There's Hair in My Fuel

After installing the new distributor I finally got the car started but it still didn't idle very well. The exhaust tone even sounded different. I tried to adjust the idle mixture screws but something wasn't right.

The procedure for adjusting the idle screws on a Holley requires a little finesse. Before starting the engine you turn the idle mixture screws clockwise until they are fully seated, and back them out 1.5 turns. This is the recommended starting point for tuning. My Holley 4150 has 2 screws on the primaries, some have 2 more on the secondaries. Start the engine, let it warm up, and then you can begin adjusting the idle mixture.

Adjustment can be done 2 ways. First, turn one screw in until the idle slows and then back it out until you get the highest vacuum reading. Then, back the other screw the same amount. If the idle increased you should lower it back down and repeat the procedure starting with the other screw. The second method ignores the vacuum reading and you adjust the idle mixture screws to get the highest rpm.

The problem I ran into was that turning the driver side screw had no effect on the idle speed or vacuum. In fact, I could turn it all the way in which should cause the engine to stall, but it didn't. The passenger side adjusted just fine though. Holley says ineffective idle mixture screws are indicative of a blown power valve, which would dump extra fuel at all times. I'm not sure that a blown power valve would only effect one side of the carb but it's an easy replacement.

Holley power valves are rated based on the vacuum that they operate. The stock valve is a 6.5 which requires an idle vacuum of 13" or higher to function properly. The correct power valve # should be half your idle vacuum, otherwise it could open too soon, and in worse cases open at idle. In my case, I'm only getting around 9" of vacuum at idle so I've known for a long time that my power valve is wrong, so this was a good time to replace it. I ordered a new 4.5 power valve from Amazon for less than $7. A week later it's $13.84, WTH Amazon?

Since I had to remove the primary metering block to replace the power valve I figured I would drop the main jets down from 71 to 70 since my spark plugs looked a little dark.

Here's the primary metering block from my Holley 4150. You can see the power valve screws in from the back side of the jets. What I wasn't expecting to find was a 6" hair coming from one of the passages. It looked like a strand of hair but it could have been a fine synthetic fiber. I tried to gently pull it out but it was stuck. I determined it was the port for the idle mixture screw and when I removed the screw the hair pulled right out with a big clump at the end.

The screwdriver is pointing to the passage where the hair came from. The idle mixture screw is removed and screws in from the side (the bottom in the picture). When I straightened out the hair it was about a foot long. I can't imagine how it got in there and I don't know if it came from Holley that way. I have removed this metering block once before and it seems I would have noticed this hair before.

I swapped the jets and the power valve and buttoned up the carb. After priming the engine fired right up but I could tell right away that nothing had changed. Fudge. It still wouldn't idle and the driver's side idle mixture still had no effect.

At this point I felt that the carb must have a blocked passage, especially since I already found some junk in there. I'm not too confident in my carb rebuilding skills so I felt it was best to just buy a cheap spare carb and rebuild this one later. I decided to buy an Edelbrock 1405 Performer 600cfm for just over $300 shipped.

When the new carb arrived I began removing the Holley. I started by disconnecting the fuel line and draining the fuel. When I pulled off the Mr. Gasket chrome fuel filter I noticed something wrong--it rattled when I shook it. If a filter rattles then it's loose inside which means fuel is leaking around the filter material. I looked in the outlet end and saw something was blocking the passage so I used a paperclip to try and pull out the obstruction.

Several plastic/rubber chunks came out, WTH? The one piece is curved so it's probably the seal around the edge. This kind of junk would cause total havoc with the smaller passages in the carb. When I drained the Holley fuel bowls more junk came out.

The larger pieces in the center of the blue cap are about 1/8" in size. No wonder the engine wouldn't idle. So the Holley needs a full tear down and rebuild. I'm glad I bought the new Edelbrock.

Lean On Me

With the new baby Holley Double Pumper installed the engine definitely runs a lot better. It idles very consistently at 800 rpms although it still lopes quite a bit from the mystery cam. I just finished swapping the cold 160 degree thermostat for a 180 and the idle seems to have increased slightly up to 900 rpms. I guess the intake mixture was too cold with the 160--the oil should stay a lot cleaner now too.

