Advanced carb tuning

I am looking for some solid knowledge, tips, or tricks to tune my carb. There is a lot of stuff to tweak on a carb (power valves, jets, etc...) and just wondering if you guys want to throw your 2 cents in to help out.




351W
Summit Street and Strip 4 barrel (based off of Holley 4010)
750 cfm
manual choke
mechanical secondaries

Well a fella should do a short intro before getting to the

meat of things.....I'm sure you'll get a number of replies...lots of knowledge here.....what's your plans for the motor...street, street strip, all out race....makes a world of difference as to the who, what, when, where and hows of tuning a carb.... sit down a spell and do an intro and let us know what you have in mind for this ride......[P

I wouldn't touch the thing till you drive the car and check the plugs for color.
It's impossible to tune a carb until you do.
Also it's best to do an introduction first
Nice ride too

Make sure you don't drop your 'stones while tuning that carb. Keep it smooth.



And yeah, you ain't gonna know much until you pull a plug immediately after shutdown...

Intro

Sorry about that gentlemen. It is my first post.

I have been building a 51 Ford F1. When I first started it was just a cab and frame. I mounted the cab, fabricated floor boards, built a front end, etc... This is my first time doing anything other than general maintenance. I am just looking to make this a fun, reliable, and safe driver to cruise around town and to work from time to time. I am not looking for a super fast, squeeze out every ounce of horse power you can application. I finally got it driving a few days before I deployed. I pulled the spark plugs and they looked a little on the rich side. I may post a picture on here next time because I am definitely not an expert. It just didn't run that great. I just ordered a vacuum gauge to properly set the idle mixture screw. I have read about using one (on the internet) and figured I would try it. I have it timed at 13 btdc. I am kind of worried that the carb is to big for the motor, but I got it for cheap (750cfm X 351W 30 over). I have read several different views on over sized carbs - Dont jet it down, jet it down a few, just get another one, it will be fine. Like I said I am not going for gold on the track. I built my own headers from a cut up set I bought(sbc). Not sure if they give enough back pressure.

Thanks guys!!!




Blue Eyed: There has been many upon many 30 packs of stones put down in the shop - haha.

In my personal opinion that might be a little too much carb for a 351 no?
it figures out to about 600 or a little more on CFM with a max RPM of 6000

I agree with Dirtyrat but I have also heard of people jetting a 750 Holley with #80 jets on smaller cubic inch engines and running just fine. When Holley made the carbs like you have I ran one out of the box on a mild 454 and it ran great. Stepped up to an 850 Demon on the same engine and it did not make any difference. Both carbs were tested at the drag strip.

Plugs have to be checked when first put in and if you let it idle it will make them dark.
I like your ride. The way the headers are built the cylinders from front to back will run rich to lean because of back pressure or the lack of.
Don't change them though they too look kool.
Those carbs have a tendency to run a little rich.
You can most definitely jet it down.
First pull the jets see what they are to make sure someone else has not jetted it up. If they have go back to the factory jet, if not.
I would start with the front jets only.
Drop two sizes (one size is not enough) run it see what ya think, then if it needs more go two more sizes.
You may also check the power valve to make sure it's not blown.

I too fear its too big, but hope to make it work.

I have #77 on the primary side and #79 on the secondaries. I just don't want to go to far and screw something up. If I go two sizes at a time how will I tell if I have gone to far? I assume plug color??

The power valves appear to be fine. When, if at all, would you look into playing around with power valve sizes? I have 9.5Hg on primary and 6.5 on secondary.

I haven't thought about the cylinders running differently. I have thought about welding a large washer inside the exhaust and may be drill a few additional holes in it. What do you think?

Thanks for the inputs guys!!

I say no to the washer idea. Do a Google search for "back pressure" and read up on exhaust scavaging. It's not as simple as it first appears.

Keystone Light said:

I too fear its too big, but hope to make it work.

I have #77 on the primary side and #79 on the secondaries. I just don't want to go to far and screw something up. If I go two sizes at a time how will I tell if I have gone to far? I assume plug color??

The power valves appear to be fine. When, if at all, would you look into playing around with power valve sizes? I have 9.5Hg on primary and 6.5 on secondary.

I haven't thought about the cylinders running differently. I have thought about welding a large washer inside the exhaust and may be drill a few additional holes in it. What do you think?

Thanks for the inputs guys!!

Click to expand...

The engine will act as if it were starving for fuel (surge under light to medium throttle)
If you're afraid of screwing something up then you're not the one to be messing with it.
9.5 seems to be to much 8.5 would be better on your application.
Sam is right no washer

Sound advise coming at you...

There are a myriad of factors that you have going on there....750 carb on a stock 30 over 351 is a bit much.... you've tried jetting it and still runs rich...I'm not a Ford expert but as was said....your plugs will tell the tale..... you could keep jetting till you get it close....but it could be long process....I'd heed some of the suggestions fired off already....or simply might want to consider selling off the 750 and going with something a bit smaller...JMHO

Everything on the Holley tech web site indicates no more than a 600 to 650 carb and they recommend a vacuum secondary unit....not sure if your's is or isn't vac secondary..... I'd check out the Holley tech site...might save you some time and money.....again JMHO

I tune bike carbs almost daily, some advise I can give is that back pressure does indeed play a part. You may want to look into some small tubes (torque tubes)that we put in harleys that run straight pipes between the headpipe and head that helps produce some back pressure. Again read the plugs!!

Old Iron: I will just go down 2 at a time and see where it gets me. I think if I use a little common sense I will be fine. The only thing I have had anyone else do on the rod thus far is cut and balance the drive shaft.

SgtPontiac: I have mechanical secondaries. Would the idea of a restrictor plate be crazy? Just want to see (and learn) if I can make it work. I will probably get a smaller carb down the road. Who knows!!

From the Holley web site they recommend taking your manifold vacuum reading and dividing it by two for the proper power valve size. I will have to wait on the gauge and then start diving into it.

I think I am going to fabricate a mechanical exhaust cut out and run a two into one single rear exit exhaust with a muffler (Ill post some pictures when I get it started). It may help out?? Besides I don't think they will let me on base with it as loud as it is right now.


Thanks for the comments!!

Glad you decided to try it
You'll be alright doin that, it's the only way to learn for yourself
What you will notice is the response of the engine will be crisper until you go too lean then it'll have a hesitation or surge, when you throttle it.
Power valves can cause a rich burn and sluggish engine also, if they are not the right number.
You'll probably end up at 6.5 to 8.5 on the power valve.
Good luck and let us know what you end up with.