Vacuum secondary 750 carbs - Team Camaro Tech
Troubleshooting Diagnosing problems done here.

 
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post #1 of 12 (permalink) Old Apr 29th, 02, 05:29 AM Thread Starter
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Is it common for a vacuum secondary carb to hesitate when I throttle quickly? I get about a one or two second pause after I accelerate, either from a launch, or while driving. I think I may need to adjust the secondary so that it comes in a little later. what do you guys think? A friend of mine told me to go to a smaller carb. I have a 383 w/ sportman II heads.

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post #2 of 12 (permalink) Old Apr 29th, 02, 06:06 AM
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if anything, make them open faster with a lighter spring. also, make sure the accelerator pump is giving enough fuel fast enough, but not too much too fast.

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post #3 of 12 (permalink) Old Apr 29th, 02, 06:22 AM
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I'll advise the other way... The stumble if secondary caused is because they are opening too soon. This has been my experence and is part of the Holley recomndations... Something else to look into is the accelerator pump. You may need a bigger squirt to get you past the stumble. Don't take my word on it though, have a look at Holleys Q&A section... www.holley.com/HiOctn/TechServ/TechInfo/Trblsht.html

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post #4 of 12 (permalink) Old Apr 29th, 02, 08:11 AM Thread Starter
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Thanks guys! I logged on to Holley's trouble shooting site!!! They had a scenario that suited my problem and had a recommendation that hit the nail right on the head!! thanks!!
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post #5 of 12 (permalink) Old Apr 29th, 02, 09:58 AM
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Don't be such a politician!! What fixed your problem? I don't have a problem if what I thought was wrong. The idea is to get you going and the thread here seems to have done that!!

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post #6 of 12 (permalink) Old Apr 30th, 02, 04:55 AM Thread Starter
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OPPS! Sorry DjD! My Problem was a pretty common one, either not enough fuel was comming out of the pump nozzle, or there was a blockage in my line somewhere. There was plenty of fuel comming from the pump nozzle, and the line was free of debris. I had to adjust the pump and make sure that there was .015 clearance between the pump arm and lever. Then look at the exhaust to see if there was black smoke out the exhaust. If I had a stumble and no black smoke out the tailpipe, I needed to increase the shooter size. If it stumbled and I was are getting black smoke from the tailpipe then I had to decrease the shooter size.
Step 2 was to make sure the carb didnt bog when I accelerate. I had to install a heavier spring, and move the spring back a little bit so that it came in a bit later.
So I a way, you both were right, eventually I had to do a little of both to make the carb hit just the way I wanted. The weird thing is the stumble would have been caused either way, by opening too soon, or from opening too late. Holley's web site had a few more troubleshooting notes that helps carb rookies like me understand EXACTLY what to do. If either one of you were trying to dial my carb in, it would have taken 2 minutes to figure out.

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post #7 of 12 (permalink) Old May 1st, 02, 03:05 AM
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lighter spring makes it harder to open secondaries.. I know it sounds weird, but the harder the spring, the easier it opens.

check your squirter size. you need to fill that void with gas when you romp it, if your squirter is only dribbling .then there you go. but glad you got it fixed...

[This message has been edited by Chris Edwards (edited 05-01-2002).]
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post #8 of 12 (permalink) Old May 1st, 02, 05:13 AM
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"lighter spring makes it harder to open secondaries.. I know it sounds weird, but the harder the spring, the easier it opens."

Chris, could you please explain that statement... or provide some documentation? I don't understand this and always thought it was the opposite. I'm not saying you are wrong or anything, I just want to learn something new if I'm doing it wrong!

blackvi,
What combo or squirter/spring did you end up with?

Thanks,
Bill C.


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post #9 of 12 (permalink) Old May 1st, 02, 07:42 AM
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Taken straight from the Holley website...

QUESTION I have a vacuum secondary carburetor that bogs when the secondaries come in. What will cause this?

ANSWER Bogging and hesitation are caused by the secondaries coming in to quickly. You can install a heavier secondary spring and this will prevent the secondary from coming in to soon. If the engine is sluggish in response at full throttle then the secondaries may not be opening soon enough. You will then need to go to a lighter spring.
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post #10 of 12 (permalink) Old May 1st, 02, 02:36 PM
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Lighter secondary diaphragm spring lets them open faster, stiffer spring makes them open slower. The spring in the diaphragm housing holds the secondary throttle plate closed; when an air velocity-induced vacuum signal from the primary (and later, secondary) venturi pickup tube is applied to the upper portion of the diaphragm housing, it pulls the diaphragm up against the spring load, which pulls the rod that opens the secondary throttle plates. If you buy the quick-change cover and spring kit, you can change the springs in about two minutes without removing the diaphragm housing from the main body; quick, easy tuning.

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post #11 of 12 (permalink) Old May 3rd, 02, 04:06 AM Thread Starter
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Thanks a ton fellas! (and ladies, if any were present) As it turned out, I did need a lighter spring. I didn't know that vac. sec. carbs sensed the load on a vehicle... meaning that the carb can only technically be tested on the road. I didn't know that. I do now. I had toplay around with it for a few minutes until I got it just right. now the 383 cooks the tires so well I get a warm fuzzy feeling that only means I have to find a good victim.....Thanks again

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post #12 of 12 (permalink) Old May 3rd, 02, 07:30 PM
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One more additional set-up note....if using an automatic trans...
You can increase the response time by making sure the kick-down cable is adjusted properly or a bit more agressive, meaning that it comes in sooner..
If the secondaries pop open sooner than the kick-down response, than you won't have the venturi velocity fast enough to accept all the fuel squirt...and most of this fuel squirt falls on it's face as droplets in the plenum wall...and you get a slight lean-out in which most tuners will think they need bigger squirters...this is really in-efficient since most of the squirted fuel is not making it to the chamber..and bigger squirters are a bad way to compensate..
With the kick-down timed correctly ...the increased venturi velocity will be there to gladly accept the fuel squirt and get most of it into the chamber for that instance in time, when are where it belongs...
Just my 2 cents...
Regards
Chris


[This message has been edited by cerrem (edited 05-03-2002).]
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