I've got a 250 that ran great til about a week ago. I was on the freeway and the car started to bog down up the hills. After that it started to having trouble shifting and was making a noticable shhh sound like it couldn't get the exhuast out. I ran a vacum check and it makes good vacum at idle, but loses it at higher rpm begining around 2000 and losing more as it increases. I don't have a cat converter and I unhooked the muffler to see if that was clogged. Nothing!!! I am told that maybe there is something in the manifold that can clog. Like a butterfly valve or something that could restrict the exhasut. I've never seen a 250 manifold off to know what I am looking for. It just seems that it's a pain in the ass to take off. And I don't want to tackle it if I don't have to. So anyone ever had this problem or know a fix or test I should try? Symptoms Good Idle and vacum at idle, Revs fine and sounds healthy til about 2000 rpm then starts to struggle. Makes a noticable shhhh sound when reved. Won't shift when on the throttle. "Let off to shift". Exhaust it very very HOT!!! Plugs don't look like the car is running lean though.
Change the fuel pump. That's the way they act. carb runs low of fuel at higher engine speeds because the pump is failing to pump enough until you slow down again. There may be some water in the gas also.
I had a car one time doing the same thing and come to find out it was an exhaust pipe. I took it to the muffler shop and the guy put a new system on it and it was fine after that. He showed me that some older exhast pipes are a double walled and the inside wall after time will seperate and from backfires or downshifting like from second to first manually they will suck closed. he started cutting the pipe until he found the place where it was restricted to about the size of a half dollar. It could also be fuel related but I've never heard them sound any different, just stumbling. Its sounds like an exhaust restriction to me. Have you ever had a car pass you really fast on the Interstate making that whooshing sound ? The late 70's Chevy Caprices were bad about sucking the pipes together. Also try advancing and retarding your timing to see if that makes any difference. It should be about 10 degress BTDC, but 13 will make it a little more peppy.
look for a Potato in the pipe ..did that to a guys GMC he took to the dealer never could find the problem ........he had the trans rebuilt cause it would not shift after it warmed up ....guy told him the rings was gone in the engine never did find the Potato ..he traded it in
did you check you fuel filter. my car was doing that. thought that the float in the carb was sticking. but it was just a cloged fuel filter. try that because if its that it's just a $3.00 fix.
I've done the fuel filter, points, cap, rotor, plugs, and condensor. I've pulled the muffler off, but not the entire exhaust. I was changing the fuel pump tonight, but before I do I want to run a pressure test on it. I was thinking that maybe the manifold is plugged, but I need to disconnect the pipe at the manifold to find out. This is really starting to suck, but I thank everyone for their imput.
Don't get discouraged and give up! There is a reason this is happening, and ultimately, you will find it. Look at it as a challenge!
thanks again to all who contributed advice. It turned out that the exhaust had collapsted inside the tubing. Basically the inner wall of the exhaust tube fell apart and clogged the exhaust. So right now I'm running just a manifold. ha ha. It's so loud!!! Hope to take it to a muffler shop in the morning. Anyway on the way home from working on the car some guy in a Datsun pickup revs on me. Stock 71 comet with no exhaust. I laughed and we go at it from the light. Man it was slow and speeds probably didn't exceed much over the speed limit, but the Comet owned him!!! Man makes me almost want to scrap my 450+hp Cleveland build and run with the 6cyl. ALMOST.....
