ELECMAN

I just had the mech install new 455 exhaust manifolds in the car and a new carb the old owner never replaced the frame mounts and the exhaust manifolds and left the 350 exhaust manifolds in it and frame mounts bad move.i had the correct ones put in now.the problem they are having is the car idles to lowand wants to stall.they said that the timing is to far advance and it's also burning rich.they said the lifters may have to be torked down i have a set of g heads does any of this makes sense input would be appreciated.is there adjustment on the lifters and where should the timing be set at a little advance or not

70 cutlass s

None of it makes sence.
How is your timing set and what is your idle speed.
What size is the carb. and what make of carb.
What distributor do you have.
Is the engine stock.
What else has been done to the engine.

ELECMAN

I have a HEI dist and the original rockchester 7042251 carb remanufactored.GOING TO HIM TOMMOROW TO HAVE THE TIMING CHECKED.THIS WAS A 455 REBUILT FROM THE BLOCK UP HAS 2000 MILES SINCE THE BUILT,LIKE I SAID IT HAS G HEADS AND A F BLOCK.THE ORIGINAL DIN'T HAVE A HEI DIST.

svnt442

Where is the initial timing set? With a stock HEI it should be 20° @ 1100 RPM. If they are Chevy mechanics tell them to blow about where the timing is set because they don't have a clue.
The rocker bridges get torqued to 25 ft lbs when the cam is on the low part of bot lobe on that cylinder.

mugzilla

The new motor mounts have changed the static level of the floats ...

Let some air out if the front tires if that doesn't work try over inflating ...

Just kidding...

A dyno tune with some distributor curving might be in order ...

Don L/442

Do I understand correctly that you have 350 exhaust manifolds on a 455?

They will bolt up but wont flow right for a 455(back pressure)could be a problem.

Don L/442

Misread thread-My bad

compedgemarine

if the timing were advanced (all thing equal) then the idle would be high and it would be lean. on an Olds there is no adjustment to the lifters if it has the original style rockers and bridges. changing exhaust manifolds would not have a big effect on the idle but you state that they put on a new carb which sounds like the problem if it idled OK before. you should check the timing and anything past that is probably the carb itself

ELECMAN

Before i put the new carb in.the car engine use to shake like it does now.i was told if i want the engine to run smooth i may lose horsepower is this true.the original owner said he had a big time northen texas mech build the engine he told me that he thinks the pistons are 30 over and a little better cam then the original.ever since i changed the manifolds wow what a difference in horsepower and performance.from what i can gather the mech has his freind coming by sat to adjust things on the engine he has over 30 years exp building race cars for what he says.what svnt442 said im going to pass the word on about the timing at 20 degrees.thanks for all your input guys with out this website i wouldn't have got my engine right.i tip my hat to all you guys thanks.

mugzilla

He's going to build and tune the engine, I'm impressed. Some off these guys like silence when they work. Genius at work ...

blueRAYwhale

I have had and currently own two 1976 Oldsmobile Cutlass's with 455's in them. I,ve wrenched on Olds. for quite a while. I thought the 75-76 3-bolt exaust connection manifolds were interchangeable between the two motors, they have the same cast # on them. Maybe on the older ones(two bolt conection), that had a dual exaust option they had different port sizes between the two motors. Do the gaskets look the same?

ELECMAN

The existing manifolds that were on the 455 engine came of the 350 engine that was the original engine in this car.The gaskets are the same.But for the longest time I started to get exhaust manifold leaks in the gaskets ,after changing them 5 months before.I put up a previous thread.and I was told to put w and z manifolds on.After putting these manifolds on I have increased in horsepower and pick up that I was lacking from the old manifolds.The previous owner said that the 350 exhaust manifolds will work,yes if it was a stock 455 but there were cam,lifters and piston work done over.but side by side the exhaust manifolds chambers look the same but they are different.but the same gasket.The only down side is the existing manifolds pipes were different, The passenger down pipe was 2 1/4 inch and the driver side was 2 inch changing over after the 90 degree bend to 2 1/4 back to the mufflers.The only thing is the exhaust flanges bolted up fine by the drivers down pipe hanges a little lower then the passenger that I was told in a previous thread.But going to change it so the drivers side is 2 1/4 all the way.

blueRAYwhale

Take all of your manifolds to a machine shop that can perform milling/grinding/sanding operations to them. Have them bead blast them, then remove whatever amount of material they typically do to correct a warped condition. If you've checked the manifolds and they appear to be flat do it anyways. Manifolds can be warped and not leak if they are left undisturbed at/on their original factory instalation/location or they can appear to be flat(as checked with a machinest straight edge), and still leak with or without a gasket. This may or may not correct the problem with a given set of pieces however if you mix and match them you may discover a combo. that is satisfactory. After doing this loosen the motor/trans. mount bolts and exaust pipe connecting bolts near the muffler. Start the car so it alligns itself(motor/trans.), in the frame, tighten down the motor mounts, trans. then exaust system bolts/fasteners(car off). Drive the car a see if you got any vibrations or got rid af any. Retighten the exaust manifold bolts, sometimes torquing them isn't good enough. You have to hand-tighten things(not over-tighten), until you get results. Maybe remove the gasket if the facory didn't intially/originally use one. Make sure all the factory exaust mountings/hangers are in place. If any of the exaust system ever got bashed on something while driving maybe the heads are messed-up, check that.