So I got my car back from the local Porsche shop and installed a rebuilt MPS from Bowlsby but the car is still not running right. Flat spots and intermittent dead fire misses. She starts right up, idles well and drives ok but when you get on it it has intermittent flat spots and when shes just cruising it misses once and a while. Seems like the advance is not ramping up like it should. Also, the mechanic left a vacuum line off the distributor. The line from the inboard connection on the pot is hooked up but the outside one is off. I understand that one of these is advance and one is retard.

Just need a list of crap to check

What year is it?

I always liked Rich Towles way to find vacuum leaks which is what it sounds like.

BUT first check all the mechanicals.

1. Take off ALL the vacuum hoses and plug them. Check all mechanicals, valves, dizzy advance plate.

2. Plug each one back in to find the leak.


Thirdly....I would find another "mechanic".

1974 2056. New injectors, rebuilt MPS.....

TPS -
mine did the same thing and i always thought it was something else but since i swapped out a new TPS assembly i got from Bruce Stone, it now accelerates smoothly with no bucking or intermittant hickup like you are describing- none, 0, zilch! i can say, like new. and i had tried a replacement board but i still had an issue until the one went in. Unfortunately - i am no fighting a different d-jet issue...

Timing light? Everyone correctly goes after vacuum but I would also check wiring. Two nights ago I had the lid open and saw arcing between a spark plug wire and a vacuum hose! I kept thinking I was making a mistake and it was another spark plug wire. Didn't really want to grab the offending item so I shut it down. Sure enough: hose for the vacuum advance. Black silicone rubber. I can only think that the black is carbon black, which I suspect might conduct high voltage. Misfire went away when I moved the hose. Maybe new HT wires soon?.

At any rate I would check all the components including the wires that go to them such as the aux air, the throttle sensor, the temp sensor connections, and of course the MPS. One problem I had once was a leaking cold start valve that dribbled gas into the induction system at random. A flashlight peak through the throttle body would show wet gas in the plenum. So many piddly little things. Good luck

BTW, how does Rich look for the leaks? Propane?

yup, TPS, Advance plates greased, vacuum leaks, and timing.
That may explain the flat spots on accel.

Cruise skipping typically is TPS.

Also, check the CHT both warm and cold....that contributes.

Rich

Tommy probably knows the solution

i believe i am suffering from this same issue(the leaky cold start valve) - WORN, can you tell me, did it have a hard time starting sometimes, like it was flooded out? if i leave the car for more than a day, it will be difficult to start, but anything from 15 minutes to half a day it fires right up... just wondering.

Check your fuel pressure.
That may be a leaky injector, and you need to check all five.
Typical of a leaky cold start injector to relieve all the fuel pressure, drip raw ful into the plenum and make it hard to start.

I always turn the key to run three or four times before I crank it when the car has sat for a long period of time.

Rich

Rich,
fuel pressure in line gauge says its at 30. stock setting should be 28, correct? would that be enough to cause this?

Mike, I'm assuming you don't have an O2 sensor & A/F Gauge installed, right? I gathered that the O2 sensor and an adjustable MPS might be pretty important for getting the 2056 to run optimally. Having said that, based on my limited experience I agree with most of the other suggestions (TPS, advance/retard function, etc.), but the sensor can help you troubleshoot.

[quote name='r_towle' date='Mar 16 2015, 10:13 AM' post='2160821'] right up... just wondering.
[/quote]

Check your fuel pressure.
That may be a leaky injector, and you need to check all five.
Typical of a leaky cold start injector to relieve all the fuel pressure, drip raw ful into the plenum and make it hard to start.

I always turn the key to run three or four times before I crank it when the car has sat for a long period of time.

Rich
[/quote]

Yeah, i turn the key and listen to the fuel cycling through.
Sometimes if it is quiet i will listen to the injectors click as i slowly press the pedal to the floor. Quick checks that tell you that so,e parts are alive.
The sure sign of cold start leak is gas showing up in the plenum. My car will hold normal fuel pressure for more than half an hour with the engine turned off. If you have a leaking injector that pressure will slowly be dissipated as the fuel leaks wherever.

