Fluctuating RPM: Ignition Box Tach Wire to MS2 Fuel Only

[Chree]

I really, really hope someone can help me with this, I've put so much time into the project and learned a lot, but this problem is unbearable. I have a 10 to 20 RPM fluctuation that causes the engine to be rough at any steady throttle. The issue isn't noticeable when accelerating.

MegaTune 4.00 MSII Rev 2.89000
MS2 built by DIYAutoTune - no modifications / relay cable / relay board / LC1
Tach signal coming from white wire of ignition box (Holley equivalent of MSD 6AL) to relay board
406 Small Block Chevy / 5 speed manual
8.3 to 1 CR
Solid roller cam, .555 lift, 114 LSA
Holley EFI intake, 36lb injectors

I have a digital tachometer also using the ignition box tach wire, but the fluctuation is still present when the tach is disconnected. Other than the fluctuating RPM everything else works fine: startup, acceleration, etc. (Thanks to Matt from DIYAutoTune for his help getting me this far!)

I appreciate any ideas.

[Mr Gadgets]

Chris,
what does your timing look like at idle and above?? Is it rock solid or does it fluctuate?

Reason I ask, is because I have been chasing a timing issue for 3 months now. I did find that when I used my older MSII, the timing is rock solid. If you are using the VR circuit, you may be having the same problem I am seeing. Would be nice to compare notes. I am using the MSD 6AL and MSD crank trigger. What are you using for you trigger source?

Thanks.
Dick

[Chree]

Dick,

I saw your other posts and that prompted me to see if anyone had figured this out yet, since I'm not the only one with the issue. I meant to list my distributor, it's an MSD 8361: Pro-Billet with Vacuum Advance made for use with ignition boxes - it does not have an ignition module, the 8361 is just a trigger. I believe my timing is steady, but I should put a light on it to confirm... it's snowing here now, which is a rarity, so I'll have to wait.

My MS2 is configured as it was when DIYAutoTune put it together, I haven't made any changes. The default is Hall sensor I think?

[Matt Cramer]

Generally, a 10-20 RPM fluctuation isn't going to cause drivability issues. Could you post a data log of the problem?

[Chree]

datalog200908161626.msl

You'll see all readings are jagged. The LC1 stops working about half way through after a half throttle run through the gears. Interestingly the LC1 seems to be working lately (or maybe I have EGO correction turned off, can't remember!).

I may be understating the RPM fluctuation, it's enough that you can tell the engine is not running correctly. Perhaps an MSD tach adapter could help, like the 8910 or 8920?

Thanks for the help.

[Matt Cramer]

I've seen engines run fine with jaggeder readings. The real issue may be the X-Tau; it's not going to work very well until the VE table is spot-on.

[Mr Gadgets]

Chris,
my motor is in a boat, so it is a different animal than a car. The boat has a constant load at speed, and the throttle is very stable. Previously I found I had a tach signal variation of 120-250rpm above 4krpm. I chased it for a long while, but in the mean time I kept the motor running on the rich side because I could see the PW vary with the large jumps in tach signal. Things I found were.

LogViewer.. is self scaling. I chased a problem for a few days thinking my rpm was all over the place, but I had logged it, only at an idle. The variation was full scale, I couldnt believe it. Then I discovered that if I reved the motor to 3krpm.. the variation was much less.
After that I look at varing data with the log in Xcell. The easy way for me to see tach signal delta was to add a column next to the tach signal column and then add a formula that would subtract the next lower cell from the top one.. EX: =SUM(C3-C4) Then I copy this down the whole column. It will show you how much it varies.. very easily. I pg dn and it goes quite fast, but you can see bigger numbers, etc. It helped me with my problem.
I did bench testing and could see this happen, so I played with my setup on the bench. I was spinning a spare MSD dist with a DC motor, coupled to the dist gear with a piece of hose. The dist was fastened loosely to a board. Long story short.. The more secure the dist was to the board the better things got.. The last thing I did was to move the two hose clamp screws on the coupling hose, so they were 180* apart. That helped balance it and took out most of the variation. I would now see about 30-60 rpm variation, but it was stepped.. before I would see it change 150rpm in from one sample to the next.
I then went to an MSD crank trigger on the engine and saw even better results. Now I can see it vary, on the engine any where from 2-30 rpm.. but it takes about 10 or so steps to move that much.
My conclusion was the timing chain, cam flex, dist gear and the small amount of play in the upper dist bearing was all adding up to a huge variation. With the VR pickup in the MSD dist, at idle I would see the timing jitter about 3-4*. With the crank trigger it was rock solid. I could add a degree on the advance table and see it move.. take it out and it would move back.
I took your log file and added the colum and your variation is not bad. But I do see other things changing. Your battery voltage looks like it is not as solid as mine is.. but it may not be a problem.. I really not sure what is happening when it varies. Mine would stay very steady until I hit the trim button for the drive.. and it would take a dive. Then pop back up when I let off the button.
Try looking at the log this way and see if it helps you see anything.

Another thing is I have my dist locked out and run about 28* at an idle and 32* total. That helps see things by reducing the variable.

My problem now seems to be the new MS I built is not working like my previous one. I switched processors from one to the other and with the old chip in the new unit.. the timing is still very unstable.

Keep looking at your data.. you will find it. Another thing... can we shut off the Xtau to see if it is causing a problem... My old one did not have that style of enrichement.. but it isnt as inportant in a boat as a car..

Hope this helps..
Dick

[Chree]

Thanks for the advice guys! I turned X-Tau off and most of the fluctuating / stuttering went away. Then I saw that I did have EGO control turned off, so I put it at 10%. Much better, see the datalog:

datalog201003031448.msl

I still have a ways to go with tuning; I don't know that my VE table will ever be "spot-on". I turned X-Tau on at some point and noticed an improvement... and haven't thought about it much since.

This engine was built for boost, but I'm starting to wonder if I will ever be versed in tuning enough to make the leap.

I appreciate the help!

[Mr Gadgets]

Chris,
Glad to see that helped.. I will try the same thing on my combination. Could I see a copy of your msq? I would love to see what your AFR table looks like. I hope to be going to the dyno with mine at some point. Just need to get the bugs out. Once I get it tuned, I can share my msq with you. It might help in your tuning under boost.
I have not been into boost yet, so it will be a learning experience..

Thanks
Dick

[Chree]

megasquirt201003041451.msq

Dick I'm happy to share my msq. As a caveat I am far from an expert, as I only make time for the project in spurts, so it's hard to remember the details in between. The VE table is a combination of the default table (set up for a small block 350) and the table I had been working on. The only issue I have now is afterstart enrichment is too high when the engine is warmed up. It will flood on a warm day.

Again I appreciate the input and hope what I've posted will help others... I rarely ask questions because I assume my questions are redundant... but as you can see something as simple as "turn X-Tau off" can make all the difference.