Question: Car is a 1970 SS 396/350hp all stock.

On the Highway today I hit the throttle hard, got great response then when I came down in rpm's (exiting the highway) she seemed to run rough like not hitting on all cylinders.

Well, drove it home (17 miles) running really rough. Its almost stalling at idle then when I come out of first gear is shakes like as if I had severe tire shake at a drag strip. Temp, oil is good.

Normally at 60 mph I run about 3000rpm, well now at 60 mph I run about 2600 rpm.

My guess is fouled plugs haven't checked yet, don't think its timing issue but not ruling it out. Vac lines at ok.

Can a cracked intake manifold gasket produce the same symptoms.

Thanks

------------------
Bruce
70 Chevelle SS (http://home.attbi.com/~bppare11/bppare11.html)
"It's not the men in your life that counts, it's the life in your men"

Sounds like a dead cylinder, and yes, a bad intake gasket can cause these symptoms. Check the plugs and do a compression test. Let us know the results.

Thanks Randy,

Any recommendations on spark plug type. My setup is a stock 1970 396/350, drive is once a week and yes I do get into it from time to time.

Thanks

Ran a compression test and replaced the plugs.

Compression results:

CLY COM CYL COM
8 145 7 140
6 105 5 140
4 145 3 140
2 126 1 143


So what does this indicate:

1. Compression rings gone.
2. Dropped a valve
3. Gaskets? Head, Intake
4. ????

Looks like a good winter project

Edit: Still runs terrible

Any suggestions.

Thanks

------------------
Bruce
70 Chevelle SS (http://home.attbi.com/~bppare11/bppare11.html)
"It's not the men in your life that counts, it's the life in your men"

[This message has been edited by Chevy_Rat (edited 09-13-2002).]

Almost sounds like a vacuum leak, but if you say you checked it ok, I would also check vacuum advance, have you done any significant engine work recently that you may have rerouted vacuum line to wrong ports on carb? Good Luck.

Jim
------
71 Chevelle frame off 462/560 convertible, red with white stripes, interior & top
------------------------------------------
1952 Chevrolet Deluxe Sedan 216.5 CI with RAJO (racing Joe)Power head (performance Cylinder Head designed by Grandfather in the late 40's )
http://chevelle.ogleonline.com/jim.htm

#6 is noticeably lower than the others, so start there. I'd pull that vavle cover and look at the pushrods and springs, then have someone crank the engine over while observing the travel of rocker arms. If you observe one that's not moving up and down as much as the others around it, you may be looking an collapsed lifter at the least, or a wiped cam lobe at the worst. But make sure you don't have a bent pushrod or broken valve spring first.

A vacuum leak will not cause a lower reading on the compression gauge. It only causes the fuel/air mixture to lean out for a particular cylinder or group of cylinders. But, since I alwasy profess checking the easy things first, you do owe it to yourself to check for a vacuum leak first just to rule it out. You can do this by spraying around the base of the carb, the vacuum hoses and their connections, and intake to head mating surfaces with WD-40 or an aerosol carb cleaner like Berryman's. Do is short bursts so it doesn't get sucked in the inlet, which will give you a false indication. But if you don't find a leak, proceed on the valvetrain.

[This message has been edited by Randy Mosier (edited 09-14-2002).]

Though I've never done it, I've read that if you put a little WD40 in the cylinder (through the spark plug hole) it will seal the ring for long enough to find out if the compression loss is through the valve.

If it's a valve, the compression won't come back up. If it's a ring, it will come up.

It makes sense. Maybe there's a better fluid to use rather than WD40.

That will at least save you from pulling the valve cover if it isn't in the valve.

------------------
My Web Page (http://www.landry-family.com) (updated 06-21-02)

"Long Live Freedom!"

Chad Landry
TC Member #643
ACES Member #4556
'68 El Camino, 357, L31 Vortec heads, 700R4,
8.2 10-bolt, 3.55 gears, Auburn Pro Posi.


[This message has been edited by cjlandry (edited 09-14-2002).]

In addition to the compression check, do a leakdown test on your "weak sister" cylinders. Use the WD or a squirt or two of motor oil.

Don't foget to check for a broken valve spring also. You might want to put a vacuum guage on a manifold vacuum port, run the engine up to about 2800-3000 rpm and note the reading, if the needle is jumping 3 or so inches of vacuum you probably have a spring issue. besides it's alot easier than tearing your engine down

Lets try a simplistic approach to this.

1) You did not mention any problems with the way the motor ran prior to this problem so I assume it's been ok.

2) Next your crusing down the road and decided to wind it up a little for some fun and it runs fine when you do this.

3) Then when you slow down the car runs rough and almost stalls at slow speeds/idle.

Since the car ran fine just priro to jumping on it something had to have happened very suddenly to cause this.

