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Better to use a Carburetor Specialist or Do-it-Yourself?

When money is tight, some engine builders try to save money by doing their own carburetor tuning rather than farming the work out to a specialist, says Steve Zepeda of Zepeda Carburetors in Pearland, TX. That can be a costly mistake.

“If you’re building a $20,000 performance engine for a customer, you shouldn’t be pinching pennies on the carburetor. I get calls from engine builders who wonder why an engine they just built that should be making 600 horsepower is not. The problem is usually the carburetor. It hasn’t been jetted and set up correctly. It may be running too rich or too lean.

“I’ve also had engine builders coming to me for parts and advice because they are trying to do their own carburetor dyno tuning. I tell them let me do the carburetor for you so it’s done right. That way it will run right and everybody will be happy.”

Zepeda said setting up a carburetor takes a lot of know-how and experience. Some of the old carburetor experts have passed on, leaving a void in the industry that can’t be filled with off-the-shelf products. Custom carburetor building requires selecting the right combination of parts, then jetting and tuning the carburetor to match the application. A carburetor that is not set up correctly will never develop peak horsepower. It may also have flat spots, or hesitate or stumble when the throttle is floored, or stall when you let up on the throttle. And on a street-driven vehicle, some off-the-shelf aftermarket carburetors run too rich and get poor fuel economy.

“One of the things we’ve noticed in our area is that carburetors have to be tuned for the kind of fuel that’s available. Oxygenated gasoline that contains alcohol does not develop as much power nor deliver as good a fuel economy as ordinary gasoline. Race cars running straight alcohol also need special coatings on aluminum carburetor components to protect them against corrosion,” said Zepeda.

Zepeda said his shop can setup almost any type of carburetor, or custom build a carburetor from billet aluminum components to suit a customer’s needs. “If you want performance, we can build performance. If you want pretty, we can do that, too.”
I have used Steve on a 1977 SOB that uses the same setup as the Barth and the difference was astounding. He checks the carb numbers against the exact engine that was installed on the original chassis and then makes the necessary adjustments and improvements to maximize the system. On mine we changed to an electric choke that immediately cured one leak around the butterfly shafts and then he did air flow testing on his bench system to balance the jets and metering rods. He spends lots of time doing NASCAR carbs for the race teams. His web site is : http://www.zepedacarburetors.com/
Tom

quote:

Originally posted by Tom K:
I have used Steve on a 1977 SOB that uses the same setup as the Barth and the difference was astounding. He checks the carb numbers against the exact engine that was installed on the original chassis and then makes the necessary adjustments and improvements to maximize the system. On mine we changed to an electric choke that immediately cured one leak around the butterfly shafts and then he did air flow testing on his bench system to balance the jets and metering rods. He spends lots of time doing NASCAR carbs for the race teams. His web site is : http://www.zepedacarburetors.com/
Tom

Tom, if you don't mind me asking, approximately what was the ballpark cost for this service?

Also, what was the turn around on his services?

Thanks
-Curtis

Jim tried to talk me into dropping the rear tank after I dropped the front one but It was a daunting job I didn't want to tackle. The filler hose that goes from the tank over the chassis rail was a bear to put in according to prev owner. The hose had to be squished(it does not back up when we fill)over the rail & it looks almost impossible to do. We'll check the fuel pump while we're there. Do we cut the hoses before we lower the tank? I don't want them to pull & break something.

We are also going to replace all the lines with the marine grade fuel lines, the steel lines, the fuel pump. We are having the carb rebuilt by Lee's grumpy old buddy Otis Heritz. Jim talked to him yesterday & he has been doing it for 47 years. I will also replace all the vacuum lines while the carb is out. We will blow air back through everything making sure there isn't any trash. I guess I need to take some pictures so I know how it all goes back together.

Tere

quote:

Originally posted by Jim and Tere:

We are also going to replace all the lines with the marine grade fuel lines,

Good idea. This is something every old gasser will need, sooner or later.

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I will also replace all the vacuum lines while the carb is out.

Good idea. NAPA has an orange silicone rubber hose that takes MH doghouse heat very well. Mine took H467, 5/32" ID.

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I will also replace all the vacuum lines

Hi Tere,

As long as you're re-doing all the vacuum lines, give some thought to replacing the vacuum modulator on the side of the tranny....easy one-bolt & slip-in job, and about 20 bucks for a good adjustable one (B&M #20234). The old one might be the same age as your lines, and it's got a rubber elbow & internal diaphram that could leak too. A screw adjustment allows you to easily change your shift points and get crisper shifts if ya want.

....And not to worry about your carb job....Old Man Heritz's carb skills are exceeded only by his business ethics and fair dealings. Fortunately, his career and market demand for carb work are ending at about the same time

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Did you remove or change the position of the heater/plenum hose to the carb?

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Where is the heater/plenum hose?

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give some thought to replacing the vacuum modulator on the side of the tranny

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Thx Lee will take a look

Rusty, I thought of the sock filter and when we drop the rear tank, I'll replace it. But the problem also appears when running on the mid tank. My unprofessional guess is that when we ran the rear tank dry, then filled it with gas, we disturbed a bunch of trash that somehow made it through the two in-line filters and the small filter in the Q-Jet. As Tere said, we're going to start by dropping the rear tank, sending it out to get it coated etc., and redoing the whole system including the Q-Jet. New fuel pump, everything. One question is should I replace the metal fuel lines with new metal fuel lines or with marine grade hose? The marine grade hose would be easier but I don't care about easy, I care about best as in no more breakdowns due to old fuel stuff.
Jim

Unless the filters collapsed (and the one on the carb body is sintered metal, and won't fail - but will clog), dirt would not get into the carb bowl. Cleaning the carb on the road would have fixed the issue if that were it.

