Idle surge

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sox and martin

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My 340 has a surge at idle when it's hot. It will idle at 650 then jump about 100-150 rpm every 3-4 secs then fall back. Last night I noticed that when deccelerating and popping it into neutral the idle would drop to 1000-1100 and stay there for 20 secs or so. At that point it would idle down and carry on as usual with the surge. The engine is relatively new (600) miles. 10.7:1, eddy heads, MP performance electronic replacement dist.,msd blaster coil,Crower stage 3 hydraulic cam, rpm air gap (no end gaskets just silicone),brand new holley 750 vac, 727 w/2500 TCS convertor. Fuel pressure starts high when cold about 12 psi, but drops to about the 5-7 psi range when hot. The fuel pump is a mechanical unit. The other strange thing is that when the car gets to temp, it almost idles smooth?? This is a .500 plus lift hydraulic with 227-237 at .050.
 
vacuum leak? Maybe the pcv? I have seen bad pcv's make engines do wierd things. Just a thought.
 
The condition you describe is an indication of a lean condition. Vacuum leaks as suggested is one place to look but the carb may need to be richened up too. The idle mixture screws should start at 1.5 turns out then go from there in 1/8" turns until you get max rpm. If you get any of the surging at light throttle crusing you will need to go up in main jet size.
 
Easy to check for vacuum leak. With engine running, just spray carb cleaner around the sealing and flange areas.
 
If you dont have adjustable rockers, the lifter preload may may a little off too. No big deal, but the heads expand as they wamr up. So in many cases, an aluminum headed car will run rougher when cold. If it's fine when at operating temp, I wouldnt worry about it. After the vaccum leak checks, you may also want to check your timing curve. Too light a spring can cause the timing to raise a little, raising the idle speed when it does. It may also just be a little out of adjustment carb wise too.
 
I pulled the intake. What I found was oil on the gasket between it and the aluminum head on both sides of the engine? I redid the lifter preload. The rockers are adjustable. I did not find anything out of the ordinary.I would like to know who uses the end gaskets on the intake and who uses silicone? THe carb had been enrichened on the idle circuit. That did not seem to help. Upon reassembly I wiil swap to a second carburator just to see if it is there. I am hoping that the intake was the culpprit. The advance springs did occur to me, but I figured that since it is a new out of the box unit and the car idles 600-650, that it wasn't the cause. I will check for that with my timing light. Thank you for all the replies.
 
Check the thickness of the gaskets on top and bottom of the ports. It's common to have the lower section not seal well if the heads or block had more than a touch of milling done. I dont use the end seals in most cases. Either Ultra Black, Ultra Grey RTV or "The Right Stuff".
 
What kind of gas are you running in this engine?Also 12 pounds of fuel pump pressure is way to much.
 
dodgeboy413 said:
What kind of gas are you running in this engine?Also 12 pounds of fuel pump pressure is way to much.
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The fuel is 94 octane pump gas. The fuel pressure drops to 5 psi when up to running temperature. I need to put a reg on it. I will also have to check for compression on the intake gasket. The heads are after market and the deck needed alot of straightening. They removed a fair bit of material. Thanx.
 
94 octane pump gas is no good for a 10.7:1 engine.You will in all likelyhood crack a piston.Or have a complete piston meltown.Good luck.
 
sox and martin said:
...the deck needed alot of straightening. They removed a fair bit of material. Thanx.
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Then they needed to remove the same amount from the intake surface of the heads, or from the intake, so it fits properly. Maybe they took some off, but not enough? You can use clay to check and see if the intake hits all spots evenly, or if it doesnt hit the bottom so well.
 
dodgeboy413 said:
94 octane pump gas is no good for a 10.7:1 engine.You will in all likelyhood crack a piston.Or have a complete piston meltown.Good luck.
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Not true at all!!!!

With the aluminum heads he is running he could run 11.5:1 on pump gas. 10.5:1 on a well though out iron headed engine is not a problem at all.

FYI, the more duration a cam has the higher the static compression ratio has to be just to keep the cylinder pressure in the same range as a stock engine.
 
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