As far as your comp goe's your 1 cylender thats at 105 or so is low but should not cause a miss. When you get below 100 psi like at 80psi or below thats where you begin to feel a miss at idle from the low comp. 100psi and above will not cause the car to miss,shake,run rough etc all the sudden like in your case.

Also,an intake leak is unlikely because this came on too suddenly,intake leaks generally come on more slowly and don't suddenly appear bad enough to cause the motor to run terrible like your did from 1 minute to the next.

You have to remeber that your motor was running fine then all the sudden acted up after you got on it so give these items below a try.

A) I have seen (first hand) carbon break off pistons and get deposited into the sparkplug gap/s shorting the plug/s causing the motor to missfire so check the plugs.

B) You could have disloged some dirt in the carb which can get stuck in the small passages for the idle circut and partially clog a main jet too which will cause this problem which i have also seen (fisrt hand) too. With the motor running slowly cover the primary side of carb and if it starts to idle better chance are you have a carb problem.

C) Less likely but still possible is if the timing chain is streached out and loose enough it could have jumped time(skipped teath) when you suddenly let off the gas after winding it up a little so verify that this is ok.

D)I am assuming that if your running a stock ignition that you have already checked your points and in general your dist cap/rotor to ensure they are not damaged as stock GM dist sometimes can break rotor/caps if the dist shaft is worn and has play in it and the rotor tip crashes into the dist terminals and things go bad from there.

E) You did not mention if you motor still ran quiet even though it ran rough. Is there any valtrain noise? Also you could have overeved the poor old 402 and lost a rocker are or two which i have also encountered more thaqn once ove the yrs too so check that out if all the above checks out.

You could save some work if you start the motor and pull the plug wires off one at a time to find which cylenders ar not firing/running and if your lucky they are on the same bank. You would then only have to pull of one valve cover to check out the rockers on that side and not both. You can also do a rough check on your cam at the same time to see if all the rockers are moving approx the same distance up/dwn to verify the cam ok. I dont really suspect the cam because because your problem came on very suddenly but you may as well check it out while your valve cover/s are off for piece of mind.

Please let us know what turns out to be your problem.

Scott

[This message has been edited by SWHEATON (edited 09-15-2002).]

Well Guys,

Did a leak-down test on # 6 no change, as a matter of act it was reading 0.0

Pulled the valve cover off and found that the intake valve spring on # 6 was broken.

Every other spring is a-ok.

Guess its not too bad after all. http://www.chevelles.com/forum/smile.gif I was meaning to replace the valve cover gaskets anyway. http://www.chevelles.com/forum/smile.gif

Now, isn't there a way to pump air into the cylinder thru the spark plug hole in order to keep the valve in position while removing the valve spring?

If there is, can a person rent a air compressor with the fitting attachment for a do-it-yourself project without tearing down the engine.

While I'm at it I might as well replace the valve seals to.

I'm assuming that the pushrod and lifter are ok but ya never know. What are your thoughts on this one.

Thoughts?

Thanks for all the help guys. Man this site Rocks.



------------------
Bruce
70 Chevelle SS (http://home.attbi.com/~bppare11/bppare11.html)
"It's not the men in your life that counts, it's the life in your men"

Your pushrod/rocker should be fine,but i would definately have someone thats changed springs with the motor in the car and knows how to also adjust the valve train to help you out. It sounds like you have some idea of what's going on but that you have not done this before so get nthe experienced help to avoid any disasters when the spring is an easy fix at thias point.

It's not hard to replaces springs but it could be tricky and you don't want to drop a valve into the motor and have to remove the head to get it back.

Yes you can use a compressor to do the job by pressurizing your cylender.

You should really spend the 50-70 bucks replace those tired springs before you damage something if another spring lets go. You can also replace your valve seals too which is a good thing to do on a 32 yr old motor.

Your motor will likely rev better with fresh springs and also put out a few more h/p too.

Good luck.

Scott

Thanks Scott,

I was thinking the same thing about some professional help on those springs. Sure don't want to chase a valve down a rabbit hole.

There is a guy that owns his own mech shop (and he's a 69 Chevelle SS owner) which I'll turn to for the pro help.

Guess I'll replace all the springs and valve seals

Thanks again.

Sounds like you got this one whipped! Now it's just a matter of having an experienced hand guide through the spring replacement. The pushrod can be checked by rolling it around on a flat surface. Glass coffee tables are great for this, but don't let the wife/mom/girlfriend catch you doing it. http://www.chevelles.com/forum/smile.gif

Randy,

You and A36chevy were exactly right about the valve train.

Thanks for steering me in that direction.

Now just sitting here thinking..... should I pull both heads, do a valve job replace lifters, rods, valve seals.

And what could I do to these heads increase a little bit more HP without changing else to the engine (all stock block) or fear of increasing hp and breaking something else.