I'm thinking TPS is one issue. I'm not opposed to getting another one.
I hooked up the outboard (retard) vacume line on the distibutor. Car ran a little better but idles quite high. I could hear vacuum. Removal and plugging of the line corrected idle and stopped the leak sound. I'm thinking a bad vacuum pot too. Flat spots feel like a lack of advance and they are intermittent.

Remove the advance vacuum line and plug it.
Drive it.

Also, remove the distributor cap and rotor.
Hook up the advance vacuum line to the little can on the distributor.
Remove the DECEL line so the port is open.
Take the other end of the advance line and suck on it, see if you can move th advance plates.
If yes, when you let off do they SNAP back to no advance???
Rich

Do what Rich says..........


AND find a new mechanic.

Ok, sucking on the advance moves the plate and it pops back nicely. I'll plug it and drive it. And report back.
I cleaned the TPS. It looks pretty iffy. Cleaning it did help the surging about 50% but caused a low idle.

So Jeff set up the MPS for a 2.0. Now we need to set it up for my 2056. Question is, do I take it back to the same guys?

Is your MPS adjustable (does it have the adjuster kit from Tangerine installed)? I'd install an O2 sensor and AFR gauge and tune it myself. But that's just me.

So what happened when you drove it?

Set tps like this at idle.
There is only one way to set the tps.

All mps' are adjustable. Chris'kit is convenient, but not totally necessary.

Cool, I've learned something new already this morning. Time to go back to bed before it happens again.

Here's a repost of my comments in my build thread:

So I had the local independant Porsche shop looking at my car over the last six weeks or so. They did allot of good work making the engine run properly, but they never got it right. The car would buck at various RMP. Mechanic said that the fuel pump might be to blame so I say, "replace the fuel pump." He says, "we want to make sure that's the problem." I tell him if you charge me $135 an hour to figure out that it's the issue then you might as well buy a new pump for $130 and install it. Then I'll at least have a new pump to show for it. I also tell the guy that it's probably the TPS making it buck.

I get a call a few days later that my car's "ready and running great". No fuel pump and no TPS installed. I pick up the car and guess what? It's still bucking and running like crap.... sheeplove.gif

Soooo.....

Last night I installed a new fuel pump. Car runs allot smother but still bucks

Soooo.....

This morning I installed a TPS that Rich sent me last week. You guessed it, that fucher's running like a fresh spun top. Why won't mechanics listen to people? I mean I obviously know a little bit about my car, I built the thing from scratch right?


People..... headbang.gif

So now she's running pretty sweet. I am hiving a high idle issue. It idles at around 1600 rpm. It feels like the adjustment screw on the intake is mostly all the way in and I get no change from turning the adjuster on the computer. I checked for vacuum leaks with map gas but found nothing. I'm going to fiddle with the TPS some more.

Any other ideas?

Maybe check the timing again?

Maybe the advance is sticky?

It is definitely worth checking timing. Checked the AAR?

this could be the alignment of your TPS still. - i ialso had the bucking issue for 5 years but once i got my repalcement TPS from Bruce stone its been like perfect ever since- except at first it was idling a bit fast, but that was cured by making 2 adjustments; one was the TPS, the other was the MPS was set too lean so making just a bit more rich helped, then reset the timing as well, and i cheated a bit here, and backed off the timing a few degrees and took it from 1000 rpm to 850 and has run perfect since!

good luck

High RPM is almost always a vacuum leak. There are so many hoses it is hard to find them sometimes. With your car I would suggest that you install an A/F meter and place the gauge where you can see it as you drive, rather than down low like one idiot I know personally did. The original MPS settings aren't going to be quite right and after you mess with the car it will need some adjustments. I also added a CHT sensor and found that when it is running well close to lambda = 1 at around 14 the heads are heading towards the recycling bin. Richening the mixture has cooled them and there is a sweet spot where the mixture is around 12 most of the time and a bit leaner at idle.

I know that the gauges are expensive and a pita to install. Especially the bung in the exhaust. But as you say you built it from scratch so you know your way around the block.

