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 Carb recommendation 
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Joined: Tue Nov 12, 2013 2:17 pm
Posts: 10
Hey folks,

I spend a bit of time on the CT website but a number of recent posts from Mark over there has me signing up here for some opinions.

Engine is a mild BBC 489, 234/244 sized hydro roller cam. Brodix RR-O heads, and a mainfold that I've not yet decided on. 4000 pounds + car, 4 speed with 3.31's. Street only, 6000 RPM max.

I've played with CFM calculators to help determine what size unit I need. Numbers are all over the place. It "seems" like a 750 ish will do the trick but all the posts I've read say it needs more.

I do not want a double pumper, VS only. I think I like QFT's HR series VS because of their ajustability, and I don't need, or want, all the "bling". But the largest VS HR series carb they offer is a 780.

I've read zillions of posts on CT about people having problems with VS carbs, a delay with the secondaries opening. Also idle issues with both DP's and VS's (recent post from Mark regarding drilling the blocks to lower the idle feeds).

So, what's your take? Will the QFT HR 780 do the trick? Should I be looking for one of the Holley 850 VS that comes on the ZZ502?

Many thanks,

Dan


Sat Nov 23, 2013 1:35 pm
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Joined: Fri Feb 08, 2013 3:51 pm
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In my opinion a rochester quadrajet would probably be best suited for your situation.


Sat Nov 23, 2013 4:15 pm
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Joined: Fri Feb 08, 2013 5:50 pm
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For what you are doing you won't see any benefit from larger than a 780, or a properly built Quadrajet. The Quick Fuel 780 is fine, I would just relocate the idle feed.


Sat Nov 23, 2013 4:52 pm
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Slowpoke,

I actually like QJ's, a lot. I've been running an SMI 800 CFM QJ on it but controlling fuel pressure isn't a QJ stronge suit, especially with the high flow needle and seat assembly. SMI wants no more than 5 PSI, and replacement mechanical pumps run in the 7 to 9 psi range. I am not going to run a regulator. Additionally, SMI warned me that idling could be a problem when idle vacuum gets down to the 14" range - the engine is making 12-13" now. The QJ is on a factory CI manifold so I'm probably giving up a little power with that. I can't fight with it anymore.

Mark,

Have read your posts about the idle feed. In one you mentioned the "thicker" metering blocks. Can the regular blocks have the idle feed relocated? I ask because I intend to run factory fuel lines, either the 1965 L78 set up with separate filter, or the 1966 L72 set up with the bowl mounted filters, and I wonder if the thicker blocks would cause the factory lines to not fit.

Is there a better alternative than a QFT HR 780 VS? BTW, needs to have an electric choke.

Thanks again,

Dan


Sat Nov 23, 2013 7:02 pm
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Thicker or the BLP wide body blocks are only for a Dominator and only to retain the intermediate circuit. For any 2 circuit 4150 or Dominator standard metering blocks work fine with the idle red lowered. And of course the rest of the calibration correct.

As far as the QF 780 VS, it's as good as any to work with. If you want billet metering blocks and baseplate the SS-780 VS would be the one to get, I wouldn't expect any difference in performance.

As far as the Q-Jet, put a standard N&S in it so pressure isn't an issue. It is a street car, right?


Sat Nov 23, 2013 7:24 pm
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Joined: Tue Nov 12, 2013 2:17 pm
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Yes, street car only.

SMI built the QJ to my engine specs. First time it would barely idle, too much fuel pressure. I replaced the pump which made no difference. Sent it back and they installed the regular N/S. Also drilled the primary T blades to help with the idle. Still too much FP for the regular N/S. According to the wide band it was idling at 14.3 but lean everywhere before the secondaries, never dropping below 15.2, even after I installed 77 main jets in place of the 74's that were in it from SMI. I know I can get it richer by juggling the main jets, the rods, and the APT but it appears I can't resolve the FP issue without a regulator. I could possibly use a larger float but that's at the expense of fuel volume in the bowl. Last time I had it off the carb to air cleaner gasket was wet.

