need advice on what could cause this slight engine problem

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71dustar

71dustar

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Whenever im cruising and i hit the gas to like wot the car hesitates then pulls. Curious as to if i'm running too rich/lean or if its just because when i go wot instead of gradually going wot that the fuel is dumping into carb faster than the air causing the hesitation.
 
if it cuts out and then go's....it's lean on the pump shot.

if it bogs then go's....it's too fat/rich on the pump shot.

What carburetor?

It's a pump shot problem
 
1wild&crazyguy said:
if it cuts out and then go's....it's lean on the pump shot.

if it bogs then go's....it's too fat/rich on the pump shot.

What carburetor?

It's a pump shot problem
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Maybe, maybe not. If it's a vacuum secondary Holley or T quad, etc, could be the secondaries are popping open too fast Could be the carb is WAY to big for the engine, and a few other things.

We need more information:

Engine, carb cam, timing, etc. Did it EVER run right, and therefore "just started", or is this a new build or maybe you just changed some things?
 
IMO, check the pump shot first. Wild&Crazy has good diagnosis.

67Dart273 is right in saying that premature opening of the secondary will also cause the engine to stumble when jumped on. However, it will let you sneak up on WOT. There is a certain carb that earned the handle of Thermobog due to this trait.

If this is an off-the-shelf carb that is generically tuned or tuned for a car other than the one under consideration, check jetting and general tuning of the carb.
 
Try the quick double pump when going to WOT. A quick short stab of the pedal, let the throttle close, the go to WOT. Do this as quick as possible. Does it still stumble?

If the pump shot is lean, this very quick stab nearly doubles the volume of gas from the accelerator pump and if it gets rid of the stumble or reduces it, You just proved that the pump shot is too lean.

Remember, these carbs were designed to run on 100% gasoline....not the 10% ethenol that we have now. You need more volume of fuel when using ethanol.
 
I'd also put a vote down for the pump shot, but what are the spec of the engine?
 
Sorry for not giving the engine combo setup. i thought it was a simple general question.

This is a new engine build. took a 5.9 mag and stroked to a 408. eddy alum heads, 241/247 duration 750ccfm holley 4160 that has stock primary jets with #80 jets in back instead of meter plate with jet extenders. motor has about 500 miles on it. Reason why i went with #80 jets in secondaries is because when i would try to pull in 3rd my engine would fall on its face at 4000rpm. so i tried to richen up the mixture plus the extenders and it pulls great now.
 
71dustar said:
Sorry for not giving the engine combo setup. i thought it was a simple general question.

This is a new engine build. took a 5.9 mag and stroked to a 408. eddy alum heads, 241/247 duration 750ccfm holley 4160 that has stock primary jets with #80 jets in back instead of meter plate with jet extenders. motor has about 500 miles on it. Reason why i went with #80 jets in secondaries is because when i would try to pull in 3rd my engine would fall on its face at 4000rpm. so i tried to richen up the mixture plus the extenders and it pulls great now.
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Try changing out the secondary vacuum spring, Holley makes a spring kit for tuning the timing of the secondary. While your at it get the quick change top cover, it makes it easier to swap springs. If it has the bare color (Factory) spring it now, it will open really slow, and maybe not fully, depending on vacuum. My WAG is start with a yellow spring!
 
1st I hope the timing is dialed in decent.
the jetting sounds good, however...even if the vac sec was not opening, it should not fall on it's face, maybe run out of rpm for lack of flow/cfm, but not drop out and fall on it's face.

And how I would tune the vac sec springs...is put the weakest spring in , that should get you a bog when getting into the secondaries..now you just put a stiffer springs in till it doesn't bog, you want to use the spring just before the one that made it bog.

really though....what shooter is on it, whats the number stamped on it?
 
The engine's way lean. Without worrying about the timing curve (which I agree is important, but not posted) I can tell you the 750 is out of the box way lean and you should be able to run any spring int he secondary and a 4" arm engine won't stumble. So without knowing the timing, I'd say...
- make sure the timing is at least 12° initial, preferably more like 18°, but you don't want more than 32° total.
- Up the primary jets to 78s to start, and I think you'll end up at 80s in the front too.
- Make sure the power valve is where it needs to be. The 6.5 supplied in the carb is probably too low depending on the camshaft and compression. So get a vacuum gage at the parts store and see what you are pulling for vaccum in drive at idle. Then get a power valve rated 1.5" lower.
- Up the secondary jets to 82 and see what it does witht he rest of the changes.
It should pull cleanly to around 5500 if you have the cam and heads for the stroke.
 
71dustar said:
Sorry for not giving the engine combo setup. i thought it was a simple general question.

