Rick Staples suggested that I share with the GMCnet my recent
experiences with getting the Qudrajet recalibrated, so here goes.
As some of you might recall last winter I was having severe fuel
enrichment problems with my carb after having it rebuilt. On a trip
last fall to New Mexico the engine spewed black sooty smoke out the
exhaust pipe and could not pull the coach over any passes much more than
7500 feet in altitude. Fuel economy was poor in the 6 to 7 mpg range.
I greatly improved on the black smoke problem when I got home and
removed one of the two air horn to float body gaskets that the rebuilder
had installed in the carb. However rarely could I get over 8 mpg, even
when trying to drive conservatively.
Having read Wes Caughlan's articles about the need to properly
recalibrate the carb using an engine exhaust analyzer, I was able to get
to his shop in Sandusky this fall to have the carb checked out.
Cinnabar has a "five gas" electronic portable analyzer that looked to me
as being a very sophisticated piece of manchinery. The mechanic using
it said that it cost several thousand dollars and was kept calibrated by
the factory techs who service the unit periodically for Cinnabar. I can
say that here in S.E. TN there are no shops that have anything like this
instrument or even hinted they had ever heard of one. At the shop
here's what I remember of the work that Cinnabar did on my carb:
> I was with the mechanic most of the time that he was working on the
> carb. I tried to observe and remember what all he did but he lost me at
> times and I didn't want to interupt too much
.
The analyzer which was about the size of a large suitcase and was
carried inside the coach for all tests. It had a long hose that ran out
through one of the side windows with an exhaust pick up nozzle clamped
inside the end of the tail pipe. It had a pump that pulled samples of
exhaust into the on-board unit where the gasses were analyzed for for
it's constituent gases. An LED read monitor out was in the hands of the
mechanic would read and take note of the numbers calculated by the
instrument. The road tests were made by driving the coach on a level
road by starting out slowly then accelerating hard upto about 55 to 60
mph where the cruise control would be set and the a constant speed held
for a couple of miles to let the engine and carb stabilize for the
readings. There would also be tests with the engine just idling in park
at the shop.
> I know that from the readings the mechanic got on the first test drive
> with the exhaust analyzer attached (I was driving and he was watching
> the LED readouts on the monitor) the engine and carb were producing a fuel to air ratio of 10.8. The mechanic
> said that they (Cinnabar) have learned to try to get the ratio as close
> to 14.8 as possible which I suppose is the best setting for the
> elevation in Michigan (about 800 ft msl, I believe.) I do not know what
> the inital CO, CO2, HC, O2 and NOX readings were but he said they were
> way off as well.
>
> The mechanic next quickly removed the carb from the engine and started
> removing parts from the carb. He immediately found that someone in past (not
> me) had changed the primary jets and metering rods to something other
> than the stock combination which Rochester put in for the MH carb. He
> said the pieces were what is commoningly used for a car (I suppose he meant
> the Toranado). These he said would cause much of the fuel enrichment
> and poor F/A ratio problem. He changed the rods and jets to the ones
> that Cinnabar is using now for their carb rebuilds. He showed them to me
> but the numbers and letters were too small for my eyesight to read.
>
> After putting the carb back together and reinstalling it on the engine,
> we went for another ride and test with the analyzer. The F/A ratio
> improved significantly but not to the 14.8 value and the other numbers
> were not satisfactory either. The mechanic shook his head some and took
> the carb off again and returned with it back to the workbench. He took
> parts out again and examined each one carefully again. He decided that
> the float was not right and installed a new one and put the carb back in
> the coach.
>
> The next test drive revealed more improvement, but things were still not
> right. There was some careful adjusting of idle circuit needle valves
> and more test drives. The last ones involved removal of the carb top (air horn) to
> replace the float valve and seat and tweaking of the float arm to get
> better fuel shut off and float level. The mechanic often would hang
> over the carb while the engine was idling to observe the fuel flow in
> primaries. He would exclaim that "its still puking gas" at times.
>
> Finally, after about two hours of this rather hot and cramped work on
> the mechanic's part, we took our last test drive. I saw the analayzer
> reading a F/A ratio moving around between 14.9 and 15.1. This satisfied
> the mechanic but he said he would perfer that the readout stabilze
> better and not jump around. The other numbers were acceptable as well. Anyway the F/A ratio was leaner than he wanted but as close as
> he felt he could get on my carb.
>
> The mechanic warned me that he had too much trouble in getting the carb
> adjusted and he was not confident that it would hold its calibration very
> long. He said that because of the wide heat and cooling cyles these
> carbs go through or poor torquing of the mounting bolts he as seen some carbs with slightly warpped bodies. This
> along with the wear in the shaft holes for the throttle vlaves will
> result in drifting calibration. All he could say was when he was
> finished was that the carb was adjusted to Cinnabar's specs. and I would just
> have to drive it to see how it holds up.
Well, after leaving Sandusky I drove about 5,500 miles to and from
Wyoming and Colorado crossing several 10,000 ft. passes on the way.
After the carb recalibration, the first two tankfuls of gas yeilded over
10 mpg at 55 to 60 mph in Michigan and Wisconsin. I was impressed! Fuel
economy in the Rockies dropped but never below 7.0 mpg and I'm getting
about 9 at present, so it looks like the calibration is holding
> O.K. for now. I would like to see how the F/A ratio reads now with the exhaust analyzer,
> but its too far from here to Sandusky just to see. I suppose that when
> this carb "craps out" again, I'll seriously look at fuel injection once more.