Can't think of a single reason I would need, or want, billet anything, especially since it is going to be covered by a gigantic air cleaner. Have to admit, though, that their "Black Diamond" series does look pretty cool. Too bad they cover it will all their QFT stickers though.

A friend with the same car and very similar engine - with with an automatic, is using the ZZ502 850 VS and he says it works great. But then I read over on CT that several guys can't get the 850 VS to work on their combos, the dreaded transition to seconday flat spot. I am tempted to just go with the 850 but it doesn't have the adjustability of the QFT (or others). Then of course there's the whole debate about how large a carb (CFM) is necessary. I don't want to over carb, but I don't want to leave 20 HP on the table either.

Anyway, at this point I want a carb that will allow the engine to run at it's potential, and one that I'm not constantly fighting with.


Sun Nov 24, 2013 11:56 am
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I know a B/S Camaro that ran a 10.36 with a Quick Fuel SSR-780… You may lose a little peak power but you can lose more low speed power if you go too large on the carb. As far as the QJet… Those things are plundered by several that think they know them, and they may work fine on a race only engine but not always so much on a street car. Tuner has a better handle on those, has calibrations from the 60's up and knows what will get you in the range as long as they have not been plundered too bad. As far as fuel pressure, seems to be an issue with a lot of the newer mechanical pumps going too high, a regulator is the only solution in that case.


Sun Nov 24, 2013 2:20 pm
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Joined: Tue Nov 12, 2013 2:17 pm
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Thanks for all the feedback, Mark.

I don't think the QJ has been plundered. SMI is a fairly well respected QJ builder. They did warn me about the FP, and about the potential idle issues when the vacuum gets down to the 14" and lower range. I thought/hoped I could get it to work. If I could find an unobtrusive, factory-ish looking regulator I'd consider it but ones like that don't exist. The stock-ish 750 QJ on the previous stock-ish 396 ran pretty well, despite having had my hands in it many times.

Sounds like the QF HR 780 VS will do the trick. If you know of any better alternatives please let me know, or if you know of any tricks/mods to ensure it works right I'd love to hear them.

Dan


Mon Nov 25, 2013 1:11 pm
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Do you have the number drill sets 1-60 and 61-80? If you can measure some orifices in your QJet I can help you make it run better.

The reason you are having trouble with the rich idle on the SMI QJet is possibly not the fuel pressure, but is a rich calibration of the idle circuit. SMI (and nearly every other “custom QJet builder”) is raping the idle circuit similar to the modifications suggested in a popular QJet book published in recent years which advises enlarging the idle jets and idle down-channel restrictions, two modifications which convert a good QJet into a plug fouling mess. (In fact, I am working on two just such raped-plundered-pillaged-run like crap but they look good QJets today, LOL.)

The carbs I have repaired for people sourced from custom builders or the special “Stage ?” units which are purchased from Jeg’s, Summit, etc, have oversize idle jet, air bleed and down-channel restrictions, large enough to be too rich with a 2” butterfly in a Dominator, but a QJet is only 1 3/8”. The idle circuit fuel flow calibration areas must be appropriately sized to match the air flow area increase of throttle position change, which is a function of the butterfly angle and throttle bore diameter.

If it has a brass float in it change to a standard nitrophyl float. The brass floats are heavier and smaller, which means they have less displacement force to close the inlet. The standard QJet float has far superior float level control than the brass, especially with large inlet valve. The small volume of the QJet fuel bowl is a good thing, not a deficiency. Less fuel sloshing around causes less disruption of the float level control. The bowl is narrow and relatively deep, compared to length and width, so the jets stay covered and there is less mass of fuel to slosh and cause the float to move up and down independent of the actual amount of fuel in the bowl.

