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Unread 07-30-2013, 10:49 AM   #1
WSS
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Q-jet getting too hot? Too much fuel psi?

Chevy guys,

I have a SBC in my CJ5 with a new (carb shop) Q-jet set up for angles. It is considered a stage 2 due to the fuel curve mod they did to be optimized at 4000ft.

The new problem (after 300 miles) is the engine does not want to stay running AFTER it is warmed up and if you stop to get gas or pick up ice or even do a little side hike on the trail, it is flooded when you start it again. Sometimes it will stall when you push in the clutch, not fun in the rocks. You can smell it is running rich too. I put a new fuel pump, a new distributor and re-routed the fuel lines to be away from heat. Did not really help much. Another problem is the float sticks fairly often but carb shop says its normal. I have never had it happen before but I have not rebuilt thousands of them either.

This engine does have the original (ish) manifold that uses the exhaust crossover and it gets really hot. I have not used a IR temp gun on it but I would say it is too hot. The engine runs at 180 when just cruising but if you work it hard, it will climb to 220. Running down the road is usually about 190 due to air flow. I am running a 3 core top flow and should probably swap in a 4 core cross flow radiator.

Would it be wise to put in a airgap type of manifold or at least block off the exh crossover? I am leaning toward a edelbrock airgap performer (non-rpm).

Any thoughts?

Thanks!

WSS

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Unread 07-30-2013, 11:09 AM   #2
Newtons3
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I would never advise against an air-gap manifold. They are great, but I dont think that will solve your problem (although it may help). If you feel part of the problem is heat, I would install a phenolic carb spacer. 1/2 inch should do it. It sounds more like a float/fuel pressure problem. How much fuel pressure are you running? How large is your needle seat? What kind of float are you running? Any fuel baffles or fuel well plugs or has all of that ben removed? Early or late Q-Jet?
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Unread 07-30-2013, 11:13 AM   #3
Newtons3
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BTW- What carb shop? Ive built and raced these for years...a float sticking is not normal.
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Unread 07-30-2013, 12:35 PM   #4
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Quote:
Originally Posted by Newtons3 View Post
BTW- What carb shop? Ive built and raced these for years...a float sticking is not normal.
Used to be owned by Brad Urban. It is the "Carb Shop" in Ontario ca.

I have not taken a fuel pressure reading, but it is a rebuilt mechanical (stock) fuel pump, I imagine it is about 4psi.

It has a electric choke but I don't think it is a later electronic model. I'll get a number off of it. I'll have to call them and get the info and just deal with the "this guy is so stupid" routine I get when I call.

I was told this would be a perfect offroad carb when it was done ($500 later). I know they epoxied some areas that were prone to leak and put in some special needle/seat configuration.

When they are running properly, is the Q-jet a good carb off road?

Thanks!

WSS
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Unread 07-30-2013, 12:53 PM   #5
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Ok, got some info.

It has a larger than stock seat which is .134" the reason I was told is it fills the fuel bowl quicker and allows for better mixing (I did not know it was mixed in the bowl) of fuel. The fuel curve was set by using different rods and jets I was told today, however when I ordered it I was under the impression that the valve body was milled or modified somehow. The float is set up using a nitrophyl (spelling?) float and factory weights and fulcrum point.
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Unread 07-30-2013, 01:25 PM   #6
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OK. First, The Carb Shop does great work. Q-Jet will be ok off road. I run a smaller needle and seat than that on a stock-ish engine like your application. I try to stay around 0.120 or less. The bigger the diameter of the seat, the harder it ir for the float to shut it off and the more crucial fuel pressure is. 4 psi would be ok, a little low, but you need to check it. If it spikes to 8 or 9 with that inlet, you will have issues. I shoot for 6.5 to 7. The nitrophyl float is good. If it is a late-model, most people take the fuel baffles and well plugs (if it had them) out. I leave them in most of the time to help with fuel control. If (?) you take the top off, look at the primary metering rods. They are on a piston and may be stuck up. That piston needs to move freely. Make sure the top gasket doesnt interfere - at all. Obviously, make sure the choke and all its linkage is working well.
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Unread 07-30-2013, 05:16 PM   #7
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I agree they do good work, I have had a few done by them though the years, My first was in '87 and Brad and his wife Ruth were it. The second one I had built in '95 at there shop by a guy named Shuan I think. Both of those worked fantastic, that is why I am suspecting something else. They have gotten big now, they do A LOT of carbs.

