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Discussion Starter #1
OK!! Here is my question but I will start from the beginning.. I broke the baseplate on my carb right after replacing the intake and heads,, so when I ran it with the vacuum leak it ran great,, here is the dilemna I replaced the baseplate with a new one and now getting a weird problem. when I slowly press the gas she goes but once the secondaries start to open it bogs for about 2 seconds then takes off like a bat out of hell but if i try again it would bog then backfire this happens frequently since the new baseplate and never happened before when the old plate was good my primary squirter is a 37 and so is my secondary but the front pump is a 50cc as my back one is 30cc my timing is 12 degrees with a total of 35 @2900 rpms my primary jets are maxjet size 81 and my second is 83 with anything smaller it would backfire constantly and tell me it was too lean the intake is a single plane for 2500-6500 rpm low profile design, the heads are l98 vette heads with 165cc runners dual exhaust 1 3/4" primaries into true dual straight back into glasspacks msd ignition Can you give me some pointers? By the way the main body is a proform 750 with adjustable air bleeds down leg boosters no choke this carb is a double pumper..
 

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Discussion Starter #3
It's mechanical secondaries so I can tell when they start to open they bog like it is lean although I know it isn't. It's only when the secondaries start to open I'm running a 30cc pump in the rear with a 37 squirter. Other forum says I'm running too rich because of the size of the jets but if that was the case why do I get a lean backfire?
 

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I was just wondering if you got that linkage correct for the secondaries...hmm...can you return the whole carb back to factory specs and see what happens?
 

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What is the rest of your combo like? Just looking at what you have posted, I would think you are fouling the plugs so fast it wouldn't even run! So a small block running the l98 heads, your jets are way too rich... also, your squirters and primary pump are too big for a small-block. That carb would feed a very healthy big-block!

I would be willing to bet that what you are experiencing with smaller jets is not a lean mis-fire, but rather the engine loading up on fuel and sputtering.

To give you another idea, a factory Holley 750 for a big-block application comes with 70 jets in the primaries, 78 or 80 secondary jets, and .028/.031 primary/secondary squirters.

My recommendation is to find a 600cfm, or re-jet your present carb WAY down. You will probably need to be in the 64 primary jet range and 68 to 70 secondary jets.

Best of luck,
Bill C.


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Bill C.
Colorado Springs, CO

68 Camaro... someday!
71 Chevelle SS
70 Nova
 

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I agree with Bill.
My 454 has a 750 vacuum with 72's in the
primary and it's a little rich.

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'86 Firebird,ladder bar
suspension,9 inch ford w/4.56,10 point,440 bbc,
400 manual valve/brake.

'86 3/4 ton 4 wd 454/400 turbo
 

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If your backfiring, your too rich. If your engine bogs, its lean. Pull your spark plugs and see what they look like. They are a good indication of the health of your fuel system & proper fuel/air ratio.

I had this same problem backfiring through the carb and it ended up being the secondary fuel bowl level was set way too high.

Dave
 

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Discussion Starter #8
I'm using for jets the Maxjets by Moroso so the primary jet is equivalent to a 72 holley jet and the secondary is only 4 sizes up from that to 76 maxjets go by the actual opening orifice not the holley number because holley jet number 72 has a .079 orifice and maxjet is exactly .080. So the only thing I did was go down jet sizes and that's about when she gave me grief. Why does my engine require so much fuel? It's only a 327 with K.B Domed pistons running 11.97 compression? How come you guys with big blocks have smaller jets and run good and when I do that she runs like s#&t? I'll put it back to factory condition and start from scratch but I'm sure she is not going to like it? On a side note I am absolutely positive there is no vacuum leak, I covered every hole on the engine with racers tape and blew smoke in the venturi and none came out anywhere, I even plugged the rear of the carb where my vacuum lines go in and nothing still.

[This message has been edited by grygst76 (edited 04-30-2002).]
 

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Discussion Starter #9
I'll give exact specs on my combo---
327 30 over with Keith Black domed pistons with a 10.5:1 ratio with 64cc heads, but I'm running 58cc L98 heads. block was zero decked
so according to my builder my compression is 11.97 pretty high but no detonation due to running on 94 octane. The intake is a weiand stealth low profile single plane 2500-6500.
The Cam is an Extreme energy 284 with specs as follows--Part Number 12-250-3
Engine 1955-present Chevrolet
262ci-400ci
8cyl.
Grind Number CS XE284H-10
Description

--------------------------------------------------------------------------------

Intake Exhaust
Valve Adjustment 0 0
Gross Valve Lift 0.507 0.510
Duration At 0.006 Tappet Lift 284 296

