Any carb tuning guys on here with a mild 500 inch build/ 850 DP??

[69wannabe]

I have a 493 ci big block in my 68/69 charger. The specs are basically 9.8 to 10.0 to 1 with diamond dish pistons, the cam is 241 at 50 degrees with 507 lift on the intake and 246 at 50 degrees and 510 lift on exhaust. The heads are 346's with 214/181 valves and mild porting, RPM intake and a holley 850 Double pump carb. It has hedman 1 3/4 headers and 2 1/2 exhaust out the back. it has a MP distributor set at 12 BTDC and 35 total timing. I had a proform 850 on it but didnt like the way it acted and after alot of changing jets around and swapping power valves it didnt seem to improve so bought me a holley. The holley runs alot better what little bit I have driven it since I installed it and it just seems a little rich/thick on there. It is more of a solid feeling carb especially while cruising at highway speeds tho. Just wondering if anybody else has something similar to my mild strocker build and if they have made any changes to their carb if it was needed. My plugs looked ok when I pulled them but maybe a lil on the rich side. I'm prolly going to go down a couple of jet sizes and go down on the power valve too and may go down on the primary pump squirters too and see if it is a little crisper. I have never gotten into changing pump cams and I have several of them laying around but dont know which would be a good one to try. I do most of my own work on my charger and I like to pick at it and pick at it til I get it just like I want it to run. It idles great but when it revs it seems to have a little lag like it has a lil rich condition there. Thanks for any info!!!!

[Chryco Psycho]

Start with float levels & the power valve , check the vacuum idling in gear if auto & make sure the PV is rated 1.5 " lower then the vacuum reading you get , , if you change jetting by 4 sizes or so you should feel if you are going the right way , it will lose responsivness immediatly if it doesn't like it . Squirter changes how fast fuel is delivered so I would work with that first if the lag gets worse with a smaller squirter go the other way , cam controls total volume of fuel delivered everytime the throttle is opened to WOT & can change the volume of fuel delivered with partial opening of the throttle , white & Black are the smallest cams , there is a chart in the Ulrich written tuning book showing totaly volume & volume / degree of throttle movement , I do not have my book handy .

[69wannabe]

I have set the floats at the bottom of the plug sight hole. Fuel trickles out if you rock the car a little bit so the floats seem pretty close. Set the idle mixture and thats all i have done so far. I have just put the carb on bout two weeks ago so I have decided to run it awhile and keep a check on the spark plugs and then if any changes need to be made I will start tinkering with it. Drove it out today a couple of times and it cruise's good even at low rpm's and it has plenty of pep when you need it to get up and go. Thanks for the info!! I haven't put a vacumm gauge on it yet but I will to see what my reading is. It was actually bout 11 when I had the proform on there so it's prolly bout the same but I do need to check and see. I have a couple of extra 5.5's and a few new 6.5's so I should have it covered with power valves if it needs to be changed.

[firefighter3931]

That engine needs more initial timing. I would set it up with 16-18* at idle and keep the total @ 36* and see how it responds. Manifold vacuum should increase and make tuning easier. 


Ron

[69wannabe]

I think if I could get 14 or 15 degrees BTDC out of it it would be good to go. About how much would you move the adjustable stop in the MP distributor to keep it from advanceing too far? maybe a 16th of and inch or so or I guess I could mark the position of where it is at now in case I need to go back closer to that spec.

[heyoldguy]

That engine needs more initial timing. I would set it up with 16-18* at idle and keep the total @ 36* and see how it responds. Manifold vacuum should increase and make tuning easier.  


Ron

Ron just gave you excellent advice, I was thinking exactly the same before I got to his post. Argue if you will, but he is pretty sharp.

[69wannabe]

I havent ever ran so much initial timing before thats why I have second thoughts about pulling it up that high. Does that much initial timing not put a strain on the starter when the engine is hot?? I pulled the distributor today and took it apart and set the stop in a little bit more to limit my total timing so I should be able to pull it up higher and still not have too much at full advance. I kinda figure this will be trial and error so I took the heavier spring off so I could see the total advance at a earlier rpm than 3000. Maybe this way I can make adjustments and with the heavier spring off total timing should be in around 2000. Dont know if this is a good idea or not but i'm gonna try it and when I get it where I like it I will put the heavy spring back in there. I also put a thicker shim on the shaft since it seemed to have excessive up and down movement. I got a box full of old distributors and one had a slightly thicker shim than the one thats on there so I swapped it out and it still has some play but not like it did have. Havent started it up yet but I will tomorrow and see what my timing looks like.

