Holley 850 stumbles and stalls

[67 B-body]

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.

I've done the same thing with the throttle blades on low vac engines to get them to idle correctly. Keeping the throttle blades out of the transition slot at idle is VERY important!
I'm not sure how everyone else does it, but I like to locate the holes as close to the edge of the throttle blade as possible, and also lined up perfectly with the idle mixture port in the base throttle plate. I figure this gives the in comming air a chance to pull fuel from the idle circuit more effectively!

 

[451Mopar]

I think some of the BG carbs had an adjustable air bleed, so you wouldn't have to drill the throttle blades, but I just drill 1/2 way between the throttle shaft and front edge of the primary throttle blades.

 

[john.thompson068]

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.

451, drilling the throttle plates is not a problem for me as the motor idles very easily at 750 rpm in drive and about 1000 in park. I had a guy I thought could help me re-curve the distributor based on what I told him my motor was. He sent me a card with my modified distributor which said it had 20* initial, and 34 total at 3000 rpm. I have no idea what that means. Fuel is delivered via a Holley mechanical pump with 1/2 line, fuel pressure regulator, and guage.

Here is the biggie. I was using the vacuum port on the metering block instead of the port on the base plate. I actually didn't even remember there was one down there at the time until I took off my fuel bowls to install my 1 hg power valves. So I took a reading with the correct vacuum port and the motor actually pulls 5 hg. So I reordered 2.5 hg power valves. I now own 1, 2.5, and 3.5 power valves. Great. I will get out there this weekend and test everything out.

 

[MoparLife]

Be careful with the "vacuum port on the metering block". When carbs are meant to be used on a blower application that port is used to signal the power valve using manifold vacuum under the blower. I would recommend you run a line from that port to your intake or carb spacer to insure your power valve works correctly. I had a customer with an HP Holley that had thesame issues as you are having, and the cure is as mentioned above.

Good Luck

 

[john.thompson068]

Still having problems but good news. First the good news. I found an industrial park close by where I was able to conduct my testing. I started by installing my 2.5 power valves. Then I went to the industrial park and tested the car to see when it was stalling. It would stall when braking. I lowered the floats two flats of the adjusting nut which lowers fuel level 1/16". The problem still persisted so I lowered the floats another 1/16". No more stalling when braking or turning or anything else. I held the rpms at 5000 rpm in park for about 10 seconds and let off and the motor died. It did not stall after performing the same test at 4000 rpm so I concluded it was running out of fuel. On the way home to get tools it ran out of fuel quickly while doing a full throttle acceleration in second gear. I checked my fuel pressure back at the industrial park and the guage was reading 3 psi. The adjusting screw on the regulator was about half way out. I think what happened was when we were checking the fuel pressure last, we did not realize that the fuel pressure takes a while to stabilize on the guage and show a true reading. After realizing this and waiting patiently for the guage to show the stabilized fuel pressure after making an adjustment, it finally settled down at 5 psi at idle and 6 psi when rpms increase. I performed the same tests as before and no more starving for fuel. I thought it would begin to stall again after braking and that I would immediately have to lower the float level again, but it still did not stall.

Question

This time however I decided to try some really hard braking to see what would happen. If I do a panick stop it will stall. Is this normal, or should I be concerned and lower the floats another 1/16"? I am debating whether or not to do this tomorrow night and depending on what you guys say probably will just to see what happens.


Bad news

The car still has a stumble off idle, but I think there may be a slight improvement with the larger squirter. It not only does it when in neutral, but can also happen when rolling on the throttle from a dead stop or really slow roll. Funny thing is though, if I rev up the motor a bunch of times, the stumble goes away for a while. I haven't read the plugs yet, but according to Holley, the carb is factory jetted for my exact altitude and weather in Florida. I think the next step will be to take a look at the number one plug to see what color it is after various engine loads, and also to order the accelerator pump cam kit. I will probably have to play around with some different cam profiles. I have no idea if I should go up from the current .035 squirter to the .037 or .040 squirter first or order the cam kit and play around with different accelerator pump cams.

 

[sstocker31]

In post #4 you mention that your exhaust is really "smelly"... enough to affect clothing. I had the same problems on my ( don't hate me) Corvette.
The cause of this is usually a lean idle circuit. It's so lean that the mixture can't burn completely, so you get lots of exhaust smell. Sounds to me that all off idle operations are OK, so i'd suggest richening up the idle mixture screws and try a little testing.....I love testing....

