1981 FI Rabbit - Timing Vacuum Advance - Hesitation/Stumble

It might be a fuel delivery problem. Make sure your fuel delivery plate is correctly adjusted and clean. The plate should sit at a certain height. Check your frequency valve with your dwell meter to find out it's reading.

I have found that certain vehicles with 02 sensors will hesitate slightly if the mixture is too rich or way too lean. I suspect your engine may need a CO adjustment to lean it up a tad. CW turning of the CO screw will lean it up AND please go slowly like 1/8 or 1/4 turn and wait for things to adjust.

OR I will hope that your engine does not have deposits inside that absorb the fuel as it is trying to enter the combustion chamber. Some good Top Engine Cleaner could help reduce that. Or some SeaFoam in the gas tank will give you a slow cleaning process of both the engine and the fuel system. What do the spark plug electrodes look like? And which plugs are you running?

Thanks for the response!

The throttle plates (butterflys) are as clean as a pin, as is the throttle chamber (assembly wherein the butterfly's are located). The actual intake air distributor (THE BIG assembly to which the cold start valve is attached) is dirty, with what i would assume to be normal soot.

The dwell on the frequency valve is 45 degrees +/- 4 degrees of fluctuation.

The plugs are autolight 63, with about 2000 miles on them and they look good, with uniform gray deposits -- not glazed, not too hot, not covered with ash or carbon deposits.

I have run two bottles of chevron techron thru it over the last 2000 miles.

I have not used any other engine cleaner. I usually run 87 or 89 octane in it, usually 87. The vehicle gets around 26 mpg. I have always wondered if this was an indication of anything. good-bad-lean- rich.

When I got this car about 9000 miles ago, it had been sitting in a barn for 14 years, and it had a lot of problems with fuel delivery, i.e. return line obstrucyed, gunk in the gas tank, rust in the tank. I was changing the fuel filter every 200 miles and the gas in the filter was brown with a lot of tiny rust particles. That has all been fixed and now i get an appropriate flow of clean fuel.

I added three more feet of vacum line to the ported vacuum line and it diminished the stumble, ( went from a bad stumble to a slight stumble).

Do you have a specific recommendation to what you refer to as an engine top cleaner.

Is it important to have the vacuum advance.

You probably have crud everywhere the fuel flows. I bought an 84 with no hood or gas cap. The cylinders had corrosion. I replaced the injectors twice on that car. Replaced the CPR. Replaced the fuel distributor. Replaced the engine but that was another story.

I think he asked if you cleaned the crud off the sensor plate which sits on top of the fuel distributor.

Have you done the flow test for the injectors as per the Bentley?

Have you removed the injector inserts replacing any broken inserts and using new orings when reseating the injectors? Are you using the proper Locktite when replacing the injector inserts?

Yes you need the vacuum advance. Your off idle performance suffers greatly with no vacuum advance.

Thanks for the interest in the subject matter!

Before I ever started the engine I flushed the tank of the debris (tarballs. etc,) cleaned the in tank screen and lines in the tank, but as you know it is impossible to get every speck of rust out. I have an in-line prefilter installed that filters out the rust specks before they get into the fuel pump, fouling it ang the parts upstream, i.e. the check valve, accumulator, etc.

The output of the pump is 24oz in 30 seconds at the fuel return line, while 30 would be perfect, 24 is acceptable, i think. i have run the 4 corona bottles test and the result revealed one less than adequate injector which was replaced, a second test revealed that the output was OK.

I installed new plastic injector inserts, & injector o-rings (the green ones) when i was eradicating all the vacuum leaks. I did not use loctite, ( a bottle of hi-temp air sealing loctite costs more than i paid for the car) and there was no leak present, using the propane, carb cleaner and Stereoscope ( vinly tubing nserted into my ear) test at each injector.

The fuel distributor (& air flow metering plate) is as clean as a pin

-- i removed it and cleaned it before starting the engine, and it is still clean 9000 miles later.

WUR/CPR in this car is right on spec --- original one is in my "83 and it runs fine. i swaped them out to see if there was a difference.

Put some seafoam in the tank today, given the number of miles i drive it will take a while to go thru a tank of gas.

Well you took care of the basics then. The issue of the Loctite is twofold. First to seal up tiny leaks and the second to reduce the chance of backing out.

The stumble can still be from the air flow plate being gunked up or the plunger in the fuel distributor binding up a bit.Since the air flow meter plate is clean that is one less thing to look at. If it cannot move quickly responding to the throttle changes then you will have some stumble. The Bentley shows the process to remove the plunger to clean and inspect.

When you were do>Thanks for the interest in the subject matter!

Well you have done a fantastic job so far.

Two things..

1. Did you try setting the dwell of the frequency valve down to 30 degrees?
2. You have to DRIVE IT!!! And driving it hard should help since I still think the engine might have deposits in it.

I have seen one cylinder head so clogged with buildup that I was shocked the vehicle made it to me. After a mechanical cleaning of the head at the machine shop that car, an '81 or '82 Audi 4000, was getting over 32 mpg.

