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Brian, Service Engineer
Category: Car
Satisfied Customers: 1227
Experience:  Master Tech, Bachelor's of Science in Automotive Technology, Ford Factory Trained
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1978 olds cutlass supreme: i rebuilt the engine..ran having

Customer Question

i have a 1978 olds cutlass supreme with a 260ci engine. i rebuilt the engine less than a year ago and its ran great but now im having a problem. its an on and off thing, lately more on than off. at idel it runs very roughly the rpms drop and raise, it stals often when i brake and lacks power to idle up a small incline. my plugs keep fouling up so i know im running too rich. ive changed the jets, needle and seat, in my carb(2300 holley 500CFM) and it ran better for a short time but im having the same problem again. ive checked the cap, rotor, wires, module, and changed the plugs but nothing has worked so far. do you have any idea what the problem could be? M. Karll
Submitted: 10 years ago.
Category: Car
Expert:  Brian replied 10 years ago.
It sounds like there could be a vacuum leak, which will cause an
overly lean mixture at idle. If the mixture is then adjusted to
compensate for the vacuum leak, it will be too rich off idle. A vacuum
leak allows such a small amount of air to enter the intake manifold,
that it does not affect the engine at higher rpm, but it will throw off
the idle mixture since it adds too much air at idle for the engine to
run right.

I recommend looking for vacuum leaks by checking for cracks in rubber
vacuum hoses, uncapped ports on the carburetor, bad intake manifold
gaskets. Some people will use propane or carburetor cleaner spray to
help locate vacuum leaks by allowing a small flow of propane around the
vacuum lines and possible leak sources. If the propane gas is ingested
through a vacuum leak, the engine will rev up a bit as the engine
receives the extra fuel to go with the extra air from the vacuum leak.

Once the vacuum leak is resolved, reset the idle speed and mixture.
Also make sure the vacuum advance works properly (no leaks in vacuum
diaphragm, no stuck mechanism), and check the ignition timing.

Customer: replied 10 years ago.
ive checked for vaccuum leaks and cant find any also when i adjust the mixture screws on my carb the passenger side screw has no effect whether all the way in or all the way out
Expert:  Brian replied 10 years ago.
The problem may be a result of the engine running too rich for a long
time. Carbon deposits may have formed that are inhibiting the engine's
airflow, and therefore affecting carburetor performance at low rpm. In
effect, the restriction caused by the carbon deposits could cause the
engine to pull less air through the venturi of the carb, and the effect
is like having a carb that is too large. Adjustment may not help. If
carbon is built up on the valves and combustion chambers, try running
some SeaFoam through the engine. It is similar to a product used by GM
technicians called "Top Engine Cleaner". It will soak into the carbon
and allow it to be loosened. A big cloud of smoke is normal during
treatment with SeaFoam, as the deposits burn off. The engine should
breathe much better afterward, and then you can adjust the carburetor
once the engine is verified to be running well with no vacuum leaks.

If the carburetor was running too rich, it could be due to the jetting
of the carb. It may be necessary to install smaller jets and keep an
eye on the spark plugs to make sure that a good mixture is present.
Adjusting the jet size is a trial and error procedure unless you have
access to a dynamometer and wideband oxygen sensor test equipment. The
Holley links at the bottom will take you to some info on adjusting the
jet size for optimal mixture. The mixture screws on the carburetor only
affect the mixture at idle, since the jets are not flowing at idle.

The idle mixture is adjusted by two screws, one on the driver's side of
the metering block, and one on the passenger side. These screws are
supposed to be adjusted equally.

The following info is from the Holley website:

When tuning the idle mixture, you’re actually tuning for the best
manifold vacuum. Idle mixture needles are found on the primary metering
blocks. If you change one idle mixture needle, you must change the
other idle mixture needle by the same amount. Here are the proper steps
for setting the idle mixture needles.

  1. Attach the vacuum gauge to a manifold vacuum port.

  2. Adjust each idle mixture screw (Figures 10 & 11) 1/8
    turn at a time, alternating between each screw. Turn them equally,until you achieve the highest possible vacuum reading without adjusting the curb idle speed screw. Turn screws in to lean the mixture. Turn them out to richen the mixture.

Figure 9

3. Now that the idle mixture is set, it may be necessary to go back and reset the idle speed using the curb idle speed screw,

as shown in Figure 9.

4. If a vacuum gauge is not available, use a tachometer to obtain the highest RPM.

Figure 10

Figure 11

ROUGH IDLE AND VACUUM LEAKS: If a rough idle persists after the engine
has been started and the mixture screws adjusted, check for manifold
vacuum leaks. These could result from unplugged vacuum fittings or a
carburetor flange gasket

that was torn during installation. Recheck for proper attachment of all vacuum lines and check the lines for cracks.

