Here's a stumper for ya

[chevyaddict]

Ok you Chevy Gurus...... I need some help. I'm at my wits end here. All I ask is that you read through everything I've stated BEFORE you reply so you don't reply with something I've already tried.

OK, working on my 71 RS Camaro 350 motor. The symptom is a sporadic, what *appears* to be a misfire. Doesn't do it when its cold. Only after the car has warmed up. Now, if it is warm and I don't catch street lights - no problem, runs smooth as a whistle. The problem only starts after its been idling for a couple minutes. Once I get up to speed, it slowly works out of the misfire.... So, in other words, the longer it idles, the more it sounds like it is running on seven cylinders. Then, as I get up to speed, providing I don't hit the next red light, it slowly smooths out and then will run perfectly smooth. Ok, now for all the things I've tried or thought of:

Bad plugs - changed them - no change
Bad wires - changed them - no change
Bad cap/rotor - changed them - no change
Fuel? Well, this is what I'm thinking it might be as I put an Edlebrock carb on it a few years ago (which I'll never do again; hate the damn thing). Its always been tempermental with the Edlebrock, but for the life of me I don't ever recall a fuel starved situation making it run *as if* it was misfiring, AND you would think that the opposite would happen, it would be starved under load and not at an idle. I richened the the idle fuel mixture and that didn't help. Its NOT that it is too rich, I just took the car through emissions and it is not running rich AT ALL; if anything the car may be running a tad too lean.

Some other things I've thought of:
Collapsing lifter? Perhaps a bad lifter wouldn't have enough oil pressure at idle, causing what would appear to be a misfire. Then, under load, it would slowly fill back up and smooth out. And, this would make sense, I think, to only happen when the car is warm because when it is cold, oil pressure is higher.... the problem is I dont have a *tapping* noise as if the lifter was collapsing.

Loose timing chain and/or bad vacuum advance? I don't think the former because if that was the case it wouldn't matter if it was cold or hot. The latter the same deal - I don't think vacuum shifts enough in the motor from hot to cold to make the symptoms I'm having ONLY when the car is warm.

Vapor locking? Well, this car runs cool and has plenty of ventilation in the engine compartment (I have a 3 1/2 inch scoop on it) so I doubt it although this is what a friend of mine has suggested. He thought putting a baseplate underneath the carb might fix this problem.... what do you guys think?

Now, the car runs so strong when it works out of this *misfire* or *sputtering* that it is making it really hard to diagnose. I'm frustrated at this point. I've even already bought a new holley carb which I'm waiting for to see if it is just this Edlebrock carb.... if not no biggy - I'll put the Edlebrock on something else!!! HELP!!:confused :ugh

 

[gmjunkie]

I had this problem 2 weeks ago on my daughters Z-71!! The Longer it would run the worse it would mis-fire at Idle!! But you hit the throttle and no miss and haul azz!! Turned out we had #7,6,4 exhaust valves just a little too tight!! We had just done a head job on it at 180,000 miles,1/4 turn back and she's on the road now!!! If you are running unleaded gas and have the stock 71 valve seats,they are softer and ware more with unleaded fuel,and tend to disappear and tighten valves!! I had this problem with my LS6 BB in my hauling truck!! Reworked the heads and installed harder seats for unleaded fuel and that was the last of my problems!! P.S.Run's great on regular or premium fuel now with no spark knock or difference in fuel mileage or performance!!

 

[g-force junky]

I go along with the lifter theory, try a little lifter adjustment (i know its a dirty job and a little smokey if you drip on the exhaust manifold) but you may be able to detect and correct it. just my 2 cents

 

[warren s]

I had a similar issue with my 70 GTO. The float in the carb was sinking, as I drove and there was some foward motion and the car ran fine, when I came to a stop the car started to sputter and mis.

It was at fast idle when cold so the choke would keep it running, but when it warmed up the problem would start.

