Dbl Pumper/Tuning - Step In
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,456
Likes: 102
From: Piedmont Va
St. Jude Donor '11-'12-'13,'19-'20
Dbl Pumper/Tuning - Step In
Greetings all,
I have a 73 coupe with a custom built 383 and TKO-600. Currently have a Dragon Fire HEI with medium springs. Timing is all in @3200.
The carb is a Holley Ultra 750 DP w 4 corner idle. I can get this thing to start, idle and run excellent with the following anomaly;
During medium acceleration, when transitioning from primary to secondary, there is vibration and strange harmonics coming from the motor. Light acceleration and cruising on the highways within the primaries the thing is smooth as glass. Go to WOT, and everything is good. This occurs as the secondaries are transitioning in, blades partially opening, with the initial pump shot. Once I get beyond that it smooths out.
My theory right now is this requires some tuning on the secondaries, but I'm not sure where to start. Would this symptoms indicate too big a pump shot maybe? That is the area I was going to start, but dont want to shoot from the hip.
Could this be more of a timing curve issue? I'm thinking not, as it seems to be more affected by throttle position.
Thanks in advance for thoughts and guidance.
I have a 73 coupe with a custom built 383 and TKO-600. Currently have a Dragon Fire HEI with medium springs. Timing is all in @3200.
The carb is a Holley Ultra 750 DP w 4 corner idle. I can get this thing to start, idle and run excellent with the following anomaly;
During medium acceleration, when transitioning from primary to secondary, there is vibration and strange harmonics coming from the motor. Light acceleration and cruising on the highways within the primaries the thing is smooth as glass. Go to WOT, and everything is good. This occurs as the secondaries are transitioning in, blades partially opening, with the initial pump shot. Once I get beyond that it smooths out.
My theory right now is this requires some tuning on the secondaries, but I'm not sure where to start. Would this symptoms indicate too big a pump shot maybe? That is the area I was going to start, but dont want to shoot from the hip.
Could this be more of a timing curve issue? I'm thinking not, as it seems to be more affected by throttle position.
Thanks in advance for thoughts and guidance.
Popular Reply
Apr 28, 2016, 10:06 AM
Le Mans Master
Joined: Jan 2006
Posts: 6,506
Likes: 139
From: Butler Pa
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
Greetings all,
I have a 73 coupe with a custom built 383 and TKO-600. Currently have a Dragon Fire HEI with medium springs. Timing is all in @3200.
The carb is a Holley Ultra 750 DP w 4 corner idle. I can get this thing to start, idle and run excellent with the following anomaly;
During medium acceleration, when transitioning from primary to secondary, there is vibration and strange harmonics coming from the motor. Light acceleration and cruising on the highways within the primaries the thing is smooth as glass. Go to WOT, and everything is good. This occurs as the secondaries are transitioning in, blades partially opening, with the initial pump shot. Once I get beyond that it smooths out.
My theory right now is this requires some tuning on the secondaries, but I'm not sure where to start. Would this symptoms indicate too big a pump shot maybe? That is the area I was going to start, but dont want to shoot from the hip.
Could this be more of a timing curve issue? I'm thinking not, as it seems to be more affected by throttle position.
Thanks in advance for thoughts and guidance.
I have a 73 coupe with a custom built 383 and TKO-600. Currently have a Dragon Fire HEI with medium springs. Timing is all in @3200.
The carb is a Holley Ultra 750 DP w 4 corner idle. I can get this thing to start, idle and run excellent with the following anomaly;
During medium acceleration, when transitioning from primary to secondary, there is vibration and strange harmonics coming from the motor. Light acceleration and cruising on the highways within the primaries the thing is smooth as glass. Go to WOT, and everything is good. This occurs as the secondaries are transitioning in, blades partially opening, with the initial pump shot. Once I get beyond that it smooths out.
My theory right now is this requires some tuning on the secondaries, but I'm not sure where to start. Would this symptoms indicate too big a pump shot maybe? That is the area I was going to start, but dont want to shoot from the hip.
Could this be more of a timing curve issue? I'm thinking not, as it seems to be more affected by throttle position.
Thanks in advance for thoughts and guidance.
Le Mans Master
Joined: Mar 2001
Posts: 5,738
Likes: 644
From: Lincoln, CA
Even without a wide band I find this an easy to follow guide with just a Vacuum gauge.
http://www.burtonmachine.com/tech
http://www.burtonmachine.com/tech
Race Director
Joined: May 2006
Posts: 18,740
Likes: 2,583
If the carburetor is a 76750 list# the power valve is a 6.5 according to Holley. I would start with checking if the secondary pump spring is correctly adjusted. With the throttle held wide open a .020 feeler gauge should be able to be inserted between the pump adjustment screw and the pump arm.
Last edited by MelWff; Apr 26, 2016 at 02:20 PM.
