Bending pushrods on 351C …

Mike Braddock · #1 (**permalink**) 07-23-2007, 09:10 AM

Greetings from sunny Florida

. We’re having a problem with our engine that we can’t figure out. We’ve got a mild 351C with a flat-tappet hydraulic lifter cam and stock rocker arms. When trying to tune the first carb (700 cfm 4150), the engine backfired and bent a few pushrods, which seemed quite odd. We replaced the damaged pushrods, buttoned it back up with new plugs and a fresh 4150 from Jesse at BIGS Performance and the engine ran like a clock. We just put the car on the road this weekend and drove it 100 miles on Saturday with no problems. On Sunday we tried to start it, the engine backfired a few times so we cleaned the plugs and tried again. When we tried to restart the engine, it had a big miss and when we revved it to around 1500 rpm, we could hear an audible tapping noise coming from the valve covers. Next, we pulled the valve covers and found 5 more bent pushrods. Have you ever experienced a backfire that resulted in bent pushrods? We’re very puzzled. Thanks for any insight you can offer!

Mike

Roscoe · #2 (**permalink**) 07-23-2007, 10:52 AM

I think you should make or buy a pushrod measuring tool and get an accurate sizing of what you need. Then call these guys

http://www.pushrods.net

Roscoe

CobraEd · #3 (**permalink**) 07-23-2007, 11:10 AM

It sounds like before the engine is warmed up, the clearance is tight. The valves must be hitting the pistons. If this happens you slightly bend the valve and it will not seat. The result is a backfire. So, My guess is that you get the backfire as a result of contact with the piston, not the backfire causing the problem.

Mike Braddock · #4 (**permalink**) 07-23-2007, 02:01 PM

The engine ran fine for the 100 mile test drive though. And it already had about 7k miles on it before we bought it. Thanks!

Mike

Jac Mac · #5 (**permalink**) 07-23-2007, 02:28 PM

I would suggest that your valve springs are too light, particularly in regard to seat pressure, along with the distributor advance curve and accelerator pump adjustment/shooter size.
The 351c with its relatively large inlet valves is susceptible to having the lifters pump up at low rpm when cold if a backfire occurs. The explosion in the manifold lifts the inlet valve of the seat and the hyd lifter naturally takes up the resulting clearance. The pushrod then bends due to either the valve contacting the piston/ the spring becomes coil bound/ the rocker arm binds on the pedestal/stud/ the pushrod may 'jump' out of its socket in the lifter-rocker------ or a combo of any of these things.. At the same time there is a good chance that the power valve diaphragm will rupture and an extreme over rich fuel mixture will result.

Jac Mac

Historybuff · #6 (**permalink**) 07-23-2007, 04:05 PM

They prep Panteras which have a 351-C stock.
They manage to get some high hp. but I think they also sell lots of
more modern engines if you want to play with over 500 hp.
The Panteras that were in European racing in the 70s all blew their engines
even though they had a NASCAR builder building the engines at one point.
I'd love to see a Pantera with a Boss 302 in it, that was one of the most exciting American engines I ever drove, though it had a narrow torque band

Mike Braddock · #7 (**permalink**) 07-23-2007, 05:20 PM

Jac,

I was wondering if that could happen. You explain it very well. Thanks for the informative reply. If it was a clearance problem it would have happened prior to this.

Mike

Woodz428 · #8 (**permalink**) 07-23-2007, 07:04 PM

Everyone has given some suggestions, so what the he!!. I did like the comments about tight guides, but don't think that's it....it would get worse quickly instead of better and likely bend pushrods each time it was cranked. I would check a few things, first is lifter pre-load, because of the pump up issue already mentioned and then spring bind or keeper guide interference. If either of these 2 are close and the lifter pumps up because of poor pre-load settings it would create a bent a pushrod everytime. Head/valve machining will reduce preload and needs to be reset. Since the rockers are non adjustable there are 2 ways to correct it. The first is the old factory way with shorter/longer pushrods or the newer(far better IMO) shims. With the guide height and the hard valve stem seals it takes little to bend a pushrod when bind occurs...the leverage is in favor of the valve end not the pushrod.

Mike Braddock · #9 (**permalink**) 07-30-2007, 08:27 AM

Sorry for the delayed response; work was very hectic last week. First, let me apologize for posting this in Shop Talk instead of the Small Block forum; I didn't realize the other forum existed when I made the original post...

When removing the 6 bent pushrods we noted they were all on intake valves. We inspected the remaining pushrods and rockers and they were in good shape. We replaced the pushrods and two scored rockers, reassembled and checked for spring bind. Everything looked good. Next we verified the timing mark on the damper was at TDC. When we pulled this apart last week we noted the plugs were indicating a very rich mixture. So I dropped the primary fuel jets two sizes from a #75s to #73s.

When we cranked the engine ... nothing. No spark. It seems to help to plug back in the MSD pickup wires

. Hit the key again and it fired right up. We put a timing light on it and the timing was sitting at 4-degrees BTDC! After resetting the base timing to 14-degrees the engine was (understandably) taking the fuel a lot better.

So I think you guys nailed it when you suggested it was a combination of fuel and timing causing a backfire in the intake that lifted the intake valves off their seats and allowed the pushrods to jump off.

The car's definitely more responsive now. Thanks for your help with this question!

Mike