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Mech.Fuel Pump - Pressure test at Colin's request on O72 Bus
Posted: Mon Aug 02, 2010 11:06 pm
The last thing Colin told me before he left my place was to check my float levels and fuel pressure using a fuel pressure gauge. This was due to the symptom of slight surging/hesitation at the lower range of second, third and partly fourth gear that we couldn't get to go away despite our best efforts within the time allowed.
Advancing the timing just 2 degrees to 30 BTDC at max advance pretty much solved the problem, but it was decided that was possibly masking a problem somewhere else at the expense of the health of my engine and a couple things would need to be verified before anything different than 28 BTDC should happen.
A check of my float levels appeared to be correct, and that left me with the remaining task to test the fuel pressure output of my mechanical fuel pump.
Tonight, I did just that and I have a few questions
Bentley says the MAXIMUM pressure is 5 psi for the mechanical fuel pump on the dual carb bus. The MINIMUM rate is not a pressure measurement, rather a cc volume per minute. How does that equate to a minimum PSI measurement on my gauge??
I got a 2.5 psi measurement at idle and a couple hard revs show that it drops slightly to 2 psi and returns to 2.5. I have not tested this while driving, because I need a partner to view the gauge while I drive but from a preliminary evaluation, what do you guys think?
Here is a video that might help.
http://s491.photobucket.com/albums/rr28 ... _15421.mp4
Any thoughts? Are my mechanical fuel pump days over and I need to go to an electrical fuel pump now???
Posted: Tue Aug 03, 2010 7:34 am
You need a new fuel pump. Is yours rebuildable? Mine was, and I did. It needs to be around 4 PSI for a type1 so I'd imagine it's about the same for you.
Posted: Tue Aug 03, 2010 9:16 am
hambone wrote:You need a new fuel pump. Is yours rebuildable? Mine was, and I did. It needs to be around 4 PSI for a type1 so I'd imagine it's about the same for you.
Are the later pumps like the Type 1's where you can remove a gasket to get more pressure? Even if you only had a single gasket under the base you could pull it, see if you got more pressure, and see if that solves your issue. I'd check that before dropping $ on a new pump altogether.
Posted: Tue Aug 03, 2010 9:41 am
Just paper shims. It usually takes a few of them to affect pump pressure noticeably.
Mine was 12 psi before shimming! Crazy. Fuel pressure is probably a good thing to routinely check on a new engine build.
Posted: Tue Aug 03, 2010 1:42 pm
Perhaps this is why many use electronic fuel pumps with regulators. Just saying...
Posted: Tue Aug 03, 2010 2:19 pm
A piece of cardboard is about $0.01, free if taken from the recycle bin. Just saying.
Posted: Tue Aug 03, 2010 2:40 pm
I used cereal boxes.
Posted: Tue Aug 03, 2010 4:09 pm
hambone wrote:You need a new fuel pump. Is yours rebuildable? .
Back in the day it was, but finding a rebuild kit for a type 4 mechanical fuel pump is pretty close to impossible. Can't really shim a type 4 mechanical pump to my knowledge and futhermore, my pressure is low so I guess you would make the thick plastic block thinner potentially - too much hassle and no guarantee it would help.
I've pretty much just decided I need to nip this in the bud and I'm going with the electrical rotary Carter pump from CB Performance in the 3.5 lb variety for $65.00 plus shipping. Get ready for a bunch of newb wiring questions from this homeboy to get it installed. Should get it in 3-5 business days. Sure hope this solves it all - I hope this is the final missing piece of the puzzle and I can start laying rubber at stop lights again
Posted: Tue Aug 03, 2010 8:11 pm
Oh Jim - always good for a couple laughs (and a little gentle scolding)
Disclaimer --- beer thirty was several hours ago -:drinkers:
Now please excuse me while I go find some green paper and see if it works
Posted: Tue Aug 03, 2010 9:26 pm
I agree with the new fangled electric pump option. Cleaner and finite results. Though a backup pump in the bus-box-o'-parts may be useful
I do wonder if opening up the pump and cleaning and looking around in there would reveal anything. Could be just plain ol' tuckered out pump but could it also be something not doing it's job cause it's gummered up?
For clarity sake...Adding shims moves the pump away from the rod, thereby reducing stroke length. The mech pump does the actual "pumping" on the release of the rod stroke...I think. If you open it up are there parts that must be changed or can it be cleaned and reassembled with the existing parts?...
What is the part number on your pump? All hypothetical questions fer me own learnin', not intended as another path to take. yer on the right one with the new electric, IMO
Posted: Tue Aug 03, 2010 9:41 pm
For clarity sake...Adding shims moves the pump away from the rod, thereby reducing stroke length.
Yep - that is correct.
I'm kinda wondering if the pushrod has become shortened with wear over all these years.
If you open it up are there parts that must be changed or can it be cleaned and reassembled with the existing parts?...
Back in the day when Elvis played, there were apparently new diaphrams and a whole kit that would get you back to optimized pumpitude. Those are long gone now. I actually have two mechanical pumps and this is a different one than the one that was in the bus originally and the same one that was in my parts box prior to the rebuild. One day I might try this other pump (hard to change with the engine in), but I'm just fed up with it because I feel I've had similar symptoms with both mechanical pumps. Sooooo - I'm gonna try ELECTRICITY and see where that gets me. How pissed I will be if this doesn't fix what ails me.
Posted: Wed Aug 04, 2010 5:09 am
Gypsie wrote: For clarity sake...Adding shims moves the pump away from the rod, thereby reducing stroke length.
Yep - that is correct.
Right... I was wondering if you had a single gasket in there already - you could try removing it to see if the pressure went up and see if that solves your hesitation problem. Not good long-term but could be a useful piece of diagnostic information before you spring for a new pump of any variety.
Posted: Wed Aug 04, 2010 7:32 am
I don't think we were advocating shimming his current pump; rather, a NEW mechanical pump can be shimmed for proper PSI if too high.
This would be my preferred method IF the mechanical pumps were still available due to originality and simpicity.
Go back and read it again, 2 pts. extra credit for comprehension.
Posted: Wed Aug 04, 2010 3:14 pm
Troy, I can help you on the install of the pump. Keep in mind the "correct" and SAFE (you do have kids, yah know!) thing to do is to put a relay in so that the electric pump only runs when the fan is on aka the engine is on. The fear is that in an accident, the pump will continue to unleash petrol on your burning vehicle. There is a plus and minus with this as you will need to manually prime your carbs occasionally, but safety helps you sleep at night (unless you are on the top bunk at Crater Lake. Your head ok yet?).
We can go over this via phone or setup some time to get-a-crankin.
ps- don't forget you are going to have to "plug" your current mechanical pump post install.
pps- carry a backup el cheapo fuel pump with you. My carter was the reason I was having load issues this winter. I ended up ditching it for a rattling box generic style via a FLAPS.
Posted: Wed Aug 04, 2010 4:57 pm
I'm planning on going this route - even though it barely makes sense to me how to put this in action
http://www.paultaylorimaging.com/VW/Fue ... Relays.htm
Apparently this way incorporates two relays - one that runs the pump while cranking and the other that runs the pump while the engine is actually running.
I'm going to be leaving the mechancal fuel pump in place and loop a 6 inch piece of fuel line to the inlet and outlet for now. That will serve as my backup to limp home if need be. It is my understanding that removing the mech pump and pushrod and putting in a block off plate can cause an internal oil leak of sorts because there is a gallery that lubricates the pushrod.
I'll need to figure out where to tap into the power and such - lets schedule a time as I could use help if you've done this type of thing before. Electrical stuff has never been my strong suit.