I have to add that I am running the resistor from the 1973 model to richen the mixture by fooling the ECU into thinking the car is still a bit cold. Sounds like you have a really nice engine there. Good luck!

high idle is vacuum leak AND/OR timing.

So, remove everything from the plenum and cap it off with rubber caps available at the local hardware store plumbing isle....

Leave JUST the MPS connected to the plenum.
Its all you need.
Dont be lazy and leave the AAR on there....
Remove all of them..

Does it still do it?

Add one system (vacuum line) back at a time.


First I would quickly check timing...it may simple be that..
3500 rpms, tie off the accel on the throttl body with a piece of string, steady at 3500
Then set the timing...
vacuum line off the distributor...Dwell already set correctly.
Again, dont cheat...

Rich

Hi ho, hi ho, off to the garage I go
I'll report back as soon as humanly possible.

Following Rich's advice above I determined that:
A: the AAR was staying open causing high idle
B: the timing was set to 13 deg at 3500rpm causing a flat spot at acceleration.

I plugged the AAR and set the timing. Car ran pretty good and did not flatten out while accelerating

Now I'm worried that the shop set up my MPS with bad timing and a leaky AAR.
Now she wants to idle at 1200 rpm. Improvement but not perfection.

I'm going to fiddle with the AAR or get another one. Then start looking for a friend with an O2 sensor I can use.

I removed my AAR long ago....it's not really needed.

Does the air bypass screw do anything now? Can you tune the idle with it?

If it's not an LE, it's not worth fixin.

It's screwed pretty far in. to the point that I have to use a screw driver (I.E. remove the air cleaner) to adjust it in.
If by "now" you mean with the AAR removed, I haven't tried messing with it yet.
In chilly Maryland I don't need the AAR? Do I plug the back of the 5th injector?

By "now" he means you have eliminated once source of a vaccum leak.

Next:

Can you adjust the idle speed with a screwdriver. Stay focused...don't worry about the AAR right now.

If you cannot significantly change the idle with the air bypass screw, and almost make it die by turning it all the way in, you still have a vacuum leak.

Sources of leaks.
Head----intake manifolds
injector seals
intake runners to plenum (large rubber hoses)
anything plugged into the plenum and all rubber vacuum lines (cracks)

Throttle body gasket
Cold start injector gasket and injector itself

I figure there are more I forgot, but I take a VERY methodical approach to finding the leak....remove all (except the MPS)

If idle can now be controlled, one of those systems you removed is at fault.
If idle cannot be controlled, check intake from plenum to head.


Think of it this way.

If I put 10 holes in your house, all over the place, 1 foot square each in size....you would have a 10 square foot leak in your house.

Same thing applies here....lots of small leaks add up to a big leak.



Rich

I had some time to mess with the FI last night. I found a vacuum leak around the throttle body to plenium gasket. Not having a new one handy I pulled the TB and cleaned the surfaces with cleaner and some sand paper. When I put it back together and started the engine it revved out of control. I reset the TPS, pulled and cleaned everything again. Same issue.
I got in it this morning and heard a click when I pushed the clutch in. The throttle cable was stuck under the clutch armature
Now the idle can be adjusted with the adjustment screw and she runs a little smoother. It wants to die when the screw is too far in.

Next I want to sort the AAR. I like how the car starts with it working.

I also bought an LM2 exaust gas analyzer. I'll double check all the vacuum stuff again and see what it's reading.

Good job! That would be a hard one to find.

I did basically the same thing last night and had the same issue. I replaced the TPS board and when I put the throttle body back on and started it I got a horrific whine, almost like a turbine, and the engine would start to rev out of control. Sounds like a massive vacuum leak. Sure enough, when I put the throttle body back on I let one of the washers from the 13 mm bolts that hold it on the manifold get under the throttle body instead of on top. All better now and I think the bucking is gone.

A quick update as I have had home projects and summer taking all my time.

I replaced the AAR with a known good unit. Now the car starts easily and warms up with no help from the throttle. Car ran pretty good. I was able to adjust the idle to 1k. Happy me. Now on hard decel the idle drops into the weeks. I hear the 2056 like a little higher idle...

