Holley carb tuning advice

[edbux]

750 vac sec on a 360. To get it to rev without stumbling from idle or getting a single backfire, I had the idle adj screws out 4 3/4 turns, feels like they want to drop out. My question, is an increase in primary jets, different power valve or squirter the answer? Stock 70 pri jets, 6.5 power valve and .031 squirter with a 30cc pump and stock pump cam. Vac at idle is 11 pounds.

I have the whole jet kit in the garage, different power valve would have to be mail order--not much in the way of selection here in Caldwell Idaho--and I have extra squirters I can drill out.

thanks for any and all advice.

 

[dgc333]

What's your timing? With only 11"Hg sounds like a fairly radical cam. You need to dial in the initial timing for max vacuum before messing with the carb. The initial will likely wind up in the 14-18 degrees BTDC. You always have to get the timing right before messing with the carb.

The mixture screws should be about 1 1/2 turns out. Make sure the accelerator pump is adjusted properly. The lever should make the pump arm move the instant the throttle is moved and should not bottom out the pump arm at WOT. The carb should be pretty close out of the box so resist making any large changes.

 

[edbux]

The cam isn't too radical, .470 lift with 224 dur @.050. The initial is setup at 16 deg per FBO's instructions. Moved the inital between 10 and 20 deg and no change. I'm even running Rhoads lifters, more so for the mechanical lifter sound than the low RPM benefits. I'll be swapping over to Magnum heads in the next week or so. I'd like to get this issue solved before then. Swapping out the power valve and jets is easy enough, think I'll try a 5.5 PV first. Thanks

 

[C130 Chief]

11 POUNDS of vacuum!?! Holy cow thats a lot. 11 INCHES is more resonable ( I have 14 inches but I dont like to brag). Depending on the LSA of that cam 11" may be all you are going to get. No worries, at your altitude, thats fine.

Power valves arent too far away, the Scmucks on Overland in Boise has a fair selection, and any store can get one in overnight. But, the stock PV should be fine (you are running a 3310 right?). If so, I have put that exact same carb on several engines in the Treasure Valley running similar cams and they typically work fine right out of the box.

Also, it sounds as if you are lean for some reason, if you really need 4.5 turns at idle. Holleys generally come fairly rich out of the box, so, lets take a look at whats really happening. Does it take that many turns to get a smooth idle, or just to mask a flat spot? If your idle quality does not improve past 2.5 (normal), then you have some sort of a vacuum leak. If it takes 4.5 turns to get a smoth idle with that cam, I am betting you have a vacuum leak (at 2.5 turns, hold your hand over the primaries, does idle speed up?). Also, a flat spot off idle, especially one that you can "step through" is a classic symptom of a ported vacuum leak (like a bad vac advance). To eliminate this, cap the ported vac at the carb. Any change? Also, this is how you should be setting your timing, with the vac advance plugged. You must have stock compression if they recommended 16 degrees. If the above does not fix it, dig deeper. If it is backfiring through the carb, something is very wrong. Either you are over advanced, have vac advance hooked to manifold vacuum, or your float level is seriously low (you DID check that right?) You will eventually trash the power valve by letting it backfire, and suddenly the whole thing will be silly rich which can also wreck your throttle response.

You may have a vacuum leak at idle. Check the obvious, and replace the PCV valve. To see if you are leaking internally, cover the breather (non-PCV valve cover) at idle, and let your crankcase build a vacuum. If the idle changes even a little bit, you have an internal vacuum leak either at the ports or intake valve guides.

Holleys are simple, robust instruments, and they are capable of performing well on a wide variety of engines. If the carb is in good shape, I would look elsewhere long and hard before I start monkeying with jets and such. I bet your problem is a whole lot simpler than that.

 

[djwhog]

Does it seem to idle ok at 1 1/2 to 2 turns out on the mixture screws? If not I say vac leak. Do you have vac advance type distributor, if so is it connected to the ported vac port on the carb and the base timing is set correct with no vac advance at and least about 15 degrees initial timing and 32-36 total.

If all these are correct, and there is no major vac leak, sounds like a lean transition circuit. You may need to go up 1 or 2 sizes on the main jets. Have you also checked the basics like the float levels?

I have seen a lot of low float levels casue this issue too... If you had access to a air / fuel meter you would be ablel to find out easier.