JMHO If your cylinder head has deposits...............the deposits will absorb more fuel coming into the intake area especially when the engine is warm and the fuel mixture is leaner. Setting the valve at 30 degrees and DRIVING IT should help get things cleaned up quicker. Or I think that link I had included in a prior post claims that some people slowly pour SeaFoam into the intake to clean things up almost instantly. I doubt if your fuel system is clogged since you have tested it well, so maybe the deposits are causing the last bit of trouble. ;-)

BTW I saved a non-running Scirocco from the junkyard and it cost $1200 to get it running perfectly. A quick turn of the key and the engine would start and purr perfectly at all times.

Thanks for the relpies!!!

In the very beginning I pulled the fuel distributor and shot a lot of carb cleaner into the outlets and inlets, I also let the plunger slip out (of it's own weight) and cleaned it. I cleaned every part of the air flow mechanism and relubricated the moving parts, with the result being that it was and is as clean and pristine as it can be. I also tried a completely different air box/fuel distributor from a working '82, thnking that the stumble was the fuel distributor, It was not. The replacement unit exhibited the same problems. After tyring that I put the original unit back on, and moved on to the vacuum leaks, which have been since been eradicated.

Hate to keep asking the question, is the vacuum advance that important, as it runs great without it.

I agree with checking timing and vacuum routing. 26 mpg is very low for this vehicle, i'd expect more like 35.

The retard port on the vacuum advance unit (the vacuum port on the back of the vacuum cannister closest to the distributor) is hooked up to the main manifold vacuum -- the same vacuum outlet that goes to the brake booster. The front port of the vacuum advance unit (the one that is on the outside of the unit -- the front side -- pointing toward the radiator) is hooked up to the venturi port on the back side of the throttle body. The vehicle has a nice diagram on the underside of the hood, and it is hooked up correctly.

The timing is exactly as described in bentley, there is a small O for top dead center and a v shaped goldish colored mark to the left of the O --- to the left three degrees. The label under the hood says 3 degrees plus or minus (+/-) 2 degrees.

I wish I had it mistimed, after the timing belt was installed by a good mechanic, who timed it, I checked the static timing (with the marks as described in Bentley) just to make sure that it was on correctly, and at the static timing points, the O symbol for top dead center was right in the middle of the timing hole. The three degree mark was also viewable to the left the the top dead center mark.

Always wondered what decent mileage would be on a CIS engine which was running correctly.

I got 30+ with my 80 Rabbit that did not have good mileage 4th gear. My 84 GTI with an economy 5th would get 34 with more power.

My Bentley figure 4-11 says 3 degrees after which is to the right of the 0 mark. The Bentley on page 68 states that the mark to the right about 6mm is correct for 3 degrees retarded. I do not see anything in my manual that says 3 degrees advanced. I do see some get 6 BTDC, 7.5 BTDC and 3 ATDC. B is to the left, A is to the right. Right meaning the firewall and left meaning the radiator.

I check the flywheel timing mark with a long screwdriver in cylinder

1. I slowly turn the crank and watch for TDC. I c>The retard port on the vacuum advance unit (the vacuum port on the

Re BTDC and ATDC --- I goofed in the prior post --- My top dead center is a very clear O, and my timing mark is a very pronounced and clear V cut (goldish in color) to the right (toward the firewall). I goofed when I said left. In any event the engine is timed to 3 degrees ATDC as per the diagram and bentley.

snip snip

Yes you have that correct, except that it may not burn off the deposits, it may only compensate for them! The cleaner will help rid your engine of deposits, if there are any.

It is important!

"dave AKA vwdoc1" is the man. I followed his instructions and used the seafoam. I drove the vehicle for a week, (about 130 miles) (using about 5 gallons of gas) with the seafoam added to the gas tank, and today used the thru the manifold method of cleaning out the top end deposits, slowly pouring about 5 oz of the seafoam into the intake via the vacuum pipe which leads from the manifold to the valve cover. Then shutting it off, waiting the requisite 5 minutes and restarting it and then running it hard. The stumble hesitation problem disappeared. This was on a short test run, but it disappeared and I could not replicate it (the stumble). Usually I can make the vehicle stumble or hesitate by barely opening the throttle to cause the vacuum advance to advance the timing. Isn't in nice when there is a long thread, with a solution at the end, rather than leaving a reader hanging.

WHO ME? Did I win anything? lol

Yeah let us know what happens over time. ;-)

It is the "Italian Tune-up" that helped....really! I always think that the VW engines need to be driven hard instead of pampered. My '83 GTI engine still runs smooth and powerful after 230K miles. I drive it hard but not abusively and it does not need those cleaners. AND personally I like the inexpensive fix! ;-)

congrats and thanks for the followup report. dave (One out of many daves)

On the subject of vacuum advance and this car, i was running a mirror of this subject on bentley's forum. I put up some photos of the advance unit i took off this vw a week & 1/2 ago, (bad unit) and a good unit. I also posted photo's of stationary, retarded and advanced positions on a junk yard distributor with the good vacuum advance unit mounted thereon. Again I thought in the beginning that my sole problem was the advance canister. I realize that you know good from bad, but if you have the time the photo's are decent and hopefully helpful to other readers. Also appropriate attribution was given.

I mess up links all the time but I think this is the link.