Exhaust gas analysis

1950 - 1952, M38, questions, discussions, regarding anything related to the M38.
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Scott in NM
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Exhaust gas analysis

Post by Scott in NM » Wed Apr 20, 2022 5:57 pm

Got my emissions tested today on my stock M38 (if it is in the ORD 9, it is on the jeep), and if not, then not. Hydrocarbons were running 6%, I've heard that 4.5% is more normal, so a bit rich of a mixture. Seems like it goes through gas rather quickly. Readings didn't change much when adjusting the screw, as expected. So, being a bit hydrocarbon rich, I could either have a weak spark or the main jet is oversize for my altitude (7000'). No black smoke out the tailpipe. Spark test OK, nice bright blue about 1/2" long, plugs are a dry gray-brown. Is it possible to get a main jet that is a size or two smaller than normal? Carb, fuel pump, and engine recently rebuilt; spark plug wires and plugs new replacements; distributor rebuilt with new condenser and coil. So assuming all these parts are OK, is it possible to replace the jet? Just wonderin'. (And yes, timing is spot on, starts within 1/2 engine RPM).

Scott


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dpcd67
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Re: Exhaust gas analysis

Post by dpcd67 » Wed Apr 20, 2022 7:20 pm

Fortunately in this state we have neither exhaust gas nor any other type of required vehicle inspections. Just pay the money and get the license plates. And we don't want it either.
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Wolfman
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Re: Exhaust gas analysis

Post by Wolfman » Thu Apr 21, 2022 4:51 am

Guess I am a little behind the power curve.
Is the exhaust analysis test state mandated or just something you did ??
And is the 4.5% level for a 2022 computer controlled engine ??
Early 1950's engines ran rich. Exhaust analysis testing came much later.
Engines were set up to do what yours is doing. Start and perform well. Nobody stuck a test tube up the exhaust.
As for your question. How to lean the mixture ?
Easiest might be lower the float level a little.
The idle screw only adjusts the idle mixture. Has little or no effect on higher RPM mixture.
I suppose there are different size power jets. The one the metering rod goes into. There is usually a number on the jet. ??? No idea what that should be ? Maybe Scout Pilot knows. I put back what I take out.
Altitude could easily come into play. Carburetor's are driven by internal differences in barometric pressure and at 7000', that is going to be lower and a possible factor.
Maybe move to the Florida Keys. Just kidding !
There are air bleed ports in the carb. that mix air with the fuel going through the passages. Maybe one of these air bleed ports are blocked or restricted ?? Small ports at the top of the venturi, on the side. Maybe a shot of spray carb. cleaner into these ports might help. Others are internal. Some disassembly required.
There are high altitude jeep engines. One of the things was a higher compression head. Maybe a difference in the carb. jetting ??
Personally, if the jeep performs OK and you are not driven by some other factor, I would go for a ride & be happy.
Mike Wolford
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dpcd67
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Re: Exhaust gas analysis

Post by dpcd67 » Thu Apr 21, 2022 6:15 am

I just looked up the NM laws; yes, they do require exhaust gas analysis, for cars 36 years old and newer.
Just drive your jeep. You have no problem.
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Scoutpilot
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Re: Exhaust gas analysis

Post by Scoutpilot » Thu Apr 21, 2022 8:22 am

The first thing to do is proper tuneup. Using the vacuum gauge instead of the timing light will put the motor into its sweet spot.

https://oldjeepcarbs.com/thread/29/time ... ming-light
'70-'71 'A' Batt., 377th FA, 101AB
(Slicks and LOH's)(Col's LOH Pilot)
'71-72 CHARLIE TROOP,
16TH AIR CAV, 1ST AVN BDE (AEROSCOUTS)
LOW LEVEL HELL.
'46 CJ2A
'47 CJ2A
'48 CJ2A
'48 B1PW126
'69 CJ5

Scott in NM
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Re: Exhaust gas analysis

Post by Scott in NM » Fri Apr 22, 2022 10:57 am

Appreciate your inputs. My responses -
1. Gas analysis not required, I just wanted to get some baseline numbers after my engine rebuild and troubleshoot what I consider low power.
2. Power is a little low. I've had this thing for 45 years and know it better than my own family members. Compression just like new engine, timing done by both vacuum and light, so no need to go there.
3. I'll check into the bleed ports.

I've been suspecting the ignition, but after new plugs, wires, condenser, coil, and points, and everything else looks good, I still don't trust the ignition system and was hoping a gas analysis would tell me something to conclusively confirm or deny.

And still open to other suggestions. Second car I've ever owned, it deserves any TLC I can give it to ensure it runs better than other 70 year olds out there. :D

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Re: Exhaust gas analysis

Post by Wolfman » Fri Apr 22, 2022 11:07 am

So, this is the same jeep and engine you have had for years and it did not have a power problem until after the rebuild. ???
Something changed during the rebuild.
Mike Wolford
CJ-2A
VEP GPW
Comm./Inst. SEL
AOPA ( 50 yrs)
EAA ( 49 yrs)
4th Inf. Div. - 5th Inf. Div. - 2nd Armor Div. - CIB

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Re: Exhaust gas analysis

Post by Scoutpilot » Fri Apr 22, 2022 11:19 am

I must agree with Wolfman. What did the cylinder size change to?
'70-'71 'A' Batt., 377th FA, 101AB
(Slicks and LOH's)(Col's LOH Pilot)
'71-72 CHARLIE TROOP,
16TH AIR CAV, 1ST AVN BDE (AEROSCOUTS)
LOW LEVEL HELL.
'46 CJ2A
'47 CJ2A
'48 CJ2A
'48 B1PW126
'69 CJ5

Wolfman
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Re: Exhaust gas analysis

Post by Wolfman » Fri Apr 22, 2022 11:35 am

Still thinking.
Was there a power issue before the rebuild or did this start after ??
Two things come to mind if this started after. Cam timing and valve clearance.
Cam timing would show up on a vacuum gauge. Low reading but at your altitude, the reading is going to be low. Question is, how low is normal at 7000 feet.
I have a friend with an M-38. He adjusted the valves and afterward, the jeep would start and run OK but out on the road, the farther he drove and the warmer the engine got, the less power it had. But never missed out. He set the valve clearance way too tight. He brought it over. I adjusted the valves and it was back on line.
If this was going on before the rebuild, I am wondering about timing advance. Set the timing and the engine starts great and purrs like a kitten but up to speed, under load, it does not have enough power. For some reason the timing is not getting to full advance and the engine power level comes up short.
The advance could be checked with a timing light. Mark the front pulley for 5 degrees and use a degree wheel to mark the advance point. How much ? I am thinking 28 degrees ?? Need a little back up to confirm that one.
Or if you know someone with a distributor set up machine. Put a distributor in the machine. Spin it up. Check the parameters and set them the way you want and put the set up distributor back in the engine. Speed shops and racing engine builders usually have these.
Mike Wolford
CJ-2A
VEP GPW
Comm./Inst. SEL
AOPA ( 50 yrs)
EAA ( 49 yrs)
4th Inf. Div. - 5th Inf. Div. - 2nd Armor Div. - CIB


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