Holley Vacuum Secondaries Not Opening

[Brigand]

I've fitted a brand new Holley 1850 carb on my 302 engine. It is running at about 19" of vacuum at idle. I've put a cable tie around the secondary diaphragm plunger to see how far they are opening and after a fast run down the road it appears that they are not opening at all.

When the engine is not running, should you be able to open the secondaries by hand by pushing on the plunger? I ask because I can't and am wondering if the mechanism is seized.

I am considering changine the springs in the diapgragm housing, but the Holley website/instructions repeatedly say that the carb should be pretty well set up out of the box.

Thanks

 

[tiger331]

Hallo,

did you go to WOT.
You can only open the secondary by hand when the mechanics are wide open.

Hans

 

[Brigand]

Hi Hans,

Thanks for the quick response.

I just tried to operate the secondaries by hand with the throttle open as you said and they worked fine.

Yes, when I took it out today I had the throttle wide open, both in second gear up to the red line, and in forth gear up a hill at about 50mph.

 

[Brigand]

Quick update: I took the car into work this mornig and drove it hard (in fact I had a nice little battle with an Audi TT, but that's another story ) and the secondarys have opened, but only slightly. The cable tie is now about a quarter of an inch down the diaphragm plunger.

 

[0neoffive]

Inputs It

Quick update: I took the car into work this mornig and drove it hard (in fact I had a nice little battle with an Audi TT, but that's another story ) and the secondarys have opened, but only slightly. The cable tie is now about a quarter of an inch down the diaphragm plunger.

The 1850 series is a fine carb design. You may have simply found the optimum amount of food that the motor will eat using those particular primaries. You can always add more cam, more head, more exhaust, more piston, more stroke, etc; all in the quest to open those secondaries . . . .

 

[Duke Mk1a]

Get mechanical secondaries.....problem solved.

My recommendation -

 

[cadreamn67]

Just curious, what is the CFM rating of the carb you have?

Also, if you want to play with different diaphram springs, I strongly recommend installing the quick change kit. That way you can just remove the lid and pop in a different spring. No need to remove the linkage, canister, disassembly, getting the sealing washers right on reattachment, etc.

Perhaps you know this already.

Also, conventional wisdom is that if you can feel a power boost when the secondaries open, they are actually opening too late. Go to the next lighter spring and try again. Should be a smooth transition. You might also want to experiment with finding at what rpm the secondaries start to open under load. Since you will be running about 3000 rpm on the highway, it is helps mpg if the secondaries do not open until a little north of that. If mpg is important to you!

Also remember that you cannot get the secondaries to open on their own even at WOT when the car is stationary, their opening is load dependent.

Sorry if I am telling you stuff you already know. You know the little cable tie trick so you obviously have some familiarity with these things. But, hopefully there is something in all of the above that you can use!

Gene

 

[Brigand]

Thanks for the replies!

Oneoffive; That point had occurred to me... The manifold vacuum reading at idle is high which I believe suggests that the engine is running a mild cam (I have no idea what has been done to the engine in the past) so as you say, maybe the engine just doesn't need any more fuel. Then again, it does splutter at high revs.

Duke; I hate to tell you this but remember I said that the carb was brand new? Well that's because I ditched the double pumper that was on there. The problem was that it drank fuel and the carb had generally had it (stripped threads) and after some internet research I gathered that a vacuum operated secondary carb was better for road use than a double pumper.

Cardream; The carb is a 600 cfm. Thanks for the information. I hadn't considered the point about the secondaries not opening whilst cruising. Good point!

So from all this, I guess maybe the best thing to do would be to fit a lighter spring & see if it improves matters? If not, just go back to what is fitted now.

Does anyone know what spring the 1850S comes from the factory with?

 

[67 Tiger]

You can control the vacuum with springs. Read this.
http://www.holley.com/data/Products/Technical/199R8219-2.pdf

 

[wag123]

Sputtering at high RPMs usually indicates that you are running too rich (a flat spot, hesitation, or surging usually indicates that you are running too lean).
A stock (or nearly stock) 302 in a light weight car like the Tiger won't open the vacuum secondaries very much on a 600 cfm carb. The same engine/carb combination in a much heavier pickup truck will open the secondaries up much wider.