Vapor lock issues on a 1972 Roadrunner 400 engine

[kcracegirl]

Push rod was replaced when we rebuilt the engine. Just checked to

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It appears we are getting fuel in the carb - we did worked the throttle with the engine off and it is getting fuel. So here is our thought: could it be the distributor causing an electrical issue that just gives the appearance of a vapor lock issue? The current distributor is a mopar performance and was put on when we rebuilt the engine 3 years ago. Should we consider an MSD or would we be wasting 450.00?

 

[Propwash]

When you shut the car off when it's hot and go to start it a few minutes later, is it really hard to start and keep running? Or, is this something that only happens when the car is running? Is it an Edelbrock carb? Are the heat crossovers blocked off? Did you use a metal or composite type riser? Are you using fuel with Ethanol?

 

[kcracegirl]

It is not hard to start at all, it just runs rough - makes the tach jump then appears to have a miss. It only happens once the car has ran about 13 miles and warm. If you let it cool back down for several hours it will go another 10-13 miles then the same thing happens. We have a new Edelbrock carb with a composite riser. We have been using the 100% unleaded NO ethanol fuel.

 

[Ron Ward]

Just for giggles and grins, swap out the coil from another car to see if it solves the issue. I once had a coil that would break down and cause problems only when warm. I replaced it and solved all my headaches.

 

[kcracegirl]

We replaced it about 3 months ago thinking it was the problem but it didnt change anything. Will try another one though just in case. Thanks!

 

[Ron Ward]

The tach jumping around has me thinking this isn't a fuel issue.... Scratching my head....

 

[Propwash]

I wouldn't guess vapor lock if it's starting normally after shutting it down when it's hot. Usually that's half the battle with vapor lock, and it get's even worse after it sits for a minute or two. Being it doesn't happen until full operating temp, I'd guess something with the ignition getting hot or a vacuum leak issue, possibly at the intake manifold gaskets. Spraying some starting fluid around the base of the carb and/or the intake to head area, and then noticing a change in RPM is a good indicator of a vacuum leak. As far as the ignition, i'd double check nothing is getting scorching hot, (like mentioned your coil), ECU ect..ect.. Also check the inside of your dist cap for arcing/burn marks across the plastic inside. Good luck...Unfortunately there's quite a collection of different things as well of combinations of things that can cause the same similar issue. Just gotta check'em off till you hit the nail on the head.

 

[Budnicks]

I was thinking the same thing as Propwash & Ron Ward

Are you running a ballast resister, if not your coil is probably overheating

Is the ECU grounded well ??

check all your ignition wiring, for good ground & good connections at the terminals,
some dielectric grease on connections will help too

Sounds like ignition issues possibly, {but could still be a fuel delivery problem}
if it runs fine until warm, it's getting weird after it warms up, you have no problems starting when warm, or fuel delivery,
it possibly/probably the coil or bad ignition controller or both...

could be cross-firing/arching between wires or in the distributor cap
are your wires separated, you can see the arching really easy at night
is there any orange dust in the cap ?? means cracked cap
are the contact points at each plug connection on the underside of the cap all white-ish/crusty oxidized looking ??
you could scrape them off & try running it again, if it helps buy a new cap & rotor
is the contact point on the top of rotor look good on the cap & rotor ?? is it at all black ??
if so replace both cap & rotor try to buy a quality brand cap & rotors, with bronze/brass contacting points,
much better than cheapo/bargain brand silver-ish pot-metal or aluminum types of contacting points on the rotors or caps

did you set the air gap in the distributor to 0.08 inch with a non-magnetic/bronze feller gauge ??

What carburetor & what intake are on this engine ??

Do you have the choke stove/heat-risers blocked off ??

Is your fuel line anywhere near any heater hose or exhaust pipes/manifolds ??

Does the engine water temp overheat or get on the hot side ??

Have you checked for vacuum leaks ??

or possibly it's the carburetor lean surging,
because it's way too lean past the idle circuits,
especially with Edelbrock Performer 4bbl Carbs,
they are notorious for these type problems {some AFB & AVS Carters too},
especially right "Out of the box" & not properly tuned, for your specific combo,
especially if your engine's been modified, beyond stockish...

Just spit-balling here,
But we might need more detailed information & all symtoms/spec.'s, to correctly diagnose the problem...

 

[kcracegirl]

Thank you for all the info! Will get right back to you with all the answers to your questions. I really appreciate the help....

 

[kcracegirl]

-Its a dual ballast resister
-The ECU appears to be grounded well
-The gap is set at .08
-The intake is a mopar dual plane for a 400 cubic inch motor
-Carb is edelbrock 650 cfm AVS Thunder series
-Heat risers are closed
-Fuel line is a descent distance away from headers and exhaust, with minimal rubber connecting hoses, the original metal fuel line is heat wrapped.
-the water temp is good, is controlled by electric fans that turn on at 185 degrees
-have not checked for vacuum leaks yet but will get that done
- have not adjusted the new carb as it seems to run good until it gets hot in the engine compartment
-it was rebuilt, bored 30 over, everything is either Mopar or Comp Cam products from Summit Racing.

Thanks again for all the help! We do appreciate it...

 

[Budnicks]

have you checked your sparkplugs, plug wires, cap & rotor or cross arching yet ??
also the plug & plug-wires connections,

did you get water on the ignition at any time ??, moisture under cap, causing oxidation etc.

Plugs could be fuel or oil fouled if it's a newer build

bad wire resistance/less voltage getting to the plugs, especially when the plug wires get warm,
can also cause some similar type issues too, crappy wires or bad connections or contacts,
some quality ultra-low ohm resistance plug wires are a good idea also,
don't skimp on tune up parts, there's allot of inferior/cheap crap {AKA Bargain Brand/Chinese made usually}
out there today, that doesn't last very long

I added some content about the plug wires, distributor cap & rotor, to my original post, after I thought of a few more things

just spit-balling ideas
for stuff you can easily check

 

[kcracegirl]

We replaced all spark-plugs last week with accell to eliminate those, we checked all the wires for arching, didnt see anything on the tester. The plug wires are also accell 8mm. No water or moisture in the cap or ignition that we could ever tell. THANK YOU again for all your help! We are really trying to figure it out. I want to cruise and hit some car shows, just not on a rollback.. Lol

 

[SlinktRR]

[we pressure tested the fuel and we are getting 7-8 psi consistently. Thanks for the idea though. We do appreciate it.

QUOTE=WP29440SE;910098389]how do you know its actually vapor lock and not weak fuel pressure or flooding?

7-8psi is pretty high for the eddys. I have the same carburetor as you guys, the 650 AVS, and fought stalling for a good while. Turned out it was flooding, would start up and idle fine but would stall out while driving at around town speeds. Edelbrock recommends 5.5 and these carbs are known to potentially flood with high pressure. Try running a regulator in front of the carb, the dial types can be found at most auto parts stores. Hope you get it resolved!

 

[TRAMO]

borrow or buy a15.00 12v coil and eliminate that.i had the very hard warm start problem and the new coil cured it so far.