 

[G Winter]

I had this problem 2 weeks ago on my daughters Z-71!! The Longer it would run the worse it would mis-fire at Idle!! But you hit the throttle and no miss and haul azz!! Turned out we had #7,6,4 exhaust valves just a little too tight!! We had just done a head job on it at 180,000 miles,1/4 turn back and she's on the road now!!! If you are running unleaded gas and have the stock 71 valve seats,they are softer and ware more with unleaded fuel,and tend to disappear and tighten valves!! I had this problem with my LS6 BB in my hauling truck!! Reworked the heads and installed harder seats for unleaded fuel and that was the last of my problems!! P.S.Run's great on regular or premium fuel now with no spark knock or difference in fuel mileage or performance!!

I find that intresting. I am reminded of a Ford 460 in a pickup with high miles back in the early 80s that was doing somewhat the same thing. It would miss when it warmed up and eventually missed all the time. It turned out the valves were sinking and the 460 had now adjustable rockers. The sulotion, we used a 5 in grinder and took a little off each valve stem sounds a little back yard but it worked and it ran for a lot of miles after that without trouble.
I think something with the valves is on the right track.

Glenn

 

[bossvette]

Cut out an old valve cover to keep the mess down, not pretty but it works

I usually back them off till they just start to "click" then tighten up 1/2 turn thats on the loose end of the recomended 1/2 to 1 turn for hydraulic lifters.
Good luck

 

[Tom Bryant]

I've run into the float problem before too with the composite floats. Seems like I read in Hot Rod a while back about someone having a sinking float problem with an Edelbrock carb causing an excessively high fuel level in the bowl resulting in a sputtering at idle. Higher rpms would compensate until the float got so fuel logged that the bowl flooded over. About the only way to tell would be to have a new one to compare weight against.

Valve recession is more prevelent in a truck or race ingine where it is subjected to greater loads but I wouldn't rule that out either. Adjusting the valves would compensate for the stem growing in length on the spring side as the valve head recessed into the head. If adjusting cures the problem I would suggest a head rebuild with hardened seats in the near future. '71 was the year that the compression was first dropped so engines could be run on unleaded. Many people confuse that with '75 when cat convertors were introduced and you had to use unleaded. Your machine shop can tell by inspecting the heads when they are off if the seats are hardened or not. I'm running '72 Monte Carlo 350 heads on my engine I built for my truck and they were hardened from the factory. I'm fairly confident that your original '71 heads would have hardened exhaust seats too.

Tom

p.s. Do you ever get down to Green Valley or do you steer clear of the geezers? My wife's mom and aunt live down there. They don't consider themselves geezers though. :L

 

[chevyaddict]

Hey Tom - I used to go down to Green Valley alot in the good old days trying to circumvent having to go through emissions! DMV got smart though; no getting around that these days!

 

[chevyaddict]

Ok, these are really good suggestions - now another question for ya'll. *If* it is a valve issue, wouldn't I have some evidence of something awry on the spark plugs?????

And, I did order a new carb - its on its way so I will obviously try that first before I go pulling the heads off. I'll also try adjusting the valves first before pulling the heads as well. If I pull the heads I'm going to rebuild them because I know they have not been rebuilt at all.

 

[gmjunkie]

Ok, these are really good suggestions - now another question for ya'll. *If* it is a valve issue, wouldn't I have some evidence of something awry on the spark plugs?????

Not necessarily!!! My LS6 had 5 exhaust seat's Gone!! (1,3,5,7,8)It still had good compression,it ran good, had a random miss at idle, it rattled a little in the top end,and was a little short on power!! But the plugs looked good!! I ran it 3 trips to Florida before I tore it back down!! But when I took 1 look at heads I could see the exhaust valves recessed up in the head!! The valves were seating on head castings!! Luckily it did not screw them up bad!!
Mine is out of a 72 SS 454 Chevelle!!

 

[chevyaddict]

Well, there is something odd going on with number eight plug; I've never seen anything like it - I think it is carbon buildup; certainly isn't oil fowled. The rest of the plugs were textbook perfect - but there was certainly something going on with #8. I surmise that if there is a valve issue it is happening in that cylinder. I suppose I should check the compression in that cylinder regardless just to see what is up.... it always seems #8 is the susceptible ones on chevies!!!!

