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URGENT!!!!! PLEASE HELP!!!

536 views 7 replies 2 participants last post by  Junk Yard Genius 
G
#1 ·
Well, my rebuilt 258 finally fired up today but it doesn't idle well. It is running straight pipe, rebuilt carb, K&N, RV upgrade. A mechanic timed it and tried to set the carb but without much success. He said it could be manifold leak, (checked it, its not!), the exhaust (no backpressure), or the PCV not routed into the carb (its pluged right now) What are your opinions?
Thanx

1987 YJ Laredo
Rylan D.
 
G
#2 ·
Carefully pull one sparkplug wire off of the distributor at a time, take note of how much each cylinder is contributing. I often find it helpful knowing if the problem is specific to a certain cylinder(s) or affects all cylinders. Every once in a while I find the wires on wrong and fix the problem with this process.
problems related to a specific cylinder:
firing order, plugs & wires, rocker arm adjustment, push rods, tappets, valves, piston
problems affecting all cylinders: carburator, fuel filter, fuel quality, vacum leaks, catalytic converter, cam timing,
did the engine have this problem before rebuild?
could a previous failure be related? i.e. plugged catalytic converter?
good luck, jjc

 
G
#4 ·
Rylan,
Make sure you work with known facts. For example, before you dismiss a vacuum leak, measure the vacuum with a vacuum gauge....something your mechanic should have done. One of the problems with a re-build when so much has been done, is isolating problems. This is why I like to rebuild the carb before I do the engine.
As stated above, check and make sure all the plugs are fireing correctly....I cheat by using a scope. The type of idle problem should help you narrow down the problem. If it runs well at higher RPM's, then ignition is ok as well as fuel delivery. If its sick on all RPM's, then it could be cam timming...being off 1 tooth is hard to find....

Just go through each system and make sure it is working right....personally, I would get the exhaust hooked up as soon as possible....not good on exhaust valves.

Please let us know what you find.

John......southern CA
84CJ7, 3"lift, 32"BFG, 4.10's, ARB Locker, Solid Axle's, Durabak
 
G
#5 ·
Well, the carb is newly rebuilt, and the idle is the problem. It hesitates at low rpm and fluctuates up and down in idle. It also deisels when shut off. When you gun it, it revs high and smooth, decelerating is kinda sloppy when you gear down which makes me think exhaust is being a problem. Thanks for any input guys!

1987 YJ Laredo
Rylan D.
 
#6 ·
You just described the symptoms of a distributor in 180 degrees out...
The 258 will run with the distributor in at TDC on the EXHAUST stroke, and it will produce the symptoms you just described.
You stated that you just started a fresh engine, so it is entirely possible.
We are averaging this very same problem here on the BBS about once a week now, so you might want to check it.

I agree with the DAWG again about taking nothing for granted.
You have to verify EVERYTHING your self, or take it to someone that will...

This could be a lot of things, but I would say this is the most likely...

Let us know,
Aaron.

"I Have The Body Of A God... Buddha"
 
#8 ·
It will time right up with a timing light when the distributor is 180 out...

I just re-read the posts, and you said that you had everything done...

You may want to go over some of the stuff yourself...

Beg, borrow, or steal a vacuum gauge, timing light, and get a friend you can trust...
AND...

A. Disconnect the ignition coil connector.

B. Take out the #1 spark plug, and stick your finger in the hole.
Have the friend bump the engine over until you feel compression.
(Make sure it is compression and not the exhaust stroke... If you crank it over several times you will quickly learn the difference)
STOP CRANKING. Do NOT go past TDC. (Top Dead Center)

C. Use a ratchet and socket on the crankshaft bolt.
ALWAYS turn the crank pulley CLOCKWISE when turning the engine over.
Stick a thin wooden dowel rod in the spark plug hole until it touches the top of the piston.
(Wooden dowel will not scratch the cylinder wall, or damage the piston)
Turn the engine by hand until the piston comes to TDC (Top Dead Center).
--------------------------------

When the piston 'says' you are at TDC, do two things...
1. Check the harmonic balancer to see if the timing mark on it matches the timing marks on your timing tab on the front cover.
This verifies the balancer mark.
If it doesn't, you have a bad balancer...

