MFI Part Throttle Surge
 

My car is a '72 "T" with about 15k on a total engine and MFI overhaul. It has great compression and leakdown numbers and I just did a complete tune-up. The engine has had a part throttle surge at about 4,000 RPM for as long as I have owned it, about four months. I have downloaded the CMA bulletin from this site and have the 101 Projects book, so I am beginning to familiarize myself with the ways of MFI. I have the adjustment tools and synchronometer on the way from our friendly sponsor even as we speak. I've looked through the archives here, but can't seem to find much that directly addresses this situation. So, can any of the resident MFI gurus help me narrow down where to look first? I get the impression that the CMA must be followed all the way through, in order, without skipping any steps. Is this the case, or is it possible to target one specific malady without running through the whole drill? Is this a known problem with a known fix? The car runs extemely strong through the whole rev range, and pulls hard past this point under load even at part throttle. The surge only surfaces when cruising under a very light load within a very narrow range centered around 4,000 RPM, plus or minus a couple hundred. Thanks for the help in advance, guys. Let me know if more information would help.

It sounds like "lean surge" to me. I'd try the following since it worked for me.

1) Buy a notebook and pen and store them in your glove compartment -- next to the part throttle adjustment tool.

2) Record the initial conditions -- basically what it feels like now.

3) Richen up the part throttle adjustment by a couple of clicks. Note the date, the number of clicks moved (right or left).

4) Drive it for a few days. Record any changes in the condition.

5) If the condition continues, adjust the MFI again by a couple of clicks in the same direction and record it in the book. If it goes away, move half the number of clicks in the opposite direction and record it in the note book.

6) Drive the car and log the results.

Repeat steps 5 and 6 as needed until you reach the best compromise.

Jeff,

I will second john’s analysis and recommendation. That common symptom is one of the most difficult to resolve.

A exhaust analyzer/ CO meter is also an indispensable tool for basic ser-up or fine tuning MFI.

When you go through CMA, pay particular attention to checking the opening pressure, spray pattern and sealing of the injector nozzles. Test them at the lowest flow possible.

What ignition do you have? What sparkplugs? The OE Bosch CDI has a notoriously short spark duration. A “fatter” spark and more gap may help.

Anything you can do other than just setting it rich, is the better solution. You might not want to get rid of all the surge rather than paying the price in poor fuel economy and oil dilution. Try driving at 3000 in 3rd rather than 4000 in 2nd.

Keep the reports coming.

Best,
Grady

The reason the steps are taken in order is becuase it's pointless to make fine adjustments until the basics are in order. It won't hurt to check out the rod adjustments and butterflys even though they are probably OK. At least you'll know what is what and be able to troubleshoot your system for as long as you own the car. Once the fundamentals are known and out of the way, you can begin as stated above. You will finish up by compensating for your adjustments for part load by fine tuning the idle.

What type of exhaust are you using? Some people have had trouble with radically different exhaust and MFI in the past.

I will second what Milt says about messing with the mixture untill you are certain that all the basic adjustments are in order. I have seen throttle plates out of synch also cause a surge. Usually the surge in MFI cars occurs around 3000rpm under light load but your 4k surge sounds the same as the one Grady refers to above. It is very common on MFI cars but can be elliminated. There are many variables in the MFI equation and only when they are ALL within spec does the whole system have a chance to operate as designed. The mixture is the last thing to be adjusted during MFI set up.

Good luck, Scott

Power,

You are right on. I screwed up big time not discussing exhaust systems. I haven’t seen every exhaust. With my lame and limited experience I have never found an exhaust for MFI that worked as well as the Factory OE muffler.


Jeff,

If you don’t have a Factory muffler, go borrow one for your CMA. Once everything is working properly you can always change back and then try and re-adjust.


Milt & Scott,

I agree on the rigor of CMA. However, there are issues that CMA doesn’t address and, once you have verified basics, there are several sub-loops that speed adjustment. I would like so see an addendum to CMA that reflects all that has been learned since ’74. Another book for Wayne.

