Carb troubleshooting

[Chilibit]

Over in Madison County south of Lexington.

There is a Snake and Eggs on Sunday in Louisville. We will be there. Details on location and time on the KCC site.

Ciao

[MrMagoo]

BTW, here is a picture showing the AFR head size versus the header flange size. You can see it's a considerable difference, which I am sure would not have been the case when the carb was originally jetted by Keith Craft:

[wtm442]

Originally Posted by MrMagoo

The pressure seems to be staying steady at 6.5. The engine is just up to normal operating temperature when it honks out. Although I think, in general, my fuel temperature issues need to be addressed, I am now leaning towards jetting as the main problem. Carb tuned for max power, I have a restricted exhaust, hence too much jet for the air I can pump.

Overly rich jetting is NOT going to make the engine quit in 5 minutes. Put in the carb spacer before doing any carb changes.

Then adjust your float levels. Check float levels again after a 5 minute drive where you open the secondaries a couple of times. I know that will be a tough job.

Now check the condition of the plugs. It is probably time to change them. Now set the idle screws and take it for a ride. If it still smells rich, then change the jets.

When I first bought my Cobra (used), it smelled very rich. I adjusted the idle screws and the smell from the exhaust was eliminated. Then I had one dyno pull, just to see what the mixture was like. It was LEAN. I had to go up in jet sizes - 3 steps on both the primaries and secondaries. What a difference!

The 7 PSI at idle is OK. If you bought a Holley Fuel Pressure regulator, the factory setting is 7 PSI. Of course, you should also have at least 4 PSI when running flat out ... thru the 1/4 mile traps, for example.

[Randy Rosenberg]

Originally Posted by wtm442

Overly rich jetting is NOT going to make the engine quit in 5 minutes. Put in the carb spacer before doing any carb changes...

I agree... When the engine quits after 5 minutes, what is the fuel level in the bowls?

[MrMagoo]

The primaries were too high, the secondaries were too low. QuickFuel specifies 1/2 way up the sight glass. The primaries were 3/4 or higher, the secondaries too low to tell, maybe at the bottom. Fuel pressure as I said was normal. I am going to put in the spacer first before attempting the reinstall.

[MrMagoo]

I wasn't able to get the car to the shop on account of the weather, so I removed the float bowls myself just to take a look in there. The primary float was stuck. I freed it, and adjusted the level and now I am idling fine, so I can proceed with tuning. I now have a stumble off idle from a stop in first gear which I am trying to eliminate.

[wtm442]

Check the accelerator pump. The squirter should spray gas into the primaries as soon as the throttle is moved.

[Ron61]

You seem to be pretty well on the way to having your problem solved but I am not sure about the fuel being half way up the sight glass. I always adjusted mine so it was just at the bottom of the plug hole and would splash a drop or so out if you shook the car. I tried the sight glasses and took them out as I really didn't care for them. Hope by now you have everything going great.

Ron

[MrMagoo]

Ron, this appears to be Quick Fuel specific. It says right in the Quick Fuel installation instructions: "Half way up the sight glass", as other Quick Fuel owners have also mentioned. Clearly, this would be impossible with plugs. It would be really interesting to find out why the Quick Fuel carbs are set up this way. On other odd thing I noticed when I took out the float bowls: the primary float was brass, the secondary float was nitrophyl. It doesn't matter to me since I won't be running methanol, but that seemed odd, I wonder if this is done for any specific reason.

Other questions for carb gurus: I bought a tech paper from Don Gould at 4secondsflat.com (although it deals primarily with Demon carbs, it is really a well written and informative paper for general fuel system issues; I highly recommend it) thats says the relationship between primary and secondary jets should be 8#'s, with the secondaries being obviously larger. Anyone else know about that?

I used Don Gould's paper to get my 4 corner idle mixture screws balanced and my take off problem is basically solved. The exhaust smells a little rich now, but not too noticeably so.

WTM442, Don Gould's method has you check the pump lever clearance at idle, start full lean, back out 1 turn on the primary screws and 3/4 on the secondaries, then you tap the accelerator pump to see if it increases RPM, in which case you know which direction (rich or lean) you need to go. I did this, and I am in a fairly good place right now, so I am assuming the pump is working, it was just my idle mixture that was screwed up (too lean, I think).

[wtm442]

Sounds like you are a lot better off than last week.