2. Mark the position of the #1 plug wire tower on the distributor housing.
Flip the distributor cap, and see if the rotor nose if pointing at the mark you made.
This verifies the distributor install position.
If the rotor nose is pointing away from the #1 tower on the distributor cap, the distributor is in 180 degrees out...
--------------------------------

If these two tests come out alright, it's cost you nothing, and it verifies two potential problem points...
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Now, Put the #1 spark plug back in, put the distributor cap back on, and reconnect the ignition coil connector.
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I don't know what computer control was used on the 87 model year, or if you still have it connected...
This could affect carb and ignition...
Find a good service manual, and use it to set timing as per it's instructions...
OR...
Ask some of the guys here to tell you how to do it correctly.
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Idle mixture is normally set by,
Gently turning in the idle mixture screws all the way in to LIGHTLY bottom them out, then IMMEDIATELY turn them out 1-1/2 turns.
Start and warm up the engine.
Connect a vacuum gauge to intake vacuum (not the PCV line).
Turn the screws TOGETHER, both in or both out...
Use about 1/4 turn movements of BOTH SCREWS to acquire the highest vacuum reading.
(Usually in 1/4 turn at a time)
When you get the highest vacuum reading, you have the mixture set correctly.
--------------------------------
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Here is a reprint of the vacuum gauge as a tool document I published a while back...
----------------

VACUUM GAUGE A MANDATORY TOOL?
Vacuum Gauge A Mandatory Tool? You Bet Your Life It IS!

The entire combustion process demands continuous high and low pressure cells to operate.
A vacuum gauge lets us 'see' the invisible low pressure cells inside the engine, and with a good working knowledge of a few basic principals of nature, we can figure out what is wrong with an engine.

This is mostly for those of you with stock engines, or engines that have been slightly modified, which is most of you...

1. Vacuum readings are mandatory to tune ANY carburetor. The most tuneable carburetor on the market is a Holley, and you will learn to live and die by the vacuum gauge tuning one of them.

2. Nearly every fuel injection system has a manifold pressure sensor. That Manifold Absolute Pressure (MAP) sensor, combined with input from other sensors, determines fuel delivery volume and ignition advance in modern vehicles.

3. Many millions of transmissions have vacuum inputs. Those inputs can be tuned by knowing what the vacuum is doing when the transmission is supposed to be shifting, and how to change the input of the vacuum information into the transmission.

4. Nearly every single older ignition system has a vacuum advance canister. It is impartial to know what the vacuum in your engine is doing when the distributor advance curve is programmed into the distributor.

5. Knowing what your 'Normal' engine vacuum is supposed to be is really the key in finding problems easily with a vacuum gauge. Taking vacuum readings, especially idle vacuum, is real easy, takes virtually no time, and will be a life saver when something goes wrong.
Taking regular vacuum reading at idle and through the RPM range up to about 2,500 RPM is the ONLY way to know exactly what is 'Normal' for your vehicle, especially if you have modified the engine in ANY way...

----------------------------------
WHAT TO USE,
Use an UNDAMPENED or UNDAMPED vacuum gauge. None of the fancy and expensive 'buffered' or oil filled gauges, they are worthless for our purposes.
Most of the better vacuum gauges intended for automotive service are satisfactory.
Try to get a quality gauge, Sears or better. A cheap gauge can not only miss a lot of things, but can give false readings.
-----------------------------------
WHAT TO LOOK FOR,

(paraphrased from a rebuild manual)
1. A steady reading between 16 and 22 in.Hg. at idle...
This is normal for most vehicles.
Radically cammed engines have lower, less steady readings.

2. Normal range at idle, with sporadic drops below normal,
This could indicate a sticking valve.

3. Normal range at idle with needle vibration of about 2 in.Hg.,
This could indicate an ignition problem.
Check plug gap, dwell, cap, rotor, and plug wires.
(yes, electronic ignitions have dwell too)

4. A steady reading slightly higher than normal.
This can be caused by a dirty air filter, or overly advanced ignition timing.

5. A steady reading 3 to 12 in.Hg. lower than normal.
This could indicate one or more of the following conditions,
Intake or carburetor vacuum leak, Late ignition or cam timing, Worn piston rings.

6. Gauge needle drifts slowly over a range of 4 to 5 in.Hg. at idle,
This could indicate an idle mixture that is too rich or too lean.

7. Gauge needle fluctuates rapidly between 10 and 21 in.Hg. at idle,
This could occur when one or more valve springs are weak or broken.

8. Gauge reads normal at idle, but drops slowly as engine speed is increased to 2,500 RPM,
This could indicate a restricted exhaust system.

9. Gauge reads below normal and fluctuates rapidly over a range of about 3 in.Hg. at idle, then the needle becomes steady as engine speed is increased,
Worn intake guides usually cause this reading.

"I Have The Body Of A God... Buddha"
 
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