340 has me stumped

exile uk

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Need some input and help,since i got this 1970 340 i have had trouble getting her to run good or consistant.Gas tank is fairly new,vent line clear,new sender /pick up,cleaned fuel hard line with carb cleaner blown through with air then gas blown through with air,dirty now clean,all new fuel hoses,newmechanical carter fuel pump,new edelbrock av2 650(also tried 2 edel 600cfm off working cars)new air cleaner.new fuel filter clear glass type.new plugs,leads,pertronix dizzy cap,rotor,new igniter module,pertronix flamethrower distributor,new coil,voltage regulator,starter/ignition relay,new earth leads,alternator bench tested good,wiring checked and replaced if looking worn.
Just put vacuum gauge on inlet manifold and got steady 22hg at idle,i dont know cam specs but very mild.Idles great mostly,drives great on a 30 mile run,slight bog on wot but nothing too bad,then all of a sudden it will start stumbling and as i appoach a stop sign it wants to stall so i put it into neutrel ,i have taken gas cap off but no better,checked fuel filter and full of fuel,fuel lines not hot,car temp 185 deg,oil pressure good.compression test on all 8 good and even.timing 10 deg -12 deg static checked with 2 strobe lights.all 3 carbs fail the same.no engine smoke or coolant loss.starts first time.it seems a fueling issue,but tried 3 pumps all same.ill try find my fuel pump gauge and see what ive got?Trans fluid level is good and new.trans seems smooth.kickdown needs adjustment again.ive tried to include as much as i know etc,hoping someone has had a similar issue n can help.cheers jon
 

TT5.9mag

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It’s going to be difficult and time consuming but youre going to need to do the diagnostics WHILE you’re having the problem. Drive it until the stalling starts to happen and try to keep the problem “alive”. Check anything and everything you can during the stumble. A few things come to mind, heat related electronics failures (coil, ballast, etc) or fuel pickup sock clogging, Both common. You tried three carter style (edlebrock) carbs, all of those have fuel boiling/percolating issues. If you can try a Holley style carb.
 

exile uk

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It’s going to be difficult and time consuming but youre going to need to do the diagnostics WHILE you’re having the problem. Drive it until the stalling starts to happen and try to keep the problem “alive”. Check anything and everything you can during the stumble. A few things come to mind, heat related electronics failures (coil, ballast, etc) or fuel pickup sock clogging, Both common. You tried three carter style (edlebrock) carbs, all of those have fuel boiling/percolating issues. If you can try a Holley style carb.
 

exile uk

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Thanks tt,i have a spacer under carb and a large working hood scoop fitted,checked pick up filter and tank was flushed and very clean,no rust.once it starts stumbling it continues all the way home and is a fpita!do you think it could be ignition switch related causing weak spark?
 

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Keep doing the Process of Elimination.

Looks like your fuel delivery system is good.

See if you can round up a points distributor and a different 12 volt coil. Drop that in and see if your problem goes away. That will take your electronic ignition system out of the picture.
 

exile uk

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Keep doing the Process of Elimination.

Looks like your fuel delivery system is good.

See if you can round up a points distributor and a different 12 volt coil. Drop that in and see if your problem goes away. That will take your electronic ignition system out of the picture.
i shall try and borrow a working dizzy off my mates 73 challenger n try it,thanks
 

TT5.9mag

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Thanks tt,i have a spacer under carb and a large working hood scoop fitted,checked pick up filter and tank was flushed and very clean,no rust.once it starts stumbling it continues all the way home and is a fpita!do you think it could be ignition switch related causing weak spark?
Intermittent stuff is a pain in the *** but you can figure it out with good diagnostics. Sounds like you’ve been through the fuel system, so have you checked spark condition (quality) when it’s acting up? Should always have a nice blue snappy spark. That would be my next test. Or while it’s stumbling, just take a temporary wire from positive post on battery straight to the coil and see if it helps.
 

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It’s going to be difficult and time consuming but youre going to need to do the diagnostics WHILE you’re having the problem. Drive it until the stalling starts to happen and try to keep the problem “alive”. Check anything and everything you can during the stumble. A few things come to mind, heat related electronics failures (coil, ballast, etc) or fuel pickup sock clogging, Both common. You tried three carter style (edlebrock) carbs, all of those have fuel boiling/percolating issues. If you can try a Holley style carb.


The pro's know their stuff!

I have had to do a few things to get my stock 340 to run right

#1 I am quite sure the avs2 is as vulnerable as my stock avs 5lbs of fuel pressure TOPS and as stated in many build posts dial your fuel pressure in !st then your float level//to much and you can push the needle off of the seat-stumble might be flooding..

