carb tuning help

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younggun2.0

younggun2.0

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I am new to carb tuning and need some guidence. i have a proform 750 mech. secondaries carb. it is running super rich. it burns my eyes in a matter of minutes when running. I know that changing the jets to smaller ones will lean it out but what do i need to change or adjust along with them. the carb is stock out of the box.

engine specs.

383 with mildly ported 84cc eddy heads.
mech. flat tappet cam .525 lift 294 duration
110 degree lobe seperation
9.5 compression
mopar m1 single plane intake
tti 1 3/4" headers

the carb specs from the factory.

primary jets: 74
secondary jets: 84
idle air bleed: 70
high speed bleed: 33
primary nozzle: 31
secondary nozzle: 31
needle and seat: 110
power valve: 4.5

the engine runs smooth and transitions from part to full throttle smoothly. no bogs or surges. runs nice and smooth just really rich. plugs are black.

was thinking i should step down at least 3-4 jets on primary and secondarys. from the little bit that i do know and have read the 4.5 power valve is pretty close.

any input would would be awesome guys. recommendations, explanations of how thing work, etc. i ordered a holley tuning book from amazon today so that will help also.
 
Timing FIRST!

If it doesn't have at least 20* initial you aren't in the ballpark is my guess.

I'll guess the cam has 245ish at .050 for duration?

Get the initial squared away, then total number and then start in on the carb idle settings. If you give the distributor a little bit of a clockwise twist and your idle RPM increases, it wants the timing.
 
Make sure all the base settings for the carb are right, float level, fuel pressure, etc.
 
I'm really surprised your that rich with your combo. A regulation 750dp usually comes with about a 70/71 jet, but i'm sure that carb out of the box wouldn't be jetted much richer then it needs for however it's set up. A few jet sizes lower, 2 at most, may clean it up. But i have to wonder if your fuel pressure or float level isn't too high?
 
Comp 294,254 @ .050 Cracked .Slow mellow cam,rampwise...
 
What do.you have,for ignition? Backfire,check that power valve.Replace with a 6.5.
 
crackedback said:
Timing FIRST!

If it doesn't have at least 20* initial you aren't in the ballpark is my guess.

I'll guess the cam has 245ish at .050 for duration?

Get the initial squared away, then start in on the carb idle settings
Click to expand...

i have the engine timed at 36* total timing all in by 2500 rpm. my msd dizzy has 21 degree of mech. advance built in. i have the medium springs in the dizzy.

based on that it looks like i would have 15* initial. i can change the bushing and bring it down to a total of 18* mech. advance in the dizzy and set the total at 36* and initial at 18*.
 
Not close and it will run dirty with that initial. Anything less than 20 is going to be a huge compromise.

Don't drive it, but, turn the distributor advancing it until the engine has a hard time cranking and back it off just a bit. Start it, readjust the idle speed and see where you end up. You WILL need to make a bushing to limit your MSD distributor, or buy one.

I run stock 340's at ~15*, no way that 383 is going to like the same for initial.

254 at .050 is even more than I thought... 24-26 initial and the rest mechanical. Minimum idle around 1000-1100
 
OldmanRick said:
I'm really surprised your that rich with your combo. A regulation 750dp usually comes with about a 70/71 jet, but i'm sure that carb out of the box wouldn't be jetted much richer then it needs for however it's set up. A few jet sizes lower, 2 at most, may clean it up. But i have to wonder if your fuel pressure or float level isn't too high?
Click to expand...


fuel pressure is at 9 psi. i have a regulator with a guage right before the carb. looks like i need to bump the fuel pressure down to 5-7 psi? the float level has not been set yet. when i first plumbed the fuel system and turned on the pump the rear float was too high and dribbling fuel out. i set it a little lower and it stopped.

to adjust the floats do i drain the bowls and then pull the plugs on the side of the float bowls and turn them up slowly until the fuel just barely dribbles out of the float bowl holes?
 
IMO, timing an engine, especially a street driven one, using the total method is a rotten way to do things.

You need to know where the total number needs to be to run best, BUT, that's the second piece of your puzzle.
 
Sorry, i stepped away for a late night pizza snack.....lol.

Cracked is right about finding your sweet spot on the initial timing. As far as the carb......9lbs is too much. I'd lower it to between 6 and 7, with seven being the max. After that, as far as the floats, seeing how they seem high out of the box, i'd lower them a 1/2 turn or so to get you at a starting point. They can be low at idle and that will let you pull the site plugs without making a mess :D. For a regulation setting, i like to go just under the dribble point. And i mean just under.........And don't rule out raising your idle speed a hair. I watched your vid and it sounds like you have plenty of converter by the way it sounded pulling into the drive. A low idle enhances the cam sound, but not always good for drivability.
 
crackedback said:
Not close and it will run dirty with that initial. Anything less than 20 is going to be a huge compromise.

Don't drive it, but, turn the distributor advancing it until the engine has a hard time cranking and back it off just a bit. Start it, readjust the idle speed and see where you end up. You WILL need to make a bushing to limit your MSD distributor, or buy one.

I run stock 340's at ~15*, no way that 383 is going to like the same for initial.

