Carb Problem?

[David Kirkham]

Fellow Car Nuts:

I may know how to turn a Coke can into a car...but I am lost when it comes to carbs. I have built several engines, but always left the carbs up to Thomas.

My brother Thomas isn't able to help me right now...so I have been left out on my own on this one. The weather is so nice, I can't stand to not drive...but I do have a problem.

A couple of days ago, I changed the jets in my car from 76 to 72. (Holley 850 double pumper). The car stunk like a gas can when you drove it and the pipes were as black as the mines of Moria. A quick check of the plugs suggested I had pulled them out of a coal mine. Remember, we are at 5000 feet here.

I called the engine builder and he thought I might have the distributor phased backwards...sure enough, it was. Reversed the wires to the distributor. MSD has had this problem for a while I have since heard. Re-did the timing...35 degrees. Seems to come in quite late though...at about 4000.

So, one step hotter on the plugs...put in Autolite #24's up from 23's and put it all back together again. While I was adjusting the timing, the car seemed to run fine. Then, of course, the car ran out of gas and backfired a little through the carb...not too much though.

NOW,

The car doesn't seem to have too much power from 1000-3000 rpms. (427 side oiler with 428 crank). Step on the gas a little at 2000 and there is a huge hesitation then it lights off. Seems to run normal at 4000-6000 rpms but maybe not as powerful as usual. (Who knows at those rpms anyway?)

The car seriously stumbles at 2000-3500 and now that I think of it seems to miss all the way up to 6000. Again, hard to tell because I really don't drive up there anyway.

The car sure smells better...but runs worse.

Any suggestions?

David

ps Jamo...good news, the experiment was a success. Car will be finished in days...stay tuned.

[Whaler]

Hello David

You are going through exactly what I am with my carb/engine now. Whether u were following it at all, there are good tips on carb tuning posted on a thread I started asking about total CFM for the dual 4 crowd

Anyhow I have some questions for you.

1) Is your carb the Holley 850 with man or vac secondaries?
2) Doe it have a choke- manual or otherwise?
3)What is your base idle set at ( Intitial )
4)What heads and cam...is it cammy?
5)What fuel? octane wise?

6)If you have the MSD, what springs did you choose for your advance curve...this will help solve the 4000rpm question

I too suffer a pop or 2 on initial startup. I have a manual choke thats wired wide open on my Holley 850. After a warm up I have idle at 900-1000 with 10 inches vac. when I shut it off it sometimes stops cleanly and other times it has a slight run on ( you know 2-3 more sputters) And mine too smells like fuel.
My initial is set at 14 as a start point and I don't know to go up or down with the slight run on as it is. I hear that never less than 10 and never more than 20 is a good ROT

Is yours a new problem? Did the car run well before? What had changed?

A stumble could be lean or delivery problems.
I take it you have fooled with the idle air/mixture screws? The place to start on those is 1.5 turns out from close on each corner ( if you have 4). Out richens and in leans
Adjust these out or in, until the car just falters.
Also adjust the screws equally and if you have one, use a vacuum gage, and adjust for peak vacuum. This adjustment some times changes your idle RPM and that may have to be reset after.

Hows the float bowl level? The sight plugs should either just have fuel visible at the edge or starting to TRICKLE out.Ahigh level here can cause flooding and richness in the idle if it even starts.

There are good tips in the links I had included in that earlier post.

Tim

[John A. Simpson]

David,

You may need a different power valve or your backfire may have damaged the existing power valve. The power valve opens at a specified vacuum level to admit more fuel, if your car doesn't make much vacuum (less than that required by your particular p valve) the power valve will open creating a rich condition, a blown valve will do the same thing but both should clear up at higher rpm when they would normally be open. Is your fuel pressure OK, 5-7 psi, if its too low you may have a miss, if its too high the pressure can override the needle/seat assembly and be too rich. I don't know of a simple test for power valve integrity without removal.

