Tuning MFI - patience or immediate redo ?

[Greg D.]

I have a 72T (MFI) recently rebuilt, new TBs too, which was running great, IMO (honestly it felt like it made 180hp and sounded like a banshee) but admittedly it was hard to start after a couple of days of rest despite the hand throttle (like a carburated car with dry bowls), it also clearly ran too rich and had high idle.

I took it to my long time specialist (whom I've had nothing but good experiences with, and worked at a dealer when these things were new). The plugs he pulled out were as sooty as I ever saw, no idea how they fired, so he readjusted all linkages on the TBs, played with the pump, broke out his special tools and notes, etc... He did warn me I'd have to come back after a couple spirited drives, for more fine tuning once things sort of settled down with the new settings. Hmm, not sure I understand how MFI settings can change after a few drives but OK...

Problem is, the car now feels like *crap*. I feel like I lost 40hp, 3200 rpm passes by in engine stutters that completely kill the acceleration (fuel delivery?), no banshee roar whatsoever, it's like a castrated engine... but hey, lower idle, and it probably does run leaner now. I trust the guy and he'll make it right, I'm just wondering if there is ANY benefits of me driving it "as is" for a couple days or if I should immediately take it back? It's my first MFI car (prior were CIS or carbs) so I profess ignorance here, but I really don't see a mechanical injection magically fixing itself to this extent (esp. as it was running great before, so it's not like something was stuck and is gonna loosen up more, seems like bad settings to me). Should I ask for slightly richer settings again? Just trying to not ruffle feathers here, in case I'm wrong and there's something to be gained from some driving prior to a redo... Wasn't cheap either, but the customer isn't always right, so I'm checking here first ;-)

[mobius911]

That's a bummer. Once the MFI is tuned properly it's very stable. But there are so many things that can be out of adjustment that it can be tough to sort out when it goes wonky on you. Without knowing exactly what the mechanic did, it seems that you have to get back to a decent starting point and go from there. This requires going through the Bosch/Porsche "Check Measure Adjust" booklet and doing what I think they refer to as the "basic setting". This means correct gap/dwell, plug condition/gap, ignition timing, pump drive belt timing, correct (to .5mm I believe) linkage length for the connector from the throttle crossbar to the pump, initial settings or the idle air screws, etc. It's all in the book, and that is available in pdf form either here or on Pelican.

It's not clear what the mechanic did when "playing with the pump". Hopefully he actually went through the basic adjustment process first, and then did some fine tuning. But something is clearly very off.

If the parts are worn out you may be chasing your tail on this. The pump may not be operating properly, the thermostat rack may not be working right, the throttle bodies may be worn out, etc. You have any idea how many miles are on the pump and throttle bodies, either since new or since rebuilding? But it sounds like you had a pretty decent set-up before the mechanic started messing with it, so unless he broke something it should be possible to get back to at least that state of tune.

And I wouldn't drive it as is. There's really nothing to break in so it won't fix itself. Better to try to get it close to a decent base setting first, and then go from there.

[Haasman]

Greg- MFI when correctly adjusted is immediate, obvious if anything needs correction.

Jeff is correct,

going through the Bosch/Porsche "Check Measure Adjust" booklet and doing what I think they refer to as the "basic setting". This means correct gap/dwell, plug condition/gap, ignition timing, pump drive belt timing, correct (to .5mm I believe) linkage length for the connector from the throttle crossbar to the pump, initial settings or the idle air screws

MFI doesn't fix itself. Yes, sooted plugs can clean themselves, but thats it. Ask your tech to throughly go through the Check Measure Adjust steps. Often its more than one problem.

Keep us posted.

[vicrola]

When Ed Mayo tuned my MFI, it was an all day affair, adjusting this...drive...adjusting that...drive. And there is a specific order of testing that should be done! MFI once set is super!

Vic

[csbush]

Agree with the comments above- it won't get better on its own. My lesson with these systems is that you don't have to accept compromises, and some (otherwise) really good mechanics just don't know how to adjust them properly. My engine suffered for years- through many different shops, then I finally found the right guy who fixed it in a day.

[RSTarga]

For the 10 years or so that my car was serviced at dealerships, my car never really ran right. Sooty plugs, crappy midrange etc.
Then my current mechanic got everything set up, and since then it has run flawlessly. These systems are really easy to screw up if the proper steps aren't followed. I don't know where you are, but you need to find someone who knows what they are doing.
Many of these cars were needlessly switched to carbs for lack of a good technician.
PS. As Jeff says, if anything is worn out, you will chase your tail forever.

PPS: Chuck and I share the same mechanic

[Rusty]

Is it possible to undo what your mechanic did, ie, back to original rod length etc?
It sounds like a typical too cool of running. I recently installed a system back on a 72 and over rode the warm up regulator.
so the injection pump thinks it is warm all the time. Most of the time we just don't run them hot enough and over rich is a result as well as fuel dilution. Make sure all your other mechanical systems are to par, points if you still run them, timing, valve adjust the ususal, then tackle the MFI again. Good luck and when you get it right it is the only way to go.
Ruty

[Greg D.]

Ok, pretty much what I expected, but good to hear nevertheless... I didn't want to stick my foot in my mouth but I could not fathom how that would possibly fix itself either ;-)

Yeah everything is new, engine, throttle bodies, rods, except the pump itself but since the car was running really well before, performance wise (aside from the aforementioned slightly high idle and sooty plugs) I'm gonna hope the pump is alright and it's just the new settings. I think "by the book" is alright but not 100% for my car. As I said he's got more experience with those MFI than anyone else in the area, so I think I rushed him or something - I picked it up immediately after his test drive, still that was a pretty obvious miss.

Got a bill for plugs, reset points, lube distrib and cam, reset timing, recalibrate fuel pump, reset air flow linkages and corrections... Almost 8 hours of work...
Gonna take it back tonight. Previous settings made for amazing torque, but lots of soot on the plugs, so either way something had to be done - it's a process I guess ;-)

[Dmill]

Vic is correct....the guys who know how to tune MFI will literally spend an entire day fussing with a car.....because changing one element of the system can have an upstream and downstream effect on the entire system. Oftentimes you will ultimately make a compromise decision....a little rich gives you that kick in the pants you might like, but a little lean results in a cleaner and more efficient burning engine. If your mechanic does not have a tail pipe sniffer....he should invest in one....that way, he can get instant feedback regarding what he just changed and the effect on the engine.

Lots of folks want to immediately start going after the pump....I have heard Ed say on many occasions....it's generally not the pump that is the problem....I have pulled pumps off the shelf that haven't been used for years...cleaned them up, installed them and they work just fine

As you have experienced...nothing like a small displacement high compression engine breathing through MFI.......screaming like a scalded dog.

[mobius911]

Agree with all the above comments that it's an iterative process and takes time and test drives to dial it in (the kick-in-the-ass meter is a very valuable tool here). The Check Measure Adjust (CMA) settings are a good starting point, but you'll tweak them to get it running where you want. For example, the CO settings in the CMA are low for where I run it; these systems do prefer a rich mixture. I don't know whether that's due to today's crappy gas, or that the settings in the book were intentionally low to satisfy emissions requirements. Probably a combination of the two. The CMA basic settings are a necessary starting point to give you a solid foundation.