CARB PROBLEM

[LAS VEGAS DAVE]

I was wondering if anyone with the same carb as the one in my 38 Special has ever encountered this. I have three carbs, two are rebuilt and one is the original one that came on the car, all the carbs do the same thing. These carbs have a vent hole about an inch above the top of the float cover. When the outside temps are cold or cool there is no problem. When its hotter outside than 80 and I drive in traffic the underwood temperature gets very hot, the radiator temp is still only around 170 but when I raise the hood its hard to put my hands around the air cleaner because its so hot. The problem is that every now and again the carb will spit gas out the vent hole. If I let it sit and idle for the length of a stoplight and then blip the throttle a spray of gas will come out, its about a fifth of a shot glass, its not a lot but it lands on the manifold  or even a few drops might reach the inner hood side panel. After it spits a little then it won't do it again for maybe a minute but as it gets hotter if its left to idle it will do it again. I thought the fuel pump pressure might be to high but it is only about 3 lbs. Remember that this can not be made to happen if the underwood temp is just warm or not real hot. I have tried richening the idle jets but that does not seem to help. The engine seem to run good regardless of this but I don't like the hot gasoline landing on other hot parts. I am thinking of running a small vent tube from the vent and exiting it out the bottom of the car. I am also wondering if I tried gasoline with no ethanal in it if that would cure it.

[Beemon]

Last year when I got the car back on the road, I would have issues with the carb boiling or percolating due to the ethanol fuels. Every fill up I dumped a full can of Seafoam fuel additive into the tank and the issues ran away. I also run 92 Octane every fill up. Don't know if this will help, it seldom gets up to the 80s in Washington.

[DonMicheletti]

I have run the '38 Buick in hot temps and all kinds of traffic and havn't had that problem.

Also, I cant find the vent you speak of - is it this one?  If it is, the hole in mine doesn't go anywhere.

 

In any case, the air cleaner shouldn't get that hot. Is the insulator between the carb and the intake manifold there?

 

[LAS VEGAS DAVE]

Yes Don that is the one. The protruding casting just below the hole is hollow and leads to one side of the float chamber, there is a matching one on the engine side of the carb that is also hollow and leads to the engine side of the float chamber. The hole you pictured goes over the top of both the hollow protrusions and is the vent. About the insulator between the carb and manifold, I don't have one. I don't think they were stock, do you? If they are stock I wonder if anyone sells them. How thick is yours? I could fabricate one if it would cure the problem. I have read lots of stuff about boiling gas and it seems that in the old days the gas did not boil easily but the ethanal gas does. 

[1937-44]

Hi Dave,

 

 I would think the carb to manifold gasket would help and there was probably one on the car originally. You can get one from Bob's and probably CARS. Here is a link to the one Bob's sells;

  http://bobsautomobilia.com/shop/carburetor-items/carb.-base-gasket-3-hole-1934-49-cbg-350.html

 

                                                                              Carl

[LAS VEGAS DAVE]

Upon closer inspection and after I ordered the spacer from CARS I see that I do have the stock one. I put a small magnet to it and it seems to be slightly magnetic so I thought I didn't have one. I don't know why it is SLIGHTLY magnetic unless the magnet is just getting near to the carb base or manifold. I drilled the carb with a 5/16 drill about 3/16 deep at the vent hole and bent a piece of 5/16 tubing and JB WELD it in the recess. The air cleaner hides it as it does the vent hole so it won't be seen. The tube is bent at a 90 degree angle and a 5/16 rubber tube can be slipped over it to vent any gas that might come out of it to go under the car. In the event that I can't solve the problem at least I won't worry about a fire. The carb does not spit the fuel very often and I don't if or when its happening as I'm driving. I noticed it while I was in the garage after coming home from a drive in traffic, I had opened the hood and was just going to adjust the idle jets while it was good and hot. When I revved the engine I saw some gas exit the vent. I could make it happen every once in a while by letting it continue to idle for a couple of minutes and then revving the engine. 

