72 stepvan - slow up hills.. rough idle

[Mgicm]

HI
Hoping for some advice on this 72 chevy stepvan i have. Motor is a 250 six and its been to no less than four mechanics. Problem is rough idle, and slow as heck when encountering even the slightest hill. I lose about 15mph going up a hill when traveling 60 mph. It has always seemed to be like this and nobody can figure it out. We have changed plugs, wires, cap, rotor a few times. Had three mechanics time it which I dont think any of them did right. Vacuum checked out ok so Im told.

The rough idle is odd. Its only when you start of from a stop or at gear change that it is sputtering or shaking. Once you get into the gear it seems to run well. Changed fuel filter 2x and pretty much the same. Changed gas, and gas grade with no effect.

I cant tell if it is a fuel issue (pump, carb, line, filter), vacuum issue, or more to do with timing/distributor. How would you diagnose or troubleshoot?

Sure appreciate it.

thanks GP.

[geezer#99]

First thing I'd do is check the timing!!
And not by a mechanic!
Do it yourself.
Get a good digital light and learn how to use it!

[Jurassic-1]

I would pull the cap and check condition of the points. Also if it is sluggish under a load itcould have bad plug wires. Start it at night and see if they arc. I think a good tune up is the ticket. I recommend a compression test when you remove the plugs for inspection. If the compression is good it is all in the tune up.

[Mgicm]

on third set of wires... but will check at night. one odd thing yesterday. I was in a parking lot where the exit had a big hill to drive up to exit. From a stop the van did fine up this hill. Now later same day i was on interstate doing about 65mph. Came upon a hill in the road...a long upward hill but not that steep. truck lost easily 15mph. Was going...but just lost so much speed. Has two chest freezers in the truck which cant weigh more than a few hundred pounds plus me so its not like theres much weight in there. used to have a big freezer which weighed 1000 pounds so i dont get it.

thanks

[factorystock]

Originally Posted by Mgicm

HI
Vacuum checked out ok so Im told.

I would look into vacuum leaks and compression loss a little closer. Check the intake manifold- exhaust manifold gasket, they both share one very long gasket, it is very common for these to warp or blow when the bolts loosened up on these manifolds. IMO, this is why GM went to the integral head and manifold system in the mid 70's. Carb to manifold gasket may be loose also. Is the engine original with high miles? These engines were designed to run on leaded gas which has more lubrication. Unleaded gas in older engines over time tend to wipe out exhaust valves, resulting in rough idle and loss of power. I'm not sure on when GM went to hardened valve seats on 250's or 350's.

[67 chevelle]

hows the transmission

[hugger6933]

My fist thought is vacuum leak. Spray some carb cleaner around the base of the carb and along the intake manifold where it bolts to the head while running and if the idle changes you have found your problem. My first thought was vacuum leak [at the carb mounting gasket or the intake to head mating gasket]My second thought was loose carb. Also on a side note when spraying the cleaner on the intake be a little careful as the intake [since it is mated to the exhaust] is gonna be hot.
Also can you tell if the RPM go up as your climbing the hill? Like in a clutch slipping or do you ever smell anything funny? That is far down my list but there just throwing that out there.
Also like has been said do a compression test it will let you know if you have problems starting inside the engine. Jim

[slomotion]

I'd take off the rocker cover and watch a rocker arm while I moved the crankshaft back and forth. What you want to prove is that the timing gears haven't worn several degrees of slack in them causing a retarded condition under load, and an irratic timing under no-load condition. You can do the same thing by pulling the dist cap and watching the rotor. How many degrees does the crank turn before the rotor moves? If slack is there, your timing will jump around on the damper mark.

[Wrenchbender Ret]

I agree. The next thing you need to do is a compression check. A weak valve will cause these symptoms. If that is OK look into the timing. Try advanceing it a little more then specs. I would not be concerned about the timing gears. They seldom give a problem on these. Although you could pull the valve cover & check for broken valve springs or a valve that sticks up farther from the rest would indicate a recessed seat. These would not show up in a compression test. This engine should pull good & smooth but you can't expect too much in a step van. I don't know what size it is or weather it is aluminum or steel body. These trucks are heavy & not aerodynamic. You can't expect too much from a small 6 cyl. like that.