Carb spacer question
Thread Starter
Instructor
Joined: Apr 2005
Posts: 136
Likes: 41
From: Arvada CO
Carb spacer question
A buddy made a 1/2 inch carb spacer and I wanted to see if there was any difference when I installed it. Pure for the hell of it. The idle seemed to smooth out a bit, and it seemed a little smoother overall so all of that seemed positive, but not a huge difference. The big difference was a noticeable stumble off idle when I floored it. That stumble was not there before. Took the spacer out and stumble went away. Nothing else changed. Litteraly loosened carb bolts,installed and started up.
What might cause this? Lean off idle? Rich? Accelerator pump weak?
Details: 383, new 625 street demon carb, performer intake, 11:1 compression, 33* all in by 2500 rpm.
I am just getting into fine tuning the tune, but thought this an interesting situation. I am ordering the tuning kit and thought this stumble might lead me in the right direction on main jets.
Thanks
Brad
What might cause this? Lean off idle? Rich? Accelerator pump weak?
Details: 383, new 625 street demon carb, performer intake, 11:1 compression, 33* all in by 2500 rpm.
I am just getting into fine tuning the tune, but thought this an interesting situation. I am ordering the tuning kit and thought this stumble might lead me in the right direction on main jets.
Thanks
Brad
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
At your altitude it's very easy to get too rich on everything.
The accel pump squirt needs to be leaner, more idle air less idle fuel, smaller main jets etc.
I always default to leaning rather than enrichning a circuit due to my altitude, and usually that is exactly what the engine wants. It can be hard to detect the difference between lean bog and rich bog at times.
Keep tweeking and eventually you will get it right.
Some engines want spacers some don't.
Open spacer vs 4 hole spacer changes the velocity directly below the fuel nozzles and can cause mixture changes.
Experimentation seems to be the solution, you just have to see what your engine wants.
The accel pump squirt needs to be leaner, more idle air less idle fuel, smaller main jets etc.
I always default to leaning rather than enrichning a circuit due to my altitude, and usually that is exactly what the engine wants. It can be hard to detect the difference between lean bog and rich bog at times.
Keep tweeking and eventually you will get it right.
Some engines want spacers some don't.
Open spacer vs 4 hole spacer changes the velocity directly below the fuel nozzles and can cause mixture changes.
Experimentation seems to be the solution, you just have to see what your engine wants.
Safety Car
Joined: Jan 2009
Posts: 4,000
Likes: 299
From: New York
The reason this spacer caused this could be as simple as a larger plenum area and hence a bigger shot to compensate for it. That is where I would start.
Up the rear squirter 2 sizes and try again..
Up the rear squirter 2 sizes and try again..
Melting Slicks
Joined: Dec 1999
Posts: 2,116
Likes: 156
From: Mid West
Cruise-In III Veteran
I tried an open spacer on my ZZ4 and it didn't really like it. Switched to a 4-hole 1/2" spacer and it was more responsive at lower RPM's. I just liked the feel of the 4-hole spacer, and its been on there for years now.
Last edited by Jason Staley; Jan 31, 2016 at 06:59 AM.
Thread Starter
Instructor
Joined: Apr 2005
Posts: 136
Likes: 41
From: Arvada CO
Thanks for the input guys. I will experient with it and let you all know what happens. We are supposed to get a good snow storm here so it may be a bit before I can get her out.
Cheers
Brad
Cheers
Brad
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Verdad Gallardo
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
I have a collection of wood thermal spacers. Actually the only way to go to keep carb heat down. Like after shut off and carbs that you can hear the gas start to boil in the bowls or you see vapor rising out of the throttle area. You are just venting away MPG
I use a 4 hole 1/2 inch wood from summit racing. The four holes match my throttle bore size on a 830 cfm double pumper, giving the best signal and additional distance to my single plane intake plenum.
way back when I tried two hole and open in dual plane intakes. To very intake rpm choices
I use a 4 hole 1/2 inch wood from summit racing. The four holes match my throttle bore size on a 830 cfm double pumper, giving the best signal and additional distance to my single plane intake plenum.
way back when I tried two hole and open in dual plane intakes. To very intake rpm choices
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
I did 3 runs each back to back testing on open versus 4 hole at the strip, the open netted better times and mph, this was with a dual plane, i have recently went to a single plane because of the results and like it better than the dual. I will experiment with the spacers this next summer.
Racer
Joined: Jan 2008
Posts: 295
Likes: 58
From: Arvada Colorado
Carb Spacer
It's definitely the accelerator pump system. There is a difference between a "stumble" and a "bog", One the pump nozzle is too small and the other is it's too big. I also live in Arvada. I had to adjust nozzle size and also fool with the pump cam to get mine right. If this is an open spacer, it is mimicking a single plane manifold in some ways. (assuming you don't have one already)