Holley supplies carbs where the secondaries could be the same size as the primary jets, and some with as much as 10 jet size difference. The best way to size jets is to put it on a dyno. Then the air/fuel ratio will tell you what to change. Then when you are in the ballpark and the A/F ratio is correct per the dyno, go up 2 sizes on the primaries and see if the car feels better. If not, go back to the size you had. Then try it on the secondaries ... go up 2 sizes and see if the "feel in the pants" is better.

I've got a 750 CFM Holley, and right now I have the secondary jets 6 steps up from the primary size. My carb runs best with idle screws about 3/4 of a turn off the closed seat.

I am a little confused by your last paragraph. The accelerator pump spring should be adjusted (with the carb at idle position) until there is clearance. Then relax the spring until it hits the arm and there is no clearance. This makes sure that you get gas thru the squirter as soon as you step on the gas pedal. Then open the butterflies to WOT and check to make sure that there is at least 0.010 clearance (ie additional stroke) by pressing on the pump lever in the center of the diaphragm. This is to insure that you do not "bottom out the pump" at WOT.

As far as the best position for the idle screws, I use a vacuum gauge to get the best vacuum at idle, and then drive it to see if its OK. Many times a slight adjustment is required after setting by the vacuum gauge.

I've never heard of the "tap the acc pump" method for adjusting the idle screws. But if it drives OK, don't worry about it.

Might be a good idea to change the spark plugs.

[vettestr]

Thanks for the heads up info on float height specific to Quick Fuel. The setting does differ from Holley and the other "patterned after" carbs. A 1/2 inch or so increased float level is significant. The Quick Fuel carb is not a standard holley copy.

The issues with the (see through - solid plastic) bowl plugs has been leaking if they break/crack the outer lip of the plug. The plug has a surface that seals against the bowl as it is tightened in place. The brass plugs are tough but the solid clear plastic plugs can be break and leak. I do not know if the plugs were torqued too much or just a defect but the solid plastic ones seem pretty lame to my standards anyway. The metal body plugs with a clear window in them are much tougher and work great.

[MrMagoo]

I probably didnt paraphrase it too well, as far as the initial settings for the acc pump, the way you said it is what's in the Don Gould tech paper. The business about tapping the acc pump lever is kind of unique, I have never seen that anywhere else. The theory behind it, (again, I'm paraphrasing, and this is my understanding of it) is that introducing the fuel from the acc pump and observing the behavior of the motor tells you a lot about your idle circuit: how fast the air is coming down the venturis, how well the extra fuel is getting atomized, how well it is being burned.

Agreed about the plugs. They were new, I'll just clean them up.

[MrMagoo]

Just got back from the dyno session with Jeff Roell at Danbury Chassis Dyno. He got me dialed in. Wow, what a ride. He welded an A/F bung in, tweaked my idle settings, and changed the accelerator pump nozzle to cover a bog when I hit the throttle. No jet changes. I pulled 435HP at the rear wheels at 5600 RPM, which I am real pleased with.

Thanks to everyone for advice and suggestions. If you are in the area, check out Jeff, his shop is a great resource for the local cobra community.

[gene8084]

Originally Posted by wtm442

With the engine running and the sight plugs removed, when you rock the car gently (just a bump) a little gas should just trickle out of the hole. Forget the plastic sight glasses..

First, read your post...and immediately ordered the book "Super Tuning and Modifying Holley Carburetors by Dave Emanuel.

Second, while I'm waiting for it to arrive...I have a boat, Thunderbird Formula 180 (I/0) with a 1974 Mercruiser 188 (Ford 302), Holley 6317-1 (looks like a 4150), double barrel...

I'm far from an expert, but I'm all I've got.

1. I rebuilt the carb.
2. After the rebuild I had some trouble starting and keeping it running. (start-quit, start-quit)
3. The float adjustment was too high, flooding and quiting. (my assessment)
4. I adjusted the float (1/3 clockwise) and now it starts so so.
5. No plastic or glass sight plug in this model, it's a screw.
6. Was worried about pulling the site plug and spilling gas on running engine, so that's my Q below. Need some advice on the how to.
7. Once running it idles smoothly.
8. If I accelerate slowly, it runs fine from 800 RPM up to 2500+ and runs smoothly.
9. If I accelerate quickly, under load, in the water, it stalls.
10. Starts again easily.
11. I don't think I had it stall with the muffs on accerating quickly, but that's a moot point I think, in the water counts.

