Holley 850 stumbles and stalls

[john.thompson068]

• 66 Satellite
• Full weight street machine
• 446ci 440
• Eddy RPM heads/intake
• Holley 850 double pumper with mechanical secondaries
• Comp 305H cam (.525/.525) 305 Duration (253 @.50) 110 lobe separation
• 1 7/8 headers 3" exhaust
• 10.5:1 forged pistons
• 13* timing
• 727
• 4000 rpm PTC convertor
• 3.91 gears
• 29" tall tires

Now that you have helped me set the timing, there is still two more drive ability issues which must be resolved asap. First and most important. Car can stall after turning, accelerating, then lifting at low speed. Like when pulling out of a gas station and immediately hitting a red light. Car can also begin to stall after turning and getting on gas harder but then needing to lift off because of traffic. The car comes out of its stall as long as traffic keeps moving so I can keep on the gas. These symptoms do not happen every single time these conditions are met, but happened one time each on a 5 mile outing today for example.

Second, car has low speed stumble when rolling on throttle from light throttle cruise speed. Stumble is even worse if throttle is rolled on faster. From light or no throttle to wide open throttle results in immediate rapid acceleration with no stumble.

 

[kidmopar]

check the float level, with engine idling, remove plug and fuel should trickle out of the bottom of the plug hole , have a rag nearby to soak up gas, its a lil dangerous but caution should prevail, adjust float with 5/8 wrench and a large flatblade screwdriver until level is good, replace plugs with sight plugs so u can see it. also your front shooter could be too small if it stumbles under hard acceleration, make sure u have 6 -7psi fuel pressure to ensure u have enough fuel flow to the bowls,also screw side mixture screws in till it lightly seats and back out one and a half turns. this is ball park, put a vacuum gauage on and fine adjust the mixture screws for the highest vaccum reading. if it still does it, check the power valves, the blow on a lot of them if u ever had a backfire. hope this helps u.

 

[arizona satellite]

that pretty much covers all the base's. one thing to check is the acc pump spring, you should have a zero clearance between the arm and the spring. as for the power valve if it is an older holley then yes do check but if the carb is less the a few years old the now have anti backfire valves installed. but I think you will find either the float is not set correct OR you could have a stuck needle and seat. good luck.

 

[john.thompson068]

Okay, I will double check the float levels again. I can also check the idle mixture screws since I had adjusted those when the timing was improperly set. I adjust those with a digital tach for the highest rpm, and you are right that around 1.5 turns out is optimal. I can also take a look at the fuel pressure which I believe I set at like 6 psi.

Do blown power valves cause the stalling as I mentioned? If so, this may be the problem as the car has had quite a few backfires at both ends while I learned all about setting timing correctly.

The low speed stumble happens at low engine speed when just stabbing the throttle or rolling on quickly to like 1/4 or 1/2 throttle from low engine speeds. I am thinking maybe a shooter is too big.

Also, the exhaust is pretty smelly at idle, so maybe we can work on that problem next. I can easily get to the number one plug for reading but that is the least of my worries.

 

[john.thompson068]

that pretty much covers all the base's. one thing to check is the acc pump spring, you should have a zero clearance between the arm and the spring. as for the power valve if it is an older holley then yes do check but if the carb is less the a few years old the now have anti backfire valves installed. but I think you will find either the float is not set correct OR you could have a stuck needle and seat. good luck.

The power valves have blow out protection, but I wonder if that is full proof. My car has backfired hard through the carb and caught on fire a number of times in the carb while discovering the importance of proper timing. I also wonder if I even have the right size in there though. I remember calling Holley when upgrading the power valves to the newer ones with blow out protection and they recommended a certain size but I can't say for sure what it is. Something like 6.5. If someone could tell me how to calculate the proper size that would be great.

 

[john.thompson068]

Reminder to self: a needle and seat may be stuck.

