My 250 sucks goats

[Michael A. K. G]

I've been trying to tune up this 1972 250 since I got it a few months ago. So far, I've replaced most of the ignition system (except the coil), rebuilt the carburator, reconstructed the mutilated air cleaner (still no blend door, though; it's stuck on full cold), timed the ignition, and replaced the manifold gaskets. It is still unacceptable.

Here are the current symptoms:

• Engine stalls if idle is set below 900 RPM.
• Vacuum is only 15 inches (low elevation), with a tiny jitter (< 1 in) at all speeds.
• Throttle response is very, very slow (but it doesn't stumble).
• Power is barely enough to pull the truck's own weight over the hills I live in.
• Occasional stalls at stop signs, especially coming off high RPMs.
• Exhaust manifold seems very hot; 1200 deg manifold paint is bubbling and flaking off.
• Compression is a somewhat high -- 155-170 PSI (110-130 PSI on first stroke, mildly improved "wet").
• Blowing compressed air through the plug holes makes hissing in the crankcase only (but this was not a calibrated leakdown tester, so I don't know if it was excessive).
• Adjusting the idle mixture screw seems to have no effect whatsoever.

Dwell is a nice steady 33 deg, and timing is 4 deg BTDC. All the spark plugs are firing (checked with a timing light on all six wires).

I'm at my wits end on this; I've done everything I can think of. I've rechecked the metering rod adjustment on the Rochester Monojet carburator several times, as well as the various choke adjustments. I replaced the choke pull-off and vacuum advance because both were leaking vacuum. Most recently, I replaced the throttle plate, since it had large gaps at the pivot edges when completely closed.

Is it possible to leak vacuum from the charcoal cannister? Does the power brake booster not hiss on this vehicle when it's leaking?

Is there anything obvious I'm overlooking?

TIA.

[Hooter]

well it is a 250 in a big chevy truck. those motors barely push a chevelle. I would go out and buy a whole new rotchester monojet

[Hooter]

take the chacoal canister off. I hate those things. you are probly getting a vacum leak through that thing

[tom hand]

I would try running more timeing. Forget the light, run as much as the motor will tolerate. Advance it till it "pings" under load and then retard it till it just stops. You may be surprized how much you can run.

[cableguy0]

to me it sounds like its starving for fuel. did u put a new fuel filter on it. or does it even have one. also as tom hand said bump that timing up u should be able to run at least 8 degrees advance with no probs

[stelth2002]

I would increase the timing like they said above. I can run almost 40 degrees on my 292, thats with the intinal set at 13. I would most dinfinatly take that hunk of junk charcol can off and that will most likely emilinated any vacucum leaks. Possibly look at your power brakes. Thats why i like my truck manual brakes and steering. Part of your problem might be your exhaust valve that heats the intake. Maybe try to bypass that and then just let her warm up a few min before you go and drive, see what that does. I would try and find a differnt carb. They are not a excellent carb to begin with, and make sure that your choke is opening all the way. This could lead to some drivability problems and high temp. Maybe give me and email at nt_expert_at_hotmail.com Good luck, my 292 has gave me its fair share of problems, but I am quite satisfied with it now. It has great torque and economy is above some of the trucks now.

[Desert Rat]

do those carbs have that little stone filter inside them?if so check that?sounds like you have some vacuum leak issues.Carb bushings wore out,float level set right?Id play with timing and ignition sytem.If motr getting really hot sounds like a lean condition,has it been sitting?if you dont have a clear fuel filter on it,pull metal line see if it gushed mass amounts of fuel through,or crawl up and make sure the squirters are shooting good streams of fuel.If you want a good solution a 250 is a dinky motor for a truck about 204 more cubes might be in order??

[racedvl]

Sucks goats!!!! LMAO!!!!

