Engine and Transmission Tech

I recently got my MGB 3.5 running, with an Edelbrock 500CFM, and I noticed that when coming off the throttle (only when in gear driving, not running in neutral), the exhaust would make a few popping noises. I didn't worry about it much, but it's becoming more prevalent. Yesterday I went from my home at 5000 feet on a drive up to 10000 feet. On the way down, the issue became chronic and happened under compression braking, acceleration, and even a little bit while cruising. At first I assumed I was now running too rich, and that must be the source of the problem in general; however, leaning out the mixture screws had no effect I could see. But are those only for idle, and therefore irrelevant? I also played with the ignition timing. Advancing it seemed to alleviate the problem a bit, but I wouldn't swear to it. At the times when the popping occurs during acceleration, it seems to be accompanied by slight power hesitation.

I also noticed that now whenever I step on the throttle, the engine hesitates briefly and jerks, unless I ease into the throttle REALLY slowly. I assume this is just me getting ahead of the accelerator pump, but I don't think it was happening when I first got the car running.

Can anyone tell me if I need to change my jets and/or metering rods for life at 5000+ feet? Or if something else may be up? I went through the Edelbrock manual, and it provides a bunch of charts on what components to change for a desired mixture, but I don't know enough to understand what I need to do to my mixture.

Thanks!
Billy

Our engines may need different tuning due to different cams, compression ratios, headers, etc... but at least we're at similar altitude. I'm no carburetor expert, but I suspect you're running rich. The idle screws may have a small effect, but it's the combination of main jets and needles that are going to matter most. I installed an air/fuel mixture gauge this spring, and I found it very instructive/helpful. (Taking the car to a tuner and/or putting it on a local dyno might have been better.) Anyhow, here's the jetting I'm currently using: 083 main jets, 089 secondary jets, 1460/065x052 needles, and yellow springs.

Billy, how many miles are on your engine? I don't want to be the carrier of bad news, but it sounds suspiciously like a wiped cam lobe. Do you run a zinc rich engine oil or ZDDP additive? Modern oils do not have enough zinc for a flat tappet engine and will cause the cam to destroy itself.

Jim

Thanks, guys. Curtis, I re-read the manual a 10th time and finally figured out where I was misunderstanding it (must be poorly written!). I came up with the same primary jet / metering rod combination you're using; I'll pick up the jet tomorrow and try it out. I'll definitely keep the secondary jet and the spring in mind, thanks.

Jim, the engine has only a few hundred miles since a rebuild with a brand new camshaft; I'm hoping I haven't managed to trash it already. If the carburetor fix doesn't straighten me out, I will definitely keep that in mind. Also, thanks for the heads up on the zinc-rich oil.

> I noticed that when coming off the throttle (only when in gear driving, not
> running in neutral), the exhaust would make a few popping noises.

If the carb is set up for lean cruise, it's not unusual to have some
popping during compression braking. Make sure the initial timing is
sufficiently advanced and the total isn't too large.

> At the times when the popping occurs during acceleration, it seems to be
> accompanied by slight power hesitation.

Hesitation can be momentarily too lean or too rich. Check your
accelerator pump operation. There should be a clearance spec in the
Edelbrock manual. The pump should squirt for very small throttle movements.

> Can anyone tell me if I need to change my jets and/or metering rods for
> life at 5000+ feet?

In general, yes. There are correction formulas for altitude compensation
but, if you just bolted the carb on out of the box, you need to do some
initial tuning. I've attached my tuning notes below.

> Billy, how many miles are on your engine? I don't want to be the carrier
> of bad news, but it sounds suspiciously like a wiped cam lobe.

That is very possible and will make popping sounds, along with a loss of
power.

