Ok guys, tell me if this sounds right. My engine (big bore 1911), which we are starting on the ground, is backfiring through the carbs under acceleration. We adjusted the valvles, engine cold, to .006 all the way around. New plugs, gapped to .024, new wires, rebuilt the carberators (dual Weber 34 ICT). Someone told me that I need to rejet the carbs, that backfiring through the carbs is a sign of running too lean.

So when I go down to the VW parts store to buy larger jets, the guy tells me that my valves are off - that the too lean stuff is bull. He tells me that the exhaust valve's are set differently than the intake. I thought they were both .006??

hmm, i think you are right; valves are .006 all around except on the 2.0 (?).

when you find out what the problem is, please post it as i will be trying to figure out my newly mounted 34 ICTs, and i know nothing about them right now.

Well, there is truth in both of those statements. Yes, if you run too lean, you will backfire. If your valves are too tight, you will backfire. But assuming your valves are set correctly (.006 should be just fine) then I would say try re-jetting the carbs. I had one car poping and farting all over the place until I took out the air correction and someone had drilled them out HUGE. Soldered back up and drilled out smaller. Much better.

The 4cyl cars you can adjust the intake to .004 and exhaust to .006. When I do the 356/912 engines, that is what I use. The 914-4's I use .006 straight across the board. Seems to work just fine.

but setting all to .006 should work just fine. in fact, i got a bit more punch out of my 1.8L when i set them all to .004 ...

Andy

Do the ICTs have a choke circuit? If not (as in, they're like all other Webers used on the Type IV), then they'll pop and bang some until the engine is warmed up. Just idling with no load on it may not warm it up enough to stop this. If it is still doing in after being warmed up, then look at the idle mixture screws and the idle jets. You're not even going to get it up on the mains without driving it, and certainly not up onto the air correction jets.

Idle jets also plug up easily from even the slightest about of dirt in the fuel circuit.

What do the plugs look like after it's been running awhile? Black and sooty, or tan, or white/light grey?

You gotta look at the plugs to see if it's lean or rich.

Rich can cause backfiring up thru the carbs... Running lean will make it *SNAP* up out of the carb as well... If it sounds like a heavy pop, it's probably rich.

However, you gotta pull the plugs and see.

When webers are cold, a nice health snap when it's running cold is a good sign (IMHO). It should be running lean since there's no choke.

Later,
Tom

My plugs looked blackish. But, what do you mean by "you won't get it up on the mains without driving it"? Why isn't hand reving it enough? The carb just goes by airflow, Not the load the car is under, right?

The engine gets plenty hot on the ground, so I know I am up to operating temperature.

When I say it backfires, it does the "snaping". It higher RPMS, holding the throttle steady, it is more of a rumbling backfire. Almost continuous.

You did not mention what the timing and dwell are set at? Is the cam stock or a hot rod ground? If not stock then possibly the valve adjustment could be different than stock and can cause backfiring. Other things such as type of push rods can affect valve adjustment clearance. Things such as worn throttle shafts can cause air leaks that will result in false mixture settings. Finally you could even have arching in the distributor cap that could cause the backfiring? Good luck

Load does matter, actually. Intake vacuum changes significantly with load.

I'd triple check to make sure you have no intake leaks, and I'd also triple check ignition timing. Over-advanced ignition will cause what you're describing. Also make sure all of the plug leads are on the correct plugs. The engine will often start with two plug leads reversed, but it will run very poorly, again, much like you're describing.

You got me thinking about timing now. I had a guy help me with this, and you know, I didn't see him set the timing. I saw him marking up the fan with chalk, and I asked him if he was going to use a strobe light, and he said no, he was going to set it "static". Do you have to use a strobe on these engines?

Get a strobe and check the timing yourself. If you can borrow (or can afford to buy) an adjustable timing light, get one. You want about 10dBTDC at idle, and about 30dBTDC at 3500rpm. The exact numbers will depend on which dizzy you have on there. If the timing is way off those numbers (like more than 5d at idle and more than 10d at high speed), then you have a timing problem.

Since you didn't set it yourself, you should also double check the plugs are on the cap in the right order. 1-4-3-2 is the firing order, and the dizzy rotates clockwise.

Ok guys, I am getting real discouraged. On my lunch break today, I went home and played with the timing on the car. I set it statically first at about 6 deg. BTDC. Started the engine, and gunned it up to about 2500 rpm. The engine was doing it's thing: poping and rumbling a noisy song, this time really from the exhaust, not so much the carbs. Next, I advanced the timing by hand - no better. Then I tried retarding it. No better. Although the engine seems like it has good power, and is string, it still poped and rumbled like crap out of the exhaust. I have determined that my problem is irrespective of timing. What else can I do?

I have changed points, plugs, plug wires, rebuilt the carbs, adjusted the valves, and did a compression test. What the heck is wrong with this engine?

