Quick Fuel Slayer 750 Jetting
August 29th, 2015, 06:06 PM
#1
Quick Fuel Slayer 750 Jetting
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.
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.
August 29th, 2015, 06:33 PM
#2
Registered User
Join Date: Nov 2008
Location: Central Fl
Posts: 7,824
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.
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.
August 29th, 2015, 07:14 PM
#3
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.
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.
August 30th, 2015, 10:50 AM
#5
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Join Date: Jun 2010
Location: Rogues Island, USA
Posts: 3,613
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.
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.
August 30th, 2015, 01:08 PM
#6
Registered User
Join Date: Nov 2008
Location: Central Fl
Posts: 7,824
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.
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.
August 30th, 2015, 05:30 PM
#8
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.
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.
August 30th, 2015, 05:36 PM
#9
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.
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.
August 30th, 2015, 09:54 PM
#13
August 31st, 2015, 05:34 AM
#15
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?
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?
August 31st, 2015, 06:40 AM
#19
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.
September 2nd, 2015, 09:00 AM
#20
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)
September 2nd, 2015, 12:22 PM
#21
Registered User
Join Date: Nov 2008
Location: Central Fl
Posts: 7,824
I changed the air bleeds and got a better reading with the screw out .5 more than before- 13inHG at idle. So it's now almost 3 turns out? 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)
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)
September 2nd, 2015, 01:19 PM
#23
September 8th, 2015, 09:42 AM
#26
Administrator
Join Date: Oct 2009
Location: Poteau, Ok
Posts: 40,521
September 9th, 2015, 10:50 AM
#28
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
I'm getting mixed reviews about A/F adjustment- does the car need to be in gear or not?
Thanks
September 9th, 2015, 12:10 PM
#30
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.
Last edited by coppercutlass; September 9th, 2015 at 12:14 PM.
September 9th, 2015, 10:41 PM
#31
September 9th, 2015, 10:46 PM
#32
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.
September 10th, 2015, 07:10 AM
#34
September 21st, 2015, 06:32 AM
#35
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.
October 8th, 2015, 09:00 AM
#36
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!
-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!
October 10th, 2015, 06:59 PM
#38
October 10th, 2015, 07:29 PM
#39
Administrator
Join Date: Oct 2009
Location: Poteau, Ok
Posts: 40,521
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.
October 10th, 2015, 07:35 PM
#40
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
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
