OK Carb gurus.... HELP!

Still working on getting my '79 308 running smoothly after the belt change. I am running stock distributors w/dual points & condensers.

The timing is set right on (3* ATDC for the R2 points and 7* for the R1 points on both banks...). And the distributors were gone through about 6 months ago and the advance seems to be working just fine.

The carbs have been sync'ed, and MAN does it sound pretty at idle and during SMOOTH acceleration all the way up to about 6k rpm (didn't go higher than that yet.

Here is the problem: under fast acceleration (below 3k rpm) I am getting LOTS of popping through what appears to be just the rear bank. Again, it is fine under smooth accel and at idle.

So it's running lean, right? Well, I am running 55's on all the idle jets (the front bank appears not to be popping with the same 55's) and I adjusted the idle mixture screws out a few turns from where I thought they were rich.

I am running 125 main jets with 220 tubes. The mains may be a little lean, but again, the popping is only on acceleration at and below 3500rpm.

Just double checked the timing and it's right on the money. The centrifugal weights seem to be working smoothly. (They were both overhauled less than six months ago).

Could a slightly cracked distributor cap cause this? or wires? I thought if it were wires it would not just be at acceleration....

Anyway, here is the picture of my extremely over-rich idle mixture setting (trying to make up for the popping.)
Could it be
Image Unavailable, Please Login

 

I'm not an expert but here's what I would do:

Make sure all of your accell pumps are firing about the same - check the injectors and the diaphragms for leaks. If they are clean, your 55 idles are rich enough.

Make sure you have all the same F36 ET tubes - don't laugh, it's happened and your car is near the swap point. There are some who would say switching to F24s might alone help your malady.

Go to some 130 or 135 mains - with modern gas formulations being different than the 1970s, 125 is really just not the best.

Oh yeah, buy an MSD 6AL ignition for those plugs...

More as I think about it, but I am sure others will chime in...

 

Sounds like it goes lean when you "blip" the throttle and before the main jets are on line.

This is probably an accelerator pump problem. make sure your pump jets are clean and that the stroke is sufficient. It's generally okay to go a bit bigger on these, it just affects mileage mostly as long as they are big enough.

 

Hi
I hope you meant to write BTDC, not ATDC?

Gerrit
http://dino308gt4.com

 

3* ATDC for the r2 points
7* BTDC for the r1 points (10* advance between the two sets of points)...

I think I may have a problem with a leaky accelerator pump on one of the back carbs. But both the back carbs are "popping".

The f36 tubes... are those the tubes the main jets are attached too (sorry for the stupid question, I am new to webers....)

Thanks.

 

Just confirmed with previous owner; the disributors were completely rebuilt and run on a test machine for a few hours. Working 4.0.

Also, he did not have any problems with popping, so it is something since I did the belt change. The only thing I can think of that makes sense is that one of the carbs has a leaky accel pump. It was gummed up quite a bit when I bought it. Since then, I have hit it with quite a bit of carb cleaner. Maybe that only made the problem worse? Possibly loosening things up an causing it to leak enough to make it lean on acceleration?

Thoughts?

In the meantime, I have a project for after I get back from work on Monday: re-building a few carbs and maybe re-jetting to 130 on the mains....

Does anyone else think that I can disregard any ignition issues since it only happens on acceleration and the fly-weights/advance are all good?

 

Yes.
There is a thread I did in the tech section on how to re-jet Webers with LOTS of photos. Perhaps search on ' jetting Weber carbs' with me as the thread starter.

As you said, the carbs are the area of interest after you have excluded the ignition.

DCNF accel pumps use a diaphragm pump, and if they have never been rebuilt, these things may get a bit troublesome - a rebuild kit often comes with a replacement diaphragm. You may be able to see an outside leak around the pump. My personal check is to use a mirror and see if the spray looks about the same in the barrels when you open the throttle plates rapidly.

A rare problem may be mismatched accel pump cams - the black plastic ramps that activate the pump on one side of the throttle shaft.

best
rt

EDIT:
Found the helpful threads:
http://www.ferrarichat.com/forum/showthread.php?t=55165&highlight=weber+carb+jet
http://www.ferrarichat.com/forum/showthread.php?t=42363&highlight=weber+carb+jet
Good luck

 

Funny how it only seems to affect the rear bank. If you're certain the ignition is acting properly, I wonder if the cams on the rear bank are properly timed. You did replace the cam belts. Did you check the cam timing?

Bill

 

Yes. Checked the cam timing and verfied about 5 times. Even posted the pictures and put hard copies in my receipt/maintenance book that I started (along with all past receipts from 2 prev. owners and the original bill of sale!

Looking closely at the back 2 carbs, there is quite a bit of grime around the acclerator pumps, linkages, etc that is not present on the front two. Right now, I am "leaning" (no pun) toward rebuilding the back two and seeing where we are then....

 

I was about to suggest you pull those two carbs and rebuild them. Let us know!

 

Those plugs look rich & fouled. Hows the front set look? Fuel pressure? Fuel Evap & power brake booster both run off rear carbs, any leaks? Idle mixture screws out 3 turns?

Just a thought
Bill

 

Pretty much look like standard appearance of plugs using 55 idle jets with conventional ignition coils if you ask me.

An MSD and/or one step hotter plug is my first thought.

 

OK JR so before you did the belt change did the car run OK? If it did I would not play with re-jetting the carbs. It ran before and it should run after a belt change. So you did something during the change that screwed up the carbs. So go back and remember what you did and see if you can figure it out. You may have the back bank 180* out so recheck that dizzy and see if it is firing correctly. Also if you did any carb adjustments check that too. I assume that you also put back the same type of plugs and you did not change the plug wires or the points. The more stuff you changed without verifing the effect will compound the problem. Think back.

