Technical SBC gurus I need some opinions.

I am having a problem with Mechanical fuel pump failures. The thing is its not the diaphragm that is failing. It keeps breaking the pivot pin for the pump arm out of the housing. The pump still works but will not supply enough volume for the engine when running hard. I first found this last year at the Meltdown drags my car ran a 12.89 @109 next pass it slowed and acted like it was starving for fuel after checking all the usual things and finding nothing wrong.
I drove it to the HAMB drags (400+ miles). It did the same thing slowed to 14's. When I got home I pulled the pump off to find the pin pulled out of the housing. New pump (Napa SBC mechanical) pump took it out and it ran hard again. Fast forward to yesterday at the Hunnert Heads up Drags First pass was pulling hard and right before the traps I felt it stumble a little it still ran a 13.00 @ 104 next pass back to the same thing just past half track it starts starving for fuel again. got it home and same thing only this time only one side of the pin was pulled out of the housing. The pushrod is a new Chevy Performance parts chromoly push rod. But when I measured it it measures about .020 too long. 5.770. Do you think this is enough to cause that? They cant be set up on that tight of a tolerance for pump lever travel can they? Any ideas? anybody heard of this before I sure haven't. I guess I could grind and polish the pump end of the push rod down a bit and try it again or maybe double the gaskets up on the plate and the pump to gain a little room. Whats your thoughts. Thanks in advance Mike. (and before anybody replies that I'm breaking them on installation, I'm not)

 

Must be bottoming out the pump lever

 

There are a few different pump rods available and the BBC is not the same as the one for SBCs. If you have an aftermarket camshaft it may have a slightly bigger lobe. I like the rods from Comp Cams that have a bronze button on one end for cam wear.

 

I have seen them break on occasion. Not often. Regular daily driver cars. It might be a problem that an aftermarket cam has a larger lobe and the rod being a bit too long. Maybe put 3 or 4 gaskets on it first to see if that cures the problem. If the gaskets push out it might be still bottoming out. I seem to remember a car on occasion having a spacer plate between the pump and the block.

 

There is a plate between the pump and the block ,Right?

 

Yes, but its aftermarket. I just measured it it is .115 thick. If some one has an OEM one that could measure it maybe the after market one is thinner and that combined with the .020 longer push rod is the culprit. (although I still cant believe they are on that tight of a go/no go tolerance)

 

Just checked a stock plate= came in at .140 w/o a gasket. Might be your problem. You could stack gaskets, but that's a pain in the ass. I'de look for a stock plate!

 

What kinda RPM are you turning ??
dave

 

6200 max I try to shift at 6000.

Thanks! .045 between the pushrod and the plate. I would thank they would allow a bit more tolerance than that, but that's a possibility.

 

if that .045 is with the rod off the lobe, and sliding out, that's not going to do any good...... The rod has to be on the lobe at full peg outward as actuating the pump arm. Just say the pump arm itself will compress 1/4 inch, but with a longer pump rod/thinner plate it will see 5/16 or more..... You have a stock push rod, but the plate thickness.... Not too drastic in the plate, or rod, but sometimes thousand's come into play.... Then there's the thin and thick gaskets which I've seen. Then the cam, but that would be a fluke, but seen crazy shit before..... Possibly start with stock stuff that's been around forever. I know a PIA, but a starting point. Hate to see a fun day at the track turn into a shit sandwich..........

 

the pump in my brother's wagon failed like that last year. Put a new replacement in, and it's been fine so far. I thought it was just defective. I guess it's a thing now?

 

Jim, I have thousand of miles the failed pump even after it failed they still work for normal driving. They just lose enough volume that they wont keep up when I'm racing it, which isn't very often. Hell I drove it 400 miles home from the HAMB drags like that (the one pictured) last august. The new one I believe failed during my first pass yesterday as I just felt it at the end of the run. Could it float the push rod hard enough to break the pump? I wouldn't think so but maybe? I hate it when I cant find an exact cause of a failure. I have the same pump on my blown 327 for years without a problem.

 

This one was acting like that, then it slowly got worse, then died completely. Looked like the crimp on the pivot pin was weak or something.

 

A crappy picture.

