Caburetor help - Vacuum secondaries won't open

[92saturnsl2]

I have a 1985 Ford F-250 4x4 w/ a 460 and a Holley 4180C 4-barrel carb on top. I've owned this truck for about a year and a half, and it appears the vacuum secondaries have not been opening on the carb.

The truck was very low on power when I bought it-- I did a full tune-up and rebuilt the carb with a geniune Motorcraft rebuild kit. It's the first carb I've ever rebuilt (I'm far more competent in working on fuel injected vehicles), but despite that, I didn't encounter any real difficulty; it was pretty straightforward.

The engine perked up, but I noticed it still runs out of breath around 3k RPM. I also noticed the secondary throttle plates were accumulating varnish, an indicator that they weren't receiving any fuel that would otherwise keep them clean. As a test, I tied the primary & secondary butterflies together, much like mechanical secondaries (except they open simultaneously, not sequentially). At WOT, the difference in power is night and day. The engine pulls like a mule clear to 4,500 RPM and makes wonderful noises that only a big block V8 can make, that weren't present before.

Something is amiss here and I'm looking for suggestions / input on where I should look first, what things I can try to fix this. Thanks in advance!

 

[Lapham3]

So, with vacuum activated secondaries there would be a diaphram-did you check that? good luck

 

[92saturnsl2]

Yes, the rubber diaphragm was replaced during the rebuild. I guess it could open up the cover and check it again.

 

[Skid]

Also, there will be a port on the primary side to control the secondaries... make sure that it's not plugged.

 

[Kool1]

If the secondaries are opening late or not enough, you can try an old hot rodders trick which is to put a screw secured with a washer in the secondary travel path to force the secondaries to open sooner.

 

[MCompact]

Putting a screw in the linkage usually just causes the car to bog when you slam open the throttle. Fix it properly. Check the diaphragm/spring/etc., as noted above.

 

[MrBeachcomber]

From my experience with vacuum-operated secondaries on Holley carbs, the usual culprit is lack of vacuum either due to a torn/non-sealing diaphragm, blocked vacuum port in the secondary bore, or torn/missing cork gasket between the main body and the vacuum secondary diaphragm unit.

Not sure if this model has this feature; however, I'll mention it as it has caused problems before. The early Holleys had a small ball in the vacuum source port that had a flat which allowed the vacuum to build gradually, but steadily, making the vacuum secondaries open more smoothly. The problem with this setup is most rebuilders replaced the stock ball with a steel ball w/o a flat. This resulted in almost no secondary actuation. Later model Holleys just left out the ball entirely.


Finally, since this Holley has probably been rebuilt previously, another area I would look into is the secondary throttle blade adjustment. There is a small adjustment screw in the base plate underneath the vacuum secondary rod's attachment point that is normally one full turn in to keep the secondary throttle blades from sticking in their bores. (It also uncovers some secondary idle ports to prevent lean tip in.)

Hope this info helps.

 

[CT8]

The how to hotrod Holly carbs is a great work book to have.

 

[Fred_Garvin]

As been said check all the passages from the diaphragm housing to the primary venturi. Also don't overlook the simple things like vacuum leaks, timing, accelerator pump setting, general tune of the engine.

Here is a video that covers it pretty well from Holley. Try the trick at the end with the paper clip.

https://www.youtube.com/watch?v=4xOiJQj6CSU

 

[Chris142]

Remove the vacuum diaphragm and suck on the hole ,see if the vacuum pod works.you can change the spring to make it open up more,sooner etc. If your just revving it in the driveway they won't open

 

[CharBaby]

Although I don't know your particular fuel system/carb, I have fixed these issues by finding another vacuum line to tap into with a "T". It can be difficult to locate the right line but, can be done! Once you've found the vacuum line of choice, the secondary may not be perfect(a little delay) but, it'll be better than what you have now!

 

[The_Eric]

Tons of excellent info above. Only wanted to chime in to second them all.

Char Baby- the secondary diaphragm is operated by an internal vacuum source, not external.

 

[440Magnum]

Its things like this that make me a Carter carb guy...

Seriously, good advice on the vacuum path above. Also, you can have different spring rates to delay the engaging of the vacuum secondaries on Holley carbs, maybe yours are too stiff (although you'd think a 460 could open ANY dang secondary spring...).

My bet is that the vacuum path is simply blocked. Something as simple as an incorrect carb base gasket can cover vacuum passages and make things no-worky.

 

[CharBaby]

Originally Posted By: The_Eric
Tons of excellent info above. Only wanted to chime in to second them all.

Char Baby- the secondary diaphragm is operated by an internal vacuum source, not external.


Thanks!
Yeah, like I mentioned, IDK that particular carb. Worked on some old Honda carbs(Keihin)