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Discussion Starter · #1 ·
hey guys my buddy and i built him a 40 over 350 10.1 compression, comp cams bigg matha thumper (295) 486"/500" adv duration 295/313, okay high rise dual plan holley intake, and a holley 770cfm truck avenger, now the problem it has aratic ideling we pulled the vac it has 5in we adjusted the mixture while the vac gauge was hooked up, then went to idel it down it lunged down way too low ok if we idel it back up alil its way too high, the timing is off i think he has the wroung damper bc looking at the distributor it looks tobe retarded too much, put the light on it and its no where on the timimg tab its more looking straight down behind the water pump soo we pulled it down till it said 16 on tab still not running no better( adjusting the mixture on the carb helped) but didnt help the ideling problem......any help will be greatful guys thanx..
 

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trouble shooting 350 chevy

You need to purchace a bolt on timing tab any parts store should have this yhen remove #1 plug REMOVE COIL WIRE carefuly place finger over plug hole have someone bump the starter little at a time until you hear air pressure probably will blow finger away from hole now BY HAND place a socket long pull handle on balancer bolt place a long phillils screwdriver in plug hole SLOWLY ROTATE ENGINE UNTIL PISTON REACHES TOP it will stop then start back down STOP MARK BALANCER SOON AS IT STARTS DOWN THEN TURN ENGINE IN OTHER DIRECTON until it reaches top and then starts down STOP mark this spot on balancer measure half way between these 2 marks this is TDC top dead center noe you can set your time correctly with this cam i would start @ 6 degrees you will probably have to change your power valve if you are running a holley 1.5 hgs vacumn less than your readings be sure you dont have vacumn leak or readings will ga wrond did you degree your cam set it straghit up /advanced or retardet if all else fails you may have to drill 1/16 or 3/32 hole in butterfly where you can adjust idle if you use idle screw to try to make iy idle you uncever idle slots makeing idle mixture screws useless hops this helpe GOOD choice on cam sounds GREAT
 

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carb might need a couple mods to idle with that cam.It will also idle about 1000.get the timing issue fixed then come back to carb(float level,does it have an electric pump? regulator?,,, power valve number? modify primary throttle blade) takes a little tweeking,,,it will come
 

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hey guys my buddy and i built him a 40 over 350 10.1 compression, comp cams bigg matha thumper (295) 486"/500" adv duration 295/313, okay high rise dual plan holley intake, and a holley 770cfm truck avenger, now the problem it has aratic ideling we pulled the vac it has 5in we adjusted the mixture while the vac gauge was hooked up, then went to idel it down it lunged down way too low ok if we idel it back up alil its way too high, the timing is off i think he has the wroung damper bc looking at the distributor it looks tobe retarded too much, put the light on it and its no where on the timimg tab its more looking straight down behind the water pump soo we pulled it down till it said 16 on tab still not running no better( adjusting the mixture on the carb helped) but didnt help the ideling problem......any help will be greatful guys thanx..
The deal to making the thumpr cam work is timing related. They require a LOT of initial timing. I do not recommend locking the timing at full advance for a street-driven vehicle very often, but this is one case where that is going to be the easiest thing to do.

That said, any time you can allow for a mechanical advance- even if it's only 10 degrees- you should use it. Locked timing is like a stopped clock. It WILL be correct... a couple times a day.:mwink:

There's some info here that you can take a look at to get some idea of what's required. In your case, I would start w/using 20-26 degrees initial w/the rest coming from the mechanical advance. The total timing depends on the chamber design and head material, etc. With this set up you'll want to use a vacuum advance connected to ported vacuum as described in the linked-to page above.
 

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Discussion Starter · #8 ·
the heads are just a sets he had laying around 883's open chambers 2.02/1.60 valves, he used 150 domes to get the cr he wanted, the cam was degreed to the cam card dont ask idk, keep in mine this motor is in a 82' chevy 4x4, with a 4speed granny, 3" exhaust with super44 flowmasters...the carb is stock right out of the box....
 

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I just time by ear.Then if it fires on one click warm and don't kick back and don't ping when I stand on it shes in range.Then play with weights and springs and see what happens.
Course I'm cobbler........lol
 

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the heads are just a sets he had laying around 883's open chambers 2.02/1.60 valves, he used 150 domes to get the cr he wanted, the cam was degreed to the cam card dont ask idk, keep in mine this motor is in a 82' chevy 4x4, with a 4speed granny, 3" exhaust with super44 flowmasters...the carb is stock right out of the box....
You've got a piss poor combination and you shouldn't expect that cam to run at all with those heads and no ignition timing specs. If you're bent on keeping those snarly crap heads then you really need to replace that cam with something in the 215* duration range and call it a day.

If you want to run that cam prepare to replace the heads with something that will flow some air, use a piston stop to determine TRUE TDC and get a timing tape and bolt-on tab, limit your mechanical advance to 10* and set your initial at about 26*, then add 10* of vacuum advance with full manifold vacuum. Once this is done you'll be able to tune the carb to handle that cam.

It never ceases to amaze me how people use junk heads because they have them and then expect the mill to run with a bottom-of-the-page cam. :spank:
 

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Discussion Starter · #14 ·
well :smash: i didnt expect that, us country boys dont have all the money for those flashy heads that flows for race cars and such, this is a 4x4 truck a (toy) not a musle car or race car or a drag car, i set the timing by ear it dont ping it runs like a race car down the road, put the foot down and she is all over the place, when i got the timing in the rite area the carb settle down some what, the rest of that idle issue i beleave is due to it loading up from haveing the wroung size power valve in it, when he gets one ill let yal know how it goes...
 

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They say to get a power valve 1" below idle vacuum for best results,I'm yet to have a need to swap one but I may try it myself when I start super tuning my ride.
If the engine is loading up clean/change the plugs,take her for a WOT blast,shut it down right away and read the plugs,try jetting down a couple sizes if its rich and repeat till the plugs read correct.
I'm not the most elaborate tuner in the world but I just play with everything and anything till I feel the engine has no more to give.Works well for me,prolly not the method one would wanna do on a $10K engine,but on my mild street mills it allways gets it done.
BTW I have similar crappy heads 993 castings on my engine 1.94s though,they will work fine for a street engine though up to 350HP/5500rpm.Your cam is a bit big for the heads flow,it will take away throttle response in exchange for top end the heads cannot support,but to say it will not run at all is wrong just won't run upto the cams potential.Idealy a cam in the [email protected] up to [email protected] range(in a light car or with stout gears)woulda been best.
I do get a little pissed when people knock stock heads,like anyone who doesn't have a pocket full of bills for aftermarket can't make power.
 

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Where is the location of the timing tab on your timing chain cover? Are you using a stock 8" balancer? Another thing the 305 cid chevy timing chain cover has the pointer at the 12:00 position. Can you get the casting number off the passenger side back of the block? There should be a number on the pad behind the alternator too. Small block casting date is located on driver side rear of block on flange forward of bellhousing. Once you know the date you know the position of the timing pointer or balancer. You can then find TDC. Don't forget to read this link from post #7. Great for timing engines. http://www.crankshaftcoalition.com/wiki/Hot_rodding_the_HEI_distributor#Ignition_advance. Here is another article on the different balancers and timing chain cover pointer location. http://www.crankshaftcoalition.com/wiki/Timing_tabs_and_damper_TDC_lines_SBC
 
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