I have tried to eliminate a bogging condition experienced during hard accelaration beginning at 3,000 RPM in third gear particularly. The secondeies kick in then it just falls and can't get up until I let off the gas for 15 to 20 seconds. I have gone through the bowl levels for and aft, the floats diaphram,spring etc in the carb all are fine and the distributor checks out. Now I suspect the brand X mufflers installed by the "other guys" insurance company while I was not looking after a rear ender. Could these unknown mufflers be the culprit? How do I check this out? The SB Base engine has been hard to time which seems to be due to the modified cam which I don't have the history on. Is there a quick way to determine the cam specifications short of taking it out and looking at i($$) ? Car runs fine otherwise.

 

My apologies if my questions may seem off base, but you didn't state the year of your car so I'm not sure what carb you're using (even assuming that wasn't changed with the cam). Here's some food for thought:

Why not try adjusting the linkage to eliminate the secondaries, and/or to open the secondaries sooner and later to see if you still have the same acceleration flat spot at the same place (ie, maybe they're pouring too much gas into it, or they come in too early or too late). How do you know the secondaries ARE actually kicking in at 3000rpm?

Is there any indication that the plugs are misfiring while it bogs? ...If not, it sure sounds like the secondaries are not functioning correctly. ...If so, it still could mean the plugs are getting fuel-fouled, I guess, and that could explain the need to get off the throttle for an extended period to clear the problem up.

Is anything happening at the tailpipes while you're having the problem (ie, white or black smoke)?

What do you mean by your statement: "the distributor checks out"?

Any way you could temporarily use a replacement-type carb for testing?

Once you let off on the gas and recover, will the engine then accelerate past 3000 rpm; ie, are you saying you can go above 3000 smoothly if you use mild acceleration?

I can't think of an effective way to check the mufflers, but I wonder why they would be affected only under hard acceleration. Is the exhaust heat riser functioning?

You might check to see if the gas tank cap vent is clear, or if there are any weak rubber hoses at either end of the long tank-to-pump gas line, or in the vacuum systems. Also install new fuel filter(s), if you haven't changed them recently.

The cam can be "spec'd" without removing it by using a degree wheel and measuring lift at the tops of a pair of pushrods while rotating the crank. Still not an easy task.

 

Check your accelerator pump. Make sure the squirters are squirting. The diaghram may be torn, etc.

 

A Chevy V8 with a 4V carb can accelerate past 3000 with a single exhaust.

IMO:All of the above questions need answers for more help. All of the above suggestions are great. 90% of all fuel problems turn out to be ignition. Change your plugs, wires, and timing before buying/rebuilding the carb.

 

Hi..
Sounds like classic fuel starvation.
Some common causes:
- If Holley, and still have the internal bronze filters, "chuck 'em" and
install a good external filter. (This is a known problem)
- Check any or all fuel filters. (Internal/external)
- Fuel pump. (maybe, but not likely)
- Hole in fuel line before the pump. (Picks up air - you usually smell gas or see the leak)
- 'Sock' in the tank is partially plugged.

If it is ignition, you don't usually get the "big bog", you just get breakdown,
missing, poping in carb. etc.

When you do find the problem, please post the results.

Thanks
Barry

 

Waynec your questions are good ones have me thinking. Mag Red Man,magicmachines and Barry's70LT1 points are helpful. Here are more details. The engine is as base 66 SB with the stock holly carb .
I assume the secondaries kick in because of the sudden increase in power when I first step on it. On gradual accelaration it gets past 3,000 rpm and more no problem and also when it clears after bogging.
The distributor was just recurved and during that work the cam question naturally came up since it became obvious that it was not a stock cam . The heat riser is not frozen and moves freely( at passenger side exhaust manifold neck, right?)
I don't see any white or black smoke during the bogging. I am not sure if it misfires at this point what would be the symptom ?
I have a new acclerator pump and new diaphram. Now that I write this I remember that before the new diapham was put in this never happend but it never had a real surge of power when punched at 40 to 55 mph. I am going to check plugs and filter (the stock bronze internal one and maybe the next to go) Next would be to spec out the cam. I welcome more suggestions.


[Modified by midyearinterest, 5:10 PM 12/5/2001]