carburator trouble

[et1]

I 750 holley double pumper that bogs when I snap the thottle but it ok when slowly rev it. Could be the accerator pump ?

ED

[Jeff_in_Atl]

Re: carburator trouble

Originally posted by et1
I 750 holley double pumper that bogs when I snap the thottle but it ok when slowly rev it. Could be the accerator pump ?

ED

Either that or the secondaries are opening too soon because of too light a spring in it. I've also seen this happen when the check ball is left out of the secondaries.

I'd put a pump on it first. It's like 5 bucks.

[Joe 1320]

Could be several things. A mechanical secondary is not spring actuated like a vacuum secondary.... it is a progressive mechanical linkeage with an accelerator pump for the primary circuit and another accelerator pump for the secondaries. Double pumpers are a little more tricky to set up because of the mechanical operated secondaries. A vacuum secondary has changeable springs that taylor when the secondaries open. Even if you go WOT, a vacuum secondary when properly set up won't bring the secondaries in until the engine demand warrants it. With a double pumper, it gets a little dicey. If you have too much carb for the engine, it's going to be tough dialing in the secondaries..... if the airflow is low, velocity drops and the motor wants to bog, even with additional fuel. For me to help you dial it in, additional info is needed. What engine, exhaust, ignition, intake, trans, gearing, etc? That's going to determine how you are going to dial it in. There are pump kits that tailor the amount that the pump squirts in order to match the sudden increase in airflow when you suddenly go WOT. If anything is out of whack, it will bog when you punch it, but rev nicely when you slowly tip in the throttle. I grew up tuning this stuff, it seems to be a forgotten art in today's world.

[Jeff_in_Atl]

Originally posted by Joe 1320
Could be several things. A mechanical secondary is not spring actuated like a vacuum secondary.... it is a progressive mechanical linkeage with an accelerator pump for the primary circuit and another accelerator pump for the secondaries. Double pumpers are a little more tricky to set up because of the mechanical operated secondaries. A vacuum secondary has changeable springs that taylor when the secondaries open. Even if you go WOT, a vacuum secondary when properly set up won't bring the secondaries in until the engine demand warrants it. With a double pumper, it gets a little dicey. If you have too much carb for the engine, it's going to be tough dialing in the secondaries..... if the airflow is low, velocity drops and the motor wants to bog, even with additional fuel. For me to help you dial it in, additional info is needed. What engine, exhaust, ignition, intake, trans, gearing, etc? That's going to determine how you are going to dial it in. There are pump kits that tailor the amount that the pump squirts in order to match the sudden increase in airflow when you suddenly go WOT. If anything is out of whack, it will bog when you punch it, but rev nicely when you slowly tip in the throttle. I grew up tuning this stuff, it seems to be a forgotten art in today's world.

OOPS. That is what I get for scanning. Missed that it was a dp.

[et1]

Hi Joe1320,
I bought the car it's a 1969 firebird with a 400 motor, freeflow airfilter, crane cam (don't know size) headers, stock dist. but accel hei coil and wires, edlebrock perforamer intake, automatic trans, stock gear 3.27. I try change the spring in the advance spring in the dist. no change.

It idle very nicely and warm up start up great. Last night I check the timing the repair manual said 9 degrees BTD. I pull the vacuum line and checked it with the timing light and it was about 2" pass the 12 degrees mark (30 degrees). I don't who or what size cam is in there but this seem a little wrong.
Hope that help.

ED

[Jeff_in_Atl]

With the car off, take the air cleaner off and look down into the top of the carb. Now move the throttle not slowly and not quickly. Do you get a solid stream into all 4 bores?

Another thing, where do you have the vacuum from the distributor plugged into the carb? Make sure it is plugged into the distrbutor port and not the manifold vacuum port.

[et1]

Hi Jeff,
There is 4 streams of fuel squirting inside the carb. The vacuum line is plug in the front of the carb not the intake manifold.
Is there anything else I could try ?

[Jeff_in_Atl]

The only thing I can think of right now is to put a vacuum gage on the hose coming from the carb. It should read 0 at idle and increase as the throttle is opened.

