Carb Problems?? - Need Help

[hkelley]

Here's the scenario...460, big cam, Holley 870 w/vac sec.

Car starts fairly, easily runs great until reaching full operating temps after about 10 - 15 minutes. After that the car will begin to stumble at anything other than a heavy throttle blast. The car will buck when throttle is applied and will try to stall. Usually you can push in the clutch and really rev it and that will clear it for a while. But the problems comes right back and the car will then stall. The car then needs to sit for a couple of minutes and then, with the throttle pushed to the floor, it will restart and still run the same as mentioned above. The car is flooding...you pull off the air cleaner and you can see it.

All thoughts and suggestions appreciated.

Thanks.

Joe

[Rick Parker]

Have the floats adjusted when engine is warm.

[wtm442]

Is this something that just started happening, or has this been a problem since day 1 ?

Is the gas percolating in the bowls? That's what it sounds like. Do you have enough room between air cleaner and hood scoop to install a thick phenolic heat dissipation spacer under the carb? If not, try one of the metallic plates or a turkey pan.

http://store.summitracing.com/egnsea...115+4294814811

[hkelley]

[quote=wtm442]Is this something that just started happening, or has this been a problem since day 1 ?

There have been a variety of carb issues...this is the latest...fix one thing something else crops up.

[undy]

Check your fuel pressure. Holleys don't like much more than 7 psi. If you have a single fuel line and a high flow elec pump that might cause problems. It did with mine..

[wtm442]

Joe
Sounds like gas boiling in the fuel bowls. Got to get the heat from the intake manifold away from the fuel bowls. In addition, float level may be too high.

Place a small dish towel or hand rag all the way around each fuel bowl. Make sure the rags are under the bowls too. Start the engine and let it warm up. As soon as it starts to stumble, pour some ice water on the rags and see if the problem goes away for a while.

[mrmustang]

Dump the Street Avenger 870cfm carb and find yourself a new Holley 650 double pumper 04777C. Add on an electric choke kit, then find yourself a chassis dyno who's owner can properly tune the carb to the engine. I'll bet with what I suggested, and a reset of the engine timing, and that motor will scream like never before..........

Bill S.

[hkelley]

Originally Posted by undy

Check your fuel pressure. Holleys don't like much more than 7 psi. If you have a single fuel line and a high flow elec pump that might cause problems. It did with mine..

I am running right around 5-6 with an aeromotive fuel pump.

[hkelley]

Originally Posted by mrmustang

Dump the Street Avenger 870cfm carb and find yourself a new Holley 650 double pumper 04777C. Add on an electric choke kit, then find yourself a chassis dyno who's owner can properly tune the carb to the engine. I'll bet with what I suggested, and a reset of the engine timing, and that motor will scream like never before..........

Bill S.

That is where I started. I had a 750dp that kept flooding. Dennis Olthoff had it for a week and couldn't get it sorted. It was Dennis that decided to put the 870 on.

[wtm442]

You said "The car is flooding...you pull off the air cleaner and you can see it."

Is that when you shut the engine off you can see gas dripping down into the throttle bores? Still sounds like gas percolating in the fuel bowls to me, especially if you have had the same problem with two different carbs.

[mrmustang]

Originally Posted by hkelley

That is where I started. I had a 750dp that kept flooding. Dennis Olthoff had it for a week and couldn't get it sorted. It was Dennis that decided to put the 870 on.

The 750CFM was too big a double pumper for your engine, the same goes for the 870cfm carb. A properly tuned 650cfm double pumper would be your best bet. Keep in mind that it can be as simple as adjusting the metering plates and jets once you have the proper size carb on the engine. Then it's a matter of balancing the timing and the carb adjustments until you get it just right.
Been there, done that countless times and learned a bunch from those experiences once I got time on a computerized chassis dyno.

Bill S.

[hkelley]

Originally Posted by wtm442

You said "The car is flooding...you pull off the air cleaner and you can see it."

Is that when you shut the engine off you can see gas dripping down into the throttle bores? Still sounds like gas percolating in the fuel bowls to me, especially if you have had the same problem with two different carbs.

That's right. I think its a good chance this is the problem. I have not had this same problem before. With the 750 it ran real rich, was tough to start, etc. They put on the 870 and initially I still had trouble getting it started, but once going it was fine. Took it back to Olthoff to have them take another look at it and they found the vacuum advance was off and he changed the jets. Drove it home and it ran great for an hour and 15 on the freeway. After I was on surface streets, stop and go for about 15 minutes it started bucking and it has gotten worse every time I have driven it since then.

