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Old 09-03-2005, 07:10 PM
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Too much carb?

So I just got my engine put back together and in the truck and im trying to get it tuned. I'm starting to think that I'm over-carbed or something isn't tuned right with the carb. I'm running a Holley 750cfm 4160 dual feed vac. secondary carb on a 350.

My symptoms are: Seemingly weak low-low-mid range. By weak I mean that it feels like it has a hesitation sort of through the lower rpms. If I punch it in 5th gear at around 2000rpms it has great response but doesn't pull like I would think it should. Feels like it doesn't lose any power, perhaps picks up some power if i let off the gas some (like from full throttle to half). Once I hit 3000rpms the thing screams all the way to redline. Top end power is awesome.

The other symptom is that when the engine is fairly cold (before the t-stat opens), the engine will actually bog and start to misfire if I floor at at low rpms in a high gear. Once again the top end is great and after it warms up, it goes from a bog/misfire to just a dull pull.

I'm not sure what to think other than maybe it's too much carb for the engine since it sound like its bogging. But then I also heard that carb will meter themselves and that you could put a 850cfm carb on a stock 305 and it would run ok just not good response and low end. Is this true?

I'm running #72 jets (which seem pretty standard?) Should I try to do some adjustments with the carb? Should I jet down? Or should I just trade for a 650 instead?

My engine specs:

350 std. bore
10:1 cr
#450 heads, 2.02/1.6 valves, 60cc, fully ported, 1.6 roller tip rockers
Comp XE274, 230/236 @.050, .490/.490 (.522/.522 corrected for 1.6 rockers)
Edelbrock performer RPM intake
Hedman 1 1/2" shorties
3.73 gears


My truck only weighs about 2800lbs, doesn't seem like it should dog it much.


Any ideas?

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Old 09-03-2005, 07:19 PM
steve t's Avatar
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might be your sec open too soon , try differn springs on your vacumn, 750 is alot of carb
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Old 09-03-2005, 07:24 PM
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I had a similar problem with almost the identical carb setup on my 327 chevy. Simple answer is that it is way too much carb for the setup. Mine was running sluggish and felt like it was bogging down.

My solution was to replace the 750 with a 650. Problem solved. Much better response alot more get up a go. Might be an expensive solution but it worked for me.
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Old 09-03-2005, 07:28 PM
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Here's an amateur's thoughts

I've been working with a 4160, albeit a 390 cfm. The low end lack of response can have to do with your shooter/accellerator pump cam.

For a 350 you need a pretty good shot. To give you a comparison, my 4160 is on 175 ci. which is half your engine. It uses a #25 shooter with orange cam at #1 setting.

The standard spring in the vacuum secondary should not be far off. Again by comparison, I use the lightest yellow spring on mine.

The other thing to check is your timing advance, but I expect you are on top of that.

Good luck
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Old 09-03-2005, 07:41 PM
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Quote:
Originally Posted by Arns85GT
I've been working with a 4160, albeit a 390 cfm. The low end lack of response can have to do with your shooter/accellerator pump cam.

For a 350 you need a pretty good shot. To give you a comparison, my 4160 is on 175 ci. which is half your engine. It uses a #25 shooter with orange cam at #1 setting.

The standard spring in the vacuum secondary should not be far off. Again by comparison, I use the lightest yellow spring on mine.

The other thing to check is your timing advance, but I expect you are on top of that.

Good luck
Thanks for your reply.

I think that my accelerator pump is working fine, I feel an instant reponse when I punch the gas... it's more along the lines of a few seconds after that initial punch that it feels dead. It's the point which my foot has been held down.

I suppose it could be my vacuum secondaries. I'm not sure whether this means they're opening too early or too late (sounds like too early?) But maybe I could mess with that if I can figure out which way to go with it.

750 is a lot of carb... I suspected it might be a bit much but I thought that after all the work I did on my engine that it could take that gob of carb and use it. Maybe a mech. sec. 650 carb is right up my alley? Bummer

Anyone want to trade? :-D
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Old 09-03-2005, 07:47 PM
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I have a 750 double pumper on a 350 of mine that has a similar setup to yours but with aftermarket heads and a slightly larger solid lifter cam. I too have a lack of low end power but it's not terribly bad and I think it's mostly because I have a Victor Jr intake on it (I know it's completely unnecessary but I had an RPM on it for a long time and decided to try something new) The engine is also in a truck that weighs 4500 lbs.

You should be able to get that vacuum secondary work if you tinker with it enough. I too would think that maybe changing the spring out on the secondaries would be something to try first. Keep them from opening just a little bit longer. A 750 is getting big on a 350 but they can be made to work pretty well and they are great for top end too (I love my double pumper at 6500 rpm he he)

2000rpm is at the very bottom of the powerband of that cam too, how does the engine react if you downshift to 4th and get on it?
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