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 Fuel Pump Question 
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Joined: Thu Sep 15, 2011 6:48 am
Posts: 160
Location: Covington, Louisiana
Post Fuel Pump Question
I have been trying to track down an engine performance issue and without getting into too much detail, my question is this... when the engine is running should the fuel filter be full of fuel? I have a clear filter so you can see into it. Both at idle and when you press on the accelerator, the gas merely drips from the intake side of the filter. You don't see any fuel in the filter as it must be getting sucked through the filter element itself. The engine runs OK at idle but when you put it under load it stumbles and back fires. One possible cause is a failing fuel pump. I have tested it and got 2.5 psi on the outbound side of the pump; which should mean it's good. But, the filter is not full of fuel when it's running (in fact its hard to see any fuel in it at all. It literally is dripping into the filter housing). Hopefully someone will know the answer to this crazy question :D

Thanks for the help! John


Wed Feb 01, 2017 8:55 am
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Joined: Sun Dec 10, 2006 7:38 pm
Posts: 1214
Location: Southwest Wisconsin
Post Re: Fuel Pump Question
The fuel filters on both my 710's are also clear plastic (Mann WK42/1 or MF1010) and both are 1/2 to 3/4 full when running.

A couple of thoughts: 1) the diaphram in your fuel pump has a hole in it and is therefore not sucking as it should, 2) there is crud in the bottom of your tank keeping the fuel pump from sucking adequate fuel. A strong led flashlight shined in the fuel filler may reveal any tank issues. You could also remove the line from the tank at the filter and blow into it to clear any debris around the fuel pickup and then see if it fills the filter better after reconnecting.

Good luck!

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Paul C.
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'73 Swiss 710M
'72 Swiss 710M
'89 Puch 230GE


Wed Feb 01, 2017 10:08 am
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Joined: Wed Sep 30, 2009 11:06 pm
Posts: 81
Location: Carlsbad, CA
Post Re: Fuel Pump Question
Everything Paul said... and minor indicator of crud on the fuel pick up is a bunch of red flakes in the fuel filter. If so, there are likely really big flakes on the pick up. Being a noob, I replaced my fuel pump (easy) before I figured out that my tank needed a good scouring (easy, but time consuming).

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1971 710K S.... Hmmm, flat paint is great but shows everything. I'm thinking semi gloss will be better and more aerodynamic...


Wed Feb 01, 2017 2:10 pm
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Joined: Thu Jul 29, 2004 2:12 pm
Posts: 716
Location: Indio, California
Post Re: Fuel Pump Question
Be sure to run your Pinz out of gas and then drain the tank annually. I have done that for 13 years and never had a tank issue, and that could just be coincidence. Who knows? Just like charging the batteries individually every 6 months and bleeding the hydraulics annually, its the maintenance items you do to prevent future problems.


Wed Feb 01, 2017 5:34 pm
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Joined: Sat Oct 24, 2009 5:17 pm
Posts: 850
Location: arizona
Post Re: Fuel Pump Question
if you search gas issues you will find that the fuel hoses have a tendency to collapse inside. This is not always obvious on the outside. New fuel lines and a rebuild kit for the pump along with verifying your tank pickup line is clear should satisfy these issues.

You have not mentioned if you see visible red flakes in the filter, again a good indication the lining in the tank is starting to delaminate (common problem with these trucks).

You also need to verify that the fuel flow, not so much the pressure is adequate. remove one of the fuel lines at the carb and turn the engine over. It will only take a short time to know if there is a bunch of fuel coming up to the carbs or just a minor flow.

If all is good, fuel flow and lines are clear then a quick check of the shut off solenoids is in order. When not working properly the engine can idle fine but hesitate under acceleration and perform poorly. Not as likely as the earlier items but definitely don't over look them.


Wed Feb 01, 2017 7:11 pm
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Joined: Tue Jul 05, 2011 6:35 pm
Posts: 246
Location: Paraguay
Post Re: Fuel Pump Question
i always wonder why my fuel filter is the same as John L. i never see any fuel in my filter looks empty but runs fine...

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1985 710MS
1985 712MK
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Thu Feb 02, 2017 4:59 am
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Joined: Thu Sep 15, 2011 6:48 am
Posts: 160
Location: Covington, Louisiana
Post Re: Fuel Pump Question
Hey guys, thanks for the input. I do not see any red flakes in the filter. I did find a rock in the gas tank a while back; but the tank seems clear now.(How it got there I have NO idea!) I have changed out all of the fuel lines from the tank to the carbs. The one from the tank to the pump did seem a bit spongy; which prompted the change. New fuel filter. New intake manifold gaskets and carbs. I'm running the Pinz SSI with new plugs and wires. I tested the psi of the fuel pump and it was 2.5; which the book says is good. But, when the engine is running the fuel barely drips into the filter. When I put it under load it bogs down and backfires, as if it is starving for fuel. Since the pump tested fine, I looked elsewhere. But now I noticed the fuel just dripping into the filter which made me send the post. I'm thinking of changing the pump in the hopes it solves the problem. If the filter should basically be full of fuel then this is a source of a problem. It would be either fuel intake is clogged or the pump is on the fritz. Any other thoughts?

John


Mon Feb 06, 2017 11:17 am
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Joined: Wed Apr 14, 2004 9:58 pm
Posts: 1344
Location: Idyllwild, CA
Post Re: Fuel Pump Question
You should also check the coil voltage, especially if you have the original 4500rpm box (as most trucks have, unless it has been disabled). A low coil voltage due to a dying rpm box tends to idle just fine due to limited fuel and air in the cylinder allow the weak spark to still ignite the idle mixture, but as soon as the throttle is opened the increase in air and fuel will actually prevent the weak spark from sparking! This creates a miss in the engine and injects raw fuel mixture into the exhaust (since it wasn't burned) that creates the backfires.

Put a voltmeter between chassis ground and the "+" coil terminal while the engine is idling. Correct value should be in the 16-18 volt range. When my rpm box started failing, I had the same bogging/backfiring symptoms you describe and eventually found that I had only 8-9 volts at the coil! If so, unplug the rpm box (deck of card sized box under the dash, with a 6 terminal connector). On the connector (wiring harness) you have two rows of three terminals; they are common 1/4" spade terminals in the plastic housing. Make a jumper to connect the middle terminal of each row together; the rest of the terminals can go unconnected. Now recheck to see you have the expected 16-18V at the coil and be amazed at the improvement in performance! I am pretty confident from your posts that this is very likely to be the issue.

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Jim M.
712W and 710M


Mon Feb 06, 2017 1:40 pm
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Joined: Thu Sep 15, 2011 6:48 am
Posts: 160
Location: Covington, Louisiana
Post Re: Fuel Pump Question
Thanks Jimm. I may tackle that project tonight. It seems simple enough. The weather where we are right now is perfect Pinz weather and I hate seeing her stuck in the garage. :D

John


Mon Feb 06, 2017 1:50 pm
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Joined: Wed Apr 14, 2004 9:58 pm
Posts: 1344
Location: Idyllwild, CA
Post Re: Fuel Pump Question
Well, was it low voltage at the coil? My reputation is on the line here, I have to know... :D

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Jim M.
712W and 710M


Wed Feb 15, 2017 10:19 pm
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