Thanks
Thanks

BTW,

Any recommendations on valve springs, retainers, seals

I'm not trying to talk you out of anything, but if you just want to replace the broken springs and don't have a compressor, you can turn the engine over by hand until #6 is at BDC on the intake stroke, feed a bunch of rope through the plug hole, turn the engine some more so the piston pushes the rope up against valve faces, perform the religious act of your choice, and proceed from there with spring replacement.

Then again, since one broke you'll probably just be waiting for more to break if you don't change them all.

For a bone stock engine, I'd go with stock replacement springs. I don't think I'd go with one with a very high pressure. Yours are old and fatigued, and new replacements should be adequate. Here's a link to Scoggin Dickey's spring selection. (GM Performance Parts)
http://www.sdpc2000.com/cart.asp?action=catalog_start&catid=961

[This message has been edited by Randy Mosier (edited 09-16-2002).]

To answer your other question, how is your oil consumption? I wouldn't go into the engine unless needed. For now, you're better off just replacing the springs and stem seals, unless you have an oil consumption problem. If it's burning a little oil due to worn valve guides, now's as good a time as any to address it. If the oil consumption is acceptable to you, then I wouldn't roll the heads off. Just take care of the springs. I would recommend a spring kit as opposed to buying just springs though. The kits will come with new retainers as well as springs, and you should always install new retainers.

I agree with Randy, replace all the springs. Maybe just the one right now, but buy them all and change them all as soon as you can. The first time I broke one, I just chaznged that one, and 3 weeks later another one broke. If one has broken due to fatigue, the rest aren't far behind.

I just broke one about 3 weeks ago. My solution is a new short block for the new heads I have on the bench, that's where I been keepin my new springs http://www.chevelles.com/forum/biggrin.gif

chris

Randy,

I don't have an oil consumption problem, I change the oil every 3k miles and have never had to add oil, and exhaust is clean. http://www.chevelles.com/forum/smile.gif

I'm planning on changing all springs with the spring kit. I was poking around at Jegs and Summitt looking for the valve seals, no dice. Guess I'll go to the local parts store or Chevy dealer for those.

Edit: BTW, When I was an Army Recruiter back in the early 80's I was stationed in Weatherford, TX, right next door to FT Worth, man, I loved that area http://www.chevelles.com/forum/smile.gif We did our shopping at Carswell AFB when it was open.

bulb122, I like the way you think http://www.chevelles.com/forum/biggrin.gif I was thinking of a ZZ502 to correct my small problem. LOL J/K

Again thanks for all the help with this problem. My learning curve has significantly leveled out since I joined this fantastic site and I am constanly learning for guys like you all.

All The Best.....

Very Respectfully

Bruce

[This message has been edited by Chevy_Rat (edited 09-16-2002).]

Pardon for jumping in here on the tail end of things.....I didn't read where the low cylinder pressure on #2 and #6 was resolved. Low reading on #6 is probably the valve spring. I'm assuming with no oil consumption, the rings are ok.

FWIW, Don't know your expertise, but consider pulling the heads and getting a valve job. It's realy not much more work than installing 16 springs with the heads on. Nice car!

------------------
-Mark "Some of us got it and some of us ain't." -Deputy Sheriff Barney Fife
TC# 717
ACES #1641
1967 Chevelle SS396/375 (http://www.chevelles.com/showroom/1BadRat2)
1967 RailVelle (http://www.chevelles.com/showroom/1badrat3.jpg)
1964 Chevelle 283/195
1975 Corvette 396/425 (http://www.chevelles.com/showroom/1BadRat1.jpg) "Wow, that's a mighty strong 350!"
Rat Garage (http://www.chevelles.com/showroom/RatGarage.jpg)

1BadRat,

I found a broken intake valve spring on #6,
First time I did a compression test it was 105, second time it was 0.0

I'm guessing a weak spring on #2 as well.

If one goes the others can't be too far behind.

Plan is to R & R all springs, retainers, and stem seals, conduct another compression test and if all goes well then thats it, if not then pull heads and valve job.

Thanks for the compliment on the car, and of course your advice (Much Appreciated).

Thanks

Chevy Rat,if your motor has a fair amt of miles on it you could still do the valve job and be ok if you atr not usingmuch oil and you dod a leak down test for some indication that the rings will take it.

No,it's not a good idea to put new lifters on a used cam,could end up wiping out a few lobes and sending metal material through your oiling syetm.yuk.

If you decide to do the valve jod it's a perfect time to dump the whipy stock cam that comes in the 350hp 396 and step up to a cam that will still give a decent idle/vacumme (if your running auto)but will aslo give you some xtra umph. I have a 69 396/350hp and stepping up from the stock cam 214/218 deg dur @ .05 .461/.480,115 L/S to 222/226 dur @ .05,.525/.525,112 L/S with stock int/ex manifold,man did it wake this motor,it made a big dif.

If you do the heads try the manley race flow necked down swirl polished valves alnd also have them do some mild pocket porting with a little cleanup in the intake/ex ports too for some free xtra H/P.


Let us know what you decide to do and it you need some help with cam selection.

Scott