I recently bought a 1970 f250 w/ a 302. It ran great when I first bought it 3 weeks ago, but now it runs like crap. At full throtle it runs fine but when I lightly depress the pedal the truck lurches and sometimes backfires and shoots flames out the tailpipe. I can almost hear it just spitting fuel instead of a constant spray. Is it time for a new carb, or a simple fix?

 

How does it idle? Does it smoke?? black?? Does it miss??

Sounds like possible carb issues. Something as simple as a sticky float maybe. Need some other info to make a better diagnosis.

It could be timing(distributor) issues if it is mis-firing.

Larry

 

Doesnt smoke. sometimes it idles nice, sometimes very rough. I dont know if its missing, how do I tell? I dont know how to set the timing, I cant find any refrence point to set it to

 

Childsplay, I would encourage you and anybody new to this hobby (infatuation?disease?) to get a good manual or two or three. If you plan on keeping your rig for any length of time it will be the best money you spend on your truck. Any good manual will get you familiar with your truck and simple procedures like timing and tune ups. Don't get me wrong, the folks on this site are the best. They will bend over backwards to help you with almost any situation you have but you'll find that all of this will make a lot more sense if you read up on the basics of your truck. Not to mention a manual will give you all the specs at your fingertips, step by step diagnostic procedures AND an idea of what tools you'll need to live with your truck. Diagnosing problems (especially long distance) can be a hit and miss adventure and as anyone on this site can tell you it's really easy to spend hundreds if not thousands of dollars and countless hours chasing the wrong problem.

P.S. Sounds to me like your timing is creeping, but if you list ALL of the symptoms it 'll make it easier to figure out.

 

I compleatly agree on buying manuels. I have a chiltons on order from amazon(local autoparts store laughed when I asked) I know these are stupid questions that could be easily fixed with a guide and a little know how. I have looked for timing marks that I cannot find. I have found the specs on settings, I just dont know how to adjust it when there is nothing to adjust it to. Maybe Im just blind and am not seeing them.

Im doing my best to list all symptoms:

sometimes runs rough at idle, sometimes great

usualy on the freeway when I apply light gas it makes a poping noise from the exhaust

when I apply medium to heavy gas the noise goes away and the truck runs great

Im just tring to get this problem licked before a trip I have coming up this weekend. Thanks ahead of time!!!!

 

Float set too high and causing a bit of flooding- only at certain angles- maybe a vacuume leak, and possibly a bad vacuume advance on distributor along with incorrect base timing. Get the manual first and look in there before you start throwing parts and adjustments at it, cause if you chase the wrong problem, it can be hard to get that set correctly when you do find the original mistake, IE carb adjusted out of wack, when it was the timing all along, but the bad cab settings hide the now corrected timing issues.

 

It sounds like a combination of issues to me, both carb and ignition. The most frustrating part of it is the intermitant nature of the problem.

I'd start looking at the obvious solutions first, checking distributor cap and rotor, plug wires, plugs, fuel filter etc...When making adjustments, adjust only 1 thing at a time and then test the vehicle to see if if made a difference or not.

THe first place I'd start is with the timing, both mechanical and vacuum.

It sounds a lot like a problem I had on an old GMC a few years back. Turned out a couple of cam lobes were flat.

 

Don't know if you know, so bear with me. Not sure if you realize some things so I'm going to start at the basics. If you know more than I realize I apologize for my misconception.

To set the timing, you need a timing light. There should be a timing tab on the bottom passenger side of balancer(hooked to block/timing cover. It should point at the balancer itself. If you clean up the balancer and look carefully you will find TIMING MARKS....like TDC and 0,10,20,30 (TDC=top dead center).

With a timing light hooked to #1 plug wire(front pass. side) truck should read 10 degrees(give or take one or two-but for conversation this is a good starting point). This should be at IDLE. If you rev it up it should INCREASE to mid 30 something. This may be hard to read do to poor marking of balancer. Watch for bouncing around of timing as you watch it move, or while at idle. This is telling you weather things are good or bad inside the distributor. Loosen the dist. with a 1/2" wrench and turn clockwise to increase, CCW to decrease timing.

You should also need to check the points gap in distributor. This is where a manual will help with pictures so you can find things easily.

Once you have the distributor all in line and accurately set you need to drive the truck and see if things got better. If things are still bad, then the carb is the next thing to start looking at.

Hope this helps you get started in the right direction.
We'll do our best to get this truck running smooth again.

Larry

 

well... I spent most of today rebuilding my carb and setting the timing. still nothing

guess I got to just keep trying

 

oh ya and we had to set the air mixture screws waaaaaaay out( about 4 turns) for it to run smoother

 

I sounds to me that you still have timing issues. Timing is involved with both the ignition and the camshaft.

