|01-15-2013 12:17 AM|
|Greg T||What is the final reaer ratio and tire diameter? It makes a huge difference.|
|01-14-2013 01:42 PM|
There is the option of buying an MSD distributor then forking out for custom bushings ($33), but that's a lot of money.
|01-14-2013 09:08 AM|
So I played with the truck a little this weekend and to my suprise it does start with 23* initial(no comments needed here). I unplugged the vacuum advance and its all in at 36*. Now come the questions.
Are my only options for limiting my mechanical to braze or stick something in the hole on/next to the center plate? Is there anything I can purchase for my HEI that would limit/ allow my to have only 10-12* mechanical?
Is it normal for my timing to bounce around slightly when check it with the light?
I didnt get to drive it without the vacuum advance plugged in to see if it shutters/bucks/bogs but will sometime this week. Just for reference when I built the motor I used a piston stop to find 0 on the dampner and have timing tape on it so its accurate. Thanks to everyone that posted on here. I will be back when I have more info.
|01-13-2013 07:54 AM|
Its just that easy. 26deg inital timing 10deg mech curve 36deg total.
10deg of vac advance for cruising at part throttle.
You will never get it right until you give the motor the generous base timing it needs to idle correctly.
|01-13-2013 07:27 AM|
This is all good info, but none of it is relevant until the timing set up properly for this cam. Thumprs are very difficult to set up for the street and if you don't have the proper initial timing and short curve it will never run right. If the timing too far retarded at idle, the throttle plates will need to be opened more to allow it to idle. This causes a tuning problem if the carb is not utilizing the idle circuit because of the throttle blades being opened too far. The timing MUST be set correctly before the carb is tuned or this set up will never run.
|01-13-2013 06:54 AM|
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.
|01-13-2013 02:09 AM|
|68NovaSS||Try a little self help, use the search function for one, this site has an awesome database.|
|01-13-2013 12:15 AM|
|cadillacdave1||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|
|01-12-2013 10:34 PM|
|Greg T||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.|
|01-12-2013 08:19 PM|
|tcarlx||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.|
|01-12-2013 07:45 PM|
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.
|01-12-2013 11:42 AM|
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.
|01-12-2013 08:29 AM|
|tcarlx||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.|
|01-11-2013 10:57 PM|
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.
|01-11-2013 06:21 PM|
|cadillacdave1||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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