T4 D-Jet Throttle Shaft Slop

MGVWfan · Post by **MGVWfan** » Wed Jan 19, 2005 5:36 pm

Anyone have an opinion on wheter slop in the throttle shaft needs to be fixed? It does represent a variable air leak, but one the MPS should know about and may be able to compensate for. It could cause idle speed variations, and maybe difficulty in setting the idle switch, too.

The reason I ask is because the Nomad's throttle shaft is a bit loose, enough so if it was an SU carbie on an MG I'd re-bush the carb. I reamed the carb body on my MGB's SU's to allow bushing installation (uses a chucking reamer with a shank the same size as the throttle shaft as a pilot in one end to ream an opening for the other end's bushing, then the reverse), and I'll bet I can come up with something similar for the Bosch TB. Is it worth it, though?

raygreenwood · Post by **raygreenwood** » Wed Jan 19, 2005 8:01 pm

Yep...it can be a "slight" problem, especially if you are tuned very closely/tightly. This is the primary reason I went to the later model and larger 1.8/2.0 TB. That is teh one that is used withL-jet primatily...and has the fato-rings idle screw. It is 2-3mm larger and uses a better seal against the plenum that is still available at the dealer. I also found that the extra diameter was excellent on the 1.7. Now check around. There are numerous variations. Ther are some....with a short shaft, meaning no provision for the TPS.....but....allof them have the casting flange to mount theTPS...and most (but not all) already have the screw holes drilled for theTPS. AND....the plug in the bottom of the shaft hole...is a small freeze plug. It pops right out.

Whats nice about these TB's.....is that they have a little blue replaceable throttle shaft seal at the top. I recently found that a good replacment for it....is the clutch throwout seal for a 90 model cabriolet...4 or 5 speed (or any golf for that matter). They are available anywhere..and are about $4.
The shaft from any of the TBs with a TPS...will drop right in. Its also niceto have that late model radial throttle return spring. Just simply take the nut off the top, remove the spare lever underneath...and install like your old TB.
What I found, is that among teh late model L-jet TB's...there are some that are larger than the 1.7 TB...and some the same size. The one Ifound that was larger had the short shaft (no TPS). So I found that one of the small diameter L-jet TB's had the throttle plate screws on the same center and dropped right in. Ray

MGVWfan · Post by **MGVWfan** » Wed Jan 19, 2005 8:25 pm

Next time I get close to an L-Jet TB in the junkyard, I'll snatch it out. BTW, anything other than an ACVW use the same TB (easier availability)?

Thanks Herr Doktor Gruenesholz

raygreenwood · Post by **raygreenwood** » Thu Jan 20, 2005 8:07 pm

The ones I am talking about are all air cooled. They are on vans, vanagons some 914's...anything with L-jet or Digijet/early digifant...sorry i did not clarify. When i get to dallas tommorrow...Iwill get you an exact part #. Ray

MGVWfan · Post by **MGVWfan** » Thu Jan 20, 2005 8:28 pm

Thanks for the P'N, that'd help!

I was afraid it was ACVW's only, harder to find than LH or Motronic Volvos, BMWs, etc.

Hey, I just had a thought...I wonder if the Hitachi version of L-Jet used on Datsuns in the early 80's has a similar TB? There always seems to be a big cache of those out in my favorite establishment.

MGVWfan · Post by **MGVWfan** » Sun Jan 23, 2005 11:13 am

BTW, I re-bushed the 1.7L TB yesterday. Took about 30 minutes. It turns out the throttle shaft is the same diameter as the ones on my SU carbs in the MGB, and there's enough aluminum around the shaft bore to ream for sintered bronze bushings, so I could use the same tool and bushings I used on the SU's a few years back. I happened to have extra bushings, and the modified reamer, so now I've got a TB with no slop. It's really cheap to do, if anyone's interested, I can post a dissertation.

raygreenwood · Post by **raygreenwood** » Sun Jan 23, 2005 9:39 pm

Very cool!. The slop is not really a problem...but the vacuum leakage is. Thats why L-jet cam with seals. Ray

MGVWfan · Post by **MGVWfan** » Mon Jan 24, 2005 5:59 am

True, the vacuum leakage is a problem, but I think that slop at fully closed may be, too. I found as I was testing the TB with the switch in place that the idle switch got "teased" by the monkey motion of the shaft, even though the throttle wasn't fully closed. After re-bushing, the switch behaves as it should.

raygreenwood · Post by **raygreenwood** » Mon Jan 24, 2005 11:56 am

Aaaah...gwassshoppah....you have actually trancended to the level where you find the "slop"....in the TPS! Yes, there is a little slop in the throttle shafts. It mainly causes vacuum leakage. Bear in mind, that the coil spring keeps the throttle shaft loaded toward the rear of the car at all times. Chances are...any slop you are noticing via the TPS....is the TPS. I have made some mods that really help.

READ THIS CAREFULLY

OK...take the top off the TPS. Look at it. See the floating fork at the top...with the two brass 3/32" pins.....with the floating whisker switch in between? That is the problem. The plastic fork...attached to the shaft...is designed to move...with the just the minutest throttle movements....at any throttle position.
When the plastic fork moves back towards closed....the whisker switch
moves from one pin to the other. On most late model switches.....and this is D-jet only.....the closed position pin for the whisker switch (rear) (terminal 17 generally)....cuts off the injection.....or at least stops any enrichment that may happen from the trailing whisker switches on the circuit plate down below...where the two rows of 10 contacts each side are. A small movement of the throttle....and the rear whisker switch seperates from the "pin"...and the foward whisker switch makes contact with the forward "pin".....so enrichment from the trailing whiskers on the circuit plate contacts down below....are now engaged....allowing any throttle movement that sweeps over the plate contacts...to give extra injections. So....whats the problem? Several things.

