1970 911S low RPM cut out, drops back to 1000

bigcity911s · 03:08 AM

Thank you for input in advance -

Great day in Houston, start the 911, warm her up with hand throttle, once warm if I keep foot on accelerator a little, under 2000, about 1500, she will cut out and drop back to around 1000, then climb right back to where she was? Has never done this before, she has sat for about 5 days. She drove fine, about 30 miles after this, but you can feel it sometime when in traffic.

CDI Box issue? when I was back at the house, doesn't always do it, intermittently while I tried to replicate for 4-5 minutes.

Thank you.

Marc

john walker's workshop · 02-27-2022, 07:04 PM

Is the adjusting screw prematurely depressing the micro-switch, like screwed in too far?

bigcity911s · 03:10 AM

JohnWalker,

I thought about that, but I didnt make any adjustments to that screw or linkage... I operated the linkage by hand and it doesn't happen nearly as much. I will check engagement of he micro switch later this week, and the screw. Doesn't happen every time, it will not happen a couple times thru a testing cylce, but your idea makes a ton of sense. I have put about 700 miles on the car in last two months, only had 800 in last three years before I purchased.

When it drops, it then climbs immediately to where it was, like she is shutting down and then climbs right back.

Thank you for the idea!

Marc

bigcity911s · 02-28-2022, 08:33 PM

The more I read I am thinking it is rated to the Speed With on the MFI S? Any thoughts? The RPMs line up pretty close to the 1500/1300 bands for that switch. I never touched the micro switch screw, could the Injection Speed Switch / RPM Relay be going bad and its being energized somehow? Maybe someone with more knowledge of the workings of the system can help?

john walker's workshop · 02-28-2022, 08:42 PM

Pull the wire off the pump solenoid and see if things change. Maybe a clue to help diagnose.

bigcity911s · 03-01-2022, 04:08 AM

JohnWalker

I will try that, that should be one of the two solenoids on the MFI pump itself? It has only happened once, I will check again when I fire her back up this weekend for a drive.

Marc

356RS · 356RS's Garage

Quote:

Originally Posted by bigcity911s

JohnWalker

that should be one of the two solenoids on the MFI pump itself?

Marc

If you have 2 solenoids on the pump cover then you have a 69 pump. The lower solenoid is the one John is suggesting. A 70S should have only 1 solenoid on the pump cover. If just 1 solenoid on the cover, pull that wire off as John suggested.

bigcity911s · 03-01-2022, 12:01 PM

John and Mark,

Thank you BOTH. Stayed up late last night looking at the circuits online. I have a true 70S pump, just one solenoid. I had also had a "stumble off idle" when applying the throttle so when I started and warmed up today, I couldn't get the same issue to repeat. I did clean ALL the contacts in the system, my CDI ground was in need, so did the others. I turned the micro switch screw out one full turn and locked. When warmed and then drove, I had no more "stumble" and I could not get the same issue to reoccur so maybe that first thought you had John was correct, or the contract was bad in the system somewhere. Thats my thought or a combo of the two since screw had not been touched, but was engaging early it seemed to me.

Also looks very like someone has been in there before (MFI Pump) by the paint markings.

Will keep you both updated on future issue, but learned a ton in the last 48 hours, THANK YOU!

Marc