--------------------------------------------------------------------------------

Valve Timing At 0.006
Open Close
Intake 36 68
Exhaust 82 34

--------------------------------------------------------------------------------

These Specs Are For The Cam Installed At 106 Intake CL
Intake Exhaust
Duration At 0.05 240 246
Lobe Lift 0.338 0.34
Lobe Separation 110
Following that I have a TH350 with shift kit backed by a 3000 stall running into 3.42 gears. The carb is a proform 750 double pumper with adjustable air bleeds, down leg boosters, no choke. The dilemna started when I got rid of the vacuum leak I had and lowered the jetting because I fattened it because of the vacuum leak I had, one of the ears where the primary squirter is was broken and you could hear the air rushing into the intake. It kills me because it NEVER did this before when I ran it on (holley part numbers) 76 primary jets and 80 rear. My plugs are a little black but only a little there is no black on the resistor but I'm running a .45 gap on platinum bosch racing plugs. I think I will change the wires to 8mm anyways because they are stock plug wires but those are new. Also I am not using vacuum advance since I always had problems connecting it and running, I would use stiffer springs due to the engine surging and I did not want to lower the timing since when I did, I lost power. I hope this helps a little.
 

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Ok, then sorry and thanks for the education on the Maxjets... didn't know they were so different.

Although I still think you are way too rich even with 72 and 76 equivalents in there... that's close to big-block numbers.

One thing you mentioned was the close-to 12.0 compression and 94 octane gas. Seems to me that you would need to use a higher octane gas with compression that high. Have you tried bumping up the octane rating and then tuning it?

After that, I really don't have any more ideas... the motor is fairly radical, but I don't think it should use that much gas. Have you tried a different carb?

Best of luck,
Bill C.
 

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I'm running a 750 vacuum secondary Holley on a 396 big block and I'm only using #70 jets. I tend to agree you're pouring the gas to it.

The first cardinal sin is using non-Holley parts on a Holley carb.

Are your fuel bowl levels set properly? The level should be just under the level screws.

Is your timing good? This makes a BIG difference!

Your plugs don't lie. If they have a black powdery coating, your too rich. Tune your jets so that you get that tan appearance.

Are you running the correct size powervalve? Rule of thumb is 2 under your normal cruise vacuum.

Set your idle mixture screws to achieve the highest vacuum.

Next you can turn your attention to the shooters. You want just enough shooter to cover up the bog. No more. This can be set by adjusting your accelerator pump - changing cams, or changing the squirters.

You should pick up Dave Emmanual's book on Supertuning Holleys. It really was an eye opener for me.

Good luck.

Dave

[This message has been edited by Scooby Doo (edited 04-30-2002).]
 

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Discussion Starter #12
I will have to say thanks for all your help guys I will go "back to the basics" and go back up from there, it does seem weird that mym otor wants so much fuel, I have always noticed that even when I had a 650 installed on it it wanted 73 jets in the primary and 77 in the secondary with 37 squirters, and that advice was given to me from holley after telling them the same scenario when it was at factory, the thing just did not want to stop backfiring and I know the rule of thumb with holley jets is go 2 sizes up and back down when you notice a loss or power then 1 size from there, it's very weird
 

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Discussion Starter #13
CRAPPP!! I think I might have the answer in front of my idiotic face!! when I went to take the primary bowl off and disconnect the fuel line it would not come off and twisted itself so the line is bent I think the line might be restricting fuel up higher and thats why it is dying for fuel!!!! its pretty bent up too maybe enough to do that its one of those adjustable fuel lines I will just replace it and see what happens, probobly won't solve my rich jetting problem but maybe thats why they have to be so rich, because the line is restricted?
 

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Discussion Starter #14
Well it ended up being a kink in the fuel line to the carb now she still has a little bog but thats because my squirters will need to be a little bigger in the rear to .040 so I went back to factory settings and she did not like it at all (bogging and spitting, throwing a tantrum) and I ended up right back where I started with the big jets in the front and rear so thanks for the help.
 

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OK...so the fuel line was the problem...
If you get a new one ..try the blue anodized ones from EARL's ....

Also...you say you have a bit of bog still there...and you are running an automatic..
Many times this is simply the kick-down cable not properly adjusted... You need that kick-down cable to jump down a gear quickly and timed with the throttle opening so that the venturi velocity increases apropriately, this way the added squirt of fuel gets sucked in and properly utilized when needed...if the kick down is late dropping the gear with repect to the secondary throttle base plate timing..then the venturi air velocity will be lazy ( actually it will have a charge back from the cam overlap, thus really making for some bad air velocity) and all that squirt will turn to droplets on the plenum and runner walls, not much of it will make it to the chambers and there you have a lean-out, BUT now you hit the pedal again and now you get too much fuel since the second time you have the acumulated droplets summed with the addition of more squirt pouring into the chamber....
Regards
Chris
 
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