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I give up - Surging issue
https://www.ausmini.com/forums/viewtopic.php?f=2&t=64610		 Page 1 of 2
Author:  jester99 [ Sat Jul 24, 2010 9:30 pm ]
Post subject:  I give up - Surging issue
Well i have given up now. i have got the S#^^&s with this engine right now :lol:

spent the last 5 hours trying 4 different needles and 3 different springs, and about 20 different timing settings to try and get rid of a surging issue at highway speeds.

i have been playing with it on and off for over a week now. after rebuilding the engine i have been babying it somewhat as i run it in, loading up etc to bed the rings in but not giving it a hard time. i have done about 450-kms in it now so am trying to tune it a little better so i can at least drive the thing :lol:

if i get the mixture right at idle, with a number 5 needle, and set the timing to what i want it to be, 10 degrees at idle and a max of 22 degrees (a safe setting right now) the car then drives well at low rpm with light loads, but at highway rpm, 3500-4000rpm, the thing surges and carries on when going uphill. it is fine when no load is there but surges with light load at highway rpm. if i then richen the mixture there is no change to the surging, if i lean it, no change, if i advance or retard the timing, no change. well there are changes, just no to the surging.

can anyone shed some light on what may be causing this issue.

i have tried many needles and have found #5 to be the closest so far. it is also the best with the red spring and dextron 3 atf trans fluid in the dashpot.

but i cannot get rid of this surging. i am about at wits end now. this is a similar problem i had with this engine before i rebuilt it, the only way i got rid of it was with a harder spring and a super rich mixture, and retarded the hell out of the tming, but this was all due to the engine being very very badly worn out.

i also have noticed the idle mixture seems to change every time i drive the car. if i set the mixture to what i think is right, then go for a drive around the block, when i test it again it is different. i also tried going around the block and adjusting it time and time again and i never seem to get it exactly right so it doesnt have a change needed. maybe the carby is shot? everything seems pretty good though.

any advice guys?

engine specs

1098 bored to 1220
standard 12G295 head
731 cam timed in at 108 degrees ATDC
A+ sintered rocker arms
10:1 compression, flat top pistons
1.5" SU on alloy minispares manifold
pulsar DR85 dizzy
LCB RC40
Author:  drmini in aust [ Sat Jul 24, 2010 9:37 pm ]
Post subject: 
I reckon you need more advance. 22° total ain't enough in most A series, even standard. And with bigger cams you do need more usually.
My 1412 runs fine at 32° total (RE13 cam), my 1360 needs 38° total (because it has a bathtubbed head and GR's RE282 sprint cam).
Author:  knuts2au [ Sat Jul 24, 2010 9:41 pm ]
Post subject: 
What about fuel starvation? checked filter, fuel lines, tank breather, fuel pump?
Author:  Tombo [ Sat Jul 24, 2010 9:43 pm ]
Post subject: 
Pretty weird problem.
One of my diagnosis rules is 'If you make an adjustment and nothing changes, you are adjusting the wrong thing'.
Maybe look at the distributor advance mechanism.
Does it do it un loaded? or in a lower gear at the same revvs?

Good luck.
Author:  jester99 [ Sat Jul 24, 2010 9:46 pm ]
Post subject: 
i will double check the operation of the dizzy mechanism again tomorrow but i have checked it twice already.

if i set the dizzy to give a max of 30 degrees at 4000rpm, the surging is no different.

new fuel filters fitted, fuel pump good, fuel pressure and pressure reg ok, getting 2psi at carb, float bowl level good, new needle in float bowl.
Author:  drmini in aust [ Sat Jul 24, 2010 9:54 pm ]
Post subject: 
Any air leaks at manifold, that will cause it.
Author:  justminis [ Sat Jul 24, 2010 9:56 pm ]
Post subject: 
Do you have access to a std mini dizzy that you know works OK? Swap it over and see if it still does it. I agree with Kev, 22* isn't enough, whether Pulsar or mini dizzy.
Author:  DTrain [ Sat Jul 24, 2010 9:58 pm ]
Post subject: 
whats the condition of the carb?
Author:  GT mowog [ Sat Jul 24, 2010 10:01 pm ]
Post subject: 
Sounds very much like fuel stavation / low delivery.

2 psi fuel pressure is at the low end. What fuel pump are you using and have you measured the fuel pressure when running and in particular when surging. Put a Tee in the line so you can connect your fuel pressure gauge while driving.

Ideally 2.5 to 3.5 psi would be better, but more important is how much fuel your pump is delivering. Free flowing sould do aroumd 1 litre in a minute.

Also, if you have a vacuum advance, try driving with it disconnected and blocked off.
Author:  jester99 [ Sun Jul 25, 2010 10:45 am ]
Post subject: 
i will test fuel pressure again today and check for air leaks.

i tested the fuel pressure with a guage and tee a few months back before the engine build when i last had a similar issue, and fuel pressure was 2 psi after the pressure reg.

the fuel pump is the quiet walbro type that produces 3psi, and the pressure reg is the adjustable crap cheap type, which set at 5psi, currently produces 2 psi at the float bowl.
Author:  BALLISTIC [ Sun Jul 25, 2010 10:45 am ]
Post subject: 
Im with Drmini and GT Mowog on this one it is either an air leak or not enough fuel pressure.

Are you using an electric pump or is it mechanical?

Prior to the rebuild did this carb spend abit of time at around 4000rpm? (normally freeway speeds), if so it may have a slightly worn throttle shaft but it would also benefit you to check the fuel delivery from the pump.

Cheers Scott
Author:  DOZ [ Sun Jul 25, 2010 10:50 am ]
Post subject: 
Check the earth strap - I've found under load issues are sometimes caused by poor contact and the coil not producing enough sparkage.

Even with wrong needle etc I have often found poor fuel delivery to the carb as a cause so check fuel pump/filter/line - and needle/seat in carby, particularly if this issue was there before rebuild.

Daniel
Author:  jester99 [ Sun Jul 25, 2010 10:58 am ]
Post subject: 
sweet thanks guys, your feedback is greatly appreciated. i will go out and tackle it again today.

8)
Author:  1275LS [ Sun Jul 25, 2010 11:00 am ]
Post subject: 
A side question regarding dashpot oil. It is availble readily and costs about $10 for a 500ml container. Why use ATF and the like?
I'm in Brisbane so I realise it may be a temperature thing down south but if they can use the same oil in the UK as Brissie, why not use it?

BTW I doubt that is the poblem, surging will be fuel or air starvation. I'd be looking at air first. It is a case that the either the air is flowing freely and it takes a bit for enough fuel to get in the system to build revs up so it surges once there is fuel there and dies down as it drops off. WIth Air starvation the air drops off and comes back with a surge when suction gets high enough to drag a quantity through then the suction falls away and the engine dies again. You see this on mowers when the air filter is clogged with oil and dust.
Author:  jester99 [ Sun Jul 25, 2010 11:05 am ]
Post subject: 
ok sweet as 1275LS, where do you get your dashpot oil from? what brand etc?
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