308 front bank backfires/pops

Cam timing can be externally checked on late carbed cars and all fuel injected by a factory fixture that lays over the shaft just behind the gears. If your car is an early 308, perhaps a helpful mechanic has witness marked your cams for you. Engine needs to be at top dead #1 for this check.
Also make certain your 5-8 distributor timing is not overly advanced.

 

I would consider the points and condensor 1st . Check the point "heads" to make sure they are not loose. Then check the dizzy advance on that bank. Make sure that it is advancing correctly. Do we assume that the idle points set is disconnected? Also check the plugs and see what cylinder is causing the popping. If it is a lean issue you should be able to adjust that out with the mixture screws. Last would be the wires and extensions breaking down. My 2 cents

 

did you do anything to your car before this happened?

 

EVERY carb 308 has dyno'd way down on power at our "dyno days" in our local FCA chapter. In every instance the cars have either distributors not advancing, carbs set way too lean, or an ignition miss (from bad plug wires or incorrect plug gap). Sometimes it is a combination of these.

Are you absolutely sure your distributors are advancing correctly?

 

The popping through the carbs is a very common indication of running lean. Either you have a vacuum leak, a plugged idle jet, a plugged carb orifice someplace or something that is making the front bank run extremely lean. The fact that you have measured the mixture and it confirms the lean condition pretty much points the finger. If the jetting is the same on the front and back banks, you need to look more specifically at the front carbs and see what is up. It is possible that the cams have moved a tooth and this is causing the problem too. If nothing obvious turns up in the carbs, you may have to remove the front bank cam cover to check the cam timing.

My car had a similar issue with running lean, popping through the carbs, down on power (only dynoed at 150 on a dyno day) and generally ran like crap. I rejetted, rebuilt the carbs, added an electronic ignition and the car is much much more powerful and runs 1000 times better. I have not had it on the dyno again, but plan to next time there is a dyno day here.

Birdman

 

By the way, you don't mention at what RPM you checked the mixture, but the idle jets do all the work from 0-3000 RPM so they make a big difference. The idle mixture screws only affect the mixture at idle. The minute you push the gas pedal and open the butterflies, the majority of the fuel comes from the progression holes which are fed directly off the idle jet. The mixture screw doesn't affect them. As you open it up more, the main jets come into play. Around town, with off and on throttle driving, you are using the idle jets 90% of the time. Getting the right idle jet is important. The '78s were jetted lean to pass emissions (first year with cats) and with modern fuels they run way too lean, especially if the exhaust has been modded.

Here is where I would go with this. Others may have other ideas:

1. Pull the carb tops and check float heights. (procedure on my website www.birdmanferrari.com)
2. Pull all the jets and soak overnight in carb cleaner. Blow dry and visually inspect. Pay particular attention to the idle jets and find out what size they are. Report back. I'm betting they are .45 or .50.
3. Blow out all the orifices in the carbs with carb cleaner then air while they are partially apart (tops off)
4. Reassemble and synchronize. (procedure on my site). This is VERY important.

If you still have a problem, it may be time to check the cam timing.

I'm thinking that if your front bank is way off from the bank, it may be a synch issue, a plugged jet, a badly set or defective carb float or a combination.

Birdman

 

Have you pulled and cleaned the idle jets?

 

Aaron,
I'm gonna have to say that you are looking at a cam timing thing. If the front and rear banks were grossly out of synch, you could get a very lean condition on the front bank if the rear bank was open a lot more. Meaning, the rear bank was doing most of the work while the butterflies in front were too far closed. But this condition would go away as RPM increased and the butterflies all open. At that point, the differences in flow would be negligable.

Put the engine on TDC and look into the oil filler hole to see if the rear cam timing mark is "on". But you are going to have to pull the front cam cover to check the timing. Some of the 2V cars have marks on the cams near the pulleys but I don't know which ones or where. Mine doesn't.

Good luck

Birdman

 

Sounds like a timing issue to me. The cam cover will have to come off for checking the 5-8 bank. Set it to TDC and see where the cam lines up. There should be one hash mark on the cap and the cam.

Then, if that is right, you want to check the distributor timing next. Use the timing light synched to number 5 wire (closest to the US driver).

When both are right, then we can move to other things.

 

Holy cow, that was a great discovery! Good thing you found it before you pulled half the engine apart or much of your hair out! Good find.

176 RWHP, not bad at all on a USA '78!

Birdman

 

That is a timing problem allright.

 

Excellent numbers for a 78 308!! Good show!

 

Aaron your car looks sharp!!! I think I will be looking for a 308. What were the years of that series?

 

I just bought a blue 308gts 1979 which I am also having to go through since it hasn't been driven in the last three years....I've been following your posts and it seems we are covering a lot of common ground at the same time... I only wish I had a dyno to put mine on....';<(........ Was wondering if you could share how to synce carbs.... I'm following the progress with delight since I'm also making great progress on bringing mine back up to speed.


Another thing is I was wondering if you had to take the distributors out to clean the points...I was told by a mechanic that the points couldn't be changed with the distributors in. Did you pull your distributors? and if so....any tips ?..


thanks

Rok

 

You could do it with it inside the car but it takes contortionist body parts, and it is 10 times quicker with the dizzy outside the car.

 

You can service the points in place...it's the advance that's mechanical and cannot be 'adjusted' without a calibration machine.........it follows the curve based upon the springs installed...

Pulling the distribs is a 'once every 30K miles' thing, but you lube them at 15K intervals.......

They'll stick from disuse and old grease though......

 

It takes just a few minutes to pull the distributors off. I couldnt even imagine trying to set the points with them in the car. Once they are off, you can service or replace the points, oil the advance pins, relube points cam shaft, and set dwell and phasing. If nothing else, use a drill motor or a degree wheel to set dwell/phasing.

Points require a better secondary ignition system to work properly. Any weakness anywhere will show up as a miss much sooner with points than with an electronic system. Check all your wires for resistance, as well as your extenders. I also was having trouble with low speed missing with BPR6ES spark plugs, until Scott reminded me that points systems dont like resistor plugs. When I put in BP6ES plugs the sputter was gone. Now it runs virtually perfect, with no missing or sputtering no matter how high it revs. Today I am running BR5ES plugs.

Birdman wrote a very good article on carb syncronisation, and its well worth taking the time to read. I would also recommend you get some books, such as the Haynes Weber manual, as well as Pat Braden's Weber manual by HPBooks. Between those two books and Birdmans article, you will know enough to be dangerous. But I will offer some further advice. Once you have the carbs closely synced, open the mixture screws up three full turns, minimum, before idling it down, just like it says in the OWM. I was having a hell of a time getting mine to idle down without sputtering, but I had mistakenly started with the screws at two turns out. This required more throttle as it was just to lean at that setting. Once I screwed them out, I could then close the throttles down considerably, and it nows idles wonderfully smooth.

My car had more things screwed up, yet still ran and drove pretty good. But as I have corrected each one in turn, it has continued to run and idle better and better and gain higher performance. It had cams out of time (one 34 degrees advanced), excessive valve clearances (over 7 valves had clearance in excess of .016" inch), poor ignition (three poor wires, two shorted extenders, resistor plugs, poor dwell), ignition timing quite retarded (flywheel mark off 3 degrees, distributors off time, one as much as 10 degrees), lean carbs (lean jets), and poor sycronisation. I would imagine ANY car you guys find thats not been driven or maintained in some time, will need all of those items addressed before you could ever expect it to run properly. This is one of those instances where sweating the petty things really pays off.