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

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