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Questions for Holley carb guys

Seventee

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Dec 7, 2012
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Loc.
In the sticks of MT
Do you think it is worth pulling and checking that? I’m going through my timing excises after i eat some turkey. The bouncing vacuum gauge really adds some validity to Mark’s comments above about timing or a miss. I need to investigate that. I always assumed it was just carburetor issues. The surge is gone and now the idle mixture screws are responsive now. Being new at this, I’m not sure how “responsive” the idle mixture screws should be.

If it is worth pulling the carb, what is that procedure called, so i can google? “Secondary stop screw adjustment”?

Edit : I think mine may be good. I just watched the Holley how to and it said don’t touch this adjustment unless you have a big cam with low vacuum. It said to verify the flaps cover the groove and when I had if off yesterday, I did see that the vertical groove was not visible when the flap was closed. I adjusted the primaries to have the square and saw the secondaries didn’t have a visible groove at all when closed. You can kind of see it in the pic below.


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18bf931e8daa5e08eb3c278d5fa47fc5.jpg

From what I can see in the pic, your secondaries appear to be closed all the way. Others are correct that ordinarily you would not need to adjust this screw as it would be set correctly from the factory. However, in my case, I bought a used carb and someone had monkeyed with it, so I had to readjust back to spec after experiencing similar idle issues on my 460.

Timing and potential vacuum leaks should always be addressed first before trying to tune a carb.

Both my 750 and 650 Holleys are sensitive to idle screw adjustment +/- 1/8 turn when you are near the ideal setting. The idle air bleeds are particularly sensitive to clogging, especially with poorly maintained K&N filters. In cases where I've had difficulty maintaining proper idle, I've removed both screws and used my rubber tipped blow gun to give each screw port a quick shot of air, and also do that to the air bleeds.
 

rmk57

Sr. Member
Joined
Feb 24, 2016
Messages
580
A piston stop would help you find tdc, then you could make a temporary timing pointer so at least you could time it accurately until you find a replacement.
As far as jetting I would put everything back to stock. Holley should have a pdf on all their carbs such as jetting, power valve, secondary spring and so on.

Also try and keep the mixture screws opened an equal amount or as close as you can get. Like I've said Holleys tend to run rich anyways, I guess to keep the majority of users from damaging engines from a lean condition.
 
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DC_Gearhead

DC_Gearhead

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Dec 28, 2015
Messages
564
Ok. Missed my speed shop to get a timing pointer but it looks like I am just a moron. I have a pointer that is cast in the water pump and degrees in my harmonic balancer. Once I cleaned off the rust and muck. I scraped out the degrees and filled with black paint pen. I also marked my pointer with white and the 15 on my balancer.

My balancer looks like it is in rough shape with rubber falling off. Something to worry about or not?
cb63cc31f821bf4be23f1e4df1deaa03.jpg





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DC_Gearhead

DC_Gearhead

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Dec 28, 2015
Messages
564
Did this just start happening or has it always done it? As pointed out above 75% of driveability "carb" issues are ignition or vacuum leaks. Without having the timing set right you are really wandering around in the dark but maybe we can help with that too. I would just buy the pointer it bolts on in about 10 minutes.

Things I would try to isolate the problem:
Adjust the idle mixture screws until you get the highest vacuum. Then go back and make sure they are both turned out about the same by gently screwing them in while counting the turns. Turn both side out the same amount. If you get one out 3 turns and the other out 1/2 you can get a hunting idle

Set the idle where you like it. Put a timing light on the engine on #1 and try to see the line on the balancer. You may not have a timing reference but look for the line. Watch the line as the engine idles and hunts up and down in rpm. If the line keeps moving around a lot you may have something in the distributor like leaky vacuum advance can or sticky weights.

Remove the vac advance hose and plug the port. Did the hunting stop? If so your advance can is probably wonky. If not put it back the way it was. In a stick shift vehicle I like manifold vacuum for the advance but not for an A/T it makes for a big drop in rpm from park to drive and then the idle gets unstable.

