Hello CO,
I need to increase the jet size on my carb and I've never done that so I wanted to get some advice.

The car is surging at cruise- quick fuel slayer 750- 72cc primary, 82 cc primary.

My plan is to get 6-8 ft of hose and set up my vacuum gauge so It can be read in the car. I want to get a vacuum reading while it is surging. I was going to buy 75, 77, 79 sized primary jets and test until I achieve highest vacuum reading and surge is gone.

Questions
-What is the most intelligent way to drain the carb? I have to remove the front of the carb to access the secondary metering plate and screw in new jets.

 

Just an fyi, incorrect timing can cause surging as well.
If you're sure the timing makes sense then I'd go up 2 sizes on the primaries and go from there.
To drain them properly, loosen one of the lower screws on the bowl. Have a cup that fits under the bowl ready to catch the fuel. Normally works pretty well.

 

Thanks Mark.
To clarify- I should order #74, 75, 76 to start?

I feel pretty confident about the timing as I've experimented with total timing from 36*-44* and the surge has always been present. Right now I'm at 38* and getting the best vacuum @ idle and no starting issues. The car also doesn't surge throughout the RPM band just constant (my tach isn't accurate but it sounds like 2,000-2,400.

 

How many turns out are your idle mixture screws?

 

A vacuum gauge isn't going to help you at cruising speed. It's designed to work at idle. It can't tell you what your air fuel ratio is or what jets are best. You'd need an air fuel ratio gauge for that. It will give you the best idle air fuel ratio but not cruising.
If you're positive that your ignition system is not the problem then it sounds like you have a lean mixture at cruise. I would go up one primary jet size at a time until the surge goes away and then maybe go up one more jet size from there.

 

Why would you go up another size once the surging is gone? That's counter productive. Once it's gone you're now at your best lean cruise.
And a vacuum gauge will work at cruise the same way it works at idle, why wouldn't it?
It'll be more difficult than with an air/fuel but it can still be somewhat useful. I routinely use vacuum readings to tune EFI while driving/cruising with excellent results.

 

2.25 turns

 

I was going to ask about that. the whole drive home from work I am thinking "....What wouldn't a vacuum gauge work always..? I see them installed in cars long term all the time"

 

Very sure that the issue has nothing to do with ignition..though..when I ordered the jets last night I treated myself to a new timing gun with all the bells and whistles

 

What is your timing at 3500 with the vacuum advance connected?

 

I'll have that number for you when my new gun gets here.

 

Go down 4 numbers on the idle air bleeds BEFORE you change the fuel jets. The idle air bleeds are the ones on top and the outer perimeter.

 

Ok, doing so will richen the idle but will that translate to more fuel delivery at cruise?

 

Yes.

 

There are 4 up there Mark. To be clear

Just the two on the outside? They read .039. I'm gonna over night .035s to the house and give give it a try (small heart attack when I almost fumbled one into the secondary bowls!)
I'm going to assume that I'll need to re-adjust the a/f screws for best idle quality and that will be in? Will I need to readjust timing as well?

 

Yes, yes, yes and no.

 

Ok great- I'll be back with results and timing information as soon as the parts arrive. Thanks

 

Hmmm aren't there 4 idle air bleeds in that carb? They'll all most likely be the same number.

 

I wasn't sure if it was all 4 or just the 2 on the outside that controlled the emulsion. I called summit back and got 2 more (4 total) .035 air bleeds.
I like to understand the theory of things as well- in decreasing the size of the air bleeds (which I assume let air IN) is this effectively decreasing air flow in terms of CFM? Or is the CFM static and these control the mix or air/fuel?

Is the surge a need for more fuel or the product of improper air/fuel emulsion? I guess what I should ask is; What do these do?

Also
http://www.quickfueltechnology.com/f...talog-2014.pdf
This exploded view makes it look like idle bleeds and high speed bleeds are different.

 

I changed the air bleeds and got a better reading with the screw out .5 more than before- 13inHG at idle. I took the car for a ride and the surge was dramatically decreased.

