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Old 05-24-2010, 02:50 PM
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Need advice and all input on carb jetting

Hello guys I changed my plugs about two months ago and I have not checked them since then but took three plugs out today. Engine is following
350 sbc 9:1 compression
World sportsman 2's
Crane hydraulic roller 222/230 @ 50 509/528 lift 1.5 roller rockers
Edelbrock performer rpm intake
Holley 600 cfm 1850 carb jetted 70 front 76 rear silver spring for secondaries
1 inch 4 hole carb spacer
Hei 50k distributor with ngk 6630 plugs gaped .040

Ok everything is new on motor with all wires etc. I had some issues with missing here and there and found out the problem is some how carb related. I came home from work last Saturday and my truck never sat there and idled any I just shut it off in the driveway. The one plug I pulled out was a little wet but not fuel fouled but I did smell gas on it. The ring base was black with soot but other then that driving the truck it did not give me any problems just the occasional miss here and there. Did compression test on both cylinders and they are both good at 160 psi. Other side gives same result.

The carburetor came with 65 front and 68 rear which I figured right off it would be to lean for my engine seeing that is should be pushing 375 to 400 hp and issues I had with the holley street avenger being lean.

The reason I am getting to that is because my Dad has a 302 ford with same carb and his engine runs excellent with that jetting with nice tan plugs but his motor is a factory 302 all stock about 200 horse power. Am I wrong to assume that that same jetting would be lean for my engine? Brothers 1987 stock 305 chevy has same carb with same 65,68 jetting as well and his idle mixture screws are out just 1 turn and it runs excellent there. His engine runs good as well.

I had a holley street avenger 670 aluminum one last fall and out of the box I re jetted it to 68 front to 74 rear and it did not have any surging during cruising but looking at new plugs I put in they where white looking for the time being but eventually got black around the ring base. I have posted before pictures and it seems I can't keep my plugs clean.

Idle mixture screws are currently 1.5 turns out and if I try to lean them out more my engine will start to spit and run rough more. I also have secondaries cracked open some to for extra air flow but no transfer slot showing in rear and in front with current idle setting the transfer slot is about .040 or so.

I have my timing set and checked at 16 initial with vacuum advanced unhooked and with it hooked it is around 28 with full manifold vacuum.

Total timing is 34 all in by 3000 to 3500 rpm. Hopefully Fbird and others can give me there opinions here as what would be a good base area for jetting on this thing. I don't want to be looking at my plugs with the ring base looking rich and and end up jetting this thing down and then will still think its rich but will actually be to lean and burn my engine up over time.

The porcelain part where the fire happens its a grayish tan color and looking clear down in the plug at the base of the porcelain its dark but white the rest of the way up with tan and gray at the top part area.

I don't want to wash my cylinder walls down here and I don't also want to make my engine burn up being too lean but I am not reading plugs right.

I looked at different charts as to what a good plug looks like and mine does not look to bad but trying to get it matched up perfectly like them is kind of hard. I would like some suggestions on jetting here of what should be the ball park here. I know Fbird has ran some excellent engine combos here making over 400 horse power etc. I figure you guys have some much experience you will know better on a good estimate on what my jet size should be on this engine combo

I only have three sbc's under my belt for working on. Is the size of my cam playing in too effect here cause of some minor overlap as compared to a stock cam? Engine idles nice at 850 900 and in gear about 700 rpm. Anything lower and it does not run well. Hope to hear from you guys.
Eric

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Last edited by eric32; 05-24-2010 at 02:57 PM.
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Old 05-24-2010, 04:05 PM
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Out of the box jetting is usually the best place to start. The basic carb works with lots of combinations and should only be changed if it doesn't work. You may be surprised how well a stock carb works on a pretty stout engine. Over jetting should be avoided just like over carbing.
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Old 05-24-2010, 05:01 PM
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Yeah I know what you mean but I did not over carb my motor like I have in the past I can't decide on how much jet size I should go down though. I was thinking going down to 68 front and leaving the rear at 76 or would that be too much for the rear as well? The most I would probably go down in the rear is 72. On most holley carb settings on peoples motors they have a 6 to 8 size difference between the front and rear jets. I will have to go and take my carb off but I want to get some more feedback cause its a messy job to get it on and off with all the gas that has to be spilled out from the carb itself.

