Big Block tuning problem.

**frog67** · 03-01-2021, 05:04 PM

Hi All, i am hoping the some of the chev guys can help with a tunning problem i have.
i will give some general info and hope fully that will be enough to get it rolling and answer questions with detailed info.
I have a big block engine that i have just put a FAST fuel TB on (took off a 950 double pumper), it is driven by a Link GX4 monsoon ECU. the ign is controlled by a MSD 6AL not the ecu.
the problem is, upto 2000rpm it tunes well resonding to fuel map changes as expected, then be tween 2k and 3k it has massive changes in A/f from small change in the fuel table.
A/F can read 16:1 so increase the fuel by 10-15points and it will. on occasion drop to 12:1?.
with the trans in neutral from 750 rpm @ 5% tps, as the rpm and tps riase to, say, 2000rpm and 15% tps fuel has to be taken out so it doesn't run rich, so at idle it will have 16pionts of fuel and when it get to the 2500rpm area it may have 12points.
once it gets past 3000rpm it needs 30 pionts and then up to 140 at the top.
the pump, regulator, TB are all new
the pressure has been between 42 and 70 psi with no desernable difference, and doesnt fluctuate on the gauge.
coincidentally, 2000rpm is where it used to come off the cam and smooth out with the carbie and 3000rpm is where the 6AL stops the mulit spark CDI and goes back to single single spark.
Is the 6AL interfering with the ECU during multi spark? all the logging doesnt show any thing concussive.
is there a chance of a vaccum leak under the manifold, is this common on dart pro1 heads? the map sensor loggs don't show any major changes but i can find the cam specs to begin to guess at the correct map value.
i have been playing with this on and off for months but need it off the dyno soon.
Any ideas would be muchly appreciated
Dave

**monaroman76** · 03-01-2021, 07:47 PM

Hi Dave, I have no experience with Link ECU's, but a few things come to mind..
-Verify that leads and plugs are good, an intermittent misfire can have you chasing your tail with A/F problems. Try removing spark leads one at a time to see if one of them has minimal effect on AFR.
-Have you verified the output from the MAP sensor?
-Is this strictly open loop? Do you have a single wideband 02 sensor, or one for each bank?
-Is the outcome repeatable, or is the AFR erratic from one run to the next?

**immortality** · 03-01-2021, 07:59 PM

Out of curiosity, why not control the ignition timing with the Link?

What do you mean by the MAP logs don't show any major changes? Where is the MAP sensor taking it's reading from?

Depending on the cam, I would expect to see somewhere in the area of 30-60kpa at idle and at WOT 100kpa.

**frog67** · 03-01-2021, 09:22 PM

the plugs were replaced when the ecu and Tb were fitted but 60 lts of fuel has gone through since then.
the leads have done 3000km since new and are OEM Boss Hoss, so i will check them. i know #1 is okay as that one checked the timing

the Map sensor is internal of The ECU and appears to be reading okay and show about 70kpa at idle, i will post the log
it is running a speed density system so it is TPS over RPM, no O2 sensor one it is tuned
the A/f is erratic from run to run
the map sensor is reading from under the throttle plate on the body of the TB
I am using the mechanical advance in the MSD distributor as the curve was plotted to this engine on a engine dyno. I have no way of loading the engine on a chassis dyno to do the ign map with out cooking or breaking something, namably the trans

Dave

**immortality** · 03-01-2021, 10:13 PM

That has to be a decent cam, that is **** all vacuum at idle.

Need to be sure the vacuum source under the TB is manifold vacuum and not ported vacuum from the TB.

Have you spoken with Link? They are quiet helpful and have their own online forum IIRC.

**frog67** · 03-01-2021, 11:01 PM

i will look for a port in the manifold to be sure.
i have spoken with link and they are exceptionally helpful, we have altered a heap in the file and each time it gets better, but now i think it is something simple and mechanical. i havent had much to do with BBC's so i though i would ask here before pulling it apart.
as for the cam, i have attached the dyno sheets from the original build with the carbie, says it all for a bike engine.Agrona dyno graph.jpg Agrona 601cubes.jpg

**frog67** · 09-01-2021, 02:27 PM

it was a miss right in the roughest part of the cam i could only just detect it on number 8, pulling leads at that rpm i could only just notice the tiniest difference in engine speed and sound, so i changed leads and plugs any ways and it still sounds like it is farting, coughing and missing but the A/F isn't jumping around and it revs straight though now.
Thanks for your help.