Hi All - Here's my problem:

Speed Demon 750 Vacuum Secondary

I bought this carb about 2 years ago and it had never given me problems until a few months ago. One day when I went to start the car as usual, it wouldnt start and the engine was completely flooded. I waited a few hours and tried again, the car barely ran, flooded and fouled the plugs. After doing the usual checks, I came to realize that both fuel bowls are draining into the intake and letting raw fuel into the motor when it sits overnight(sometimes it takes a day or so). I have rebuilt the carb 4 times, using new gaskets,PV,needle/seat, etc....and still have the problem. Right now the carb is sitting on a stand in my garage, and over a weeks time the fuel level goes from the top of the site window to the bottom. (normally the floats are set with fuel at the bottom line on the window, i just filled em up to the top for testing purposes) If i put the carb on now, it fires up and runs fine....but let it sit and the same issue.

I know it is not a fuel pressure/pump problem, b/c i have an old holley on the car now, and all works fine - car starts up everytime w/o issue. I have used Holley rebuild kits with the blue gaskets and the barry grant kit with red gaskets. I cant figure out what is wrong....I am thinking that fuel is just trickling out of the transfer slots somehow as it sits...Both bowls will drain, but the primaries seem to go quicker.

Anyone had a problem like this before, or any ideas why its leaking? Any ideas are appreciated except "buy a holley/Qjet" THANKS

With it leaking, I would figure you would see tell-tale signs of fuel.

During your 'bench test' for leaks, you might add some food coloring for a better leak detect to see the problem. Lay the carb on a sheet of paper and check the location of wet spots. If sheet shows spotting, wonder if colored lamp oil could be sibbed for leak detector?

If it takes a week to drain the diameter of the sight glass, the fuel could be evaporating. Surface temp of carb bowl with a temp gun?

Might be too porous of material used. Gravity is a powerful tool.

Not familiar with the Speed Demon, but if it has power valves, have you changed them ?? If the diaphragm is ruptured, they may be able to seep fuel.

I can't answer your question, but if this has been happening alot, you might ought to sniff for fuel in your oil...just to be careful you're not gonna have another problem due to this one.
Scott

(706) 864-8544

That's the product support number for Barry Grant carbs. I'd call them................

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We forgot ......... Welcome to the Club!

We forgot ......... Welcome to the Club!

YUP...:thumbsup: ...didn't notice it is your first post !!

I think the Demon's larger, shiny fuel bowls have a greater tendency to evaporate fuel into the engine after shutoff than other carburetors. Did you recently change gas stations or did they do anything to their mix? I simply cannot run cheap gas in mine. Fuel today isn't designed for a carburetor... in my opinion.

Thanks for all the replies and welcomes.

-I actually bought the food coloring early in the week to do the same test you spoke of everett...b/c gas isnt showing by itself on the sheet of paper it sits on now.

-I have changed the power valve about 4 times.

-Yes, fuel did get in the oil, and I have drained and changed already.

- I called the BG tech line and got no help...the guys response was "thats odd, dont know how thats happening" As we discussed, the only thing he said was to send it to BG for a full rebuild & flow testing($225 + shipping) and the carb will run like new. Considering the carb will work now, i am worried that they could bench test it and see no issues..only to send it back to me with the same problem. I then asked him if there would be a full warranty on the rebuild (like i want a new carb if the same issue occurs) and he said if it happens again it must be the car. I hung up.

- I have used the same Shell gas station for years...and only run 93 octane. I wouldnt know if they changed anything.

-Also some more info, when the carb is on the car and its running.....it tends to backfire back through the carb when snapping the throttle open quickly. With the 650 holley on the car - i dont get any backfires. Not sure if this will help anyone think of a solution, but im throwing it out there.

Sorry for the miracle sized post, but wanted to answer all questions. Heres a bit about the car...'69 Camaro - 383, 9.5:1, AFR 195cc heads, edelbrock performer air gap intake, .512 lift cam 232 duration, accel HEI, 700r4 and 3.73 rear.

