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Old 09-07-2013, 09:31 PM   #1
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Poor Jamboree gas milage

Hey Guy's,
I have 1989 24' Jamboree class C with the Chevy P-30 and 350/TH400 combo. It's very clean and have been doing some upgrades, Trans cooler, synthetic oil, new water pump extc.
My question:
It does not have OD just the 3 speed. I added a tach. and it runs about 3000 rpm @ 60 or so, does that seem right? I baby it getting off the line and do not go over 60 mph but I only get 7 or so mpg. It seems low for a 350, it has 91,500 miles and runs good, smoke a bit when cold but I think that's carb. that needs adjustment. I wish it had OD though. It has true dual exhaust that seem original but with a valve of some type on the drivers side exhaust just after manifold that seems to be operated by vacuum.
Just wondering if this all sound normal? I was hoping for at least 9/10 mpg from a smaller class c like this.

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Old 09-07-2013, 09:46 PM   #2
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Have you had it tuned up? You are also running a carbureted engine vs. EFI. That will also account for less mileage. What weight are you at? That will also affect mileage. Air pressure on tires. How many miles have you driven to get the mileage you have. Mine varies depending on many factors.

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Old 09-08-2013, 06:58 AM   #3
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I try to do as much myself as possible. I replaced the air cleaner, check the plugs and they look good. I might do the wires and start there. It's at 9,500 lbs. it was mostly loaded for a trip I took from Tampa, Fl. to S.C. round trip was about 900 miles and it never got to 8 mpg the whole way. Got as low as 6.4 on one fuel up stayed around 7. I inflate my tires to 80 psi. Yes, it's got the thermoquad carb. I guess what bothers me most is the RPM's I'm puling 3,000 @ 60 mph ? is that about right for this rig? As I mentioned I looked up the info tag numbers on the glove box that's on the motor cover and going to a GM site it has the TH 400/3 speed tranny and the HD 350 motor.
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Old 09-08-2013, 07:12 AM   #4
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That's about right rpm and a good power making rpm for a small block Chevy. And the mpg is very normal for that type of rig. Just drive it and have fun and don't worry. We used to run trucks like that 80 mph all day long at 4000 plus rpms with no problems. If you had the four speed auto with overdrive then it would not pull it in od all the time and it would be hunting for the proper gear on every grade. And fuel mileage would be no better as I've had them also. Some days we only get 4 with our rig if its windy and I run it hard.
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Old 09-08-2013, 04:03 PM   #5
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Hi qrider.

You are actually getting good mileage for a carbureted, non-overdrive chassis pushing a 12,000 pound brick down the highway at 60 mph.

To get your desired 9-10 mpg, you'll need to step up to a newer (by about 10yrs) chassis.

I suggest you enjoy your motorhome and keep your eyes closed at the gas pump.

BTW - the vacuum valve on the exhaust is closed when cold to force hot exhaust up thru the intake manifold crossover passage. This warms the carb quickly when cold for better cold performance. It should fully open when warm.
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Old 09-08-2013, 07:38 PM   #6
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Thanks Nick,
Good information on the valve, I am going to start it cold and get under it and watch that the valve is opening correctly just in case.
I guess for the year and technology I should not expect more with the weight as you said. I am going to try my old high school tricks on her since it is carburated to try to squeeze a little more power and mileage by getting the carb. tuned or rebuilt, ordering a K&N air filter and going to a local muffler shop I have here and have them add a cross pipe and swap out the stock mufflers for some flow trough type. In the old days better breathing added up to power and economy if your right foot did not get carried away. It just seems like allot of rig for a 350 and I was hopping the smaller motor would equal better economy.

But we are loving it and are enjoying the RV lifestyle and now that it's going to cool down a little here in FL. are looking forward to allot more of it.
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Old 09-08-2013, 07:46 PM   #7
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I feel better about the RPM, thought it was bit high but never had a rig like this. It is dependable and simple to maintain not having the EFI, at least for me. As Nick said "keep your eyes closed at the pump and enjoy".
I wont worry about it so much and just do a few mods to Improve drive-ability.
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Old 09-10-2013, 01:08 PM   #8
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I have a 2013, 32' Class C, and get about 7 mph average. Dealer told me ahead of time that would be the average.
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Old 09-10-2013, 06:14 PM   #9
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7.7 mpg on a good day.
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Old 09-10-2013, 08:03 PM   #10
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Ok, I don't feel so bad..
We just need that price of gas to drop then..
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Old 09-10-2013, 08:14 PM   #11
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I had an 84 Jamboree, Chev 350, same trans, etc.

You are doing fine!

I caught a tail wind from MN to PA and dang near got 8. :-)
Dave and Nola, RVM1
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Old 09-10-2013, 09:41 PM   #12
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The valve in the exhaust is exactly as stated by Nick. It has a bimetal spring that closes the valve when cold so it directs exhaust across the intake and under the carb when engine is cold. It helps heat up the intake charge so fuel atomizes better and helps cold driveability a bit. You can either remove it or just wire it open full time if you do not operate in really cold climates. As long as it is actually opening up, it does not have that much restriction, you could also remove it completely if you really want.

A 350 small block Chevy can run at 3000 rpm all day long without any concern, besides the noise and poor mileage. Your issue is that if you swapped to an OD trans, the engine may not have sufficient torque to pull the lower gear ratio. The trans you would need is the 4L80E, which is an electronic OD version of the turbo 400 you have. Besides the cost for the swap (approx $3500 or more conversion cost) it may require many years to pay back in fuel savings.

Your smoking when starting up - is it light grey smoke that lasts for a 3-15 seconds when first started in morning and then goes away? If so you just have the very typical slightly worn valve guides. Oil gets past the guides when sitting overnight, and then causes slight smoke when started the next day. So common in SBC's, nothing to worry about.

Mileage is basic physics working against you. You have lot of weight and also air resistance. It takes power to overcome these forces. Power takes gas.
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Old 09-11-2013, 07:25 PM   #13
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I had a 1997 Jayco 3230, I bought it new, It got 8 mpg. It was on the Ford/V10 chassis. It only made 6.5 if I was pulling a toad.
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Old 09-11-2013, 08:59 PM   #14
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We had a 1985 26' Class C with a Ford 460 (carb and dual air pumps). At sea level we got around 6 mpg and it dropped to about 4.5 in the mountains. We have a lot of mountains around here!

Since the floor plan was such a POS, we got rid of the thing after two seasons. Our average night's camping cost about $425!

After a 4-year hiatus, we bought a Georgetown 325 (an '02) three years ago, from the original owners. We really like the Class A view of the world and this rig gets about 7.5 mpg regardless of altitude. GT isn't the greatest in the industry for quality, but this rig will see us through to bagging the life-style altogether. I'm coming up on 72, with DW 2 years astern and we don't see much more than maybe 4 more years.

Frank Damp -Anacortes, WA,(DW- Eileen)
ex-pat Brits (1968) and now ex-RVers, as of 08 Dec 14.
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