Take your timing light and point it at your hand and watch the strobe it should be even and consistent. Take the timing light pick up off #1 and put it on the next cylinder and point it at your had and watch. Repeat for each cylinder - you are basically checking the soundness of your engine ignition system wire by wire. If the light is inconsistent (flash-flash-pause-flash-pause-flash etc) you may a bad wire, plug, cap, coil. If it is only wonky on one cylinder swap the wire from a "good" on to the "bad" and see if it fixes. If so replace the plug or wire. If it is inconsistent across several wires your coil or cap are probably bad.

If you have too much timing you can impact the carb tuning because you expose the transition slot as someone pointed out above. But that will get you effect the transition as you pull away from a light. Buy the pointer, set it to 12-14 with the vac advance disconnected.

You may have a vacuum leak under the intake manifold or along the end seals. Google or search this site for 302 intake vacuum leak there are about a zillion guys who had that problem. When I had it my plugs oiled up like bad valve seals. If you pull your plugs an done or two look messy I would think about intake gaskets.

Finally to the carb. Idle circuit is really simple with very few parts to fail. Take out the idle mixture screws careful not to lose the cork seal and shoot a jet of carb cleaner into the hole. It should squirt out into the throttle bores and may come out of the air bleeds on the top of the carb. Spray carb cleaner into the air bleeds on top of the carb it should also trickle out of the idle mixture screw hole. Down in the throttle bore you could get some build up but I have never really seen that. Idle bleeds usually get filmed up from poor tuning and sitting a lot.

If you can take a few pics of your carb sometimes we can spot something helpful. That carb is pretty solid and reliable once set up.

My guess from 3,000 mile away is your Chinese distributor is giving up the ghost and you are getting random mis-fires.


I went through all of my wires with the timing light. All seems consistent with no jumping. When setting timing there was no “hunting” that I could see. Very solid.

I have a couple things that cause me questions. I set my timing to 12. When I pull the plug out of my vacuum advance the idle goes up but timing stays the same. When I connect it to my HEI vacuum can, it stays high and timing goes to 40. My rig also doesn’t start after turning it off if I connect to the vacuum can. If I disconnect it from the vac advance can and plug the vacuum line it starts up perfectly.

Any thoughts about what I am doing wrong?


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jckkys

Bronco Guru
Joined
Mar 15, 2012
Messages
5,212
The books I recommended explain exactly why the various carburetor circuits do what they do and how. The 750 CFM has overly large throttle bores so you get too much air before the added fuel from the transfer slot starts to contribute to the mixture. So you don't see any of the slot. There's a reason why Holley has a carb size selection guide. It's in the books I recommended and online. 450 CFM is what they say is a match for your engine. You're learning the why the hard way. The 750 looks like a speed shop knucklehead recommendation.
 

pcf_mark

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Jun 11, 2010
Messages
3,614
"I went through all of my wires with the timing light. All seems consistent with no jumping. When setting timing there was no “hunting” that I could see. Very solid."

That is a good indicator your ignition is solid.

"I set my timing to 12." - perfect

"When I pull the plug out of my vacuum advance the idle goes up but timing stays the same." - that does not make sense. The idle should drop down if you remove the vacuum to the advance can. Assuming it is connected to manifold vacuum. If ported it should make very little difference. Did I understand you correctly?

"When I connect it to my HEI vacuum can, it stays high and timing goes to 40. My rig also doesn’t start after turning it off if I connect to the vacuum can. If I disconnect it from the vac advance can and plug the vacuum line it starts up perfectly." This tells me your vacuum advance can is probably fouled up. There is a way to limit the travel of the advance arm but it depends on the distributor. If you double check it and it does go from 12 to 40 just by connecting the vac advance then you need to get that fixed. It will cause all sorts of part throttle problems. On some it is a screw / nut inside right on the arm others it is an allen screw inside the nipple on the advance cam.

