It’s time to get serious with my engine problem.

I have been trying to tune my VW engine. The engine is backfiring when I drive the car in first, second, third and fourth gear.

when the vehicle is warming up or idling there is no backfiring.

Any suggestions?

There can be only one...!

Original Post

The real question: does it backfire under part load below around 3000 rpm? Or does it smooth out under full load above 3000? If both are true, you've got at least one plugged idle jet, and maybe need a cleaning, synch, and mixture adjustment.

If it backfires no matter what load or rpm, look at spark.

Before you do any changes, wait until it's ICE cold, and adjust the valves. After you adjust the valves, do a compression check, all cylinders should be within 10%. If that's true, move on to spark. Check or change the plugs, and check out points, distributor cap, and wires. 

You need three things for an engine to run. Mechanical, spark, then fuel. In that order

Good luck.

What valve clearance do you recommend?

By the sound of it, i need to check the idle jets and air fuel mixture. I think the Carburetors are too big for the engine. It seems to run rich.

To do a compression check I need to warm the engine and then remove the spark plugs?

 

 

 

Highlander356 posted:

What valve clearance do you recommend?

By the sound of it, i need to check the idle jets and air fuel mixture. I think the Carburetors are too big for the engine. It seems to run rich.

To do a compression check I need to warm the engine and then remove the spark plugs?

 

 

 

It depends. Aluminum pushrods: .004" intake and .006" exhaust. Chromoly steel pushrods: "loose zero" i.e.: 0.001" clearance, or you can just spin the pushrod with your finger ICE COLD engine.

Chromoly pushrods don't expand much, but the engine does, hence it will gain clearance engine warm. Aluminum pushrods expand at the same rate as the engine, so they tend to maintain the clearance set when cold.

Yes, compression check warm.

Your engine specs might help us to help you: displacement, cam, rocker ratio, carbs(including jet sizes) and exhaust, and heads(valve sizes). Spark plugs too, and ignition system(which distributor, points/points eliminator). Ignition timing set at?

Agree with what Danny P has said, I had the same issue and a dirty idle jet was my main issue. Took them out and changed them. 

Another area you may want to check is the bolts from the webber carbs to the intake runners. One side of mine has come a little loos allowing air in and I would get a few pops on acceleration and also deceleration. 

Last thing I did for mine was change the linkage to the dual carbs to a CPI unit, was pretty $$ but the car stays in sync more with the Heim Joints then the post in post cup of the CB units. Not your issue but will help keep the car in tune after. Really worked great!

Good point Eric, I always forget that one. I cured my intake manifold problem permanently with thin, non-compressible gaskets, heavy steel backing washers, and those special long steel 11mm nuts on the studs. I also use Loctite 517 anaerobic sealant and primer on both sides of the gasket. Never a problem for me. They came right off too, but were still sealed after over 30,000 miles.

The thick gaskets some vendors sell will squish out eventually, causing a vacuum leak. 

A vacuum or exhaust leak can cause the backfiring you're experiencing.

Last edited by DannyP

Thanks 

Engine number AH064320

Its a 1973 VW Engine (California)?

i have been under the car, I can only see one fin. The other one is blocked?

66E21D01-41C7-466D-AD29-F2AEA358C39983602F9A-F775-4286-A9D0-93EC19F7B37C0AC06C73-3DDC-4682-B4A5-AC97D0C6E607

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Ok, so you have twin 1 barrel carbs. I can't tell what brand they are, but I'll wager in the 34mm or so size. Take the air cleaners off, and then you'll see. 

I can also see an alternator(good) and is that a Berg breather?(if so very good) I also see a fuel pump blockoff with a hose going to the breather(good because you have an electric fuel pump, but need to check your fuel pressure). I can also see a hose running manifold to manifold for a vacuum balance tube, also ok to have.

Ok, here's where more info is needed. Carbs: If they are Weber or baby Dellorto, 3 psi for pressure. If they are Solex, NO MORE THAN 1.5 pounds! Exceeding this will overwhelm the float valves in the carbs, and flood the engine with too much gas, and also, dilute your oil and ruin your motor. The fuel pressure needs to be verified, engine running, with an actual test gauge. You can't guess and say oh, I have a regulator set to 3, because you really won't ever KNOW what you have. You need to start with the basics, and check stuff in the order I listed above. When you get to the carbs, check the pressure, it's really that important.

