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I had the same distributor and same problem so I contacted MSD and they sent me out a prototype limiter you could try but it's not quite as good as a better can with the Lars limiter that LeMans guy was describing, something about the quality of the vacuum signal I can't remember.
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Discussion Starter · #22 ·
Thank you very much for the information gentlemen. I'll report back when I make some progress on it.
 

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Discussion Starter · #23 ·
My solution involved $3 in parts from the local hardware store. Needed a couple of adjustments as the metal bar limited the travel to 4 degrees at first so I shaved a little off the corner and got it to 8 degrees. I set my initial to 14 so I have 22 at idle with the vacuum to a manifold source. The mechanical advance is still at 23 degrees which seems a little high. I guess you have to buy new weights that are different shapes to change the limit on that? One model of this MSD uses different size bushings but I don't think that applies on this one.
 

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Yes you would need new weights from a curve kit. But 23 is perfect, you won’t gain anything unless your cam is so radical that it needs 28 to 30 degrees idle timing.
You are running 37 degrees total timing, ok as long as you are not pinging. If you are dial the base back 2 degrees

good work on that stop!
 

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Discussion Starter · #25 ·
Thanks, I'm going to retighten my screw and use some blue Loctite unless you think I should shave off a tiny bit to get the vacuum advance to 10 degrees.
 

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8 to 12 is good……10 is the median, if you decide to shave it you can go up to as much as 4 degrees at Idle, but 10 would give you 24 that would Be smooth and cool. 8 is ok 10 a hair better on your set up I would say
 

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Discussion Starter · #27 ·
I wanted to update the folks interested in this. I'm not one to just leave something alone after it's running ok so I decided to tinker again. I took the mechanical advance down to 18 degrees so I could start with more initial. I went with 18 and it starts and idles just fine. Of course I messed with my vacuum pot as well and it's pulling 12 degrees now. I'm at 36 total and my initial plus vacuum at idle is 30. I know that's a little high but I don't have any audible knocking but I'll check the plugs after a bit. The engine seems to run much nicer with these settings. I had the feeling that 14 or 15 was a little low for this motor and it was contributing to a small but noticeable off idle stumble. I like to tune my Holly idle screws to max vacuum per the guidelines but it always seemed to want another 1/4 turn to get rid of the stumble (accelerator pumps are set correctly too by the way). That fixed it but I always had the rich idle partially burned gas smell all over me when I took a cruise. Well, a little timing and a little less fuel did the trick. No gas smell and no stumble. Now it's time to get my catch can and wagner pcv valve into the mix. Thanks for the guidance on this.
 

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Great job whitt 30 is Ok. As long as the starter does not kick back and the idle is smooth it will be nice. An off idle stumble from a stop is usually indicative of the transfer slot not being perfectly set. The carb transfer slot actually is the first thing that adds fuel/air to the idle circuit as you transition from stop to moving. A slower pedal does not engage the accelerator pump them and if the transfer slot was not right it can stumble.

When you set to highest vacumn it sometimes causes you to close the throttle and thereby open the transfer slot too much to the manifold. Now the slot is already adding mixture at idle, already drawing it. So when you transition it does not help, because you are already drawing it.

So setting the vac gauge say 2 HG below highest can help to make it right. Another way is to square the slot with carb turned over and don’t touch it at all. Just use the mixture screws.

But for you, you now have it right, I would not touch it drive it and enjoy that perfect timing!
 

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Nice job, I ended up putting a 14° bushing in my MSD from FBO ignitions, set the base at 20°, all in at 34° about 2500 rpms and pulling 11° vacuum so 31° at idle to burn off the fuel and the plugs are looking good, still starts good even after letting the temperature peak and runs like a beast with no detonation....now if I could just eliminate the secondary hesitation from idle to wot I could sleep better. Tried everything, cam position, bigger tube nozzles, floats are right on as are the accelerator pumps, tried secondary idle adjustment, went up a couple jet sizes, have a 5.5 pv. It's an 850 QFT on an Performer RPM with a 1/2" spacer but just can't get the air moving fast enough to start pulling fuel when I mat the peddle after that split second it goes like a scalded dog.
 

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Have you increased the size of your “Power valve channel restrictors? PVCR’s are two mini jets behind each power valve. On a Quick fuel they are easy screw in just like a jet.

when you drop below 5.5 HG on the power valve those are what feeds the fuel… they come in various sizes. At WOT all your main jets, primary and secondary and all of your PVCR’s are open and pulling. If you have two PV you have 4 PVCR.

Normally hesitation like this is a transfer slot or accelerator pump, the cam or nozzles or adjustment of same. But you say that tried those, so you might want to look at the PVCR they may need to be larger.

pull the bowl they are right under the PV they have a number just like a jet. Your carb sheet that comes with it will tell you the size as well.
 

