Black Smoke

M606BR · Post by **M606BR** » Sat Oct 06, 2018 4:34 pm

I have a M38 and it runs and starts great. I have been driving with the summer top on for a couple of years and recently I decided to remove it. Now I can see a black smoke coming out from exhaust pipe only when I quickly push all way down the accelerator pedal. However, there is no black smoke at idle, at normal speed or when I slowly push all way down the accelerator pedal. I know that black smoke means rich mixture, but I was wondering if anyone else might know what is the most common cause of this specific problem. My carb is a YS637S.
Thanks,
Felipe

Post by **wesk** » Sat Oct 06, 2018 6:14 pm

Only 2 good reasons for it. Your carb mixture is too rich or your distributor advance governor is not functioning.

If your governor is stcu or rusted stuck then the engine spark doesn't advance fast enough to use all the gas that is coming at it.

If the carb is metering too much fuel you may have a float setting issue or a leaking carb leaking fuel from the bowl to the venturi.

Or you may have a manifold heat riser stuck open so your carb intake air is way too hot (expanded) resulting in a very over rich operation with rapid accelerator movement.

M606BR · Post by **M606BR** » Sun Oct 07, 2018 12:31 pm

Wes,
Thanks for showing me the way to troubleshoot it. The distributor advance stuck or the float setting issue are good bets to start with. As my M38 doesn't have a manifold heat riser, this option is eliminated.

Post by **wesk** » Sun Oct 07, 2018 2:50 pm

Unless you fabricated your own intake and exhaust manifold all the L134's have the heat deflector valve. Yours may just be missing it's external operating parts and the flapper inside may be in the heat on position.

If you have absolutely no remaining heat riser parts then your manifold is being heated all the time.

M606BR · Post by **M606BR** » Mon Oct 08, 2018 8:07 pm

Hi Wes!
You were right! Today I checked it out and confirmed that only the heat control valve shaft is still there, but it's stuck, probably in the heat on position. Unfortunately, there is no counterweight, spring, spring washer and spring stop anymore. I'll need to buy a heat riser repair kit and install it. Any tip on how to replace it?

I didn't know that all L134's have the heat control valve. As I had never seen the counterweight lever in my M38, I thought that only early L134 engines were equipped with heat control valves.
Thanks.

Post by **wesk** » Mon Oct 08, 2018 10:45 pm

You need to choose a repair kit wisely. One that matches the correct M38 spring orientation.

Make a close review of my L134 engine sub-photo album. Pay particular attention to the intak vs exhaust manifold fitment.
http://www.willysmjeeps.com/v2/modules. ... _album.php

You will have to remove both manifolds if you cannot free up that flapper valve shaft. If you do have to remove manifolds exercise caution and patience. Soak liberaly with PB Blaster several days then use a torch to get the nuts red hot before you try removing them.

M606BR · Post by **M606BR** » Sat Oct 13, 2018 11:03 am

Hi Wes.
Supposing that the cause for the black smoke is really a manifold heat riser stuck open, is it possible that black smoke happens even with the engine still cold? This morning, after starting the cold engine, I noted the same black smoke coming out from exhaust pipe every time I quickly pushed all way down the accelerator pedal. I think there is no sufficient time for the heat been transferred from the exhaust manifold to the intake manifold to cause black smoke with the engine still cold. Maybe there is something else causing the black smoke... What do you think? Carburetion? The float level seems to be okay because the engine runs and starts fine, cold or hot, carburetor never floods. What do you think about a possibility of having an issue at pump circuit? Maybe something missing or stuck there causing excessive charge of fuel during a quick acceleration...

mdainsd · Post by **mdainsd** » Sat Oct 13, 2018 5:54 pm

Have you serviced the air filter recently?

M606BR · Post by **M606BR** » Sat Oct 13, 2018 6:48 pm

No, I haven't. The last time was some months ago. Anyway, I will take a look on it in order to check oil level. I will also remove the hose between air filter and carburetor to confirm if it makes any difference or not.
Thanks.

M606BR · Post by **M606BR** » Sat Oct 20, 2018 5:29 pm

mdainsd wrote:Have you serviced the air filter recently?

Hi!
Today I removed the oil filter and the black smoke still happens during quick acceleration, so such hypothesis is definitively discarded.
Thanks.