Part throttle miss and blue smoke on deceleration
 

Wondering if you can suggest a cure..... 455 HO 4spd 71 GTO.
Starts easy...runs smooth at idle and quarter throttle 80 mph cruise.
Engine misfire and slight backfire at slight throttle 50mph.
I added 10oz of Seafoam to 10 gallons of gas.
Although slight it’s there and can be eliminated with your help.
Also noticed Quite a bit of blue smoke on deceleration.
Engine doesn’t load up or run rough ever.
Your suggestions are greatly appreciated especially if you’ve experienced the same and overcome it.
Thanks in advance,
Bruno

It's junk...sell it to me cheap! :D


Back fire like into induction system or out the exhaust??

IS this a pretty stock car? Factory air cleaner assembly etc?

If known...when did you last do a tune up?

Plugs, cap rotor WIRES! points fuel filter PCV.valve,etc. Time and miles.


Pull the spark plugs, keep the in order, and take pictures and post.


Smoke on decel is likely from the valve guides.... Valve Guide Wear or deteriorated seals.
~Possibly poor crankcase ventilation.

Further diagnosis on smoking:
Does it smoke on heavy acceleration?
Does it smoke from valve cover oil fill hole with PCV valve removed and revving engine?

LOL....we’re still on our honeymoon,
I’ll dig deeper Monday and report back as soon as I can.
Thanks for the input.
Bruno

:wife:

Honeym:eek:n?

:popcorn:

LOL.... I Recently bought the car..... still in love!

Your valve seals are gone and the oil usage is lowering your fuels octant level and making it ping and knock at hard throttle applications beating the Pistons around, this then unseats the rings and makes for more oil usage and knock .

It's a bad circular issue!

If its still using the factory carb and intake it simply may need a slight recalibration on the primary side of the carb. I've re-tuned hundreds of HO carburetors over the years and the 1971's are the worst as far as fuel delivery at light engine load........Cliff

Thanks
 

Thanks

Thanks Cliff..... do I add fuel? Larger jet?

A different jet/rod combo is what I do for them here. The factory carbs are pretty lean on the primary side to please the EPA, and that was on better fuel than we have now......Cliff

Swapped out carb now she’s running so smooth. No slight backfire out the exhaust and no part throttle misfire.
Now wondering if my blue smoke on deceleration could be PCV issue. No smoke on startup and no smoke on full throttle acceleration.
Discovered that the carb is original and so is the block. I assumed that it was all exchanged as it’s got service replacement heads as noted by the X on centre ports.
How rare is a 455 HO Ram Air 4Spd A/C in 71?

SO what carb did you swap in...and out?
I'm not familiar with this X designating service replacement you speak of on cylinder heads.
Some later heads actually have an X suffix on the # there, but hey are not round exhaust port heads.
Some production #'s and specs
https://www.gtoheaven.com/gtostats/71gto.html

You should be able to find how to diagnose a PCV system online, eliminating a long explanation.

Thanks for the links. I had a spare 9263 carb from a 69 that I had rebuilt years ago. I always thought X designated service replacement.

Blocks. Are the heads round exhaust port?

Yes

Installed new PCV valve. Smoke on deceleration is now much less than before but not cured completely. Will try adding 5oz of Seafoam and see if it helps any. Leak down test to follow.

The 7029263 carb is not a direct replacement for the factory 455 HO carb. I would correct the lean issue with the HO carb and put it back in place. It is a special carb made just for those engines. Once the lean part throttle condition is corrected nothing out there will work better on that engine.

Also make sure that you plugged the hole in the air cleaner base after the carb swap. The large front vent tube on the HO carb sealed off any unfiltered air that would enter the engine as that hole is after the air filter.

I've seen a boatload of Pontiac engines ruined over the years and the 1971 thru 1979 Q-jets (and 1970 CA emission engines) used a large front vent tube that protrudes up thru the air cleaner base.......

Thanks for the tip Cliff. I’ll have the original carb back on next week and let you know how it works..

I guess I’m not getting it anytime soon? 🤑
🤔

I’m dialing it in for you LOL