Engine Bogging

[chevyc1068]

I got a '68 c-10 with a 307 in it. The only problem is when you step on the accelerater it will bog down and sometimes dies, only if you barely press on it will it rev up but any amount of acceleration will bog it. This is all just sitting out of gear not driving. Although it does the same when driving. Now the funny thing is with the choke on it will run perfectly. Even when it warms up to normal temp it is exactly the same. I put a new accelerater pump in and replaced the carb gaskets along with cleaning the whole carb out. Put in new plugs the other were worn out and black. Put in a new condensor, have new points just have not got there yet. The plug wires don't look like they would be a problem. I have tried tuning it but all it has is two mixture screws but maybe I just have not tuned it quite right yet. I have a new coil for it (again not got there yet).

Please any help on what to replace or what to test or what might be wrong would be very appreciated.

[bryanw1968]

If it runs fine with the choke on you're way too lean. Using the air fuel mixture screws tune for highest vacuum. You should be able to get it pretty close by ear if you don't have a vacuum gauge.

What kind of carburetor is it?

Good luck,
B

[70rs/ss]

vacum leak, look around the carb base, use carb cleaner with it running, spray it around the base and if it revs or dies, you found your leak. Not an idle setting issue if it just started, and the running with the choke on is the tell tale sign, cause the leak is allowing too much air, the choke then adds a ton of fuel which is why it runs OK with the choke on. Start by checking the carb bolts for tightness, then follow the carb cleaner steps.

[bryanw1968]

Good point. It does sound like a vacuum leak since it just started. I got thrown by the "I just rebuilt it part".

[chevyc1068]

It is a Rochester 2g with manual choke.

I have noticed that there is a bit of liquid around one of the bolts on the intake manifold, but i haven't got around to checking the torque of that bolt yet. Do you think that could be the problem?

Thanks for the replys keep them coming!!!

Oh I forgot to mention that this pickup has been sitting in the shed for three of four years and I am now trying to restore it so it will drive. so its not like it " just started ".

[PanelDeland]

Could be a manifold leak since you have the puddle on the manifold bolt.Maybe coolant to the cyl. or just air into the intake.

[Boog]

If this truck has power brakes pull the end of the vacumn hose to the booster off and plug it. The brake booster diaphram may have a small hole in it leaking your vacumn or the hose itself may be cracked. See if it makes any difference with it plugged.

[chevyc1068]

It doesn't have power brakes, but good idea.

[chevyc1068]

Can you tell me where some common vacuum leaks are located?

?o you think it is ignition timing or something advanced, or can i take that off the list of probable causes?

[Werewolf]

It really does sound like the truck's running way too lean. A hot truck, choked, should start killing itself in short order. If it were an external leak, and if you had a decent exhaust (not loud as hell under the hood) you should be able to just get an ear near the carb and hear the typical sucking noise.

Not to offend, but do you have experience rebuilding carbs? Did you use a good rebuild kit? Did you make sure everything was good and clean before putting it all back together?

I had a problem once where I was changing jets out on a Rochester 2bbl like yours and I didn't want to buy the whole rebuild kit, so I just dabbed a bit of gasket maker around the float bowl. Some got into the bowl, and clogged one of the jets, giving me this same exact problem.

[chevyc1068]

No actually I don't really have any experience in carb rebuilding. This is my first project. But yes I made sure everything was cleaned out real good. The carb rebuild kit I used was from the LMC catalog so I don't know what quality it is. the manifold gasket didn't really look like it had any metal in it, but maybe it was just covered up. I tried tightening the manifold bolt but that didn't help. I keep forgeting though to adjust the micxture screws so I will try that.

[Werewolf]

I don't think the mixture screws are responsible. It sounds like you've got a pretty good vacuum leak or lack of gasoline going on. Did you replace the big thick gasket that runs between the carb and manifold? Did you replace the accelerator pump? Did you remember to put the spring under the accelerator pump back in?

What was the truck doing beforehand to make you rebuild the carb? If it's not the carb, it could be the fuel pump, fuel filter, etcetera.

[chevyc1068]

Yes I replaced the gasket between the manifold. Yes I replaced the accelerator pump and put the spring back under it. I don't think that it is the fuel pump or related because remember it runs fine with the choke on. This pickup previously was a repair truck, one we took to the field when something broke down. Then it was put back in the shed and wasn't used for three or four years. I looked for a vacuum leak in some places like around the base of the carb and on some manifold bolts using carb cleaner to spray around with but noticed nothing. I don't know of much else to do except keep looking for vacuum leaks. What do you think?

[Solver]

Just a shot but you may have put the accelerator pump lever in the wrong hole as there are usually two holes . Try switching it to the other hole. It changes the amount of gas shot in on accel

[chevyc1068]

Good thought I'll take a look at it tomorrow.

[Jerry Jones]

I agree ...By the way welcome to the board... you got pics of your rig???

[Werewolf]

And you might want to change the fuel filter again, too, if you haven't already.

[chevyc1068]

Okay I will try that. But how does that explain that it will run good with the choke on?

[Werewolf]

If the gas isn't burning as well as it should, it may take more gas (or less air) to really get the engine to fire properly. In addition, old gas will cause a varnish to build up inside the tank and in the lines. If this varnish is thick, it could block the flow of gasoline, clog up a fuel filter, and maybe even goof up a fuel pump. I don't know about all of that, I've never had that type of bad experience with old gas at all. I know a friend who's been dealing with the remnants of 7 year old gas in his tank; for a while he would have to change his fuel filter every 2k miles or so until the new gas finally got the old crap out.

Basically it sounds like what you've got is an engine running way too lean. Only so much can cause that, these aren't very complex vehicles. A lean condition is caused by too much air, or too little gas. You've got one or the other. If the gas is old, only some of it might burn in the mix. I don't know how likely it is, but it makes a certain amount of sense.

[chevyc1068]

True that does make sense. I will have to try that sometime when I have some time.

If it has caused build up in the lines or even a little bit in the carburetor do you think that if I stuck a bottle of the STP carburetor cleaner in the tank that it would flush out the lines or just the carburetor, or do you think that kind of stuff actually works? I've heard some people say that it really does work, but others have said its just a gimmick to make some money.

[chevyc1068]

Ok I put some new gas in it and it seemed to help to some degree. i changed the timing on it to about 8 degrees BTC from 14 but i don't know if that is correct. It still doesn't run very well without choke. It does recover if you put it all the way to the floor, it used to just die. I also put new points in.

So what do you think I should do now?

[chevyc1068]

I just remembered I want to know if the myth that one good backfire can kill a powervalve. Because I did'nt have one to replace it with when I rebuilt it, and through a buch of this testing I have had quite a few GOOD backfires. So maybe the powervalve is shot and thats my problem???
Also I didn't replace the jets.(LMC's rebuild kit didn't have much in it)

[oldgold70c10]

Some other things you might want to check- PVC valve and crank case breather, if they are leaky, they will cause vacuum leaks. Check the little diaphram (vacuum break) that connects to the choke lever (manual choke carb may or may not have this) and the hose attached to it, if it is leaky, you'll have vacuum problems. Sounds like you may also have insufficent feul delivery, since it runs fine either in idle or with the choke. Did you check the float level? It might be too low, clogged or wrong feul filter or bad pump. If you disconnect the feul line from the carb and crank the engine over 1 or 2 turns (be careful doing this and don't let it start) you should be able to see gas squirt out the line. If it does your pump is OK, check the filter and the float level. I doubt its the power valve unless it look damaged or pitted, or something else obvious. Do check the spring loaded plunger in the top half of the carb that pushes on the power valve, it is easily damaged.