I want to go down in the jets, readjust my secondary's open just a hair more so I can close my primary throttle blades a little more. Then I will try to lean out my idle mixture screws more as well but I won't be able to do much more with them.
Thanks
Eric
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Old 05-24-2010, 05:18 PM
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What size shooter are you using? I would start with 66/73 and a 31 shooter and see how it runs. Secondaries are best left alone except for spring changes. It is far easier to screw them up than to improve them.
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Old 05-24-2010, 05:19 PM
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I don't think anyone can give you accurate jetting instructions, you don't just say I have a 400 hp motor so I need "xx" size jets, every engine combo reacts different. Even if three engines are built with identical parts the same jetting might not be the same in three identical carbs, due to production tolerances in everything in the engine and carb. Even same size jets have a variance.

75% of the run time the engine isn't even using the main jets anyway, it is running on the idle circuit and transfer slot unless you are down on the gas pedal a bit. Cruise rpm is the fed by the transfer slot most of the time.

The only way to accurately read plugs is a "plug chop". Put a fresh plug in one hole, kill the ignition at the end of a WOT pass, no idle or cruise at all, otherwise all you are looking at is a very rough averaged color. Plugs on an unleaded fuel motor should be pretty much white to light gray, and you will need a lighted magnifier to look down into the base of the plug for the fuel ring near the base of the porcelain.

70 and 76 jetting is the factory jetting for the original 780 Vacuum secondary carb, likely to be way too rich on a 600 carb. You have increased jetting 13 steps total, a huge amount! Nearly 8% front and 12% rear.

Go back to say 66-67 front and 70 rear and test, I frequently find the 1850 carb to end up with leaner jets in the front than sent from the factory, but every situation is different. Holley sends the 1850 and other general performance replacement carbs out slightly rich to start with, on most engines.

The new Street Avengers seem to be an exception to this old standard, they seem to start out lean from the get go.
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Old 05-24-2010, 05:41 PM
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Thanks ericnova72 for the input. The shooter is a 31 so its fine and I get no issues with it unless I give it a very quick WOT shot then it has a slight pop but nothing really major. I got some 35 shooters but that is something else to mess with after I get the other stuff sorted out first or I will have this carb all messed up. I will try and start with 67 in front and 72 in the rear.

As far as doing the wide open throttle test ericnova I have not done that test before so I pull over my truck pop out an old plug put in a new one and start it up and just floor it for a good quick run then let off and then turn it off correct? That will tell me how the secondary jets are doing AF ratio wise correct? Other then that for main jetting have read to start up vehicle then cruise down highway for about 15 minutes or so then shut off with no idle time and pull over and check plug.

I will have to get it off later on this week and post some pictures and you guys can give your opinions. Thanks again
Eric
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Old 05-24-2010, 10:11 PM
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65 pri 73 sec jets 6.5" power valve. purple sec springs.

remove the carb and reset the pri and sec throttle/ transfer slot exposure, even .025" pri and sec. reinstall, set the idle mix and speed and leave it alone.

NGK BPR6FS .035" gap.

Last edited by F-BIRD'88; 05-24-2010 at 10:24 PM.
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Old 05-25-2010, 04:41 PM
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thanks Fbird I was hoping you would reply. I tried to do the .020 on both the primary and secondary's before and my idle was like 1000 rpm and it would not go any lower then that even turning down the primary idle screw all the way. It was kind of like when you have a vacuum leak but I didn't. I had to take it off and close the secondary's some and that allowed me to not have such a high idle and was able to change it again.

Should I just open the secondaries just to where it almost exposes the secondary transfer slot and readjust the front ones to show only about .020? I will change my vacuum secondary spring to a purple and see how it goes.

My carburetor already has a 6.5 pv so don't have to change there. Also on those ngk plugs are they a colder plug then the 6630 by chance? Also I hope that they are the same length cause I have 3 header tubes that need that particular length to clear my plug boots from getting burned.

I had some colder plugs in my engine previously which was equivalent to autolite 25 and they fouled out pretty fast as to where the hotter plug stays clean better. I don't do much highway driving mostly town with some highway. Cruising in town I average 1200 to 1500 rpm in drive and 2200 to 2500 rpm on highway. Do you think that is part of my plug problem with my town driving?
Thanks for input and to others. I will get the carb off later on this week and readjust everything. Also will be hooking back up my pcv valve as well. Take care

Eric
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Old 05-26-2010, 07:01 PM
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if the idle is just too high with the throttles/transfer slot exposure at .025" close both down evenly a bit as required. Check for a vacuum leak. faulty PCV valve, leaking hose, leaking P/B booster, leaking trans modulator. gasket leak.

It's best to keep the primary and secondary throttles evenly opened as much as possible for even pri/sec idle flow sharing and fuel distribution at idle.

