air cleaners and improving air mixture on 250 six

[old cool]

So, the weekend is coming to a close and its now time to putz with my 66 GMC 250 manual. I'll try to keep it short but need some tips before I start turning screws.
Carb is new Rochester monojet. Gas mileage is so-so. (heard that's typical).
Truck burns rich and needs absolutely no pumping to start (choke closed when cold). Some bluish gray exhaust when started. Currently I have an original oil bath air cleaner. An acquaintance said a papered air filtered set up might give me a boost in air mixture which made sense. I know there's an air mixture screw but before I start trying to fix something that ain't broke, thought I toss this out there for some TA. In addition, anyone know where I can pick up one of those original wide body air cleaners for the above carb? Got to be a ton out there.

[TJ's Chevy]

You talking about the single snorkel versions? Yeah there out there...you can get re-pop single barrel carb filters. If I was you I'd find a snorkel version and do a cold air intake setup..little more power and mileage that way.

[old cool]

Thanks T.J. I have not found an original looking air cleaner on ebay. Not been able to match any casting numbers other than block. Searching for GMC data is a big struggle
(for me anyway). Think I'll head to library tomorrow.
As a quik switch, I'd consider an aftermarket air cleaner. Any suggestions for nice fitting unit for Rochester one barrel?

[66Submarine]

The "GMC" 250 is exactly the same as the Chevrolet. Not sure how much better or worse an oil-bath type would be, but the later and more common paper type should be easy to find. Besides all the cars and trucks that came with sixes and one barrel carbs, you could also look for other filters with the smaller diameter opening, or use an adapter with one with the 4 barrel sized opening. (One of my trucks has a generic air filter like that on it; IIRC, the only thing to really watch for is clearance for the choke linkage.)

[66Submarine]

And I'd adjust the carb, too. If the truck is running rich, then the adjustment is "broke" (so to speak). FWIW, I have mine set just rich enough to idle smooth when warmed up, and (IIRC) that's just a little leaner than the peak RPM/vacuum reading for me.

Blue smoke indicates oil, BTW.

And FWIW, I haven't really experienced what I'd consider poor mileage with 250's (I think they are pretty thrifty engines, actually).

[TJ's Chevy]

Originally Posted by old cool

Thanks T.J. I have not found an original looking air cleaner on ebay. Not been able to match any casting numbers other than block. Searching for GMC data is a big struggle
(for me anyway). Think I'll head to library tomorrow.
As a quik switch, I'd consider an aftermarket air cleaner. Any suggestions for nice fitting unit for Rochester one barrel?

LMC and The Truck Shop sell mini air cleaners for the single barrel carb..not to expensive either..several people have used them, but like said..it'd be good to adjust your carb a little leaner.

[old cool]

Thanks guys. Here's a few more details. I have a new, B model Rochester. I would be surprised if I get better than 12 mpg. I was told my rear end is 342. I'm sure that affects mileage. I hope you don't mind the constant questions, but here are my exact concerns: below ave mpg's, start ups spray some black stuff, doesn't require any pumping to start; some spark plugs show rich/dark carbon. Upon buying the truck, I was told by the seller to error on the side of rich. So, I have held off on messing with the mixture screw. The other issue is that the dang truck runs quite well under load and cruising. It doesn't ping, or diesel. I guess I can always return the adjustment screw to where it's set, if I experiment. I just like to be conservative before messing with something. Oh, two other points, the inside of the distributor is Petronix and the plugs are Delco 44 xls. Otherwise, all looks original.
I will adjust mixture screw and get back soon. In the mean time, any tips are welcomed.

[66Submarine]

I'd expect a lot more than 12MPG! When my father was driving my C10 with a 250 last, he was averaging about 18MPG. The taller 3.42 gears should be good for economy, too.

IMO, it's WAY too rich if you see soot coming out of the pipe! That's bad not only for mileage, power, and hot restarting, but also engine wear. The seller sounds like he wasn't too good of a mechanic IMO.

I'd definitely adjust the carb before doing anything else. And don't worry about making it ping; idle mixture shouldn't do that, and these are very tolerant engines with low compression and low timing when set to GM spec. I have mine as lean as it runs smoothly and jacked the timing up a good bit.