See you Friday

John

[Jamo]

Thanks David.

[Whaler]

Depending on old that Holley is, they say the carbs built in the last 6 years have backfire protection built in. Inline check valves now save the holley of today.

http://www.holley.com/HiOctn/TechSer...fo/CI-121.html

"Many of the new Holley performance carburetors built today now incorporate "power valve blow-out protection". With this provision, the power valve diaphragm is protected from damage due to engine backfire, by a check valve that is located in the throttle body. This check valve is designed to be normally open but quickly seals to close off the internal vacuum passage when a backfire occurs. Once closed, the check valve interrupts the pressure wave generated by the backfire, thus protecting the
power valve diaphragm."

Tim

Actually David pick any on the links off the bottom of this page they all touch on what I asked about

http://www.holley.com/HiOctn/TechSer...nfo/CI-44.html

[David Kirkham]

Tim,

Sorry, I did not follow any of those earlier carb threads. I have never really followed them...I guess I need to now! It always seems the problems you face in life are in the area you need to study the most.

I think the carb is an 850, but the more I think about it, the more I don't know. Like I said, Thomas has always been in charge of the carb department and I wasn't smart enough to pay attention. How do you tell what kind of carb you have? What should I look for?

I do know the following.

4 idle mixture screws.
Air horn has been milled off.
Manual secondaries
No choke.
Base idle is about 800-900 rpm
Aluminum Edlebrock heads
Cam unknown, but not too radical. Nice street motor. Power on the dyno starts to fall at 6500 so it can't be too radical.
91 octane fuel.
9.5:1 compression
I didn't do the distributor, engine builder did. I know the springs are probably a little too stiff as the advance is a little late for what I would think.

Any suggestions on when full advance should come in? Full advance is set at 35 degrees right now--around 4000 rpm.

I am not sure what the idle timing is...I didn't check it.
The car ran fairly well, but progressively went down hill. That is why we suspected the distributor was phased wrong. We checked the rotor and surely, the burn on the rotor was on the trailing edge. The coil contact on the cap was completely burnt away as well. Changed the cap and rotor.

A lean stumble makes me think the power valve is toast. The carb is a brand new Holley, so maybe it has the blow out protection. How do I check if it has the protection? How do you check a power valve?

I fooled around with the fuel mixture becasue the car died under hard braking. The fuel mixture needle is now 3 1/2 turns out on the back and 1 1/2 turns out on the front. That is the only way the car won't die under hard braking. All that was done according to the engine builder's suggestions. I do trust the engine builder, but he is a long way away and trying to help from a long way away. Of course, you are too, but as we write things down, there is a certain amount of thinking we have to do as well. BTW, I truly thank you for your suggesitons and time.

Fuel bowl level has been set to the screws. Just barely will run out if you slightly jiggle the car.

Main complaint is the stumble on acceleration and the seeming miss at all rpms. Seems to be low on power.

John,

Looking forward to seeing you. When do you think you will be in?

Fuel pressure is 6-8 psi always.

David

[Eddy K]

I am experiencing that same problem right now. I am no carburator expert but I do have a Holley book and have also talked to Jegs tech line about this.
From all I can find out, this is a common problem with 850 cfm double pumper Holleys with manual secondaries.
When you hit the throttle it creates a lean condition which in turn creates a stumble. You need a bigger shot of fuel to overcome this.
I have ordered a 50cc accelerator pump to see if that helps and you can also install bigger squirters. The stock size is about a 31 and the tech guy recommended 37's.
I am going to give this a try tomorrow and see how it works.
The only way to stop this or lessen it is to just experiment.
Actually, the best thing to do for on the street use is to get a carb. with vacuum secondaries.

[David Kirkham]

Eddy,

Any suggestions on the best carb for the street?

David

[concobra]

If I may make a suggestion: Please consider contacting Mr. Patrick James, owner of Pro-systems.

