Tuning a 5.7 Hemi with a Carb HELP ***

[Uncle Don]

Hi

Ive been running a dual quad 6.1 with the MSD box for 8 years.

I’d like to examine your timing curves, both RPM and MAP.

I have a suspicion your MAP curve is not set up correctly and is pulling timing from your RPM curve.

I’ve added a pic of my timing curve - note there is nothing below the 0deg mark.

View attachment 1312611

I have conflicting advice on this one. I did have my MAP sensor plugged in, and another guy with Hemi and a carb said it is not needed, and not used because it can't do anything to a carb'd engine? with no computer to add or subtract fuel etc.. and a post above says similar that on carb'd engines it messes with the adjustments on the carb?? so i am not sure here whats right or wrong.? i uhooked mine and it made zero difference in how it ran i can attest to that.

 

[Uncle Don]

I am puzzled as to why you installed a modern motor and vintage fuel system. I have a 5.7 with a mild cam, Hemi 6, convertor, 2.94 gears, 4150 style intake and a bolt on EFI. My desire was a OE looking highway cruiser that is maintenance free. I have used the factory tune and am tickled. It will break the tires loose and it loves the Interstate.

thought it would be cool, and no one around had done it in my area 7-8 years ago. i wanted to be the first....well now hindsight 20/20 wish i would have left it EFI. but when i win the lottery and down the road i want same set up as you yes. thats my final plan. or yank this and put a 383 with 727 in it and leave the hood closed lol.

 

[Thrashingcows]

Never done anything with the modern Hemi's but I have had the wrong combo of parts that sure affected performance.

After reading through all the comments and replies first is that air intake hood on your carb....that thing is super restrictive and should be replaced with a big wide air cleaner....I think engine/carb height might be an issue though but they do make low profile air cleaners.

What are the specs on the cam....what RPM does it start building torque? You need to take that into consideration with transmission torque converter stall and rear gearing. I swapped a bigger cam into a 383 years ago thinking it would build more HP and performance...well the motor turned into a bog machine, wouldn't get up and go until I got the RPM up over 4000..then it would take off like a rocket!

What year is the 904 and does it have part throttle kick down? Also make sure your kick down linkage is set up properly.

 

[furyus]

If you backed off the throttle pressure adjustment, and didn't make sure it was set properly, you will have a burned up trans as a reward. Fix the cable, and sort out the drivability before trying to do burnouts. Maybe watch some YouTube videos on carb tuning.

 

[Uncle Don]

updates:......

discarded the restrictive breather hat thingy...its now on the wall as a conversation starter.

put a mechanical secondary 750 holley with 82 jets set up for a big block 440 on my hemi last night and WALLAH....it ran soooo much better. actually pulled hard in second and drive and acted like it had a little horsepower for once. amazing. so lends me to think its been running pretty lean and not getting enough fuel with my old 770 street avenger. hence the cough and fall on its face when smashing the go pedal.

i need to adjust the cable on my kick down, its a lokar cable kit from classic industries, and right now its adjusted to far out i assume as i have zero kick down action. and makes me think this is why it feels soft going into second gear at full throttle. i am assuming this would make a big difference in the transmission working properly. my trans is the original trans to the car 67 model.

i still think i need more timing reading some of the comments above from guys have similar set ups.

 

[furious70]

The 770 without the hat night be fine too

 

[Thrashingcows]

updates:......

discarded the restrictive breather hat thingy...its now on the wall as a conversation starter.

put a mechanical secondary 750 holley with 82 jets set up for a big block 440 on my hemi last night and WALLAH....it ran soooo much better. actually pulled hard in second and drive and acted like it had a little horsepower for once. amazing. so lends me to think its been running pretty lean and not getting enough fuel with my old 770 street avenger. hence the cough and fall on its face when smashing the go pedal.

i need to adjust the cable on my kick down, its a lokar cable kit from classic industries, and right now its adjusted to far out i assume as i have zero kick down action. and makes me think this is why it feels soft going into second gear at full throttle. i am assuming this would make a big difference in the transmission working properly. my trans is the original trans to the car 67 model.

i still think i need more timing reading some of the comments above from guys have similar set ups.

Step in the right direction...

The original 67 904...well first off get that kick down linkage cable set up properly, that alone will help with driveability. Yes that slow slushy shift into 2nd and probably 3rd is because you do not have enough throttle pressure on the kick down lever. I'd also look for a replacement valve body from a mid 70's 904 with part throttle kick down....that part throttle kick down will help a lot in the driving characteristics and performance of the motor and car.