Thanks again for the help.

pull it apart, and check all the flat surfaces with a straightedge.
i don't know what this will accomplish, but it sounds good to me..
oh, yeah......

get a q-jet..
hehe

I can't think of any way for fuel to get into the engine when off other than the orfices in the metering plates. Should be same as holley . I have heard stories that their metering plates have more porosity? How about wrong gaskets on the block to body side? Wonder if you could, put the holley meter plate on the BG on the bench and test? Seems fuel has to go thru the meter plate to get into the ventri or transfer slot and into engine. How about the base plate screws, are they tight? Is the bowl vent unplugged?

I don't have the answer, but I had the same exact problem with that same carb.

It would drain the primary bowl when sitting over night, and gas would run down and puddle on the intake.

I tried everything too. The blue gaskets, power valves, and checked all the surfaces with a straight edge. Everything looked fine.

This was on a brand new Demon and still under Summit warranty.

Porosity was my guess, but I couldn't find it.

I sent it back and went with the usual Holley carb.

I should also mention that I have installed 2 other Demons on different cars with no problems at all, other than finding chips/filings from the machining inside the float bowls.

I'm sure your probably doing this right but, are you sure the gaskets on the pwr valves arn't getting caught on the edge of the valve and preventing getting a good seal.

Thanks for more thoughts....especially ZZ430DropTop67RS - it makes me feel a bit better that im not the only one.

I have done the power valve so many times, i doubt i screwed it up everytime.

zdld17, why did you ask if the vents are unplugged? I never really did anything with the vents...

I just set up my food coloring test to see if I can nail down exactly where the fuel is coming through. After that I will begin to test using different parts with some holleys I have. Im going to try using the metering blocks and base plates interchangably next to help nail down the problem.

Thanks for more thoughts....especially ZZ430DropTop67RS - it makes me feel a bit better that im not the only one.

I have done the power valve so many times, i doubt i screwed it up everytime.

zdld17, why did you ask if the vents are unplugged? I never really did anything with the vents...

I just set up my food coloring test to see if I can nail down exactly where the fuel is coming through. After that I will begin to test using different parts with some holleys I have. Im going to try using the metering blocks and base plates interchangably next to help nail down the problem.
Only think I can think of was after motor was hot, and fuel bowl was plugged at top vent, it would build pressure and force out thru main jets and on to ventri?? Looking at bowls. there is no way for fuel to escape fuel bowl, UNLESS , you have a accellorator pump diaphram with a hole in it and the little red check valve flapper is dried up. But you will see it on the manifold in drops unless it dries up and never see anything unless you put some thing like a paper napkin to pick it up. Thats my best guess.

UNLESS , you have a accellorator pump diaphram with a hole in it and the little red check valve flapper is dried up.

That's a possibilty, but on the one that I had, I changed both of those too and still had the same problem.

Actually, I tried everything known to modern Man except trying different metering blocks, which sounds like plan to me.

A tech from BG posts on another board, I'll try to find out how common (or not) this problem is.

Rossi1369,

If the carburetor worked properly before and now this problem has occured there are only a couple of things that can cause this:
-One would be if the PV gasket is not seating properly or if it is torn.
-Another potential cause would be dirt in either the air bleeds or metering block. This would cause the fuel to siphon through the carburetor.
Two quick questions; 1) Have you installed new PV gaskets each time you've changed the PV? 2) Do both fuel bowls seem to drain the same amount?

Rossi1369,

If the carburetor worked properly before and now this problem has occured there are only a couple of things that can cause this:
-One would be if the PV gasket is not seating properly or if it is torn.
-Another potential cause would be dirt in either the air bleeds or metering block. This would cause the fuel to siphon through the carburetor.
Two quick questions; 1) Have you installed new PV gaskets each time you've changed the PV? 2) Do both fuel bowls seem to drain the same amount?

To answer your questions:
1) The PV gaskets were replaced with every PV change
2) The primary bowl definately drains faster than the secondary. Once the fuel level goes underneath the site plug, its hard to tell what level each would stop draining.

When rebuilding the carb, the air bleeds and all passages got a healthy amount of carb cleaner and compressed air through them.

zdld17 - I replaced the accelerator pump diaphram already, so I think we can rule that out.
I will have to check to see if the vents are plugged in any way, but I would think that it wouldnt have any effect when its on the bench, in which it still leaks.