Tel.: 231-798-6397
www.pro-system.com

Patrick built my carburetor for my FE Ford. Like yours a 427 with 428 crank. When we took the engine to the dyno the carburetor settings were spot on. We could not improve on the carburetor settings. Fuel curve over the entire RPM range was as close to perfect as possible, idle quality excellent, throttle response so good that others in the shop walked over and looked in the dyno room to see if the engine was injected.

I could go on but suffice to say that Patrick truly know his business. Plus he is a pleasure to work with.

[David Kirkham]

Muchas Gracias for the info. You can be sure I will be giving him a call on Monday.

David

[David Kirkham]

Concobra,

What kind of carb did you get and how much did it set you back?

Thanks for the help,

David

[PatBuckley]

David -

Did the problem start after you re-jetted? There may be a clue there...

Check the plug wire resistance and for a good connection - as for the stalling under hard braking - put a tube from the one bowl vent to the other and put a venting hole in it at the very top.

I have always had good luck with the Barry Grant carbs - for my stroker in the ERA I used an 850....

Pb

Happy Easter!

[Excaliber]

,,,that backfire thing sure does point to blown power valve all right.....

I never did figure out a "consistent" way to check them other than "replace".

As far as accellerator "pump shot" you can look down the throat of the carb and SEE it injecting gas, which it better be doing in front AND back. Manual secondaries would indicate it is in fact a "double pumper" (two acclerator pumps, fore and aft). My guess is the double pumper would NOT have power valve protection! Thats actually a fairly rare carb., most were vacuum secondary.

Ernie

[Steve R]

David,
I note you have had confirmed ignition problems. If I were you I'd steal a known good carb off of another car.... Yeah, you have my permission to rob it from you dad's car Anyway, slap the known good unit on and see what happens. You might have multiple problems here, which is not a good time to take carb tuning 101.
You really need to take the time to sit down and educate yourself on the basic theory of the Holly carb. This will allow you to reason through the problem if it is the carb.
BTW,
The power valve has a little diaphram in it that the vacuum acts on to close it during high vacuum conditions. When you drive the car and accelerate, the vacuum drops and a little sping in the power valve valve opens the power valve to to supply additional fuel via a paralelle path.
Anyway, a blown power valve (ruptured diaphram) would cause a rich condition when in low load/ rpm conditions because it would then be open all the time.

Steven

[SNAKE CHARMER]

David In your original post you make referance to a problem MSD has had with distributor"phasing". What is this? How did you fix? Thanks in advance for reply. STEVE

[mr0077]

David, my first impulse is that your current problem is due to having the carb too lean now, after the jet change...did you change all four jets? Has the car always had the rich odor and sooty plugs? If it ran okay then went rich, maybe your jets were close to right but you blew a power valve (causing it to go rich on you), and giving you the indication that the jets were too rich? And, the late advance curve and reversed polarity on the MSD certainly didn't help with the drivability problems.
One (not very tasty) way to check the power valve on the cheap is to pull it out and suck on it (clean it off first!), or, get a tester (where?) or just change it. If it has a vacuum leak it's no bueno if the needle moves off the seat with no apparent leak, it should be working "okay"...
Too bad the choke horn is milled off...if you could find the Holley part number you could look through their online tech info and see what jets and power valve it should have.
What vacuum are you pulling at idle? Make sure the power valve is rated a couple of inches lower (smaller number) than your idle vacuum reading.
If the power valve is good, I would think it is accelerator pump problems, or a combination of lean jeting and pump shot(s) too small...
If you have an MSD distributor, I'd recommend recurving with an MSD blue or silver stop bushing and the light blue and silver springs (MSD curve "E"), and set total advance about where you have it at 35...in any case, getting the advance all in by 3000 RPM should improve the drivability.
BTW, tell us a bit about the reversed polarity issue...what are the symptoms of reversed polarity, how do we check that? I'm like Snakecharmer, hadn't heard about that.

