/ distributor cap, points, rotor and condenser, and ignition coil. The gap and timing are set accordingly to our Chiltons Manual.

One Last thing, there is also a slight ticking noise at idle, don't know if that relates to the backfire or not. And if anyone could hook me up with a comprehensive vacuum diagram I would greatly appreciate it as well, stupid emission controls

Thanks for your help!
-- Stephen


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Ayuh,... Fix the accelerator pump in the carb,... Sounds like it's going Lean on quick acceleration..

It's the accelerator pump... Every Cleveland I've built has liked a nice fat accelerator pump shot. I had a Edelbrock carb on one that I never could get the pump shot dialed in, they're not as flexible as the Holleys.

Here's what I've got on the vacuum routing.

Attached Thumbnails

   

Updates:

1) Carb rebuilt, replaced diaphragm in the accelerator pump.

2) Crownver, thx for the diagrams but they only have 1/2 as many hoses. I have 3 or 4 T's in my lines too. Don't suppose you might know where I could find a more comprehensive one?

3) Someone made the suggestion since I am running a 600cfm carb on an engine that was supposed to have 750, I may want to try and swap out a bigger pump nozzle. This sound like a valid solution?

So, I am the proud owner of a wiped cam. Interestingly enough, the heads have been machined recently and adjustable rockers were put in. Now to get a new cam and figure out why it got worn in the first place...

With your next cam make sure you install it with lots of break in lube. Get a zinc additive like GM's EOS or ZDDP to mix in with your oil. I would suggest using Shell Rotella, it has been recently reformulated but still contains more zinc than your of the shelf valvoline from what I understand. The Zinc helps the lifters and cam lobes wear in together without smoothing the lobes off. Stay away from synthetic oil. Don't even consider reusing the lifters, even if they're new. If it won't go above 3k RPM I would imagine you've got more than one lobe flat. If it was my engine it would be coming apart for a thorough cleaning, but I'm guessing your not gonna go that far. Change your oil shortly after break in, and a couple more times shortly after getting it running to get as many of the metal particles out that you can.

How did you set your lifter preload? If the heads were milled you may need shorter or adjustable pushrods, your lifter plungers may be fully depressed.