... a bit lengthy ...but I tried to include the whole thread so those
who haven't seen it may learn from this.
So many people have responded I can't even remember
who to thank anymore so the whole list gets it.
Heres' the ORIGINAL MESSAGE Revisted
comments the list made, and at the end what's been tested
and the results. I hope this helps.
********* original message: *******************
* I need the tech'ies to come out of the wood work please!
* I've rebuilt the dual SU's 3?4? times, tuned 'em several times.
* Changed to K&N filters and back to stock filters (open like K&N)
* now back to stock air boxes.
* Changed from AAE needles to ABD's to #6's and now AAA's,
* ignition. changed from stock to xr700 electronic w/ new sports coil
* plug gap widened from .024 to .028 plugs look great at any speed when
checked
* centrifugal adv. and vacuum adv. checked independently and are OK
* Brand new SU fuel pump and pressure regulator**
* This bugger still stammers at a constant cruising speed.
* THIS IS THE ODD PART OF IT!
* It sounds like it must be the ignition right? wrong! If it were the
ignition system....
* why would pulling the choke out make it smooth right out!!!???
* any wild ideas crazy suggestions welcomed at this point.
* All thru heavy acceleration and driving things are
* fine. Just cruising sucks.
*******************
Lawie replied:
It sounds as though you have a vacuum leak somewhere. In the PCV valve,
perhaps? Spray something volatile around the manifolds, etc. and see if you
can trace a leak that way.
*******************
Sean Muldoon said:
well paul your not alone my 73B does the same thing, and im also running all
the same stuff. so if you get a good answer please forward to me.
*******************
Rick Mason said:
Sounds to me that you have a vacuum leak which is leaning out the mixture.
Pulling the choke enriches the mixture.Under heavy acceleration, the
manifold pressure is closer to ambient - less vacuum and less leakage.
Several places to look:
1) Throttle shafts
2) all the gasketed surfaces between the carb and the head (three gaskets
between each carb and the inlet manifold, one gasket between the manifold
and head).
My guess is if you properly rebuilt the carbs - it's a leaking gasket.
*******************
Chris Kotting said:
Heavier oil in the SU dashpots might help (by damping the piston more).
If that doesn't do it, check the following:
Valve Lash
Timing Chain
*******************
Bob Wiseman said:
Lawrie is probably close to right. Pulling out the choke changes the RPM
and
Fuel mixture ratio so if it is a fuel/air problem it changes. A small air
leak
at the manifold will make a difference.....
On the other hand you changed to electronic ignition. Did you still use the
same base distributor? Is the drive dog OK (no play)? Is it the correct
distributor...I don't mean model 25D4 or 45D4, but correct "part" number..
My
MG's and Land Rover both use a 25D4 but a different "part" number, the
difference being in the centrifugal advance.
Check the air leaks as Lawrie suggests then look at your distributor.
By the way go back to the K&N's.
*******************
Ed (JustBrits) said:
Because Paul, you didn't rebuild the carbs CORRECTLY
(as most folks don't).
Cheers........
*******************
results:
Since the original problem was posted......
I took off the SU's and put the DGV in place while waiting for the new SU's
the engines from YB and OB were swapped (O/D transmission as well)
Brand new SU HS4 (AUD405's) installed with red, white, and orange springs
tested, popet valve removed and soldered closed , ABD AAE and
AAA and #6 needles tested
20/50 wt and Magic Mystery oil tested in the carb dash pots.
THIS IS A DIFFERENT ENGINE AND CARBS AND STILL STAMMERS AT CRUISE
I installed a vacumn gauge to monitor the engine.
My previous bench tests show my vacummn advance (VA) comes in at 3-5Hg
and ends at 15Hg for a total VA of 10 degrees. So watching my VAC gauge I
can essentially watch my VA position. IE: 5 to 15 Hg of vac. = 0 to 10
degrees
of advance.
If I ever so lightly let off of the gas at cuise, the stammer/stumble
dissappears. At this point the VA is at 15Hg or more, max'ing out the VA on
the
Distributor. With the power on (registering under 15Hg of vac), I'm in the
variable range
of timing due to the VA curve.
If I put my foot down as in passing, the stammer/stumble dissappears.
At this point the VA is probably close 3 Hg or less, barely starting to
effect the VA on the Distributor.
Apparently this sub 15 Hg vac JITTER is what's floating my ignition timing
around at crusingspeeds making the car feel like it's stumbling.
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