If the manifold was changed, a manifold vacuum leak could occur at the
cylinder head/manifold surface due to damaged gaskets or improper
torquing. Frequently, manifold vacuum leaks occur from the valley side
of the manifold. These are very difficult to detect, unless a
discernible whistle can be heard.

NOTE: In most cases, when rough idle occurs after a carburetor/manifold
change, they result from manifold vacuum leaks similar to those
described above. Assuring a proper manifold installation rather than
assuming the carburetor is not

functioning properly will ultimately save time.

Take a look at the Holley site, there is more info available at the following links:

Holley 2300 Installation, Tuning and Adjustment

Inside Your Holley Carburetor

Exploded View of 2300

Please remember to select "Accept" if the answer is helpful. If you need more info, let me know.

Customer: replied 10 years ago.
the problem is on and off it will run fine for a little while then it will run like crap. it does it mostly while in gear and stopped. tho it is somewhat noticable in park. why would it be an on and off problem?
Expert:  Brian replied 10 years ago.
An intermittent problem could be due to a vacuum leak that is
opening and closing. Sometimes a hose or gasket can develop a leak that
will open and close at random. Idling in gear creates more manifold
vacuum than idling in Park, so the problem is more likely to occur with
the stronger vacuum if it is a leak. I know you said you have checked
for vacuum leaks, but if the check was done with the problem not
present then the leak may not have been present during the check. It
might be worthwhile to check again with the help of a partner so that
you can test it while idling in gear (be very careful doing this, set
the brake and block the wheels).

Also, check the vacuum advance mechanism, it might be binding up
intermittently and preventing the engine from running with the right
amount of ignition timing. Vibration and heat may cause the binding to
come and go.

Customer: replied 10 years ago.
ive got mechanic i know who is going to put a scope on it to see if it has something to do with the ignition but ill have to check the vacuum in gear too hopefully thats it. ive got a lot of people stumped on this one but uve got some good ideas. ill let you kno how it goes. im also going to try the sea foam because it has been running rich for a while.
Expert:  Brian replied 10 years ago.
I hope you get it fixed up. Sounds like a neat car to drive.

My truck ran rich for a while and I did the BG Chemicals throttle body
and intake cleaning system, which is similar to the SeaFoam, and it
helped out a lot. It was having trouble starting cold, and it fixed
that problem. I wish I could have seen a before and after to see how
much carbon had built up.

Please select the "accept" option if you have found the answers helpful. Thanks!

Customer: replied 10 years ago.
i tried the sea foam. it smoked out the tail pipes for a bit but it didnt really change anything. it runs good, then runs bad, then runs good, and so on. ill have to check the vacuum idea again. im also using more fule than i normally did before the problem started. when this whole problem started it was a cold day and it was almost like a switch was flipped and it began to run REALLY rough. i used between 1/2 and 1/4 of a tank of gas driving only about 25 miles. it could be a number of things and ive checked everything. i guess ill have to check it all again and again until i get an idea of exactly what the problem is. let me know if you have any more ideas. thanks
Customer: replied 10 years ago.
another question i had is what would caust the idle adjustment screws to have little to no affect on the idle of the engine. the driver side has some effect but the passenger side can go all the way in and all the way out without doing anything. what do you think?
Expert:  Brian replied 10 years ago.
They should be adjusted together, one is for the left barrel of
the carb and one is for the right barrel. So they end up feeding into
the same airspace. The idea is just to have them balanced. Adjusting
one screw after the other may not result in the same change in rpm. I
suppose it is possible that the passageway for the passenger side screw
is clogged, but you would have to do some tests on the carburetor or
just disassemble it and check the passage to know for sure.

I think if you had the right one adjusted to the slowest engine speed
possible, then you should be able to raise the rpm by changing the
passenger side screw. If it never responds to any changing of the
passenger side screw, I would consider rebuilding the carb and doing a
thorough inspection.

Expert:  Brian replied 10 years ago.
If you can unscrew that passenger side screw and take it out, try
spraying some carburetor cleaner in the hole to blast out any debris.
Maybe you can get the passage flowing again if it is clogged.

Customer: replied 10 years ago.
Relist: I still need help.
Customer: replied 10 years ago.
a mechanic that i took it too looked at the car and every time he drove it the problem didnt occur so i drove it with him in the car with me and it happened in less than 5 minutes. he seems to think that it is a vacuum leak as well but isnt sure where. the only place i could think of is at the front of the intake. i have a oil pressure censor there and it leaks oil. if the oil saturated through the rubber gasket at the front valley of the intake manifold would that cause a vacuum leak? i was thinking that as the car heated up the oil would become thinner making it easier for the air to get through there. is that possible even though i have the intake torqued to the proper specs?
Expert:  Brian replied 10 years ago.
If there was a leak between the intake manifold and the engine
block at the front, it would just be an oil leak. A vacuum leak would
only occur between the intake manifold and the cylinder head.