 

[john.thompson068]

Thanks for taking the time to read through all of this. Actually, exhaust smell is probably normal for a 440 ci race motor without catalytic converters and fuel injection. The smell hasn't actually caught my attention since putting the last tune on the carb. The testing will continue with playing with the different holes on the existing acc pump cam. Maybe because the car has a 4000 rpm stall I could use on of the other holes which are actually to help with motors that must idle over 1000 rpm due to cam size. I will experiment with both size squirters that I now own. I will also lower float levels to see what happens to the panick stop stall situation since I don't know if this is avoidable or not.

 

[70rr-brian]

If I am not mistaken in a previous post regarding timing you implied you had a vacuum advanced distributor? Are you sure you don't have any vacuum leaks?

 

[john.thompson068]

No vacuum advance distributor. No vacuum leaks. Checked all around base of pretty much brand new carburetor with carb cleaner and no vacuum leaks. Probably by this weekend I will have gotten out to the industrial park for some more testing.

 

[coloradodave]

Re; Vacuum port off the side of the metering block: This is called 'timed vacuum' and is taken above the throttle plates. Little vacuum at idle and increases as RPM increases. Was required for 'smog motors'. For tuning, you are correct to use the manifold vacuum from the base plate of your carb.
As for the panic stops: Remove the air cleaner, take a piece of 1/4" rubber hose and connect the two vent tubes together. Then cut a small slice out in the top center of the hose, so they can continue to vent. Be sure that these clear the top of your air cleaner when you replace it. What is probably happening is that when you panic stop, the fuel in the secondary bowl runs forward and up the vent tube and dumps down the venturies, stalling the motor. Holley makes a part called a 'vent whistle' that you can install in the rear metering block to help with this, but the rubber hose trick works too. As for the gas smell at idle, there is nothing like a wide band air fuel gauge to help you 'tune' this problem away. I also wonder why there is only 5" of vacuum with that cam. It's not that radical. Have you taken a compression or leakdown test on the motor? If so, what was the result? Good luck on your quest to get it squared away.

 

[john.thompson068]

Re; Vacuum port off the side of the metering block: This is called 'timed vacuum' and is taken above the throttle plates. Little vacuum at idle and increases as RPM increases. Was required for 'smog motors'. For tuning, you are correct to use the manifold vacuum from the base plate of your carb.
As for the panic stops: Remove the air cleaner, take a piece of 1/4" rubber hose and connect the two vent tubes together. Then cut a small slice out in the top center of the hose, so they can continue to vent. Be sure that these clear the top of your air cleaner when you replace it. What is probably happening is that when you panic stop, the fuel in the secondary bowl runs forward and up the vent tube and dumps down the venturies, stalling the motor. Holley makes a part called a 'vent whistle' that you can install in the rear metering block to help with this, but the rubber hose trick works too. As for the gas smell at idle, there is nothing like a wide band air fuel gauge to help you 'tune' this problem away. I also wonder why there is only 5" of vacuum with that cam. It's not that radical. Have you taken a compression or leakdown test on the motor? If so, what was the result? Good luck on your quest to get it squared away.

Just looked at the vent tube whistle and carb already had one on each metering block. I will take note of the exhaust smell next time I take her out, but I think it is pretty normal now that the timing and idle mixture screws have been fine tuned. Lastly, the motor is practically brand new and was built from all new parts not many thousands of miles ago. I have no idea about the 5" of vacuum either. The lift on the cam is not huge as you say, but I think it might have a lot of duration. Maybe it is the duration? The cam generates incredible torque numbers though with over 535 ft lbs of torque starting at 3800 rpm at the beginning of the pull, 560 ft lbs at 4600 rpm, and finishing with 500 ft lbs at 5700 rpm. Peak hp for this cam is 543 hp at 5700 rpm.

 

[coloradodave]

You put your motor on a dyno or were these figures from the manufacturer? Have you asked the manufacturer about what range of idle vacuum you can expect with this cam? As an aside, if you don't know the history of the carb, it's possible that a racer modified the idle circuits to enrichen them. Borrowing a known good carb from a friend would help diagnose the problem. Hopefully, when you test the car all will be fine. Hang in there..........

 

[coloradodave]

Went to Comp Cams site to check out your grind...........the following is from their site:
305H Series
The 305H should really be considered for Pro-Street type cars where actual streetability is not a major concern. The design of this cam also makes it ideal for all bracket racing engines. Major engine modifications are needed as well as transmission and driveline improvements. The 305H works best in large cubic inch engines and has a radical idle. This is the only hydraulic cam in the series that does not have an advance made into it and will need to be degreed in.
Whoops.........................................

