Ok 355 10.2-1 AFR 195cc heads 64cc comb chamber comp xe284 cam single plane holley intake and 750 demon carb. Having a horrible time getting it timed. not sure to use constant or ported vacuum. won't idle under 1000 rpm and timing seems way way too far advanced to make it idle decent like 46*-48* total at 4000. and still cranks over like timing is too far retarded. It is pure crap up to about 3500 then all of a sudden takes off like a rocket and blows the tires off. I keep advancing the timing but I'm scared of pre ignition hammering the rings.

 

maybe the vacuum advance isn't adjusted right and the advance springs are too stiff? If you know it's 46-48 when it's all in then pull it back from there and work out the other issues afterwards. At least if your max advance is set to an acceptable level you aren't slamming stuff around.

 

I did make some runs with timing about 38* and it still ran like junk down low and seemed to crank freely for a long time before it would fire. had to give it gas to fire everytime. with it advanced it would fire almost instantly with no throttle. I'm starting to worry that my cam timing is somehow wrong

 

try to get a rough estimate of timing when cranking or around idle-ish if you can.

 

Ok set initial while cranking at 10* car seems to idle smooth at 1100 but still has 48* advance at 4000. It's just a stock HEI

 

Is that 48 degrees advance with vacuum advance hooked up? If so, thats fine and normal.

I'm assuming your cam is the flat tappet cam that is 240/246 @.050 duration? If so, that cam is going to want a lot of inital advance. Might be around 20 degrees initial. Add in vacuum advance (to full manifold vacuum) you might have close to 40 degrees advance at idle. The cam is going to bleed off a lot of cylinder pressure at low rpm so this amount of timing is ok. Try to get your total timing around 35 degrees for now with the vacuum disconnected. This should happen by 3000 rpm or less. Then mess with the distributor so you have at least 15 degrees advance initial with the vacuum advance disconnected. Then hook up the vacuum advance and see where its at.

 

k I'll try that right now. main problem is the car really won't idle low enough to get rid of the mechanical advance

 

Assuming the idle on the carb is adjusted right, you will probably find that with a little more initial timing, the car will idle lower.

Edit: just went back and relooked at the specs on the cam. Thats the same cam I use to have before I went solid roller. I know my motor would idle fine at 700 to 800. It did want a lot of initial timing though. When I had it at 8 degrees initial with no vacuum advance, my headers would actually glow within 5 minutes of turning the car on becuase the combustion process was continuing into the exhaust. That was due to not enough timing.

 

Ok well with vacuum disconnected initial is 18* and 35* at or before 3000

 

Would you possibly remember what jets you ran. I'm sure mine's a rich pig right now maybe thats the cause of low power at low rpm and maybe the crappy idle too

 

sounds perfect. I would hook vacuum advance up to full vacuum. Not timed or ported vacuum.

I had the stock jets for a 650 mighty demon. I really knew nothing about tuning then so thats one reason I had a bunch of problems then. I think its going to run a little rich regardless at low rpm with a cam like that.

Check your idle mixture screws. Demon says to have them out about 1.5 turns, but on my new cam (236@.050) I found 3/4's of a turn is best.

 

turn your mixture screw(s) in either direction a little - if idle speed goes up.. continue until it evens out. If you have multiple mixture screw you will want them to be at known starting points and turn them evenly. (I didn't check what you have for a carb)

 

we're running 82 primary and 84 secondary with a stiff spring to keep the secondaries closed as long as possible. the last motor for some odd reason we could not get enough fuel on the dyno (was runnin like 15-1). Not sure if that was the right way to go.

 

ya all 4 are between 3/4 to 1 turn out screwed them all in as far as possible without having the idle drop off

 

It may be too rich. I know I just had an issue with an off idle stumble while driving and richening up from 69 primaries to 72 took it away. Your running 10 sizes bigger than that. Is this a vacuum secondaries carb?

 

First off, you are showing the same signs I was seeing when my cam was installed a tooth retarded as in seemingly low cylinder pressure that excessive timing would kind of fix and that it makes huge power up real high in the rpms.

Like others have said, it will want a bunch of timing at idle so you need to play with the dist. so that you can set idle timing to something in the 20 degree area and still only have maximum timing of 36-38 degrees. Might be usefull to try locking out any movement for advancing in the dist. just to eliminate it as a problem and just run timing in the low 30's no matter what. I know mine, cam is 260@ .050, can idle around 700 but it likes lots of fuel and timing to do it confidently asside from the 8.6:1 compression that doesn't help me.

 

You may not have been getting the right signalling to the carb to deliver enough fuel before ie: leak or improperly setup. Perhaps go down a couple sizes on the jets if it seems right off idle.. I think stock is 76-80 or somewhere around there. I think you will continue having problems with a stock dizzy.. you may want to just shutoff the vacuum advance and work from there for right now.. stock HEI setup sucks on advance. needs springs and the stock vacuum advance sucks

 

I was thinking the only reason it was taking that much timing was because the cam timing was off. but I know when we put it together we went over it again and again to make sure we had it right and inline. even comparing the new timing set to the old timing set to make sure we had the right keyways.

 

I'm not saying its not possible its a tooth off, but a cam with that much duration, in a 355 with avearage compression ratio is going to bleed off a lot of cylinder pressure at idle. Even with my new cam, that is less duration, I run 20 degrees initial plus 15 vacuum. At idle I show 35 degrees advance and at 3000 rpm its a total of 50. Thats on 93 octane in the heat of central florida. Car runs great with no detonation.

 

So then I should be able to run a compression test and see if the compression is low? What should a 10.2-1 motor run for compression numbers?

 

haha! that depends on the cam and it's timing

EDIT: Sorry it's getting late... lol - well if it's less than 100PSI youve got issues, check a cylinder.. I'm not familiar with your cam. A stock vehicle at 9:1 could put out 150-200 PSI, change the cam and ? - could go anywhere depending on valve overlap and such.

 

I've used this calculator before and it was dead on: http://www.wallaceracing.com/dynamic-cr.php
Using rough guesses on a few numbers, you are probably looking at 160-170psi.

 

ok I came up with 186 with my specs

 

I'll check that first tomorrow and I hope thats not it then just work on jetting at the dyno

 

check for a vacumm leak i had the same problem and it was just a bad hose

 

Turns out I'm a moron it ran just fine when I set total timing at 38* without the vacuum advance. dynod fine at 348rwhp and 331 ft lbs

 

Glad you got it all worked out

 

With a XE284H-10 cam it will idle much better and have a lot better throttle response if you lock out the advance and set the locked timing at 34-36deg. Auto trans 3500 stall converter

Then modify the vacuum advance for a limit of 10-12deg max occuring at hiway cruise speeds., not all at idle.

If it is hard to hot start with locked out timing, install a spark interupt switch to aid starting.

manual transmission:

Other than locked out advance the motor will want a minimum of 26deg base at idle.
Shorten the mechanical advance travel stop limit to 10deg to allow this.

GM HEI distributor mechanical advance mod to limit the travel:

Remove the rotor and weights. see the plate that the rotor bolts to and the rotor mounting screw holes.
See the extra holes closer to the center. A machine screw in one of these holes will limit the mechanical travel.
The head of the machine screw must be stubby enough to not interfer with the bottom of the advance weight.