Stumble off acceleration

[Bobcherry13]

I have a 69' GMC c-10 with a crate 350 (not sure on how exactly how crate it is, did not do any "engine" work to it as of yet do to just bought it), edelbrock performer manifold, holley 700 cfm double pumper (mechanical) with the choke pulled out, new Mallory ingnition, not sure of cam specs but is a mild cam. the carb has 31 and 28 sized accelerator pump discharge nozzles in it. stock fuel pump. slammed to the ground with 31" by 18.5" Hossier radials on the back. upon stomping on the gas the truck stumbles, then picks up and accelerates pretty good. everyone says that the 700 is too big of a carb but I am planning on putting new smaller cc heads on in the future. also, I would think the engine is getting too much gas but the tuning book says the accelerator pump discharge nozzles may be too small and not providing enough gas. another guy said it could be the power valves. I hate carb tuning. please give me suggestions, I was just going to buy two 35 nozzles and try that.

[Bobcherry13]

Either this is a very elementary question that I should have not asked or I guess no one can answer it or I answered mt own question with the bigger accelerator nozzles.

[LONGHAIR]

This is tough to diagnose....either can cause a stumble.
I would "guess" rich because of the double pumper. A 700 double pumper is too much for a "mild cammed" 350 as it comes "out of the box". It can probably be made to work, but a smaller vacuum secondary carb will suit you better.

[turbohacker]

I dont know what to tell you, i am having the exact same problom in mine with an edelbrock 650 carb on a mild 350 with double hump 202 heads, i was going to rebuild an old holly just like yours.

[WorkinLonghorn]

I would try the bigger nozzles but I wanted mention also that I have seen this happen when a guy removes the thermostat,has no exhaust heat crossover in the mani, and runs around with a cold engine.I see you removed the choke,did you also remove the thermostat?

I had mine 383 tuned on the chassis dyno, it has a mild rv cam, 600 holley. I can get all the power out of the engine with a 600, it did 210 hp at the wheels at 5000 rpm.
i think you got too big of carb for that engine. Try to find a speed shop with a chassis dyno, and they will tune it right on for about $300.

[Sinisterspeed]

I have this same problem in my 57. It has a pretty much stock SBC400 and the PO installed a carter 750 on it. It runs just fine but get REALLY crappy mileage and stumbles when you first put your foot in it. I was planning on installing a 600 cfm to cure the problem since in my case I know it's plane and simple to much fuel at once.

[hotrod 80]

Typically a stumble is a lean spot in the transition from the idle circuit to the primary jets . try the bigger squirters . Also make sure the accelerator pumps begin to squirt the instant you move the throttle . Also solved this problem on a 650 DP by installing Jet extensions in the rear bowl . What jets are in it ? Check the vacuum and make sure it doesn't have a power valve for a low vacuum car in it , Like a 6.5 " power valve with 16" of idle vacuum .

[Slammed70]

How come no one has said timing yet?

[LONGHAIR]

How come no one has said timing yet?

'cause that's an even bigger can of worms.........LOL

[Bobcherry13]

The front nozzle is a 31 and the rear is a 28. I bought two 35's to go in but still need to do it (working late). I had a "mechanic" set the timimg, but did not mess with the advance in the distributor. I did not remove the thermo after taking the choke off. I really appreciate (spelling) all of the help. I plan on putting the nozzels on today and I will update the results.

[piecesparts]

Originally Posted by Bobcherry13

I have a 69' GMC c-10 with a crate 350 (not sure on how exactly how crate it is, did not do any "engine" work to it as of yet do to just bought it), edelbrock performer manifold, holley 700 cfm double pumper (mechanical) with the choke pulled out, new Mallory ingnition, not sure of cam specs but is a mild cam. the carb has 31 and 28 sized accelerator pump discharge nozzles in it. stock fuel pump. slammed to the ground with 31" by 18.5" Hossier radials on the back. upon stomping on the gas the truck stumbles, then picks up and accelerates pretty good. everyone says that the 700 is too big of a carb but I am planning on putting new smaller cc heads on in the future. also, I would think the engine is getting too much gas but the tuning book says the accelerator pump discharge nozzles may be too small and not providing enough gas. another guy said it could be the power valves. I hate carb tuning. please give me suggestions, I was just going to buy two 35 nozzles and try that.

As you may have surmised from the replies, there are a lot of things that can affect your engine and the resultant stumble from the acceleration. I can relate to you some things that I found in my efforts to set up my latest engine build.
First of all, you say a mild cam--do you have a good lope to that cam or is it an easy idle at a stop (vehicle in gear)? The reason I ask, You didn't state if you are running an automatic transmission. I would guess that you are, here goes. Do you have to set the idle high enough to keep it running while in "Drive" at a stop? Meaning that the idle is setting at 900 to 1000 RPMs. This would mean that you are already transitioning out of your "Idle Cicuit" and going into the "Power Circuit" of operation.

