carb backfiring

[Woodie]

I think I am very close on finalizing the settings for the carbs. I reset the choke yesterday and the car starts much easier today ( a cold and damp morning to boot!) However while driving at easy speeds once the car was warmed up, I back fired through the carbs occassionally. It did not do it yesterday when I balanced the carbs (got the idle holding nice and steady at 950 -1000) I reset the timing yesterday now that I have the pertronix ignition installed to 10 degrees BTDC and the plugs NGKBPR6ES are gapped at .032

I thought I had the mixture ok. So what caused carb back fire. And how do I tell which crab it is as it only happens when driving?

 

[DrEntropy]

May be a bit lean. One flat and try it some more. Good for an excuse to take it out again! /bcforum/images/%%GRAEMLIN_URL%%/laugh.gif

 

[jlaird]

I would say one carb is too lean as well.

 

[Trevor Jessie]

Yes, the carb may be too lean, but does it happen when the car is totally warm? If not, you may be coming off the choke too early.

What type air filters do you have?

 

[Woodie]

all right lets go with that, I have never had any luck in pushing up on the little buttons under the carb to test for lean /rich condition. It seems no matter how much I lower the jets and then push the button, the engine wants to slow and stall.

 

[Woodie]

Yes trevor, I was almost all the way to work a ten minute drive and it was doing it.

 

[Woodie]

Air filters are twin K&N cones

 

[Trevor Jessie]

What method are you using to set the mixture?

 

[Woodie]

12 flats down, adjust the balance, then try to use the [censored] buttons.

 

[Trevor Jessie]

What rpm is your idle at when your are trying the lift the piston method?

 

[Woodie]

steady at 950-1000 before I push the button, then It drops and the cars stalls, doesn't matter which carb. and If I lower the jets , say 2 or 3 more flats it still stalls.

A question, when I push the button do I push it all the way in.

 

[Atrus]

If I remember correctly, you'd only want to raise the piston an additional 1/16" over where it is at idle. If you are doing it all the way, that may be why it's stalling. I know for mine, the pin lifts far more than 1/16" in total, so I don't push it all the way. I am new to this too though, so I could be wrong.

 

[Woodie]

Oh, yes I am pushing the pin in all the way. 1/16 inch huh,,, yeah and how do you measure that. With the K&N's the filters wwould be off but the backplates would be on, that measure would not easy to determine.

 

[Atrus]

Shouldn't be a problem, look down the carb bore and the piston should be right there. Look at a ruler, get a good idea of what 1/16" looks like (it's more than what it sounds, 1/16" is large) and push the pin until you see the piston move up that much. I don't think it needs to be precise, you're just trying to add more air to the mixture and see how the engine reacts.

 

[DrEntropy]

I'll go out on the limb: I'd be trying to lower the idle a bit first. Closer to 600~700 RPM, no more than 850. Then try the piston lift. Start at the 12 flats, carbs balanced for air intake as closely as you can get them. Air cleaners off... you can raise the idle to compensate for the filters after the exercise.

 

[dklawson]

I also think it sounds like a lean burn condition. I seem to remember the manuals using an even more hard to observe pin-lift of 1/32". Is it really supposed to be 1/16"?

I also have a sidebar to this topic since the thread is moving into the arena of carb tuning (the task I loathe). So all the manuals talk about balancing the airflow but they focus on doing it at idle. When I've checked the carbs off idle I've found they seldom stay balanced. I've started holding the throttle part way open, balancing the air flow at 2000 RPM, then tightened the linkages. After dropping the car back to idle I use the idle speed screws on each carb to balance the idle air flow. I presume this will give me balanced flow through most the RPM range compared to what I was observing. Any comments?

 

[Billm]

Two things-
One- is the backfire from the carbs or out of the tailpipe? My car used to pop out of the tailpipe at every shift when I still had a (bad) gulp valve in the system.
Two- I have always adjusted my dual carbs one-at-a-time. Loosen the clamps connecting the two to set idle mixture (I would set the idle stop to "zero" and adjust the other one). I didn't think that you could set them (lift the button) if the carbs were still connected.
Bill

 

[Woodie]

one thought i had was the plugs ar gapped at .032 with pertronix sport coil and ignition and NGK 8MM wires. Is this to big and would that affect the backfiring..and the backfiring Bill is through the carbs. Since the pin lifts the piston it does not affect the butterflys or the linkage

 

[jlaird]

Twelve flats is ok for first start up. BUT..........

Then you seperate the linkage on the carbs so they are seperate, raise the idle on the back carb and lower the front untill it is only running on the rear carb. Raise the front piston with your finger all the way up, Idle all the way out, ie butterfly closed. Adjust rear carb mix for best running. Make sure choke is all the way closed while doing this.

Now do the same with the front carb. With rear piston up and butterfly closed ie idle off.

Adjust both idles to about a turn or so out where it idles best then and only then balance, and check idle and bal againg, rince and repeat.

Tie the carbs back together and you should be good to go.

 

[Dave Russell]

The lifter pin has it's own return spring. As you push the pin you will first feel the resistance of this spring. Since there is some slack between the pin & the piston, this first part of the pin lift will have no effect on the piston. When the pin moves far enough to contact the piston, you will feel the additional resistance of the piston return spring. It is from this contact point that you lift the piston the prescribed amount. Easy to see with the filters off, can go by feel with the filters on.

If the carbs are perfectly synched at idle, throttle blades open exactly the same amounts & the mixtures are the same, the pistons should rise the same amounts at 2,000 rpm or higher. If they don't, it means that the piston return springs are not perfectly matched. Provided that the dampers or needles are not dragging.

You can find the spec for spring tension at given lengths in many books. For example - blue springs are rated at 2.5 ounces at 2 5/8" length. Yellow springs are rated at 8.0 ounces at 2.75". White springs are rated at 18 ounces at 3 5/8". With a little experimenting, you can get both springs perfectly matched. The actual spring tension is not nearly as important as having them matched.

A fairly simple way to match the springs is to put a spring in a close fitting plastic tube, put calibrated weights on top of the spring, & measure the spring lengths.

I guess few folks go to this much bother.
D