A Little Help From Holley Users

[shotsy]

Hey All,

Still having probs with the holey 750 (4160 vac sec) on my 351C. It's running rich after dropping the jets from a 72 down to a 68, Is this still way to big for a 351C. I've got the timing set at 13 deg initial and the mixture screws are only 1/2 turn out from the bottom, pulling 8 inches of vac at 700 rpm idle. Floats are set perfect, It's not burning my eyes as bad anymore, but it's still running too rich. So I guess my question is to you guys with similar 750 CFM carbs on 351 C/M motors. What size jets are you running? Should I drop down to 66's or 64's?

Thanks

 

[iskwezm]

what about the power valve???

 

[shotsy]

havent gotten that far yet, it's gotta match the vaccum at idle right or it just opens and dumps gas? am I close?

 

[iskwezm]

havent gotten that far yet, it's gotta match the vaccum at idle right or it just opens and dumps gas? am I close?

well if you car had a backfire,it could blow the valve out and then it would just dump gas.You power valve is supposed to be like -2 less then your vacume,i think thats how it goes.

What about your floats????

 

[shotsy]

hasn't backfired once since I put the motor together, and the floats are adjusted to where the gas is just to the bottom of the sight plugs? do 68's sound right? I so I may have to check the power valve, so if I'm pulling 8 inches of vaccum I'd need a power valve that rated at 6?

 

[10secgoal]

8 at idle seems kinda low. Is it a real wicked cam or what. Mine was 11 with the old cam, and is was a pretty stout cam. If you have a 7 or higher, the power valve is more than likely open, or has broke the seal ( slightly opened ). If your eyes are burning, you may want to look into an exhuast leak. Is it burning your eyes at idle ? if you do take the power valve out, makes sure to replace it with a holley paper. Some gaskets come with alignment holes. And if you have the later style PV, they seal with a ring, and gas can leak past the PV even if the PV is still shut

 

[shotsy]

Yeah it's a pretty lopey cam, and the exhaust at the moment is dumping under the back seat cause I dont have the 3" pipes to go over the axle yet, SO you guys are think it sounds like the power valve? so 68's on the jets should be right?

 

[iskwezm]

68's are probably close.Why dont you take it for a drive,run it hard and then pull a plug and see how it looks?????

 

[shotsy]

I'd love to but without the fenders hood and doors I'd probably go to jail I guess that I can wait untill I get the paint done and get it all back together before I mess with the carb anymore. Thanks for all of the help, I'll check the power valve and go with the 6 . Anyone in OKC wanna help sand off a bunch of 30 year old paint!! I'll buy the

 

[ddonaca351]

Ok, so youre saying the car is running rich just sitting there???

If its rich at idle you need to check:
-float level ... ( thats good you said )
-the idle mixture screws
-the condition and rating of the PV
thats it,
-the jets DO NOT CHANGE IDLE MIXTURE
if they do, then youre idle stop screw is set too high allowing the throtle blades to be held open and causing the carb to run on the transfer circuit / primaries instead of limiting it to the idle circuit.

8 is very low vacuum for idle. (i think thats about what my dads dragster made...) Most good street motors I have seen still pull 10-12 at idle and mild street motors can be 14 or higher...
Are you sure you have the cam and ignition timing set correctly?

Dave-

 

[shotsy]

Here's all of the specs for the cam Look at PN 32-225-4
http://www.compcams.com/Technical/CurrentCatalog/HTML/92-97.asp
cam timing was set by the machine shop that built the motor. Should be right on the money.

The timing is set at 13 deg advanced with the vac advance disconnected and hose capped off.

Mixture screws are both 1/2 turn out from the bottom (everything I've read says should be about 1 1/2 turns out?

Idle is set at about 700 rpm cause it sounds so damn good

and pulls about 7 inches of vac at the bottom of the carb, when reved to about 3000 rpm in park the vac will climb to over 25?

 

[iskwezm]

have you checked for any vacume/intake leaks????That 7" sounds low,me new cam is a lot bigger and pulls more then that.

 

[shotsy]

I'll check for leaks, another question, I've got the PCV tied into the big vac port on the manifold under the rear of the carb, should I pull that and cap it as well, I've also got the big vac port on the back of the carb hooked to the power brake booster, shouldnt be a prob? But the PCV might be hooked to the wrong place, I didnt want to run it to the air cleaner cause of the oil/soot crap makes such a mess?

 

[chepsk8]

Tuning a Holley Carb

I had that carb on the 428 SCJ. It took alot of tuning to get it right, but once dialed in, it was awesome!

Keep dropping down the primary jets. I cannot remember my final jetting, but I think it was at least 5 steps down from the jets out of the box. I also dropped the power valve down one notch, and changed the vacuum secondary spring to the lightest one available, as well as put in a better cam profile on the accelerator pump.

Once I got it set, nice medium grey at the tailpipes, and I could get 12-14 mpg If i really babied it on the highway!