Rossi1369,

OK. The accelerator pump diaphragms shouldn’t cause an issue with the fuel leaking down. You could actually have the René valve out of the float bowl and it wouldn’t leak down, but it wouldn’t function. Am I reading your post correctly that you can get fuel to leak through the carburetor with it off of the engine? If so try filling the bowls independently so we can determine which side of the carburetor is leaking down.

Hi again - so I finally got some results from my bench leak test. It isnt much, but I can see that there is some fuel leaking out of the hole that is right next to the rear middle base plate screw (right in front of the rear manifold vacuum port). It really doesnt seem to be enough to flood the car, but the carb leaks slower on the bench than on the car, and I have to assume it would be due to the pressure from the fuel pump). W/O tearing the carb down, I dont know where this passage in the base plate leads to....any thoughts BG Tech?

I still have both bowls filled, and the primaries go down faster, but i only see signs of leaking in the rear.

any thoughts BG Tech?

IMO, I would say it would be evaporation of fuel from the bowls. There wouldn't be any fuel pressure problem because pressure would bleed down over a short period of time, say a couple of hours.

Do you smell any vapor?
Is the carb in direct sunlight during the day?
Is the garage warm, above 75*F?

I hope there isn't a gas water heater in the same room.

You might cap off all available ports and the two bowl vents on the airhorn.
Don't forget the air bleeds in the airhorn also need to be plugged capped. Idle transfer slots as well.

Might you have another carb to do the same experiment and set it next the BG carb? Compare results.

but the carb leaks slower on the bench than on the car,

I still think its an evaporation problem. I've seen many Demon carburetors have this problem. On the engine after shut-down, things are hot. On the bench, not.

My Demon had this problem and the only solution was to try different gas stations until I found one that worked. Turns out it was the cheaper the gas, the worse the problem. Also, anything with ethanol will be more volatile.

Hi All - The car is in a garage when it happens, and this has been happening in the cold NJ winters (probably 50 degrees in the garage at best). The carb is sitting on an old intake with a rag covering the top of it as tighlty as possible. A holley sits on the car in the same garage and no issues.

The evaporation thought does not really explain the problem of flooding I have. I dont mind if the fuel evaporates out of the bowls, its when I start the car and it wont run - and plugs are soaked in gas.

To futher explain: the last time i rebuilt the carb - i put it on the car, cranked the ignition to fill up the bowls, and the car fired right up. I warmed it up and then took a short drive. I came home, shut the car off and it sat for 10 minutes, the car fires right up again. When I try to start the car the next day - Barely start, black smoke out the pipes. Pull a plug and it is SOAKED in gas. I can pour gas out of the plug. The issue is raw fuel getting into the motor, not gas evaporating out of the bowls.

I experienced the same problem and could only fix it by going to different gasoline. My theory is that the volatility of the fuel somehow contributes to it pushing fuel through the metering plate into the engine. I agree its a stretch, but, switching sources did fix it. Also, I added a phenolic spacer between the carb and engine. That helped a little but switching sources helped more.

Note also that gasoline is regularly reformulated, so there is no guarantee that gas from the same place won't change.

Anyway - that is what worked for my engine.

Best of luck solving this!! :)

Install carb on engine/car. Drive it as you would normally. Park the car.
Remove the accelerator linkage. Remove air cleaner, open throttle plates and check the dryness of the intake floor with flashlight. Replace the air cleaner.

Come out the next day or whenever, remove air cleaner and open throttle plates and check dryness of floor again.

If floor is wet, then carb is leaking.
If floor is dry, then fueling too much when choke is on. Adjust choke.

Fuel recipe has alot to do with it also.

Hi again - so I finally got some results from my bench leak test. It isnt much, but I can see that there is some fuel leaking out of the hole that is right next to the rear middle base plate screw (right in front of the rear manifold vacuum port). It really doesnt seem to be enough to flood the car, but the carb leaks slower on the bench than on the car, and I have to assume it would be due to the pressure from the fuel pump). W/O tearing the carb down, I dont know where this passage in the base plate leads to....any thoughts BG Tech?

