Type 4 misfire reasons

[darindeb]

We have a 72 411, that has about 500 miles on shop built motor with hydrallic lifters. Marvin (cars name as its green), has a runability issue. Starts and accelerates well. But while running under steady throttle, wants to cut out, as if missing. Valves have been adjusted and are correct, new injectors and fuel filters clean.
Any suggestions as we are stumped? Really dont want to swap to carbs. But can as we have an extra set.
Any help would be greatly appreciated.

Thanks, Darindeb

[raygreenwood]

[Lars S]

Agree with Ray, we should have more spec info to get this right.

Also has the car been running with this symptom all since the rebuild or has this occured lately?

The 1st thing I would look at with the symtoms your car have is the throttle switch (TPS). The car can be run with the TPS electrically disconnected (it will accelerate slower and idle poorly) but if it runs without bucking when unplugged the fault is likely in a worn or dirty TPS.


/Lars S
_________________
Porsche 914 -72, Bahia Red daily driver
VW411 2-d -70, White, sold
VW412 4-d, -73, Gold Metallic, daily driver
Suzuki T500, -69, Candy Gold, sold
Suzuki K50, -77, Black, daily driver
BMW R69S -69, White, sold
Husqvarna 118cc, -47, Black, Sold

[raygreenwood]

What Lars said!

To explain a little better about the issues with D-jet and hydraulic lifters.

The problem is not primarily the fact that the lifters are hydraulic. By a certain respect.....keeping effective valve lash down to 0 or near 0 can actually be a positive thing.

The problem is in two areas:

1. The fact that the lifters are hydraulic requires a softer ramp and nose design to the cam lobe to operate. This changes the injection timing in a very subtle way....to our eyes. If it also requires slight changes to ignition timing to compensate.....that right there changes the injection timing slightly.

While these two very slight timing changes have no effect on fuel mixture on the later L-jet system or with carbs......these subtle changes to injection timing affect the vacuum signature.......and D-jet uses this signature to set your main fuel mixture via the MPS.

This can cause slight lean or rich running.....and frequently an alternating cycle of lean to rich at idle and low rpm. That cycle.....changes the vacuum signature even more.....making the lean/rich spikes worse.

This is why cam changes do not have the same effect on L-jet and later injection systems. They do not use vacuum as the load measurement method.

If having hydraulic lifters wers the only factor with the camshaft.....it could be adjusted around with some tweaks to the MPS and the trigger plate.

2. The big problem.....is being able to find a hydraulic type 4 camshaft that has the proper intake valve timing and lobe center. The lobe center is not very hard to find in aftermarket cams. Web cam has several that are in the 106-108° range.

The critical part is finding one with the correct intake valve timing not an excessive amount of lift and duration.
D-jet does not have perfect injection timing. At least two valves are closed when the iniector fires. This alone is a slight idle smoothness issue.......and has some effect on vacuum signature smoothness......but it is carefully distributed to have acceptable enough timing between injection event and intake valve open points at low rpm and idle and still maintain smooth vacuum signature. Above about 1500 rpm.....its all happening so fast that the timing matters less.

But.....cams not optimized for D-jet can have vacuum signature issues all through the range and cause poor adjustability of the MPS and hesitation at shift points.

Cams that are known to work well with D-jet because of the timing are the Webcam #73, the Raby 9550, 9590 and I think the Webcam 124 or 142.....its one or the other on that one. I cannot remember. Its a 914 2.0L grind.

And.....all of those are solid lifter.

The issue....which is why I ask who built the engine....is that too many people who build engines that only have bay window bus experience....have no knowledge in general of D-jet....and consider a 411 or 412 engine to simply be a bus engine with different specs so they use what works well for a bus.

In reality.. .its the other way around. The bus engine us an adaptation of a 411 engine. What works with a bus injection system.....camshaft wise....does not work for a 411/412/914.

Ray

[darindeb]

This is so much information for me to take in, but l do understand. Will be getting with the shop that built it as well as last owner. Most likely last owner first and then shop.
We have put 400 miles on this motor and he had put 80 on it.
After the valves were adjusted and readjusted, we were able to get Marvin to 70 mph. Thats impressive considering the automatic trans. Its just the constant throttle that gives hiccups. Idles smooth, and accelerates well.
Will be asking these questions to last keeper and mechanic /shop.
Although a question we had has been answered. No to find what lift/duration cam has. If it is one fact one that you mentioned, is it possible to just put in solid lifters?
Thanks to all you knowledgeable Dubbers.
We will get this right,
Darindeb
P.S. we installed air shocks at rear for proper ride height since locating correct ones was not possible. Cant tell, it looks so good.

[dawie]

If your hydraulic cam is the bus-type, thats a "lost case". Original hydraulic bus cam is "lazier" than even solid lifter bus cam, because of different timing (in addition to being hydraulic). It has negative overlap at 1mm, and early closing inlet valve. That cam pulls a much higher vacuum at light loads/low rpms, and restricts airflow at higher rpms. Specifically designed to lower hydro-carbon emissions at the expense of reduced torque.

D-jet's control unit is hard-wired to have mixture match longer overlap duration, higher lift, and later inlet valve closing time which is specific to the D-jet cam.

[raygreenwood]

[darindeb]

Thanks Ray and dawie.
Will look more into your advise. And double check all lashes, connections and vacuum lines.
This is a great little car. Just want it to perform without hurting it.
Will keep you posted
Darindeb