Do not look at the plugs for main jetting inspiration if you mostly idle around town and rarely get over 2000rpm.

At 2000 rpm and lower you are not on the main jets. You are on the idle and off idle circuit of the carb.

Get a AFR gauge if you want to see the idle/off idle afr.
The color of the plugs means nothing other than to warn of a extreme condition way too lean, way too rich, oil fouling, detonation.

The off idle circuit in your carb is not adjustable unless you are real good with drill bits. It is controled by the idle feed restrictions and the idle air bleeds.
small .001" to .002" changes make a big difference. Do not mess with it unless you know what you're doing and have accurate instrumentation and the time and patience to get it right.

If you want to see when (throttle opening and engine rpm) your engine is actually running on the main jets in the carb get a mirror and look down the carb with the mirror. Open the throttle and watch the venturrii for fuel flow.
Notice the rpm, manifold vacuum and throttle opening required to get fuel flowing from the venturrii. Read the AFR gauge and you will see the crossover between the idle circuit/off idle transfer and on to the main system as you open the throttle.

Even a simple low cost narrow band (heated) 02 sensor/afr gauge or DMM is very informative for carb tuning.

Last edited by F-BIRD'88; 05-26-2010 at 07:16 PM.
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Old 05-27-2010, 02:49 PM
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Hello Fbird all is noted. I did not have any vacuum leaks before when I did the even front and back secondaries being the same. I wish there was a way I could adjust my secondaries while my carb is still bolted on. Is there a way or trick to be able to do that with a small screwdriver? I won't ever mess with my air bleeds. I know there function and how they work but I am not about to start drilling them.

I will just have to take it off and readjust things and go down some jet sizes in the front as you stated and readjust idle mixture screws as needed. As far as the AFR gauge goes I don't have funds for one now and I would also have to get a bung welded on my exhaust to be able to use one as well. I will be pulling it off tomorrow and re jetting it.
Thanks
Eric
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Old 05-27-2010, 02:56 PM
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Remove the carb and back out the little set screw that adjusts the sec throttle idle, from the bottom side. Screw it back in from the top of the hole.
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Old 05-29-2010, 09:00 PM
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Hello Fbird thanks for the tip on the screw. I never thought of that. Anyways i took my carb off yesterday and I checked out and my secondary's I cracked them open just a tiny bit more but not too much cause its getting closer to showing the transfer slot. Also on the primary side of things my transfer slot was just a tiny box and I would say only about .015 was showing.

So for idle wise I was not really able to close the main throttle blades anymore as with anything less then what the transfer slot was showing it would not idle good. I had a problem with the stupid dual fuel line hookup bending and then leaking on me so I have to replace it. I will also have to put some new plugs back in again cause I had a problem with a stuck float so got some extra fuel into the intake. Also I am going to ditch the jegs chrome steel covers and was wondering what would you recommend for a set of aluminum covers that is not name brand expensive ones that has a good baffle in them and are the taller style?
Thanks for the help
Eric
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Old 05-30-2010, 12:23 PM
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Well guys I got things all hooked back up today and finally got a new fuel line assembly to hook up and work after fighting with it for a few hours because of it leaking. Fbird if you read this and hopefully you will give your opinion but if not no biggy. I got my carb readjusted to where the back is not showing the transfer slot and I also got it set to where the transfer slot is not showing on the front when looking at it. I got it idling at 900 and I readjusted the idle mixture screws to about 1 1/4 turns out. I put it into drive and the emergency brake on and I kept turning them in to almost when the engine would die then I backed them out about another 1/4 of a turn to get the best lean idle mixture. I now have the front and back almost dead on the same like you said to do.

I also put the purple spring in but how can you tell if you can go even lighter on the spring? Can it make your engine burn a tiny bit richer if they open to fast by chance? I did not get any bog or anything by giving it half throttle or anything. Well thanks guys for input your suggestions sure helped.
Eric
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Old 05-30-2010, 02:31 PM
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The best instructions for the secondary springs are the ones that come with the kit. If secondaries open early or late it will kill your performance. There is NO advantage to quick-opening secondaries.
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Old 05-30-2010, 04:25 PM
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The only way to determine what colour sec spring is best is to go the dragstrip and test. Last time I did that with a vac sec carb the standard default plain steel medium tension spring and the purple spring were the best.

Seat of the pants "butt dyno testing" will not tell you much but a smooth powerfull transition into the secondaries is a good sign.
If you can feel the secondaries crash in, the spring is either too light, too heavy or the diaphram is leaking or the little O ring vac passage gasket between the housing and carb body is leaking or the check ball is sticking or missing.
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