Carb troubleshooting

[MrMagoo]

Hey guys,

I have a new crate motor, with a new Quick Fuel SS-780 (vacuum secondaries) which is basically a Holley 4150. The motor starts up easily, idles pretty smoothly with 13 inches of manifold vacuum, but it quits when it gets to temperature (after about 5 minutes).

The fuel level in the sight glass for the primaries is 3/4 or more of the way to the top of the glass. The fuel level on the secondary sight glass looks like it doesnt make it up to the bottom of the glass, but there is fuel back there. QuickFuel documentation says they should both be half way up.

Fuel pressure is good, 6.5psi, coming out of a mechanical fuel pump.

After the motor dies, it won't maintain idle again until I wait quite a while, like when it cools down. One other symptom: it doesn't appear that fast idle is kicking in like it should after I start up.

I know about backfires and power valves, and I did have a backfire due to a loose spark plug wire when I first started running the motor, so I was wondering if this could be a symptom of a blown power valve.

I just bought a holley book and borrowed the DVD, since obviously I'm going to have to learn something about these carbs, but I was would love to hear from everyone about where to start and what to look at.

Thanks!

[blykins]

Did it backfire out the carb, or through the exhaust?

One thing to check: are your fuel lines running close to the exhaust? Could it be vapor locking?

Both float levels should be about halfway up.

Fast idle should only be when the choke is on....on a cold start up.

[wtm442]

The fuel level on the rear sight glass looks like it doesn't make it up to the bottom of the glass, but there is fuel back there. I know they should both be half way up.

Wrong. 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. They are just a fire hazard in my opinion.

After making the levels correct, go for a short ride. Stop and remove the sight plug and check the level again.

Now go and adjust the idle screws for smooth idle and best drive-ability. Usually that is at the max vacuum point, but not always.

I doubt that you have a blown power valve since it idles smoothly. It appears that you also have to set the fast idle screw and/or the choke setting.

Sounds like you are percolating the gas in the carb bowls due to heat from the manifold. Adding a spacer between the manifold and carb will help cool down the gas in the bowls, as long as you currently have some clearance between the air cleaner and hood. You can check by putting wet towels underneath both carb bowls and see if the continues to run longer than 5 minutes from a cold start.

One of the best Holley carb book IMHO is Super Tuning and Modifying Holley Carburetors by Dave Emanuel. After you get that book, read it ... and then read it a second time.

[MrMagoo]

Thanks WTM, blykins. I am going to put my sorry ass old edelbrock on there, since it was known good when I pulled my motor. That will tell me whether the problems are carb related or fuel supply issues... or any of the other problems that masquerade as carb problems. My friend always says "carburetor is a french word for spark plug".

[Bobcat]

Agree 100% with what wtm442 said ... but want to add a few comments . Get rid of the plastic bowl plugs , they will give you a false reading on your float levels . The fuel will appear to be higher than it actually is . Go back to the old tried and true method of taking the brass plugs out and seeing where the fuel level is .... and it should be just at the bottom of the hole so the threads are barely wet . Check the level with the engine running . If you are worried about fuel spilling out , do a preliminary check with the engine off to see if it squirts across the shop.
Another good book , if you can find it is by Alex Walordy ... Carburetor Power Guide , Super Tuning Holley Carburetors ... old , but has some useful info .
On the Power Valve , I`ve seen the fiber gasket under the valve separate ( it`s a two piece composite ) due to the chemicals in the fuels today and provide a leakpath under the PV . It`s solved by going to the nylon gasket . I had this problem on my 427 two 4`s Galaxie .

Good luck

[MrMagoo]

Put the old "Edel-Brick" on to test for non-carb problems, and lo and behold it actually runs pretty well, so I am back to "it IS the carb". Edelbricks are easy to tweak, I stepped up 2 stages rich in power mode with a rod change, and fattened up the idle mixture and darned if the old brick isn't in a reasonable state of tune for the new motor. I was getting pretty desperate to just get the thing out of the garage and on the road since I have to test every major system on the car, so I'm glad the Edelbrick is there as a backup. Now I can take my time trying to debug the Holley. I'll take everyones advice and get rid of the sight glass and have another go.

[MrMagoo]

The heat in my fuel lines coming out of the fuel pump is really hot, almost boiling. I am thinking that my Holley problem is really a heat related issue. I just bought a carb heat shield and some other thermal protective stuff from summit, and I am going to try to get that problem resolved before I attempt the Holley again.