Let it warm up to operating temp (drive it) and adjust the idle mixture screw in until it runs rough, then back it out slowly until it just smooths out (some will set to max vacuum reading or RPM reading instead, or just lean of the peak readings). You may also need to readjust the idle speed when you do it, and make sure it's idling low (IIRC the spec is 450-500RPM) like it's supposed to when you adjust it. YMMV as they say, but I bet the truck will run a whole lot better after that.

[old cool]

Thank you again for the advise and words of encouragement. I'm looking forward to tweaking the settings and hopefully coming up with an air cleaner.
66SUB - In the order you described, I will adjust carb as follows:
1. Run truck to operating temperature
2.Set idle screw to register 450 to 500 rpm on tach (fortunately I have one on truck)
3.Slowly rotate mixture screw inward until it begins to run rough
4.Rotate mixture screw out until the engine is smooth and stop there
5.Advance timing
Note: I think timing on '66 orig 250 was 4 degrees btc. Later 250's were 8 degrees. I think. I'm not sure what my engine came out of so I'm willing to experiment with timing. Any tips? What setting do you recommend?
6.Do I reset idle to greater then 500 rpm?

[66Submarine]

That's pretty much it.

I really just noted that I increased the initial timing on mine to emphasize that you shouldn't have any trouble with pinging or anything. I think they seem to run better with more timing, but I haven't really done any A-B-A testing at this point. IIRC, mine is at 10º or 12º BTDC now instead of 4º or whatever GM said. I went higher than that and it seemed to like the additional timing, but I decided to just set it there for right now and work out from that. So far I haven't been able to make it ping, so I'll probably do a total timing check and advance it a some more.

You probably don't need to mess with the timing at this point, though. If you do check or mess with it, remember to disconnect and plug the vacuum advance and check that the engine's idling slowly so that the mechanical advance isn't doing anything.

The idle speed doesn't need to be faster than 500RPM when you're done, you just want to make sure it's idling nice and slow like that to make sure the carb is running on the idle circuit. As the speed increases the carb begins running on the main circuit, which would obviously make the idle adjustment harder to make. On stuff with a working choke I set it for about 500RPM at a hot idle. You could set it higher than that if you want, but there's no need to IMO.

[TJ's Chevy]

You don't have to idle it down so far either...Mine idles at 500-650 rpm..I adjusted the mixture screws at 650 rpm...you want to set your mixture..then reset you idle to were you want..then re-adjust you mixture again..a tad richer...good way to tell if your dead on..is..fire it up and drive it hard..don't let it warm up..but take it for a short hard drive, then pull over and pull a plug.

[66Submarine]

Yeah, I just like a low idle (in part due to non-synchro first gears and the like). If something doesn't have a choke I have also set the hot idle to 800RPM or so (that way it will idle before it warms up). I think it should be set low to adjust the idle mixture, though; once you start opening the throttle you may begin to get into the main circuit, which may make adjustments/changes less apparent. (Obviously, something with a larger carb will not be as bad about this.)

Originally Posted by T.J. MCCAULEY

..fire it up and drive it hard..don't let it warm up..but take it for a short hard drive, then pull over and pull a plug.

Actually, that's for the main/power circuit. Also, you are really supposed to do a long wide-open run (pulling a hill or something) and shut it off while still running wide-open, then coast to a stop to check plugs. I certainly recommend a warm up before WOT runs , and the carb should really be at operating temp, too.

[old cool]

Well, I'm moving in the right direction but thought an update was warranted. I was unable to time the truck as the vacuum advance and #1 were out of placement. V-advance was hitting block so I couldn't advance timing. Had a friend lift distributor and reset. We were able to advance timing and I adjusted carb to a leaner setting (about a quarter turn improvement). I need to change my carboned plugs to see just how much improvement I actually have. I'll get back on that.
It's really great we have this forum, but it's also kinda awkward that I don't seem to be much of a problem solver. My helper ran out of time and I'm not sure the distributor is perfectly set. The distrib. hold down screws are at 11 and 5 oclock when looking down from passenger side fender (inline 6). Pictures that I have seen shown hold down screws at noon and 6 oclock. Also, timing was done by ear as the pulley mark was a couple of inches above saw marker.
Go figure.