[Knoxville71shortbed]

I had most of the same problems with mine. Be patient but persistant. I didn't have any luck until I bought a new carb. And I would strongly recommend buying one set up for manual choke. If you are comfortable using a manual choke, they work a lot better on these engines than the automatics. I would also remove the canister. If you haven't installed a new fuel pump, you probably should. It's not that expensive, and at least eliminates the possibility of weak pressure. Replace the fuel filter at the same time. And I agree 100% with Tom Hand's recommendation regarding timing. Throw away that timing light, and use the method he describes. If you spend the time, you'll be surprised at how well these I-6s really do run. I drive mine through the curvy, hilly, roads of east Tennessee, almost 100 miles every day. Never had a need for more power. Of course I have the 3-speed tranny, and my truck is a SWB. I can't say how a LWB with an automatic tranny would perform. But I really like my I-6. Trust me on the new carb with manual choke. It made a world if difference with mine.

[Bernie]

I had much the same with my 250. If you have a vacuum gauge you can turn the distributor to get maximum vacuum, then back it off a bit. Also, I could never completely cure a vacuum leak where the manifolds meet the head. If you spray carb cleaner around the joint you will be able to hear if the engine speeds up.

[Michael A. K. G]

BTW, this is a 3-on-the-tree, on an LWB.

I'll try the timing suggestions, and try disconnecting and capping the charcoal cannister. The vacuum-dependent timing trick is something I've heard for adjusting modified engines. Any idea why this would be necessary for a stock 250? Retarding the timing isn't very good for emissions.

I can check the power brake booster for leaks with a big pliers in 5 sec (just squeeze the @#$% out of the vacuum hose and listen for changes in idle), and I will do so. Converting to manual brakes where I live is not an option. I wouldn't be able to even back the truck out of my driveway without it getting away from me.

The EFE valve works fine, and the carburator has been rebuilt with the Borg-Warner kit (including checking float level and metering rod adjustments). The cute little fuel filter was changed at the same time.

Thanks.

[DJH]

sounds like you need a big block!!!

[Michael A. K. G]

Why? Got one cheap?

If the 250 needs a rebuild, I will chuck it and rebuild a junkyard 350 instead. But I don't think that's the case.

[chevy lover]

I helped a friend of mine build a 250 just for sh!ts. Damn thing ran like a rocket. It was in a 71 Nova. We put a th350 behind it. Just to give you an Idea of what we did withoput going into it :

[COBALT]

Yeah, either that or get a refurbished 4bolt main 350 from Cummins.

I had some similar problems with my thrown-together setup in my truck. It's an '80 350 with a '69 327 2bbl intake on it. The carb had some problems so I rebuilt that, swapped the intake for an identical one that wasn't having oil seep up around the intake ports every damn minute, and advanced the timing as far as I could push it. Turns out it's always starving for fuel and air. That's why I had to advance it as far as I did. There wasn't enough fuel/air charge in the combustion chamber when it came time to burn it. Down side to advancing is not completely burning the mixture. You get dirty pipes.

Now the thing runs like a top. My next move is to graduate to a 4bbl performer intake/carb. I'm thinking about doing that after I get the transmission resealed next week. After that I'm going to rebuild a small-block for the first time by myself.

Baby steps.

[Michael A. K. G]

Well, I've eliminated the charcoal cannister, power brakes and CEC solenoid (disconnected all three from the carb and capped off the nipples; no change). I could try the PCV system, but all the parts are new.

I tried the timing trick. It seemed to increase the vacuum and the RPM (wildly). Backing off the throttle stop made it stumble and stall at 1000 RPM instead of 900. So I still can't make it idle right.

These are nice suggestions. They just aren't working.

The fuel pump suggestion has me skeptical; it's the job of the fuel pump only to get fuel into the bowl. Every time I've pulled the air horn off, I've found a reasonable amount of fuel in the bowl, so I'm having a hard time believing this is fuel starvation.

It makes me think I screwed something up in the carburator rebuild. This is my first carburator; I'm much more comfortable with electronic fuel injection. Are there any obvious pitfalls I may have jumped into?

[Fred T]

Lets see if we can help you out. Don't think that I'm trying to insult you, just making sure all the bases are covered.

