The Stovebolt Site Forums The Shop Area Engine Shop Fueling or Ignition

After searching and reading all the posts on an engine with a miss, I'm in sort of a debate what my issue is.

Symptoms: The truck runs well 90% of the time, I put it out on the highway, and climbing up a long hill, it started to miss and backfire. I was running it in 4th gear, giving it a fair amount of fuel. Once at the top it eventually smoothed out and ran fine down the hill and across the valley to the next hill where the missing and backfiring started again. when its missing the whole truck surges. I don't notice any problems on side roads, or when accelerating. It seems like only when there is a long period when the accelerator is depressed more than usual. Another symptom that may or may not be related, after it runs a while and I shut it off, it will either not start up or start up and soon bog down and die.

I'm suspecting plugs, but certainly won't rule anything out. Engine is 235 and all parts are suspect of condition/age. I'm not interested in doing a 'full' tune up right now, but I will probably do so piece at a time. Truck has been ideling more than normal, adding to my suspeting the plugs, perhaps they have been gunked up from ideling.

I know my main options are overhaul carbureator, or plugs as most likely, hopefully someone can narrow it down to one or the other. Thanks!

Sounds like a fuel starvation problem to me. Check the usual culprits: clogged fuel filter, weak fuel pump, kink or crud in the fuel line.

I cast my vote with Waldo! Could there be a dead frog in the gas tank?

Stuart

Three votes for starvation.

Alright, I'll check my lines, and filter. If not those, would the carb itself do this?

asilverblazer,

I don't think the carburetor itself would be the culprit. If its bowl is kept full, which is seems to be until a long hard pull, it functions fine.

Something isn't allowing the system to keep it in fuel when the pedal is to the metal. A low float setting would affect you all the time and be pretty apparent. Your carburetor works fine most of the time so it's good! It's the supply that is in question.

That's my take on it.

Stuart

check for worn actuator lever on the fuel pump, been there

Bill

Took it for another drive yesterday, trying to duplicate the same conditions, only this time the truck was loaded. There were times when I was having to give it quite a bit of pedal to maintain speed. It *might* have missed a small amount, nothing like the other day. It was hard to tell if it was missing or if it was road imperfections causing a jerking feeling, because I couldn't hear the motor real well, but in any case I don't think it was missing.

The only think difference was the temperature was a little bit cooler.

Power seemed a little down too. I've had a heavier load on it before, and it pulled hills better than it does now. I had to go all the way down to 2nd on one. I'd guess total load weight was, 3000-4500 lbs.

Maybe I do need to just do a full tune up, not like it would hurt anything...

How does this sound?
Plugs (Any recomendations?)
Wires (Any recomendations?)
Dist. Cap
Rebuild Carb. (Kit Recomendations?)
Clean out the oil bath air filter
Fuel filter
Coil
Fuel pump?
Vacuum Advance?
Any parts associated with Dist, Fuel pump, coil...
Adjust valves
Check timing
...uhhh...
Anything else?

I would start with a simple tuneup. Plugs, points, check or replace wires & dist cap. Check timing. Those roch. model B carbs are easy to put a kit in. Making shure the power valve is working & the foat level is correct can help a lot.

So what is the new part number for a AC Delco spark plug, the ones I bought do not match the ones I pulled out.

AC 44 for light duty use probably will be alright. AC 42 or 43 I think is the original specification. AC C42 or C43, if you can find them are commercial plugs, made for heavy duty operation. The only difference between commercial and standard duty plugs of the same heat range (number) is the commercial plugs have larger elecrodes than do standard plugs.

What ever you use for spark plugs, check the color after a few miles- 50-100 or so. Dark means too cold, too rich or burning oil. Bright white porcelain, like new, or shiney elecrodes mean too hot, or maybe too lean with fuel. A good color is light tan or slightly off-white, electrodes not quite shiney. I know that last bit is not very clear; you kind of learn what the electrodes should look like. A little too cold is better than too hot for a larger truck. Spark plug temperature goes up with engine load and throttle opening.