 

[john.thompson068]

You put your motor on a dyno or were these figures from the manufacturer? Have you asked the manufacturer about what range of idle vacuum you can expect with this cam? As an aside, if you don't know the history of the carb, it's possible that a racer modified the idle circuits to enrichen them. Borrowing a known good carb from a friend would help diagnose the problem. Hopefully, when you test the car all will be fine. Hang in there..........

When the motor was built I asked them to dyno it just to see how much power it made. Also, the carb was brand new out of the box. Pretty soon when I feel like going out again I will lower the floats another 1/16 of an inch at a time to see if that cures the panic stop stall. As far as the stumble, it is definitely related to the accelerator pump being tuned properly so that is just a matter of trying out different squirters and cams. The 305H cam is perfectly streetable though in slight contrast to Comp's warning. I have never had any driveability issues related to the cam, and the idle is pretty rough but nothing out of the ordinary. It goes good with the rest of the car which is caged and tubbed on 10.5 W's.

 

[coloradodave]

More info helped. Think you will cure the bog/hesitation working with the pump cam and shooter size. When you change pump cams make sure to adjust all the play out of the arm so that any throttle movement moves the arm. As to float level, as explained by one of the prior readers, car on level ground, rag under float site plug, remove plug. If fuel flows out continuously, adjust the nut down until flow stops and normalizes. Then slowly adjust nut up till fuel just wets the threads of the plug hole. Stop at that point and rev motor a few times to get level to drop. Wait and see if level returns to where you set it. Do this for both bowls. Also make sure that your vent whistles aren't 'bent' down. Also try to raise the idle speed and see if that helps with the stall problem. If so, drilling holes in the throttle blades may help and get the idle down where you want it. There are a few tips on that already given. Good luck.

 

[john.thompson068]

Lowered rear float another 1/16 of an inch and the panick stop stall went away. I have learned so much about my carburetor that this is great. What is not great is that I tried all three different holes on the pump cam with both my #31 and my #35 squirter and holes number two and three create the worst stumble ever. Hole number one is correct just like Holley says for motors that have a low idle speed such as mine. I still have a stumble coming off idle, and have a hesitation when going from part throttle cruise to instant full throttle. I need to try out more squirters and cams.

 

[john.thompson068]

So I take the car out tonight. First the shift light on my Auto Meter Monster Tach wasn't working, but later mysteriously began working again. Then my car runs fine the first few times laying into the throttle. Then suddenly anytime I layed into the throttle it would either starve for fuel for 30 seconds after letting off the gas, or it would fall flat on its face and then starve for fuel for a half a minute. Last weekend and the one before this I was all over the throttle and it was running beautifully. The bog is really annoying also. It is especially bad when wanting to pick up speed from a slow roll in third when stuck in some traffic. After cruising home barely touching the gas, and idling through my apartments and into my garage, I pulled the number one plug. It was a perfect light tan color. What is going on inside my fuel bowls? Also, look at where my fuel pressure guage is set at? Before I remember the set screw being about half way out or a little way out to get about 5 or 6 psi fuel pressure. But last time I looked at it the pressure only at 3. So I immediately suspected the guage was bad because the guage was not moving at all when I made an adjustment. But then the guage started to move but the movement was delayed. And when I was able to make the guage read 5 psi I had the adjusting screw all the way in. So I thought maybe this is where it was before my step dad started messing with it over the summer. I wonder know if it is too much fuel pressure and my guage is bad. The excess fuel pressure could be holding the needle and seat open and flooding the car. Or maybe I am not running enough fuel pressure and the bowls are running empty. But why would it have run perfect last weekend? I am suppossed to screw in the adjusting screw to increase fuel pressure correct? I am going to be adding this fuel pressure regulator topic to my research list. Yeah, after uploading those pictures, there is definitely something wrong about having that fuel pressure regulator screwed all the way in like that. What do you think?

 

[67 B-body]

yep,,, all the way in and barely 5psi is not right.
Is this a stock mechanical fuel pump your using with a regulator?

 

[john.thompson068]

yep,,, all the way in and barely 5psi is not right.
Is this a stock mechanical fuel pump your using with a regulator?

It is a Holley mechanical fuel pump rated at 110 GPH. Says it maintains 110 gph free flow at 6.5-8 psi maximum pressure and maintains top performance without requiring a regulator. I am so broke, I will probably start messing with the adjustment on the regulator to see if this helps even if I never know what the pressure is. A new guage is going to cost me over 65 bucks for the cheap one if I have to order from Jegs.

 

[70rr-brian]

Are you sure your fuel lines from tank to pump are clean and don't have **** in them?