One big issue I still have is drivability at low rpms. If I open the throttle too quickly at say 1500-1800 rpms the engine will buck and backfire. The timing is fairly close to where it should be, and for now I have to assume the camshaft timing is correct, so Google says that my carb mixture is too lean. Now, I was actually thinking that I had the wrong power valve for my application which was making the mixture too rich when I cracked open the throttle. My Holley 4150 came stock with a 6.5 power valve which requires 13" of vacuum at idle to operate correctly. Since I'm only getting 8" I need a 3.5 or 4.5 power valve. But Google keeps telling me a backfire means lean not rich. So I pulled a spark plug to take a reading.

This plug looks very clean (except for the oil on the threads WTH). Too clean actually. It should be a light tan color and not this ghostly white. So she's definitely running lean. We had just driven the 'Stang back from a cruise-in so the engine had been running steadily at 2500 rpms for the last 30 minutes. So this plug is pointing to the main jets being too small. My Holley is stamped 4776-5 which means it came with 66 primary and 73 secondary jets. My old carb is stamped 4778-8 which should have 69 primary and 78 secondary jets as long as they weren't changed in the past. I think I'll remove the jets from the old carb and see what size they are.

The primary and secondary jets are located inside the front and rear fuel bowls, respectively. I removed the 4 retaining screws of the primary bowl on the old carb and then removed the jets. Instead of finding the stock 69 jets they were stamped 72! One of the side-effects of using a large carb on a small motor is that airflow is much slower than optimum, especially at idle and part throttle. The larger throttle plates don't have to open as much to supply sufficient air to the engine. This means that air moves slower through a large carb than it would through a smaller carb for any given volume, assuming that the smaller carb can handle the maximum flow required for the engine. Since carbs rely on airflow to "suck" fuel from the bowl, reduced airspeed means less fuel is being drawn in, making the engine run lean. So someone had previously increased the primary jet size to compensate for the slower airflow. Unfortunately that means I can't use these jets in my new carb. Jetting up from 66 to 72 is way too much. However, in a stroke of luck, I found a Holley jet kit on Amazon with same-day delivery! With this kit I'll have plenty of jets in all sizes to jet away to my heart's content.

While I'm waiting for the jets to arrive I reassembled the old carb and started removing the jets from the new carb. This is more difficult since the carb is still mounted on the engine but it's doable. I first siphoned as much fuel as I could from the bowl through the sight plug--the rest drains out when you remove the bottom screws.

I was able to remove the primary bowl with the fuel line still connected since it has a braided hose. Then I could just barely move the bowl out of the way and reach the jets with a screwdriver. 

While I was removing the jets the metering block came loose so I just removed it from the carb.

This made it a lot easier to change the jets. I debated back and forth about how big of a jet to go to. Holley recommends going two steps at a time when changing jets, but I'm not sure 2 will be enough. So for now I swapped the 66's for 68's.

While I was draining the fuel from the carb I figured it was a good time to swap out that glass fuel filter before it breaks and starts a fire. I picked up a cheap chrome filter for now. One of the cheap hose clamps was stripped so I had to make a run for a replacement. I also wanted to replace the two short pieces of rubber fuel line but the local Autozone didn't have any on the shelf. Eventually I'd like to replace it with braided line like I have feeding the carb.

With the larger jets in place I started her up to go for a test drive. It might have been psychological but the throttle response seemed much better, allowing a smoother start from standstill. However, as soon as I hit 3rd gear, a quick stab at the gas pedal produced another backfire. So I continued around the block and parked it without letting the engine fully warm up. Since it was still backfiring I figured it was still "way" lean and not just "wee" lean. I didn't want to go all the way to 72 since that would mean I could have used the primary jets from the old carb and saved the $60 for the jet kit. So I swapped jets again, but this time I jumped up 3 sizes to a 71. 

I didn't get to test drive the car for a few days until we went to a cruise-in at the local Rock 'n' Brew. She started right up and didn't backfire once on the trip there and back. The clutch still chatters a little, but not nearly as bad when I first drove the car. Right now the only drivability issue is when I try to crawl through a parking lot. While in 1st gear at basically idle speed the engine starts to buck. It's easily stopped by pushing in the clutch, increasing the revs a little, and letting the clutch back out. 

