Expansion of Tuning Procedure...

[DougE]

This is my first tuning effort; both electronic or mechanical. (I admit it... I"m guilty of bolting on a carb and going with it.) The tuning procedures provided in all the sources I've accessed are clear and understandable, but operate from the assumption that one is starting with a known, running engine.

I'm working on a Chevy 292 inline six with a ported head, oversized valves, and removed bolt bosses. As this is a siamese port engine, it isn't a good candidate for port injection, and consquently I'm running three TBI units, one sitting on a runner to each port, with large balance tubes, all part of a homemade sheet metal intake. The engine is also equipped with a Chinese knock-off of a TO4B turbo, feeding through an intercooler to an airbox above the TBI units. The cam is a turbo special; moderately long but iwth little overlap. The center TBI incorporates the TPS and the IAC. Ignition is a MSD driven by the MegaSquirt.

I ran the engine after assembly with a factory intake manifold and a stock carb to perform cam break-in. Now, after assembling the truck around the engine, I'm attempting to tune it. I can get it to idle, but not well. Clearly, I'm starting with a lot more variables than the normal tuning procedure assumes,

I can get the engine to idle, but it misses badly. Some plugs are black, some are white which I would think implies poor air and or fuel distribution at idle. Missing badly, of course the O2 sensor is off and isn't a reliable guide to overall AFR settings. I can't rule out mismatched injectors, or vacuum leaks. Before assembly, I set the TBI throttle blades very nearly closed, assuming that the IAC would control the idle. I plan this evening to take off the air box and attempt to set the throttle blades with a feeler gauge; it will probably be too much air, but at least the flows should be consistent. I'm further assuming that I might be advised to run the IAC full closed and leave it there until I can acheive a decent idle quality. I can't do an ether test to see if I have vacuum leaks until the idle improves. Too many interdependent problems...

Guidance on basic tuning (including the mechanical aspects thereof) would be greatly appreciated. I haven't shot myself in the foot yet, but I certainly seem to be aiming in that direction.

[DougE]

A embarassing update... Shortly after posting this query, I realized why I'm having idle problems; for non-port-injected engine, one can't tune for 14.7 AFR, but instead must allow for the charge-robbing, and so should target for somewhere in the low 13's. No wonder it was snorting and bucking. I knew this, but somewhere along the line managed to misplace that datum.

I was disgusted enough with myself that I didn't work on it last night. I think I'll continue with the plan to check the mechanical part of the air equation; e.g. the throttle blade opening, before loading a hopefully more realistic AFR table. I'm assuming that the lower AFR is needed for the idle bins only, and that I should transition to more normal AFRs within the first additional 1000 RPM and 50 kPa or so.

Ideas and suggestions are still needed for help in sorting out the vast numbers of things that 'could be' wrong when starting with an unknown combination.

Thanks!

[DougE]

Okay...

This post can be entirely disregarded except as an object lesson and reminder to not automatically blame the 'new thing' and to watch the basics.

In a stunning display of applied stupidity I managed to transpose two of the numbers in the firing order after carefully looking it up. No wonder it bucked and snorted.

All together now... 1... 2... 3... Duh!

[kholman]

an object lesson and reminder to not automatically blame the 'new thing' and to watch the basics.

Excellent observation and not the first time its happened.

[DougE]

By way of follow-up...

Even after correcting the firing order, I was never able to get the installation to idle well; it continued to backfire and run erratically, and I was consequently unable to get it to idle well enough to attempt to begin the actual tuning process. Going back to basics, I've made one discovery and surmised a couple of other issues that I'm now working to correct.

I disassembled one of the TBI units to obtain injector part numbers and verify flow rates. I had specified 45 lb/hr injectors, but it turns out that the supplier had given me 33 lb/hr injectors. I could no doubt get it to run using the 33s, but have opted to replace the injectors to get 45s as I don't want to take a chance on leaning out under boost. Becasue I had the MS set based on 45 lb/hr injectors, I was already starting much leaner than desired.

While attempting to idle before, the oxygen sensor always indicated an AFR of 12 or so. This poor abused engine was backfiring and snorting enough that the inside of the throttle bores are blackened. I suspect that the backfiring was pre-burning enough fuel in the intake tract that there was no oxygen left in the mix actually going into the cylinders. It is easy to forget that the AFR dial is not an actual reading of the effective fuel mixture; it is instead an interpretation of the amount of remaining oxygen to derive a calculated AFR. I suspect that my efforts to get a better idle were always in exactly the wrong direction.

I'm also surmising that my initial choice to set up the MS was a contributing problem. This installatino has three TBI units, each sitting on a six-inch tract leading to each siames intake port. I had in mind that 1 squirt per cycle was based on ignition events. A more careful re-reading of the Megamanual indicates that it is instead one alternating squirt per engine cycle (two complete revolutions) which would result in long gaps between squirts, and the near-certainty that charge-robbing would be an issue.

Hopefully, after getting the needed replacement part installed, I'll be able to get enough idle quality to actually begin to work on tuning. I undertook this first-time fabrication tuning process as a learning experience. I can cheerfully report that I've now discovered dozens of new ways to shoot myself in the foot.