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Haven't touched those and don't know the size but no one has suggested that before also I was mistaken the PV is a 6.5 with the secondary pv blocked off. FBO thought it might be out of balance so they said turn the main idle up 200 rpms then use the secondary idle screw to bring it back down, tried this twice with no luck so I reversed the procedure just once. From about 1500 rpms there's no hesitation and none up to the secondarys. The builder doesn't have any more suggestions and I guess I shouldn't have used Ebay for this but his stuff looks professional with a 100 feedback and I talked with him a few times and seemed to know his stuff since it's all he does but I suppose every carb needs to be tuned to the motor it's on, he says it's not to big either...here's the build sheet and sorry I'm drifting off of the timing post so maybe I'll start another one but I did change one advance spring back to how I got the motor one light blue and one light silver instead of two light blues to see if it was an advance problem, now I'm all in at 34° @ about 2500 so hopefully that's ok. Got a couple of shows this weekend so I'll see how it feels.
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Have you increased the size of your “Power valve channel restrictors? PVCR’s are two mini jets behind each power valve. On a Quick fuel they are easy screw in just like a jet.

when you drop below 5.5 HG on the power valve those are what feeds the fuel… they come in various sizes. At WOT all your main jets, primary and secondary and all of your PVCR’s are open and pulling. If you have two PV you have 4 PVCR.

Normally hesitation like this is a transfer slot or accelerator pump, the cam or nozzles or adjustment of same. But you say that tried those, so you might want to look at the PVCR they may need to be larger.

pull the bowl they are right under the PV they have a number just like a jet. Your carb sheet that comes with it will tell you the size as well.
Don't know if you got a chance to look at my build sheet on the carb but will it have pvcr's if the secondary power circuit is blocked off? I tried one lighter advance spring and driving it this weekend it still hesitates on wot so don't think it's timing.
 

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Yes I see it is a double pumper with one 6.5 power valve with one blocked off. So under the one power valve up front you have 2 PVCR’s, yes the valve is high flow, but the PVCR are part of flow too. You can look on the quick fuel tech website and see what the stock one is. You can call your carb builder and ask him what it is, or you can pull off the front bowl unscrew the power valve and look for yourself. They are two small screw in mini- jets.

These are part of fuel fuel during any operation where the PV vacumn is below the rated number, 6.5 HG. It basically opens below that and allows fuel to flow thru the PVCR, they come in various sizes. They are not normally a “Hesitation” thing, so not sure it will solve your issue, but since you tried everything else and they are in the fuel equation when you go to WOT that extra fuel may be needed. It sounds like a lean bog. But again if this does not happen under load and only in neutral not sure it is a valid concern.
 

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Yes it's happening under load all the time but there's no hesitation stabbing the throttle up to the secondarys only when I put the pedal to the floor quickly so does that mean the problem is in the secondarys and will there be PVCR's in the secondary if the PV is blocked off? I went up two jet sizes in the secondary to 84 and up one to 75 in the primary, I know one size doesn't do much but it's what I had in stock now I bought some #76 and #86 I could try. The carb builder wasn't that friendly the first time I called and even less the second time so I'm done with him. I emailed with a Holley site tech because they own Quick Fuel and they were no help and stopped answering me.
 

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Right no PVcr in secondary because no Power valve, just two up front. You could up the jets watch the AFR on a meter…12.2 for power…

you also have high speed air bleeds on top of the air horn 2 for the primaries 2 for the secondaries. On QFT those are mini jets as well and easy to change. But you are getting into some sophisticated tuning and an AFR meter is really needed, temp or permanent.
 

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Ok, don't have an AF meter ..I did increase the rear jets from 82 to 84, would it be a jet problem that quickly into flooring the pedal? The plugs are on light side after about a 100 miles and I don't ever remember seeing black smoke when this happens so maybe I'm still lean, I did try larger pump nozzles but they do have a shorter duration so maybe I should try a smaller nozzle for a longer squirt duration?
 

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Did you have this hesitation before you changed the timing per FBO?…..the 14 Centrifigal and 31?…..

FBO says the carb is not too large, but you say that you cannot get the velocity thru the Ventura’s to draw the fuel…

..those two things cannot both be true…..

it has to be one way or the other…..
 

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Yes it started hesitating last year soon as it got hot and humid so I called the builder who wasn't to helpful and he said I could try up jetting it, then called him this summer and he said check to see if the fuel was boiling out of the squirters which it wasn't so I asked him if it was to big he said no the 750 has the same bores, FBO never said it was to big when I asked him. FBO said they can't help me anymore without putting their 5 gas analyzer on it so idk how I would accomplish that. I'm just guessing on the air velocity speed theory by what I've read in the Holley books I have, and like I mentioned I changed out one spring to see if getting my advance in a little sooner would help but it didn't, it feels good and I don't hear any detonation so I think I'll leave it there.
 

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I have a good video of the problem but I haven't been able to post videos here so I could put it on YouTube or if you want to PM me with an email I'll send it that way.
 

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Well now that you have identified it as the secondaries, the most likely culprit is the secondary accelerator pump. Richer in some form, bigger, longer duration, different cam….or some combination.

a 5 Gas is great an AFR is almost as good lot’s easier to read. The goal of the 5 gas is to get to the perfect AFR anyway, but both tools together is great. As particular as you are I would put wideband 02 sensors in each exhaust and get a dash mounted gauge set. I use Daytona digital meter. Then you easily see if you are too lean or too rich in the secondaries and adjust according.
 
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