Engine miss fire @3,500 rpm ???

[scooter55]

It starts no problem in an hour. I tested the pressure and it does go from 30 to 40 when I block off the vacuum .

[majicwrench]

When it does not start you have an opportunity to find out why!!! TEST THINGS WHEN IT IS NOT STARTING. Does it have spark?? Do you have pressure at the cold start inj?? Do you have inj pulse??? DOn't just let it sit. Find out why.
Keith

[scooter55]

Keith, thanks that was my plan I was waiting for my wife to crank the motor over while I tested those things. While I was waiting I decided to change the temp sensor on the front of the motor. I had changed it to an old one I had during my first problem trying to find what was causing the miss at 3,500 rpm when I changed it the first time it did not solve the problem,so I left it in. Well guess what it must have just craped out because I changed it back to the first one and it runs fine. I took it for a test ride and it did not miss. I have the cold start valve blocked off for now. I am thinking it could have caused the miss because I just filled the tank and found I got 14 mpg. Now granted it is a fresh rebuild and I am sure the motor is tight . But I can't help think that the cold start valve was leaking. I will drive the car with it blocked off for a while and see if the miss returns. If not I will hook it back up and see if it returns. What do you think?

[majicwrench]

Methinks you have already watched the CSI and it does not leak. If it did leak, it would cause issues way before 3500 rpm.
Methinks you have already dinked with temp sensor before, and if it was bad, would cause an issue throughout rpm range, not just above 3500.
With it running so funny no way you can figure MPG, why even try? Well built motor should not be "tight" and should not affect your mileage, urban legend.
Hopefully tho something has magicly straighted out. But just randomly changing things, like back to the old temp sensor, is not the way to go. Get that wife on the job when car quits. Methinks it will again\, but keep fingers crossed.
Keith

[scooter55]

Keith, an up date I have been driving the car and it has not yet miss fired. The cold start valve is blocked off. Here is my theory on what is happening. I think the sensor that tells the cold start to fire is heating up and firing the cold start valve . Only way to tell is probe the sensor or replace it and hook the cold start up and keep my fingers crossed. What do you think of that theory?

[scooter55]

I just re read my past post and realized that I already did drive the car with the CSI pluged off and it did miss so I guess my theory is not sound. I will drive the car and expect it to have a problem because nothing I did should have fixed it for now it is running good . I will put more miles on it and check the plugs. Or break down on the side of the road!
I will keep you posted

[So Cal Mark]

is the #3 plug still fouling? The fact the motor was recently rebuilt somehow escaped me. Was the head rebuilt also? Can you list the procedures done to rebuild the motor and head?
The temp sensor is affected by a loose connection. If the pigtail is not in excellent condition it can cause lots of trouble

[chrisfiat]

Keith, an up date I have been driving the car and it has not yet miss fired. The cold start valve is blocked off. Here is my theory on what is happening. I think the sensor that tells the cold start to fire is heating up and firing the cold start valve . Only way to tell is probe the sensor or replace it and hook the cold start up and keep my fingers crossed. What do you think of that theory?

the cold start valve i run by a thermo timer, which only gets it's power during cranking, so unless you have a major elec malfunction it will not energize while driving.

[scooter55]

Mark I will try to get more info on the head work. I know new valves. Guides were good , decked a minimal amount. Cam lobes checked , new seals .
Chrisfiat thanks for the info on the cold start. I thought my theory had some holes in it. I did not have time to run the car today so no new updates . I will let you know if it still has the miss after some more testing . I will also do a plug pull after I get some miles on the new set of plugs.

[scooter55]

An up date and new theory . I took the car for a long ride and it did not miss at all. I duplicated all the things that made it miss before and to my surprise no miss. I still have the cold start injector blocked off, but I don't think it is the problem. What I think it was is a bad connection on the #3 injector or the ect connector The reason I think that is that pulling off the fiel rail and switching the #3to#1 injector or the etc connector was the last thing I did befor the miss went away. I will put more miles on the motor and then check the plugs. If all is fine I will hook up the csi and try that.

[majicwrench]

As a wise man mentioned earlier, bad connectors at injectors is common. With engine running give em a gentle wiggle. They are simple and cheap to replace, if you need the part # let me know.
Glad to hear it's running, oh what a feeling..................

[crazyfiatguy]

Hi - for my 71 1608cc, I had bad misfiring upon accelleration and found my centrifugal advance mechanism gummed up; maybe in your case it gets partially stuck and doesnt release untill higher RPM? Just a thought. Distributor dissassembly & relube is a good thing to do anyway.

Hank