The Stovebolt Site Forums The Shop Area General Truck Talk Spitting and sputtering - what's your take on the problem?

I have a 1955 GMC 450 series with a 302 engine.

Here's the problem. The engine idles fine and sounds good but when you give the gas to her fast it coughs up through the air intake and sputters, almost stalls, in some cases it gets caught up and the engine races, in others it would stall. When driving you have to plan ahead and give the gas slow or no go.

I was thinking timing but not sure, what are your thoughts?

maybe the carb has some issue's, have you been tinkering with it or did it start doing this all at once,,,,,,,,,,,,

Well I rebuilt the carb myself. It was having the problem somewhat before but it hadn't run all that much before I rebuilt it. I was hoping that it would point towards a particular thing and that someone with more experience than I would point me in the right direction.

So yes, that could definetly be the problem.

you are probably a better mech then I but if I had re-built the carb I would know for sure where to look do you know of anyone close to you that you might borrow a good carb to run on it ,,how does it start up,,have you fooled with the timeing or dist. at all??also check to make sure you have a clean gas filter,, I had one so clogged up one time that my truck would not hardly run at all, in fact it acted the same way,, replaced the filter and all was well,,good luck,,,,Fred

With the engine off, look down the carb throat & see if the accelerator pump is squirting fuel when the throttle is moved. You might have left out a check ball or weight. Brian

It definitely sounds like carb issues. What is it? A Rochester 1 bbl?
If it idles fine, set your timing.
Then hook a vacuum gauge to your intake at a port as close to base of carb as possible. Dial your mixture in using vacuum gauge. Look for highest STEADY reading. Then if necessary, re-adjust engine idle.
If this doesn't work, you may have to take her apart again and check your measurements. Or, if it is a Rochester, do as Carb King says: Remove the carburetor and hire someone to sit in engine compartment and pour fuel into intake 'cause it'd undoubtedly work better than a Rottenchester 1 bbl.
-David

You,ve may have a flat lobe on the camshaft. Pull the valve cover & watch the valves to see if they all go up & down about the same height. If not that it could be timing too slow. Make shure you have the float level set right.

Forget that about the camshaft. I didn,t read your post thurouly before replying.

The carb on it is a Stromberg 2 bbl. I don't believe I left out a check ball when I assembled it. The truck idles fine and I did set the timing last fall.

That sounds like a plan, check the timing, then move onto check it w/ a vacuum gauge. I am working on getting a mechanic to it since hauling that beast would cost a kidney or two.

Thanks for the info guys

Timing issues...backfire through the carb is too advanced, through the exhaust is too retarded. Since yer headin to the distributor, look at the points. You say it hasn't run in a while so points could be corroded and a piece of sandpaper and a reset could solve this also. Everything else being OK, IE, carb, points, timing, put in a new condenser. Good luck and let us know how ya did.

Aschubox.Not being a smart butt.Butt I know what your problem is about 99%.Frist get the carb ID tag or brand of carb.Go to NAPA auto parts and order a accerator pump for your carbrator.This is all ya need.Your original accerator pump has a leather plunger, it is worn out.Before installing the new pump,put the pump in a cup of gas so the new leather expans for 10 min.Newer pumps are made of neophrim,no need to soke.careful with those old gaskets.I can't tell ya what I did yesterday,but 40 years ago seems like yesterday.Ha Ha.happy truckin One more thing;if you can buy Seafoam at the parts store buy a can and add to gas tank.This stuff is great and will clean the carb well your driving.Its excellent in everthing.have a nice day

My 283 in The Ox does that too... but usually just once or twice before it gets hot, until i can get the 4 barrel to kick in.. then it runs fine. I'm using an Edelbrock 500 cfm carb.

