Happy Spring, aka fix your car so you can drive it all summer!

Having an issue tuning my Weber 40's.

The car does run, and idles quite well, sounds smooth and even. If I rev beyond 3k, I get backfires. Starts fine too. Ordinarily I would assume it's a simple matter of resetting and resyncing.

Passenger side carb is as it always has been, fine. Mix adjustment screws work normally, idle adjustment also. I see the squirt from the accelerator jets.

Driver side, not so much. It's getting gas. For some reason, the mix screws have little to no effect. This is on BOTH cylinders. The only time I notice any change in the engine's sound is if I back them way way out, then I get backfiring and carb throat sneezing. Otherwise, I can have them set just about anywhere, including fully seated, and there is no difference. I've pulled the carb off, (three times now) torn it down and blown everything out a couple of times already. I did see some blockage in the idle jets and mains before I blew them out.

I also noticed that I didn't see any squirt from the accelerator jets. I tore it down again, and fiddled with the pump (new last spring) and reassembled it. Accel jets now appear to work normally.

It doesn't at all help that my kids decided to reorganize a ton of things, and I now can't find my Weber manual.
Can anyone help? Thanks!

I overhauled both my weber 44idf's this year. After I put them back together it still ran like crap. I pulled the idle jets (again) and they were clogged. Check the idle jets again. It's easy after you've done it about 50 times. These help a lot. https://www.cbperformance.com/product-p/7557.htm

At around 3k rpm's you are transitioning to your main circuit. Are you certain you have the correct main fuel and air jets?

Adjusting the idle mixture screws should have little affect. Also you might want to look into your ignition system, I have had that be the culprit in the past. Check for good wires, plugs connections, points, etc...

Goodluck!

On the offending carb that the idle mixture screws do not respond on, does the car idle lower if you manually pull the linkage closed more? If so, the carb is not operating fully on the idle circuit, and therefore the mixture screws won't have much/any effect. But first -?

When is the last time you adjusted the valve lash? Do this first.
What is you idle speed?
Have you synchronized the carbs at idle with the linkage disconnected? Do this next.

If so, then try and synch the carbs again at 2500-3000 rpms, or at least get a reading. I am leaning towards one carb being ahead of the other.

Is the backfiring coming from one carb only? If so, then you need to verify your floats are working and set properly.

If not, then suspect low fuel volume across the entire system. Do you have a fuel pressure gauge installed? What kind of fuel pump?

Ok, time for an update, and some replies.

Elliot, I have pulled the idle jets a few times, they're still pretty clear.

Justin, yes, at 3k I'm moving to the mains. I replaced the jets last spring to conform to what is listed in the Weber manual. Now using 55 idles and 115 mains. Adjusting the idle mixture screws is necessary to set the carbs to best lean idle, isn't it? Cylinder 1 does nothing. Cylinder 2 gets rough if I open it WAY up, like 2 or 3 full turns. New plugs are going in in a few minutes, will check wires (though they're new last spring also) and timing, etc.

George, so many questions!
There is one prog port exposed below the throttle valve on Cylinder 2. Always been that way, hasn't had too much effect in the past. Idle screws worked normally. Yes, idle does drop a little when rotating the linkage bar manually, also always been this way, and the throttle valves appear to close fully. (Looked at them while it was off the car)
Valves were adjusted late last summer.
Idle speed was at about 1200, I have it at 950 now, hope to keep it there!
Just synched the carbs, disconnected. Backfiring is actually worse now.
Can't tell which carb is backfiring. Just reset float on driver's side, as it seems to be the issue. Will check passenger side in a bit when I'm pulling plugs.
I get a nice stream of gas when I disconnect the line (only 3.5 psi) using a mechanical rotary pump I bought from CB Performance. I don't believe that is the issue. It's 10 times the crappy Facet one it replaced!

what are you running for ignition? are you sure it is not breaking down by not advancing? sure you are timed correctly?

