85 mack r686 surging

chrisdvorak · Oct 29, 2017

85 r686 is surging at lower rpm between 1000-1200 is the worst. smooths out ok above that but very annoying driving around the yard when almost any throttle will start a surging/bucking show. throttle linkage is tight and return spring on pump is intact. I seem to remember a service bulletin about the ambac pumps with governer/flyweight issues, can the governor assembly be separated from the pump while still on the truck? and any idea what parts need to be replaced? the number off the pump is GVB250/850E-7507A1

any input would be appreciated

-thanks

funwithfuel · Oct 29, 2017

I seem to recall some item mounted on top of the rear of the trans . It was somehow tied into the fuel system. It would cause all sorts of shenanigans with the old maxidyne and ultra dynes. Is your puff limiter still operable?

funwithfuel · Oct 29, 2017

I'm sure truck shop or pumpguy will chime in

DMiller · Oct 29, 2017

When I worked at Mack the old units always were very Sensitive when not working hard. Learned to set side of my boot against the floor hump what ever there was of it to keep from jockeying the throttle.

Delmer · Oct 29, 2017

https://www.heavyequipmentforums.com/threads/1990-mack-em-6-injector-pump-troubles.67799/

Does this help any?

thepumpguysc · Oct 29, 2017

Gov. removal is possible w/ pump still on truck.. you'll have to grind down the outside of a 7/16, 1/4" drive socket to get to the 2 bottom corner bolts.
I would guess that the low idle spring just need to be shimmed about .010" to stop the surging..
That's done thru the round back cover..[4 bolts] The low idle spring is the larger outside spring.
BUT, its possible the flyweight bushings are worn..?? Good luck.

chrisdvorak · Oct 31, 2017

would I be a able to pull and inspect the governor assembly for wear without having to recalibrate the pump? or is it kind of all or nothing once its apart? Id like to be able to inspect the flyweights first to rule them out if possible...
thanks for all the advise- very helpful!

thepumpguysc · Oct 31, 2017

Yes very possible.. just don't mess w/ the adjustment screws..

mitch504 · Nov 1, 2017

Another thing to check is motor mounts, the old R-models were bad about that causing this. Does it surge when you aren't moving?

chrisdvorak · Nov 1, 2017

it wanders a bit at idle but not bad. things get crazy when I get to dirt/rough roads then the throttle is basically uncontrollable in any gear. I checked IP lever again no grooves caused by the spring and very little left to right slack. I will check motor mounts cab is air ride. might try putting a stronger return spring on the pump too

mitch504 · Nov 1, 2017

The worst is an air ride cab with a hard linkage on the throttle. All you can do with one of those is think of the throttle as an on-off switch, or change it to a cable .

td25c · Nov 1, 2017

mitch504 said:
The worst is an air ride cab with a hard linkage on the throttle. All you can do with one of those is think of the throttle as an on-off switch, or change it to a cable .

Amen brother !

R model with air ride bags on the rear of the cab .

Cab a shakin & engine going Romp Romp Romp !

Sounds like a Mack with a high performance camshaft ...... LOL

DMiller · Nov 2, 2017

We converted a lot of the R series Mack fire engines in ST. Louis to cable as they had hard linkages and the engineers had trouble with that.

crane operator · Nov 2, 2017

So in converting to cable, did you just make a short push pull cable in the engine compartment, between inj pump and the solid rod somewhere, or did you run it all the way to the pedal? My governor was shot on mine, but it's not back from the inj. shop yet. I haven't checked engine mounts, mine is not air ride cab, but it is solid linkage.

mitch504 · Nov 2, 2017

Just a short cable from the firewall

DMiller · Nov 2, 2017

Mack used to offer a kit, of course was twenty some years ago, more than likely not anymore. There are control cables one can convert as for heater or PTO, just have to engineer a eye loop maker(a wire winder) to set it up for length. Making the cable end fixtures for straight cable, can buy PTO cable nuts or build from any brass stock where can be turned to make a shouldered thru nut and internally thread for a cable pinch bolt cable stop.

Similar to:

These are Dorman

DMiller · Nov 2, 2017

With no air ride may not even need a cable. Wait for the governor reinstall.

td25c · Nov 2, 2017

Good idea DMiller ! Imagine the cable fixed the problem .

td25c · Nov 3, 2017

MR Gasket throttle kit . http://www.jegs.com/i/Mr-Gasket/720/3842G/10002/-1

Parachute kit after ya get the Mack tuned up fine .....

http://www.jegs.com/i/Stroud/877/4004JEGS/10002/-1

DMiller · Nov 3, 2017

The old Mack kit had a bracket to hold the cable at firewall and another at inj pump, simple set up and was easier to set throttle break over with. Minor cab/engine misalign or bounces from worn mounts had no effect on them after the install. The Fire fighters really had problems feathering throttle prior as all those late 80's R Pumpers had air ride cabs.

85 mack r686 surging

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