[66Submarine]

Did you disconnect the vacuum advance and have the idle speed where the mechanical advance isn't doing anything?

Can't say anything about phasing of the distributor (I.E. relative position of hold down screws); never bothered paying that any mind. It makes no difference as long as you can set the timing where it should be.

Check the timing with a light. I'd set it reasonably close to GM spec and work out from there as you see fit. (FWIW, I have mine at 10º BTDC right now IIRC.) Another consideration is what the total timing (initial+mech) ends up at.

Does the engine try to kick back against the starter or spark knock (ping) or anything? (All of this would be indicative of a lot of/too much timing.) It's also possible the balancer has slipped and made the timing mark move as a result; might consider checking for that.

[old cool]

We did remove hose to vacuum advance. And yes I can adjust timing now. We set it by ear and then I tweaked the mixture and idle screw. The truck runs a little better and the only ping I get is in third gear if I step on the gas at 25-30 mph. Other wise, no pinging in 1st, 2nd, or even third if my speed is above 35 mph. My vacuum gauge reads excellent when at idle. Should restate too, plugs are very carboned up from previous richness. So, I may get an even better feel once those are switched.
I think it is worthy of a new thread, but what is your opinion on spark plug choice for engines that seem to run rich no matter what you do? I currently have factory AC 44 which are a cooler plug. I was thinking about going up to a slightly hotter plug to address excess carbon.

[old cool]

Forgot to mention. The timing marks are off (couple inches) as I originally explained. What I was trying to determine, with help from this forum, was whether or not the distributor and its hold down screws needed to run exactly as diagrams show to get the timing marks to be exactly where they should be. I'm not sure there would be a connection but thought Id ask.
Though not perfect, right now I just feel relieved that I could advance timing and lean-out the mixture screw. lol. As mentioned in previous post, plugs will be my next inexpensive attempt to get 'er running better.

[68post]

If it is really sooted badly I'd consider running some Seafoam thru it before the new plugs.

[66Submarine]

These are low compression engines, and I'll bet you have a lot of timing if it pings at all (which would put the timing mark on the pulley past the tab on the timing cover). Mine is set to 10º BTDC initial with no pinging being evident at any point, and when I had it at 16º or something I don't remember it ever pinging (although I didn't try to make it ping). If you are concerned that the balancer may have slipped, then check it.

If the engine still runs rich and soots up the plugs when tuned up correctly, you need to work on the carb. No spark plug in the world will fix a rich mixture. BTW, just let it warm up and crank the idle mixture screw in until it runs like crap, then back it out slowly until it smooths up. You are just adjusting the idle mixture--you aren't going to hurt anything.

You should also check and make sure the choke is working like it should at this point, considering the mixture problem.

Like I said, if it still runs rich and soots up plugs after adjusting, then you'll need to work on the carb (take it apart yourself or buy a rebuilt one).

[66Submarine]

FWIW, here are the plugs from my junkyard 250 with a generic (parts store) rebuilt Monojet that I haven't messed with:

[68post]

Originally Posted by 66Submarine

FWIW, here are the plugs from my junkyard 250 with a generic (parts store) rebuilt Monojet that I haven't messed with:

Excellent color !!

[old cool]

Great picture!. I have driven the truck about 80 miles since our previous threads. I advanced the timing (though pulley mark is too high). I leaned out carb by 1/4 turn. I ran techron concentrate with 11 gal of 89 octane gas. I pulled number one plug yesterday and saw that plug was no longer carboned around electrode. Still have some carbon deposits deeper in plug well. Probably older carbon.
It was great to see the plug improvement. THANKS again!

[walkerb]

Ive also found that mine runs much much better with non ethanol gas.

