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I have a Subary Brumby 1992 model that starts jerking-surging under gentle acceleration and cruising down the road under no acceleration and the vehicle becomes undriveable. The only way to keep driving is to flatten the throttle and keep accelerating under full throttle.All fuel filter's have been replaced and carby cleaned and overhauled and motor tuned including replacement of leaking vacuum lines.Appreciate any assistance regards Mick Fitz.

Make sure you don't have a vacuum leak. This includes hoses, intake manifold and vacuum diafragms. To check the manifold for leaks you can spray degreaser on the ends and see if the idle changes. I don't know if egr or pcv valves can do this, but I wouldn't rule them out either.

i have had this trouble before and have seen a couple of threads with the same problem

i replaced all filters atill did it cleand out my pump fixed it
but someone elce had this problem eaven after new or reconditioned pump
hope this helps

I couldn't sleep so here it goes: I believe it might be the EGR, there is a vacuum hose with a small metal restriction in it, I wonder if you replaced it with a normal one. I can't remember if this hose has anything to do with egr, though.

haven't got worn throttle shafts or fuel leaking from a fuel delivery line?

surging is generally caused by lean air fuel mix or over advanced ignition timing....

egr has been known to do odd things but they were common rail turbo diesel engines I was working on back then.........

if the brumby has lots and lots of k's on it the throttle shafts probably have freeplay and leak vacuuum.....

Robert.

has been successful in the past with several others including myself after doing absolutely everything else.

With empty tank and gauge unit removed from rear of tank I disconnected fuel pump from the lines and filters and blew compressed air from the front under bonnet to have air shoot out at fuel pump point - all clear, then blew from fuel pump inlet line through to tank and THOONK - a pile of small crusty rusty bits landed in the pristine clean tank and my problem was solved!! after cleaning that crud from tank. It is as if something start rusting up inside fuel line and sticks there and is worse when air temp is colder like 3am in the morning when all you really wanna do is go home - not frig with an ongoing temperamental car !! GOOD LUCK

thats definitely a possibility, that would cause low float level (lack of fuel pressure) and therefore leaner mixtures when driving, causing the surging....

Robert.

Thanks for all the advice
To date have
(1) Opened the solenoid vent valve on the carby (not working) to vent through the air cleaner (no change)
(2) Machined the uneven top of the Carby main body to a flat surface (no change)
(3) Replaced front and rear fuel filters (no change)
(4) Replaced the thermostat (no change)
(5) Adjusted the valve clearences (no change)
Still to clean out the fuel lines (here's hoping)
The symptoms are putting me in two or many minds as the vehicle runs perfectly incluging starting and idling until the vehicle does approx 18km.
Every time I have made an adjustment or change I have taken the vehicle for a run and runs perfectly until the 18km mark or should I say arrive back in town and driving throught the streets starts to Jerk/Bunny hop and can only be driven home by flattening the throttle on and off as it won't drive smoothly under normal throttle conditions.
The problem seems to be related to heat build up in the engine bay (I think) even though the temp gauge is normal.
Has anyone heard of or experienced similar symptoms.
I'm toying with the idea now that maybe the coil or even the distributor module could be affected by heat though you would think that the vehicle would not respond to flattening the throttle.
Anyway thanks for the idea's

Well you have done a lot of things beyond my knowledge but I got a similar issue when my head gasket was leaking, are you using much water? try pulling off the blow by hose and driving about and see if the problem occurs, it stopped mine getting the issue kind of annoying white plume of steam though.

what else is there left to do? That is about the stage I got to also until I blew the lines out. Still have the new coil I bought that did not fix the problem so went back for a later date , ten years ago !!

Mine would be fine during the day so long as not start too early - cool air, so was a bastard to try to diagnose. Would mainly do it at around 11.00 pm when it is dark and all you wanna do is get home. I was blaming supermarket fuel as one night I topped it up elsewhwerre and all of a sudden went like a beaute. I could get it going so long as no load so down hill route rather than uphill which was always there !!

I know what you are going through. When I bought my ute it had a rebuild of the dizzy for some reason??? I think they sold it because of this problem.

float level??? bring it up a smidge and see if it makes any difference...

Robert.