1967 Impala Drivability Problem

[MLimpert]

I'm having a terrible time with my 1967 Impala, and I am looking for suggestions. When I am driving down the road steadily or even at WOT it runs great. It also idles fine. My problem is when I have let off the gas from moderate to heavy acceleration, it begins to stumble and die. If I get back into it hard it runs fine. If i let it fall back to idle or maintain the same cruise throttle position it will die. The only way to restart it after it dies like this is by flooring it while cranking. It will usually run normally after the restart, until the next episode.



It is equipped with an original 396/TH400. It has a Edelbrock intake, and a Holley 750 double pumper. Otherwise, its pretty much stock. In an effort to fix this, I have replaced the carb with a brand new unit, as well as replacing the points with a points eliminator from pertronix.



Anyone have any ideas? I'm stumped!

 

[fkovalski]

Ya' gott yer Catyletic Converter all plugged up :-laf



Seriously- I always looked at the cheapest easiest areas to look at (and replace to cross off the list. ) You don't mention fuel lines. Are they original? Could they be collapsing?



Vacuum lines also might be dry rotted. Is it a vacuum advance on the distributor? (I am assuming timing is set correctly) Does this still have a capacitor on the ignition?



Could the float valve be stuck open? i. e. flooding out when not WOT? (You said you have to floor it to restart)





(just brainstorming off the top of me head on the info given)

Good luck- Keep us posted

 

[Philip]

Are the floats set correctly?

 

[jmeehan]

I'm just thinking out loud here, so don't be to hard on me.



I have a jeep with a 258 6 and Weber carb that does the same thing, only when it's cold. I've assumed it's the choke setting but your problem made me think.



If the timing is advanced to far, and you quickly let off the throttle you'll get a vacuum advance spike. I think. Or maybe something in the mechanical advance is hanging up, keeping the timing advanced.

How do the plugs look? It may be a plugged pasage in the carb. I know it's new, but it's still man made.



Did you do any work to the car before the problem started? Fresh tune, change in driving area (I'm thinking altitude difference)



Sounds like a sweet car, I knew a guy in high school that had a small block '65 or '66 S/S Impala.

 

[I-6DZL]

Just make a trip to MI and i'll get 'er squared away. How long has this sat. I'm assuming all winter, if not longer. How many miles ? Valves probably need adjustin. Timing needs tweakin. If alot of miles, timing chain probably needs replacin (timing light will easily verify this) Intake gaskets dry out, don't seal again, instant vacuum leak (damn echo's) Same with carb base gaskets, but since you put a new carb on it, you did put a new base gasket on too right ??? Is there a manifold spacer under the carb that may need a new gasket as well ???

When you store this car do you put moth balls around it, and also around the building it's stored in ? Do you plug the exhaust with say a baseball or something ? (won't let vermin in there with nuts, and dog food, and cat food)



Doubt its a vacuum leak, especially if it idles too good. On such a big carb, when you're WOT do you just let right off the accelerator or do you gradually back out ?? Kind of like not wanting to huff your turbo on your truck. If you back out too quick you'll huff it and take a chance on blowing it. Well your car is almost the same way, when you let off quickly it'll take all the gas, and can't burn 90% of it. Now she floods out, that's why you have to start again at WOT.



let me know

Curtis

 

[MLimpert]

Well I unhooked and plugged the vacuum advance and took it for a drive. It still has the problem. It only occurs with a totally warmed up engine and high underhood temperatures, and it is worse on hot days like today. I unhooked the fuel line and put a pressure gauge in, it has 4-7 psi depending on RPM.



It let out a nasty backfire through the carb while my head was over it looking at the fuel pressure gauge. That is something it has never done before. Besides scaring the crap out of myself, I think I might have also done some damage to my 7 hour old carb, because the cars idle speed changed after the backfire.



I haven't changed any driving habits or equipment on the car that started this problem. It didn't do any of this last fall, but as I have started driving it this spring it has gotten progressively worse.



The float levels were set to holley spec on both the old and new carbs. No fuel runs out when I open the sight screw on the float bowl.



I'm going to try to get a vaccum reading on the car in the next couple of days. I have the pleasure of working all day tomorrow, so I won't have a chance to mess with it.