Keep at it! It's it's tunability what makes a Holley carb so good, but so cursed at if you don't get it right!

 

[Crs]

I'm going through the same thing with my 393 Stroker. I've dropped the power valve down ( 7 inches of vacuum) thinking that would help but it's still rich. I think it's a jetting issue. Try dropping down to about a 66, drive it for a day or so then check a plug. Personally, i think a 750 is quite large for a 351 and most likely run better with a 650.

Keep us informed!

 

[MCABOSS]

Here's all of the specs for the cam Look at PN 32-225-4
http://www.compcams.com/Technical/CurrentCatalog/HTML/92-97.asp
cam timing was set by the machine shop that built the motor. Should be right on the money.

The timing is set at 13 deg advanced with the vac advance disconnected and hose capped off.

Mixture screws are both 1/2 turn out from the bottom (everything I've read says should be about 1 1/2 turns out?

Idle is set at about 700 rpm cause it sounds so damn good

and pulls about 7 inches of vac at the bottom of the carb, when reved to about 3000 rpm in park the vac will climb to over 25?

Idle mixture screws set at 1/2 out is where you start the idle mixture screw adjustment. Using the vacuum gage, slowly adjust the idle mixture screws to get the highest vacuun reading. Adjust one side first then the other - then go back and check the other side etc, Adjust the screws until the vacuum level starts to fall off then go back about 1/4 turn.

 

[PoliceInterceptor]

If it is a 3310 and you have not opened the secondary gap I would just about promise you that you are into the mains at idle, check the ported vacuum above the idle mixture adjustment screw. It should have none (ideally) or very little (workable) at idle. If you have significant vacuum there at idle you need to pull the carburetor and open the secondary gap a bit. The fact that changing main jets changed your idle tends to confirm that you are on the mains at idle.

68's sound lean to me (if in fact it is a 3310)

Your 8 inches vacuum sounds really really low and most Holley’s come with a 6.5 power valve so you are cutting that too close. If you cannot get the idle vacuum up, swap the power valve for a 4.5 and see if that helps.

:twocents:

 

[ddonaca351]

Well *free* advice always being worth what you paid for it....

If it was me, I would bump the idle to about 800-850 rpm. My 51017 Lun likes idle about 850-900, and my B.M.2 is about the same, the extra 100 r's might help pull the vac up a little.

I would check again for a vac leak though....

Also that strong of a vacuum source might be too much for a pcv, you could try a flow restrictor in the line and see if that helps, or just discon and plug it for the sake of troubleshooting.
The fewer variables the better when trying to narrow things down.

Also ignore the stuff about a 750 being too big for a C motor, its a vac sec carb, and a good C can use the air at 6K+. That is one HUGE good point about running a vac sec carb on the street, is that they are more forgiving of a size mismatch.

I went and checked my jets before I left for work today (thought you might still be fighting things), I have 65s in the primary side of mine and (judging from the ones missing in the jet kit) 72s in the secondaries. But remember you can have IDENTICAL motors side by side and one will like different jets than the other, so use this as a loose guide only. And make sure you have the idle circuit sorted out (see vac guage post above) and the idle vacuum level fixed, before you try to start swaping jets. Otherwise you will be putting a band aide on an unrelated symptom, and then down the road, you'll have two layers of problems to sort through.

Just my .02

Hope some of this helps,
Dave-

 

[shotsy]

I disco'd the pcv and capped it off and swapped the carb gaslket on the manifold, sprayed starting fluid all around the intake/carb and there's no vac leak anywhere? Still only pulling 7-8 in. of vac @ idle which was also bumped to 800 rpm. I've also checked the secondary adj, and it's totaly backed off, IE closed. I also bumped the timing up to 15 deg adv. pulling 36 adv & 3000 rpm and pulled her onto the street, still rich but if I stand on the gas it totaly dies? if I get her rolling and ease into the gas she'll roll smoke till I let off the gas . I've got the black cam on the 50cc accelerator pump? Why is it dying if I stab the gas? Too much timing? Anyone wanna buy a holley? JK Please help

 

[PoliceInterceptor]

I've also checked the secondary adj, and it's totaly backed off, IE closed.

Did you check for Vaccum at the ported source?

When you had the carb off, are the transfer ports in the primarys uncovered?

As was said above:

-the jets DO NOT CHANGE IDLE MIXTURE if they do, then your idle stop screw is set too high allowing the throtle blades to be held open and causing the carb to run on the transfer circuit / primaries instead of limiting it to the idle circuit.

You are never going to get this thing to run right until you get it off the mains, the way you do that without having it idling at 250 RPM is to open the secondary air gap with the little hidden stop screw in the base plate. (you can only get to it with the carb off) Open it about a 1/2 turn at a time until you can get your desired idle speed without the carb being on the main circuit.

Just be careful not to open the secondary so far that the secondary transfer ports are uncovered.