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Ok. I need help tuning my carb.
Posted By: mopfried
Ok. I need help tuning my carb. - 04/28/14 03:43 PM
Specs: 74 scamp 440. Stock bore, stock pistons, stock bottom end.
Comp 274 XE hydraulic cam with 1.6 roller rockers,
cam spec 230° / 236° at .050 Lobe Separation: 110 Intake Centerline: 106° installed advanced 2 degrees for piston to valve clearance. Lift with 1.6 roller rockers .520 in/.523 ex
452 heads cut about .100 to make them closed chambered. Home ported with templates.
Schumacher headers with full exhaust.
M-1 intake with Speed Demon 850 DP carb.
Automatic with 3:55 gears. 32-3500 stall that is fairly loose.
I'm using a Mallory 685 ignition box with a unilight dist with the timing advance locked out. I can pull 14" of vacuum at idle, but dropped it down to a steady 13. The box has a 10 deg start retard.
Total timing is about 34-36 even at idle... It will ping just a little under a heavy load, like when the tires bite hard.
I am using an innovative A/F gauge with the heated sensor in the Right bank collector
I can get the idle lean with the idle slots in the front and rear where they should be. The idle speed screws are only about 1/2 turn from not touching the lever, front and back. The four corner idle mixture screws are all responsive and all out about 1/2-5/8 a turn. It will idle nice at 14-14.5 AFR. Setting still I can rev it slowly and it stays in the high 13s and low 14s. When I put it in gear it is in the 11-13 range cursing around and at a steady speed or 40-50 mph. I don't know for sure if anything has been done to this carb.
I know the timing seems weird... where do I start?
Posted By: RapidRobert
Re: Ok. I need help tuning my carb. - 04/28/14 04:43 PM
Street car? Just me I would unlock the timing & set the initial with the vac gauge method then set the mechanical so the total (initial+ slots) is 36 then springs then vac adv (if used). then the carb: idle, cruise then power step/PV timing then secondary opening point
Posted By: Challenger 1
Re: Ok. I need help tuning my carb. - 04/28/14 05:16 PM
Quote:
Specs: 74 scamp 440. Stock bore, stock pistons, stock bottom end.
Comp 274 XE hydraulic cam with 1.6 roller rockers,
cam spec 230° / 236° at .050 Lobe Separation: 110 Intake Centerline: 106° installed advanced 2 degrees for piston to valve clearance. Lift with 1.6 roller rockers .520 in/.523 ex
452 heads cut about .100 to make them closed chambered. Home ported with templates.
Schumacher headers with full exhaust.
M-1 intake with Speed Demon 850 DP carb.
Automatic with 3:55 gears. 32-3500 stall that is fairly loose.
I'm using a Mallory 685 ignition box with a unilight dist with the timing advance locked out. I can pull 14" of vacuum at idle, but dropped it down to a steady 13. The box has a 10 deg start retard.
Total timing is about 34-36 even at idle... It will ping just a little under a heavy load, like when the tires bite hard.
I am using an innovative A/F gauge with the heated sensor in the Right bank collector
I can get the idle lean with the idle slots in the front and rear where they should be. The idle speed screws are only about 1/2 turn from not touching the lever, front and back. The four corner idle mixture screws are all responsive and all out about 1/2-5/8 a turn. It will idle nice at 14-14.5 AFR. Setting still I can rev it slowly and it stays in the high 13s and low 14s. When I put it in gear it is in the 11-13 range cursing around and at a steady speed or 40-50 mph. I don't know for sure if anything has been done to this carb.
I know the timing seems weird... where do I start?
I agree with RR.
Why the locked distributor? Start with the distibutor, it needs to be tuned right along with the carb. They go "hand to hand" Can't do one without the other.
Posted By: YO7_A66
Re: Ok. I need help tuning my carb. - 04/28/14 05:19 PM
""When I put it in gear it is in the 11-13 range""
- How much timing do you have in D with the brake applied? I am curious if it drops in this situation.
- What is the A/F reading in D with the brake applied?
- How much of an rpm drop do you have in D with the brake applied.
With an auto street car, there is some tuning that needs to be done in the above situation.