[redmist]

I have the initial timing on my build set at 21 deg. Total is limited to 35.

[oldschool]

i run 25* inital on all of my cars......

[heyoldguy]

The camshaft duration will dictate, to a large amount, the required initial timing. Your duration puts you, for starters, right in the area that Ron has suggested. I have an engine at 13.5:1 that has the distributor locked at 38 degrees. We fire it up there and run it there.

[69wannabe]

I agree that I need more than what i was running for sure!!!!! I fired it up today and let it warm up and put it on 14 and it advanced to 33 @ 2500 rpm's. Bumped it up to 16 and it advanced o 35 @ 2500 rpm's. Moved it to 15 and it advanced to 34 @ 2500 rpm's so I am pretty sure I got 19 degrees of mechanical advance now which is bout what I was aiming for. I pulled it and put the haevy spring back in there but not sure what I did wrong there but when I got it back running and moving the timing by the vacumm canister I could feel it thumping like something was hitting something in the distributor.   I pulled it back out and back apart and didnt see anything hitting anywhere and put it back one piece at a time and spun it and whatever it was isnt hitting in there now. Got it back in and set it on 16 and adjusted the carb to the timing, seems crisper reving now and it still starts fine so i'm gonna try it there and see how it runs out!! As long as it runs out good and no pinging I will be a happy camper!!!! This is within the range that Ron had suggested and it seems to run good there!! I always ran my engines around 13 to 15 as long as they didnt ping. Wasn' able to give mine a road test today thanks to he rain but maybe by the weekend I can see how it drives out and hopefully no pinging which at 35 total it should be safe!! Thanks for all the input and thanks Ron for your specs!! It really didn't take but just a small nudge from where the stop was to drop the total timing several degrees!!! Not something you would move very much at all to get a big change in your timing!!

[firefighter3931]

The camshaft duration will dictate, to a large amount, the required initial timing. Your duration puts you, for starters, right in the area that Ron has suggested.

Bingo ! Cam and compression will dictate where the timing needs to be. The 493 has a modest cam (relative to displacement) so it doesn't need a lot of spark lead but certainly more than 12* of base timing. My recommendation was based on displacement/static compression ratio and cam timing events. 

I have an engine at 13.5:1 that has the distributor locked at 38 degrees. We fire it up there and run it there.

My old 446 at 10.5:1 with RPM heads & 264@.050 solid cam was set up just like Jim's 13.5 build outlined above. Distributor locked out and set at 35* at idle. Started up fine, even in the heat with no starter kickback. 

Every engine combo is different in what it wants but those with past experience using similar builds/parts are your best source of information for a starting point. 

I agree that I need more than what i was running for sure!!!!! I fired it up today and let it warm up and put it on 14 and it advanced to 33 @ 2500 rpm's. Bumped it up to 16 and it advanced o 35 @ 2500 rpm's. Moved it to 15 and it advanced to 34 @ 2500 rpm's so I am pretty sure I got 19 degrees of mechanical advance now which is bout what I was aiming for. I pulled it and put the haevy spring back in there but not sure what I did  set it on 16 and adjusted the carb to the timing, seems crisper reving now and it still starts fine so i'm gonna try it there and see how it runs out!!

Sounds like you're moving in the right direction. The throttle response should be quite a bit improved with the increased base timing and the carb will respond better to jetting changes. When you get a chance slap a vacuum guage on it and let us know how much it increased.  Your total timing is the same so there's no reason to think that it will detonate now if it didn't before. 



Ron

[69wannabe]

Will do!!! I am betting the 6.5 power valve will be ok in there now since the vacumm has increased. It was bout 11 the way it was so it is going to be 12 or 13 by now i figure which will put me with that 6.5 that is already in the carb. My old car is pretty much a driver so thats why I kept the cam rather small for drivability and to keep vacumm for the power brakes without having to run a vacumm tank. It sounds good with the comp cam and still pulls up to 6000 with ease and thats plenty of pep for me to be driving around with. My engine is very close to the engine jegs sells thats rated 525hp with 590 ft lbs of torque. All I was shooting for was a 500hp driver which is'nt hard to accomplish with a stroked big block!! Thanks for all the ignition info!! Cant wait to see how it responds on the road!!!