 

Have you tried a higher performance type pump, like a Holley or Carter?

The stock replacement type, (like those pictured), in my 60 Impala didn't have the pivot pin break out, but one of the valves wasn't staked in well, and popped out of the housing.

 

I have a new Holley pump in a box on my bench, but I really dont want to use it. I would then have to run a regulator (AFB carb) and really don't want to get into all that. A decent stock pump should be sufficient for this car. Maybe no one produces a good stock type pump anymore. The stock type Napa pump Ive been using is the same one that is marketed by Carter I think. I am going to buy a new light weight push rod and try an OEM cover plate and try again I guess. I may stack the pump gaskets since they are pretty thin to try to gain a few more 1000ths.

 

Take an extra pump shaft and file a sharp step in it lining up with the old motor mount bolt holes in the front of the block (by the front cover). Gently turn the engine over while levering through the bolt hole to see if you are bottomed out on fuel pump arm lift. Or cut your fuel pump plate so it looks like a two bolt gasket, reach through the resulting open space, and check for pump arm bind while you gently turn the engine over.

 

Measure the total stroke available (if you haven't already) in the pump then the total travel of the push rod. Might be a pain to bump the motor to that spot but at least you'll know what's doing what. I too suspect the pump as more the culprit than the rod/sizing.

 

Those pumps are low quality, street duty. If you want to race get a high quality one like Edelbrock or Holley. The pump mechanicals are doing the equivilent of valve spring float and that is what is tearing them up. Higher quality pumps will have the mechanical pieces of such durability and spring pressure that the pump drive arm will stay in contact with the pushrod & cam lobe. I'd also suggest getting a light-weight race duty pushrod with the brass tip to act as a bit of a cushion and the lighter weight makes it easier for the pushrod to stay in contact with the lobe, a stock one is heavy and too much mass.

 

Carter has a "street" pump that is very good. It will stand up to racing. Its much better than a stock pump, no regulator needed.
http://www.jegs.com/i/Carter/180/M4...YL2o-r-5AVqdVIgOLVImgumo-AfV_vcqOgaAuEi8P8HAQ

 

Thanks for the replies in gonna call Lunati and see what they say about the cam lobe lift or if they have run across this before too.

 

The pump that failed in my picture is a stock Carter pump. I haven't had trouble with any others.

 

I like turnaround's direct measurement/binding check through an inspection hole in the fuel pump plate.

WWSD - What would Smokey do?

 

Wouldn't be having this problem if you had a Ford motor in it...........

 

Good ol' quality parts from off shore. I have not had the same problem you describe, but...... I replaced my fuel pump, which was leaking fuel, about 3 years ago. That pump just blew a seal between the lever and diaphragm and was leaking oil. It was an Airtech.

 

Might try the lightweight rods out there. Heavy weight stock rod and performance use puts that heavy momentum on the pump lever. Circle track guys even use a rod that has a roller at the cam lobe.

 

That's true! it wouldn't run hard enough to break it haha!

 

That's the pump that I ended up putting on my Impala. It has worked quite well, without any problems. Did not need to use a regulator, either. The engine is a stock .030 over 300 hp. 327, except that it has a new 600 cfm, Edelbrock AFB style carb. By the way, the photo was taken before the Edelbrock carb was installed. But that's another story.

 

They are supposed to have a plate between the block and the pump, if it does not have one ( as many rebuilders do not) then you will have problems for no apparent reason. I usually have at least one of those plates around the shop, I keep them whenever I get one incase I decide to run mechanical.

 

It has the plate ( Aftermarket) , but it's about .025 thinner than the OEM plate. I have located a an OEM plate and ordered a new tubular pushrod from Comp Cams. I'm very limited on clearance for the fuel pump too so some of those bigger mechanicals may not even fit. ('40 Ford chassis)

I called Lunati about it. He said that's not something he's had before related to the cam lobe and the lobe height is to stock GM spec. ( although he didn't say what that was)

Like I said I suspect maybe it's a combination of the push Rod length and the plate thickness and the pushrod floating all adding up. I have the same pump on my blown 327 for almost 10 years now without a problem and I've run the shit out of it routinely.