Can I find the .120 needle/seat easy enough? I see a .110 and a .125 available from here:

http://quadrajetparts.com/rochester-...-c-128_30.html

Here is another odd thing going on, just noticed. The exhaust temps differ from side to side by 20 degrees. The passenger side is running consitently cooler, the side with the exhaust valve on the bottom of the manifold. I need to check closer to the heads and the temps around the carb itself next.

I'll do that and pop the top off the carb to look at the metering rods to see how they are moving and aslso see if any junk is working it's way around in there.

Thanks, I'll report back soon.

WSS
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Unread 07-30-2013, 05:39 PM   #8
Newtons3
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Either the 0.110 or the .125 will do. The smaller the inlet, the easier it is for the float to shut it off. As long as it will flow enough fuel, it is big enough. In other words, bigger is not necessarily better. They supplied my competition a lot of carburetors. I have raced (Dirt Track) against their Q-Jets, 2BBL Holley's and their 4BBL Holleys. They were always consistent. What do you mean by "the side with the exhaust valve on the bottom of the manifold"? 20 degrees measured where? If you are measuring at the tips then who knows. A pyrometer at the collector would be very valuable here. It is possible to get one side rich, but unlikely.
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Unread 07-30-2013, 05:58 PM   #9
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Dirt track! Big fun. I see why you have experience with these. Not exactly straight line asphalt conditions.

I measured the temp along the way from the outlet at the back of the Jeep to the collector at the manifold (right were that butterfly valve thing is on the end of the exhaust manifold). I did not open the hood as I was in a bit of a hurry. I should have some free time tonight.

Years ago they were a specialist in the Q-jet, now they move lots of Holleys. When i was in the dyno room, I could see at least 25 holleys getting ready to go out the door and only two other Q-jets.
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Unread 07-30-2013, 06:14 PM   #10
Newtons3
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Are you shooting it with an IR gun or using a contact style probe? Is the heat riser valve still in the manifold?
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Unread 07-30-2013, 08:29 PM   #11
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Quote:
Originally Posted by Newtons3 View Post
Are you shooting it with an IR gun or using a contact style probe? Is the heat riser valve still in the manifold?
Yes and yes. I have a fluke ir gun thats real accurate. The valve is still in the exhaust manifold, it is a carry over from the Kalifornia BAR requirements (smog laws). Most of the other stuff has been removed, EGR, air pump etc.

I am planning on doing a set of Advance adapters shorty headers and a airgap manifold pretty soon, so this might push me to it.
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Unread 07-30-2013, 09:23 PM   #12
Newtons3
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It is open...correct? That would cause a major vacuum signal problem, temp differential, etc. Just stating the obvious. I actually still believe you are having fuel pressure problems or primary metering rod issues.
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Unread 08-01-2013, 10:05 PM   #13
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Newtons3,

It is opening but I don't think enough. I did a cold start this morning and let it idle for 15 minutes with choke on 1200 rpm. I looked at the valve and it was bouncing but not open. I took some ir temp readings at the muffler and the temp diff was over 100 degrees. On the manifold, the temp had the same difference, 480 on the driver side and 560 on the passenger side. The middle of the intake were the crossover is, was 359 degrees, less than an inch from the carb gasket. I ordered a fuel reg and guage combo today, a quality one.

I "think" the riser valve is not opening unless it is a hi rpm and this could be the cause. What do you think?

I ordered the .110 needle/seat today as well. When I get them I will pop the top and check the primary rods and swap the seats.
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Unread 08-02-2013, 05:53 AM   #14
Newtons3
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Get rid of the riser valve.
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Unread 08-02-2013, 08:39 AM   #15
WSS
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Yep, Good enough excuse to get some shorties put on there. That will be on the list right after the internal carb peek.

Thanks!
WSS
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