Posted By: mopfried
Re: Ok. I need help tuning my carb. - 04/28/14 05:46 PM
The timing is so high at idle it wont allow for any advance. I marked my balancer when the motor was apart. its about at 36 idling! If I drop it down much it seems to get rich and expose the transfer slots. It will run like that but I lose the adjustability of the idle mixture screws. something's not right. With no advance it will ping slightly now. I shouldn't have too much compression.
Posted By: mopfried
Re: Ok. I need help tuning my carb. - 04/28/14 05:49 PM
Quote:
""When I put it in gear it is in the 11-13 range""
- How much timing do you have in D with the brake applied? I am curious if it drops in this situation.
- What is the A/F reading in D with the brake applied?
- How much of an rpm drop do you have in D with the brake applied.
With an auto street car, there is some tuning that needs to be done in the above situation.
I don't have a tach. It doesn't drop in rpm very much, the converter is pretty loose. I meant when I'm driving it just off idle it is in the 11-13 range.
Posted By: YO7_A66
Re: Ok. I need help tuning my carb. - 04/28/14 06:35 PM
""Speed Demon 850 DP carb. The four corner idle mixture screws are all responsive and all out about 1/2-5/8 a turn. It will idle nice at 14-14.5 AFR.""
- All of this seems good.
"" its about at 36 idling! If I drop it down much it seems to get rich and expose the transfer slots. It will run like that but I lose the adjustability of the idle mixture screws.""
- If you lose the adjustability of the mixture screws, then your idle speed is too fast and you are then into the "transition" of the main circuit.
Posted By: mopfried
Re: Ok. I need help tuning my carb. - 04/28/14 07:11 PM
I will try it again tonight. how low should I let the vacuum go, or how many turns should the idle speed screws (front and rear) be turned before the transfer slots are exposed?
Posted By: YO7_A66
Re: Ok. I need help tuning my carb. - 04/28/14 07:53 PM
""how many turns should the idle speed screws (front and rear) be turned before the transfer slots are exposed?""
- You may need to remove the carb, flip it over, and look at the slots from the bottom side. Then adjust your idle speed screws to achieve the "square" hole look on both front and back. This may not be your issue but at least you can say that you have checked them and they are both square.
Posted By: 383man
Re: Ok. I need help tuning my carb. - 04/29/14 01:29 AM
Quote:
Street car? Just me I would unlock the timing & set the initial with the vac gauge method then set the mechanical so the total (initial+ slots) is 36 then springs then vac adv (if used). then the carb: idle, cruise then power step/PV timing then secondary opening point
I would also agree with this. I did not see you say if your dist has a vacum advance ? Its tuff to run a locked out full advance dist on the street using pump gas if the eng is not set up just right like quench and the right cam and so on. Ron
Posted By: mopfried
Re: Ok. I need help tuning my carb. - 04/29/14 04:13 AM
Its a Mallory unilite distributor. It does not have a vacuum advance. Its adjustable from 0 to 28 degrees (Mallory YH advance mechanism ) I can remove the pickup plate and loosen two screws and move a tab and adjust how much advance it gives. I can also change the advance curve by changing springs.
I am a bit under the weather today so I didn't mess with it tonight after working 9 hours.
I have tried different settings and the advance is too much. It will run really nice until you get into it hard and it will rattle like hell. So, I started out from scratch and had no advance. I set the timing to get the best vacuum at a idle - 1" vacuum.. I As I stated before the idle screws are only opened about 5/8 turn on the primary and the same on the secondary. All four mixture screws are responsive as I can see on the wide band meter. I guess I can slow the timing down some and try to keep it from pulling from the transfer slots but I have tried before.
I have a 750 Vac holley and a 750 vac demon. I did check to see how much 5/8 or a turn would open the butterfly's and I would say I could go about a turn and the transfer slot looks square.. I will try that. I know it will speed up the idle, so I will take a little timing out to slow it back down. From what I understand as long as turning the mixture screws make a difference I am still on the idle circuit.
This is frustrating. I'm not a pro or anything but I have tuned on carbs all my life. I haven't had problems like this.