Bosch blue coil, 009 dizzy. Just checked the timing. 27 BTDC (2.0L)

Just replaced plugs 1 & 2. 1 looked normal, 2 was black with carbon. I expected that, because of that prog port being exposed as I mentioned earlier. Checked timing and dwell. Timing is good, dwell is a tad off, but at 41 degrees so not horrific. Plug wires all seem like new.

I get no backfires or carb sneezes when at idle. I only get backfiring when I rev over 3k. Going to go pull the passenger side carb and clean it out. I've been convinced that the issue is on the other side because of the mixture screws, but, it wouldn't be the first time I was an idiot! Won't be the last.



ok, just pulled and put back carb #2. cleaned idle jets, mains were clean. Adjusted float. Backfiring is better, but still present. Also now getting some stumbling on accel and popping and crackling on decel. Vacuum leak? This is getting really frustrating!

Not sure what you mean by "breaking down by not advancing" ?

ok, new update:

Just pulled off both carbs again, cleaned them out, again, also pulled intake runners and checked gaskets there.

One issue has apparently been resolved: the air/fuel mix screws are working normally on both carbs, all 4 barrels. Yay!

Still backfiring once revved to 3k+.
I checked all of the wiring to the coil, plug wires are all seated properly.
Timing is good, set at 27 BTDC at 3200. Dwell is 41 degrees, so could be better, will reset with new points.
Idle is a little high however, about 1300. Idle speed screws are backed out all the way. I should have that around 1k, I think?

The car runs nicely when idling. No popping, sneezing or backfiring. I only get the backfires when revving over 3k. (Main jets are clear) When I release the throttle, I get a little popping as it drops back down to idle.

Now I'm turning towards a valve adjustment (just did one a few hundred miles ago last fall, perhaps I did it poorly?) and check for exhaust leaks.
Any more ideas?

To be clear. The idle speed screws are backed completely out, the arms are against the stops and the idle is at 1.3k?

Yes. A (mal)function of timing? I can easily knock it down to 1k, but then I'm running with retarded timing. I've always had to fight with it to keep the idle down, but never like this! Usually no more than 1200 on a bad day, 1k when I get it just SO.
I'm also wondering if there is an issue with my coil or dizzy causing me to not get a good enough spark.

Idle now at 1200, Both carbs are adjusting. Still backfiring.

To me sounds like you have an ignition problem.

Last year towards the end of the season mine started to run like crap. Skipping at idle, really crappy running at mid-range and occasional backfiring through the carbs. I pulled the idle and main jets. Replaced the fuel filter. Still ran just as crappy.

Threw in a new set of plugs (old ones were 8 years old), bingo. Ran perfect.

I find many "carb" problems, aren't carb related.

when was the condensor last changed? it could that simple. they are cheap, used to part of any tune-up kit and they do fail.

Thanks for the ideas, guys. I've changed plugs 1 & 2 (got dark). Going after points tomorrow morning. They and condenser were new last spring, but yeah, cheap so will swap out condenser and rotor too.

I just find it really odd that while the car idles fine (ok, a little high but no other issues) it backfires only when revved. Another 914 gremlin I suppose... Maybe after another 20 years I'll get this down pat.

Thanks again!

Cars will backfire when they have a vacuum leak because the mixture gets too lean and detonates at the wrong time. Cars will pull more vacuum at higher RPM, so sometimes vacuum leaks don't show up unless you're at higher RPM. Have you checked for vacuum leaks? Try this. Get some starter fluid, and spray around your intake system. Be careful not to start a fire, but look for a sudden subtle increase in RPM when you spray the starter fluid...that's a clue that you're spraying close to the location of the vacuum leak.

If you did this already, I apologize...just trying to help.

I had a problem with my electronic points once where it idled fine and then when I tried to put power down it backfired. Ended up being the points got loose inside the distributor, man that one was tough to find. Probably a mute point here since I think he has mechanical points... but figured I'd share.

