MFI part load adjustment

TargaT · 05-08-2007, 05:52 AM

Hello, I have determined that I need to make an adjustment to my part load on the pump. I have removed the Allen bolt and made the long adjustment tool as described but cannot seem to make a good connection with the adjustment screw. Can anybody give me any pointers or should this be fairly straight forward. I believe the screw inside is a thin slotted head screw about 4mm across? I just can't seem to find the slot and can't tell whether I'm actually turning the screw or not. I understand it is supposed to click but I can't feel any. It seems to be a little gritty in this cavity.
Any insight would be appreciated.

thanks, Joe

304065 · 05-08-2007, 04:46 PM

Joe,

You need to have the screwdriver absolutely horizontal in order to fit it into the screw inside. Your screwdriver must have parallel edges on the tip in order to fit.

Have you gone through Check Measure Adjust? Remember "I Lean Left" for "idle mixture lean to the left" and then you remember that part-load is the OPPOSITE of the idle setting, so you turn it to the RIGHT or CLOCKWISE to lean.

Now, HOW exactly are you going to measure the impact of whatever changes you make? You really need an LM-1, either with a sensor in the bung or with a temporary sensor on the tailpipe, in order to verify that you have the correct mixture under load.

Search under "ultimate MFI resources thread" for more information about tuning MFI.

TargaT · 05-10-2007, 07:02 AM

Thanks John, I haven't had a chance yet to tweek my tool. Hope to do that today and give it another try. I made it out of a long piece of rod with a 90 on the end which extends past the fan. Maybe the tip just needs to be ground down a little more.
I've got a Gunson unit which I borrowed from a friend which I will use as a reference since I know it is only good in a stationary mode. The main reason I assume it needs to be richened is that the car was running poorly after warm up ie. stumbling popping etc. When I removed the preheater hose to the pump the car ran well and remained that way after warm up. The plan is to take an initial reading after warm up at 3200 then move a few clicks at a time and test driveability each time monitoring the change in CO. From what I've read people have indicated a CO level in the 7% area is about right under these conditions. Any comments from the MFI crowd would be welcome.

Thanks, Joe

304065 · 05-10-2007, 07:30 AM

Joe,

If you remove the paper heater hose to the thermostat, the mixture will go very rich. About 10:1 based on my experience. A properly adjusted thermostat will have the engine barely running with a very rich mixture, and then as soon as the discs hit a certain temperature, the rod expands enough to shut the enrichment off. You know it's working perfectly when you hear the engine rpm change, then turn off the engine and it cools just enough, then you start it again and it's rich, then after a few seconds it leans out again. Don't do that TOO much, however, the rich mixture tends to dilute the engine oil with fuel.

I used to have a Gunson gas tester, I found it very sensitive to setup. If I were using one today I would have a second 12V battery to power the tester in the garage, then drive the car to operating temperature, pull in and the tester should have normalized at 2.0%.

For MFI a rule of thumb I have heard is 5-6-7-8 or 5-6% CO at idle and 7-8% at 3000 rpm under load. That works out to be about 12.5:1 AFR and richer, according to this table: http://www.perfectpower.com/Technical_info/afr.asp

Way rich I know, that's what I've heard works. Of course there are tables in the back of Check Measure Adjust that give you the correct reading for a particular air intake temperature. These were when the engine was NEW, however, which is what accounts for the difference.

7% CO is an AFR of 11.89:1, slightly richer than the best power AFR.

So the engine likes it rich. Something to check is whether you have an air leak around the throttle shafts. These wear over time, and can bleed air into the throttle body, leaning the mixture out. Of course, you should run through the entire CMA routine first, because like Carburetors, many MFI problems are in fact ignition problems. The "Correlation" step comes before smog, and if you have a high airflow through one of the stacks, that should be fixed before touching the pump.

Good luck!

TargaT · 05-10-2007, 01:21 PM

Well, I still can't seem to make a connection with the adjustment screw. Has anybody encountered this before. I've tried everything from a ground down metal rod to a flattened coat hangar. Does anybody have a pump laying around to measure the dept to the screw? With my home made tool it bottoms but it won't find the slot. When I use the coat hangar with a little more pressure it seems to go about an inch deeper. Am I going around the screw? I wish I had a good picture of the screw. Shining a light in there doesn't buy me anything as it is too far.

Help please!

Joe

TargaT · 05-10-2007, 01:37 PM

I think I'm going to have to take the pump out to see what's going on.....
Joe

304065 · 05-10-2007, 01:41 PM

Try a small electronics screwdriver. There's definitely a slot in there.

I strongly recommend that you take a baseline measurement of the CO before adjusting anything.

TargaT · 05-11-2007, 06:25 AM

Got it! right screwdriver, right angle of approach. Taking the air cleaner off helped. Next problem I need a working tach to do my CO measurements. My mechanic told me mine was faulty so I got a used one through Pelican I hooked it up last night, still no RPMs. So do I have a second faulty unit? or do I have a different problem? Seems to be lots of info regarding this on the board so I will search that first. At the same time I installed a Pertronix points replacement unit. From my understanding this should not affect the tach?\

Thanks, Joe

304065 · 05-11-2007, 06:56 AM

What does the tach look like? What is the date on the back? Need to know the year and model of your car in order to tell you whether it works. I have written extensively about tachometers here:

Tachometer Compatibility Chart

TargaT · 05-12-2007, 11:47 AM

Well, I don't know how much 3 clicks on the pump equates to but I richened it up by that amount and the car is completely different. No flat spots no popping, just pulls clean right through the power range. I never did a base line with the analyzer since I was so excited just making the connection on the adjuster. I will take some measurements tomorrow.

Joe