2) 93 + better gas with lucas octane booster

3) a cool carb spacer plate , insulated carb to fuel pump line, insulated fuel filter...

4) good plug wires and pay attention to the routing

// a holley is worse than an avs on heat soak, those big bowls grab Alot of heat off of the intake...// I ditched mine.. MA mopar had it right with the thermoquad !!!

not saying that it is but I haven't had any problem with my pertronix dual point conversation
 
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Murray

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Check plug wires individually with ohm meter, wiggling as you go to detect a broken wire.
 

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Allow me to throw in my .02 on this.. Do you know if the heads have had hardened seats installed? Are you using any fuel additive to compensate for the loss of the lubricating lead that used to be in the fuel? The problem you describe parallels the issues I have had with my 1969 Toyota Corona. The car would start/run/idle fine for the first 20-30 minutes of driving, then quietly develop a serious miss/stumble when returning to idle. Being a pushrod 4 banger of low compression,, this issue couldn't hide among the other 4 jugs like your engine can. I would pull up to a stop, pop the hood and start pulling plug wires... sure enough,,there would be a dead jug.. problem was,,, the dead jug wouldn't always be the same the next time it acted up. Sometimes, the idle smoothed out before I could start pulling plug wires. I deducted that to the valve and seat cooling enough at idle to cure itself. I took off the head, lapped the valves, reinstalled, set clearance ,,, Ran flawlessly and the problem went away for about 1,200-1500 miles.,, Slowly the symptoms would return, the car would never stall,,,but stumble and miss more frequently as miles added up. Please let us know what you find out.
 

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What I would do:
Let it stall! Quickly open the bonnet & activate the throttle lever & watch for a fuel spurt from the acc pump nozzle into each pri throttle bore. Also check that the boosters are not dripping.
If you get the spurt, it is a safe bet that the fuel delivery is ok.
Dripping boosters means the fuel level is too high in the fuel bowls. Could be leaking needle & seats, or fuel pressure too high. These carbs can withstand 10 psi, despite some BS to the contrary. Since you have tried a number of carbs, it is unlikely to be an internal carb problem.
If the above checks out, I would look to ign.
Get a small 3w lamp like a dash light. Hook one end to coil [+] & ground the other end. Make the wires long enough so that you can have the globe sit on top of the dashboard. Drive the car until the problem occurs. If the light goes out, you are losing power to the coil. Repeat the process, but connect the lamp to positive side of the ign module.

If all the above checks out, I would replace the module. There is a bloke, Dave Ray, on the net who reckons he has 55 gal drums full of dead Pert modules.
 

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Another place to look for electrical problems is the bulkhead connector where all the wires go thru the firewall. Look at the big red and/or big black wire where they go into the connector. If the plastic is melted around the wire, the connection inside the plastic block is bad. You can splice it thru the block until you can get a new harness or just make a permanent repair.
 

exile uk

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update-ive checked more and double checked,but still same issues.decided to unwrap all wiring around firewall and the large connecter block mounted on firewall.traced wire going to my positive terminal on my ht coil and it was a mix of 10 gauge,12 gauge and 18 gauge???connections are looking poor too.just found some other wire of odd gauge pieces poorly joined too.So tomorrow i will cut out junk wires and rectify step by step.What gauge wire should be running from ignition to coil positive?).0.6 ohm flamethrower 2 coil.Also mounted on the firewall is the voltage regulator,one green wire in from the alternator and one blue wire out to my ht coil positive?is that correct?it was wired that way by PO.Thanks for advice,its been big help,lets hope i get her sorted,ill update again soon
 

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Also mounted on the firewall is the voltage regulator,one green wire in from the alternator and one blue wire out to my ht coil positive?is that correct?it
no that is NOT correct.
The Dark blue wire is the Run wire from the Ignition switch.
If you have a points Distributor, it should be powered from a ballast resistor mounted on the Firewall. That resistor should be powered by the Blue wire from the Ignition switch, and that very same blue wire should be connected to the regulator.
The way it is now, it appears that the coil is receiving battery voltage at all times. It will run like this until the points burn up. If you alternator starts sending out an A/C signal, IDK if the coil will continue to work.
 
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exile uk

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no that is NOT correct.
The Dark blue wire is the Run wire from the Ignition switch.
If you have a points Distributor, it should be powered from a ballast resistor mounted on the Firewall. That resistor should be powered by the Blue wire from the Ignition switch, and that very same blue wire should be connected to the regulator.
The way it is now, it appears that the coil is receiving battery voltage at all times. It will run like this until the points burn up. If you alternator starts sending out an A/C signal, IDK if the coil will continue to work.
Its running pertronix ignitor and flamethrower coil,no ballast resistor
 
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