You might also consider a bad coil--often a bad coil will idle but will not allow the engine to run much over 1200 RPM. However if the engine will run at higher RPMs if coaxed, I would then lean more toward a bad accelerator pump, as indicated in previous posts.

the coil check to see if OIL is leaking out of it.There mounted upside down.Just pull the coil wire out see if it's wet with oil.If so change it.Just an old thought again.Im glad there are no questions on the new trucks,Hell I can't even open the hoods and one look at the engine,where do ya start

How about manifold heat? The '51 used to have problems similar to that (not quite as bad) in all but the warmest weather until I got the heat riser sorted out.

Also check to make sure the advance weights in the distributor move, and that the springs aren't broken.

The distributor is a centrifugal advance type. I checked that as described in the maintenance manual and all looked fine.

I checked out the timing with a timing light. If it is off it is real close, I need to get in there with a mirror so I won't take my head off with the fan blade.

We thought an air leak somewhere like in a vacuum line might be the cause. I tried the old trick spraying starting fluid around the joints to see if it gets sucked in a revs up with no luck.

Again with the problem, it idles fine but if you stomp on it it will just die without a fight. Almost like the gas has just been taken away all in one shot. While it idles it will cough and it is not in any pattern....

hmmm..

I've been fighting this exact same problem for monthes. I posted a while back but I was mostly ignored. I'm curious how you manage to sort it out.

Listen only one more time change the accelrator pump in the carb.do we speak english.If you want to try instead of paying $6 for a pump. Take the pump out and streach the leather boot backwards,then back to it same shape.this will work for a few days,then it will all start over again. the same problem.I give up.just bring it to my place and I'll fix it for $300.seems people like to be screwed always PS check replace all vaccum lines hoses.there about 45cent a ft

dan-pa - the reason I haven't gone right out and replaced the accelerator pump is because I rebuilt the carb completely last fall, the truck ran throughout the winter though it didn't go far. You think it could have still dried up?

The vacuum line fron the check valve to the hydrovac is brand spankin' new, a solid l;ine straight through. I'm worried it might be sucking air in through the fuel pump.

may have dried up a bit.won't hurt to pull the top of carb off ,look for Water, and check the pump at the same time.also soak the pump in some fresh gas.when replacing the pump slowly spin it back into place so the leather boot don;t curl up.Also try the Sea foam additive to the gas tank.This will get rid of the condinsation water in the gas tank from setting over the winter.The fuel pump should have a vent hole on the side of it.This is for the pump dyafram to breath.If the dyifram goes bad the fuel would be squarting out of this hole letting ya know the pump in bad.your fuel pump pressure should be between 4 to 7lbs when checked with a vaccumn gauge.Good luck on your troble shooting .always back track when your looking for a problem.sometimes these things will drive you up a tree.Do you have a sediment bowl on the fuel line going to the carb?Check for water in the bowl.This could be a problem over looked very easy.3/4 water and no room for fuel.Please try the Seafoam it works

I have seen accel pumps dry out in six months, seems to be some problem w/new fuel or cheap pump materials. Brian

Though the accel pump appeared to be AOK it still isn't working right. I popped her off and brought it to a pro.

He agreed that something was wrong, either a bum accelerator pump or it was gummed up somewhere, maybe a little dirt somewhere.

We'll see if that cures the problem.

Time to work on something else for now.

Well the carb is back... plugged up to beat the band, I don't think that I got it all when I rebuilt it, a touch of muratic acid and he had her cleaned out. I see what you guys mean buy "squirting" out.

She runs smoothly at an idle. When running steady at a higher speed it still coughs every now and then. Giving it gas hard usually causes a cough but it keeps up.

Question about the timing... using a timing light I can see the mark at an idle, when I increase the engine speed the mark can be seen moving and will remain steady at it's new spot as long as the engine speed stays the same. Is this ok?

I would still like top get her running a little smoother.

thats the way it should do

Its working ..when you set your timming disconnect the vaccum line,and tape it so it 's not sucking vaccum.Now set your timming.Now replace your vaccum line back to the distributor vaccum advance.Set carb idle screws to max RPM ,using a tack or a good ear.Or turn in screws till idle rough,then back off 1/2 turn