 

[67 coronet]

Reminder to self: a needle and seat may be stuck.

if the carb has caught fire a few times you may want to pull it and check all your small rubber parts to make sure they arent melted.(power valve,accelerator pump diaphrams,needle and seats,for example)if you are smelling a"bad smell"out of the tail pipe,the carb is out of adjustment or leaking fuel inside.(blown power valve for example)if you are doing this all yourself,you may want to get one of the holley"how to adjust holley carbs"book or dvd.i think jegs and summit sell them,they give great info on all the ins and outs of fine tuning.

 

[john.thompson068]

Yeah the exhaust smell really rich and fumy. It will begin to sting my eyes inside my tiny garage even though the tailpipes are blowing the exhaust right out of the garage door. After driving the car my clothes reek of exhaust.

 

[lewtot184]

you'll probably need a 4.5 or lower power valve with that cam. if your using 13 degrees initial timing, thats probably not near enough. that cam has about 85 degrees overlap, thats a whole bunch with hydraulic lifters, so tune will be critical.

 

[john.thompson068]

if the carb has caught fire a few times you may want to pull it and check all your small rubber parts to make sure they arent melted.(power valve,accelerator pump diaphrams,needle and seats,for example)if you are smelling a"bad smell"out of the tail pipe,the carb is out of adjustment or leaking fuel inside.(blown power valve for example)if you are doing this all yourself,you may want to get one of the holley"how to adjust holley carbs"book or dvd.i think jegs and summit sell them,they give great info on all the ins and outs of fine tuning.

Okay 67, I ordered the Holley Carburetor Handbook 4150 & 4160 from Jegs for a total of 16$. I will keep this post updated as to my findings over the next few weekends.

 

[67 B-body]

Power valves should be installed to match your engine. Summit could not tell you with any certainty what this should be. You will need to measure manifold vacuum @ idle and purchase a power valve about 2" less vacuum than your base reading. This is because they are used to determin when that extra fuel gets supplied before you reach the main metering circuit. Think about what happens to manifold vacuum when the throttle blades open.... It drops off sharply, so this is an indicator to the carb that your throwing stones onto the neighbors porch.... It says hey,,, give this guy more fuel because things are about to go lean untill the main metering circuit can begin supplying the correct fuel ratio. That is the sole purpose of Power valves....
Now with your new found knowledge, you can imagine how this is something you should not overlook when setting up your carb........ Now I'll realy mess things up by telling you that I dont have a rear power valve..... Mine is plugged! I can only do this because the rear jets are larger than what you would run using a power valve. Not very practical for street use,,, but I can get away with it good enough.......
One other thing to offer as a suggestion,,, if you feel stumble after hard acceleration or cornering, you can add jet extension tubes to prevent fuel from rushing away from the jets. I have them in my carb, and simply fit tightly over the rear jets.

 

[Plymouth_Bob]

i would recommend the the the jet extenders maybe? they are used in racing applications to remedy when the fuel sloshes to one side of the bowl of the carb and leave a jet exposed to air, and if you have a stumble off idle on a holley u either need to adjust the squirter pump cam to engage sooner or up the size of the squirter

 

[john.thompson068]

i would recommend the the the jet extenders maybe? they are used in racing applications to remedy when the fuel sloshes to one side of the bowl of the carb and leave a jet exposed to air, and if you have a stumble off idle on a holley u either need to adjust the squirter pump cam to engage sooner or up the size of the squirter

Yes. The stumble happens at idle. When sitting at idle if I rev the motor up
with a half push on the throttle it will bog before it revs up. However if the rpms are up a little bit it has instant throttle response.

The stalls does not come after rapid acceleration or during the process of turning quickly. It happens after the car has already turned slowly during normal driving followed by slight acceleration and then letting off the gas.

My book arrives tomorrow and I plan check all the basic things again tomorrow like float level, fuel pressure, and adjusting the idle screws. Then take it from there. I have a vacuum guage so I can determine if I have the correct size power valves. I am pretty excited to get this book and fine tune my carburetor.