 

[Uncle Don]

small updates....which is a win win. and i do thank you all for the advice and direction and conversation...please keep it going. im learning new school ways on old school stuff lol. i like the comments good or bad.
got car running a ton better... most of my problem with my carb was the accelerator pump was adjusted all the way out, meaning the nut was almost completely off. not sure how that happened, or who what where when but i adjusted it to where it needed to be and bam...the 770 street avenger works pretty dang good. its no race carb but im not looking for that.
other update is i got the passing gear and linkage adjusted fairly close in the trans, shifts good, firm, and pulls good. i found that whomever installed the trans before me thought it was a 727 and so it had 2 quarts to much fluid in it....drained that and its working much much better now. so with trans pretty close and carb pretty close its alot more fun to drive and works much better.
i do think id like to mess with timing a little as you all have suggested in earlier post, see if that helps it or hurts it, doesn't hurt to try. also im curious to find out if the map sensor does anything to carb'd engines? and tuning? does it really effect the timing curve? pull from it? not sure how all that works...but should that be a non issue since its not fuel injected? over my head on this one.

all in all i drove it to work last week, interstate miles, back roads, about 150 miles in one day... did great, ran good, and pretty happy so far.

 

[Uncle Don]

First off I think that is the worst intake for carbed application. I run 30 degrees from 1,000 rpm to 3,500 then slowly ramp back to 24 degrees. 65 Coronet 2005 5.7 260 comp cam, 727 trans 2.96 gears 275/60/15 rear tires.15 mph rolling stomp the gas and leaves 2 black marks. Also I don't use the map sensor on the Hemi6 it seemed to mess with the carb adjustments the way it varies the timing. I started with the XV motorsports single plane intake it was ok but I now have the Joe Ocone dual plane and it is way better.

Sam65 is this on pump gas 87 octane? 93? race gas? i will probably only be running pump gas 91-93 at most for now.

 

[Uncle Don]

 

[Uncle Don]

 

[Uncle Don]

this is what i am currently:

 

[Uncle Don]

the last screen is where timing and MAP are.
test drove a couple of times and is running the best it has. getting much funner to drive.

 

[lxmodguy]

The great thing about the internet is you can get tons of free advice and it is a great value for your money. All kidding aside, you don't fuel and time gen3 motors the same as old small blocks. Lots of interesting advice in here. Those early truck Hemi's didn't make much power. That said, there is still something way wrong with the fueling and timing because it should be significantly better than your old motor. Id keep playing with the timing and the fueling. Have you considered a Wideband sensor to see what the actual AFR is? Gen3 motors will surprise you with how rich like to be for peak power and how little timing they need.

 

[DGE330/NC]

i have been waiting on one of the Joe OCone intakes for a while now myself

just recieved mine 3 weeks ago . man that thing is a thing of beauty

 

[70bigblockdodge]

Accelerator pump issue, either the check ball or rubber check valve may be not sealing correctly or late. Vacuum secondary Holley's need help they set the universal carbs so slow as to not make a bog, and unhappy customers that it is like driving a 2 BBL. If it is rich and stinky at idle it might have a internal problem, leaky power valve, float height adjustment. Anything Holley street is going to be very lean on primaries at cruise, they do this for the people that put that carb on a stock 305 or 302. If your moving air you need to keep up with fuel, even that early 5.7 hemi is better at moving air than a 1980 monte Carlo with said 305.

 

[Rebelrouser]

anyone have experience on timing these carb'd hemi's? like to have a baseline or what has worked not worked.

When I build an engine I use a computer program called engine analyzer by performance trends. Lets me know if a combination will have any issues. The program predicts things like idle vacuum, cranking compression, and of course HP and torque. It also gives you a timing curve. Several times on fuel injected engines with timing control, I simply input that curve into the spark maps and it is always very close. And on a real dyno the program in within 10%, 99% of the time. Plus if you are interested in how an engine really works the manual that comes with it is worth the price.
I would also be looking at the cranking compression, since you installed a camshaft, if you bleed off too much cylinder pressure it will make an engine a pig. Take a cranking compression test if it is below 140lbs your cam has too much duration for the static compression.


https://www.performancetrends.com/Engine-Simulation.htm