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,456
Likes: 102
From: Piedmont Va
St. Jude Donor '11-'12-'13,'19-'20
Thanks gents! I am pretty sure the PV in the primary is 7.5. Vacuum is 15* @ 900 RPM which is what the cam card calls for. Im not sure about the exact PN of the carb nor the component sizes in the secondaries. I'll pull the carb hopefully tomorrow evening and report all sizings, and check the lash on the pump spring as well.
I gave the article a quick read. Unfortunately I'm not set up for wideband at the moment but will try and follow along with the vac gauge. May have to go there before it's over though.
ps: What am I missing with TBTR or MO? Geniuses I take?
I gave the article a quick read. Unfortunately I'm not set up for wideband at the moment but will try and follow along with the vac gauge. May have to go there before it's over though.
ps: What am I missing with TBTR or MO? Geniuses I take?
Last edited by CheezMoe; Apr 26, 2016 at 02:26 PM.
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Race Director
Joined: May 2006
Posts: 18,740
Likes: 2,583
Carburetor number
Thanks gents! I am pretty sure the PV in the primary is 7.5. Vacuum is 15* @ 900 RPM which is what the cam card calls for. Im not sure about the exact PN of the carb nor the component sizes in the secondaries. I'll pull the carb hopefully tomorrow evening and report all sizings, and check the lash on the pump spring as well.
I gave the article a quick read. Unfortunately I'm not set up for wideband at the moment but will try and follow along with the vac gauge. May have to go there before it's over though.
ps: What am I missing with TBTR or MO? Geniuses I take?
I gave the article a quick read. Unfortunately I'm not set up for wideband at the moment but will try and follow along with the vac gauge. May have to go there before it's over though.
ps: What am I missing with TBTR or MO? Geniuses I take?
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
Thanks gents! I am pretty sure the PV in the primary is 7.5. Vacuum is 15* @ 900 RPM which is what the cam card calls for. Im not sure about the exact PN of the carb nor the component sizes in the secondaries. I'll pull the carb hopefully tomorrow evening and report all sizings, and check the lash on the pump spring as well.
I gave the article a quick read. Unfortunately I'm not set up for wideband at the moment but will try and follow along with the vac gauge. May have to go there before it's over though.
ps: What am I missing with TBTR or MO? Geniuses I take?
I gave the article a quick read. Unfortunately I'm not set up for wideband at the moment but will try and follow along with the vac gauge. May have to go there before it's over though.
ps: What am I missing with TBTR or MO? Geniuses I take?
Instructor
Joined: Apr 2016
Posts: 104
Likes: 7
From: Denver Colorado
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
It just needs some tuning. This is what you hear when some one can't figure out how to tune the carb to the engine, i know Fish has got it figured out and maybe ya'll say this cause you figure that tuning it to the engine is over the OP's head, but I think that sure there might be better suited carb sizes, but there nothing wrong with a 750 double pumper on a stock 350" as long as it's tuned correctly. I can't recall ever running a secondary power valve and the secondary tip in having a slight stumble and surge is an indication that it's lean when the secondaries start to open, two causes for a double pumper are missing adjusted accelerator pump/wrong size squirter or power valve wrong in size, it's free to check the adjustment, cheap to plug the power valve and jet up and not that expensive to buy an assortment of squirters and experiment to see which responds better.
Last edited by bluedawg; Apr 27, 2016 at 08:07 AM.
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,456
Likes: 102
From: Piedmont Va
St. Jude Donor '11-'12-'13,'19-'20
It just needs some tuning. This is what you hear when some one can't figure out how to tune the carb to the engine, i know Fish has got it figured out and maybe ya'll say this cause you figure that tuning it to the engine is over the OP's head, but I think that sure there might be better suited carb sizes, but there nothing wrong with a 750 double pumper on a stock 350" as long as it's tuned correctly. I can't recall ever running a secondary power valve and the secondary tip in having a slight stumble and surge is an indication that it's lean when the secondaries start to open, two causes for a double pumper are missing adjusted accelerator pump/wrong size squirter or power valve wrong in size, it's free to check the adjustment, cheap to plug the power valve and jet up and not that expensive to buy an assortment of squirters and experiment to see which responds better.
The motor spec:
350 block 4.040
Scat 9000 3.75
Scat I beam 5.7 rods
Diamond forged flat top piston -5cc
Howards Retrofit Roller .510/234 .530/238
RHS 72/200 Aluminum Heads
Dart dual plane intake
Dragon Fire HEI
Static Compression 10:1
DCR 8:1
Thanks for the input to date.
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,456
Likes: 102
From: Piedmont Va
St. Jude Donor '11-'12-'13,'19-'20
I was warned, and you are officially now ignored noob!
Last edited by CheezMoe; Apr 27, 2016 at 11:20 AM.
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
Ok too broke, are we going done this road again. Like I said, sure there are better sized carbs, but he's got a 750 double pumper which can be tuned to run good with his engine.
Le Mans Master
Joined: Jan 2006
Posts: 6,506
Likes: 139
From: Butler Pa