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Old 01-11-2013, 01:54 PM
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SBC with vortec heads and thumpr cam timing/carb questions

First of all I am new and wanted to say thanks to everyone who makes this site awesome. I have used Hot Rodders for much of my sbc build but now i am down to the nitty gritty and have detailed technical questions about my build i hope someone can answer.

The build:
stock 4 bolt sbc block and pistons with standard bore and stroke
stock 096 vortec heads with upgraded springs and retainers(NEW)
COMP Thumpr Cam (advertised duration 279/297, lift .479/.465)(NEW)
Rhoads lifters (originals not V-max)(NEW)
Petronix HEI distributor(NEW)
Edelbrock 600cfm(NEW)
Guess on compression and hp is 9.3:1 and about 360hp. (reference only please)

Disclaimer- Please do not fill this post with "you shoulda done this or you should have bought this." I spent time and money on this build and would like to use what I have. Relative questions are more than welcome. Thanks for understanding.

Question time.
With the cam and lifters I have in this setup what would be recommended for initial,mechanical, vacuum, and therefore total timing? I am looking for a solid starting point for further tuning. I know the basic rules for sbc's but am hoping for a more specific starting point.

What are people with similar builds setting up there carburetors like? I'm asking about needles, seats, springs, how many turns out on the air mixture screws? The more detailed info on this the better. I have tried multiple configurations with the edelbrock 600 and non of them are quite right.

I will wait for some responses before I give more details. I dont want to unintentionally get people off track with possible unrelated problems build wise.

Ok thanks to everyone in advance for the help and expertise.

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Old 01-11-2013, 03:16 PM
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I'll offer this from Edelbrock to help you with the carb tune.....
http://www.edelbrock.com/automotive_...ers_manual.pdf
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Old 01-11-2013, 03:21 PM
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Thanks for that. That is a great guide I am just looking for more detailed specs/info.
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Old 01-11-2013, 03:44 PM
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Trouble Tuning the Edelbrock Performer 1406 Carb? Read On... [Archive] - Four Eyed Forums

Found this interesting for tuning edelbrock 1406 carbs. Will try this over the weekend and report back.
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Old 01-11-2013, 04:50 PM
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What is the carb or car doing? Does the engine stall, run rich, bog. Let us know what it is doing so we can give you some reasonable feedback. The (new) carb should run right out of the box. Of course it should need a little fine tuning. The carb may be a bit small for your combination. I would think a 650 would be a better fit. Edelbrock carb run well and are easy to work on. However. most folks that know how to tune a carb are more familiar with holley style carbs. Usually people using an Edelbrock carb put it on adjsut the idle mixture and never touch it again. They are good reliable street carbs.
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Old 01-11-2013, 05:21 PM
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Also careful with the cam break in as that is a flat tappet cam. Follow the break in procedures or you can wipe the lobes on the cam.
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Old 01-11-2013, 09:57 PM
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Well, I know you said you didn't want to hear it, but I'm never one to keep my mouth shut so here it goes.

For the life of me I can't figure out why anyone would buy a Thumpr cam with it's wicked sounding idle and so-so performance, and then soften it up with a set of crappy Rhodes lifters. The ONLY reason to buy a Thumpr is for the sound because there sure are a lot more cams out there with higher performance for less money. So buying a Thumping idle and then taming it with leaky crap lifters is, at best, asinine. Rhodes lifters will cost you big on top and take away a lot of the choppy idle you just paid big money for.

If you're hell bent on making it work, then limit your mechanical advance to about 10*, use 25 or 26* at idle and have it all in by 2500 rpm. MAYBE, then, you'll get the carb to run.

There, I just said what everyone else was thinking.
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Old 01-12-2013, 07:29 AM
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keep the info coming people. The Rhoads lifter choice was for my father. He had them in his old camaro and he asked me to put them in my setup. I agree that it hurts my build and have thought about replacing them. I will try your timing suggestion Greg T. As far as issues. The truck runs and drives, its just has a shutter/shake in cruise mode. I have eliminated the possibility of trans due to enriching the fuel intake but then it over fuels at idle. I could tell you tons of details on where I have been with this and what makes me think its timing and carb but I'm sure noone really wants that. So my main issues is either over fueling to the point of gas in oil and/or I have a shutter/shake in cruise mode. Still would like to hear more timing suggestions.
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Old 01-12-2013, 10:42 AM
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Quote:
Originally Posted by tcarlx View Post
keep the info coming people. The Rhoads lifter choice was for my father. He had them in his old camaro and he asked me to put them in my setup. I agree that it hurts my build and have thought about replacing them. I will try your timing suggestion Greg T. As far as issues. The truck runs and drives, its just has a shutter/shake in cruise mode. I have eliminated the possibility of trans due to enriching the fuel intake but then it over fuels at idle. I could tell you tons of details on where I have been with this and what makes me think its timing and carb but I'm sure noone really wants that. So my main issues is either over fueling to the point of gas in oil and/or I have a shutter/shake in cruise mode. Still would like to hear more timing suggestions.
Rhoads will probably destroy that cam. I think they are made for long duration long ramp cams with the thumpr is a newer style cam.

