350 Qjet Motorhome

[rfoll]

Not only a sad level of comp with those era heads, but the chambers burn rate leaves a lot on the table also!
I would also be looking to ditch those Exh Manifolds.

You might find 1 mpg by tuning, but making significant changes to the engine will make the most difference. The trick is to make changes that don't cost more than the potential fuel savings. Those 76 cc heads are dismal. The Edelbrock manifold can be had for $40-50 at any swap meet. A tuned up set of 1.84 valve 305 heads will kick up the compression. Another 10 degrees of cam will likely help also.

[pamotorman]

Not only a sad level of comp with those era heads, but the chambers burn rate leaves a lot on the table also!
I would also be looking to ditch those Exh Manifolds.

i have driven a MH at night with the dog house off and the exhaust manifolds got so hot they were transparent as you could see the outline of the exhaust ports in the heads

[rfoll]

When I started using Vortec heads, I was amazed to see the drop in engine temps. I have to plumb heater coolant through the intake to warm the carb, and in the winter block off the radiator with cardboard just to get it to come up to 180 degrees. There's much to be said about a well designed combustion chamber. The significant increase in power and economy make it worth the extra effort.

[olescarb]

It would be worthwhile hooking up a vacuum gauge to find out how hard the engine's working under typical cruise conditions. If it's really low, say <6-7" Hg., there's probably not much you can do; I don't think shorter gears would help, given the current RPM. If it's reasonably high, say over 8-9" Hg., I'd pull the APT (adjustable part throttle) plug at the top front of the carb and turn down the adjuster 1/2 turn at a time until a slight surge is felt, then back off 1/2 turn. If this results in a turn or more leaner setting, the effect should be noticeable.

As MadBill noted I would first check the cruise vacuum, most Chevy truck q-jets used a power piston spring that caused the power system to start enriching the mixture at 10 inches of vacuum and to be in the full rich position by 6 inches of vacuum. the primary metering rods also did not go as rich as a passenger car q-jet would when the metering rods were in the full rich position. We have had q-jet equipped Chevy trucks fail the smog test in California because the vacuum was to low (with the load put on by the dyno during the smog test) causing the fuel mixture to be too rich for the emission standards.


Henry @ olescarb

[dfarr67]

4L80E

[rfoll]

It would be worthwhile hooking up a vacuum gauge to find out how hard the engine's working under typical cruise conditions. If it's really low, say <6-7" Hg., there's probably not much you can do; I don't think shorter gears would help, given the current RPM. If it's reasonably high, say over 8-9" Hg., I'd pull the APT (adjustable part throttle) plug at the top front of the carb and turn down the adjuster 1/2 turn at a time until a slight surge is felt, then back off 1/2 turn. If this results in a turn or more leaner setting, the effect should be noticeable.

As MadBill noted I would first check the cruise vacuum, most Chevy truck q-jets used a power piston spring that caused the power system to start enriching the mixture at 10 inches of vacuum and to be in the full rich position by 6 inches of vacuum. the primary metering rods also did not go as rich as a passenger car q-jet would when the metering rods were in the full rich position. We have had q-jet equipped Chevy trucks fail the smog test in California because the vacuum was to low (with the load put on by the dyno during the smog test) causing the fuel mixture to be too rich for the emission standards.

Is there a source for QJ power valve springs? I have now way to test them for stiffness, and they all look the same to me. I would love to be able to add this as a tuning aid.
Henry @ olescarb

[olescarb]

Edelbrock offers a package of power valve springs part # 1994, we use aftermarket equivalent of the Rochester 7036019 (8-4 inches of vacuum) and 7029922 (7-3 inches of vacuum) that we get from our supplier plus we often use a 43k metering rod that has a .044" cruise diameter with a .026 power tip to enrichen the power system from the truck metering rods that have a .035 power tip (for the late style q-jets).

Henry @ olescarb

[F-BIRD'88]

The vortec heads do work very well as mentioned.
059 is 305 vortec 062 or 906 is 350 vortec

On this motor a modded 081 (305 tpi) or 083 (5.7l TPI)
would also work well if replaceing a 76 cc 882 or 462624 open chamber "smogger head)
These don't require a vortec intake swap too.
The chamber is half way to a vortec chamber 58 or 64 cc
with the plug boss pushed further into the chamber
like a vortec is.

Does your MH motor have a 76 cc smogger head?
882 or 883 or 993 or 462624
Even the swirl port truck heads are more efficient than these type. (305 and 350 TBI)

[HDBD]

Having dabbled with similar stuff over the years I found a combination of all the above to work well
Vortex heads, a higher lift short duration cam, higher compression all help as does the Qjet tuning especially the APT. Don't forget timing changes either, they help.
Don't discount a transmission change to OD either, probably the best way to deal with that rear end gear while still remaining relevant as compared to the wind and weight drag.
Headers always bit me due to maintenance in heavy duty applications. If the chassis will allow maybe some corvette rams horns are a slight improvement.
Dates me but I knew Doug Roe in the late 70s and learned from him how to make the QJ work. Always took a mileage hit when I changed to spread bore Holleys even with extensive tuning.

[tjz450]

Since it hasn't been mentioned, service the distributor. Clean and lube the weights, verify the advance curve is correct for your RV. I would shorten the vacuum adv, and run more initial timing. Those engines have vacuum lines everywhere and most of them are rotting. Make sure the choke doesn't creep on while going down the road.

[Headcase]

I went through a similar experience with my 84 chevy class c...terrible gas mileage.... 6 and change no matter what ...Even on the cruise control tucked behind a big rig for entire tankfull....6 mpg...tried screwing with the carb... disconnecting the secondaries... no matter what 6 mpg...after years of trying finally gave up...and eventually the 2 inch exhaust rotted off...I put on headers and 2 1/2 duals with a simple in front of the rear tire dump...Went from 6mpg to 11mpg!!!!....Headcase

[Blakeslee]

Gear Vendors overdrive? https://www.ebay.com/itm/P-Type-Gear-Ve ... .l4275.c10

[MadBill]

Wow, I appreciate all the great replys!!

Due to age/health and the M/H only gets used about 600 miles per year any work will be minimal...

We S/Ters are all 'tear into it and change stuff' people, so it's a struggle to point out that 600 mi./yr. @ 6 M.P.U.S.G. is around $CAN 400, so even a 25% mileage gain would only save you $80/yr....

[F-BIRD'88]

A BIG Chin Spoiler mounted under the front bumper that extends wide enough to direct the air around the front tires will make a big difference. Make from plastic or aluminum. Bigger the better.
Improved engine cooling. Improved body aerodynamics.
It depends on which motorhome body you have as to where exactly is the best place to mount the chin spoiler. Either behind the bumper directly below the radiator support structure. Or mounted to the bumper.
Depends if it is a "bottom breather" or not.
Bigger wider and extending lower is better. Ya you have to watch parking curbs etc.

Add a can of MolySlip E Oil Supplement to the oil and drive it.
www.molyslip.com

Increase tire air pressure.

Cost effective simple improvements that you can do.

[tresi]

Since it hasn't been mentioned, service the distributor. Clean and lube the weights, verify the advance curve is correct for your RV. I would shorten the vacuum adv, and run more initial timing. Those engines have vacuum lines everywhere and most of them are rotting. Make sure the choke doesn't creep on while going down the road.

While you're going there check that the mechanical advance is actually working. the bushings in the shaft like to gum up. It's not too hard to pull it out and take it apart and clean them. A zero dollar fix.