(1) The plastic fork and collar...gets sticky. It must be cleaned squeeky clean...deburred...and lubed with a dry teflon lubricant.
(2) The gap....between the two pins in that plastic fork....is not very ...precise. So depending on the distance from the throttle shaft apex...to the pins at the end of the plastic fork..the number of degrees of angle the throttle shaft must move...in order to either cut on enrichment...or cut off injection....can be large. If you have a tightly tuned system, it can require more throttle movement to cut on or off....than you are needing to maintain speed. THIS IS ONE OF THE MAIN CAUSES OF THE BUCKING SYNDROME.....THAT....AND THE PCV VALVE.
(3) The methods in the books for adjusting the TPS....ARE INCORRECT...more later on this.

What to change?
(1) Make the switch more sensitive to minute throttle movements.

Go to the hobby shop. Get some thin wall brass tube. They make some that will just slip over the contact pins at the end of the plastic fork. It should have a wall thickness of no more than .010 total. They make some fine tubing that is about .005" wall thickness. Slip a piece tightly over the forward pin. You have now increased the diameter of the pin.....allowing quicker contact, with less throttle movement. Make sure the wall section is not too thick. You do not want it on all the time inadvertantly.
(2) Disconnect terminal # 17 in the five pin plg. Check the correct contact with your VOM. There are numerous switch models....and some have two rows of pin #'s. Its the one that goes to the rear pin on the forked metal contact on the floating plastic fork.
Why? Because...it was simply an emmissions device...and is redundant. The deleration valve...and the vacuum retard will both promote smooth running. The difference in saved fue on deceleration is less than 1 mile per gallon at best...and only in the city. It makes driving a chore when that function is hooked up.
Where the bucking came from...was that during part throttle cruising....the minute throttle moevments alternately either...caused the whisker switch to move to cut-off...causing a cessation of injection...and then rattled forward causing a resumption....causing a "burbble". Its one thing to not be enriching...its quite another to actually cease injection while rolling. Its unnescessary.

You now have a switch that is more sensitive to minute throttle movements...but will not be rattling and inadvertantly cutting injection off and on.

TPA ADJUSTMENT>: On the type 4...this can only be properly done with the TB in your hand. Take it out of the car.
(1) get a pair of old connectors. Put your VOM on connectivity (circuit). I can't remember off hand which two to connect...but its one half of the contact strip down below....most specifically the half that connects to the very lead most contact strip that the wiper arm will first touch.
The other goes to the contact....that accesses the wiper switch that connects to the forward pin contact on the plastic fork. It runs around the inside edge of the circuit pattern....ok so far?

Hold the TB with the TPS face up. Holding the TB from the manifold side so you can close the butterfly with your fingers...and hold it closed tight. With the screws on the TPS loose....rotate the switch body....so the forked contact up in the plastic floating fork.....moves toward the rear contact. This will also take the wiper contacts down below off of the strip of contacts toward the rear...to a small square, copper circuit pad...that is a park position. The VOM should be reading no contact...as the fork switch is out of contact with the forward pin.
Now rotate the plate on the TPS.....very slowly....while holding the throttle plate closed tight. Rotate until the forked connector in the floating plastic fork....just touches the forward pin. The VOM should beep or the needle should swing showing contact. STOP

Now look at he wiper arm down below. It should JUUUUST be about to touch the first contact on the series of 20 contacts on the plate. Rotate the TPS plate forward JUUUUUSt a hair more...until the wiper whisker just touches that first contact in the series of 20 on the TPS circuit plate. Now tighten the screws....done!

NOTE: The book actually gives the opposite directions to turn the plate many times...because it is set up for type 3 posistioning...and is usually talking about the old style TPS. Also...the book tells you to simply move one more notch (which is 2 degrees) once the VOM gives contact signal. The problem with that is...that....with slop and production variation in the TPS....the wiper switch may not be in contact with the first encrichment contact in the series...on the plate....forcing you to give 2-3 more degrees of throttle moevment before starting enrichment. This will cause flat spots.

Now when the plate is tightened, open the throttle plate all the way...and close it again. Hold it tight. If its properly adjusted....the forked contact in the plastic fork.....will be on the rear pin...and the wiper whisker down below...will be maybe .010 or less from the first enrichment contact point. It should only require 2 degrees of throttle opening to start enrichment. Previously it would have been 4-5 degrees.

When adjusting throttle cable tension....make sure to make it taut enough to compress the spring on the chuck that goes into the arm....about 1/8" before locking the allen set screw. This is also why the later model L-jet TB's are better. The wound coil spring has less of a habit of leaving the throttle slightly open when the tension on the cable is properly adjusted. But adding an extra strong spring to the standard D-jet TB to keep the throttle tightly closed...makes the driving feel sluggish. Ray

MGVWfan · Post by **MGVWfan** » Mon Jan 24, 2005 7:37 pm

Woo hoo, I tapped the moterlode of TB knowledge!

Danke schoen herr Doktor. I owe you a brewski for that one!