Dan Jones

********************************************************************************

I recently went through a tuning exercise on my Triumph TR8 using a wide
band O2 sensor. I highly recommend using a wide band O2 sensor to check
your mixture. Besides making the process of tuning a carb go much faster,
it's found problems I didn't know I had. When I started tuning the Rover
3.5L in the TR8, it was rich in 1st, 2nd and 3rd but would go lean at the
top of 4th gear and in 5th. I figured there were 3 possible causes:

1. Fuel pump capacity insufficient
2. Carb inlet needle and seat too small
3. Fuel filter too restrictive

Turns out the fuel filter I had (one of those metal mesh filters in a glass
tube) was too restrictive. Holding it up to the light, I could see through
the mesh so it didn't appear clogged but the wideband knew better. By the
time I hit fourth, the fuel bowls had drained enough to cause the mixture
to go dangerously lean. A larger diameter clear plastic fuel filter
(Purolator F21111) fixed the problem. Since the Purolator filter has a
plastic body, I moved it to the firewall to keep it away from direct engine
heat. The fuel filter originally on the car was maybe an inch diameter with
a glass body, a metal mesh filter element and 3/8" inlet and outlet.

I kept a detailed log of my tuning. Here's a summary:

The engine is a stock low compression (8.1:1) Rover 3.5L V8 engine with
the following modifications:

Edelbrock triangular foam air cleaner (needs to be replaced with a real filter)
Edelbrock 1404 500 CFM four barrel carb (Carter AFB clone)
Offenhauser/JWR Dual Port intake manifold
Crane hydraulic flat tappet cam (unknown specs)
New but stock rate TR8 valve springs
Rhoads fast bleed rate lifters
Rimmers tri-y headers and dual exhaust (H-pipe, no catalytic converters)
Mallory Unilite distributor with vacuum advance
MSD 6AL spark box
MSD Blaster 3 coil
NGK BPR5EY11 spark plugs gapped at 0.040"

I installed the Mallory, MSD and plugs. Everything else was installed by
the previous owner and, if I were buiding the engine from scratch, I'd have
picked different parts.

The Edelbrock 1404 is a clone of the Carter AFB. The easy stuff to change or
adjust includes:

idle stop
idle mixture screws
primary side jets
primary side rods (controls primary cruise and primary power mode mixtures)
secondary jets
step-up spring (controls when the transition occurs between cruise and power
mode mixtures)
accelerator pump lever hole
float drop

Instead of a power valve for enrichment like a Holley, the Carter/Edelbrock
AFB's use a rod and jet arrangement. A 2 step tapered rod, controlled by a
vacuum-referenced, spring-loaded piston, moves up and down in the jet to
provide a two different area main metering orifices. This allows the carb
to adjust the air-to-fuel ratio for differing loads, as sensed by the vacuum
level. When the vehicle is cruising, the rod is on the lean step. Under
high load, low vacuum, conditions the rod moves to the rich step. The
metering rods and step-up springs can be changed without removing the top
cover but the cover has to come off to R&R the jets. There's a series of
screws that hold the top cover on but you must first remove two small clips
that hold the accelerator pump and throttle lever linkages in place. These
are very tiny and easy to drop, so keep a spare pair on hand and cover the
carb when removing them. I used a very small pair of needle nose to remove
the accelerator pump clip but found a dental pick with curved hook worked
better on the throttle linkage. You should also remove the rod and pistons
(uses torx headed screws to hold the covers on). Unlike a Holley, there's
no need to drain the float bowls when changing jets. Use a screwdriver with
a tip that is the same width as the jet, otherwise it's easy to scrape a
little aluminum of the carb body.

