Unusual MFI Cold Start Issue - Help Appreciated

drvee12 · 10-23-2008, 08:08 AM

I know there is tons of MFI tech help on the board, and I swear I have done my best to see if anyone else has ever posed this question, but no luck.

I have a 73 T bone stock 2.4 MFI that ran like crap when acquired. Hours spent following the CMA and all related suggestions have yeilded a pretty good running car except for the cold start, which is nearly impossible to get running in mild So Cal weather. I have repaired the cold start solenoid, works like a charm, so I turned my attention to the thermostat and this is where the questions arise. I have the typical and apparently correct number of expansion discs, (24)a couple of extra spacers than most but nothing unusual. But no matter how cold the thermostat is, when it is installed in the housing it sets the pump plunger to full lean with tremendous pressure. I have removed all of the spacers, still full lean, I have put the thermostat in the freezer for full contraction, still full lean. The thermostat pack will not allow the pump plunger to move to even slightly rich unless I remove 4 sets of disks and all of the spacers, and this cant possibly be correct. The assembly is missing the spring retainer, so I suspect that perhaps someone might have installed an incorrect spring?

Can anyone check my pics below to determine if I have an incorrect spring that is causing too much tension? The digital scale is a little hard to read, but it takes a little more than 13.5 lbs to fully compress the spring. Alternatively, if the spring is correct, do I need to start tinkering with the pump plunger to increase its force against the spring? Dont want to go there before I confirm the thermo spring is correct.

Thanks!!!

pjh69911 · 10-23-2008, 09:20 AM

I once dissambled a pump where the spring retainer was improperly positioned and the pump enrichment was off. The spring retainer should be positioned on the forward end of the spring. The spring is only to prevent excessive force of the expanding discs from breaking the pump mechanism, a relief valve of sorts. Maybe this helps? Paul.

kwikt 911 · kwikt 911's Garage

You sould have the spring cup then a thick then a thin spacer. At least that's how mine is.

drvee12 · 10-23-2008, 09:35 AM

OK, stupid me! Paul, your suggestion is obviously exactly what is going on here. The assembly has no spring retainer at all, the spring is pushing directly on the discs! I see there is a shoulder in the thermo housing which would limit the force of the spring if a retainer was used, but since mine is missing it is using all 13+ lbs to force the enrichment device to full lean at all times. Now, where do I find a spring retainer??

Thanks for the direct hit on my problem.

304065 · 10-23-2008, 10:35 AM

25 pairs of discs, not 24

MFI thermostat data. . . and distaster!

Remember when it gets hot it gets longer.

Correct orientation of spring:

http://www.pelicanparts.com/techarticles/911_MFI/thermostat.jpg

When it gets hot it gets longer. The circlip is on the rod and the discs expand and push against the circlip, pushing the rod out of the housing. The spring is only there to keep the discs together.

drvee12 · 10-23-2008, 10:55 AM

yep, actually have 25 pairs of discs, so that checks out. No retainer pretty much makes the whole thing useless, so once I find one I should be in good shape. Maybe I will need to make one by finding a washer that will just fit the housing and stop at the inner flange for the spring retainer

pjh69911 · 10-23-2008, 11:19 AM

you certainly could make a spring retainer if you can't find one. The thickness is really not critical as it is merely an overload protection function.

kwikt 911 · kwikt 911's Garage

But then again the thickness of the 2 washer spacers are critical or at least in my mfi. Too thick and it hesitates too thin and startup is way rich. So I check mine by removing the top cap on the mfi pump directly behind the thermostat. When fully hot try pushing down gently on the armature with a small flat blade screwdriver. If there is any play( downforce movement) add a slightly thicker shim. Check it out!!!

Kevin

pjh69911 · 10-23-2008, 05:07 PM

Kevin,
you are correct to note that the thick and thin washers are important as they contribute to the total stack height of expandable plus thick and thin washers. The spring retainer, however, is larger diameter and its thickness is not important as it does not contribute to the stack height.

drvee12 · 10-24-2008, 10:32 AM

a 14 mm washer seemed to be a pretty good replacement for my missing spring retainer, so I put it all together and the cold start issues have been resolved. This board and its invaluable MFI section having been my main resource, I was able to completely transform a 911 that would not start, would not idle, missed at all rpm's and backfired relentlessly into an absolute joy to drive, quite possibly better than new! What a great community!

Grady Clay · 10-24-2008, 10:47 AM

Quote:

Originally Posted by drvee12

a 14 mm washer seemed to be a pretty good replacement for my missing spring retainer, so I put it all together and the cold start issues have been resolved. This board and its invaluable MFI section having been my main resource, I was able to completely transform a 911 that would not start, would not idle, missed at all rpm's and backfired relentlessly into an absolute joy to drive, quite possibly better than new! What a great community!

CONGRATULATIONS!

It is great to hear another success story.

Best,
Grady