I'll break this into two posts to keep things from getting tangled.

Occasionally, the roll pin holding the distributor gear to the shaft will shear. This allows the gear to spin, which ends up retarding the ignition timing too much. If you do a tune up and adjust the ignition timing, but find it "walks out" of the settings you left it at, then you need to look at this possibility.
You will have to pull the distributor, which is not a job for a novice without some study and assistance.

Now there is the possibility that your distributor is shifting in the block - whch will have the same effect. Make sure it is secured correctly before jumping to any conclusions. Same goes with the points and the rotor. Eliminate the simple and obvious problems first!

Now, adjusting the ignition timing ONLY sets the timing with respect to the camshaft. If the camshaft is out of time with respect to the crankshaft, changing the ignition timing (by moving the distributor) will have no effect on the camshaft timing. This means you may have additional challenges to uncover and to solve.

I'll address this issue in my next post.

 

I assume you found your timing marks. Are you using a timing light to set your timing? If so are your timing marks stationary at idle or are they moving? What happens to the marks when you increase the RPM's?

 

The camshaft timing is a different animal altogether. It is statically set by the position of the timing gears and chain. However, both the gears and the chain wear and stretch over use. The gears installed by the factory, aluminum with plastic teeth, were soft, designed to eliminate noises. The teeth often fell off, and the aluminum wore down, retarding the camshaft timing.

The chain accumulates all this wear. The following is an easy check to determine the amount of slack, slop or wear, in your timing chain.

It is not necessary to remove any components as long as you can see the timing marks clearly. It can be done with one person, but a second person can be helpful if the distributor cannot be easily seen while turning the cranskshaft from the front of the engine. This test cannot tell if a chain has jumped; it can, however, tell you if there is enough slop in the chain to have let it jumped.

Tools: Breaker Bar with socket to fit the front crankshaft bolt

1. Mark the TDC or 0 mark on the damper clearly. Ensure the rest of the timing marks can be clearly seen; clean them if necessary. You may find it useful to scrape a little chalk over them to highlight them.
2. Remove the distributor cap and place it out of the way. You must be able to see the rotor while turning the engine with the breaker bar. It usually is not necessary to remove the spark plugs, but some high compression engines may require it to get an accurate reading.
3. Turn the engine in its normal rotational direction until the TDC or 0 mark is lined up with the pointer.
4. IMPORTANT: Do NOT turn the crank backwards just to line it up. Doing so will give erroneous results. You MUST rotate the crank in ONE direction only. If you overshoot, just go around again. This is where it may be helpful to remove the plugs.
5. Once the TDC and 0 marks are lined up and while watching the rotor, rotate the crank in the opposite direction. When the rotor begins to turn, STOP. It may help to have a helper push against the rotor's direction of rotation. (Remember, it will be going backwards at this point.) They can feel it begin to move more accurately than you can see it begin to move.
6. Note the value of the timing mark the pointer is indicating. It will be BTC, because you went in reverse of normal rotation.
7. This value is the amount of slop your timing chain has in degrees of rotation. Anything less than 10 is safe, but performance begins to fall off after 5.
8. If you have anything over 25, STOP driving that motor and get the chain and gears replaced! I have seen motors run quite well, and for a long time with up to 45 degrees - but I wouldn't risk it myself.

 

If the 302 came with an EGR vavle, check that for correct operation.

 

4 turns out on the idle screws would indicate a lean condition. That egr could be the culpret. I don't know if you replaced the power valve on your carb rebuild but if it has been backfireing it may be toast. One good backfire can take it out. Unless you have an obvious carb problem like puking gas, you should always make sure that all the ignition system is working exactly like it was intended to, before attacking your carburator. This is a common mistake that most amatuer/hobbists make. Then when they finally do find the problem they have the carb so screwed up that it can't run right anyway. Over the years I have probably dealt with hundreds of these scenarios. Most of them come into the shop barely running or on the end of a tow rope. I have seen carburators "rebuilt" by well meaning uncles or nieghbors when all the thing need was plug wires or new points. I could go on but I think you get the drift.
And my take on the manuals, get the factory one. Chilton's and Motor's are fine for the specs, but I will guarantee that when you hit a snag and really need to know how something comes apart, it will just say" remove such and such". Or you need to know how to put something back together and it will just tell you how to get it apart and then say"reverse order to install". You also have to weed through engine removal just to find out how to get one little part off and back on. And those cheap clymer's manuals are the worst. They don't even show you the actual model of axle or whatever. They just show generic "typical" models and procedures.