Most recent update:

Just replaced points, rotor and condenser.
Dwell at 45 degrees, timing set at 27 BTDC at 3200 rpm.
Idling at 1400. Idle adjust screws are backed off of the stops. Only way I can get it lower is to manually pull up on the droplink (hard) on driver's side carb. This gets it down to 11-1200.
Still getting popping (not full blown backfiring!) when revving, so that is an improvement.
I suspect that I need to dismantle the one carb and find out why it isn't closing up completely at idle... Could this also be the cause of the backfires?

what is the condition of the bush of the distributor shaft? if the bush is worn, you will have ign.problems as well. it sounds as if it is ign. because you gained improvement by tuning up.

Dizzy is practically new.

Complete update and status...
Since I neglected to mention this previously - 75 2.0, dual IDF 40's.

New plugs, rotor, cap, condenser and points. Checked wiring for faults. Found none.
Wires, coil and dizzy new last spring.

Dwell is at 46.7 degrees.
Timing set to 34 degrees BTDC at 3200 rpm, as per PP article on timing for carbs. Had it at 27, but thought I'd try the new number as it came from PP. Now the idle sits at 1700 !!! Throttle valves apparently not fully closing, as I can get the idle to drop a bit if I pull up (kinda hard) on the droplinks, whether connected to the linkage bar or not. Therefore, not caused by the linkage. Odd thing is that when I had them off the car, I checked this specifically and was unable to get ANY movement from them. I can drop the idle down to a normal range if I retard the advance enough, but then the timing is way off.

Still getting popping and/or backfiring when revved over 3k.
Sprayed carb cleaner around areas where there are gaskets that could be causing a vac leak. Nothing found.

So, it appears that I have two separate issues. High idle and backfiring.
Valve adjustment (again) is next, I think.
Any other ideas???

At my wit's end...

Hi Randy,

The only reason that the idle would be that high is you have a vacuum "leak". I would concentrate on finding the cause of this problem first.

The reason I put "Leak" in parentheses as well as classic vacuum leak leaks from leaks at the various intake gaskets (carbs to manifold, manifold to heads) a cause could also be through the carb(s). Two possible causes at the carb could include butterflies that are not fully closing or an air bypass that is wide open. This seems likely since you stated that pulling up on the linkage on on of the carbs drops the idle somewhat.

I would start troubleshooting by disconnecting the linkage completely from both carbs. Make sure that the butterflies are closing on both throats of both carbs. You may need to check the idle adjusting screw.

If all of this checks out I would cover, one-by-one, each carb throat, a hand will work for this. Covering each throats should result in a drop in the idle speed. If one does not result in a drop that is the carb throat to concentrate on.

If none of this checks out you must have an external vacuum leak. You may be able to use the starter spray method localize the leak.

Okay, this one might seem out there, but it's worth a shot.

On my car I had replaced the pushrod tube seals, but I didn't get the retainer wire in correctly. A couple thousand miles down the road, I had a similar backfiring scenario, except it was only cylinder #3. It wasn't until I pulled the motor that I realized the wire had rubbed a groove into the pushrod, and put just enough tension on it that it would hold the valve closed (or perhaps open) at high rpms.

Just a thought, but glad you at least got the mixture screws working.

Remember you have throttle stops on the back side of the carbs as well. I usually back them off and use the idle screws to adjust the idle and to synch, then screw the stopper screw back in tin it just touches. Also check for worn throttle shaft and accelerator pump.

From behind my computer, I'm going to guess that your main jets are too small, or the main circuit is plugged or gunked up.

Is it at all possible that you've swapped the air bypass screws and mixture screws?

I've referenced this post several times over the years, here's a cut and paste, this was written for 3 barrels, but the tuning principals are the same:

Assumptions: the carbs have the optimum jet and venturi package (good luck on this one), the float level in the carbs is correct, the cams are correctly timed, the valves are properly adjusted, the ignition timing is dead on, you have the proper spark plugs for your engine, the linkage is good, the fuel is good, the engine is good.