Production vehicles had no trouble controlling float level with 7-8 lbs. pressure and a .130" inlet with production nitrophyl floats.

Much of the trouble we are having with fuel pressure nowadays is because the ethanol in the fuel causes a very high vapor pressure at only 100° F.

I rigged a vapor pressure tester with a Snapple bottle inside a peanut butter jar with a holes in the lid, one for the thermometer stem and one to allow the Snapple bottle lid to stand proud but keep it from floating in the hot water in the jar and a hose to a pressure gauge pushed through a hole in the Snapple bottle lid. Gasoline sample in the Snapple bottle, 100° F hot tap water in the jar around the outside of the Snapple bottle and read the pressure gauge.

15 PSI at 100° F is a common occurrence.

If your system does not have a vapor separator type fuel filter with a return line, or your fuel pump does not have an internal bleed to bypass the high pressure, the heat in the fuel line between the pump and carb will raise the fuel pressure to the vapor pressure of the fuel at whatever is the temperature of the line.

For the last several years, especially since the ethanol content went up to 15%, the little Holley fuel pressure regulator has become a necessary component when using pump gas with ethanol.

Do you have the number drill sets 1-60 and 61-80? If you can measure some orifices in your QJet I can help you make it run better.

The 850 VS crate engine carb is good. If you get one, be sure it has the little nipple that boosts the vacuum to the secondary diaphragm in the choke side primary bore. (as shown here http://racingfuelsystems.wonko3.myfunfo ... 1522#p1522 ) I worked one over for a B&M supercharged SBC about two weeks ago. The lean flat spot on the secondary opening is because they have a very small PVCR in what appears to be a metering block for a 600 CFM carb on the primary. That is easily fixed with a .070” drill bit.


Mon Nov 25, 2013 6:32 pm
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Joined: Tue Nov 12, 2013 2:17 pm
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No, Tuner, no "numbered" drill bits. I'm really not a carb builder, pretty much just an internet reading hack. ;)

I'm pretty certain FP is an issue. When I bolted the QJ on the first time - with the high flow N/S, fuel was oozing out in all sorts of places. Changed the pump, no difference. Sent carb back. SMI changed the N/S, drilled the primary blades, and installed different mixture screws, in an effort to clean it up (I thought it was horribly rich at idle; eyeball watering, bit it might have actually been very, very lean).

I finally hooked up the wide band and took it for a drive. The idle AFR was at 14.3, not horrible from what I read, (and I could probably lean it out a little with the adj screws), but the idle RPM became erratic again during the drive. Pulled the air cleaner - traces of fuel all over the top of the carb. It does have the nitrophyl float in it. The drilled primary blades probably lower my 750 or 800 RPM idle vacuum by 1" or so.

Your comments about the vapor pressure make sense. My stock fuel delivery set up does not have a return line, and uses factory steel line from the pump to the carb.

I've got at least 400 into the QJ at this point, and an unaccountable number of hours fussing with it. Do I send it to someone else to have a crack at it or do cut my losses? That’s where I stand now. I need a new intake manifold. If I decide to stick with the QJ then I need a QJ manifold. If after that purchase I still can't make the QJ play along then I'm buying another manifold. Not to mentioned the hassle of it all. Hence my decision to go Holley.

Although I am married to using a VS carb I'm certainly not married to using the QFT HR780 VS. I wasn't too impressed with their tech help via email; kept telling me to use the 830 DP. Then suggested their 880 VS - which isn't even shown on their web site. I replied that the 880 was probably too big so he returned to his 830 DP suggestion.

As mentioned a friend is using the Holley 850 on his similar engine/car and is pleased, so I'm still interested in the 850 VS Holley. So many opinion regarding proper size. I saw that thread on the little nipple(s) on the primary side. Did/do any of the 850 VS's come with them, or are they always added by someone? What about the lack of screw in bleeds like the QFT has?

Lots to consider.


Tue Nov 26, 2013 3:39 pm
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