Best,
Grady

Thanks guys, everything you mention makes sense. If this were one of my old carburated cars or bikes I would begin by fattening up the mid range. It does feel very much like a lean mid range problem, but nothing in the MFI tuning literature I've read says specifically how to change mid range mixture. As some of you mention, and I suspected from my reading, all of it is pretty inter-dependent. The car does have the stock exhaust, original Bosch distibutor with points, original CDI, and whatever Bosch plugs come in the "maintanence kit" that Pelican sells. The plugs do look a little black and sooty, and in the old days I would conclude it's running rich. With all the talk about not relying on plug readings these days, and a surge that says it's lean, I'm not sure what they are telling me. Could it be generally on the rich side with a lean condition only occuring in this very narrow range? I have already checked the rod lengths and individual butterfly openings; the only things not checked are syncronization and exhaust (waiting for the tools...). The idea of driving around and trying different adustments, after everything checks out otherwise, is what I have done for years on carburated stuff. I have always felt I can fell more through my backside, as far as throttle response and general drivablility, than I can by tuning in the driveway. Kind of "old school" I guess. Maybe I'm just too picky and the simple "drive at 3000 in 3rd rather than 4000 in 2nd" will be the best solution in the end, but it sure sounds nicer at 4000 in 2nd... Anyway, it gets great milage, about 17 average, and generally runs very strong. No passengers ever notice the surge, so it is subtle. When the tools arrive and I'm able to play with it I'll let you all know how it goes. Thanks again.

Jeff;
The surge I have often felt in the MFI cars could almost be mistaken for slightly uneven pavement; and if so, often passengers can feel it. I also agree that the old butt dyno is very acurate if it is an educated butt.

Grady;
It has been many years since we spoke. Probably 1973-4 when I was working for Don Weingarten at Performace Development and Racing in Denver. You probably remember him if not me. I went on to work for John Eisenbud after he left Rennenhaus. Anyway back on track with this thread:
I very much agree that the CMA is not the best diagnostic/adjusting alogorithm. I too think it leavesa out some crucial info. In fact over the years I have developed a modified one. My theory on the initial throttle plate adjusting/synching is that at very small throttle plate angles the relative differences in plate angles is proportionately larger than at larger angles. Different cylinders are getting different charge amounts and this difference is large at small throttle openings. I THINK that is why surge often shows up at light loads but goes away when you push it. This surge can feel like mid range leaness but doesn't go away when you fatten up the main enrichening screw. This may be why Jeff's plugs are black but he feels lean surge. I have felt this same thing many times. So that is why I recommend taking extra care to set the throttle stop screws to ensure initial throttle plate synching. I have seen this step done sloppily often enough that now I check it first.

Jeff; you sound like you have all the skills necessary to do this job.

Does this stuff make sense?

Good Luck, Scott

Since we are talking about throttle plate synchronization, what method are you experts using to make this adjustment. I've read about two methods....one is to close the plates until they barely make contact with the throttle body, then ad 3/4 of a turn to open all plates up the same amount. Second method required one to set all of the air bleed screws to the same setting( 2 full turns out from fully seated) and adjust the throttle plates to flow the same amount of air using a synchrometer.

On a newer system where wear of shafts, plates, and throttle bores is neglible, it seems sensible that method 1 would work well. However, on a system with more wear it seems logical that method 2 would be more effective since it would be more inclined to compensate for wear.

Anyone have better methods?

More great advice guys; thanks. Intuitively, it would seem the relative position of each throttle plate when they are almost closed, i.e. at idle and low load part throttle would be more important that at greater throttle openings. In other words, when they should all be 5% open but some are 3%, 6%, or whatever variation, there will be a greater impact on what charge each cyclinder is seeing than if they were all supposed to be 90% open but some were 88%, 91%, etc. I think the physical postition of the throttle plates can be identical, and due to manufacturing tolerances, still flow differently - hence the air bleed screws and syncrometer. In the end it seems to me both methods are essential; position of the throttle plates must be identical and air flow must be checked with the syncrometer. Now, with all of that said, it also seems to me that all of this could be set exactly right, and it would still surge. I'm having a hard time picturing how a surge could be a syncronization issue. Granted, I'm new at this, so maybe I'm off base a little. It seems there would have to be a lean (or whatever) condition in all cylinders, so they are all affected, to get it to surge. I'm still leaning (hey, that's a pun) towards an overall mixture issue that surfaces at a very narrow range around 4000 rpm. Does that make sense?