MFI to Zenith problem

7483cab · 7483cab's Garage

Just changed from mfi to zenith setup, installed pressure reg. and blocked all vaccume. pressure set at 6 lbs. with timming set at 12btc @1000 and carbs synced I get spitting and popping, seems to be real fat. any help? Thanks

Christien · Christien's Garage

My guess is you've got the pressure set too high. If I'm not mistaken, you need it at 4.5 psi.

Chris

Jeff Higgins · 06-08-2005, 05:17 PM

Not much to go on here. Why did you pull the MFI off? Was it running poorly with the MFI? How about some more info on your car - year and size of engine, jetting in the carbs, which distributor (12 btdc sounds way too advanced for a car that had MFI originally); stuff like that.

jim72911t · 06-08-2005, 05:41 PM

First, welcome to the board. I've done the same thing to my '72T simply because I couldn't afford to R&R the MFI (~$2000 to do it right), and I picked up a set of carbs cheap. The car runs ok now, but my jetting seems a little lean.

Do a search, there is a ton of good info in the archives.

More info on the car would help, too.

Good Luck,
Jim

7483cab · 7483cab's Garage

Thanks, the engine is a stock 2.4 911t and switched for the same reason Jim, the car starts right up but I cannot get it dialed in. the person I bought the set up from said it was commom to dissable and plug all vaccume. Maybe I will come off the timing a bit and lower the pressure?

jstobo · 06-08-2005, 07:25 PM

Your timing is way off for a 2.4. Stock setting is 5 degrees After TDC at 900 + or - 50 RPM. At 6000 RPM timing should be 32-38 BTDC. You are way too advanced. If the timing is this far off you will have trouble dailing in the carbs. What size venturis are you running. Some Zenith's come with 28.5 MM. You might have to increase to 30 or 32 for your 2.4. You can use weber venturis if you buy or make a spacer that fits on top of the venturis because weber venturis are a little shorter. This is what I did. I Increase venturis to 30mm. You may also need to change the jets. I put bigger ones in. I bored out my cylinders to 86mm to get 2.3 L. Your engine is only 41 cc bigger(2341cc). It may take some trail and error but you'll get it.

7483cab · 7483cab's Garage

These are stock zenith 40 TIN carbs, all vaccume is capped including the advance and enrichment devices, as perscribed by the person I bought these from. Tried different fuel pressures.. no differance. Set timing to 5 b to 5 atdc..no differance. Valve clearance is good @ .004 ? Thanks