First, double check your firing order. 15 is to young, 36 is to old, and 24 is just right.

Check your fuel flow. Disconnect the fuel line at the carb, and use a pop bottle to catch the fuel. Also, disconnect the coil wire. Have someone crank the engine while you watch the fuel flow. Good, strong steady stream, you're okay. Hook it back up.

Next, cap off all of your vacuum ports, yes, I said all. Now how does it run? Can you slow the idle down? If this works, try reconnecting the appliances until you find the culprit. If not, get out your propane torch.

No, I don't mean to set the dang truck on fire. With the engine running and someone in position to turn the engine off just in case, turn the propane on without lighting the torch. You are on a mission to find vacuum leaks. If you find a leak, the engine will speed up with the propane being sucked in. Check around the intake gaskets and the base of the carb.

If the carb base is leaking, it is most likely the throttle body shaft has worn the hole in the base. This can be bushed. If the manifold is leaking at the gasket to the head the fix is fairly easy. Pull the manifold and clean the mating surfaces. Put a coat of ultra-copper rtv on both sides of the gaskets and re-install your manifold set. After you have tightened the bolts, let the engine set for 24 hours to be sure the sealant is cured. I will only use the Permatex sealant, I've had to many problems with the off-brands. I have had very few sixes that this method didn't stop a leaky manifold gasket, intake or exhaust. If the manifolds are warped to bad, either scrap them or go to a machine shop and get them to put it on their belt sander, takes about 5 minutes to surface.

Another thing to check is your valve adjustment. Also, if you don't have vacuum leaks you can time the engine with your vacuum gauge. I haven't done this with an emission vehicle, but on an older engine you can get as close as a timing light will. It will at least get you close enough to know where you should be.

Hope this helps.

[Michael A. K. G]

Don't worry about insulting me. If it's a basic mistake, it's a basic mistake.

The firing order was my very first thought, and it's right (it's stamped right on the intake manifold). That mnemonic is cute.

I do have a propane torch on hand (one of the little dinky hand ones). I'll have to try what you say, though getting propane to the underside of the intake manifold will take some contortion. I presume you just need a little burp of propane to do the test. I suppose I could pop some into the air intake as a control, to see how much it takes and what it sounds like.

I already did the valve adjustment, with the engine-on-valve-cover-off method (not nearly as messy as I expected, using the Lisle oil hole plugs). If I blew that, wouldn't it have shown up in the compression test?

[Michael A. K. G]

Well, I assumed that the problem was a vacuum leak; it sure sounded like it.

I removed the manifolds, gooped up a new manifolds-to-head gasket (with aircraft goop -- nasty stuff -- on the intake ports and ultra copper RTV on the exhaust ports), and put it back together. Only a very modest difference (vacuum got a few inches higher).

Spraying brake cleaner around the base of the carburator did nothing. Removing all the vacuum accessories (including the PCV system -- if it had an intake or carburator port, I removed it and plugged the port) similarly had no effect.

But here's the kicker -- the intake ports had PUDDLES of liquid in them. Presumably fuel. And the intake is sputtering some.

At least one intake valve had deposits built up inside the intake port; I couldn't see them all, but I assume the others were similar. But this isn't enough to prevent the intake valve from seating, since I still get good compression.

And now I can idle fine down to 700 RPM, but I can't KEEP it there. It goes up and down by a few hundred RPM, over several seconds. Perhaps this is the choke pull-off responding to the low vacuum?

Cranking the timing up to 8 deg BTDC made only a modest difference. Still unacceptable; when the truck comes off a load (even a modest one), it stalls. Though it doesn't do that unloaded.

WTF is going on here?

[Fred T]

If I had to bet money on it, I would say you still have something wrong in the carb. Know of someone else that you can borrow a carb from to see if it helps? I just had similar problems with a q-jet and made the mistake of putting a kit in it. Cleaning out a tablespoon of fine red Oklahoma clay opened up vacuum leaks and port leaks, I ended up with fuel running out the lower shafts and bought a Holley.