 

[gmjunkie]

Well, there is something odd going on with number eight plug; I've never seen anything like it - I think it is carbon buildup; certainly isn't oil fowled. The rest of the plugs were textbook perfect - but there was certainly something going on with #8. I surmise that if there is a valve issue it is happening in that cylinder. I suppose I should check the compression in that cylinder regardless just to see what is up.... it always seems #8 is the susceptible ones on chevies!!!!

# 7-8 Those two cylinders run hotter than the rest,because they are at rear of Cooling system!! # 8 usually runs the Hottest!!

 

[vette-dude]

I wonder if the intake gskt might be blown or leaking on number 8 intake port. Just a random thought.

Randy

 

[gmjunkie]

I wonder if the intake gskt might be blown or leaking on number 8 intake port. Just a random thought.

Randy

That's very possible!! But antifreeze would usually make the plug clean and whitish,and vacuum would make the plug clean,whitish and eroded and lean looking!! But I haven't seen the Plugs!! My thoughts are valve starting to burn,Seat going south,loose valve guide!! Maybe even a bad valve spring or cam lobe!!

 

[vette-dude]

I was thinking more that it might be sucking oil out of the lifter valley. Had a engine that was decked when rebuilt but the aluminum aftermarket manifold didn't fit the "V" correctly. Tried using thicker gskts but still kept leaking on number 8 and sucking oil into intake port and fouled plug. Finally had manifold surfaced with corrected angle on port sides and it cured the problem. Never leaked any antifreeze as silicone around water ports seemed to seal them off. Just thought if compression check and lifter reset was okay and all plug wires are fine then there might be a random chance the gskt is allowing oil to be sucked out of lifter valley. Stranger things have happened!!

Randy

 

[gmjunkie]

I was thinking more that it might be sucking oil out of the lifter valley. Had a engine that was decked when rebuilt but the aluminum aftermarket manifold didn't fit the "V" correctly. Tried using thicker gskts but still kept leaking on number 8 and sucking oil into intake port and fouled plug. Finally had manifold surfaced with corrected angle on port sides and it cured the problem. Never leaked any antifreeze as silicone around water ports seemed to seal them off. Just thought if compression check and lifter reset was okay and all plug wires are fine then there might be a random chance the gskt is allowing oil to be sucked out of lifter valley. Stranger things have happened!!

Randy

Well, there is something odd going on with number eight plug; I've never seen anything like it - I think it is carbon buildup; certainly isn't oil fowled.

????:confused

 

[vette-dude]

Junkie,

The plug I removed on my engine certainly looked as though it was carbon fouled also. I reckon maybe the gskt material plus the oil and gas + combustion made it appear to be carbon impregnated. I ain't saying this is the problem just suggesting that it is something to think about late at night whilst you are trying to get to sleep (and leave the sheep alone as counting them won't help)!!

Randy

 

[gmjunkie]

Junkie,

The plug I removed on my engine certainly looked as though it was carbon fouled also. I reckon maybe the gskt material plus the oil and gas + combustion made it appear to be carbon impregnated. I ain't saying this is the problem just suggesting that it is something to think about late at night whilst you are trying to get to sleep (and leave the sheep alone as counting them won't help)!!

Randy

otfl Well if she go's to pull the heads,The Intake is the first to come Off!! It should the be evident!!

 

[SPANISHVETTS]

I had a similar problem once. It turned out to be a valve guide or at least I think that was the cause.:W After pulling the heads 3 times I sent them away for a total re-work and the shop told me that some of the guides were ¨well out of tolerance¨. Isn’t it funny that they did not find the worn valve guides the week before when they re-cut the seats?:eyerole

 

[TJay]

First I'd pull the vaver cover off #8 side and look to see if the valve sping is out of position. Also check the rocker stud too see if it is grooved on the side. At an idle if you have a slight gloove wearing into the side of the stud, your valve will hang up slightly, thus causing a mis-fire and not at higher revs. If it was worn badly, it would unload the spring and the pushrod would bend. But a slight wearing will create some of the issues you discribe. Also have a look to see that the rocker is sitting correct,this would be a sign of rocker wear around the pushrod or valve stem area of the rocker.