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Old 11-17-2011, 12:45 PM
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355 SBC Tuning help

Hello, first post here. I am having off-idle load up problems and nasty smelling exhaust and I am hoping to get some help. Here are my truck specs:

Vehicle: 1985 Chevy C10
Engine: 355 SBC
Cam: Big Muther Thumper #12-602-8
Pistons: Flat Top
Heads: 305 port and polished
Crank: Scat
Rods: Scat
Intake: Edel E-Performer 750 w/ Edel dual plane manifold
Exhaust: Hooker headers w/ 3inch pipes to Flowmaster 40's
Dizzy: Stock vac. advance non-functioning. Mech advance rusted solid.
Timing: Uncertain at this point as timing mark on balancer is off. Points to 10 O'clock on passenger side.

First off, I did not build this engine. It was already setup nearly like this when I bought it a few months ago. It initially came with a half-assed E-Performer 600. I bought the 750 due to a deal Jegs had on them.

Since having the 750, I replaced the step-up springs with the weaker blue ones. That has helped off idle load up after the truck has warmed up. If the motor is still cold, it'll load up then blast off. I have not changed anything else with the carb. Also, the accel pump is set to top hole.

I have idle set to 1000 RPM and it holds that just fine. I have tuned the IMS to get near 10" of vacuum. Outside of idle, the truck runs fine. I can put my foot in it and spin the tires while cruising in most cases. It is this off idle issue that I need resolved.

Another thing the truck is doing is spitting a lot of either moisture or gas out the exhuast pipes and it smells pretty knarly. Almost as bad as nitro r/c fuel.

First start of the day is a chore at times. I have to pump the gas at least 5 times to hold idle. Anything less than that, I am fighting to keep the motor running. Manual choke does work. Otherwise, it'll start right up after warm-up.

Thank you in advance!

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Old 11-17-2011, 01:36 PM
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I hope your not running a distributor with no advance. If so your headed for a heart break. Right now you don`t have enough timing at idle and the butterflies are off the idle slots on the carb, if it idles at a 1000 RPM then shine a light in the carbs throat at idle and you`ll see fuel coming out of the boosters which means lack of enough base timing and idle set too high. This is why the idle and off idle doesn`t function correctly. Thumper cams are fad cams and they need alot of timing to run decent. You`ll need a adjustable vacuum advance. Set the base timing to 12 degrees before top dead center with vacuum advance disconnected and plugged. Reconnect it to a full time manifold vacuum source, one that pulls vacuum at idle. Recheck timing, it should be 18 to 24 degrees advanced, now lower the idle speed down to spec of around 700, use a vacuum gauge to set the idle mixture. Test drive, if ping is detected this is where the adjustable advance comes in. I figure you`ll need this with the small chamber 305 heads. Mechanical advance should be 36 degrees all the way in by 3000 RPM.
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Old 11-17-2011, 03:03 PM
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What's the fuel pressure?
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Old 11-18-2011, 07:38 PM
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Going of on a hunch.

With the cam change you need more initial timing.
I estimate you need 18-22 initial. The thumper cams have a low center line with a lot of overlap, this gives the thumper sound. ( I call them whipper cams)

You need a curve kit in dist.

You reduce the amount of mechanical advance to increase your initial based on setting the total timing.

The vacuum advance advances the timing based on vacuum, vacuum is controlled b engine load. The vacuum advance needs to be disconnected during these setting, once the setting are archived, hook the advance back up.

Uploaded with ImageShack.us

Red line is total timing, the blue is initial.

On the left is 25 degrees mechanical advance, the right is 16

by reducing the mechanical advance, you raise the initial timing.

Hope this helps
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Old 11-18-2011, 09:31 PM
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Best be limiting the vacuum advance + mechanical, keep the total with the 3 amigo's all in to below approx 48 btdc no matter how you mix them up.

Most stock vacuum adv cans will add 16-20 deg advance, remember this shifts up with the initial everytime, it does not care where you limit the mechanical advance. If you have a initial @ 20 then your are going to get up to 56 btdc total when you add in stock 16-20 deg vacuum adv. (20initial+16mech+20vac)

You need to limit mech and vacuum advance

It would not surprise me if this cam liked 36 degrees at idle with 24 initial, so vacuum advance at 12 deg max + 12 deg max mechanical.

PS IMO If the compression ratio of the motor is not at least 10:1+ it wont matter how much you monkey with the timing, its going to load itself silly until its at WOT, thumpa thumpa thumpa
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Old 11-27-2011, 10:10 AM
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my stock fuel pump was putting out 10psi, edelbrocks like around 5-6psi. I installed a regulator and limited the fuel pressure to 5.5psi, really helped the idle.

Thumpr cams suck (i have the small one).

I limited my vac can to 10* with a small drill bit and installed the 375/41 weights in the distributor
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