[kdeman1@yahoo.com]

Hello guys!

I am excited to make my first post. I need your support to figure out an issue. I just can’t seem to solve. My Pontiac 400 will not allow me to go over roughly 3500 rpm in second or 3000 rpm in 3rd (Th350) and begins “popping” and bogging, basically I either can’t really enjoy the power of thing in city streets, or take it on the free way comfortably.

In my heart, I’m worried about a looped cam.

What I’ve checked,
Coil resistance -okay,
Coil ground ~1 ohm of resistance,
Fuel filter for clogs,
Spark plug gap, .05
Timing 32/34 total timing, roughly 17/18 at idle…

And I still can’t fix it!

Anyone have any ideas?

[4dblnkldude]

What carb is on it?

[kdeman1@yahoo.com]

Factory carb, engine is a recent rebuild. 30 over, but the cam / ect, all remain the same.

Another hunch, I replaced, the rotor / cap / coil but not the distributor base…. Could the components in the base be causing grief…? It’s been a big resto project.

[kdeman1@yahoo.com]

Sorry when I say “same” I mean basically stock. They are new components.

[autobahn]

sound like your ignition module

[srmmmm]

A condition I've encountered on newer cars when the core in the catalytic converter collapses.

[jonra3]

Something else to check:
transmission module vacuum line to the carb. Make sure it is connected, and no vacuum leaks.
Ive had this similar problem.
Hope this helps.
Jon

[4dblnkldude]

Originally Posted by jonra3

Something else to check:
transmission module vacuum line to the carb. Make sure it is connected, and no vacuum leaks.
Ive had this similar problem.
Hope this helps.
Jon

Good call!

[kdeman1@yahoo.com]

Thanks guy! Cheap me didn’t replace the distributor base and I pulled it apart today… gonna try and replace this capacitor… looks like Napa sells a module that looks similar tho not exactly the same I can try.

In the photo the capacitor has been removed.

[kdeman1@yahoo.com]

Sooo…

I’ve replaced the distributor ignition module, the capacitor in there, the alternator (was running high voltage)

And I still have the issue…!

Other ideas? Maybe my spark plug gap at .05 is too high?

Thanks!

[DanC]

If the engine makes a "poomp, poomp, poomp" kinda puttering sound when accelerating from a stop you probably have a cam/lifter problem.

I had a '73 GP SJ 455 that was like that, good power (I thought when I bought it) however a month or so later while going down the interstate about 80 mph the engine let out a hellava backfire and lost all power, I could barely get above idle but was able to make my exit and limp home. Later I pulled the valve covers and spun the engine over with the starter and had no movement on two rocker arms. After pulling cam and lifters I found two completely rounded lobes and all others where in various rounding condition and all lifters were concaved!!!

A new cam and lifters woke that 455 up!

DanC

[kdeman1@yahoo.com]

Ahhhh,

That’ll be a sad day! I just had the motor rebuilt !

And I went with a flat tap cam… did you by chance make the upgrade to a roll rocker cam?

[kdeman1@yahoo.com]

I took a video and let’s see whatcha think!

Looks okay to me! I video’d both

https://m.youtube.com/watch?v=RX_XoSXZ4qY

[DanC]

Originally Posted by kdeman1@yahoo.com

I took a video and let’s see whatcha think!

Looks okay to me! I video’d both

I agree that side looks fine, I presume the other side looks the same.

I did use a flat tappet cam and, at that time, Rotella T oil for the zinc properties. Now I use Valvoline VR1 20w50 conventional for the same reason.

Usually "popping" (backfire) thru the carb is an ignition issue and thru the exhaust is a fuel issue. Perhaps that can narrow it down for ya.

Long ago I had a very fine crack in the dist cap of a '77 301 that could not be seen, but showed up on a distributor machine. But I think that didn't allow the engine to start if I remember correctly. If you have an old school shop around with a machine, I'd remove the dist and get it tested. At least that would eliminate or confirm an ignition issue.

Also do you have a stock Pontiac style cam and wires hooked up in proper firing order? I once had just bought a good running '69 Mustang 351W and unbeknown to me it had an aftermarket cam. I did a tune up on it and it fired right up and idled fine, but would not rev up. After checking and double checking my firing order I decide to start pulling wires on the running engine. I wound up with an idling engine on four cylinders! So one by one I touched a wire to a plug. If no difference I'd put it to a different plug until the idle speed increased. After getting all hooked up, I wrote down the firing order and it was a 302 order. Now puzzled as I knew it was a 351W, I discovered aftermarket cams use a 302 firing order!

I hope something here can help you out.

["QUICK-SILVER"]

I've had weak/worn out or broken valve springs cause whats happening with this engine.

Found them by pushing down on rocker arms with the palm of my hand.

Clay