[Daves1940Buick56S]

Hmmm...when revving the engine? Maybe the float needle is not seating properly. Is it one of the original all metal ones or have you overhauled the carb?

 

Cheers, Dave

[FLYER15015]

Dave,

If you don't want to louver the hood, to let the heat out, you could do what I did, and wrap the exhaust pipe with heat tape.

I did mine from the base of the exhaust manifold clear back to the muffler. Keeps the master cylinder a lot cooler too.

 

Mike in Colorado

[Ben Bruce aka First Born]

Any noise reduction?

 

   Ben

[LAS VEGAS DAVE]

It might be fixed but it also might be wishful thinking. After I posted the picture of the little brass tube I bent to possibly vent the gas under the car I decided to install a fuel pressure regulator. I received one today that I had ordered from SUMMITT so I decided to install it and see what happens. After the installation I let the car idle until the temperature was 180 degrees. Normally the car runs at 170. I took the infrared thermometer and pointed it at both sides of the float bowl, it read 161 degrees which is the hottest it ever read by about ten degrees. Then I tried revving the engine while watching the vent tube, no matter what I did I couldn't get it to spit gas. I tried holding it steady at a high rpm, then letting off the throttle quickly then staying it again. When I did this before it would spit gas, nothing I tried ever got it to spit any gas and it always ran smooth. It still had the old gas in it with the ethanal as I've not put any fresh gas in it. The regulator is adjustable and it comes set to maximum 4 lbs of pressure. I adjusted it to thee pounds. I can't believe the stock fuel pump put out to much pressure as when I checked it at an idle it put out between three and four lbs pressure but I couldn't check it when it was revved up. As you can see from the photo the air cleaner hides the regulator but not the hose which connects the regulator output to the carb input. If I get some different fittings I might be able to hide everything when the air cleaner is on it. We will drive the car tomorrow on the freeway for a final test.

 

Edited April 9, 2016 by LAS VEGAS DAVE
changed photo (see edit history)

[carbking]

There is a sight plug on the side of the bowl of your carburetor.

 

You might check the fuel level AFTER the engine has idled for a few minutes. If the fuel level in the bowl is correct, fuel should just SLIGHTLY ooze out of the sight plug hole. In other words, the fuel level should be right at the bottom of the sight plug hole.

 

Jon.

[LAS VEGAS DAVE]

4 hours ago, carbking said:

There is a sight plug on the side of the bowl of your carburetor.

 

You might check the fuel level AFTER the engine has idled for a few minutes. If the fuel level in the bowl is correct, fuel should just SLIGHTLY ooze out of the sight plug hole. In other words, the fuel level should be right at the bottom of the sight plug hole.

 

Jon.

 

Thanks Jon for that bit of of info, I wondered why there was a plug there. Is the correct way to check it with the engine running while it is idling or with the engine off after it has idled for a few minutes?

[smithbrother]

You can make a heat deflector that sits on the manifold where the carb bolts, make it out of sheet aluminum, extend it out a bit, and if you can bend the sides down a bit to deflect heat away.  I have purchased carb base adapters that are approx. 1-inch thick that are of a heat absorbing materials that helps this issue also.

 

I wish you well, but feel it isn't that hard an issue to correct. IMO

 

I believe my latest issue of Street Rodder Mag address making one of the aluminum shields.

 

Dale in Indy

[carbking]

1 hour ago, LAS VEGAS DAVE said:

 

Thanks Jon for that bit of of info, I wondered why there was a plug there. Is the correct way to check it with the engine running while it is idling or with the engine off after it has idled for a few minutes?

At idle, with 5 psi fuel pressure.

 

Jon.

[LAS VEGAS DAVE]

Dale, I agree, it isn't to hard to correct. I will keep working on it till it is done. We are leaving in a few minutes for a 20 mile drive, lets see what happens now that the the regulator is on it. When I get back we will check the float level by using the sight plug. I am hopefull

that I don't need to fabricate a heat deflector, that will be a last resort as I'm trying to keep it looking stock even if there is a few mods.

