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Light throttle jerking

Posted: Mon Jun 24, 2019 10:45 am
by nickm
I took my se5 to a rolling road on Friday to set up the jetting and in an attempt to cure jerking on light throttle.
He set up the main jets as it was too rich at the top end,but a little lean at the low rpm end.
He didn't have the idle jets for my carb so he recommended 65 idles[2x DGMS carbs].
Do you think this could cure the problem?
By the way,I got 187bhp and 195 lb/ft which I was pleased with.

Light throttle jerking

Posted: Mon Jun 24, 2019 11:16 am
by scimjim
Standard mains are 145 - do you know what size he fitted?

Standard idles are 45 so as your engine obviously needs more fuel than standard, 65 sounds about right?

Light throttle jerking

Posted: Mon Jun 24, 2019 11:30 am
by nickm
He put 145 mains in[originally I had 155s].
My idles were 50s.
It has two DGMS carbs fitted so the jetting might be a bit unusual.

Light throttle jerking

Posted: Mon Jun 24, 2019 11:36 am
by scimjim
How many turns out are your idle screws then?

Do you have an AFR plot after the adjustments?

Light throttle jerking

Posted: Mon Jun 24, 2019 3:33 pm
by nickm
Just checked and they were 2 turns out which to me points to it being lean.
I have a mix of jets though,2 65s and 2 50s so I will get the other 65s and see how it goes.
I didn't get a printout of the afr but did get a power/torque printout which shows quite a dip in the torque at 2500ish.
I am running the carbs synchronously as I didn't like the feel with them set up progessively.

Light throttle jerking

Posted: Mon Jun 24, 2019 4:16 pm
by scimjim
I'm surprised he left them at 2 turns and advised bigger jets if it's lean? why not unscrew them a few turns to richen? It's safer on the rich side too.

Light throttle jerking

Posted: Mon Jun 24, 2019 5:57 pm
by AJL Electronics
nickm wrote:
Mon Jun 24, 2019 3:33 pm
Just checked and they were 2 turns out which to me points to it being lean.
Standard internet myth!

The starting point is three turns out and most end up nearer 4. I am surprised that he suggests .65 without re-running it on the dyno because that in my opinion will make the idle and progression really rich and there will be an abrupt change when it comes fully onto mains.

Light throttle jerking

Posted: Mon Jun 24, 2019 6:20 pm
by scimjim
I think I ended up 7 turns out on my first setup run with standard jets (just measuring lambda on the laptop) - 195 bhp engine with a bored out 38 DGAS. Can’t remember now what jets I ended up with (ended up drilling some out with an accurate set of drills I think) but 160/60 ballpark with idles at 3 turns out rings a bell.

Light throttle jerking

Posted: Mon Jun 24, 2019 7:27 pm
by Jasper53
When we had grief with our 924 sprint car on 45’s the Dyno guy reckoned if the screws were out any more than 3 turns the jets were wrong. We were told the idle jets or progressive jets as they are, only work up to around 1500- 2000 revs anyway then the pumps and mains take over. Ours eventually ended up at just under three turns out at the end. :D

Light throttle jerking

Posted: Mon Jun 24, 2019 8:12 pm
by philhoward
..and that’s why I’m fitting EFi whenever I get the chance... :mrgreen:

Light throttle jerking

Posted: Mon Jun 24, 2019 8:18 pm
by AJL Electronics
Jasper53 wrote:
Mon Jun 24, 2019 7:27 pm
When we had grief with our 924 sprint car on 45’s the Dyno guy reckoned if the screws were out any more than 3 turns the jets were wrong. We were told the idle jets or progressive jets as they are, only work up to around 1500- 2000 revs anyway then the pumps and mains take over. Ours eventually ended up at just under three turns out at the end. :D
Yes the myth is widespread! The rest of your comments are accurate, although they are called idle jets, they also (with the emulsion tube) handle progression until the mains are in full flow.

Light throttle jerking

Posted: Mon Jun 24, 2019 8:19 pm
by scimjim
Uneven running on light throttle and a drop in power at idle/main changeover point are both indicative of idle jets being the wrong size too. The progression circuit is holes upstream of the idle jet (and therefore controlled by the idle screws).