254 at .050 is even more than I thought... 24-26 initial and the rest mechanical. Minimum idle around 1000-1100
Click to expand...

i will order up a set of timing limiters for the dizzy. the car idles at 950 right now. it was cold in the video. my cheap elec. fans burnt up on the break in so i couldnt go far or let it warm up.
 
As referenc,e18 initial ,34-35 total. Idled,with a nice rump. Drive,800-900. Same cam,4.0 stroke slalom block.9.7 comp.
 
Ok, so i did some reading online and it seems that this dizzy actually has been causing the advance prob for alot of us mopar guys. MSD only offers the advance limiters in 25* 22* 21* and 18* so even if i ran the 18* limiter the most initial i could get would be 18*. i found a guy online that machines 10,12,14* degree limiter busings. if i were to go with the 14* bushing i could get 22* initial and 36* total. that should put me in the ball park. the dizzy came with a 21* bushing in it from the factory and the heavy advance springs. i am at
15* initial and 36* total right now.

do you think the change will help?
 
younggun2.0 said:
Ok, so i did some reading online and it seems that this dizzy actually has been causing the advance prob for alot of us mopar guys. MSD only offers the advance limiters in 25* 22* 21* and 18* so even if i ran the 18* limiter the most initial i could get would be 18*. i found a guy online that machines 10,12,14* degree limiter busings. if i were to go with the 14* bushing i could get 22* initial and 36* total. that should put me in the ball park. the dizzy came with a 21* bushing in it from the factory and the heavy advance springs. i am at
15* initial and 36* total right now.

do you think the change will help?
Click to expand...

I got the same issue. MSD should make different sets of bushings.

Don at FBO has some bushings. Who did you find that makes them? Can you post a link? I would like to have all 3 of the ones you found as part of my tuning kit.

YES it will help, the stink should go away... More initial will have your engine talking to you when you start tuning the carb. It will tell you what it needs.. You just need to listen!

Remember Timing first, then tune on the carb!

See here ... http://4secondsflat.com/MSD Distributor Tuning.htm
 
Here's an original thought, how about finding out where the engine wants the initial timing, THEN spend money to buy the correct bushing to hit your total number...

Stop pulling numbers out of thin air thinking they may be what you need, it's expensive when you do things that way. Let the engine tell you.

Get rid of the garage door springs holding the advance now too. At worst I'd run two middle springs (blue) and most likely one light sliver and one blue.
 
Mad Dart said:
I got the same issue. MSD should make different sets of bushings.

Don at FBO has some bushings. Who did you find that makes them? Can you post a link? I would like to have all 3 of the ones you found as part of my tuning kit.

YES it will help, the stink should go away... More initial will have your engine talking to you when you start tuning the carb. It will tell you what it needs.. You just need to listen!

Remember Timing first, then tune on the carb!

See here ... http://4secondsflat.com/MSD Distributor Tuning.htm
Click to expand...

that is the exact website i found them on. i found them on the msd tech pages. couple mopar guys were having probs getting more initial timing. someone chimed in with the link to 4secondsflat.com . i have a email into the tech dept asking if they only make the 10 and 14* bushings. i hope that the upping the initial will help it. i am learning as i go. i built this engine when i was 19 and that was 11 yrs ago. just got it fired and im trying to make the pile of parts run right. i have virtually zero tuning experience but i learn quick. lol. i will take all the help i can get.
 
It should. That and some good tuning with vacuum gauge and/or O2 gauge. jets and idle circuit tuning.
 
crackedback said:
Here's an original thought, how about finding out where the engine wants the initial timing, THEN spend money to buy the correct bushing to hit your total number...

Stop pulling numbers out of thin air thinking they may be what you need, it's expensive when you do things that way. Let the engine tell you.

Get rid of the garage door springs holding the advance now too. At worst I'd run two middle springs (blue) and most likely one light sliver and one blue.
Click to expand...

i dont know how to find out "what the engine wants". are you talking about checking vacume? find out where the engine pulls the most vacume? i am all ears. i am learning as i go. i built the engine when i was a kid and just bought all the fancy go fast parts and now i have to learn how they all work together. i appreciate all the help.
 
You can make one of those bushings with a piece of alum round stock a drill and some emory cloth or real rough sandpaper.

Drill center for pin on distributor and turn it down with the paper or a grinder. I've used small nuts as well to start with. If you found those threads, I believe some have the diameters you need for certain advance limits.
 
younggun2.0 said:
that is the exact website i found them on. i found them on the msd tech pages. couple mopar guys were having probs getting more initial timing. someone chimed in with the link to 4secondsflat.com . i have a email into the tech dept asking if they only make the 10 and 14* bushings. i hope that the upping the initial will help it. i am learning as i go. i built this engine when i was 19 and that was 11 yrs ago. just got it fired and im trying to make the pile of parts run right. i have virtually zero tuning experience but i learn quick. lol. i will take all the help i can get.
Click to expand...

Cracked is dead on..... You can learn a lot from a dummy!,:D
Just messiin Rob. There should be a Sticky on this very subject.
 
Mad Dart said:
Cracked is dead on..... You can learn a lot from a dummy!,:D
Just messiin Rob. There should be a Sticky on this very subject.
Click to expand...

i have to be honest and say that i am a decent fabricator but something like this i am willing to spend the $35 and know that its goin to be accurate. i dont own a lathe or a caliper. it is a very interesting idea though.
 
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