Engine misfiring when hot

[Laxy]

Hi all,

I have a fairly stock 383 running in my 68 Charger. It is a tired motor and has a little blowback out the breather. It has developed a problem where it runs perfectly when cold, but once warmed up it drops to 7 cylinders at idle and at cruise, and the odd backfire. At high revs it will fire on all cylinders again.

At first I thought it must be flooding, but I have tried 2 different proven 750 Edelbrocks plus the 600cfm Edelbrock it was already running and changing the carbs makes absolutely no difference. It's running a stock fuel pump so don't think oversupply is the issue from that.

I have also changed the coil and alternator just to eliminate these as the problem. It's running MSD ignition.

Any suggestions greatly appreciated.

Cheers!

[FLG]

Have you checked your plugs and wires??

Could just be as simple as that.

[Laxy]

Thanks FLG, the plugs and wires are still fairly new, I haven't checked plug color yet though.

[firefighter3931]

What plugs are you running ? Have a look at them for signs of oil fouling....that can be an issue with older engines that have some blowby. Run a resistance test on your ignition wires....you should be seeing less than 200ohms per foot.

Make sure you don't have any vacuum leaks. Check to make sure your carb base gasket is in good shape.



Ron

[Laxy]

Thanks Ron, I checked the plugs and I will change them, now I think about it I've probably put a fair few miles on them. Barely any signs of oil but they are burning grey/white so I'm guessing it's running lean/hot and I'll need to check my timing too. All this time I was chasing what I thought was a flooding problem!!

They are NGK BP5 S - I'm in Australia so not sure if this is a plug model you guys use there, but any recommendations welcome.

Also, no sign of vacuum leaks that I can find at the moment, and I have not tried a resistance test yet.

Cheers.

[charger Downunder]

I run the same plug bp5s (xr51x).
Sounds like its running too lean.

[Chryco Psycho]

could be coil or dist pick up

[firefighter3931]

Thanks Ron, I checked the plugs and I will change them, now I think about it I've probably put a fair few miles on them. Barely any signs of oil but they are burning grey/white so I'm guessing it's running lean/hot and I'll need to check my timing too. All this time I was chasing what I thought was a flooding problem!!

They are NGK BP5 S - I'm in Australia so not sure if this is a plug model you guys use there, but any recommendations welcome.

Also, no sign of vacuum leaks that I can find at the moment, and I have not tried a resistance test yet.

Cheers.

Yep, sounds lean....you should be seeing a nice tan color on those plugs.  With factory heads i like the NGK XR5's. Fatten up the jetting and see what the plugs look like. 


The stocking number for the XR5's is # 3332 : http://www.summitracing.com/parts/NGK-3332/

Ron

[Laxy]

Hey guys, thanks for all the help.

Finally got it sorted, seemed to be a combination of things. Firstly changed the plugs, replaced with the same NGK BP5S as these were easiest to get at short notice just before the biggest Chrysler show here in Australia! Didn't fix it but really allowed us to analyse the plugs, they showed the motor was running lean at idle and slightly rich higher in the rev range. We also checked the timing, which actually seemed ok at 18 degrees initial advance (didn't realise the big blocks loved so much timing!!).

So we chucked on a new high flow fuel filter and cranked the idle mixture right up, and the problem was almost eliminated! Fuel filter was a shit type and probably a bit old. It still has the odd miss and stumble off idle but good enough for now until my new 440 goes in. If it gets worse again I will be checking the fuel filter as thinking there may be some crap in the tank that is getting dragged up.

Ended up doing 750 miles for the show weekend with barely a hiccup!!

Thanks again, cheers

[Paul G]

  I sure hope that is tire smoke!  

Beautiful car!