I would bet you have something in the 1600-1835cc range as far as displacement. But I'm only guessing.

Twin 1 barrel carbs are less of a problem with intake leaks, they just don't weigh as much as Weber or Dellorto dual throats. So the 1 barrels put less force on the manifolds. But hey, check them anyway. Squirt some carb cleaner around the base of the carb and base of the manifold while the engine is idling. If it picks up speed momentarily while spraying you found your intake leak.

The picture is nice, but we really could use some actual specs. If you don't have them, fine, but if you do, they'd be really helpful to us so we can help you. At this age a serial number and 1973 mean nothing. It is extremely doubtful that the motor has never been rebuilt at an almost 40 year old car and almost 50 year old motor.

Last edited by DannyP

I'm assuming he's got a 1600- 1641. 1776 outside.

No matter, 5 bucks says he's got stock heads and aluminum pushrods. I'd start at the beginning, as Danny is suggesting. Pop off the valve-covers, turn the engine over with a wrench. Do the valves go up and down (all of them)? Are the springs broken that you can see? If it looks good, set the valves-- .004 and .006.

Move to the ignition. Pop the distributor cap. Look at the contacts in the cap. Do they look like they've got calcium building up? Are they carbon tracked? IF they have white stuff, scrape it off with a paring knife. If there's carbon, get a new cap and rotor. If you have points, and aren't a mechanic, and don't have access to a tach/dwell meter, put the hood down. You're going to need to trust somebody. If there were no points, and you find something under the rotor that says "Pertronix" or Compu-Fire", you've got a module, and can put the cap back on. Pull all 4 plugs. Are any of them soaking wet or oily looking? In all likelihood, they will be-- but you are looking for differences between them. Replaced them all. Resist the urge to get Iridium plugs or the like. Just get NGK motorcycle plugs. Gap 'em, put 'em in.

Then start it. Rev the engine to 3000 RPM. Get out a timing light and check the timing-- you are looking for about 30* at 3000 RPM. Where it is at idle doesn't matter.

Shut it off. Pull the intakes off the heads. You're going to tear the gasket when you do this, but you can cut new ones. Clean both the manifolds and the sealing surface on the heads. This will be no fun, but must be done. Cut new gaskets, use something (grease or oil) to coat them. Bolt the manifolds back on. Then, start and check the fuel pressure. Even if the pressure is OK, that still doesn't mean you're out of the woods on the carbs.

Were the plugs wet or oily looking when you pulled them out? It's probably flooding. KAdrons are notorious for doing this, because the needle valves are garbage. You can get a rebuild kit, but the new needle valves in the kit are garbage too. Lots of people love, love, love their Kadrons, but those people like kale too. The carbs will often be problematic no matter what.

You've got to go through all of this, or none of it matters. Don't trash talk guys trying to help with nothing to gain. Good luck. 

What about the air fins?

There is only one fin shouldn’t there should be two fins. 

This will affect the airflow and make it difficult to tune?

it looks like the engine is not sitting correctly. 

The Carburetors are Solex 40/44 EIS

371B48E9-5D73-45DA-8528-712B3360559191E86733-2B66-43F5-A004-9A7B9FD473D056579E8E-20E6-42E7-9F0A-55A4461595C1

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DannyP posted:

Ok, so you have twin 1 barrel carbs. I can't tell what brand they are, but I'll wager in the 34mm or so size. Take the air cleaners off, and then you'll see. 

I can also see an alternator(good) and is that a Berg breather?(if so very good) I also see a fuel pump blockoff with a hose going to the breather(good because you have an electric fuel pump, but need to check your fuel pressure). I can also see a hose running manifold to manifold for a vacuum balance tube, also ok to have.