 

I suspect if it is an accel. pump problem you should be able to tell by looking or listening for the fuel squirt. If the accel. circuit is OK, then I guess it could still be an idle circuit problem, as the idle circuit is active until there is enough flow through the venturi to start siphoning the fuel through the mains. When you turn the idle mix screws on the back carbs all the way in (one at a time) does anything happen? The engine should slow, but if the idle circuit is blocked, nothing will happen. As you may or may not know, the labyrinth idle passageway carries a fuel-air mixture. Not air alone or fuel alone. So, when it is blocked the idle will be lean. Did you check the idle jets for patency? Are the little rubber gaskets around the idle jets OK? "Rebuilding" carbs means many things to many people, but making shure the idle passageways and accel. pump circuit are clear would be important in this case.
My experience with general air leaks on the 308 is that they increase idle speed as a symptom, but air leaks do lean the mixture and perhaps can cause the popping. So, if the carb idle and accel. circuts are OK then check for air leaks. Are the carb hold-down nuts tight? The rear bank supplies vacuum to the flapper and vacuum tank. Are there any leaks in these lines or are things appropriatly capped off?

Let us know what you find.

 

Vacuum lines all replaced and correctly routed (there is no "vacuum bulb" however....) It ran fine before the belt change and clean-up. So, again I am thinking something with the cursory "clean-up" of those two carbs. I didn't remove the carbs, but checked the the torque on the nuts.

The idle-mixture screws do cause the engine to almost stall when turned all the way in (as they should...).

I did check and clean the idle mixture screws, idle jets and main jets.

I've been at work for the last few days, so I haven't been able to troubleshoot anymore. I am looking forward to getting home tomorrow evening so I can check the accelerator pumps again and if that doesn't work start the rebuild on those carbs.

I will make sure to check for any clog passageways! Thanks for reminding me of that. Everything seems to be pointing in the direction of the accelerator pump circuit.

Again, thanks for the help, guys. I will advise....

 

I had the same symptoms after I had reground and re-adjusted my cams. Idle and smooth acceleration was fine, hard acceleration below 4000 resulted in popping. Everyone including me was convinced it was lean popping. A dyno test showed the setting was way too rich because of the new cam timing, what caused the popping. I have installed now leaner main and accel jets. If you did not change cam timing, I would not re-jet carbs.

Regards
Harry

 

One other thought that went through my mind was to ask if you changed gasoline brands or types; i.e. does the gas have a high percentage of ethanol? JMHO, but if you are running 125s that is just too lean I believe with modern gasoline formulations and additives. I would find it highly unlikely you would be too rich with 125s and 55s. Even with the gas in the 70s, 125s were quite lean.
If you have verified that your accell pumps are all ok, I would suggest going to 130 or 135 mains -- this was a stock pre-emissions jetting as you know.

Good luck and let us know!

 

I guess I can't say it enough , so one last time. If it ran good before you screwed with it then you did something to make it run the way it does now. I would not put in new jets until you get it running back to baseline. Only then would I look at seeing if there is an improvement going to different jets. So if you played with the carbs and the cam timing is dead on and the rotors and caps are good along with the wires then its the carbs. Good luck

 

I agree. Check this before changing anything.

 

Check for air leaks, including the butterfly shafts.
When a motor is cold it will pop a lot more easly than when it is at operatings temperature.
My 75 GT4 (different cams than yours):
idles; 60 (57s available from Pierce may work, but 55s wont)
Mains; 130
AC; 200 (220 is too big) but this effects top end only.
Martin

 

I rather quickly read through the previous posts, so I might've missed this...

... but I didn't see anyone mention checking the float (bowl) levels. A symptom hand in hand with the accelerator pump/jets, as well as transition period.

While not something that should've gotten knocked around during your belt service, it's another one of those absolutes that has to be right before you can proceed with tuning.

 

+1

 

Ok. Here is what I have tried, not necessarily in order (by the way, thanks for all the great tips!)

- Rebuilt all carbs (shiny clean and no more accel pump leaks!) Didn't help...
- replaced Distributor cap (thought that would do it. expensive!) but didn't help...
- Check timing again (still right on. Tried variations.) Still didn't help....
- Checked, regaped, cleaned plugs. Didn't help....
- Replaced vacuum lines and checked for other leaks. None found. Didn't help.
- replaced Phenolic under carbs. Didn't help.....
- synched carbs and resynched after rebuild. Didn't help....

I think I'm going to have to pull the cam cover again and check to make sure a belt didn't "slip a tooth". I checked it about a dozen times before I sealed it up, but that is all I can think of. The only other thing I haven't replaced is the plug wires or extenders. But it is consistently popping, on brisk acceleration only, just on the REAR bank of cylinders. I would also call the distributors themselves into suspect. But I know they were serviced on a distributor machine w/in the last few months. Plus, I can see the smooth progression of the advance with my timing light.....

On the brighter side: I have managed to track down and fix a ground problem in my front right parking light as well as repair (yes, repair, not replace) my left radiator cooling fan!

Small victories, I guess. Hopefully, I can beat this popping problem soon.....

Cheers! \

Max

 

After reading your latest post, the only thing I can think of now is to replace those ancient Marelli coils that are still in the car. The coils get tired, and there is a big difference when replaced. My plugs on your same bank you are having an issue with looked the same, and after replacing the coils, the plugs looked much healthier and the car ran better. Any good brand coil should be fine. I personally used Bosch's red coils with great results (available from Victoriabritish.com).

Good luck -
john

 

You cannot rule out a bad plug extender, either......inspect carefully.

I went with the MSD coils as they have a high vibration one that's epoxy potted and ok to mount horizontal..

Supposedly the oil filled ones don't like horizontal but all OEM 308/328 ones are.......

Well done on the fan, anyway!!!!