[blykins]

Make sure there are not any fuel lines near an exhaust piece, and run a wood or a phenolic carb spacer.

[wtm442]

Originally Posted by MrMagoo

The heat in my fuel lines coming out of the fuel pump is really hot, almost boiling. I am thinking that my Holley problem is really a heat related issue. I just bought a carb heat shield and some other thermal protective stuff from summit, and I am going to try to get that problem resolved before I attempt the Holley again.

If the gas WAS boiling coming out of the pump, you would have problem with any carb, including the Edel-brick.

Sounds like you "should" reroute the gas line or add insulation or a shield to keep heat away from the gas line. It cannot hurt.

You need a carb spacer. The hot non-boiling gas from the pump is boiling in the Holley float bowls due to the radiant heat from the intake manifold. Use metal or phenolic. I would not recommend wood. There is enough stuff that could burn in a car without adding wood. I've never heard of a wood carb spacer.


Summit and Jeg's have a nice variety of carb spacers. As I mentioned previously, measure air cleaner to hood clearance before installing spacer.

[blykins]

Ah cmon....LOL, you've never heard of a wood carb spacer? You need an insulator of heat, not a conductor. If you pop an aluminum spacer on, chances are you're still gonna get some heat transfer from the engine up to the carb. I'd use wood or phenolic.

[patrickt]

Originally Posted by blykins

Ah cmon....LOL, you've never heard of a wood carb spacer?

He's not kidding -- wood is one of the best things for your carb. See: http://www.jegs.com/p/Edelbrock/743637/10002/-1/10289

[MrMagoo]

OK, now I have heard everything!!! I like to brag that there is nothing from the digital age in my car (a slight exaggeration since I have an MSD ignition) but now to go with wood, that takes it to a new level . Maybe it comes with a 5lb halon extinguisher ?

The holley carb heat shield I ordered comes with a phenolic spacer I believe. I did reroute the fuel lines and insulate them. Need to wrap up the fuel filter, I noticed it collects a lot of manifold heat even though it does not make physical contact. A mechanical fuel pump still presents a point of contact for fuel with the motor, but I love the absence of whining versus the electric. I agree that the biggest problem is carb to manifold.


I think the edelbrick does suffer from the heat, as it has always run doggy when the car is hot. However, it does still run. I think my problem with the Holley may be jet related. Keith suggested to me that I change the jetting. Think about it. He sets up the carb (rightly so) to produce maximum horsepower in a somewhat controlled environment of the engine dyno room. Then you get to the engine compartment of my dopey car: stock header flange mated to AFR large exhaust ports, hot fuel, etc, etc. It makes total sense that I would have to jet down, since I am probably pumping 10% (or more) less air than he was able to achieve. That will always be the case until I get new headers with two inch pipes, which may be never.

Joe Lapine from Danbury Competition Engines kindly gave me the name of a fellow named Jeff who I am going to have tune the carb for my car. Lets see, he has a chassis dyno, air fuel meters, a big selection of jets, he knows what he's doing as opposed to me, who has a driveway and a screwdriver. Advantage Jeff.

[danc30]

My QFT 950 carb needs to have the floats set so it's half-way up when viewing through the clear site plugs. I confirmed this with their customer service. The reason is because the metering blocks need it halfway up in order to work properly.

[MrMagoo]

Yeah, thats what the installation instructions on my QFT said too, half way up. That makes it impossible to *not* use the clear sight plugs.

How do you decide between a spacer with 4 holes vs an open one?

BTW do I need a carpenter to do the wooden spacer install

[blykins]

No, just a chalk line and a framing square.

[Michael P. Wole]

Holly claims the blown power valve due to back firing is old school. Just got a 4150/4777 650 dp with mech choke and the dvd that came with it said they fixed that problem back in the 70's. Phoenelic (sp) spacer is the way to go(almost wood).
Sounds like hot fuel to me...what is the float level/fuel pressure when it quits running?
How hot is it getting?

[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.

[Chilibit]

When do they switch from winter blend gas to summer blend in CT? I don't think that is the root problem but it can make matters worse. The winter blend boils easier and if temps rise before the change over we all have problems. In Kentucky it was about 3 weeks ago.

[blykins]

Kinda splitting hairs, but if you have an adjustable regulator, I'd turn it down a hair. 5-6psi is usually right where I run.

Hey Chilibit, where are you in KY?

[MrMagoo]

I am running right off the mechanical fuel pump, I'd hate to stick another gadget in the flow if I can avoid it. Supposedly, our switchover is in March.