Heres a Pic, alongside my former truck.



#ad

 

[MLimpert]

I-6DZL:



The car was driven about once a week all last summer, and about once a month during the winter when it was dry out. It stays in a nice tight insulated garage. The car has 146,000 miles on it. The engine was rebuilt about 12,000 miles ago, so all the timing chain etc should be nice and tight still. Initial timing is set almost to the end of the tab. 11 or 12 degrees I think. Carb base gasket was replaced this afternoon. Valves were adjusted about 8,000 miles ago when I changed to a roller tip rocker.



It doesnt seem to matter if I back out of it quickly or slowly. It will still do it to some extent either way. The car runs pretty good until it gets totally warmed up.

 

[AKAMAC]

Does it still have the stock mechanical FP? Did it get replaced at rebuild?



Is the distributer cap cracked? This is a warm engine problem also. The backfire is a tale-tale. Try replacing it... ... cheap trouble shooting. Mike

 

[MLimpert]

It has an edelbrock fuel pump on it. Its a mechanical unit mounted in the stock location. I'll get a cap for it when I get a chance.

 

[catoiler]

Check the float level it sounds like it is set to high. Pull the sight plugs out and adjust the floats till it just trickles out.

 

[Philip]

If you had a backfire threw that carb it might have taken out the power valve in it. That would be the reason the idle changed to.



Did you have this problem before you installed the electronic ing setup?

 

[fox]

It is rich, fuel problem. I'd say pull a plug and look at it for color. Are you sure the carb gasket is ok? ( talking carb gaskets on a diesel site )!!. Smell the gas is it fresh? At idle does the boosters in the carb drip? I'd also check the fuel ( gas ) pressure. I had one of the eledbrock fuel pumps that actually had 9 lbs pressure. This was about 15 years ago.

 

[RonaldLarson]

Uh huh

The backfire through the carb indicates a timing problem. Sounds like the chain is loose, or installed off TDC. Yes, it will still idle if these items are the cause. Also, when you changed to roller tip rockers, did you maintain the same rocker ratio?

Ron

 

[I-6DZL]

Now that you've had a backfire through the carb it's time to replace the power valve.

As mentioned above pull the plugs and check their color. Should be a medium brown color.



Do you have a timing light ?? Double check timing. Is there anything special to this engine?? Cam, Headers, stock ratio rockers ?



Curtis

 

[JeffWhite]

not solving your problem, but I bought a power valve protector one time from

summit racing I think good idea for anything with power valves. It was a little

poppet sort of thing that you drilled a hole for in the baseplate and tapped it in.

This was in the hole through the baseplate that feeds vacuum signal up to the

p valve. p. s. On a double pumper I once left out the little pin w/tapered

end that goes under the "squirter" nozzle just drops in the hole a check

valve I think. Made for some weird rich problems. But car had better

mid range than it has had before or since. I think it was pulling extra

fuel over through the nozzles.

 

[MLimpert]

The car has had a bit of a cam in it since the rebuild, but nothing too crazy. It has also run great in the past with these roller tip rockers (stock ratio).



The new holleys are supposed to come with power valve blowout protection, but I'm still going to have to disassemble it and see what happened, so I'll check/replace it. Something is hissing bigtime in the carb area now since the backfire.



The electronic points eliminator was an effort to fix this problem, not the cause. I really can't tell much of a difference overall since installing it.



I'm leaving work early tomorrow to mess with this fine GM product. We'll see what I can come up with. Thanks for all of your suggestions!!

 

[I-6DZL]

let us know!!!



BTW beautiful car!!!!



Curtis

 

[MLimpert]

Tonight I put in plugs (pretty nasty from all the poor running) and put on a new cap as well. I set the initial timing back to 8 degrees. The backfire through the carb had popped a vacuum plug off of the brake booster which I located on the floor and reinstalled. I also made a new fuel line from the fuel pump out of metal line and installed it in an effort to keep the fuel cooler. So far that stuff has eliminated my problem, although tonight was a cooler outside. I'm not convinced its totally right yet, but I think I'm on track.



Thanks again for all of your help!!