340 problems

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340 Dart

340 Dart

I don't know ****.
Joined
Feb 5, 2007
Messages
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Location
San Antonio
I put the 340 in the Dart last weekend and finally got a chance to run it last night and discovered some problems with it. I had expected high 12's and had planned on shifting at 6,300 or so. Instead I ran in the 15's and the car starts cutting out and popping at 5,000 rpm's. It is a slug under 2,000 and then wakes up until the cutting out at 5,000.

When you start the car up it want to idle around 2,000 until you put it into gear, then it goes down to about 800. You can blip the throttle and try anything you want but I couldn't get the idle down till I put it in gear. Weird huh?

It is a .40 340. Stock bottem end. Ported large valve J heads. MP purple shaft .484 lift. The following parts were robbed from the 360 when I pulled it. 750 Eddelbrock, weiand xcellerator intake, HV fuel pump, and MP distributor.

I am thinking Vacuum leak and was going to pull the intake today and reseal it. What do you think?
 
Since its a fresh install, I'm sure there is going to be a few bugs you'll have to work out. I'd just run thru all the basics until it was finetuned. Mike
 
Pulled the intake this morning and resealed it. Same problems.
Pulled the dizzy out and replaced it. Same problems.
I'll swap carbs this afternoon or tomorrow.

It is still ideling about 2000 until you put it into gear then it goes down to 800. When you put it back into park it is ok for about 15 seconds then slowly climbs back up to 2000.
 
I've used WD-40 to troubleshoot engine vacumn problems. While the engine is running, spray a little around suspect areas. Listen for any change in engine RPMs. Make sure all vacumn hose oriffices are capped. Your high RPM problem is a little mysterious. Vacumn leaks usually don't come into play at high RPMs. Distributor timing and carburation would be suspect. If you can get the symptoms at high RPMs with the car out of gear, try a dose of gas into the venturi of the carb. This may tell whether you have a leaning out problem. If all fails, I would resort to a diagnostic hookup at a good auto service facility. Just some ideas. Good luck. Howard
 
TIMING FIRST!!!

Make sure the pipe plug is in the back of your carb or you'll have a nasty vacuum leak.

That engine should idle in the 800-850 area no problem.
 
For the popping and cutting out at 5000 RPM: Make sure no spark plug wires are touching each other (especially those that are next to one another on the distributor cap).

For the High idle issue start with the obvious: Is the idle adjustment screw set correctly? Is the throttle cable pulled too tight? Check idle circuits, float level…or rebuild it.

Also, that cam likes quite a bit of initial timing…and yes, even with everything working correctly, it will be a dog under 2000 RPM (just the way it is with that cam).
 
Swapped the carb to a to a known good 600DP. Same issues.
We did a leak down test yesterday, 135-150 in all cylinders.
Cannot find a vac leak anywhere on the outside of the engine.
Initial timing was set at 24 with total at 38.
Pushrods are not too long.

Our best guess is that the when the previous owner put the cam in he didn't degree it or got something wrong so I guess I am tearing it back apart.
 
Try a different distributor. I had on once act just like yours, the advance weights started to move at 1000rpm and it would slowly climb in rpm to full advance because the springs were to weak. A cam wouldn't make the idle climb if it was off.


Chuck
 
Do you have a throttle return spring? Had that issue on the Dart Sport and all that was wrong was that the butterfles were open just enough to mess everything up. My timing was similar, swapped carbs as you did but had no double return spring to pull pressure to have it closed all the way. $ corner adjustments were all 2 out, and Idle screw was maxed out. Keep updating please, this is a good one.
 
I don`t know if you`re running a vacuum advance or not but you might try it without just to see how it reacts. Check your timing with a different timing light than what you`ve been using (may be the light). Also the initial timing may be too advanced which would cause high idle and breaking up at higher rpm when all the timings in. If you have a MP distributor check the advance in the distributor at 3000 rpm. I have one that picked up 3 more degrees from 2500-3000.
 
What diod the old intake gaskets look like when they came out? Is there raw fuel or marks from it (along with carbon) up in the intake port of the heads? Start with basics. You have compression, so it should be either fuel or spark. Is the tank full? is the fuel pump new? You set base timing. Have you checked voltage at the ignition coil? At the ballast resistor? Are you sure the plug wires are on in the right order? Dont race it until you fix it. You're just asking to break something worse.
 
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