86 720 bogging at wot

[Jethrow86]

I’ve been having issues with my 86 720 where at the top end of third gear and above it bogs significantly enough to shake the truck unless I accelerate very slowly. At wot the engine completely stops until I take my foot off the throttle again. I have a Weber conversion if that changes anything. I’ve checked my plugs, changed the fuel filter, cleaned the air filter, and cleaned the flaps to no avail. Have been trying to solve this for about a year now. Any info helps

[datzenmike]

1/ If you can floor it lower gears and it runs fine till you are revved up in third gear and it cuts out, it's running out of gas. Usually this is a dirty fuel filter but you said this was replaced so that leaves the....

a/ float set too low and not enough gas can get in to replace that used and it runs out.

b/ possibly the fuel line is restricted or the pump not supplying gas properly. Disconnect the fuel line down near the frame on the right side and direct into a suitable container. Turn ignition on (don't start) and it should supply about 1.5 liter per minute.

 

2/ The 32/36 Weber is a progressive carburetor with the secondary tipping in at about 2/3 throttle. If it runs fine below this, but acts up only when at full throttle (floored) in ANY gear then the secondary jet may be blocked or plugged and not allowing any gas in.

[Jethrow86]

I’ll give the fuel line a try. And it bogs hard at about half throttle and almost cuts out when I go wide open. Thank you for the help!

[datzenmike]

If it does this in every gear and at any time it's wide open then look at the secondary jet.

[Thomas Perkins]

It shakes.Could be the yoke is bad.Mine went bad and it would do this.I guess this is not common for our 720's.The first thing usually to check and replace when it is shaking at higher speed is the u-joint.See the groves in the yoke.I had the transmission replaced at the same time.Ronald found me a brand new yoke locally and it was 135 bucks,I have the old one as you see in the picture.I did get a new u- joint put in but that was not the problem.This was about 5 years ago.I could be wrong and it could be your Weber like Mike has said.But I have been using a Weber since 97 and never ever had any problems other than replacing the gaskets.I have taken the Weber apart many times and put new gaskets in it and cleaned it,bought a rebuild kit cause of the filters and put the other parts in.My little fuel filter has never had anything trapped in it.When ever my fuel filter by the fuel pump goes bad.It will not start.Never had it go bad while driving.If you have a oem Nissan pump,their is a fuel filter in it.Nissan still sells them.I never had mine shake cause of the Weber.Ronald had my Nissan lifted and ran it and seen the yoke shaking.Do like Mike has said and disconnect the fuel line at the Weber and stick in a glass Mason jar to see the gas flow.Also check and see if your gas lines are in the right order at the fuel pump.The gas line coming from the fuel filter goes to the fitting at the bottom of fuel pump,the top fitting of fuel pump goes to metal fitting.Their are 2 gas lines by the starter.I also use a Idle cut off solenoid.I have 382,000 miles on the speedo.Different transmission,different engine.2nd Weber.I have put 274,000 miles on it in 28 years.It had 108,000 miles on it when I bought it in 94.Our 720's start falling apart when the speedo says 100,00 miles.Right after I got it.The carb went bad.I fought it forever,then found out about Weber's.About replaced everything except the distributor and oil pump.The Nissan dealer here told me that the oem  carb was junk,they wouldn't rebuild it.Told me to get a Weber and I did.Had 3 different places try and rebuild it and they couldn't but was charged 200 bucks by each.Should of went to Nissan first.I do have a oem Nissan fuel pump that quit on me.I tried a new filter for the inside but it still wouldn't work.I still have it.Also tried new gaskets.I do have a brand new extra  fuel pump and filter.Fuel lines are 5/16 ,go and replace them all with new clamps.Also open your gas door and spray with Clorox,it will dissolve the gas residue.Rinse with water.

Edited November 14, 2022 by Thomas Perkins

[Jethrow86]

Thank you!

[datzenmike]

Driveshaft support going bad is more of a vibration because it spins so fast. Most noticeable under hard acceleration. Shaking is (well to me) more like having a flat tire flopping around.

 

If the carburetor is starving for gas, the engine tends to fall on it's face, then surge ahead, fall on it's face.... repeat till you let off the gas and more fuel fills the carburetor. Is it that kind of 'shaking'?

[Jethrow86]

Yes that’s exactly the kind of shaking it has. From the engine cutting in and out. It seems somewhat inconsistent as well, does it when it’s cold or warm but then once in awhile won’t do it at all.

[datzenmike]

Pull the fuel line off and direct into a suitable container. Turn ignition on, I think this starts the pump, but it's on a timer of 10,20?? seconds so be ready to turn ignition off and back on to keep it running. The electric fuel pump should deliver 1.5 liters per min. or half that in 30 seconds.

 

If it does deliver the proper amount, then delivery is good so must be the float in the Weber is set too low or the fine mesh filter on the inlet is part blocked.

 

If fuel delivery is poor, find the obstruction. There is a fuel filter between the tank and the pump, and one in the engine bay, correct??? The bottom of the electric fuel pump twists off and there is a filter there as well.

