Speed demon 750 Carb on a 428 Cobrajet??
07-13-2007, 12:41 AM
#1
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Speed demon 750 Carb on a 428 Cobrajet??
Got my car back had the engine rebuilt, Crower cam, edelbrock performer intake, and I put on a Speed Demon 750 Carburator, I am getting hesitation and the engine seems to be stumbling, never really making power, just stumbling. Tried all kinds of adjustments nothing works! I am thinking that I need to rejet?? but not sure which size jets to use on it? Anyone have any advice on such an application? or is rejetting the total wrong direction?
07-13-2007, 01:53 AM
#4
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RE: Speed demon 750 Carb on a 428 Cobrajet??
A 750 will work on a 428, but it's not ideal for a mild streetbuild. Sounds like it needs some serious rejetting and accelerator pump circuit tuning. A smaller carb will help too, if it has a Performer intake, it's prolly got a mild cam, so something in the 650-700 range might work a bit better, like a 670, maybe a 700. But that sounds like the carb just needs to be dialed in to that engine...."out of the box" is a relative term, and subject to a great deal of interpretation
07-13-2007, 08:30 AM
#5
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RE: Speed demon 750 Carb on a 428 Cobrajet??
The stock carburetor was a 735 CFM Holley with vacuum secondaries. According to the selection chart on the Barry Grant web site, a 750 should work. It sounds like it needs some tuning. Have you contacted Barry Grant or checked his web site for help?
07-13-2007, 09:12 AM
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RE: Speed demon 750 Carb on a 428 Cobrajet??
Stumble on acceleration isoften a situation of accelerator pump failure/misadjustment. If the carb is new, I'd say it needs adjusted. A very common mistake is an accelerator pump stroke which is out of sync with the butterflys in the carb. What you get is a temporary ( millisecond ) lean burn condition followed by a similar rich condition. Think of it like this.....
All carburetors depend on a fairly precise mixture of fuel and air, we all know that part. What many people don't realize is that if you give the engine throttle and open the venturis to allow more air, the fuel flow MUST correspond with thisincreased air flow. That's why they are called accelerator pumps. When you need to accelerate, you mash the throttle, the venturis open and the accelerator pump shoots in an additional 30-50 cc's of fuel. ANY hesitation atany step in the process will yield unsatisfactory performance.
Is the 750 to much carb ? Maybe but probably not. I suspect the problem may lie in the type of secondarys. While it is true that these engines used a 735 CFM with vac secondarys, I am assuming that you are using a mechanical secondary unit. With a vac secondary, the engine vacpulls the fuel in as the engine picks up RPM. With a mechanical secondary carb, you dump fuel into the engine whether it's ready or not. If the RPM's are down and you dump excessive fuel into the engine, it's going to sputter and stumble because you have entered a " fuel rich " condition.
One last thought. Jetting is also a very common mistake. Consult the supplier of the carb and see what he says on jet size. It will vary slightly depending on engine specs, altitude, barometric pressure, temp etc ... If you have to error one way or the other, go rich and check your plugs periodically to slowly lean it back down. Lean burn conditions will yield in a devastating effect on your engines internal components. Ever seen what happens to an aluminum piston or iron block/head under lean conditions ? It's very impressive but expensive. Whatever you do make one adjustment at a time. Making a whole bunch of adjustments simultaneously leaves you no way of telling what works and what doesn't.
JMHO from 30 + years of wrenching.
Good luck, Dean
All carburetors depend on a fairly precise mixture of fuel and air, we all know that part. What many people don't realize is that if you give the engine throttle and open the venturis to allow more air, the fuel flow MUST correspond with thisincreased air flow. That's why they are called accelerator pumps. When you need to accelerate, you mash the throttle, the venturis open and the accelerator pump shoots in an additional 30-50 cc's of fuel. ANY hesitation atany step in the process will yield unsatisfactory performance.
Is the 750 to much carb ? Maybe but probably not. I suspect the problem may lie in the type of secondarys. While it is true that these engines used a 735 CFM with vac secondarys, I am assuming that you are using a mechanical secondary unit. With a vac secondary, the engine vacpulls the fuel in as the engine picks up RPM. With a mechanical secondary carb, you dump fuel into the engine whether it's ready or not. If the RPM's are down and you dump excessive fuel into the engine, it's going to sputter and stumble because you have entered a " fuel rich " condition.
One last thought. Jetting is also a very common mistake. Consult the supplier of the carb and see what he says on jet size. It will vary slightly depending on engine specs, altitude, barometric pressure, temp etc ... If you have to error one way or the other, go rich and check your plugs periodically to slowly lean it back down. Lean burn conditions will yield in a devastating effect on your engines internal components. Ever seen what happens to an aluminum piston or iron block/head under lean conditions ? It's very impressive but expensive. Whatever you do make one adjustment at a time. Making a whole bunch of adjustments simultaneously leaves you no way of telling what works and what doesn't.