[LAS VEGAS DAVE]

1 hour ago, carbking said:

At idle, with 5 psi fuel pressure.

 

Jon.

 

Jon, we just got back from a twenty mile test. The first 10 miles were freeway. On the way home we took surface streets that had lots of lights and heavy traffic. I had placed a crumpled clean paper towel under the vent tube I made so that I could tell if it spit gas at any time during the trip. When we got home the crumpled paper towel was spotless. I tried to make it spit gas while the hood was up and I watched the tube. I worked the throttle fast, slow, steady high speed, etc but could not make it spit. It runs and idles smoothly. While it was idling I removed the sight plug and although no gas weeped out I could see the level at the bottom of the plug easily. I tried revving the engine and holding it there but the gas level at the sight plug remained level with the bottom of the hole. The spitting gas out of the vent hole seems to be cured. The only thing I did that changed anything was to add the fuel regulator which is now set slightly below four lbs pressure. I did have the fuel pump rebuilt a few months ago by Terrill Machine out of Texas, maybe that somehow raised the pressure. I am not sure if it ever happened before the pump was rebuilt or not. Once I discovered it I could make it happen whenever I opened the hood and watched the vent hole while I revved the motor right after it idled as long as the under hood temp was high, never when it was cold. IN ANY CASE IT SEEMS TO BE FIXED.

Edited April 9, 2016 by LAS VEGAS DAVE (see edit history)

[carbking]

Glad that it seems to be fixed.

 

Fuel pump output pressure can be effected by the diaphragm return spring used. There used to be a builder in the hobby whose mantra was that if the part fit, it was the right part! Have seen some fuel pumps that produced way more than 10 psi. Since we used to restore carburetors, and these pumps caused us some unnecessary rebuilds (customers blaming us for leaking carbs), we shopped around, did some testing, and have for many years suggested that fuel pump rebuilds or rebuilding kits should come from Then & Now Automotive, in the Boston area. Their parts have always been equivilent to original. This is even more true today than a few years ago, because of some vendors and  "rebuilders" importing cheap off-shore parts.

 

Since the regulator seemed to have fixed your issue, I might suggest acquiring a fuel pressure gauge, removing the regulator, and observing the actual pressure. If the actual pressure IS too high, I personally would want to correct the issue rather than using a band-aid.

 

Just my 2 cents.

 

Jon.

Edited April 10, 2016 by carbking (see edit history)

[LAS VEGAS DAVE]

Jon, I really do appreciate your posts. I love learning about this stuff. I had no idea that the spring or diaphragm could make such a difference. I will test the fuel pump for pressure although not right away as I'm going to drive the car some more in hotter weather to see what happens. I googled THEN AND NOW auto parts in MA and got their number. They are closed today but I will order a rebuild kit on Monday so that the parts will be here if the pressure is to high or the present pump quits. This car has a rare fuel pump, its a stock 1938 double diaphragm pump. It was available in 38 as an option. I have a part number, Its an AC1523687. I used a vacuum gauge to test the fuel pump pressure while I was searching for a reason for the spitting gas. It took two hands as I had to hold the tapered rubber piece on the end of the gauge tightly against the fuel line opening with the car idling, it took both hands so I couldn't rev the engine. As I remember it showed under 4 lbs but I don't know if it would have risen as the throttle was opened. Thanks again for your input.

[LAS VEGAS DAVE]

In case anyone is following this thread I thought I should add this to it. Since adding the fuel pressure regulator I've been able to adjust the two Idle air screws and the car has never run so good. The car used to surge slightly when I drove it. It felt as if it was stating to starve for gas and then almost instantly was ok and then started to starve again and on and on. It was just slight but I could feel it. It no longer does this and just pulls steady all the way up through the rpms.