any tips to tune out an off idle stumble?

mikesb · Post by **mikesb** » Sun Jan 22, 2006 4:40 pm

I'm using megasquirt on my 1970 MGB 1800cc engine. Injection is via an RC51 honda throttle body through a wet manifold to the pair of siamesed intake ports.

I've started tuning it, I'm to the point where it runs, I've gone for a short drive, and seem to have some of the basic tuning set.

I'm still fiddling with the AE, but I'm getting closer. There's a couple of problems that I'm struggling with, in order of frustration...:

#1 If I very slowly crack open the throttle from a dead idle (around 950 rpm), the O2 and MAP shoot skyward, RPMs drop, I can often cause it to stall. To start driving from idle I need to blip the throttle to get going.

#2 (I may have this tuned out, but...) Sometimes when I come to a stop, and it seems most frequent when pulling high vacuum (throttle closed, engine braking in low gear), the engine speed will drop past idle before stalling when I go into neutral. This might be gone with the little hollow I put into my map around idle, but I won't know till next weekend.

#3 Sometimes, while cold, the engine will, for no observable reason, suddenly drop idle speed to either stall, or go into cranking mode which often saves it. Then back to idle for a while with no problem, only to repeat a little later. As with #2, this might be gone with the little hollow I put into my map. I'm not using an idle air valve.

Any thoughts/tips?

MS-1, v2.2 main board.

Mike_Robert · Post by **Mike_Robert** » Sun Jan 22, 2006 5:03 pm

Mike, try richening the high load, low rpm VE bins. Try adding 5 or so to these and see if that helps. I've had some experience with problems as you describe and the abovementioned action has been very successful. The need to blip the throttle, no doubt triggering AE, is a fairly strong indicator that your high MAP/low rpm VE bins are too lean.

-Mike

arnold · Post by **arnold** » Mon Jan 23, 2006 12:32 am

Do you happen to have some pictures of your setup (hardware). I experience similar on my Austin Healey Sprite (wet manifold, Rochester TBI of an Opel Astra)

http://kmnet.demon.co.uk/~arnold/

Arnold

mikesb · Post by **mikesb** » Mon Jan 23, 2006 5:58 am

Thanks Mike, I'll give that a try. I did richen it some earlier, which did seem to help some now that I think about it. My focus was on that stalling problem that I eventually reduced with the hollow around idle. I was worried I was going too far (rich).

Should there be a jump from the idle zone or a smooth climb to the low vacuum / low rpm part of the map?

Hi Arnold, nice looking setup with the blower! I'd like to try a similar thing some day, but I've got my hands full right now as it is

.

No pictures yet but I'll grab some and post them as soon as I get a chance.

arnold · Post by **arnold** » Mon Jan 23, 2006 7:56 am

Did you mount the TBI vertical or horizontal (as I did)?

mikesb · Post by **mikesb** » Mon Jan 23, 2006 12:57 pm

Horizontal, like yours. The RC51 TB was horizontal in it's original application. I was going to use a GM TB at one point and wondered if it was okay - I never tried it but I believe it would have been.

I did a quick write-up on what I've done: http://www.7builder.com/MGB/mgbfi.html.

woh · Post by **woh** » Mon Jan 23, 2006 7:48 pm

Mike

I checked out your web site. Nice pictures.

Great idea on using the existing temp sensor on your MGB. If I may suggest you can replace your voltage stabilizer with a solid state stabilizer. They are available on e-bay, or you can build one with a three lead solid state regulator from Radio Shack.

Once I get my MG sorted out I'll change the GM sensor to the existing sensor as well for a cleaner install.

Post by **efahl** » Tue Jan 24, 2006 8:33 am

mikesb wrote:Should there be a jump from the idle zone or a smooth climb to the low vacuum / low rpm part of the map?

Mike,

Tune it so it works, and then you'll know.

Seriously, it all depends on your motor, some are smooth, others like to idle way rich and have a drop from idle to low-load.

To get rid of the off-idle stumble, do what Mike says, but I'd say be way more aggressive than he suggests, do something drastic like double the VE values for the 500-1000 RPM bins from like 65-100 kPa. See if that helps or hurts and then you'll have a couple of good reference points.

Eric

mikesb · Post by **mikesb** » Wed Jan 25, 2006 8:39 pm

It's on my job list Werner, I've got an LM317 waiting in the wings for a chance to shine

.

Thanks Eric, point taken - I think I'm starting to get the hang of it. The more fiddling I do the better my own mental model becomes and the more addictive it gets... I'm away this week, yet I'm still tempted to fiddle with the map in anticipation of getting back to the garage on the weekend

.

morris · Post by **morris** » Wed Jan 25, 2006 8:42 pm

I have a very similar set-up on an MG Midget 1500. And I was having a very similar problem. It turned out to be a PWM issue. I have a very low impedance injector, and I set the PWM according to the manual. But when I added a couple more percent to my PWM my off idle problems vanished. She pulls smooth and strong off idle now.