Backfiring

[Stewart]

I'm trying to figure out what my next plan of attack will be.

Right now the 73 with hif4's and a 45d4. I just leaned the carbs out as it was running rich as its now close to 45 degrees warmer than when they were initialy set. Before that it did do the same thing but not as bad and this started within the last few days.

In first or second gear at just over 4k it will start backfiring rapidly through the muffler with a rare pop through a carb. Once you get to third its fine to redline 4th I haven't run it that high yet. When I rev it out of gear it will blow thick black smoke at 4500 when it starts poping otherwise no smoke noticed.

I'm thinking ignition related and will be pulling the dizzy to check set point gap/condition but I'm no su guru so I dont know if that could also be it?

 

[dklawson]

Always check the ignition system before fuel adjustments. SUs (and Strombergs) don't get out of tune as much as people think they do.

When you're going through the distributor (since it will be out of the car anyway) remove the breaker plate and inspect the advance weights and springs. Make sure the weights are free and put a little motor oil on their pivots and the surfaces where they contact the dizzy cam. Make sure the springs aren't broken also. Also check that the dizzy cam (the part just below the rotor) can be turned forward and back a bit when you hold the dizzy drive dog stationary. If there is any binding of the cam, remove the felt button (if it's there) below the rotor. Remove the screw you find there and put some oil on the shaft through that hole. Refit the screw and felt and work everything by hand until it's smooth. Then put the breaker plate back in and go through the standard points adjustment. If you haven't replaced the condenser, this would be a good time to try a new one also.

 

[ronzet]

Stewart,

Since I have been collecting information about ignition anomalies (my Post ' Driving me nuts') I had a conversation with Advanced Distributor who elucuidated me to symptoms of advance problems.

I do not know if your year has a <span style="text-decoration: underline">vacuum</span> advance but it does sound like the advance is not working and is sticking in the retarded(idle)state.... the idle timing is very different (by several degrees) to that of 4,000+ rpm and would likely cause the symptoms that you have articulated.

I do not think that the point gap maladjustment will cause the severity of the symptoms you are describing, unless they are WAY, WAY off...and if so it would not run well at low RPMs anyway.

An adjustment of the lifters is also in order here and, if maladjusted, may be a contributing factor....

 

[ronzet]

Stewart,

Did you ever determine what the problem was??

 

[Stewart]

Never found anything out of place. Point gap was fine and the points are in good condition. I went ahead and did a full tune and the problem has dissapeared for the moment. Car is now running fantastic so I hope it was just out of tune.

 

[ronzet]

Stewart,

You know that is when we should worry... We fuss with it and suddenly, without a SPECIFIC problem determined, It starts to run REALLY well...

 

[Stewart]

Yep. With this car it means something else is about to break which did happen. Shortly after it was "fixed" the brake light switch got stuck in the off position. Took a while to fix that one and now I'm waiting for something else.

I think I'm going to replace the rear suspension this weekend and going to finaly put a radio in tonight (alpine deck 4" infinity speakers and sat radio) so I'm sure something bad is going to happen next or maybe it will wait till after I install the new cyl. head which I just picked up from the machine shop.