The goal of my tuning was to get it lean of stoichiometric at cruise for
best fuel economy while in the range for maximum power at wide open
throttle. 14.7:1 is the stoichiometric ratio. It's the "chemically ideal"
where there is no excess fuel or oxygen left after combustion. Leaner means
there's excess oxygen left after combustion. Richer means there's excess fuel
left. Generally, you want to run rich of stoichiometric at wide open throttle
(WOT) and a bit lean at cruise. There's no single ideal ratio that applies
to all engines. Some engines make best power at 13:1, others closer to
12.5:1. Note that the air fuel ratio is by weight. 13:1 means 13 pounds
of air are mixed with 1 pound of fuel. The usual target values for normally
aspirated 4 stroke engines are about 12.5 to 13 for WOT, 14.0-15.5 at
part-throttle cruise and 13.5-14.0 for part throttle acceleration (or
climbing a long hill, pulling a load, etc.). If you want to lean out the
mixture at cruise for best fuel economy, be aware that you'll also need to
adjust timing. Combustion gets much slower under lean conditions and if
you don't adjust spark timing, the combustion occurs much later and exhaust
temperature climbs. That's bad for the exhaust seats and valves. However,
if you adjust for MBT spark at each A/F ratio, exhaust temperature will
actually decrease relative to stoichimetric (rich will still be somewhat
cooler). For typical gasoline engines, the range or ratios is:

A/F Characteristics
Ratio
5 Rich burn limit. Combustion is weak and/or erratic.
6-9 Extremely rich. Black smoke and low power.
10-11 Very rich. Some supercharged engines run in this range at full power as
a means of controlling detonation.
12-13 Rich. Best power A/F for normally aspirated WOT.
14-15 Chemically ideal. At 14.6 the A/F is at the theoretical ideal ratio
with no excess fuel or oxygen after combustion. Good A/F target for part
throttle cruise and light to moderate acceleration.
16-17 Lean. Best fuel economy A/F ratio. Borderline for part throttle
drivability (worse than borderline if EGR is used).
18-19 Very lean. Usual lean limit (Driveability).
20-25 Lean burn limit. Varies with engine.

If your engine has a cam with a lot of overlap, your wideband may read rich
at idle, even though it isn't. In cases like that, I usually set the idle
mixture use the RPM drop method. Adjust mixture to yield maximum RPM, then
lean it so the idle drops 20 to 40 RPM. Even with milder cams on engines
with carbs, I usually only get around 13:1 at hot idle if the idle speed is
low enough. Raise the RPM a bit and the air fuel ratio will quickly go to
the cruise value. You can fine tune this by lowering the float level, if
need be. Also placement of the sensor (in collector versus near tailpipe)
can yield different results. Dyno shops using tailpipe probes will often
shoot for a leaner WOT air-fuel ratio in the 13.0 to 13.5 range. Measuring
at tailpipe tends to read leaner than if it was measured at the header.
13.2 at the tailpipe can correlate to 12.5 at the header. The best thing
to do is to correlate air fuel ratio on a dyno with maximum power for your
set-up. You can accomplish the same thing with an accelerometer based G-meter
or at the drag strip.

I started the tuning session by driving the car around to warm it up before
setting the initial idle speed and mixture. Timing and spark strenght had
been set previously. It drove well with no bogs or flat spots but the old
plugs and tail pipes suggested the mixture was rich. The carb was installed
out of the box without any jetting changes by the previous owner. The
baseline specs of the manual choke 1404 carb are:

1423 jets (0.086" diameter) in primaries
1460 rod (0.065" x 0.052", stamped 6552)
orange step up springs (5" Hg)
1426 jets (0.095" diameter) in secondaries
center position accelerator pump
0.0935" needle and seat
11/32" float height (+/- 1/4" float drop)
middle accelerator pump link hole
0.028" accelerator pump nozzle diameter
500 CFM rating

I set the initial hot idle speed to 750 RPM. BTW, with the MSD, it starts
just fine without the choke and will idle down to 400 RPM. I set the initial
idle mixture using the method suggested in the Edelbrock carb owners manual
(richen to maximum idle speed then lean to a 20 RPM drop). I then installed
a wide band O2 sensor in a bung I had welded in previously. It's located aft
of the passenger side header collector with the sensor cable routed through
the engine compartment (zip tied away from anything hot) out the rear of the
hood into the passenger side seat.



