Need help with tuning - Speed Demon carb - Team Camaro Tech
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post #1 of 14 (permalink) Old Jun 3rd, 11, 04:08 PM Thread Starter
Jamie
 
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Need help with tuning - Speed Demon carb

Hey Guys,

First post on the forums, hopefully I can get some help. My wife just surprised me with my 69 camaro project for christmas. I've had the car for 10 years but never had put it all together, now its a driver, best wife ever.

Anyway, it's got a 350 with roller rockers, camel hump heads, edelbrock intake, mallory ignition system and distributor, headers, and a speed demon 750 carb (#1402010V).

I had my uncle who's a very experienced mechanic set the timing for me which he did slightly advanced as its a high performance engine.

The problem I'm having is it seems like its dumping too much fuel in or something. I'll be cruising along in 2nd gear about 15-20 mph and then I'll jam on the gas and the engine will bog down like crazy then rev up. Or when I'm going about 30-40 in 3rd gear and I put it to the floor I can feel it hesitate. It idles great and doesnt have a miss or anything, revs up super strong, just has those two issues which dont seem normal. The car is very driveable and runs strong but just doesnt feel like its at peak performance.

The float levels check out fine, fuel about halfway up and its a new carb. Nearly everything on the engine is new except for the block and heads.

Any help would be appreciated, thanks!

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post #2 of 14 (permalink) Old Jun 3rd, 11, 05:25 PM
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Steven
 
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Re: Need help with tuning - speed demon carb

sounds like you need a bigger accelerator pump. when you accelerate fast, the size squirter you have now is not supplying the engine with enough fuel causing it to bog and hesitate.

what size came with the carb??

69 Frost Green Camaro
LS3 with TSP stage 3 cam
Tremec TKO 600
Moser 12 bolt rear with ford ends 3.73:1 Eaton tru trac differential
Speedtech pro touring subframe
Ridetech rear 4-Link


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post #3 of 14 (permalink) Old Jun 3rd, 11, 06:21 PM Thread Starter
Jamie
 
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Re: Need help with tuning - speed demon carb

Thanks so much for replying. I'm not sure what size came with it but I imagine its factory.

The hesitation is only under load, when the car is just sitting it sounds and rev's just fine. Would this make a difference in your diagnosis?
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post #4 of 14 (permalink) Old Jun 3rd, 11, 06:28 PM
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Steven
 
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Re: Need help with tuning - speed demon carb

when the car is sitting and revving under no load there is nothing putting strain on the engine ex. transmission. once driving, the engine is now turning the tranny, rear diff, wheels, you, and the car. so when you hit the gas pedal the engine needs that strong shot of fuel to make the power to get those things turning faster. your problem definatly points to the accelerator pump. very very easy fix

69 Frost Green Camaro
LS3 with TSP stage 3 cam
Tremec TKO 600
Moser 12 bolt rear with ford ends 3.73:1 Eaton tru trac differential
Speedtech pro touring subframe
Ridetech rear 4-Link


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post #5 of 14 (permalink) Old Jun 3rd, 11, 09:18 PM Thread Starter
Jamie
 
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Re: Need help with tuning - speed demon carb

Well that's good news! How would I go about fixing it? If you could provide some outline points would be very helpful or links/videos, thanks so much for your help!

Also, I just looked at your thread, your car BEFORE you stripped it is what I want mine to look like when I'm done Mine's an original lemans blue with the vinyl top but I want to add the Z28 stripes in matte black when it gets painted.

Last edited by chbrja; Jun 3rd, 11 at 09:31 PM.
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post #6 of 14 (permalink) Old Jun 4th, 11, 12:13 AM
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Re: Need help with tuning - speed demon carb

very easily. the accelerator pump is located on the drivers side front of the carb. it has a little arm connected to the throttle linkage. I think you may have to remove the front bowl not sure tho. then turn it upside down and remove the four screws holding the cover on and really all you have to swap out is the diaphragm and spring for one that either has a bigger shot or longer duration. in your case it seems you need a bigger shot. check out places like jegs or summit and type in accelerator pump.

gotta love the lemans blue. although the previous owner added the Z28 stripes. my car was originally equipped with the hockey stick stripes on the side so after im done painting the car i will add them on in a flat black

69 Frost Green Camaro
LS3 with TSP stage 3 cam
Tremec TKO 600
Moser 12 bolt rear with ford ends 3.73:1 Eaton tru trac differential
Speedtech pro touring subframe
Ridetech rear 4-Link


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post #7 of 14 (permalink) Old Jun 4th, 11, 04:44 AM
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darrell
 
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Re: Need help with tuning - speed demon carb

me personally--i would be looking at power valve in primary first and see if its correct for the motor you have- then to accelerator pump-- but thats just me
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post #8 of 14 (permalink) Old Jun 4th, 11, 07:13 AM
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Dan Ensign
 
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Re: Need help with tuning - speed demon carb

I would lower the float levels to the bottom line in the sight window, check,install #69 jets in the primaries, #70 jets in the secondaries, #6.5 power valves fro nt and rear and 28 nozzles for the accelerator pumps. This was the basic tune up I put in a lot of carbs running a similar combo to yours with very good luck as a starting point. You may need to jet up or down slightly but you should be real close.