Ok, here's where more info is needed. Carbs: If they are Weber or baby Dellorto, 3 psi for pressure. If they are Solex, NO MORE THAN 1.5 pounds! Exceeding this will overwhelm the float valves in the carbs, and flood the engine with too much gas, and also, dilute your oil and ruin your motor. The fuel pressure needs to be verified, engine running, with an actual test gauge. You can't guess and say oh, I have a regulator set to 3, because you really won't ever KNOW what you have. You need to start with the basics, and check stuff in the order I listed above. When you get to the carbs, check the pressure, it's really that important.

I would bet you have something in the 1600-1835cc range as far as displacement. But I'm only guessing.

Twin 1 barrel carbs are less of a problem with intake leaks, they just don't weigh as much as Weber or Dellorto dual throats. So the 1 barrels put less force on the manifolds. But hey, check them anyway. Squirt some carb cleaner around the base of the carb and base of the manifold while the engine is idling. If it picks up speed momentarily while spraying you found your intake leak.

The picture is nice, but we really could use some actual specs. If you don't have them, fine, but if you do, they'd be really helpful to us so we can help you. At this age a serial number and 1973 mean nothing. It is extremely doubtful that the motor has never been rebuilt at an almost 40 year old car and almost 50 year old motor.

Hi Danny

The Carburetors are Solex 40/44 EIS

How do I check the fuel pressure while the engine is running with a test gauge. I have a test gauge where do I place the hose of the test gauge?

i still need to spray some carb cleaner around the base of the carb and manifold while the engine is idling to see if there is an intake leak.

i am not sure about the engine displacement, how can I work this out?

Cheers

Stan Galat posted:

I'm assuming he's got a 1600- 1641. 1776 outside.

No matter, 5 bucks says he's got stock heads and aluminum pushrods. I'd start at the beginning, as Danny is suggesting. Pop off the valve-covers, turn the engine over with a wrench. Do the valves go up and down (all of them)? Are the springs broken that you can see? If it looks good, set the valves-- .004 and .006.

Move to the ignition. Pop the distributor cap. Look at the contacts in the cap. Do they look like they've got calcium building up? Are they carbon tracked? IF they have white stuff, scrape it off with a paring knife. If there's carbon, get a new cap and rotor. If you have points, and aren't a mechanic, and don't have access to a tach/dwell meter, put the hood down. You're going to need to trust somebody. If there were no points, and you find something under the rotor that says "Pertronix" or Compu-Fire", you've got a module, and can put the cap back on. Pull all 4 plugs. Are any of them soaking wet or oily looking? In all likelihood, they will be-- but you are looking for differences between them. Replaced them all. Resist the urge to get Iridium plugs or the like. Just get NGK motorcycle plugs. Gap 'em, put 'em in.

Then start it. Rev the engine to 3000 RPM. Get out a timing light and check the timing-- you are looking for about 30* at 3000 RPM. Where it is at idle doesn't matter.

Shut it off. Pull the intakes off the heads. You're going to tear the gasket when you do this, but you can cut new ones. Clean both the manifolds and the sealing surface on the heads. This will be no fun, but must be done. Cut new gaskets, use something (grease or oil) to coat them. Bolt the manifolds back on. Then, start and check the fuel pressure. Even if the pressure is OK, that still doesn't mean you're out of the woods on the carbs.

Were the plugs wet or oily looking when you pulled them out? It's probably flooding. KAdrons are notorious for doing this, because the needle valves are garbage. You can get a rebuild kit, but the new needle valves in the kit are garbage too. Lots of people love, love, love their Kadrons, but those people like kale too. The carbs will often be problematic no matter what.

You've got to go through all of this, or none of it matters. Don't trash talk guys trying to help with nothing to gain. Good luck. 

Hi Stan

I am not sure about the engine displacement how can I work this out?

The springs are not broken, what do you mean “do the valves go up and down”? The valves are horizontal not vertical.

The distributor cap had white stuff, I cleaned them.

what do you mean by points? I don’t have a tach/dwell meter.

How do I check the rotor to determine if I have module?

i have NGK plugs

30* at 3000 rpm ( I am assuming 30 degrees at 3000 rpm?)

Maybe i should get a single Carburetor to run the entire engine.

i agree the needle valves on the carbs are garbage.

I am concerned about the air fin,

there is only one, I don’t think the engine is sitting correctly. This will make it difficult to tune the engine?