[Jethrow86]

Finally getting the time to try this out today. I’ve never messed with the float, only the idle screw so hopefully that’s not the issue. Is it common for my fuel pump to make an almost clicking noise when the key is turned to the first position? Also, hope everyone had a good turkey day!

[datzenmike]

Electric fuel pump should come on with ignition. It's on a timer and will shut off after a few seconds. Using the starter position will also turn the pump on. Makes a whirring sound that may slow down as the pressure comes up becoming a soft chugging sound..

[Jethrow86]

Ok cool, sounds like it’s fine then. My filter inside the pump looked fairly dirty so I ordered a new one earlier. We’ll wait for that to get here and see what happens

[Jethrow86]

Finally got the new fuel pump filter in the mail and put it on yesterday. Still has the same bogging issue. I’m wondering if I should replace the pump next or look into timing. Never messed with timing before so fingers crossed that’s not it😅

[datzenmike]

If you drive away from a stop as fast as you can at full throttle when does it bog? Can you get into second or third gear or even higher???

Or does it bog right away in first??

[Jethrow86]

It’s fairly inconsistent, at full speed it will start to bog even in second once in awhile. As soon as I shift it usually has a hard bog before it starts accelerating again. Seems to hit a wall around 45 and 65. Almost like there’s an invisible governor. But it usually starts to bog when I go into third and gets progressively worse with speed

[banzai510(hainz)]

is the 2nbarrel opening up for sure or the choke plate partial closed?

Edited December 8, 2022 by banzai510(hainz)

[Jethrow86]

I cleaned all the flap connections and what not. They seem to all do what they should when I accelerate the motor manually on the carb but I’m no pro

[banzai510(hainz)]

IM not a Napz expert on those trucks but if a stock carb you could rev up the motor then put your hand over the main side(barrel of the carb) then the 2ndy side will open up with alot of vacuum. Maybe even keep you hand on the main side and put your finger on the 2nd barrle flap and see if the motor stays running.  It was just a trick I leaned on the 521 but the later trucks have alot of emmission hoses and such. Its just a best guess trouble shoot

[datzenmike]

If when accelerating you get up into second and third before it bogs this would indicate the full fuel bowl in the carburetor is emptying faster than the pump can deliver fuel and keep it full.

 

1/ Dirty fuel filter will restrict fuel flow but I think you have replaced it.

 

To prove lack of fuel delivery to the carburetor, find the fuel line from the tank where it enters the engine compartment down near the frame on the lower right side. Disconnect the line that goes to the carburetor and not the return line, put a suitable hose on the hard line and direct it into a suitable container. Turn the ignition on to activate the pump. It may be on a timer and shut off in 10 or 20? seconds, so just be ready to turn ignition off and back on to start again. If working properly the pump should deliver 1.4 liters of fuel per minute. If you get .7 liters per 30 seconds that would be fine also. This will prove that the pump is or is not delivering fuel properly and in the correct amount.

 

2/ Carburetor float is incorrectly set or inlet fine mesh filter is part blocked reducing flow.

 

It might be easier to just pull the carburetor off and check/set the float drop properly, inspect the inlet filter is clean and the needle valve not obstructed.

[Jethrow86]

so its been awhile since ive been on here. i rebuilt the engine recently and was still having the "bogging" issue. i had a fuel leak recently and the whole time it was leaking from the hose it did not have the issue. this leads me to believe its running to rich. makes sense to me being that it gets hesitant to accelerate when i hit the gas as if it was putting too much fuel in the cylinders. if this is the case how should i proceed. weber 32 36 carb

[Jethrow86]

the leak was fixed and it has started the issue again

[datzenmike]

Webers are sensitive to too much fuel pressure, always have been. I think not above 3psi is the trick.

[Thomas Perkins]

Do you have the Nissan fuel  pump on it?What do the spark plugs look like?Do they have lots of black carbon on them?Make sure the fuel line from the gas tank to the fuel filter and from fuel filter to fuel pump and two hoses by starter are not leaking.Are you using a idle cut off solenoid.If so, make sure it is not over tighten and the o ring is good. To check to see if it is working,remove wire as it is running and the engine wiil cut off.No leaks at the Weber gaskets.Both coils are good.Check your coil fuse.Like Mike said not above 3 psi,Nissan fuel.pump will be just right for your Weber.No more New Nissan pumps available.I got the last one.

 

Edited September 24, 2023 by Thomas Perkins

[Jethrow86]

im getting ready to pull the bed off and really get at everything on the fuel system so we'll see what happens

[datzenmike]

If the fuel pump pressure is the cause it would show up at idle and low speeds as well. Worth checking though. This assumes that it IS over rich. Engines will run somewhat rich much better than if running lean. The extra unburnt gas just exits the tail pipe as black smoke.

 

Going lean when the secondary tips in could very well be a part or fully blocked secondary jet.

 

Could be the float set too low and when the secondary opens it sucks the float chamber empty and bogs. I guess you could run it hard till it bogs and ASAP get the ignition off to shut the fuel pump off. This will preserve the gas level in the Weber. Pull over and take the top off the carburetor and see where the gas level is. Shutting the pump off has to be done as quickly as possible or it will fill the float chamber back up.