The wideband showed the the idle mixture at 13:1. I was initially going to
try for stoichiometric (14.7:1) but settled on 13.5:1. Then I noticed the
representative charts in the Edelbrock catalog show a 13:1 idle mixture. Be
aware that cams with a bunch of overlap can fool the wideband at idle due to
the unsteady nature of the idle (misfires result in unburnt fuel passing over
the sensor). This will go away quickly as the revs rise. Since it's
dangerous to drive and read the gauge at the same time, I had a neighbor do
the driving while I read the meter. It was immediately obvious that the base
calibration was way too rich (2 points across the board) for my engine.
Edelbrock provides nice little charts that correlates the rod and jet
combinations to changes in mixture:

[www.bacomatic.org]
rt.jpg.html

Each step is approximately a 4% change. With the mixture readings from
the meter, it's possible to calculate how many steps are required to get
a desired mixture. Be aware that not all possible combinations appear on
the chart and only the highlighted ones are possible using the tuning kit.
You'll need to purchase additional tuning parts for the other combinations.
I wrote a little program to calculate all the possible rod/jet combinations
and it helped me find a couple of rod/jet combos that worked better in my
application. I'm now cruising at 15.5:1 (lean of stoich for better fuel
economy) with transition mixtures (climbing a hill, pulling a load, part
throttle acceleration) in the 13.5 to 14:1 range and WOT in the 12.5 to
13:1 range. Pretty much ideal. When I started, it was rich across the
board. I've picked up 3 MPG and power to boot. Using the combos on the
chart, I couldn't get the primary power mode lean enough so I had to
compensate by leaning down the secondaries a bunch. Getting the right
primary rod/jet combo fixed that. Depending upon where your cruise mode
mixture is, you may want your primary power mode to go between 15 and 25%
richer.

The initial reading was so rich, I didn't even bother to write the air-fuel
ratios down. Returned home and went three steps (12%) lean on both the
primary cruise and primary power modes:

#10 on chart, #1422 jets 0.083", #1463 rods (0.067" x 0.055")
Effective Cruise Jet Area (sq. in.) = 1.8849555921538760E-03
Effective Power Jet Area (sq. in.) = 3.0347785033677404E-03
3.0348 / 1.8850 = 1.61 or 61% more area
Flow is proportional to area squared
0.61**2 = 0.372 or 37.2% power enrichment flow

That was too lean at cruise (had a mild surge) so I went to #11 on the chart
(2 stages lean on both cruise and power modes):

#11 on chart: #1422 jets 0.083", #1460 rods (0.065" x 0.052")
Effective Cruise Jet Area (sq. in.) = 2.0923007072908024E-03
Effective Power Jet Area (sq. in.) = 3.2868913138183213E-03
3.287 / 2.0923 = 1.571
0.571**2 = 0.326 0r 32.6% power enrichment

That put me near stoichiometric (14.7:1) at cruise but was rich in power
mode which also made WOT rich, even after leaning the secondaries from
the baseline 0.095" jets down to 0.080" (five 4% lean steps). The power
mode was too rich, causing me to try compensate by leaning out the
secondaries. The tuning chart doesn't show any other combinations that
would allow me to fine tune this set-up but using the program I wrote, I
came up with some additional combinations not on the chart that worked
better:

not on chart: #1422 jets 0.083", #1461 rods (0.065" x 0.057")
Effective Cruise Jet Area (sq. in.) = 2.0923007072908024E-03
Effective Power Jet Area (sq. in.) = 2.8588493147667119E-03
2.860 / 2.092 = 1.367
0.367**2 = 1.135 or 13.5% power enrichment

not on chart: #1421 jets 0.080", #1441 rods (0.062" x 0.052")
Effective Cruise Jet Area (sq. in.) = 2.0074777056438778E-03
Effective Power Jet Area (sq. in.) = 2.9028316119169689E-03
2.9028 - 2.0075 = 0.8953
0.8953 / 2.0075 = 0.446
0.446**2 = 0.200 or 20% power enrichment

