73' 400 BB Breaks up at High RPMs ? Please advise....

[BMOTOXSTAR]

Once again, thank you for the input on this. This one is driving me batty.....


Just moved to AZ from OH, this may shed some light on my questions on this...

Did a complete tune up on my 73' 400 HP  BB stock (mild cam) Rallye Charger, 40,000 original miles. Nice clean running motor, back in Ohio.
Also had mechanic install a brand spanking new Edelbrock 600 Carb. It had a 750 & was breaking up real bad...

So, she ran great in OH, did only 30 miles before the move. The carb & tune up seemed to stop the high RPM breaking up.

Get to AZ, ran great, even in the heat on a 107 temp day. No breaking up. 200 miles on Charger in AZ.

I (like a idiot) put a full tank of low octane in....now the Charger starts breaking up at high RPM's pretty bad, cruising at 60-65 on the highway.


So far not even 500 miles on car since tune up & new carb. She only breaks up on the highway   

Where should I start with this...

1. Cheap gas?
2. Carb needs adjusted for AZ climate?
3. Ignition box? I have a Orange box I was told a different box could fix this?

Thank you all once again Mopar buddies.

[RallyeMike]

Could be lots of things. Start with the easy ones first. How is your fuel filter?

[BMOTOXSTAR]

Fuel filter is good, less than 500 miles with new filter.

[1974dodgecharger]

Fuel filter, car adjust,nets i.e jets, nozzles, etc,....no more bad gas anymore premium only etc...I'm in phoenix myself glad your here enjoy the heat.

[Cooter]

Until this becomes as common place here as "Hilpag", we will need to post again.
you mentioned orange box. Swap it out with a new one. If fixes, I recommend replacing ignition with another type.

Also, try putting a timing light on it and check to see if medium weights are moving. Many times, people read here about adv. Timing to 17-18* base, and simply cannot be done with early to mid seventies distributors without major rework of adv. Rate.

[firefighter3931]

The only variable is the low octane fuel....drain the tank and get some fresh pump premium in there and see if the problem still exists. 

FWIW, It's allways preferable to buy fuel from a busy gas station. A depot that moves a lot of fuel will allways have fresh stuff available in their tanks. In some of the smaller gas stations ; fuel (high test) can sit for months and pick up moisture (condensation) as well as going stale. 


Ron

[c00nhunterjoe]

He said its a stock 400.... it should run just fine on 87 in my opinion. Can we define "high rpm"? Octane in this application should cause pinging more then anything else. Too much advance? Erratic spark? Fuel pressure? Why did a mechanic insist on replacing the old carb?

[BMOTOXSTAR]

I had a 750 on there. It was really over kill for that motor, if anything all the 400 HP has is a mild cam. I believe the motor is all stock. It is #'s matching, although when I bought it 10 years ago it did have the 750 on there & an Accel SS Ignition Coil. Maybe that is why it breaks up at high RPM's. I wanna say that my tach says 5,000 RPM's at 60-65 

[Cooter]

5k @60-65?
Something's not right. You'd have to be running like 4.56-5.13 gear for that. Back to my original post, put a timing light on it and see just how much timing is 'all in' @ around 2500-3000 rpms..

[Nacho-RT74]

or simply tach is broken

I make barelly 3700 or so  @ 160-170 miles with 3:55

[Bob]

or simply tach is broken

I make barelly 3700 or so  @ 160-170 miles with 3:55

160-170 miles with 3:55. 

[Nacho-RT74]

No sorry! 160-170 KILOMETERS! LOL... Around 100-105 MPH LOL

[StockMan]

I had this start happening after making some carb adjustments to correct some 'Surging' at highways speeds (50-55mph).  The engine (bone stock 440mag, 625AVS, with points ingition), would feel like it was running out of fuel when the carb began to demand petro at 3/4 to full throttle (4bbls begin to draw).  I added some larger primary and secondary jets finally, along with a new vaccum advance tuned in, a metering rod spring adjustment, and that solved the surging at highway speeds.  The engine idles and runs like a top at this point.  I was going to test the fuel pump to see if that was the culprit for the bogging, but then my Pertronix II system arrived, with the new coil, installed this and the bogging problem was gone.  Idle quality better, better power all the way through.  I guess with the fuel issues due to the carb tuning, I may have disrupted the plugs a bit maybe, and the weak points system couldn't keep up after that.  And note that I always had the points dialed right in.  I run the NGK plugs recommended by Ron (can't recall the number right now).

Hope this helps.