I still have both bowls filled, and the primaries go down faster, but i only see signs of leaking in the rear.

any thoughts BG Tech?

Sorry for the delay.

You may be dealing with a combination of evaporation, and leak down. If you're seeing fuel coming out of the small hole next to the center baseplate screw that would most likely be a bad PV gasket, or the PV gasket not seated completely. That small hole is where the vacuum from the engine goes through the body.

Thanks for the reply BG. With the amount that I can see leaked out vs. the amount the is gone from the bowls, it does seem as though evaporation is also occurring. The weird thing is that is is leaking out of the rear hole on the center base plate - and this is a single squirter carb...no secondary PV. I just had enough time to pull the carb apart again, and it looks as though the holes in the main body dont go anywhere - which is expected, but im not sure how fuel was getting out of that hole yet.

Rossi,

You could be running into a situation where the gasket isn't sealling properly.

What exact carb is it?
Have you changed gaskets on it yet?
What kind of fuel are you using?

The carb is a 750 speed demon VE. Single squirter, vacuum secondaries, electric choke. Its maybe 2 years old.

I have changed the gaskets on the carb about 4 times...using holley blue gaskets, and the last time with a barry grant rebuild kit from Jegs (with red gaskets - i confirmed the part number with the BG tech line).

I run 93 octane from the local Shell (which i have used for years....The first time this happened it was after filling up at a Sunoco)

Have you changed the gaskets since it started happening or just over the time of having the carburetor?

Is the fuel leaking out the little hole next to the base screw? Isn't that the hole where the PV blowout preventor is? Little check valve . BG mentioned this as its right in front of the PV chamber.. There is no way for fuel to get there except like he stated, either PV is leaking or gasket is. You have been dealing with this for a while now. Just wonder if there is not pin hole or crack in that metering block.

I have the same problem I've just started breaking down the carb for rebuild. I have a Holley 700cfm. Manual choke and secondary. My oil smells like gas. From first sight I dont see antything out of the ordinary. Too bad! I'll be watching this post.

I have changed all the gaskets 4 times since the problem occurred.

The hole in the baseplate the fuel comes through is the rear, and there is no secondary PV.

This weekend I performed a Flourescent Penetrant inspection...and there are no cracks to be seen in the metering blocks/main body/bowls.

What i did find was a pin hole in the spot where the secondary accelerator pump should go. The hole leads to the acc pump squirter passage. Also, one if the tiny brass plugs in the metering block squirter passage was very loose. There was definately some fuel coming out of this pin hole in the rear float bowl. (??Iis there a way to post pictures??)

I am not sure if this explains my issues, but I am wating on a rebuild kit to try it again.

... what do you do for a living? Failure analysis? You are amazingly thorough searching for an answer. Very impressive! :)

LOL - you call it thorough, most call it stubborn. My last job was in Quality Assurance.....but I also worked in an aluminum casting plant that my old man runs....It really got me in to root cause analysis, so I dont think my problem is solved until I know exactly what caused it. I also have a ton of people scratching their head on this and want others to learn from it.

I appreciate everyones thoughts so far...

Hopefully I will get more testing done next weekend!

What i did find was a pin hole in the spot where the secondary accelerator pump should go. The hole leads to the acc pump squirter passage. Also, one if the tiny brass plugs in the metering block squirter passage was very loose. There was definately some fuel coming out of this pin hole in the rear float bowl. (??Iis there a way to post pictures??)

If this is the leak, would the Q-jet epoxy fix work??? ( seal leak prone Q-jet casting plug with epoxy)

I've got the same problem on a factory Holley vacuum secondairy 750. I've tried all of these checks and no luck. Any more ideas anyone?

Thanks,
Doug

I went through 3 boggy grant carbs for various problems and failures one after another and i gotta tell ya their quality has gone downhill.
Most speed shops dont wanna deal with them anymore.
MY advice is get a carb from a bonafide carb shop that test each carb before they send them out.
like AED or the carb shop etc.