In the picture below, I have highlighted the points in the intake
gasket where a vacuum leak could occur. Not all of the ports would
prbably have a leak, but one or more may have an intermittent leak
related to engine temperature.

In the bottom picture I circled the areas where the ports attach to the
cylinder heads. These are the areas where the intake manifold could
have intermittent vacuum leaks (yes this is a 4-barrel carb manifold,
but the principle is the same).

Also, keep in mind that the carburetor gasket can become a vacuum leak.
So make sure to check around the base of the carburetor carefully. It
would be fairly easy to replace the carburetor mounting gasket to rule
it out. Make sure to tighten the carburetor down evenly in a
criss-cross fashion and do not overtighten it since that can cause
warping and leaks. Use a torque wrench to make sure each nut is
tightened equally.

Customer: replied 10 years ago.
Relist: I still need help.
the mechanic i had take a look at it seems to think strongly that it is the power valve. i took it out of the carb and im pretty sure that it is bad. how do i judge what size power valve is the right size for my engine size. is there a sure way or is it just trial and error? i think that the one that was in there was a 5.5.should i try the same size? since i put that carb on my car it has run rich should i maybe go smaller? what do you think?
Expert:  Brian replied 10 years ago.
From the Holley Power Valve Tech Article :

"The power valve is a key component of the power

enrichment system of Holley performance carburetors.

The power enrichment system supplies additional fuel to

the main system during heavy load or full power situations.

Holley utilizes a vacuum operated power enrichment

system and a selection of power valves is available

to “time” this system’s operation to your specific


Each Holley power valve is stamped with a number to

indicate its vacuum opening point. For example, the

number “65” indicates that the power valve will open

when the engine vacuum drops to 6.5" Hg, or below.

An accurate vacuum gauge, such as Holley P/N

26-501, should be used when determining the correct

power valve to use.

A stock engine, or one that is only mildly built for street

use, will have high manifold vacuum (17" to 21" Hg) at

idle speeds. To determine the correct power valve, the

vehicle should be driven at various steady speeds and

vacuum readings taken. The power valve selected

should have an opening point about 2" Hg below the

lowest steady speed engine vacuum observed. Holley

has a 6.5" Hg power valve, P/N 125-65, which usually

works out well for most driving situations."

Basically, the main jet size provides the tuning for regular
driving, the power valve size provides tuning for heavy load
enrichment, and when it comes in, and the idle adjustment screws take
care of idle.

If you put in a 6.5 power valve like they mention in the above article,
the enrichment would occur sooner than with the 5.5. The manifold
vacuum drops as the throttle is opened, so a lower power valve number
requires more throttle opening before it starts adding extra fuel. At
any vacuum levels above the power valve number, the power valve is not
adding fuel, it is up to the main jets to meter the correct amount.

Using a good vacuum gauge as a tool is important. If you notice that
the engine seems to bog around 5.5" Hg, (the point where the power
valve opens), then it might be due to the extra fuel which is not yet
needed. Swapping to a 3.5 or 4.5 will delay the enrichment. Keep in
mind that if the carb's jet sizes had been picked to work with the
power valve, changing the power valve will mean you have to compensate
for the change by adjusting the jet size.

Also, on your idle enrichment screws, if they still are not affecting
the idle, it could be due to the idle speed being set too high. If the
throttle blades expose too much of the idle transfer slots, the main
circuit will start adding fuel, and this overwhelms the idle mixture
screw's ability to control the mixture. So make sure that only
.040"-.050" of the slots are showing under the throttle blades at idle.
The slots allow the carburetor to transition from idle to main circuits
by providing a bit of fuel as the throttle starts to open. You may have
to adjust the idle speed down, and then try adjusting the idle mixture
screws to accomplish this.

Customer: replied 10 years ago.
Relist: I still need help.
i changed the power valve to a 3.5 and downsized the jets to 65 and it ran good at first but after sitting for a few hrs after some driving it ran terrible. it wouldnt idle, and when i gave it gas it backfired out of the carburetor every time. this was all yesterday. today i checked the vaccum with a gauge and checked the compression both of which were good. after i was done checking the compression however, i started it and i popped out the carb then out the exaust and is now backfiring out the exhaust. do you think that i need a new distributor? it is the original and has over 100k miles on it. i just cant help but think that my timing just keeps changing internally. what do you think?
Expert:  Brian replied 10 years ago.