 

[duster340]

i like all the earlier advice, but there is one mor step i would have a look at, is your accpump working like it should? just another point of wiev

 

[dgc333]

The cam isn't too radical, .470 lift with 224 dur @.050. The initial is setup at 16 deg per FBO's instructions. Moved the inital between 10 and 20 deg and no change. I'm even running Rhoads lifters, more so for the mechanical lifter sound than the low RPM benefits. I'll be swapping over to Magnum heads in the next week or so. I'd like to get this issue solved before then. Swapping out the power valve and jets is easy enough, think I'll try a 5.5 PV first. Thanks

Going to a 5.5 from a 6.5 power valve is only going to delay opening. It has no effect on mixture.

I would be looking at your accelerator pump circuit first. Before changing jets get some full throttle runs and look at the plugs.

 

[A360]

I know that I am far away but I have 750DP with 50cc pump on a 360,727 with a 3.7 spool and I have 80 jets and has proved to be a good combo.
Now 12.6@108mph

May or not be of help

Cheers

Benton

 

[ramcharger]

Was this carb bought new? has someone been in this carb? It's possible that the idle air bleeds may have been drilled causing a very lean idle condition. Has your heads and block been milled? You may have a vacuum leak on the underside of the intake manifold. How much preload do you have on the lifters? Standard lifters require .040, anti-pump ups you can run zero due to the hefty circlip preventing the lifter from flying apart. What does Rhoades recommend? If you have to much preload on the lifters, the engine will run like crap and you will have low vacuum. Did you degree the cam? It may be retarded.

Somethings definitely wrong as 11" Hg sounds low for that cam and displacement, I would think 14"+ would be about right.

 

[C130 Chief]

bear in mind Caldwell elevation about 2600 feet, so vac will be slightly lower. But 11 is still pretty low. Low enough to give any carb fits. But not stumble. Does it only stumble off idle or does it also stumble if accelerating from cruise speed?

 

[ramcharger]

bear in mind Caldwell elevation about 2600 feet, so vac will be slightly lower. But 11 is still pretty low. Low enough to give any carb fits.

C130 Chief,

I'm at 5280+ in Denver, over twice his elevation. I installed a Comp XE262 in a "well worn" 360 and was pulling 16+ Hg. That cam had 218 degrees duration at .050. He's only running 6 more degrees and is at less than half the altitiude, hence my reply.

 

[redfastback]

is it an auto car? are you measuring vacuum at idle in gear? dave is right, check timing first, initial, 2000rpm, 2500rpm, 3000, only to see what total is. some come in way later. carbs are always last. i agree with dave, check the accelerator pump.

 

[edbux]

Thanks for the replies. I made a typo, it has around 15 INCHES (ok, 2 typos) at idle in neutral and drops to 11 in drive idling around 650 RPM.. The idle is smooth at 1 and 1/2 turns of the screws, turning them out further was to try and get rid of the stumble. Pulled the sight plugs and the fuel is right to the edge of the hole. Spray carb cleaner then propane around everything and no increase in idle. Press on the accel pump arm and the idle worsens. It is a brand new carb, bought a used one once which led to an engine bay fire, learned my lesson...I was young and broke. The cam was degreed and ended up being installed 4 degrees advanced. Think that covered all the suggestions/questions.

Here is what I did, increased the squirted to an .035 which greatly reduced the stumble. Pulled plugs and they were tan. Added a 8.5 PV which I had sitting around and now it smokes the wheels again. I don't know if I went about it correctly, but it has seemed to work.

The compression according to the PO who put the mother together was 8.2:1. I have a set of magnum heads going on in the next couple of weeks which should get me to the 9:1 comp cams recomended.

Once again, thanks to everyone!!

 

[C130 Chief]

Wow, your Holley was actually lean. Yet, I still don't think it is all the carb. The clue is the BIG difference in idle settings. You have to be having an issue with ported vacuum or your advance (you arent running EGR are you). Again, plug EVERYTHING coming off the carb and drive it. If there is ANY difference, you have a problem. Also, what are you running for an intake?

 

[edbux]

I'm running an LD340 with a 1" 4 hole spacer. Stumbled with or without the spacer. Tomorrow I'll plug up everything and give it a shot. That and cover the breather hole on the valve cover.

Also, A Freudian slip, should've been "the PO put the otor together" not "mother together". guess frustation slipped thru!!

Thanks again

 

[bill paynter]

edbux
I'm having the same problem with my new 600 Holley, if the idle screws arn't turned out 3-4 turns, it stumbles and backfires thru. the carb, feels like it's running out of gas.
I'm taking it to the local speed shop today they said some of the new Holley's seem to be running lean, they said they could open up a few holes to correct, Ill let you know if it works

 

[ramcharger]

Jet that sucker up to 74 or 76 on the primaries. It won't richen the idle circuit, but will improve driveability.