Harvester

So these AC 41-602 aren't going to fit huh?
I asked for AC 44 at the parts store and they couldn't find any with that number.

Might be hard to find the older gasket seat plugs, AC has cut way back on them. Last time I checked at my parts store, the R43 was the only one available. I did look on boht Auto Zone and Advance Autos sites the other day, the AC plugs are listed as not available. May have to use Champions, J8C (or whatever the new number is).

Definitely sounds like fuel to me. Funny story that relates... Inherited my father-in-law's '76 motor home. 454, 4 bbl, auto. He always complained about clogged filters. As I drove the big box from Brandon to Calgary, I had no trouble as long as the road was level. When I started climbing, the engine started missing. I stopped and put in the spare fuel filter. I put the new one in the same way the old one came out. The engine ran well for a while, then missed again. As I drove down the road, I considerred how the filter was installed... The closed end was facing the fuel line. Made no sense to me, so I stopped and turned the filter around. TOTALLY different vehicle. From struggling up hill at 30kmh to roaring up hill at 120. Guess I found out what the old man was complaining about...

Anyway, the symtoms sound similar....

Good news, I put new plugs, wires, points, cap, fuel filter, and rebuilt the carb, and it still runs. Not so sure I got the carb adjusted right, book says fiddle with it til it sounds good, OK, did that and hit the gas and back fires... Fiddle with it some more. Any adjusting suggestions?

1. Do a compression test.
2. Do a vacuum test.
3. Do a fuel pump pressure test.
Then, re-check timing under varying speed paying particular attention to mark movement. Ensure the distributor cam lobes are not unevenly worn, vacuum advance functions properly, the points rubbing block is intact, and the point assembly is securely fastened. Next, check the air filter and inlet for obstruction. A sloppy choke plate is suspect too.
A broken valve spring, as well as a sticky valve will cause the problem you've described; improper valve stem adjustment, too. But, without 1&2&3 done first it's hunting and pecking. Harvester is right on the money about plugs - they are the 'eyes' of an engine and reveal much about its efficient use of fuel. Happy hunting.

Interesting you mention choke plate, it was improperly attached to the cable, causing it not to open fully.

You mentioned 'Hills.' I have had carbs cause similar problems when on a decent grade because the fuel was running to the back of the bowl affecting the float level and effectively starving the engine! Cleaning the needles and properly adjusting the float (and making sure the float doesn't have a hole in it...) with a carb kit typically fixed the problem...
Laterz,
Gator
>|--|~

If your truck takes AC 41 spark plugs, the 44s will be too hot. The 41s seem too cold, but if that is the correct plug, Ok. A step or two hotter, 42 or 43 would be all right, but check the color after running for a while. I agree with others, that it sounds like your hill trouble is fuel-related. Aside from fuel pressure, you need to check fuel flow from the pump- usually a pint in about 20 seconds at idle speed. More is better, a little less may still be all right. Laurence mentioned a vacuum check- if he means vacuum check on the suction side of the fuel pump, yes. I don't remember the level, but it should go pretty high at idle speed, probably above 10 "Hg.

Harvester

I think my initial problem was the fuel filter, when I pulled it off, muddy water came out (really it was rusty fuel). Now, I'm working on the idle adjustments, and checking the timing.

I also think my powervalve was stuck, but I don't notice any driving dirving difference now vs. then. It still seems the truck is more sluggish than before, but hard to tell. I suppose if the timing is off it could be the problem there.

Put a timing light on my truck last night. It should've been institutionalized for how retarded it was. It wasn't even in the window, I put it right below the needle, and it runs a lot better now. I'm tempted to keep playing with it to see how much better I can make it, or take what I got and run.

You might be able to improve it some with fuel/carburetor adjustments. Make sure the disributor advance mechanisms are working properly. If the truck has solid lifters, make sure the tappets are set properly: .010 I, .020 E, for heavy-duty operation.

Harvester

What else can I adjust carbureation wise? I thought the idle mixture and speed only affected that, idle. Advance works, lifters, I dunno. I intend to adjust them to the book, eventualy.