I'll drive it a few more times before I do any more fine tuning. Ideally, to read the plugs you should drive at cruising speed for a while and shut down the engine and coast to a stop without letting the engine idle. Then you can pull a plug while on the side of the road. This will narrow down the reading to just the primary jets. If the primary jets seem good I'll then have to tackle the secondary jets.

What I'd really like to do (someday) is install a wideband oxygen sensor. This would require welding bungs in the header collectors for a pair of O2 sensors. Then I could install a gauge that would monitor the air/fuel ratio taking all the guesswork out of jet selection. Since I can't weld I would have to either take my car somewhere and let somebody else monkey with it or remove the headers and take them someplace. Maybe if and when I have to remove the transmission to fix the leak I'll try removing the headers since I'll have to remove the rest of the exhaust anyway.

Good Carb, Bad Carb

The big Holley is probably at least 10 years old and has been sitting for most of that time so it's probably due for a rebuild. Even after spending the time and money to rebuild it I'm not confident it would ever perform well since it's so big. So I bit the bullet and bought a new carb. I wanted to stick with Holley, specifically a smaller double-pumper, because I like the looks and I could keep my fancy braided dual-feed fuel lines. I also wanted to keep my manual choke setup. The smallest 4150 double-pumper that Holley makes is a 600 cfm. I ordered 0-4776S which has a manual choke and silver finish.

The shiney new carb bolted right in since it's externally identical to the older carb. I need to replace that glass fuel filter soon--too much of a fire hazard. The engine fired right up since I had primed the primary bowl. I was able to set the idle down to 800 rpms whereas before with the older larger carb I couldn't get the idle below 1100. The engine sounded great and revved nicely but it would put out a bit of black smoke when I gave it a lot of gas so I think it's running rich. I took the car off the jack stands and backed it out of the garage for the first time in 6 weeks. Right off I noticed the clutch shudder was still there. As a drove around the neighborhood I also noticed the throttle response was actually too much. It was hard to accelerate smoothly. It reminded me of when I installed a twist throttle on my Honda ATV. I mistakenly hooked the cable to the two-stroke position instead of the four-stroke. This caused the throttle to open way too quickly. Now my Mustang felt the same way. Time to look at the linkage.

I had connected the throttle rod and return springs the same way they were on the old carb. The rod is attached to the carb so close to the pivot that there is only about 1.5" of travel with poor leverage near full throttle. So I swapped the attachment points for the rod with the return springs.

Now I'm getting about 2.5" to 3" of travel giving a much slower throttle opening with easier modulation. I still need to find a bushing for the end of the rod. A lot of other setups have the return springs attached to the bottom of the throttle linkage, pulling from the right. I might look into switching my setup that way since it looks cleaner. Although, right off it seems that having the rod and springs both pulling from the same side puts added tension on the throttle shaft causing addition wear.

Carburetor sure is spelled funny

Monster carb! (I'm not spelling that out again)

A quick lookup on the Holley website shows this to be a 4150 model 700 cfm double pumper, with manual choke and mechanical secondaries. I was told it was really a 750 so I don't know if it been modified or misidentified. Holley recommends that it be used on light-weight cars with manual transmissions and low gearing (higher numerical). You can't get much lighter than a 1st gen Mustang with its curb weight of around 2860 pounds. For comparison this is only about 200 lbs more than my Mini Cooper S, but still 300 lbs less than my wife's C6 Corvette.

Here's the transmission with tag HEH-BT. Another quick lookup shows it to be a 1965 Mustang close-ratio 4-speed toploader. This is obviously not the original tranny which is a good thing since that was only a 3-speed manual.

So even though my car meets Holley's criteria for this carb I still have a huge problem with it. It's HUGE! Most sizing calculators show I only need around 500 cfm so my 700/750 is about 50% larger than I can actually utilize. It does have some drivability issues which I can't address until I get the clutch linkage repaired, especially the poor engine idle. For now I'll do some research on Holley carb tuning and see if I can achieve an acceptable tune before I resort to downsizing.