View Full Version : 17.8 volts cause 4000 rpm cutout?

05-11-2005, 09:14 PM
Here is something goofy. Truck info:

78 F250 4x4, mild 400 ( 8.5-1,255 cam,4V intake & 670 avenger with
headers) and a c6.

Issue. When I first got it back together( the 400 was a previously run rebuild low miles- I just tore it down to check the internals were all legit and in good shape) I could spin her up to 5500 ish no problem. After running it for a while over the winter ( not driving..just running periodically as the project progressed) she seemed to start spitting fire and became slow to rev in the higher rpm range ( above 4000 rpm it had open headers) and not wanting to rev all the way. No stange noises, no other issues I can tell.

Now that she has exhaust and is almost ready for the road..she was having issues moving under her own power at the exhaust shop( backfires both carb and out the pipe)

When I got it home and could run it in drive, vacuum disconnected around 650 rpm.. noticed my base timing was out ( looked good when I put the distributor in, and when I had run it no load before the driveline was in). So I set it to 10 BTC and took it for a ride... feels funny, like it's choking ( choke is open, fuel is new, supply is good). Now maybe the new avenger needs setting ( I thought idle screws only affected it at idle ??? ). Haven't checked the ammount of mechanical advance or vacuum yet. May have to re curve this one too.. but that's another issues all together.

I noticed a "new problem" with the battery the other day. It's sitting at 17.8 volts ( not sure how that happens)...re did some wires so it is charging now ( runs with no battery but I forgot to check the voltage with the battery disconnected)

Would the overly high charge or high battery voltage cause cutting out as far as the ignition module or coil is concerned? Just looking for opinions. Once I get the brake issue fixed tomorrow, I was going to pull the cables and/or swap for another battery take it for a ride, seeing if the voltage makes a difference.

05-12-2005, 11:20 AM
Well would you look at that.... so on a tip from the wonderful folks on the net, I re checked my findings with another voltmeter....and it appears that the stange voltage issue was with my voltmeter, as with the 2nd voltmeter I get 12.56 off, 14.5 running... so all is fine. A good/bad thing.... I know one problem is non existent, now I have to look for the other one...lol

I guess I get to look elsewhere for the running issues... I will try running her at 12 then 14 BTC see if that helps some...

05-12-2005, 05:09 PM
are you running the stock duraspark or after market ignition box??


05-13-2005, 09:11 PM
Stock ignition box for now.....

Timing chain is nice and tight....I just think timing was not it was not in the right place as I didn't get to set it in drive, at the correct rpm. ( as when I first got it together there were no driveshafts in it and no exhaust on it..so running it was a chore.) but at that time it was running free with no load on it.. so I don't know why that would have made it run any better.

Another guy told me to do a compression check as I may have burnt valves running it with open headers...??? Once I get the darn brake job completed this weekend... I will re check all of the timing specs, and re adjust as required. Will also try to dial the carb in to get the best vacuum I can out of it and then take her for another run. (I've already re checked it all for vacuum leaks. )

As for the faulty meter.. I was going to try a new battery...if that does not work..chuck it.

I will keep you all posted.

05-13-2005, 11:18 PM
I went through somthing simaler and I put a crane cams digtail 6 ignition box and a ps91 coil and it went away for the most part just a thought


05-14-2005, 08:47 PM
check and see if there are different firing orders do to a change in cam i know that fireing orders can change with the 351 w not sure with the 351 m and the motor will seem to run fine but when you put under load or revit up it will caugh or back fire .
just a thought


Rebulon Refurbulater
05-16-2005, 07:59 PM
if you think its a valve do a compression test simple to rule this out...good luck

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05-17-2005, 06:33 PM
Update.... got my brakes done this weekend..so I turned my focus on the engine.
Lleft the 10 BTC timing adjustment... and went for the carb adjustment as per a recommendation.

A quick adjustment of the idle screws with vacuum guage in hand, and voila...18LBS Vacuum..and a smooth running engine. Revs smothly now to 5500.

So now I took her for a spin, 2nd time out and much better then the first time. However, not overwhelming like I had hoped ( if memory serves me the stock 400 in my last 3/4 seemed to pull stronger and it had 37's on her) Both were 77-79 vintage with 4:11's, the other was a stick. I guess I will get her driving around for a while, make some more adjustments and see how it goes. Could be the single plane intake on this engine...that Weiand Dominator came with the rebuild block..so I used it. After I drive her for a while, I may spring for a different one.

Thanks for all the help guys