Carb tuning blower engine - Chevelle Tech
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post #1 of 17 (permalink) Old Aug 20th, 19, 2:30 AM Thread Starter
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Kerry
 
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Carb tuning blower engine

Looking for some feedback on a car I’ve been working on this summer. 33 ford pro street style rod with a 350, edelbrock heads, b&m 250 blower at 6-7 psi and 1850 carbs.

I had 2 1850’s set up according to a recipe given by Tuner on fuel systems forum.

The engine runs well, except for a couple issues.
I had to go from 35 squirters and white cams to green cams to eliminate a lean pop when snapping the throttle. It would pop the blow off plate. But now if you hit the throttle hard more than once the engine loads up (doesn’t rev as high or quickly the second hit). There is no black smoke when you hit the throttle to the floor from an idle.

I have the mixture screws set for highest vacuum of 11.5” at 825 rpm idle. 1 turn. Any leaner and I can get a lean pop once in a while if I snap the throttle a few times in succession. Any leaner and the idle vacuum becomes unstable.

The strange thing is that every time you blip the throttle, or hit it to the floor from an idle or revved up to 2000, the engine will rev, but when it idles back down (and quite quickly), it loads up and the idle drops to 500 or so for a split second. Then recovers and comes back up to 800-825. There is a slight puff of black out the pipes when the rpm recover. If I lean the mixture screws this rpm recovery becomes better, but then it has a lean pop once in a while. And the idle vacuum becomes unstable as mentioned earlier.

Also, when I hold it at 2000 rpm for 10 seconds or so, then let off the throttle, the rpm will drop all the way to the point that the engine quits, like it’s out of fuel?

Timing is locked at 30, BKR6E plugs gapped at .030”. MSD dist with 6al box and blaster 2 coil. 6.5 psi fuel pressure, floats set so the fuel is at the bottom of the screw holes. If I bump the car fuel will seep out of the holes.

I haven’t tuned a car with a blower before, wondering if some of this isn’t unusual? Or any suggestions?
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post #2 of 17 (permalink) Old Aug 20th, 19, 6:50 AM
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Eric
 
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Re: Carb tuning blower engine

Try going slightly richer on the idle mixture screws--maybe 1/16 to 1/8 turn out and see what that does. Sometimes setting the mixture to the highest vacuum puts the engine right on the verge of being too lean. Most carbureted performance engines prefer to idle slightly richer than a stock engine since they're less efficient at low rpm.

My mantra: Let the engine tell you what it wants, don't tell it what it can have.

One other thing--what does the timing curve look like?

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post #3 of 17 (permalink) Old Aug 20th, 19, 7:20 AM
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Bob
 
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Open the transition restrictions a few .001
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post #4 of 17 (permalink) Old Aug 20th, 19, 9:16 AM Thread Starter
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Kerry
 
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Quote:
Originally Posted by Vintage Musclecar View Post
Try going slightly richer on the idle mixture screws--maybe 1/16 to 1/8 turn out and see what that does. Sometimes setting the mixture to the highest vacuum puts the engine right on the verge of being too lean. Most carbureted performance engines prefer to idle slightly richer than a stock engine since they're less efficient at low rpm.

My mantra: Let the engine tell you what it wants, don't tell it what it can have.

One other thing--what does the timing curve look like?
Timing locked at 30.

Richer on the screws it doesn’t recover back to normal idle as quickly after hitting the throttle.
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post #5 of 17 (permalink) Old Aug 20th, 19, 10:41 AM
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Vince
 
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Re: Carb tuning blower engine

have you tried increasing idle speed to 900-950rpm?
it may like that more; as your timing is locked at 30*

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post #6 of 17 (permalink) Old Aug 20th, 19, 1:23 PM
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Jeff
 
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Re: Carb tuning blower engine

If your getting a lean pop when you blip throttle that is a pump issue. The loading up is because you are shutting of the air supply and so it goes rich. Free revving a blower engine is not always a good thing and it is not a good way to tune them. I would invest in a wide band 02 sensor and gauge. That said what jetting? are you plugging off the power valves and running 4-10 higher jets to cover? I would have to know how you setup the carbs to better help in all truth. Blowers can be a different animal then NA because they make there own vacuum, they are draw thru and the carb needs to be setup accordingly.

78 Malibu, 402, eddy rpm performer heads, 174 B&M blower, th350 2600 stall, holley sniper EFI, 9" 3:70, Hoosier 26X8.50-15 slicks.
69 VW baja 1600cc, about to be a 2.2l or 2.5l witch ever I come by cheaper
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post #7 of 17 (permalink) Old Aug 20th, 19, 2:50 PM Thread Starter
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Kerry
 
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Re: Carb tuning blower engine

I did not increase the idle speed more. I didn't want to overexpose the t slots.

