I have a 71 225, it has a comp cam milled head and a holley 390. The secondaries open at about 2800 rpm at half throttle with no problem, but at about 4000 rpm the engine will start to spit and sputter and I lose power. I'm going to get a secondary metering block because I think it's running too rich at the higher rpm. Am I on the right track?

Vacc secondary? If so, try a slightly stiffer spring in it... Cheaper and quicker than opening it up and swapping internals out...?

_________________
"if it aint broke, fix it till it is"
78 Plymouth Volare Super Six wagon
89 Volkswagen Golf GTI 16v
92 Chevrolet K1500 5.7
98 Ford Escort ZX2 zetech

Not necessarily. I am not sure why you would get a new/another metering block.....How about a little more info?
- Is this happening at around 4000 RPM only, and does it clear up at higher RPM's or persist and/or get worse at higher RPM's?
- Does this happen with 1/2 throttle only, or does it happen with any opeing from 1/2 throttle opening to WOT, or??

This is important so we can know the probably vacuum conditions in the carb to see what circuit may be failing/out-of-tune.

What is your fuel pump and filter setup? Has the tank been replaced or cleaned out thoroughly? You could be simply running out of gas or fuel pressure.

If you donwshift so that the engine RPM's are over 4000, does it run smoothly with ligth throttle? If so, then that would tend to point more to the carb and not the the ignition system.

It will do it from 1/2 throttle to wot and it starts anywhere between 3800 and 4300 but will always continue in the higher rpms. I have a new pump with the regulator set at 3.5 because anything higher will overcome the float seat on the primarys. 5/16 line from pump to carb and a clear glass filter. The tank is clean as far as I know, I replaced the sending unit and it looked ok.

I'm running the lightest white spring on the secondaries. Tomorrow I'll throw the dark yellow on and see what happens. If I keep out of the throttle I can get 20 mpg highway.

you need 5.5 at the regulator, the needle seats should beable to handle 7 lbs at the regulator before over coming and flooding the engine. you must have needle seat or float trouble. try setting the float level,fuel just dripping from sight hole in the bowl.

5-7 psi is indeed the recommended pressure, so the observation above is right. I would work on that first.

If you still have issues then: If the engine runs well at the lower RPM's and 1/2 to WOT, then the vacuum secondaries are opening and not likely causing this issue. (Are you using the paper clip test to see when the secondaries are opening?) Going to a heavier spring will cause the secondaries to open sooner and can lead to more bogging; I am not convinced that is the issue. But it is not a lot of work to try and you may get a change in operation that will lead to a solution.

I would be looking at the ignition system if the engine runs well at, say, 3000 RPM and with 1/2 to WOT. What ignition system do you have?

If you do want to check if it is running too rich at the higher RPM, do a plug check. Get some fresh plugs; put them in only after the engine is fully warmed up on another set of plugs and you are ready to test; ideally, this is on the strip or by the road where you can start the car and then go right into testing at WOT and high RPM. Run under the WOT and high RPMs condtion, and when it has bogged/cutout for several seconds, cut the ignition off before lifting and then coast to a stop and pull the plugs to read them. If rich, you will get fluffly black carbon on the plugs; if the ignition is cutting out, then it more likley will be a bit wet with unburned fuel. (I have not done plug tests with a cutting out ignition so am just making a guess on that.)

Ok I'll take it up to 5 and see what it does. But I just replaced the secondary diaphragm because before they were not opening at all. The diaphragm was tore up and leaking. But they are opening now. The engine runs beautiful when they first open but like I said at the 4000 range it falls on its face.

Stiffer spring in the secondary pod will open them later not sooner...(more venturi vacuum required to overcome the spring tension)... I would lean on trying that next once the PSI and float settings are corrected... Its probably a combo of secondaries opening too soon, and the low pressure not keeping the bowl full when they are open so its leaning out...

Funny thing about the Holleys are thier almost limitless tuning abilities... You can even fiddle with different pump cams and change the volume and duration of the accel pump from idle to WOT as well if needed...

_________________
"if it aint broke, fix it till it is"
78 Plymouth Volare Super Six wagon
89 Volkswagen Golf GTI 16v
92 Chevrolet K1500 5.7
98 Ford Escort ZX2 zetech

Oldskoolracer:
[quote]Stiffer spring in the secondary pod will open them later not sooner...(more venturi vacuum required to overcome the spring tension)... I would lean on trying that next once the PSI and float settings are corrected... Its probably a combo of secondaries opening too soon, and the low pressure not keeping the bowl full when they are open so its leaning out...[/quote]



I agree with the above. Stumble almost always caused by secondary opening to early with not enough airflow through carb to maintain fuel flow through jets and secondary metering plate, and A/F mixture going very lean. Float level has to be correct or A/F mixture will be off, and enough fuel needs to be delivered to sustain high rpm operation; that is to say more fuel pressure to keep up with demand. To add to the fun I also see that Larkspur, CO is 6700 feet above sea level, this will also [url=https://www.youtube.com/watch?v=lS9pXtWiUL8&index=40&list=PL353239D6A7D7BFF0]screw with A/F mixture as you have less oxygen to work with than we folks at sea level[/url]. You may have to step down one jet size for every 2000 feet of elevation above seal level, so if you are using a 51 or 512 jet out of the box, a 48 jet may be closer to your needs. I’m not sure if one can acquire secondary metering plates calibrated leaner for a 390 or if a work-around for high altitude conditions is available, or if stock secondary metering plate is even an issue under WOT a mile up; that part of the equation will require a call to Holley tec line: 866-464-6553.

