70t engine question, backfiring at 80 mph+

[Surf65]

Esteemed members,

Once again I turn to you for advice in the hope that I can sound informed when I drop off my car to get fine tuned.

My car will start to backfire, bog down, etc. and the rpm gauge flutters while cranking up high speeds....I simply don't press, let it settle to 60 mph range and drive on.

my 70t engine is equipped with weber carbs.....The engine was not run for several years when I first purchased it and have seen better performance after a major tune, plugs, wires, etc....and several tanks of high octane.

Any advice on what could cause the backfiring or is it that a 70t is pushing the limit closer to 80 - 90mph. Do these old engines need a sip of lead additive or is there anything else I should try?

Best,
iphoto
Matt

[blucille]

a friend used to tell me that something like 90% of the fuel related issues were in the ignition system.....especially when thinking about an issue at high speed, under load, high revs, etc....seems more likely to me to be in your ignition system....to me, the indicator is your fluctuating tach....recheck your points, CDI and coil, recheck your plugs, too.....then start investigating carbs....

come to cars and croissants tomorrow, Gene will spin it up to 80 and see what he thinks

[Jay Laifman]

As far as carbs goes, sounds to me like float bowl levels can be off. I had a similar problem when I put Webers on my 912. I dutifully set the top float level, but never thought about the bottom setting. Turns out, after countless attempts to figure out what electrical problem I had, that the bottom end of the float was way too high. So, the floats wouldn't go low enough to open the valve wide enough to keep the gas flowing fast enough. Once I set that, it was like a whole new car.

[mike curnow]

Something easy to try is disconnecting your tack at the distributor, the black and purple wire. Sometimes the tack can cause ignition problems.

[RSTarga]

Replace your points.

[Surf65]

Thanks gentleman...

Can't make tomorrow's croissants and coffee due to kid activities but someday will.

[Harvey Weidman]

No, even 2.2 Ts should rev the entire range at speed.
Did you check the main jets? They may not have checked the size when the engine was converted to webers. I check them anyway to make sure they are clean.
My webers took several bowl cleanings after setting for 4 years...
When you tuned it up did you check the timing? What about the vacum line on the dist?
Backfiring at speed can be caused by both the ignition and carbs..
I'm sure Ed will be able to fgure it out...
H

[edmayo]

My rule is that when trying to solve a problem with multiple possibilities is to always start with the simplest checks first, plus the customers like that approach best! I'm with all those above that mentioned the ignition points as a starting point (pun intended) especially given the problem RPM range and the fluctuating tach.

[304065]

Could be the points, but I would suggest first a check of the battery voltage, both at rest and at 3000 rpm, measured with a voltmeter at the battery. The fluttering needle is most likely a symptom of a defective external voltage regulator.