What is this engine noise?

[berni29]

Hi Guys

My +2 standard weber engined car runs well apart from a frequent single spitting noise that the engine makes when running at constant but slight throttle openings. It is a definite misfire because the engine misses a beat. I have heard it on slight overrun as well as slight acceleration. I have even heard it on the motorway at reasonably high revs. I cannot decide if it is coming from the exhaust or carbs. Altering the ignition timing (I was running 15 adv, but back to 10 now) makes no difference.

Any ideas? its really annoying me!

Many thanks in advance!

Berni

[alaric]

Check the exhaust manifold studs & nuts - maybe the manifold's a bit loose on one cylinder, and under slight load, you're hearing exhaust escaping.

[berni29]

Hi

I have never checked them, so I will do. When I say that the noise is frequent I mean every minute or so when conditions are right. The car used to pop back through the exhaust but since I fixed a small leak at the Y piece it has stopped. Maybe if I took the airbox off and drove round I could pin the noise down to induction or exhaust from any relative (or lack of) change in volume of the noise.

Thanks

Berni

[type26owner]

Berni,
The early Webers had spring loaded auxiliary venturis. The spring forced the venturi up against the carb body to make a quasi vacuum-tight seal. The P-shaped springs are a joke. It's the first thing to check absolutely since all the springs relax and loose their stored energy rather quickly.

The spitting is from a lean mixture condition or an induction system airleak. It will be heard as popping in the exhaust when a cylinder partially or fully misfires.

Adjusting the idle mixture by the 'best lean idle' method also causes exhaust popping on overrun. That is the dumbest damn tuning advice to possibly do. An AFR of 22:1 which is the typical value I've measured on every car so far is going to cause some misfires for sure. Also the with the induction manifold vacuum at about 28 inches which is typical on an overrun the airleak down the thread of the idle mixture screws makes the mixture go even leaner.

When I first measured the AFR on overrun with my wideband O2 meter it was reading 40:1 and popping away along with the worst cases I'm sure. When it's corrected with no popping it's now 17:1. A well tuned Weber twinkcam should never pop, spit or backfire.

[berni29]

Hi Keith

I know what you mean about the "P" shaped springs, and they are the early type carbs. I am sure that at least one of the auxiliary venturis will be loose, but with the trumpets on why does any looseness matter? In fact it would not surprise me to find a spring missing because I vauguely remember that one is. I can understand air getting in through threads, spindles and "O" rings. I will check for leaks as soon as I can. I am also going to put some heavy grease on the mixture screws.

Thanks again

Berni

[type26owner]

but with the trumpets on why does any looseness matter?

It's a killer, huge problem! Weber went to a grubscrew later on to cure this problem. If you can reach in the trumpet and rattle the auxiliary venturi around AT ALL with lots of force then it's too loose.

When you're at cruise, the induction manifold vacuum will hover around 8-10 inches of vacuum. The airleaks become less and less important as the throttles are opened up more and more. This is why the racers can get away with airleaks big enough that it would render a road car undrivable. Put they cannot have the auxiliary venturis loose! This also is a primary cause of a flatspot.

You'll need to makeup your own replacement springs. Find a tension coil spring that is about .230" OD x .028" wire. Snip off a section that is exactly two turns and shove the sharp end into the pocket. A shim might be needed under the spring to get the right amount of interference fit in the carb body. This works ten times better then the original P springs.

[berni29]

Hi Keith

I have the trumpets off and sure enough 2 of the aux venturis are a bit loose, and the other two (on the back carb) are really loose. The good news is that they all have the "p" springs intact. I will get hold of a replacement spring as soon as the clock/watch repairer is open again, do the fix as you suggest, and let you know the result!

By the way, these carbs are the type 18, and do not have the holes in the bottom of the throttle plates. Do I need to drill some (.5mm)?

I notice that I have a thickish brown oily sludge (mayonnaise consistency) covering the bottom of the airbox. It is coming from the crankcase breather. Is that normal?

Thanks again

Berni

[type26owner]

Berni,

and let you know the result!

Don't get your expectations up too much. These improvements and repairs will be noticable but will never make a carbie the equal of a computer controlled fuel injection system.

By the way, these carbs are the type 18, and do not have the holes in the bottom of the throttle plates. Do I need to drill some (.5mm)?

I also have the 18s. The leather shaft seals on either side of the throttle spindle that are there to stop air from leaking in are likely shriveled up and doing no good at all. Replacing those seals is essential to getting it to purr again at idle. When the seals are working okay you should be able to slow the idle speed down to at least 600 rpms with the speed screw. On mine when the seals were totally shot and it would not slowdown below 1100 rpms with the speed screw completely retracted not contacting the linkage at all. Once all the airleaks are stopped only then does one drill bleed holes in 3 0f the 4 butterflies to balance the amount of air entering each of the cylinders.

Is that normal?

Yes, but there are two things happening in the airbox actually which account for about 90% of discharge. The water vapor that makes it past the rings into the crankcase will only get sucked back out of the breather at a one-to-one rate if the engine oil reaches 190F. That is the minimum temperature all oil suppliers recommend. The twinkcam is known to run it's oil way too cold at around 140-150F range. I'm headed out to the radiator shop today to have them install a loop of tubing in the top tank of the radiator so the all the oil that gets pumped first circulates though the loop and is forced to heat up to the minimum 190F temperature. The amount of oily goo will vary as the humidity does.

The other stuff is fuel from around the trumpets. You can stop this by making your own gaskets that seal around the trumpet flange that the clamp bears down on and the carb body. Find a supply of 1/64" thick neoprene or better yet viton sheet that has a fabric inside. Place the trumpet on the sheet and cutout the ID with a scalpel. Cut the OD so the radial width is about 5/32". Works at treat.