I purchased the additional rods and jets for these two combinations.
When I went to install them, I noticed the #1461 rods were machined
differently. Both carry the same part number, though it's obvious
they are from different production batches as the stamped lettering
is different. Also, the length of the machined step is different
and the tip of one of the rods was blackened which solvent failed to
remove. I put the micrometer on the rods and they varied more than
I cared for. I also have a Carter 400 AFB here and, as luck would
have it, it was equipped with rods with the power mode of the #1461
rods but a slightly leaner cruise mode. Since the rods are so easy
to change out, I swapped the rods over and took it for a test drive.

not on chart: #1422 jets 0.083", Carter 400 rods (0.066" x 0.057")
Effective Cruise Jet Area (sq. in.) = 1.9894135478857367E-03
Effective Power Jet Area (sq. in.) = 2.8588493147667119E-03
2.860 / 1.989 = 1.437
0.437**2 = 0.191 or 19.1% power enrichment

Cold, without the choke, there was some mild lean hesitation but that
quickly went away as the engine warmed. The leaner rods did the trick.
By leaning out the primary mode, WOT also leaned out without a further
seconday jet change. With the Carter rods, I'm now cruising at 15.5:1
(lean of stoich for better fuel economy) with transition mixtures
(climbing a hill, pulling a load, part throttle acceleration) in the
13.5 to 14:1 range and WOT in the 12.5 to 13:1 range. Pretty much ideal.
I got 19 MPG on the last tankful when I was cruising at 14.7:1, up a
couple of MPG from where I started even though I was mainly WOT tuning
the carb or cruising at 75+ MPH. With the latest carb changes, I think
it'll do 20+ MPG around town. Edelbrock doesn't have a 0.066" x 0.057"
rod listed. The next size listed is #1436 rods (0.068" x 0.057"). That
would be too lean at:

not on chart: #1422 jets 0.083", #1436 rods (0.068" x 0.057")
Effective Cruise Jet Area (sq. in.) = 1.7789268400952206E-03
Effective Power Jet Area (sq. in.) = 2.8588493147667119E-03

With all Edelbrock parts, the best bet would likely be the other
combination listed above that's not on the charts (#1421 jets 0.080",
#1441 rods 0.062" x 0.052")

There are some other rod/jet combos that give similar but slightly different
effective jet areas. If I was really trying to fine tune this combo, I'd
test each on the same day with an accelerometer to see which gives the best
performance. Vizard has gone so far as to put O2 sensors in each header
primary so he could stagger jet to better equalize the mixture distribution.
IIRC, he said it was worth 20 HP on an engine in the 300 cubic inch range.

Some miscellaneous observations. While it starts just fine with no choke,
you don't get the fast idle cam so it idles at 500 RPM cold versus 750 hot.
The Offenhauser Dual Port/AFB combo is very smooth and I could not tell when
the secondaries opened. It will pull 3rd gear idling through the subdivision
and is, in fact, smoother than the fuel injected 5.0L in my 1987 Mustang.
In case you aren't familiar with it, the Dual Port design has the runners
split into top and bottom sections with the plenum split fore and aft (rather
than the usual side-to-side). It's essentially two single plane intakes
stacked on top of each other with the 4 barrel carb primaries feeding the
longer path lower runners and the secondaries feeding the shorter path upper
runners. It's an interesting concept but the packaging required to fit within
a carb intake envelope does compromise the design. The Offy Dual Port has the
reputation of being a good low to mid range intake with excellent throttle
response and fuel economy (probably the best cruise fuel economy based upon
BSFC tests performed by David Vizard... I've got the dyno results here some
place). It's not a high rpm intake but Offenhauser says you can trim the
dividers back at the head flange to pick up some top end power. An Edelbrock
Performer Rover intake will make better power but is taller so hood clearance
may be an issue. The Edelbrock foam air filter has got to go. The air fuel
readings were not quite as stable as the readings I got on my buddy's 428CJ
powered Cobra replica. That might be due to the size of the motor relative to
the carb (750 CFM on a 428 versus 500 on a 215). Also, the accelerator pump
shot and/or clogged fuel filter may have had something to do with it. Make
sure to tighten the fuel line after you've re-jetted or you'll spray fuel on
the hot manifold. Doh! It's tough to read the numbers on the rods and jets
once they get stained with fuel. I resorted to the calipers (for the rods)
and a tapered rod (for the jets) to verify the sizing. I then bagged and
tagged the rods in individual zip-loc baggies (the really small snack size).
I noticed a little sediment in the bowls and cleaned it out. I'll check
again later. I was shifting at 6000 RPM and managed to bump the 6400 RPM rev
limiter once while watching the meter. I bought a dual outlet for cigarette
lighter so I could use the radar detector during tuning. The wideband O2
uses the lighter socket to provide power for the sensor heater. The primary
side venturis of the 600 CFM AFB's are the same size as the 500 CFM AFB's
though the boosters are different. Need to get a G-meter to correlate
acceleration (power) with mixture.