I'm sure I need to complete adjustments and the instructions with the kit are not specific enough for a novice (assumes you know too much).

For example:
1. Do you remove the sight plug before or after starting?
2. Do you adjust the float height with the engine running?

Interested in your thoughts while I'm waiting for the book.

Thank you.

[MrMagoo]

My understanding is that you check float levels at idle. I have heard that you can use "a cut off bottom of a soda can" or "a rag" to catch the fuel spillage.

Based on what I have recently learned... your problem may be accelerator pump related, and my guess is that is where you are going to find your fix. However, wtm442 will give you an expert opinion, I just offer this for your consideration until you get an answer from him or someone like him.

[wtm442]

Put a wet rag under the bowl to get any gas that spills out. You should check the primary float level first. After it is adjusted correctly, then check the secondary float level.

The straight slotted screw on the top of the fuel bowl is the locking mechanism, and the 5/8" nut is the float adjuster. Turning the nut clockwise will lower the level. Use a box wrench and a big flat bladed screwdriver when making the adjustments.

Ideally, the adjustments should be done with the engine running, but it can be done with the engine off which is much safer, especially for a novice.

So engine is OFF.
1) Slowly remove the PRIMARY float level inspection screw. If a lot of gas starts to flow out of the hole before you get the screw out, the float level is too high. Tighten the inspection screw. Make the float adjustment using big screwdriver and 5/8 inch box wrench. Loosen the locking screw just a little (maybe 1/8 of a turn) and then turn the wrench no more than 1/8 of a turn clockwise to lower the float. Tighten screw with big screwdriver. Most novices will make too big of an adjustment and will turn a high float level into a low float level with a 1/2 turn adjustment.

2) Now you need to use up the excessive amount of gas in the bowl that made the level too high before the adjustment. So clean up any gas spilled and remove the rag. Now start the engine and let it run for a few minutes if you have a mechanical pump. Of course, this adds heat to the carb and makes adjustments harder. Many experienced tuners will slowly remove one of the lower float bowl screws and let all of the gas run into a cup. Tricky for a novice. On the other hand, if you have an electric pump, just turn on the pump without starting the motor.

Repeat step 1 and 2 until level is correct

OR

3) If no gas came out of the inspection hole when the float level inspection screw was removed, the level is either correct or too low. The gas in the bowl should just come up to the bottom of the hole. By bumping the car a little (or rocking the boat a little), a trickle of gas should come out of the hole if the adjustment is correct. If no gas comes out, the level is low. Make the adjustment with big screwdriver and 5/8" wrench, turning the wrench in the counter clockwise direction to raise the level. Put inspection screw back in the bowl.

4) Now you need to put more gas in the bowl. Remove the rag. Now start the engine and let it run for a minutes if you have a mechanical pump. On the other hand, if you have an electric pump, just turn on the pump without starting the motor.

Repeat step 3 and 4 until level correct.

Now you need to check the secondary bowl. See next post. I'd hate to lose all this typing if there was a computer glitch in the site.

[wtm442]

Secondary float adjustment same as the primary float except for one important bit of info.

If the secondary float is too low, use the steps above for the primary float level.

If the secondary float level is too high, you cannot use up the gas just by running at idle. You have to get the secondaries to open up. That means taking the car or boat out for a ride and floor it a few times.

Of course, if you feel brave, just remove one of the lower float bowl screws and let the gas drain into a cup. You probably will have to make a special cup if the carb bowls are close to the manifold. Then replace the screw. Now start the engine and let it run for a few minutes if you have a mechanical pump.

On the other hand, if you have an electric pump, just turn on the pump without starting the motor.

Repeat the steps for the secondary bowl until the levels are correct.

Now take the boat or car out for a 10 minute cruise. Make sure the secondaries are opened up a few times.

Now check the level again, but with the engine running. Many novices make the adjustments too quick or over-adjust. This last step is important, just to check your work with the floats before you move on the acc pump adjustments, jet changes and secondary actuation changes.

[gene8084]

wtm442, Thanks for taking the time for such a detailed reply. I actually only have a single bowl on the 6317...guess it doesn't really look like a 4150.

Thanks again.