Second item: The 700 CFM carb is probalby too large as stated for the weight of your truck. The principal is to NOT over carb and still get performance. The 700 would work quite well on a lighter vehicle, like a Camaro, but will cause a bogging or slump in performance on a truck. I noted this when I went to a 750 carb on my stroker motor. The idea is to keep the airflow at the higher numbers that is needed to keep the engine performing efficiently. A 650 or even a 600 CFM carb would serve your needs better---BUT you would loose some of your top end performance in the long run.

Third, I would look at tuning the carb to a better setup on the jets and the squirters.

Fourth I would look at advancing the timing to a good number that will let you accelerate without dumping off the line. I run my engine at 35 degrees TOTAL advance and it works well for me.

[LONGHAIR]

Do you have to set the idle high enough to keep it running while in "Drive" at a stop? Meaning that the idle is setting at 900 to 1000 RPMs.

If you have a big drop in idle RPM when you put the transmission in gear, your torque convertor is too tight. You need a higher stall convertor, but your cam company should tell you that.....

[piecesparts]

I got side tracked and did not finish above. If your cam is causing you to pull at a stop light, then a stall convertor will definitely help you. However, your stumble very well could be the transition circuit on the carb and the accelerator pump or squirter size. I would eleiminate all of the timing issues and tune the carb, before going to the torque convertor. I had to go to a 2400 stall for the cam that I have, but you may do it with a lower stall.

[tim21391]

i had a bad problem like this....i needed new plugs and plug wires...new points and condensor and distributor cap....solved my problem

[Bobcherry13]

Thanks to all. Finally got off my lazy ass (plus it was 100+ in South Carolina the past few days) and put the 35 nozzle in the front squirter and the 31 on the rear. The stumble is all but gone. I wish I could answer you guys on what type of cam lobe grind I have and such, but I am having a hard time getting in contact with the guy I bought the truck from. I guess the only way to know what I got is to put a new cam in and pull the heads to see if he did any head work. The guy said the engine was a crate engine, but did not specify wheather or not it was a long or short block. I am new at truck performance, began a motorcycle guy (part of The Jockey Journal). The truck has a turbo 400, and a 10 bolt rear-end (I know I already bought a 9" I have to cut down). The rear-end is a limited slip with 31x18.5 inch Hossier street radials. The truck will light "one" up in an instant (and hold it for as long as you got the gas down) and chirp both really good going into second. Without spending $300 on a dyno, would you guys say he may have had some head work done or is this common for this type of set up.

[piecesparts]

Originally Posted by Bobcherry13

Thanks to all. Finally got off my lazy ass (plus it was 100+ in South Carolina the past few days) and put the 35 nozzle in the front squirter and the 31 on the rear. The stumble is all but gone. I wish I could answer you guys on what type of cam lobe grind I have and such, but I am having a hard time getting in contact with the guy I bought the truck from. I guess the only way to know what I got is to put a new cam in and pull the heads to see if he did any head work. The guy said the engine was a crate engine, but did not specify wheather or not it was a long or short block. I am new at truck performance, began a motorcycle guy (part of The Jockey Journal). The truck has a turbo 400, and a 10 bolt rear-end (I know I already bought a 9" I have to cut down). The rear-end is a limited slip with 31x18.5 inch Hossier street radials. The truck will light "one" up in an instant (and hold it for as long as you got the gas down) and chirp both really good going into second. Without spending $300 on a dyno, would you guys say he may have had some head work done or is this common for this type of set up.

You have just told me, in no uncertain words, that you may be definitely over carbed. First off the carb that you have performs really well at higher RPMs and your discussions on the daily driving talked about a BOG. So that tells me that you have helped by changing the internals, but your engine still has a little slump to it. The bigger carb will require a good air signal for performance, so the increase in the RPMs makes that signal stand out. If you take your heads off, after a period of time you will find that there will be deposits on the intake valves that is related to the LAZY air charge that you are getting and the overgassing of each cylinder. That is what happens if you over carburate your engine. Unless you plan to drive around above 3000 RPM, all day, then I would look to a smaller CFM rating and give up the TOP END reponse for the economy and lower end torque. You can confirm this by contacting ANY carburetor manufacturer's tech department and they will give you input as to what they see as a possible success path here.

Yes, it appears that you have a great "Peg Leg" tire burn and the cam and heads are the items that help in this. As for head work done, MAYBE, but many Crate Motors have a good solid flow design in their ports and it is possible that they came that way. A dyno around here runs $50.00 for two pulls and then more for tuning runs.

[1969-08 chevy]

start with easy stuff, like looking at the spark plugs to see what kind of burn you have, and the fuel bowl float level.