The wide-band I used is an Innovate LM1 and it's very nice. It allows you
to datalog or just watch a hand held display. On most of the cars I've tuned,
we've welded a bung into the header collector but on some we've had to go a
few feet aft of the collector. I typically use zip ties to temporarily route
the cable into the car via the passenger side window. When datalogging, be
aware noise from high power ignition systems can cause an erratic RPM converter
signal. A 50k-ohm potentiometer from Radio Shack will usually fix that problem.

As a follow up, the Offy Dual Port seems to need a richer mixture at WOT.
Even though wide band showed I was in the range I wanted to be at WOT, top
speed was down a bit so I suspect individual cylinders are leaner than the
average reported by the wideband. A recent dyno test of an Offy Dual Port
on another engine showed it to respond to richer than usual mixtures, likely
due to poor mixture distribution. The Dual Port is an interesting design
but has quite small passages inside and, if any engine does not need a dual
port, it's the small port Rover/Buick.

Further investigtion revealed the pre-load was improperly set on the Rhoads
fast bleed rate lifters by the previous owner. He installed adjustable
pushrods but they were too long to get to zero pre-load. I pulled the
manifold and removed the lifters for inspection and found they were
flat on the bottom, not radiused which indicates premature wear. I decided
to swap out the Offenhauser Dual Port and Crane H-216/285-2S-12 cam
(266/278 degrees advertised duration, 0.456/0.480" lift, 112 LSA) and Rhoads
fast bleed lifters with an Edelbrock Performer Rover intake, standard lifters
and an Erson RV10/RV15 (Wedge Shop custom grind) cam of the following specs:

280/288 degrees advertised duration (208/214 @ 0.050")
0.448"/0.460" lift
111 lobe separation angle
4 degrees advanced when installed straight up

With this combination, I had to change to #1423 main jets (.086"), #1450 rods
(listed as 0.070" x 0.042" but I measure them at 0.069" x 0.041") which is
close to the leanest cruise and richest power mode (#15 on Edelbrock tuning
chart).

The Offy/Crane/Rhoads combo was very smooth at low RPM, like fuel injection
and would pull 5th gear from very low RPM but fell off at higher RPM. Some
of that is probably due to the intake manifold design and some to the Rhoads
lifters. The small primaries of the Dual Port design maintain a high velocity
across the carb primaries giving good fuel economy and throttle response but
the design restricts flow at higher RPM. Also the lifters bleed down at low
RPM, effectively shortening the duration of the cam but testing by David Vizard
has shown they do not give the full duration and lift of the cam, even at high
RPM.

By comparison, the Edelbrock/Erson combo needs a bit more RPM to get into
its working RPM range but pulls harder and the power hangs on past 6000 RPM.
It might be interesting to try a Carter 400 AFB or perhaps an Autolite 4100.
The smaller primaries of the Carter 400 and the higher gain annular boosters
of Autolite 4100 would both tend to increase atomization at low RPM.

Dan Jones