Dan E.
69 SS396 4spd. 4.10 posi. x66 coupe
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post #9 of 14 (permalink) Old Jun 5th, 11, 10:38 PM
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darrell
 
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Re: Need help with tuning - Speed Demon carb

cant pick a power valve until you know your vacuam or it will stumble-a 65 power valve means you means you should be around 13 in vacuam at idle which is perty close to stock cam
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post #10 of 14 (permalink) Old Jun 6th, 11, 11:16 AM Thread Starter
Jamie
 
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Re: Need help with tuning - Speed Demon carb

Thanks for all the help guys. So how I'm interpreting all this is:
1. test my vacuum to see what power valve I should be using (how do i test this?)
2. ask you guys what PV I should use after I figure out what my vacuum is at
3. put my float levels at the bottom line to start
4. test the engine to see what happens?
5. then what?

thanks!
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post #11 of 14 (permalink) Old Jun 7th, 11, 03:03 AM
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darrell
 
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Re: Need help with tuning - Speed Demon carb

get a vacuam gauge and hook it to main vacuam sorce intake or pvc line or i use the passenger front vacuam port on a holley--with it idling adj idle mixture start with drivers side first go back and forth to get highest vacuam reading--take that reading and subtract 6.5 and thats your power valve number or close to it--if you 10in vacuam then that would yield a 35 power valve
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post #12 of 14 (permalink) Old Jun 13th, 11, 10:46 AM Thread Starter
Jamie
 
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Re: Need help with tuning - Speed Demon carb

Hey All, after all the research and learning over the past week to attempt to get my bog and off idle stumble gone, I made some significant progress this weekend. I wanted to share my journey.
1. started researching how carbs work in general. I made sure to get to about 80% retention with the different components and how they work.
2. I started asking for advice from experienced people who knew way more about it than I did. The advice I got ranged from: "buy a holley or quick fuel carb and dump the demon" to "i can tune it for you but I charge $100/hr" to "try the accelerator pump, bigger squirter etc"
3. To recap my issue, my timing was set correctly, the car would start hot, revved fine under no load but had a significant bog/stumble under hard acceleration in any gear.
4. MY PROGRESS:
4.a. I looked at the BG website and they said to "readjust my butterfly position". So i learned at what it should be set as which is turning the idle speed screws (the two on each side for primary and secondary NOTES:not to be confused with the idle mixture screws) to have a nice little square showing in the slot in the side of the carb for both primary and secondary. I left this as a baseline and put the carb back on.
4.b. I went up from a #31 to a #35 squirter. Super easy to change out, takes 2 min.
4.c started the engine, let it warm up to operating temp (about 160 on my car) it ran great but was idling at about 1200 RPM now up from 1000 previously. I left the idle mixture screws where they were from the previous tune job my uncle did. My plugs were nice marshmellow color so I knew my mixture was okay.
4.d I turned the idle speed screws evenly to get my idle down to about 950.
4.e. I drove the car around the block to see where I was at. The bog was totally gone but there was still a little bit of a stumble.
4.f I turned the car off and reset all of my idle mixture screws. I turned them all the way in and then backed them out 1.5 turns.
4.g I started the car again and the engine was idling okay but didnt seem super strong (im a newbie remember). I then plugged the vacuum guage into the manifold port and was reading 12.5 vacuum.
4.h I turned the idle mixture screws evenly until my vacuum read a solid 15.
4.i. I drove the car again and the bog and the stumble were completely gone. HOWEVER, it still seems that at wide open throttle that something was still holding the engine back from letting loose.

MY NEW QUESTION
Now that my off idle stumble is gone, what could be causing the engine at WOT to not be getting total power?

Thanks for your continued help and I hope what I've found so far can help someone else in my situation. So when you say you've got a Demon and someone tells you to scrap it, I only put a little time and energy and at $13 squirter into mine and got it running much much better. Hopefully I'm not too far away from getting it at top performance.
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post #13 of 14 (permalink) Old Jun 13th, 11, 07:22 PM
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Steve
 
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Re: Need help with tuning - Speed Demon carb

I tried a 750 demon on my setup and it was way too big ended up down sizing to a 650 demon and ran way better.

Steve
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post #14 of 14 (permalink) Old Jun 14th, 11, 10:17 AM Thread Starter
Jamie
 
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Re: Need help with tuning - Speed Demon carb

Yeah, I'm eventually going to get a Quick Fuel HR 680 but there are so many other things I need to buy for the car, a new carb is lower on the list. Thanks for the heads up though, I figured that may be one of my problems.
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