I'd not spend any money on a carb rebuild until the lines and tanks are cleaned, AND the fuel pump is replaced, and the old carb tried again.

As for lines, I prefer the marine grade flexible, and with a braided shield, over steel.

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Is it possible the second line with the filter was to your generator and not a return line? I've never seen a return line on a Q Jet before. Very fishy!!

Corey

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It's not for the generator, our gen runs off the rear tank. When we get below 1/4 tank in rear the gen won't run. So is it or isn't it a return line. If it is not a return line then what is it? I have two lines coming from the front tank too. They are the top two hoses on the switching unit & the rear tank is the bottom two.

See Tech Talk-topic-"valve for switching gas tank"
for pics of what I'm talking about.
Which way do these flow?

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Unless the filters collapsed (and the one on the carb body is sintered metal, and won't fail - but will clog), dirt would not get into the carb bowl. Cleaning the carb on the road would have fixed the issue if that were it.

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Looking down into the carb when gassing the throttle, the chamber on passenger side has a jet of fuel that shoots out toward the right rear of the MH, the one on the left shoots out toward the left rear. The left(drivers) side was shooting a spray of gas with no obstructions, the passenger side had a void in the center of the spray area. That is when we determined there was debris in the jets. When we took the top of carb off, there was a large clump of red gasket sealer in it. Then we saw some down in the jet itself but we couldn't get to it. We took everything we could take out & blow through & cleaned & blew through it. Might have blown some of it out but not enough to have a smooth ride. Just assume it is in the carb based on what we found in it. How would gasket sealer get in there???

We took the fuel line off the carb & tested the flow into a coke bottle. It filled it in less than 5 sec from both tanks. That is why we took the carb apart because there was good flow from both tanks.

quote:

How would gasket sealer get in there???

Somebody may have had it apart and reassembled it with sealer instead of the correct gaskets.

There were some fuel pumps on the 454 set up that had a return line from the pump to the gas tank for vapor and unused gas. It was there to help with vapor lock. Ours happens to be one of them. When looking at the fuel pump, there is one 3/8" line for intake and one 5/16" line for the vapor/unused gas return. You can follow that 5/16" line all the way back to the fuel tank. Then there is the outlet to the carburetor which is a solid metal line.

Curtis: I paid about $125 four years ago and it took about two days. But as you are reading, there are a number of other possibilities. The gas tank has four lines in addition to the filler tube. One goes to the inlet of the mechanical fuel pump, one to the generator, one is a return line from the outlet of the charcoal canister and in some cases the last is a vent line (before the age of sealed gas caps, which I found to be one problem that was solved by loosening the cap and driving it) and in other cases it is a pressure return from the outlet diaphragm of the fuel pump. One check of the carb is to attach a can of gas to an electric fuel pump and then to the inlet of the carb and run the engine. I have seen, but do not recommend, some people doing this inside the coach with a crew of two. Basically you need flow and pressure to diagnose the carb and this has to occur under load and the whole range of throttle positions. That is what Steve does. But all the checks and hoses, and potential blockages should be checked out before you blame the carb. It almost never fails suddenly, more like a slow deterioration of performance. To recap in addition to the ideas above try unscrewing the gas cap and then drive the coach. You might be surprised.
Good Luck

Sounds similar to problems discussed some time ago. Search for "emptying a gas tank" and look for my post at "Posted 09-18-2005 09:31 PM".

Shot in the dark but a possibility.

Mike

Sending pics of fuel pump, vacuum modulator, some strange thing attached to the exaust pipe?(any ideas),rear fuel lines etc.

Will be replacing vacuum lines(are vacuum lines all the same size?) & fuel lines. Some of the steel fuel lines are pretty rusty.
Is there a rule of thumb about the run.

From the rear gas tank we have a 3/8" hose hooked to sending unit about 3' long. It is then attached to a steel tube that is then attached to a rubber hose that is attached to a metal cannister fuel filter, attached to a rubber hose, attached to the switching unit.
from sw unit back to rear tank is 5/16" rubber hose to metal cannister fuel filter, to rubber hose to steel hose to rubber hose to rear tank.

The front tank from sending unit is 3/8" rubber hose attached to switching unit. Then there is a 5/16" return hose from the switching unit back to the return line of the tank.

From switching unit to fuel pump we have a 3/8" rubber hose about 18" long attached to a metal cannister fuel filter, attached to a 4" rubber hose attached to a 3/8" metal tube that continues to a rubber hose about 3' long attached to the supply of the fuel pump.

Return line from pump back toward switching unit we have a 5/16" rubber hose about 6" long attached to a metal tube about 6' long attached to a rubber hose attached to the return of the switching unit.

Up at the fuel pump I see two different type of rubber hoses attached to the return line & the supply line. The metal fuel line going from the fuel pump to the carb is brand new(yr old).



















Tere

Looks like an exhaust brake on the exhaust pipe. Never seen one on a gas engine.