Author Topic: Very slight miss at idle (Read 4182 times)

Dakotakid · « **on:** February 27, 2009, 09:57:06 pm »

I just received a used pump from an '86 N/A. I replaced the 4 most common seals (shaft, two sides on cold start pivot, and dist. head O-ring) before placing it into service on an existing car which ran fine prior to pump installation (pulled off a very good pump).

This pump runs very well except for a very slight "miss" or "stumble" at idle. Very slightly more noticeable with warm engine. Most casual owners would not even notice the miss. I slightly richened the load/mixture screw and this worked to "mask" it a little bit (improvement). But, it is still there.

Otherwise, the lines are tight, no air.....blah blah. I immediately capped off the injectors when the lines came off while switching out pumps. Carefully set cam and crank and pump at .98 mm (which always works well for me with this model of pump). Overall, runs very well with the usual amount of N/A smoke for my altitude and for my more aggressive attempts at getting down these long rolling hills highways in my area.
Any ideas?

veedubcanuck · « **Reply #1 on:** February 28, 2009, 07:08:58 am »

Are you %100 sure the other pump did not do this? If it did then you might have lower compression on one cyl. If it didn't than it is either a bad output valve, dirt in an injector, or air trapped in the line. And lastly and not likely is a scratch in the bore of the pump causing a drop in injection pressure on one port.

Dakotakid · « **Reply #2 on:** February 28, 2009, 09:15:43 am »

I am sure the identical model pump which just came off did not have this slight stumble/stutter/miss.
By output valve, you mean right there where the lines hook up to the pump...right?? I was thinking about that. I've never had to work on these before on any pump.
I could loosen the lines (while running) to indicate which one is the culpret and take it from there. I have other pumps which could be cannabalized.

Dakotakid · « **Reply #3 on:** February 28, 2009, 04:05:54 pm »

Problem solved. It was indeed a problem with one of the little delivery valves. The #2 cylinder (last one out and first one in) delivery valve body had a little bit of corrosion on it's innermost aspect. I elected to replace this delivery valve with one from another pump and meticulously cleaned and assembled and....problem solved!!!!
Thank you VDcanuck for the advice.

veedubcanuck · « **Reply #4 on:** February 28, 2009, 04:46:48 pm »

Glad to hear it was a relatively easy fix. :lol:

Rabbit TD · « **Reply #5 on:** February 28, 2009, 06:56:59 pm »

Quote from: "Dakotakid"

Problem solved. It was indeed a problem with one of the little delivery valves. The #2 cylinder (last one out and first one in) delivery valve body had a little bit of corrosion on it's innermost aspect. I elected to replace this delivery valve with one from another pump and meticulously cleaned and assembled and....problem solved!!!!
Thank you VDcanuck for the advice.

I read about those things sticking a while back and exxperienced it myself, that little plunger thing inside sometimes binds a little but some PB Blaster and a little twisting with some needle nose pliers usualy fixes them. I've found out it is the first place to look.

fatmobile · « **Reply #6 on:** February 28, 2009, 10:50:29 pm »

I think this might be the problem with my Rabbit 4-door.
It doesn't miss much while running but always misses for a little while on cold startup and smokes too much on acceleration.
Did your car have any of these symtoms?

Dakotakid · « **Reply #7 on:** March 01, 2009, 11:41:47 am »

Now understand this is on a car with only 4,000 miles on total rebuild. So, everything is fresh and I must have all my compression (never a need to test it....but curious what the numbers would say) because rotating it with the 19 mm. wrench works pretty good on the ligaments in my shoulder!!!!

It would start just fine...even yesterday in the A.M. when it was right at 0 degrees F. the night before. Immediately, there was a VERY slight stutter or miss. Many people would not even be aware of it. It slightly made the tailpipe jiggle more. As the engine heated up, it was a bit more apparent. Richening or leaning did me no good...except it acted to "mask it" a little more when richened.

But, the car seemed to run just great otherwise. I was diligent to immediately cap off the injectors orifices as soon as I removed each fuel line during the installation...so I did not think I had a piece of material in an injector.

Embarrassingly, I have never had to open up one of these pumps except for the usual seal replacements over all the years I have been driving these. I work hard to keep very clean fuel, etc...and I guess that is my reward. So, now I know about these and it was really no big deal...just new, you know???

It sure helps to have your own "junk yard" of cars lying around. The price on parts is ALWAYS RIGHT!!!!!!!!