My 1987 Class C motorhome on an E-350 chassis with a 460 has developed a problem I cannot identify, but it seems like a timing problem to me. I usually set the cruise control at about 65mph and it runs along quite well, but the last trip I would occasionally get a small backfire, that I could hear, but not feel, just a little "pop". Also, once when passing a line of semi trucks, I kicked it up to about 72mph to get around them and all of a sudden it just starting slowing down. The motor sounded smooth, just no power. I was about to jump in my Ranger I was towing and PUSH the motorhome! It dropped down to about 55, and I fell in behind the truck to let other traffic pass, and it would then accelerate and I passed the truck at about 75 or better after that. Power on the hills has dropped too, but it seems ok sometimes. I first thought it was a fuel problem, but I changed the filter and checked flow out of the fuel line and it looks good. The timing at an idle is fine, the mechanical advance seems to be working, but doesn't the Dura-spark module control timing too? Of all the Fords I've owned, the only problem I've had with a Dura-spark was no spark at all not an intermittant problem like this.
Any help would be appreciated.
Ken
90 Ranger 4X4
91 Explorer 4X4
94 F-150 4X4
87 Ford Moho (ailing)