One other tuning tip I just read in Dave Emanuel’s Holley Carburetors tuning book: Whenever main jetting is reduced power valve should be also be replaced as engine vacuum decreases as elevation increases. Each 3000 feet power valve should be delayed by 1.5 to 2 inches/ Hg. Quoting here… “For example a carb equipped with a number 85 PV (opens at or below 8.5 inches of Hg.) will typically deliver better fuel economy at 3000 feet if fitted with a 65 PV. Since altitude has reduced engine vacuum, this lower setting should not cause excessive leanness when transitioning to a heavy load condition.â€￾

Increase fuel pressure so engine won’t run out of fuel at high rpm, check float level, check main jetting & power valve rating for your elevation, and slow up secondary throttle opening or in other words raise their rpm opening point. If I recall the secondary metering plate A/F mixture on a 390 is set to be close to stock out of the box main jetting; say a 512 or in other words a 51 jet.

Do you have oversized valves, and higher than stock compression?

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

I am currently running 52 primary jets, I have put different sizes in like 46, 48, and 51 but all of them cause it to go too lean. I have also tried each one with a 6.5 and 8.5 pv but the best combo is 52 jets and a 6.5 pv. I actually live at 7300 feet and I am going to school in laramie, wy at 7200 feet. I drove back to Colorado on friday and it did fine because it's downhill on the step parts. But coming back tonight it had trouble. I could get it to keep the secondaries closed cruising the interstate at 70 mph (3100 rpm) but if I went up a steep hill they would open and make the engine stutter.

The reason I believe that they are too rich is because when I back off the throttle after having the secondaries open, it spits and sputters again like crazy until they fully close.

I did turn the fuel up to 5 psi and the float stayed true. However it had no change on my problem. My next step is changing out the spring, but that is still in colorado. It'll have to wait 2 weeks.

.

Yes pulling up a steep hill will cause the secondaries to open. My car dose the same thing on a few big hills requiring lifting throttle a bit to maintain steady speed as secondaries creep open. If this throttle connected to a big rubber band response bugs you install a heaver spring which will delegate secondary opening at larger throttle openings or higher loads. The best combination is to keep secondaries closed during all highway driving, and only accessing them during passing, or WOT. By doing this you are treating the secondary throttle as a fifth circuit much the same as the power valve comes on line to pick up additional fuel needs when under load resulting in fuel savings.

Sputtering could be from dirt in secondary circuits. Have you taken off its fuel bowl and cleaned all passages, used correct gaskets, set float level and made sure its idle circuit is functioning dribbling fuel just below the throttle plates?

Once secondary throttle is closed, air flow past the booster venturi is greatly reduced dropping vacuum the metering plate sees which causes fuel flow to stop. If the secondary throttle plates do not fully close, and are allowed to flutter there could be enough air velocity through the booster or venturi to keep pulling fuel which would cause a temporary rich condition until things settle down and primary throttle closes enough to allow secondary lock out or throttle control rod to secure secondary throttle closed. Heaver spring may help control closing speed, and perhaps a linkage adjustment

Racing is a different story, re spring so the secondaries open as soon as engine can take extra fuel; this will be a tune & test, tune and test until you can dial it in. If you have the quick change spring cover it will be easy, if not you will order one after the first change.

Tec Tip. when screwing down spring cover I always place a tiny dab Vasoline on the screws to prevent screw from grabbing the diaphragm and ripping it.

Installing an o2 sensor will help with jet selection, and general tuning; more on that here if interested. I installed one a few years ago per Bob on my car, and was able to get my 390 tuned quickly.

You should get a copies of "Super Tuning and Modifying Holley Carburetors" by Dave Emanuel, and "Holley Carburetor Handbook 4150 & 4160" by Mike Urich; both books are very helpful.

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Don't jump on me but I remeber when I first got my w150 it would do the same thing but it had a 1bbl at the time and it turned out the the carb was loose on the intake

The carb is mounted tight with a 1" spacer and 2 good gaskets. After driving it for a while the problem has almost completely gone away. ..It's weird but I'm gonna install a little stiffer spring this weekend anyway.

Has the air temperature in your driving locality undergone a general change as this improved, like getting generally cooler or warmer?

Not at the times I'm driving. It'll get colder at night but not till 8 00 or so.