[hkelley]

Here are some more details if this helps.

10.5:1 Compression ratio
Edelbrock Performer 460 intake manifold w/Holley 870 CFM
SVO Cobra Jet aluminum heads with SVO valve train M-6049-B429
Polished valves - Intake diameter 2.25" & exhaust diameter 1.76"
SVO high performance hydraulic flat tappet camshaft M-625-C460
Valve lift - .588" intake & .614" exhaust
Duration at .050" is 244 degrees intake & 254 exhaust

[hkelley]

Originally Posted by mrmustang

The 750CFM was too big a double pumper for your engine, the same goes for the 870cfm carb. A properly tuned 650cfm double pumper would be your best bet. Keep in mind that it can be as simple as adjusting the metering plates and jets once you have the proper size carb on the engine. Then it's a matter of balancing the timing and the carb adjustments until you get it just right.
Been there, done that countless times and learned a bunch from those experiences once I got time on a computerized chassis dyno.

Bill S.

Bill. Thanks. I am a bit hesitant to jump into another carb. If I have to I will but I really need to find someone local who can work on this thing. You planning on being in NC anytime soon?

[cdnus]

mrmustang
Please explain the relationship between the carb size & the "flooding" problem.
I have a pro-system 4150xe-950HP on a 408W that is absolutely awsome.
Response is absolutely clean & crisp from idle on up.
I can drop the clutch at idle & it will smoke the tires if not controlled & i can cruise in traffic @ 1500RPM in 4th very nicely. I am definetely not a carb expert but it would seem the carb you are recomending would be a little small for a "460 with a big cam"
Craig

[Cobrabill]

Ditto CDnus.CFM is NOT the issue here.

Joe-simple stuff first:

Borrow a KNOWN carb.If this cures it then look at:
Verify float level-the littlest piece of crap can cause the biggest headaches.
VERIFY fuel pressure(try another guage)
return line-highly recommended.

[Jerry Clayton]

How much vacuum does the engine have---at start up, while warming up, after warm up, and when its running poorly????

Change the power valve to a 3.5 and see if it helps----

Jerry

[hkelley]

Originally Posted by Cobrabill

Ditto CDnus.CFM is NOT the issue here.

Joe-simple stuff first:

Borrow a KNOWN carb.If this cures it then look at:
Verify float level-the littlest piece of crap can cause the biggest headaches.
VERIFY fuel pressure(try another guage)
return line-highly recommended.

CobraBill

Thanks. Tell me more about the return line. How would that be plumbed? We did verify pressure with another gauge and this is another issue. At startup the inline gauge reads 5 lbs. After the car has run a while it will intermittently read as low as zero however the car is idling fine. Is it a bad gauge or bad placement? It is placed at the end of the fuel line run.

Joe

[mrmustang]

Originally Posted by hkelley

Bill. Thanks. I am a bit hesitant to jump into another carb. If I have to I will but I really need to find someone local who can work on this thing. You planning on being in NC anytime soon?

You never know, but you would still need to find someone with a chassis dyno

[mrmustang]

Originally Posted by cdnus

mrmustang
Please explain the relationship between the carb size & the "flooding" problem.
I have a pro-system 4150xe-950HP on a 408W that is absolutely awsome.
Response is absolutely clean & crisp from idle on up.
I can drop the clutch at idle & it will smoke the tires if not controlled & i can cruise in traffic @ 1500RPM in 4th very nicely. I am definetely not a carb expert but it would seem the carb you are recomending would be a little small for a "460 with a big cam"
Craig

The street Avenger series of carbs is like using an Edelbrock carb on a performance engine, used strictly as a plug and play item good for street cruising at low only.....Too large a carb and you create issues with washing the cylinder walls down with fuel, mix enough fuel with oil and things go boom. A simple formula is attached below. As for your carb, just because it says it's a 950HP does not mean the main body, idle curcuits, jetting, and squirters are sized for 950cfm. I'm assuming someone set up your carb to your engine? if not, I'll bet you I can make more HP with a smaller carb on that engine thanb what you have on it right now as it sits. Bold statement, but I can back it up with both big block and small block stroker experience on a chassis dyno.........Not an engine dyno, but a chassis dyno.......If your's is working flawlessly, then you are one of the very lucky few, count your blessings.........

Bill S.