Here is a link to how the carbs are set up. They are boost referenced running 6.5 PV to a factory port on the backside of the blower, at the bottom of the case.

https://www.tapatalk.com/groups/raci...cipe%22#p16714

I would like to install a wideband, but as the car was put together as a show car initially, with all the wiring hidden in the frame, custom interior, etc. it is very difficult to work on and add things now. Just trying to put in a new larger battery ground cable meant being a contortionist to reinstall it.
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post #8 of 17 (permalink) Old Aug 20th, 19, 4:49 PM
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Re: Carb tuning blower engine

I have never got that far into a carb, to pug and re-drill bleeds and such that is above my pay grade. I once sent out a carb and had the mods to boost reference the power valve done but never tried doing it my self. I think I would call J mark or another carb specialist directly. I know about the balance tube setup, I just never went further into it by modifying the blocks or anything. That can and does effect tuning. So I am afraid can not be of anymore help.

78 Malibu, 402, eddy rpm performer heads, 174 B&M blower, th350 2600 stall, holley sniper EFI, 9" 3:70, Hoosier 26X8.50-15 slicks.
69 VW baja 1600cc, about to be a 2.2l or 2.5l witch ever I come by cheaper
96 chevy 1500, tow vehicle.
03 yz85 fun bike.
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post #9 of 17 (permalink) Old Aug 20th, 19, 6:37 PM Thread Starter
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Kerry
 
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Re: Carb tuning blower engine

Quote:
Originally Posted by Jebchevelle View Post
I have never got that far into a carb, to pug and re-drill bleeds and such that is above my pay grade. I once sent out a carb and had the mods to boost reference the power valve done but never tried doing it my self. I think I would call J mark or another carb specialist directly. I know about the balance tube setup, I just never went further into it by modifying the blocks or anything. That can and does effect tuning. So I am afraid can not be of anymore help.
Ok, thanks.
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post #10 of 17 (permalink) Old Aug 20th, 19, 7:37 PM
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Re: Carb tuning blower engine

Have you tried more timing to see if it changes anything?
The only blower I ever ran like timing it was a BB but it liked about 40 locked.

Jim

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post #11 of 17 (permalink) Old Aug 20th, 19, 11:57 PM Thread Starter
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Kerry
 
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Quote:
Originally Posted by 77 cruiser View Post
Have you tried more timing to see if it changes anything?
The only blower I ever ran like timing it was a BB but it liked about 40 locked.
With no timing retard, I am hesitant to run more than 30. I know the blower makes 6-7lbs, but the build specs for the engine were lost years ago. And without knowing the static compression I don’t want to risk damage. Damage from detonation can be happening with no warning signs.
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post #12 of 17 (permalink) Old Sep 5th, 19, 10:43 PM Thread Starter
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Kerry
 
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So the owner of the car finished some other things on the car. Went to take it for a drive, and he reports it will hardly pull itself? Starts great, sounds good when you rev it in neutral.
The headers are Sanderson street rod headers, ceramic coated, and wrapped with header wrap from where the 4 pipes come together all the way to the end of the collector.
He says the headers from where the header wrap starts glow red.
The car has supertrapp mufflers on the end of the pipes. I told him take them off and try it again, could they be that restrictive now that the timing, carbs are all correct?
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post #13 of 17 (permalink) Old Sep 6th, 19, 11:00 AM
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Re: Carb tuning blower engine

A friend of mine had Super Traps so blocked up he had the same issue. You might be on to something.

69 461" BBC 11.25:1 Compression
840 Rec Heads (2.25/1.88) AED 850 HO carb
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post #14 of 17 (permalink) Old Sep 6th, 19, 5:03 PM
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Jeff
 
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Re: Carb tuning blower engine

if the headers are glowing red it is lean.

78 Malibu, 402, eddy rpm performer heads, 174 B&M blower, th350 2600 stall, holley sniper EFI, 9" 3:70, Hoosier 26X8.50-15 slicks.
69 VW baja 1600cc, about to be a 2.2l or 2.5l witch ever I come by cheaper
96 chevy 1500, tow vehicle.
03 yz85 fun bike.
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post #15 of 17 (permalink) Old Sep 6th, 19, 6:08 PM Thread Starter
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Kerry
 
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Quote:
Originally Posted by rp930 View Post
A friend of mine had Super Traps so blocked up he had the same issue. You might be on to something.
That’s where I’m going to have him start.


Quote:
Originally Posted by Jebchevelle View Post
if the headers are glowing red it is lean.
I did richen the idle mixture until the vacuum dropped to 9”. Still no change in how the headers were red at idle.
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