This is a new engine build. took a 5.9 mag and stroked to a 408. eddy alum heads, 241/247 duration 750ccfm holley 4160 that has stock primary jets with #80 jets in back instead of meter plate with jet extenders. motor has about 500 miles on it. Reason why i went with #80 jets in secondaries is because when i would try to pull in 3rd my engine would fall on its face at 4000rpm. so i tried to richen up the mixture plus the extenders and it pulls great now.
Click to expand...

I hate to tell you this but, with that set-up the 3310 has business being on there, unless your a Guru at tuneing in carbs, you'll never get the metering right, its not all about pump shot & jetting, its in the "metering", those 3310s are/were designed for mild applications, they have what you would call a "conservative" metering system, they were not metered for perf. compaired to the DPers, a 950 HP with 4 corner idleing is about as good as it can get on those SB strokers, in the long run you'll be much happier, hell even a 750 HP would do better, Now don't get me wrong, the 3310 is & always will be a VG carb, just on the right appl., like a mildly cammed 360/383/440 & bolt-ons. Thats just my thoughts.

Just make sure the rest of the tune/timing curve is where it needs to be before blameing the carb, thats on any engine.
 
The timing was set for 34degrees total but not sure what the initial timing is. I'll check all those things out when i get some time.
 
the cam you have in yer stroker isn't that big, maybe if it was in a 340/360 it would seem bigger but not in yer cid bracket.

do like I said above.

on the jetting for the primary just jet down till you get a lean surge at cruise 40-60mph, then go back up 2 jet sizes, then go about 8 jets bigger than the primary jet size in the secondary.
The weakest spring in the vac sec and see if it bogs, on the stroker it might not...

then don't forget to put a power valve rating in that is half the vac reading in gear at idle.

thats how you can make WHAT YOU HAVE work good.
 
thanks for all the input guys. I plan on going to the track soon and see what this baby can run. I know there is a lot that can be tweaked to get the optimum performance out this motor itll just take me time to do it.
 
So i hooked a vacuum gauge up to the motor tonight. IN park im pulling 11 and in first im pulling 5 (both are at idle and the car is stationary) so do i need a 3.5" power valve?
 
well i changed the power valve out from the 6.5 to the 2.5 and i also installed the weakest spring for the secondaries.

I read through the piece of paper supplied with the springs about if the engine bogs or has a flat spot and seem that the engine has a slight bog but the hesitation has improved. the original spring was black (heaviest). I installed the next stiffer spring up from the weakest one and will test the car tomorrow.
 
71dustar said:
well i changed the power valve out from the 6.5 to the 2.5 and i also installed the weakest spring for the secondaries.

I read through the piece of paper supplied with the springs about if the engine bogs or has a flat spot and seem that the engine has a slight bog but the hesitation has improved. the original spring was black (heaviest). I installed the next stiffer spring up from the weakest one and will test the car tomorrow.
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cool, now if when getting into the secondaries it bogs, go another stiffer.

if it's hesitating right out of the hole, start looking at the shooter/pump shot.
 
I would be wondering why it only has 5" of vacuum at idle in gear. It should have 7-8 in gear pretty easy with the 4" crank. Check on the timing setup.
 
Ill check initial timing again and see what it's set at. I keep forgetting and never write it down.

Now, how would i go about tuning the pump shot?

Also how do you tune the spring on the pump shot since im sure there has to be a certain preload or whatever?
 
You can tune the acc pump two ways:
You can change the squirtor which will either make it spray more for less time, or less over more time.
Or... You can change the pump cam on the throttle lever to reduce total amount of fuel and the timining in relation to the throttle movement.
 
if I'm cruising at city speeds like 30mph and i step into the throttle to wot i get a slight bog but when im cruising at 50mph or higher and i do the same i dont get a bog and its very smooth.

So i'm thinking i need to change the pump shot but not sure exactly which way to go with it (sooner vs later or more vs less)
 
71dustar said:
if I'm cruising at city speeds like 30mph and i step into the throttle to wot i get a slight bog but when im cruising at 50mph or higher and i do the same i dont get a bog and its very smooth.

So i'm thinking i need to change the pump shot but not sure exactly which way to go with it (sooner vs later or more vs less)
Click to expand...

Buy the Blue cam and use it. Keep the squirters small if you can, tune with the cam first. You'll have to tune to the spring as well. Try one thing at a time.
I would have gone to a 3.5 PV personally. It sounds like you have the idle circuit lean...maybe 1 or 1.5 turns out...try 2 turns out and bump the idle up or at least 1000rpm and see if that helps.
Brian
 
71dustar said:
if I'm cruising at city speeds like 30mph and i step into the throttle to wot i get a slight bog but when im cruising at 50mph or higher and i do the same i dont get a bog and its very smooth.

So i'm thinking i need to change the pump shot but not sure exactly which way to go with it (sooner vs later or more vs less)
Click to expand...

99% of the time flat spots mean lean.

Go up one size in the squirter.
 
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