I have now installed a tested good decel valve. This helped the idle drop but only until the engine is warm. I'm thinking it's rich at idle causing it to tank when the RPMs drop. A quick touch of the accelerator keeps it going.

Car accelerates great but still has a flat spot at near full throttle. I'm thinking it goes lean at full throttle. If I'm easy on the gas, RPMs climb with power and smoothness.

My assumption is the MPS was adjusted with vacuume leaks present and the guy who did it f-ed it up.

My plan is to adjust valves, change the oil (500 mile service) and the hook up the LM2 to set the MPS. I found a good thread on the Samba that I'm adding to my study list.

Samba D-Jet thread

Mike - I feel like we're having very similar issues with the 2056. Right now on hard decel/overrun I occasionally get the RPM's to drop where the engine will sometimes actually stall. It's very intermittent, which makes it weirder for me. It's no big deal (I just pop the clutch and engine fires back up), but a little disconcerting.

Did you see the thread today about fuel pressure? I'm going to try cranking mine up a notch or two (to 31 psi) as suggested on there. I guess dialing these in to perfect (or good) tune takes some experimenting and time...

Good luck man.

I really feel like the MPS is wrong in my car. The asshats that tuned it for me didn't even have vacuum leaks sorted, plus a few of the hoses were wrong.
Another thing I'm going to sort is the PCV set up. Probably not any real issue but I want to bring the system back to 100% as designed then tune it from there.
I'm making small changes then driving the car so I can monitor each result.

I just installed a new coil too. Another measurable improvement in the way she runs.

Ok, so I'm getting really close.

Valves were adjusted poorly, set them to (almost) zero lash per Raby spec.
New fuel pump with pressure meter installed and adjusted to 29psi (it was at 40!)
Set the new TPS correctly


Please tell me the o2 numbers I'm looking for on my LM2.

I think I'm still a little rich at idle and part load 10.5-12.4
And lean at full load 13.8-15

I saw somewhere that 12.7 was a sweet spot?

Car runs good. Gets a bit lean sounding when I get on it hard and wants to die when I drop RPM for a stop.

A:F ratio, you want to see 13.5-12.9 on the gauge while accelerating or climbing, 12.8-12.5 at WOT, and 16-17 while cruising, going downhill or decelerating.

Ok, I can do that.....

It wanting to stall or slow idle cold be because it's still running too rich, or timing is still a bit off-i had the same issue(assuming the tps is set correctly). If your numbers are from a warmed up motor, you are rich at idle, part load and lean on wot. I think with air cooled and d-jet motor I would not want to be as lean at cruise as 16-17 as Rich stated. Maybe 13-14. Which is about where i have got mine to. I'm at that 12-13 at w.o.t., idle is about 10.5 cold and 11.5-12 when warmed up, I'd rather be a little on the rich side than on the lean.. I've read the power Is better right at the 13.7-14 . Check your plugs and see how the look , should give you some idea of where you are as well.

After my last adjustment that's about where I am too. Tendency to stall is less now. I saw an adjustment procedure for the decel valve. I'll try that later. 16-17 does seem lean. I'm not finding any consensus on the net about these numbers.

when i first had my MPS rebuilt then set to factory specs, i had very lean running and even surging at idle , my first corrections were too much and i was very rich, and had that low idle, stall issue. so ive been slowly working it back toward those numbers. Las year at Octeenerfest i was really , under load, too lean and as i accelerated up hill hard it was maybe 15 or so. now its more in that 12.8 range and should run more cool as well when under load. but one factor in my running rich was that i found out that the shop after setting the valves for me did not have the timing set correct. as soon as i set it for the 27'btdc, (they had it set at zero) this put me even more riche and i had to go back and readjust the mps again to lean it back out just a bit. and of course i was running hotter from being too advanced and my idle was too low - now, idle is right at 950 warmed up, it doesnt try to die, running about 10-15 degrees cooler on oil temps and A/F is just about perfect. - it only tooke me 3 years to figure it out, lol!!

A bad decel valve can cause similar problems at idle and stalling.

Make sure it is working correctly and not leaking first before you adjust it.

You've done a great job learning the system and doing it yourself.

I am going through the exact process. Getting close to getting it sorted but still a bit rich.