Really, really clean the idle jets and the float bowls.

Check your exhaust, too, esp if it pops more when you let off than it does when you apply the throttle.

And you have made absolutely sure the plugs wires are on the right plugs? You don't have 2&3 swapped, or something like that?

As mentioned in the posting below RED914 is having the same problem you're describing. I just picked up my 914 and it was/is doing the same thing. In short, I've hydralic lifters (not requiring adjustment), set my timing, dwell, new plugs, wires, dist. cap, points. And it was still popping (and you could see it while looking in the carbs throat) at idle. I resolved this by disconnecting the linkage and setting the airflow equal between the two carbs, then adjusting the fuel mixture. I also noted that the gaskets on the pass. side were leaking and fixed that. This resolved the cracking at idle but when running 2500rpm or above I get the same issue, although it seems reduced while under load at highway speed.

I guess I'm telling you all this because I wonder if you're airflow rates are equal. Again this resolved my issue except at higher rpm which I attribute to the car being in storage and probably needing the carbs rebuilt and I've not had more time to try and adjust it out. I have a flowmeter so this was easy for me to check but if you don't have one you could probably try setting the flow by ear to see if the problem resolves. This is done by simply adjusting the idle screws on each carb. I would write down how many screws in or out you turn so you can at least set it back to where it once was if necessary.

I'm not a pro so if someone thinks this is unwise please speak up....just my two cents.


http://www.914world.com/bbs2/index.php?act...2&t=16490&st=20

Is the airflow adjusted by the screw that tells the throttle how much to be open at idle? Or is it the screw with the spring on it?

Yes, i t is the idle screw. If only one of your carbs is snapping try playing with that one first.

Ok, I think we got the snaping at idle to go away, but the problem remains of the poping out the exhaust when at high rpm. Are you saying that this is normal, and that it won't do that under load, like when driving?

Nope, I'm not saying that but now you and and my 914 have the same problem

Albeit, I felt my issue was my carbs needing rebuilt as mines been in storarge for a couple of years. I'm not sure what to tell you and hopefully someone else will chime in with a thought. My understanding for dual carb setups is that the airflow must be near equal along with all the other issue that you've already covered and others have mentioned....valves, timing, etc. As I've mentioned I've not had time to work on mine further but should in a day or so.

Anybody else have thoughts as now I'm starting to wonder if mine won't still have the same problem after they get rebuilt just like thomasotten is having.

Well, I just did a big old search on the forum. It seems like backfiring through the exhaust on deceleration is an exhaust leak. But mine does it at constant speed. So, I'll keep looking and reading.

Peace of crap car...

for starters get bob tomlinsons weber tech manual from cb performance. a local vw parts store in your area may have it. you MUST know where your timing is. just advancing and retarding by ear prob isn't going to help. the carbs are running on the idle circuit until you exceede 3000 rpm. the fuel pressure must be correct. that type of pump are you running? an unregulated facet is probably over driving the fuel inlet valves. resulting in a constant rich condition. the correct sequence for your fuel flow should be, filter, pump, regulator(if needed), carbs. get the book. get the book. aircooled.net has a nice trouble shooting/setup tech article.

kevin

Just got back from playing around at lunch time again. Still runs like crap. I did noticed that a small amount of gas was seeping out of the throttle shaft on the carb body. Not really dripping, but it was wet. That can't be good. I guess there are some bushings or seals in there that need to be replaced. Which means I am going to tear these things apart again. Will keep updated...

OK guys, if you are following the story of the 1911cc with dual 34 ICT webers and it's poping exhaust at 2-3k rpm, I have some good news. First of all let me start out by saying I rebuilt the carbs, again. I just wanted to be sure I didn't have crap in their somewhere. Next, we rejetted the carbs. We went one size larger on the idle jets, and for the main jets we went larger as well. I am not at all sure if this rejetting was necessary, but we were able to get it tuned better by adjusting the linkage, as explained below. I did find one document that said my idle jet was the idle jet for a 34 ICT for a single carb application. I am starting to get the feeling that even though you buy carbs in a "kit" for a specific application, they just grab two carbs from the shelf and you get what the vw-beetle guys get.

We then started adjusting the linkage. I have found that the linkage plays a huge part in this whole thing. Trouble is, it is a pain to adjust. At one point, we had it where all the poping was virtually gone. Basically, when one carb is leading the other due to the linkage, we get the poping in the exhaust. I suppose what is happening technically, is that one side of the engine is "pulling" the other side, causing more of a depression in the weaker side's exhaust. So I am happy that I at least heard my engine run at full rpm the way it should sound. My next task is to find a way to methodically adjust the linkage. I have found that I need to figure out a way to make both carbs open by the same amount at each degree of throttle. This would be easier if the linkage was symmetrical, but it is not. I am going to do a search for linkage adjusting tips.