Remember that the mixture and air bypass adjustment screws are precision needle valves, not head gaskets. Use your fingers to tighten them, not your fist.
Start and warm up the engine.
Make sure the two drop links for the throttle linkage are exactly the same length, and disconnected. You can use a 8mm thin igntion wrench to snap them off.
Turn the mixture screws all the way in and then 5 half turns out.
Turn the air bypass screws all the way in.
Turn the idle speed screws out til it just touches, and then in 5 half turns.
Put on your hearing protection and start the car.
Use your STE and find the barrel that pulls the most. We'll call this one baseline.
Balance the barrel in the other carb that pulls the most with the idle speed screw. (if you have a Uni-syn, give it to someone you don't like and purchase a STE airflow meter)
Go back to the other carb, with the baseline barrel. You will have one all the way in, then use the air bypass screws and balance the other two barrels.
Go to the other carb and do the same thing.
Snug the jamb nuts on the air bypass screws.
All six barrels should pull the same amount of air at this point, if not repeat air adjustment proceedure.
Snap the throttle linkage drop links back on the carbs. If the idle changes then you need to barely adjust the linkage mounts so snapping the drop links on, doesn't change the side to side idle balance.
Use the hand throttle or a vice grip and rag to lock the linkage between 1400 and 1800 rpm.
Start back at the baseline barrel and adjust the mixture screw in or out, to get the smoothest running and highest idle, then turn it in 1/4 turn.
Do the same with the five other mixture screws.
If you have to turn the mixture screws more than two turns either way, you've got the wrong jets.
Recheck side to side and individual air balance, adjust as needed.
Road test the car.
If you get snapping and poping out the intake, it's generally a lean condition.
If you get heavy exhaust fumes, or pboofing out the exhaust it's probably too rich.
If you get a flat spot or popping out the intake at between 2800 and 3200 rpm, you probably need larger idle jets.

That's a rough, five minute draft of my carb tuning proceedure, hope it helps!

If it goes good it should take about 45 minutes, if not about three years.

PK

A sincere thank you to everyone that has posted, and PM-ed me on this!

Another update, as the weather has finally cleared to coincide with a window of opportunity to work on this beast again!

A week ago, I checked for vacuum leaks using the hand covering the velocity stack method. Found none, but when I covered cyl 1, it sneezed, and I saw a puff of gray smoke come from the outboard side of the carb, looked like maybe it came from the area around the gasket between carb and intake runner. Checking all cyls with snail gauge, #'s 2, 3, and 4 all read 7, #1 read 2.5. I figured I had blown the gasket out. Nothing had changed with engine sound or backfiring. A/F adjustment still had normal effect. So, I figured I'd look at it after the valve adjustment I had already planned.

Did the valve adjustment yesterday. All seemed to be correct without touching them. .008 on the ex's and .006 on the in's. At least I got a free oil change! (VR-1 was clearanced for $1, plus a $1 instant rebate... sadly they only had 5 qts in stock. Price has now gone back up!) Ok, $4 for the filter...

Pulled carb off this afternoon, that gasket appears fine, as do the ones inside the carb. Big thick one where runners go into engine is brand new, and also seems fine. Snail gauge still reads with cyl 1 much lower than the other three. No idea where that puff of smoke came from, nor why the snail gauge is reading so low on cyl 1.

Messed with the linkage, as I wasn't happy about it's position on the bar. (Driver's side)

It's a miracle! Backfiring is gone. Idle is at about 1050. Car is a little shaky when idling (I mean literally, like it's loping along) but it smooths right out when applying throttle. Going for a test drive in a little while, will report back.

Meantime, any ideas why cyl 1 is reading so low? Engine sounds just fine...
Thanks, guys!

Air bypass screw.

I'll look into the air bypass screw this afternoon. Thanks!

Everything seems to be in order. She drives well, responds well, it's as if she never left! VERY minor popping on decel, but that's just a tweak. Readjusted the timing, now it idles at about 11-1200, but is smooth. Still lopes when cold, guessing it's related to the air bypass.

Been driving all night, and all morning! MegaYAY!

What type engine and capacity?

He posted earlier it's a '75 2.0L.

With carbs.