 

[john.thompson068]

Okay. I lowered the float level until no fuel would slosh out of the sight hole as the motor shakes around at idle and the fuel pressure is at 5 psi. I verified that the car has 3.5 hg power valves. Then I started the car and after it warmed up I took a vacuum reading and it was at 4 hg at a 750 rpm idle in drive. I then adjusted the idle mixture screws and the highest rpm was at 7/8 of a turn out. But here is the new problem. Is it true that initial timing affects the engine's vacuum? And that more timing will increase vacuum? And that 4 hg is too low for my combo? My initial timing is set at 13*. Should I first try and advance the timing a little bit to get a higher hg? Then take a vacuum reading to determine the correct power valve size? Holley says to take a vacuum reading in drive at idle and then divide that number in half to determine the correct power valve size. If the number is a whole number, then use the next lower power valve size. For example, 8hg divided by 2 equals 4, so a 3.5hg power valve should be used. Also, I discovered that I need to increase the size of my accelerator pump because this can cause the bogging and hesitation off idle. The car has a #31, and Holley says to go up three sizes to make a difference so that would be a #34. I lost the gasket on the sight plug so I will need to get a new one somehow. I was hoping to go for a test drive to see if lowering the float level slightly would prevent the car from stalling after turning sometimes, but until I can get a gasket on the sight plug I don't want to.

 

[Plymouth_Bob]

O_o sounds like you are on the right path boss good luc...pics help hehe

 

[john.thompson068]

I was also able to determine that the power valves were not blown because according to the Holley website, if the engine is affected by screwing in the idle mixture screw then the power valve is not blown.

 

[john.thompson068]

Today I tested out timing and its affect on vacuum. The car was idling at a constant 180* the entire time and I readjusted the idle mixture screws and idle for each test. Idle speed is 950 rpm in park and 750 in drive. The engine runs best at 7/8 turns out with the idle mixture screws regardless of timing. I increased timing to 18* and got only 3.5 hg vacuum. The starter also had a very hard time starting the motor. I reduced the timing to 16* and still got only 3.5 hg vacuum, and the starter was struggling to start the motor. I lowered the timing down to 14* and got 4 hg and the car started quickly. I also went down to 10* and to 12* and was only getting 3.5-4 hg. So I set the timing back to 13* with the idle mixture screws at 7/8 turn out with a 750 rpm idle in drive. So with this combo the motor makes 4 hg of vacuum. I am going to order a set of 1.5 hg power valves and the bigger accelerator pump, as well as get a gasket for the site plug. Hopefully I can just get a gasket and not have to spend 11.99 for the glass sight plugs when all I really need is the gasket. So after I do some test driving I can finally let you know how everything turns out.

As far as that book goes, it didn't help very much at all. In fact, I am sending it back to Jegs for a refund. The book was published in 1980 and the illustrations in the book were almost useless. The book talked more about the history of Holley carburetor models and how the carburetor works. There was no information on how to properly set float level, adjust or properly select accelerator pump sizes or pump cams, test or determine correct power valve size, or even adjust idle mixture screws. After spending an hour or so to finish reading the book, I went to Holley's website and found much more detailed information on all of these topics.

According to one chart in both sources, my carburetor is the correct size, but according to another source it maybe 50cfm to large. Also Holley's formula is 446ci * 6250rpm / 3456 = 806 cfm.

 

[john.thompson068]

Holley's tech line said to run 1 hg power valves. Local speed parts place and engine shop says to run block off plugs and jet up 2-4 sizes. What do you say are the advantages or disadvantages. Installing the plugs seems like it would decrease gas mileage which is not the goal for my street car.

 

[john.thompson068]

Never mind. I just remembered that I can Google almost everything I need these days. So it appears I will order the 1 hg power valve. If you don't hear from me again it is because everything went well with the carb.

 

[451Mopar]

Did you check the throttle blade position at idle to make sure your not into the transition circuit? The 13 initial timing sound fine, what is the total timing, and how fast does it advance? What fuel system are you using?
When I had an 850 on my 451 engine 11:1 comp, [email protected]" 0.614" lift) I had to drill the throttle blades to get out of the transition circuit at idle. I also blocked off the rear power valve, jetted the rear up about 5-6 jet sizes, used rear jet extentions and notched rear float. Also used electric pump -8AN lines to/from a bypass regulator, and -6AN lines from the regulator to each fuel bowl.