JMHO from 30 + years of wrenching.
Good luck, Dean
07-13-2007, 10:39 AM
#7
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RE: Speed demon 750 Carb on a 428 Cobrajet??
Thanks, this is very good information, I knew some of this but there is still alot I learned!
ORIGINAL: RSRXION
Stumble on acceleration isoften a situation of accelerator pump failure/misadjustment. If the carb is new, I'd say it needs adjusted. A very common mistake is an accelerator pump stroke which is out of sync with the butterflys in the carb. What you get is a temporary ( millisecond ) lean burn condition followed by a similar rich condition. Think of it like this.....
All carburetors depend on a fairly precise mixture of fuel and air, we all know that part. What many people don't realize is that if you give the engine throttle and open the venturis to allow more air, the fuel flow MUST correspond with thisincreased air flow. That's why they are called accelerator pumps. When you need to accelerate, you mash the throttle, the venturis open and the accelerator pump shoots in an additional 30-50 cc's of fuel. ANY hesitation atany step in the process will yield unsatisfactory performance.
Is the 750 to much carb ? Maybe but probably not. I suspect the problem may lie in the type of secondarys. While it is true that these engines used a 735 CFM with vac secondarys, I am assuming that you are using a mechanical secondary unit. With a vac secondary, the engine vacpulls the fuel in as the engine picks up RPM. With a mechanical secondary carb, you dump fuel into the engine whether it's ready or not. If the RPM's are down and you dump excessive fuel into the engine, it's going to sputter and stumble because you have entered a " fuel rich " condition.
One last thought. Jetting is also a very common mistake. Consult the supplier of the carb and see what he says on jet size. It will vary slightly depending on engine specs, altitude, barometric pressure, temp etc ... If you have to error one way or the other, go rich and check your plugs periodically to slowly lean it back down. Lean burn conditions will yield in a devastating effect on your engines internal components. Ever seen what happens to an aluminum piston or iron block/head under lean conditions ? It's very impressive but expensive. Whatever you do make one adjustment at a time. Making a whole bunch of adjustments simultaneously leaves you no way of telling what works and what doesn't.
JMHO from 30 + years of wrenching.
Good luck, Dean
Stumble on acceleration isoften a situation of accelerator pump failure/misadjustment. If the carb is new, I'd say it needs adjusted. A very common mistake is an accelerator pump stroke which is out of sync with the butterflys in the carb. What you get is a temporary ( millisecond ) lean burn condition followed by a similar rich condition. Think of it like this.....
All carburetors depend on a fairly precise mixture of fuel and air, we all know that part. What many people don't realize is that if you give the engine throttle and open the venturis to allow more air, the fuel flow MUST correspond with thisincreased air flow. That's why they are called accelerator pumps. When you need to accelerate, you mash the throttle, the venturis open and the accelerator pump shoots in an additional 30-50 cc's of fuel. ANY hesitation atany step in the process will yield unsatisfactory performance.
Is the 750 to much carb ? Maybe but probably not. I suspect the problem may lie in the type of secondarys. While it is true that these engines used a 735 CFM with vac secondarys, I am assuming that you are using a mechanical secondary unit. With a vac secondary, the engine vacpulls the fuel in as the engine picks up RPM. With a mechanical secondary carb, you dump fuel into the engine whether it's ready or not. If the RPM's are down and you dump excessive fuel into the engine, it's going to sputter and stumble because you have entered a " fuel rich " condition.
One last thought. Jetting is also a very common mistake. Consult the supplier of the carb and see what he says on jet size. It will vary slightly depending on engine specs, altitude, barometric pressure, temp etc ... If you have to error one way or the other, go rich and check your plugs periodically to slowly lean it back down. Lean burn conditions will yield in a devastating effect on your engines internal components. Ever seen what happens to an aluminum piston or iron block/head under lean conditions ? It's very impressive but expensive. Whatever you do make one adjustment at a time. Making a whole bunch of adjustments simultaneously leaves you no way of telling what works and what doesn't.
JMHO from 30 + years of wrenching.
Good luck, Dean
07-13-2007, 01:37 PM
#8
6th Gear Member
Join Date: Apr 2007
Location: California
Posts: 10,468
RE: Speed demon 750 Carb on a 428 Cobrajet??
A lot of carb tuning is trial and error. And yeah, adjust ONLY 1 thing at a time. See what it does, better or worse, then adjust something else, better or worse and so on and so forth. It takes time to get a carb dialed in right for a street car
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