Popping under quick throttle opening - Ford Forums

351Powered · 04-09-2009, 20:37

Hello everyone!

Still having a problem w/ the Mach 1:

Problem: A "popping" noise/small backfire from the carb when the throttle is opened quickly and only quickly. If it is opened slowly the engine will spool up just fine and everything sounds great. The popping happens more easily when the engine is under load. For audio/visual reference please watch this clip:

YouTube - Mach 1 First Run 4 08 09

Car is question:
1973 Mach 1
4160 600cfm Holley
351 C
4 speed

I would really like to get this car to the big show at the University of Maryland (April 26). I'm thinking a valve or timing chain problem off hand?? Please help!! Thank you!!

**Big Block Ford** · 04-09-2009, 22:18

Can you give a little bit of history on the car.

Have you done engine work or has the car been sitting a long time etc.

It could be ignition timing out by a long way but I would think starting the engine would be difficult.
If that noise is definately coming out of the carby then I would have to wonder if there is a sticking valve or perhaps bent valve or maybe a broken valve spring.

A simple compression test would help pin point which cylinder is the issue if a valve is the problem.

351Powered · 04-10-2009, 17:27

1) The 'popping' is defiantly only coming out of the carb (it doesn't sound as deep IRL as it does in the video)

2) It does not only occur under load. You will hear it at idle if you give a quick opening to the throttle.

3) The timing is set and the firing order has been checked several times.

4) Put new rotor and cap on today, that seemed to help the amount of popping a little.

5) There is a slight light sounding ticking at idle which is making me think it's a valve.

6) The car hasn't run for the last 9 months with no work done to it.

7) Did a compression check:
Cyl______Reading
1________150
2________180
3________150
4________145
5________160
6________180
7________150
8________150

**Big Block Ford** · 04-10-2009, 23:14

Those compressions don't look like an obvious valve open or bent problem to me. It might pay to do a thorough check that the leads are on the correct plugs although I see you have checked the firing order. Be sure that the firing order you are checking is for the Cleveland as some windsors have a different order. Firing order is 13726548. Check intake manifold for cylinder numbering as they sometimes have it there. The numbering runs down one bank in a row with the most forward cylinder as number1 and the one behind is number 2 etc. If all this checks out then maybe taking rocker covers off may be next step. There could be a bent pushrod or faulty lifter maybe. I have seen pushrods punch a hole through the rockers before. Inspect valve springs as well and maybe start it with one cover off at a time and check valve/rocker movement.

Will be interesting to find out what has caused it but if sitting for a while then a valve may have jammed and perhaps bent the pushrod.

351Powered · 04-17-2009, 09:09

Double checked the timing and it's dead on 16 BTDC right where she's supposed to be. Also tried adjusting the accelerator pump but that didn't seem to make any difference. Gonna pull the valve covers either today or tomorrow, will post again to let you know what I find. Here is another video of idle and the 'pop' I keep referring to:

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

mil1ion · 04-23-2009, 20:38

NOTE: AN EXHAUST LEAK can sound like lifter tick.

**Big Block Ford** · 04-23-2009, 23:58

It sure sounds like it is popping out of the carby. It will be great to find out what is causing it. It also gives me the impression that perhaps it may be running on 7 cylinders when I see it idling. You could try removing one sparkplug lead at a time then refitting it while it is running. It should idle down rougher when removed if that cylinder is working and you may not notice any difference on a cylinder that is not working. This would at least narrow it down to which cylinder is the problem.

351Powered · 04-30-2009, 22:18

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...

**Big Block Ford** · 05-01-2009, 00:19

Ok at least you have an explanation now even though you didnt want to have this problem.

If it is the original cam then it may have degraded with use and once the wear gets to a certain point it just flogs the lobes off. I am assuming this is a hydraulic cam engine that someone has fitted adjustable rockers to. Perhaps they fitted them to address the worn cam or perhaps they have mismatched some parts and caused the problem. You will need to flush the engine to remove any steel particles from the worn cam and check what valve springs and rocker setup it has to match the new cam and lifters. I guess checking the oiling system would be a good thing too.

mil1ion · 05-01-2009, 19:15

do you have a high volume oil pump?

on comp cams web site they talk about premature cam failure with HV pumps