I don't understand what you mean by "air fin". Please try to explain in a different way.

I got a pair of Solex 35s to run like a clock on a tube-frame sand rail. Pull the fuel line from one carb, and install some hose and a "T" to the gauge. You'll get the pressure across both carbs. Stan is correct that the float valves are usually not that great quality. But if you make sure you're at no more than 1.5 psi you should be OK.

The only way to truly verify displacement is to take a cylinder head off, and measure the diameter(bore) and stoke of the pistons. Stan is most probably correct that you have a 1600 to 1776 at the biggest. But there is no way to know for certain without pulling a head.

One thing at a time. That's troubleshooting. Don't fixate on the tin-- the tin is for cooling, and this isn't a cooling issue. Once it's tuned correctly, if you have an issue with the cooling, you can address it then. Until then-- mechanical, spark, fuel (in that order).

Fins?  Here's what your engine looks like with all the cooling tin removed.  Each piston cylinder has steel fins around it (probably rusty) and the heads have fins but are silver aluminum.  If the car sat for a long time mice like to build nests above the cylinder tins disrupting airflow and causing a stink! 

Image result for vw bug engine

I don't see any of the '73 California smog stuff on the engine.  Transaxle is a '73.

On an engine that you don't know anything about - a compression test is always good for establishing a baseline condition.  Low compression or uneven cylinder compression can indicate worn cylinders/rings, bad valves or cylinder-head leaks.

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Last edited by WOLFGANG

You have to do some of your own thinking here Highlander. Stan said to check the valve movement and used some generic language to check the movement of the valves by asking you to see if the valves were moving up and down. Your response was, "The valves are horizontal not vertical.". Well of course they're vertical, it's a boxer motor. Stan is well aware of the engine configuration since he's built several motors. But he's also been wrenching on good ole American iron for almost all of his life. Those valves go up and down. He didn't phrase it properly but it means the same thing. So check if all of the vales are moving in and out instead of up and down. If any of the valves aren't moving in or out then you have a broken valve spring and that will cause problems.

FWIW, Kadrons are notoriously hard to dial up. My car came with a set and I just sold them and bought a set of Dellorto 40's that were "plug and play". The posts above are spot on. Intake leaks will do it. With my Dellortos I had occasional backfiring on deceleration but that was after 15 years of them being installed and it ended up being a plugged jet on cylinder #2 so that cylinder was being cancelled occasionally. After cleaning up the carbs everything went away.

I disagree. Kadrons are great on stock or near stock engines. If you want to build some serious HP on an engine bigger than a 1915 then I agree totally.

We could never get them right; they were running rich and had a notorious stumble with the .009 distributor the car had. I know some people are happy with them but not in our case. A friend of mine also had a pair and sent them to AJ Sims at Lowbugget to get them dialed in and gave up on them too. After I installed my Dellorto 40’s and saw how nice they work he bought two sets; one for his Speedster and another for his 914 and had perfect results as well. This is in our experience.

Last edited by Impala

@Highlander356 unless you really want to spend a lot of time and learn a lot about these engines and their idiosyncrasies, I would recommend finding a good, trustworthy mechanic in your area and have them sort out the problems.

While many of us are good wrenches, it's very hard to help without being there. Many others of us just like the cars and don't have a clue about working on them. There's nothing wrong with either approach, but becoming a good air-cooled VW mechanic is a long and time consuming process.

mppickett posted:

@Highlander356 unless you really want to spend a lot of time and learn a lot about these engines and their idiosyncrasies, I would recommend finding a good, trustworthy mechanic in your area and have them sort out the problems...

You're wasting your breath, Mike, there are no good mechanics in Australia. We've already been told so several times. I guess working on cars are where Aus. guys go when they're not smart enough to do anything else. He'll be the first to master complex procedures such as installing spark plugs, setting timing and adjusting valves and carbs. 

The tin has nothing to do with timing. The tin has everything to do with cooling.

If the tin was important to timing, how do you think we run drag cars without  any tin?

You should really take your car to one of those really bad mechanics in Australia and let them fix it.  lol

Last edited by LI-Rick

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