[Whaler]

From the online Holley site

"QUESTION How do I know if a vacuum or mechanical carburetor is best for me?

ANSWER For streetcars the vacuum secondary carburetor works best on midweight or heavyweight cars with an automatic transmission. They are more forgiving than a double pumper is because they work by sensing engine load. The mechanical secondary carburetor is best on a lighter car with radical camshaft and a lower gear and manual transmission or on a car that is going to be used for racing purposes. "


So gang this is how I picked out my carb I think a cobra fits description #2

The key is Vacuum!

Here's a good tuning page for Holley.

http://corvetteactioncenter.com/kb/q....php?qstId=434


Tim

[Excaliber]

Phasing.

Let me take a stab at it. The number 1 spark plug wire is actually in the number 2 hole of the distributor cap. Number 2 wire is in number 3 hole, etc. etc.? The timeing is kinda OK, the engine runs, but the spark energy is reduced?

The rotor tip should be in good alignment with the metal electrode that leads to the spark plug wire at the top of the distributor WHEN the coil fires. You can tell how well it's aligned by looking at the metal segment. If it's all burned up on one edge and NOT in the middle, this alignment is not good. Never heard it called "phasing", but the term fits very well to describe the condition. I would think this would be a problem only after some "time" had passed. On a new rotor and cap it may not be noticed right away.

Ernie

Edit: In diagnostics the "eternal" and "first" question has always been.....Is it a spark or fuel issue? Tough call when it's both!

[John A. Simpson]

David

I think the suggestion to try another carb is a perfect idea, nothing more frustrating than trying to diagnose two problems at the same time. If you have a carb with vacuum secondaries put it on until you have the ignition sorted. The HP series carbs, the ones with the choke horn milled off and the 4corner idle system are usually race items. Not that they can't be used for the street but require more 'tuning'. The proper carb depends on use, for predominantly track time you have the right carb. For mostly street maybe a carb with vacuum secondaries, for an equal mix some people like the control of mechanical secondaries, ie: a little spring doesn't decide to open the rear barrels in the middle of a corner. Your driveability may improve with a smaller carb, some people run as small as a 650 on a 'mild' big block for throttle response. A 780 vacuum secondary would be good for someone with out much carb tuning experience. The fact that the engine ran fine for the builder and now is a problem is the real puzzler!

Have you checked (with the engine off, if you value eyebrows) to see if the throttle blades are opening properly, and that the accelerator pumps are indeed squirting fuel down the throttle bores when you manipulate the linkage? Did you check fuel pressure, not too much not too little? Maybe a good idea to check filters on the fuel system.

I'm closing my second mortgage (Cobra mortgage?) Friday morning so I imagine it will be early afternoon before I can get to Provo. I'll have my cell so I can call and let you know as I get closer. Just e-mail me if you want me to call a particular number.

John

[George Snyder]

David,

A few suggestions.

To check a blown power valve is easy. With the engine ideling hold your fingers over both air tubes. If the engine shuts down it is blown. You only have a power valve on the primaries. If blown when you replace it try getting one that is 2 inches lower than your intake vacuum, rather than the standard 6.5. If your jetting seems right, but the exhaust still reeks with gas smell this may help.
To help with the backfire and sputtering during acceleration, try setting your indle more from the rear screw than the front. You will see the throttle plates opened slightly.

The timing I agree with others. Blue bushing (21) + 14 initial. Try light silver springs first and adjust from there. Remember that you can mix and match any spring combination. All in by 3000 sounds good.
Jetting can be tough, eight sizes seems to be the normal, but I have seem good results both closer and farther apart.


Good luck.
You did not mention which intake you are using? If it is an Edelbrock Performer RPM, make sure you have a steel carb plate installed between the intake and carb. They are known to be hard to seal.
Did you change the carburetor gasket each time you removed and reinstalled the carb? You may be able to flip them over one time.