[Joe 1320]

I see a couple potential problems. In order for everything to work correctly, the parts combo must match. A little more info could be helpful. The cam and the converter should be matched. If you are trying to remove a bog from a car with a cam and too low of a stall speed, you have your hands full. The springs in the distributer bother me as well. I would re-time the engine with the stock spring in place for now. Hook up a vacuum gauge and let me know what the engine vacuum reading is at idle both in and out of gear. We also have to determine if the bog is from too little fuel or too much fuel. When you suddenly go WOT, any puff of black smoke? Now the other curiosity is that a regular performer is a spreadbore pattern, to use a square bore carb requires an adapter, negating the basic design of the manifold anyway. IMHO, unless the car has headwork and a cam, I think the better carb would be a spreadbore 650 with vacuum secondaries.... especially with a automatic and a stock converter with those gears. But for the moment, we'll work with what you got if that's your goal.

[et1]

Hi Joe
at idle there is 20 inch pds. and in gear 18.
I also check the port that dist vacuum line is connected to 0 @ idle and 20 at 2000 RMP. I didn't have someone to help check to see if the exhaust is black or normal but I didn't see any.

ED

[Joe 1320]

Providing you have the timing set to factory spec, that is good vacuum. Most likley the cam low to mid range torque design. It would make sense considering the rest of the combo. Trying to dial in the carb is going to require an assistant. When the throttle is taken to WOT, either you or the assistant needs to check the exhaust for either no black smoke or a puff of black smoke at the moment it bogs. A tiny black puff can be acceptable with a good state of tune. Either way, you will most lilely be purchasing a pump kit from a speed shop/catalog to taylor the amount of the shot. You could disconnect the rear accelerator pump just to see if having no squirt on the secondaries makes any impact. There is a certain amount of adjustment on the carb itself, not in amount of shot per se, more like at what point the shot starts to function which in one way does affect the amount of fuel being injected into the stream. Make sense? If there ends up being black smoke from a rich bog, go with a smaller shot of fuel. If there is no smoke, try more fuel. Be prepared though...... you might not be able to completely eliminate it due to the configuration that you are using.

[Jeff_in_Atl]

Your vacuum to the distributor sounds correct.
You're getting accelerator pump.

I know you want to use what you have instead of "you need a different carb" so here is something else that you could use for fine tuning. http://www.holley.com/HiOctn/ProdLin...SCA/20-12.html


I had a Holley 3310 (750 vacuum secondaries) on a 70 Dodge 340 with a .484 lift .284 duration cam and it ran great. I tried a Holley 650 double pumper and I had all kinds of problems with it. I could never get the bog out of it either. I switched back to the 750 vacuum secondaries. I later put the 6 pack on it and to be honest, it didn't run as good as the 750 although it sounded like a giant vacuum cleaner when they opened up. It was a little faster with the 6 pack but it was much more driveable with the 750.
http://www.holley.com/HiOctn/ProdLin.../0-80508S.html (best choice for the street)
http://www.holley.com/HiOctn/ProdLin...C/0-3310C.html
http://www.holley.com/HiOctn/ProdLin...C/0-3310S.html

Here are some things for you to read.

http://www.holley.com/HiOctn/TechSer...fo/TI-225.html "If the carburetor is too large, the pump shot will be
consumed before the main system starts. The result is a "bog" or a "sag." "


http://www.holley.com/HiOctn/TechSer...o/Trblsht.html
QUESTION How do I know if a vacuum or mechanical carburetor is best for me?
ANSWER For streetcars the vacuum secondary carburetor works best on midweight or heavyweight cars with an automatic transmission. They are more forgiving than a double pumper is because they work by sensing engine load. The mechanical secondary carburetor is best on a lighter car with radical camshaft and a lower gear and manual transmission or on a car that is going to be used for racing purposes.

If you decide to get a vacuum secondaries car, get this too http://www.holley.com/HiOctn/ProdLin...SCA/20-13.html

Note the full open RPM on each spring. WOW.

[Joe 1320]

You go Jeff..... That's good stuff.

[et1]

Hi Jeff & Joe,
I'm planning to order some secondary springs and accelerator pump cam maybe it will fix the bog. I did noice the back barrels start to just before the squirt. maybe a heavyier spring is needed or a earlier squirt. I try it and let you guys know the results.

Have a good one

ED

[Jeff_in_Atl]

If you have a douple pumper carb, it does not use the spring kit. That is only for vacuum secondaires carbs.

They sell a bigger pump kit too. You can go to the 50 cc pump instead of the stock 30 cc. http://www.holley.com/HiOctn/ProdLin...cess.html#APCI

But I'm not sure this will solve the problem if the carb is too big.