The Holley 3310-1 series is a very good carburetor and you have the earlier good version too. This can easily be made to work right on your truck. If it was a double pumper I would not recommend.
 

jckkys

Bronco Guru
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Mar 15, 2012
Messages
5,212
I find it a little strange that Holley developed carburetor size recommendations for the carburetors they designed and people who know next to nothing in comparison to the engineers, ignore that recommendation. Instead they go with the bigger is better idea. I had a 3310 on a 460 and it proved too big for that engine. A 600CFM 4180 made for 460s made more power and got better mileage. I got the 3310 because I was young, ignorant, and it was the cheapest new carb Holley made.
 
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DC_Gearhead

DC_Gearhead

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Joined
Dec 28, 2015
Messages
564
Try running ported vacuum. I bet that would clear it up.



This worked perfectly. It starts up fine and idles normal now. I do need to investigate the vacuum can issue, but this did make it act great in the garage and timing is acting normal as well. I just need to make some adjustments and double check all of my other work and I will be ready to do a road test. That will be the real report card.

Good advice. Funny thing is that the opinion on where the advance hose hooks up is all over the place. Very opinionated on both sides. I originally had it hooked up to the metering block “ported” and moved it over to the manifold because of a Holley YouTube tutorial.


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pcf_mark

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Jun 11, 2010
Messages
3,614
I find it a little strange that Holley developed carburetor size recommendations for the carburetors they designed and people who know next to nothing in comparison to the engineers, ignore that recommendation. Instead they go with the bigger is better idea. I had a 3310 on a 460 and it proved too big for that engine. A 600CFM 4180 made for 460s made more power and got better mileage. I got the 3310 because I was young, ignorant, and it was the cheapest new carb Holley made.

I didn't say it was perfect but it is definitely workable instead of buying a new one. The vacuum secondaries may rarely open giving him a 375 cfm two barrel most of the time.
 
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DC_Gearhead

DC_Gearhead

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Joined
Dec 28, 2015
Messages
564
I find it a little strange that Holley developed carburetor size recommendations for the carburetors they designed and people who know next to nothing in comparison to the engineers, ignore that recommendation. Instead they go with the bigger is better idea. I had a 3310 on a 460 and it proved too big for that engine. A 600CFM 4180 made for 460s made more power and got better mileage. I got the 3310 because I was young, ignorant, and it was the cheapest new carb Holley made.



Everyone is entitled to an opinion. I’m just enjoying the help tuning my Bronco.
 
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jckkys

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Mar 15, 2012
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If an EB was going to be run at full throttle all the time it may make some sense. I sure Holley knows driveabillity, low end torque, and MPGs are more important to the sane. The carb size selection guide is made to consider the reality of how trucks are used.
 
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DC_Gearhead

DC_Gearhead

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Dec 28, 2015
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So I did some additional adjustments on my carb and timing. I had decreased the jets to 65s. That was a bad decision apparently. I didn’t know that until I got a wide band O2 sensor and new timing gun. I was so lean, the gauge wouldn’t read it. As it turns out the 750 is perfect for my 351w with near stock jets (74s). I was able to adjust the fuel air to be spot on at 14ish, vacuum at 19, and timing at 12. I still need to drive it wide open, but it runs great now and the fuel air is perfect. I also figured out my advance issue and made some adjustments. I have one of those cheap Chinese HEIs, but I got a Crane adjustable vacuum advance with a limiter wheel for $40 on amazon. The best thing since sliced bread. I set my timing to 12 and my total advance at 3000k with centrifugal advance is 38 (based on some other forums). Exactly where I wanted it to start. I moved my advance canister vacuum over to be ported to. I took me a while because I moved into a new house (big garage now), but it is really coming along. A great resource for Holley info is ThunderHead289 on YouTube. He was really helpful in understanding how it all works together and diagnose my symptoms. I’m in the process of making a couple of modifications based on his advice. I’m converting my 4160 to a 4150 by adding the secondary metering plate/jets and I have added a fuel pressure regulator to keep consistency. Both make mods logical sense.

I have other issues right now, but this one seems to be heading the right direction. Thanks guys for the advice. I used a little bit from everyone.


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