Carburetor Issues and Recommendations

[mattyj858]

Hi guys,


I have an old Thermoquad on my 440 that's having issues with the secondaries not opening up regularly, could be linkage. Had a mechanic look at it and the shafts are really loose cause air to seep in and recommended that I ditch the old Carter and install a 650 cfm Edelbrock with electric choke.


Called two old school carb shops and they didn't agree on the new Edelbrock and recommended fixing the old Carter. First both said it could be an easy fix for the secondaries but a possible rebuild because of the loose shaft. They both felt a 650 cfm as small for a 440, and last both said Edelbrocks tend to flood especially with the adapter plate.


I am sure their are two opinions on this issue and read a few other threads.couldnt really find straight info on the cfm for a 440.


Thanks
Matt

[Rickf1985]

650 will handle a 440 but just barely, a 750 would be better. If I were going to the Edelbrock I would also get the manifold. A good carb rebuilder can rebush the throttle shaft, the problem is finding a good rebuilder anymore. The Thermoquads had big issues with the center sections warping on them so as long as yours is in good shape there then it is worth rebuilding if you can find a qualified builder.

[mattyj858]

Thanks Rick, I agree on a matching manifold but my cash flow won't allow it right now. That being I am l leaning on towards a rebuild. I believe the body is fine, runs nice at idle and on the primaries.


I don't want to invest $400 on a 650 and find out it is restrictive.whatever I do I want it to be reliable


I am lucky that I have two old school carb guys in San Diego with 40+ years experience each. What I don't know is how much it's going to cost....

[Rickf1985]

I can tell you that the adapter will not cause the Edelbrock to flood, I have never had any issues with that on any conversions I have done. As long as the floats are set right, and they are right out of the box, then you should never have flooding issues. The nice thing about an Edelbrock is that they are infinitely adjustable. You can buy a kit with jets, rods and springs in it for about 40.00 that will allow you to tailor that carb to any kind of driving style you like from extreme mileage to extreme acceleration. (forget mileage with that one!) The adapter is fine, it takes away a little of the performance but it can be adjusted around that. If it turns out that the carb cannot be rebuilt for a reasonable price do not be afraid to go with the Edelbrock. Also, do not hesitate to call them, they are fantastic people to talk to and will work with you on exactly what you need. They will be the ones to tell you exactly what size you need. Most vehicles are actually using carburetors that are too big for them. When you do the math a 600 will handle a 440 with no problem but the guys at Edelbrock would be the experts to talk to. They have not been doing this for 70 years for nothing.

[DaveVA78Chieftain]

Use the site search function for Edelbrock or THermoquad.  There is loads of information here about them.

I also have a lot of Thermoquad Information.

[mattyj858]

Thanks guys! Bit of a dilemma as I am tight on cash after dropping $$$$ on exhaust manifolds and a complete exhaust system, yet a brand new carb would be nice... I guess the rebuild cost might determine the decision

[Rickf1985]

You are going to find that a professional rebuild is going to cost probably more than a new Edelbrock.

[mattyj858]

Ouch

[mvptarheel]

How do you know the secondaries aren't opening? If the choke isn't fully open the secondaries air door won't open. Might not be too hard to fix. I'm planning to rebuild my TQ soon. There is a lot of good info on them if you do a search.
Here is a discussion of a similar problem:http://www.v8buick.com/archive/index.php/t-56049.html

[mattyj858]

How do you know the secondaries aren't opening? If the choke isn't fully open the secondaries air door won't open. Might not be too hard to fix. I'm planning to rebuild my TQ soon. There is a lot of good info on them if you do a search.
Here is a discussion of a similar problem:http://www.v8buick.com/archive/index.php/t-56049.html

We did a visual inspection by manually engaging the linkage. The secondaries are opening intermittently which is most likely just an adjustment. The real concern is the slop on the shaft which is allowing air to enter. Still trying to figure out the more cost effective way to go, fix linkage and rebuild the TC or buy an Edelbrock and if so a 650 or 750?

[DaveVA78Chieftain]

Just remember on a TQ that while the secondary throttle plate is mechanically operated, the secondary's will not operate if the spring loaded air dam does not open.  The air dam is operated by air velocity through the carburetor.  Air dam response is controlled by adjustable spring pressure.  The venturi effect created by air flow across the air damn regulates the amount of fuel drawn from the Secondary Discharge Nozzle.  This is how the TQ varies the fuel flow based on the air flow through the carb.

FYI - The tools to rebushing a throttle shaft are expensive which is why a carb rebuilder is about the only person that invests in them.

[Rickf1985]

Just remember on a TQ that while the secondary throttle plate is mechanically operated, the secondary's will not operate if the spring loaded air dam does not open.  The air dam is operated by air velocity through the carburetor.  Air dam response is controlled by adjustable spring pressure.  The venturi effect created by air flow across the air damn regulates the amount of fuel drawn from the Secondary Discharge Nozzle.  This is how the TQ varies the fuel flow based on the air flow through the carb.

FYI - The tools to rebushing a throttle shaft are expensive which is why a carb rebuilder is about the only person that invests in them.

BINGO!

I had several thousand dollars tied up in a lathe and tooling back when I was doing them and now it is worth a lot more since you basically have to make your own tooling. And in addition it is getting much harder to find the hard parts for those carbs such as the floats, needles, jets and such.