If the backfiring only started after changing the carb adjustments, then I
think the distributor is not causing the problem. But it is very important to
ensure proper ignition timing before adjusting the carb. So make sure to check
the timing, and inspect the distributor. The timing must ALWAYS be proper before
attempting carb adjustments. Improper ignition timing can cause backfiring, but
so can mixture problems. Take the cap off and wiggle the shaft to see if it has
excessive side to side play from worn bushings. Excessive wear could cause
inconsistent timing. Replace the distributor if you feel it is beyond

Using the vacuum gauge is a good way to diagnose. Take a look at this page
for some good carb tuning info and what the vacuum gauge can tell you:

Use a vacuum pump to test the vacuum advance diaphragm for leaks, and watch
inside the distributor to see if the vacuum advance mechanism works without
binding when the vacuum is applied and released. If the advance sticks in the
advanced position, it can cause backfiring.

The backfiring was probably initially caused by the mixture being too lean.
When I was explaining about changing the power valve, I was trying to make the
point that the carb could have been adjusted to compensate for extra fuel from a
blown or wrong-sized power valve. Therefore, the jets might have been already on
the lean side since there was additional fuel all the time from the (possibly
leaking) power valve. Once you changed the power valve, the mixture was probably
leaner, but then it was leaned further by the jet size reduction. It might have
needed no jet size adjustment, or an increase in jet size.

The backfiring can cause the power valve to blow out again on older
carbs. Holley revised the design on the new carbs to incorporate power
valve protection by means of a spring loaded check ball. This type of
protection can be added to older carbs with a kit that costs about
$10-15. The easiest way to check for blown-out power valves is to
gently turn the idle-mixture screws all of the way in. If the engine
starts and idles without stalling, it's getting fuel from somewhere
else—most likely a leaking, blown-out power valve. Or too much of the
transfer slots are showing like I explained previously. I recommend
checking the power valve again, and going back to the jets that were
removed. It might be necessary to go to further tune by additional jet
changes. Check the power valve, install the protection kit if your carb
doesn't have it, and switch jet sizes to a larger number, then see how
it responds.

Since the backfiring changed from intake to exhaust, I wonder if you have the
original exhaust system with the Air Injection Reactor system to reduce
emissions by injecting air into the exhaust. There is a diverter valve in the
system that is vacuum controlled, and it is designed to stop airflow into the
exhaust on deceleration to prevent exhaust backfires. Air leaks in the exhaust
can also cause backfiring. If the diverter valve diaphragm was damaged, it could
be allowing air to pump into the exhaust. Disconnecting the air pump would helps
diagnose if it was pumping air into the exhaust and causing the backfire.

Customer: replied 10 years ago.
Relist: I still need help.
it isnt the original exhaust i put hedman headers on when i rebuilt the engine. it is doing all of the backfiring in park. if changing the jets smaller and the powervalve to a smaller size makes it too lean then why are my plugs still fouling out? they look like they are covered with charcoal dust and many dont appear to be firing because of the buildup. it doesnt take long for them to get that way either.
Expert:  Brian replied 10 years ago.
There are several things that can cause backfire. Incorrect
ignition timing and incorrect mixture are the most common. Depending on
how the backfiring occurs, it can usually be diagnosed. A lean backfire
would be more likely to occur under some load. You described the
backfire as through the intake initially, and then through the exhaust.
If it is backfiring in park through the exhaust, there is a problem
with unburned fuel getting into the exhaust and burning. Air getting
into the exhaust like I described earlier can also contribute to the
problem. Since you have the headers, I am assuming you got rid of the
air injection system that connects the air pump to the exhaust.
Unburned fuel could be getting into the exhaust because there is simply
too much fuel (as the spark plugs now indicate--make sure the power
valve is not blown), or because the ignition spark is occurring too
late, and then the fuel is still burning when the exhaust valve opens.
So checking the timing is vital, because you can play with mixtures all
day and if the timing is the problem, it will never get resolved

When setting the timing, make sure that the timing marks on the
balancer are accurate. It is possible for the balancer's outer surface
to slip and cause incorrect timing readings. The way to make sure it is
accurate is to get the #1 piston to Top Dead Center and then make sure
that the timing mark is showing Top Dead Center accurately. This
way the timing will end up being set accurately--if it is referenced
off an inaccurate balancer, the timing will end up inaccurate.

The mixture definitely sounds too rich at this point, but that is
possibly due to a blown power valve. Right after you replaced the power
valve and the jets, the car did not backfire through the exhaust. I
think the rich condition you are seeing now may be the result of the
intake backfire rupturing the power valve diaphragm.

Customer: replied 10 years ago.
Relist: I still need help.
the car ran fine after i changed to the 3.5 power valve and then when i started it after it sat for a few hours it wouldnt idle properly. it was fouling out before it started backfiring out of the carburetor then after the running condition worsened it began to back fire out of the carburetor every time i put my foot on the gas.

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