Here is what I was reading.http://gasalley.thetumbleweeds.net/tech_archive/general/DemonIdle.htm
Posted By: RapidRobert
Re: Ok. I need help tuning my carb. - 04/29/14 04:50 AM
I would set the initial as you described get max vac then retard timing till the vac drops 1 in hg all the while keep reducing the idle speed back to a reasonable in gear idle speed. Then adj the slots to get 36 total then mix/match springs staying just under the pinging point at WOT up thru the gears on your hottest/driest day with the advance starting no less than 200 RPM above the in gear idle speed. Then start on the carb: idle then "cruise" then PV ratio/tip in point (in hg) then secondaries (tip in/sec jetting. I believe on the secondaries (double check on this) that you want a smooth transition when it comes in (you cant tell when it opens) and jet for max horsepower (dyno) or max MPH (strip)
Posted By: CUDAJAS
Re: Ok. I need help tuning my carb. - 04/29/14 06:42 PM
I see it alluded to in several posts above, but not stated...all initial adjustments should be made with the car idling in D and e-brake applied (i.e., the engine should be under load).
Otherwise, once you pull the car into D, all the initial settings are meaningless!
Jason
Posted By: fourgearsavoy
Re: Ok. I need help tuning my carb. - 04/30/14 08:08 PM
I would say the carb is o.k. and any issues you have are all based on your distributor being locked out at over 30 degrees
Why wouldn't you set the base around 20 degrees and limit the total timing to 32-34 max with your iron heads.
Why would you set your timing so high at idle? I think I read your original post correctly.
Gus
Posted By: RobX4406
Re: Ok. I need help tuning my carb. - 04/30/14 08:36 PM
That should run fairly easily with ~20* initial timing and around 850 rpm.
Having to lock it out or having a big drop in rpm when in gear might be indicative something else is causing issues. Maybe a vacuum leak.
Posted By: mopfried
Re: Ok. I need help tuning my carb. - 05/02/14 03:31 AM
First off, no big drop in RPM when its in gear. I have made progress though. I set the advance to 10 deg in the dist just to start with tonight. I brought the base timing down so I have 34 deg total advance. I know that sounds bass akwards but I have had problems with pinging so bad.... I had to open the idle speed screws (front and back) about 1/8 turn to get it to not stall at idle. The vacuum gauge now reads at about 11-12" at idle. I put it in gear with the e brake on and set the 4 corner mixture screws to have 14.2-14-5 AFR at idle.. I have tried this all before and the mixture screws had no effect. Maybe an air bleed was plugged? I could move all 4 less that 1/8 turn and see a difference on the wide band gauge. I took it for a 10 mile ride. It runs pretty good, no pinging and a good stable idle. I need to take some more timing out at idle. WhenI shut it off it didn't want to crank. The AFR seemed to run 12.8-13.2 cursing 35 to 55 mph. I wont have a chance to get back to it until Monday. Thanks for all your help... When I get the intial down where it should be and still be able to make the mixture screws make a difference I will move on to the next step. Im sure the secondary jets are not 93 like it came with.
Posted By: mopfried
Re: Ok. I need help tuning my carb. - 05/02/14 03:36 AM
Quote:
Quote:
""When I put it in gear it is in the 11-13 range""
- How much timing do you have in D with the brake applied? I am curious if it drops in this situation.
- What is the A/F reading in D with the brake applied?
- How much of an rpm drop do you have in D with the brake applied.
With an auto street car, there is some tuning that needs to be done in the above situation.
I don't have a tach. It doesn't drop in rpm very much, the converter is pretty loose. I meant when I'm driving it just off idle it is in the 11-13 range.
The timing didnt change with the dropping it in gear. I dont think the springs are that loose.
Posted By: mopfried
Re: Ok. I need help tuning my carb. - 05/02/14 03:48 AM
Quote:
I would set the initial as you described get max vac then retard timing till the vac drops 1 in hg all the while keep reducing the idle speed back to a reasonable in gear idle speed. Then adj the slots to get 36 total then mix/match springs staying just under the pinging point at WOT up thru the gears on your hottest/driest day with the advance starting no less than 200 RPM above the in gear idle speed. Then start on the carb: idle then "cruise" then PV ratio/tip in point (in hg) then secondaries (tip in/sec jetting. I believe on the secondaries (double check on this) that you want a smooth transition when it comes in (you cant tell when it opens) and jet for max horsepower (dyno) or max MPH (strip)
I thoght I had this all set good last summer... but something was missing. The carb was only opening half way. Wnen I found that out it ran better but thats when the rattle started... wnen the seconcarys opened