Phil Stewart
'76 Transmode
experiences with getting the Qudrajet recalibrated, so here goes.
As some of you might recall last winter I was having severe fuel
enrichment problems with my carb after having it rebuilt. On a trip
last fall to New Mexico the engine spewed black sooty smoke out the
exhaust pipe and could not pull the coach over any passes much more than
7500 feet in altitude. Fuel economy was poor in the 6 to 7 mpg range.
I greatly improved on the black smoke problem when I got home and
removed one of the two air horn to float body gaskets that the rebuilder
had installed in the carb. However rarely could I get over 8 mpg, even
when trying to drive conservatively.
Having read Wes Caughlan's articles about the need to properly
recalibrate the carb using an engine exhaust analyzer, I was able to get
to his shop in Sandusky this fall to have the carb checked out.
Cinnabar has a "five gas" electronic portable analyzer that looked to me
as being a very sophisticated piece of manchinery. The mechanic using
it said that it cost several thousand dollars and was kept calibrated by
the factory techs who service the unit periodically for Cinnabar. I can
say that here in S.E. TN there are no shops that have anything like this
instrument or even hinted they had ever heard of one. At the shop
here's what I remember of the work that Cinnabar did on my carb:
> I was with the mechanic most of the time that he was working on the
> carb. I tried to observe and remember what all he did but he lost me at
> times and I didn't want to interupt too much
.
The analyzer which was about the size of a large suitcase and was
carried inside the coach for all tests. It had a long hose that ran out
through one of the side windows with an exhaust pick up nozzle clamped
inside the end of the tail pipe. It had a pump that pulled samples of
exhaust into the on-board unit where the gasses were analyzed for for
it's constituent gases. An LED read monitor out was in the hands of the
mechanic would read and take note of the numbers calculated by the
instrument. The road tests were made by driving the coach on a level
road by starting out slowly then accelerating hard upto about 55 to 60
mph where the cruise control would be set and the a constant speed held
for a couple of miles to let the engine and carb stabilize for the
readings. There would also be tests with the engine just idling in park
at the shop.
> I know that from the readings the mechanic got on the first test drive
> with the exhaust analyzer attached (I was driving and he was watching
> the LED readouts on the monitor) the engine and carb were producing a fuel to air ratio of 10.8. The mechanic
> said that they (Cinnabar) have learned to try to get the ratio as close
> to 14.8 as possible which I suppose is the best setting for the
> elevation in Michigan (about 800 ft msl, I believe.) I do not know what
> the inital CO, CO2, HC, O2 and NOX readings were but he said they were
> way off as well.
>
> The mechanic next quickly removed the carb from the engine and started
> removing parts from the carb. He immediately found that someone in past (not
> me) had changed the primary jets and metering rods to something other
> than the stock combination which Rochester put in for the MH carb. He
> said the pieces were what is commoningly used for a car (I suppose he meant
> the Toranado). These he said would cause much of the fuel enrichment
> and poor F/A ratio problem. He changed the rods and jets to the ones
> that Cinnabar is using now for their carb rebuilds. He showed them to me
> but the numbers and letters were too small for my eyesight to read.
>
> After putting the carb back together and reinstalling it on the engine,
> we went for another ride and test with the analyzer. The F/A ratio
> improved significantly but not to the 14.8 value and the other numbers
> were not satisfactory either. The mechanic shook his head some and took
> the carb off again and returned with it back to the workbench. He took
> parts out again and examined each one carefully again. He decided that
> the float was not right and installed a new one and put the carb back in
> the coach.
>
> The next test drive revealed more improvement, but things were still not
> right. There was some careful adjusting of idle circuit needle valves
> and more test drives. The last ones involved removal of the carb top (air horn) to
> replace the float valve and seat and tweaking of the float arm to get
> better fuel shut off and float level. The mechanic often would hang
> over the carb while the engine was idling to observe the fuel flow in
> primaries. He would exclaim that "its still puking gas" at times.
>
> Finally, after about two hours of this rather hot and cramped work on
> the mechanic's part, we took our last test drive. I saw the analayzer
> reading a F/A ratio moving around between 14.9 and 15.1. This satisfied
> the mechanic but he said he would perfer that the readout stabilze
> better and not jump around. The other numbers were acceptable as well. Anyway the F/A ratio was leaner than he wanted but as close as
> he felt he could get on my carb.
>
> The mechanic warned me that he had too much trouble in getting the carb
> adjusted and he was not confident that it would hold its calibration very
> long. He said that because of the wide heat and cooling cyles these
> carbs go through or poor torquing of the mounting bolts he as seen some carbs with slightly warpped bodies. This
> along with the wear in the shaft holes for the throttle vlaves will
> result in drifting calibration. All he could say was when he was
> finished was that the carb was adjusted to Cinnabar's specs. and I would just
> have to drive it to see how it holds up.
Well, after leaving Sandusky I drove about 5,500 miles to and from
Wyoming and Colorado crossing several 10,000 ft. passes on the way.
After the carb recalibration, the first two tankfuls of gas yeilded over
10 mpg at 55 to 60 mph in Michigan and Wisconsin. I was impressed! Fuel
economy in the Rockies dropped but never below 7.0 mpg and I'm getting
about 9 at present, so it looks like the calibration is holding
> O.K. for now. I would like to see how the F/A ratio reads now with the exhaust analyzer,
> but its too far from here to Sandusky just to see. I suppose that when
> this carb "craps out" again, I'll seriously look at fuel injection once more.
Phil Stewart
'76 Transmode