[old cool]

I located an air cleaner at a swap meet. It was in great condition. However, I believe its from a 4 cylinder engine or smaller gm six. If it is from a 4 banger, it is a clone in appearance to what our trucks use; it even has indentations for hoses and choke. It was only 5$ so I took the chance. Glad I didn't pay a lot. It appears I will never be able to use a stock air cleaner. One of the PO added a power booster and the air cleaner hits it making it unable to drop over carb. That, I assume, is why I have an oil bath unit.
In 1966, GMC used the Chevy 250 as its base engine. 1960 to 65 GMC trucks only used 305 v-6's. My confusion lies in the air cleaner. I have a simple, narrow, stock looking booster. Why would the a-cleaner hit it? Didn't GMC have boosters in '66?
Well, I'm still looking to convert to a papered filter rather than oil. But, I really don't want one of those tiny 4" cleaners sold everywhere. Must be something out there that wouldn't completely diminish the look of stock. Hope to hear back from someone.

[66Submarine]

Yeah, I remember reading that power brake trucks all had the oil bath air cleaner. I'd love to return your $5 investment and put that on one of my trucks if it helps the pain.

Also, what do the plugs (etc) look like now? I forgot to mention for some reason (stupidity), but high fuel pressure will push fuel past the needle and seat and flood the carb, making it run super rich like that. Seems like with the new mechanical pumps it's pretty common (more so than I would have guessed). Might want to check that if it's still running a little ragged.

[old cool]

Seems like you're always coming up with helpful suggestions. Thank you. I would gladly give you the air cleaner if I knew what I was doing. Since it was cleaned and repainted, I mounted it on my garage wall like a clock. Looks pretty cool next to GMC plate. I am also not sure if I have a defective power booster. It was disabled when I bought it; which may explain why I'm burning rich if vacuum is off. I changed hose and booster grommet to make booster operational. Seems a little touchy, but ok.

To answer your question re performance, I'm far from satisfied. The plugs do seem much cleaner on the electrodes. And, I seem to be getting better mpg. Both improved when I had the dizz turned, advanced timing, and was able to turn the mixture screw in some. BUT, your idea of pressurized fuel coming in is a great one. If you'll recall, I never give my truck gas when starting or it will crank for a while; choke on, or choke off. I just turn the key. I also see a wet gasket around float bowl in front only. (carb looks imperfect and somehow got passed anyway). I wondered why I have fuel showing at that point which is above float. It's never a drip, just moist; and it happens while setting or running.

So, to get to us back on course, how do I address pressurized fuel? Hope its external otherwise I'm going to need a full carb kit. I've faintly tweaked on floats and reused upper gasket. Nothing changed with float work. But then again, I looked at drop per instructions and it seemed ok.

Latest concerns re performance are liquid spay when first started ( I covered in thread called "drilling hole in muffler". The truck idles nicely in neutral and moves ok once driving - good pickup. But, it vibrates in idle while sitting at light; it also stinks like heck from tail pipe. No one tail gates or gives a thumbs-up when behind me - LOL.

So I don't put too much info here and cause others to rack there brains, here is what I've done to date (much advice from you):
1. changed plugs; using 45 AC instead of 44's - no gain
2. examined float drop with carb off - no gain
3. moved dizzy, however, I can't set to specs or even advance to get proper reading per toothed marker. Pulley mark seems top of pulley to get best sound from idle. - some gain
4. messed with mixture screw as you directed - clockwise to stumble; counter clockwise until smooth - I believe I'm getting better mpg
5. I changed flange gasket at end of exhaust manifold. - not sure of gain but the previous gasket had minor tick when putting stethoscope (hose) up to it
6. Changed grommet and hose related to power booster - not sure it impacted engine performance but I like PB.
7. Using 89 to 90 octane - think it helps
8. Wires, Petronix, and fuel filters have less than 1000 miles. I know nothing about Petronix or if it requires a specific coil.

***Just a thought, I've read that coils may not function properly. If my spark is weak when at idle; in gear; clutch in.... would that cause vibation probs?
***I still find the pressurized fuel a possibility as I have always wondered why I don't need to give pedal when starting. Maybe both are probs.

Oh well, I just hope I'm not going to one day learn I have a gear, crank, valves, or cam problems. Right now it's pretty peppy and it doesn't seem to smoke once warmed up - so.... I remain hopeful. Have a great day!