[mattyj858]

BINGO!

I had several thousand dollars tied up in a lathe and tooling back when I was doing them and now it is worth a lot more since you basically have to make your own tooling. And in addition it is getting much harder to find the hard parts for those carbs such as the floats, needles, jets and such.

Thanks guys, given the shaft is really sloppy and parts getting hard to find, might just be best to slap a new Edelbrock on it. Leaning towards a 750 rather than the 650.

[Rickf1985]

Call them and ask! Do not guess at it and then be sorry later. They know their carburetors! they are great people to deal with. Manufacturers always subscribed to the "bigger is better" theory for years and it took a LONG time for the science to catch up to the myth in carburetors and prove that was not the case. If you are dead set on not calling Edelbrock do some research  on CFM requirements for big block engines and you will be surprised. But I STRONGLY suggest that you call. They will set you up with the right size AND the right jetting for the weight of the vehicle and altitude if you are up there.

[mattyj858]

Thanks Rick, yes I agree on calling Edelbrock. I was hoping to get some experience from guys that have already been down this road. Seems like the carb shops are biased towards rebuilding and straight mechanics would rather slap a new one on and call it a day.


Seems like when I have called Summit and Jegs they were a little lost on a motor home application. I'll call Edelbrock tomorrow. Appreciate all the advise!

[Rickf1985]

Well, My past work experience is a master ASE auto and truck (Retired). When I had my own shop I specialized in carburetors and automotive electrical. So I kind of have both sides of the fence covered on that one. I can see the rebuilders point of view and I can see the straight mechanics point of view. The rebuilders view anymore is getting tougher because it is getting harder to find good cores for the rebuilds, especially thermoquads, and it is getting harder to find the smaller hard parts and gaskets. And all of that stuff has gotten very expensive. It is tough for them to say "I can rebuild your carb at the same price as a new one" when they can't even get the parts themselves for that price. The regular mechanic wants to throw on a new carburetor because new is easy to work with and chances of failure are slim. Easy and fast means profit.

[mattyj858]

  Thanks again, good advise

[Rickf1985]

Summit and Jegs are based mostly on high performance and racing and have little to no experience with heavy vehicles. They sell a lot of the stuff we need but they do not have any of the experience for motorhomes. Now if you want a thousand horsepower in your Winnie.................... they will be right there.

[beaverman]

Summit and Jegs are based mostly on high performance and racing and have little to no experience with heavy vehicles. They sell a lot of the stuff we need but they do not have any of the experience for motorhomes. Now if you want a thousand horsepower in your Winnie.................... they will be right there.

Thousand HP !!! now your talking Rick!

[Froggy1936]

Always remember with horses You have to feed them to get anything out of them  The more HP you have and use the lower MPG
Frank 

[beaverman]

Always remember with horses You have to feed them to get anything out of them  The more HP you have and use the lower MPG
Frank

NOT necessarily true , when a big block is built and tuned correctly and allowed to breath, the less it has to work for the horsepower!

[Rickf1985]

Bingo Frank, And this falls right back to the carburetors. I will have to find the CFM formulas but it took a long time for the major companies to realize that they did not need such big carburetors. An engine runs at low efficiency, not a whole lot that can be done about that. It requires a certain amount of air to burn a certain amount of fuel to produce a certain amount of power.  What they came to find was that the bigger carbs worked fine for all out racing where the higher rpm was able to draw enough air to atomize the fuel being drawn into the airstream. And you had to keep the rpm up in that 4500 rpm and above range. And the engines had hot cams and high compression. Need I say what the mileage is at that range? Then they came out with the Quadrajet and later the Thermoquad with small primaries and huge secondaries. "Best of both worlds" everyone said. Well, kind of but not quite. You still have a huge carburetor that flows more CFM than the engine needs. The secondaries on a Quadrajet never open all the way unless you are racing and have a pretty hot cam. They only open as far as needed to supply the engine what it needs, hence wasted volume. So you are still running an inefficient carburetor, albeit a less inefficient one. The best scenario is to have a carburetor that is matched to the engine it is on. These engines in the motorhomes are never going to turn 5,000 RPM. I hope anyway. W% You are not going to have a hot, high lift, long duration cam in a motorhome, if you do you will be VERY disappointed but you will have a mean bum idle! :)rotflmao You do not have 11:1 compression in the motorhome, if you do then you are towing a trailer of Avgas behind you or you own stock in the 104 Plus company. Therefor the carburetor you need is probably not going to have to be any larger than 650 CFM. A bigger carburetor will not help if the engine is not set up to handle it and engines from the mid seventies were neutered as we all know. I will find my formulas and post them as soon as I can.

[Wildman5977]

I have a similar issue, I looked at summit and found this formula (engine CID (440)) X (engine RPM (5250)) X (proficiency (.85)) = 1963500 /3456 = 568


They have a Carter 600 for under $250 that should work for my Dodge M500 Executive Elite 31 foot if that formula works, I am always getting fouled plugs after a thousand miles or so.

[Rickf1985]

But are the plugs gas fouled or oil fouled?

[Wilhelm7]

I have a 1971 D-22 Custom with a 413.  How do I determine which carb I have? Thanks.