Thumpr will usally make close to the max hp for cams in its size. Not a whimpy cam but very peaky. I like the idea but not brave enough to try it myself but if it works please let us know cause the thumpr cams with good low end would not be a bad combo.

You may want to call comp cams and make sure they are compatible. If they wont work at all best to find out now and before you wipe the cam in the motor on the side of the highway. Also if it does work it should help with carb setup and give more vacum so carb will act more normal.
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Old 01-12-2013, 06:45 PM
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Quote:
Originally Posted by tcarlx View Post
keep the info coming people. The Rhoads lifter choice was for my father. He had them in his old camaro and he asked me to put them in my setup. I agree that it hurts my build and have thought about replacing them. I will try your timing suggestion Greg T. As far as issues. The truck runs and drives, its just has a shutter/shake in cruise mode. I have eliminated the possibility of trans due to enriching the fuel intake but then it over fuels at idle. I could tell you tons of details on where I have been with this and what makes me think its timing and carb but I'm sure noone really wants that. So my main issues is either over fueling to the point of gas in oil and/or I have a shutter/shake in cruise mode. Still would like to hear more timing suggestions.
Start w/22 degrees BTDC and work up or down from there, limit the total to 32 degrees all in by 3000 rpm, sooner if it'll take it w/o detonating. More here.

Use a vacuum advance limited to 10-12 degrees, use ported vacuum. Add vacuum advance if the engine likes it.

Be sure the timing marks for TDC are accurate.
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Old 01-12-2013, 07:19 PM
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The truck bucks the starter warm at 16 initial. Is the 22 including vacuum. I am guessing not since u said to run ported. Worth a shot. Thanks for the info.
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Old 01-12-2013, 09:34 PM
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No, initial is with no vac. If it's bucking against the starter at 16* you have a problem. Either you're not at 16* or you have an electrical problem. Verify TDC on the damper by using a piston stop to be sure the damper is good and the timing tab is correct.
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Old 01-12-2013, 11:15 PM
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you may have a problem with the ignition if it shutters and shakes at cruise. You said the carb is pig rich at idle. It sounds like the carb is pulling through the idle circuit. If you turn the idle mixture screws all the way in the motor should shake or turn off. If not the motor is getting gas from somewhere else - improiper adjustment on carb. If it was a holley style I would say the secondary throttle plate is adjusted wrong, too far open revealing too much of the transfer slot. With edelbrock carbs you have toi get the right combo of metering rods and springs. If this does not work then you must change jets and start the fine ruining with the metering rods and springs again. Good luck
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Old 01-13-2013, 01:09 AM
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Old 01-13-2013, 05:54 AM
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On the Edelbrock 1406
- The idle circuit is mostly independent of the rest of the carb, so if its pig rich at idle you have an idle mixture problem to solve. It may not affect the rest of the carb very much.
- When you get stutter with an Edelbrock under light to moderate acceleration its usually due to step-up springs that are too low (e.g. 5"). The low rate springs delay the transition from cruise to power until 5" of vaccum and the accelerator pump does not provide a long enough shot to fill the transition gap. The carb goes lean and stutters until the vacuum drops enough to allow transition to power mode. Switching to a 6", 7" or 8" spring will let it transition to power sooner as the vacuum drops.
- If you are getting a surge at steady cruise it usually means the cruise step on the rods is too lean. You should be able to use a rod that is 1-2 steps richer in cruise and the same in power.

The Edelbrock carb tuning book provides a lot of good information, but does not emphasize enough that it will work much better if you do certain steps in order.
- Fix the idle mixture first - if you can't fix the idle there may be a flaw with the carburetor.
- Use step-up springs to fix part throttle problems before you start switching rods.
- Don't try to use only the accerator pump shot to fix part throttle transitions. It does not provide a long enough shot to overcome big gaps. Use step-up springs first, then accerator pump.
- Once you get the cruise mixture running right (by swapping rods), then play with other rods to get your power mode running better. You will reach a point where going richer in power has no effect.
- If possible, use an AFR meter to check the mixture while running. Sometimes its hard to "feel" lean vs. rich and you may be trying to fix the wrong problem.

Sets of rods are relatively expensive if you play with many different combinations and you buy one or two rod sets at a time. You probably have to buy them online, and $7/set + shipping can be painful if you buy one set at a time. I bought the Edelbrock tuning kit for the 1406, but it did not have very many combinations on the rich side. You may want to look at the 1405 tuning kit, considering your cam.

Bruce
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1406 edelbrock 600, rhoads lifters, sbc thumpr cam, sbc vortec heads

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