After going down a steep grade hill I came to a stop sign and when I turned right and gave it some gas the carb "sneezed" a little and took a second to come back to life? Did the bowl overfill or carb load up- Float level is 60% up the sight window. No issues after that but the surge is still present with the reduction of the air bleeds from .039 to .035.

My jets should be here today and I'm going to try the 74cc jet (current primary is 72cc)

 

Are there 4 idle air bleeds in that carb? Trying to remember. If so did you change all four? If not, change all four to the 35's. Then go to the 74 mains.

 

There are 4, I changed them all. Going to verify timing (light just got here) and then I'll get to the jets.

 

36* at 3,500 with VA
35* at 2,500 without VA

13* initial w/o VA
20* initial with VA

800 RPMS= 13 in HG

 

If you're draining a quick fuel; party cup and scissors will save you some mess.

74cc primary is another great improvement but it's not perfect. 75s tonight- will report.

 

76cc primary jets, .035c air bleeds- Good to go on lean cruise, idle, temp, start.

 

Some how this just doesn't look right. If your adding 7* at idle then you should be adding 7* at cruise.

 

I'll check timing @ 3,500 W/O VA

 

I set the timing to 35* @ 2,600 Rpms (it was at 32*) with VA disconnected, that brought the initial to about 21*, stronger idle quality and vacuum in park was a steady 13in HG- Went for a ride and the low end throttle response was much improved, hot starting was perfect (and I was sweating it).

I'm getting mixed reviews about A/F adjustment- does the car need to be in gear or not?

Thanks

 

What is your timing with the advance connected? Make sure it does not advance more when you raise the throttle up to say 4500?

 

Im running 14 inital 34 total . box stock holley 3310 . Im a fan of the slayers price and tunability but i got some really good scoop on it from a friend of mine and my holley was free lol . I have learned from experience that carbs with more tuneability can be good and bad. I had a quick fuel that would not run right with a 10 degree diffrence in temp. I had the same cfm carb in a holley double pumper with less tunability and it would work great on the street at 45 degrees or 90 degrees.

 

I'll run it tomorrow- 2,600, 3,600, 4,600 WITH VA.

And Why?

 

14/34 With out VA? Free Holley?! I'm in but I was on a budget for an entire build and I wanted- eletric choke+tuneabilty+ looks+performance and I wanted it all for under $350.00. Only summer testing thus far but I'm happy with my choice.

 

I don't used the va. And the carb was in near new condition when I got it from my friend. It has worked well so there has been no need to mess with it .

 

With VA Advance:
@2,600- 40*
@3,600- 40*
@4,600- 41*

 

Another quick question. I'm not getting the tug or surge feeling I was before but I can hear a slight change or palpitation in the exhaust note at steady cruise RPM. I'm going to move try 77cc, 78cc and check for improvement. Ordering today.

 

Ok- here is where I landed.

-78cc on the primary
-.035 air bleeds
- 38* total timing at 3000 RPMs
- 44* Total timing with the vacuum advance hooked to the timed spark port of the carb at 3000 rpms
- 20* initial timing at 800 rpms with 13inHG.

Car idles fine
Good throttle response
1st gear at 15mph dump the peddle and It just screams through the rpms. Spins right through second gear no black smoke just a ton of burnt rubber.
low throttle cruise is good. steady like you're on rails.
part throttle is good. I still feel like I am hearing the exhaust note change but honestly I've been so focused on it I may be wishing it there.
part-part/full throttle- just skates through the gears.

Will pull plugs after some miles with this tune. Launch video to come when I get a discrete camara person!

 

For a little better fuel economy at cruise you can adjust your vacuum advance closer to 48-50.

 

I know the mechanical advance is adjustable with the use of springs in the assembly. Is the vacuum adjustment similar? Not familiar.

Thanks

 

If its adjustable it will be through the port that the vacuum line is connected to. It takes a small allan wrench. It also may be limited by a stop on the rod. It is totally separate from the rest of the distributer.

 

Awesome, thank you. I'll give that a look after work.

Will increasing the vacuum advance have any adverse affects on non-cruise rpms? Just interested to know the science behind why the fuel economy will improve.

Thank you