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Old 01-04-2021, 05:34 PM
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Join Date: Dec 2020
Posts: 15
Newbie 250C carbs question

Hi gang-
I am new to MB and this forum. I have been doing a lot of reading and am going to attempt rebuilding my Zenith carbs. I may need some guidance with that. Also tracing a no crank condition since 1995.

First- the car-
I had the opportunity to purchase a tobacco brown/ tan 1970 250C from the estate of the original owner. I have most maintenance records of this car. Story goes- the car was serviced at 42K miles, then stopped running some point shortly thereafter. It sat stored indoors since 1995 and has not started. No one really knows why it didn't start.

I got the car home on my trailer. I think I traced the NO CRANK issue to the neutral safety switch. No reverse light gave it away. Checked the switch and no continuity. Opened the switch up and the contacts were junk and beyond repair. NOS from ebay on the way. Hope that fixes the No crank condition.

Removed the plugs and soaked the cylinders with PB blaster/ trans fluid. I hand cranked it over a 1/8 turn every few days. Engine is NOT stuck.
But the fuel was in the tank from 1995. Removed the valve cover (CLEAN under here) and pulled the carbs off- some of those carb studs/ nuts were NOT EZ.

Orderd a pair of rebuilt kits from our sponsor.
Will see what happens!

Thanks for any advice on the carbs / NSS in advance.

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Old 01-04-2021, 08:57 PM
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Join Date: Sep 2001
Posts: 1,556

I don't have a lot of experience with those carbs but have heard/read they can be difficult to get dialed in. Consider getting a manual for them if you haven't one.

It sounds as though you nailed it on the NSS.

Sounds like a nice car, good luck and post pictures!!!
Does a Caddy stretch? No, but a Mercedes bendz.
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Old 01-05-2021, 07:30 AM
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Join Date: Dec 2020
Posts: 15
Thanks. I downloaded the link for the Jamie Kopp Zenith carb guide. Will start there.
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Old 01-05-2021, 11:40 AM
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Join Date: Mar 2003
Location: Central Ky
Posts: 6,193
The carbs are easy to rebuild. Dialing them in however is tricky. Take pictures as you tear it down and get a good rebuild kit.
Once back on the car repost question about set up. Many threads on here about that subject
2011 Porsche Cayman - Bond,James Bond
99 E320 THE Queen Mary - SOLD
62 220b - Dolly - Finally my Finny! Sadly SOLD
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19 BMW X5 - Heaven on Wheels
14 38HP John Deere 3038E Tractor -Mean Green
84 300SD, Benjamin -SOLD
71 220 - W115-Libby ( my first love) -SOLD
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Old 01-05-2021, 07:59 PM
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Join Date: Nov 2014
Location: Las Vegas & N. Cal
Posts: 167
Good start:
Continue depending on just how Much Fluids you added to each cylinder and how much time has passed since then
If you added more the 2 OZ's per cylinder and less time then a week has passed you may need to crank out the fluids first unless they passed the rings already
Once you get the motor Cranking confirm the Valves are all moving. You Might do that by Hand is best plugs out of course .
At this point since it sat so long add about 1/2 to 1 OZ of ATF to each cylinder and run a compression test and see What you get . It well not be a normal reading most likely but should tell you if its going to run . Normal is about 150 (Dry test ) in good order if your getting 80 to 120 it should run , you can recheck it later ( wet test may Jump as much as 35 Lbs if the rings are not sealing Example : 60 Dry to 95 Wet )
You don't need Carbs to check the compression and yes they are Easy to rebuild watch out for the Thread's in the Center Top cover hold down as they strip easy

Lots of reasons why it may not crank Like a bad Ignition switch very common>Lots of time it well crank just by pressing on the back of the switch when you turn to crank as example
Be careful if you pull the distributor it may have a spacer between the Hold down and the block you don't want to lose. Have fun
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Old 01-05-2021, 08:49 PM
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Join Date: Dec 2000
Location: Long Island, NY
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To pile on, you can get your ignition system about right at starter speed and carbs still off. That should at least let you rule out the transistorized ignition box, the two ballast resistors and the points as being your next problem. At "starter speed", having the timing set to about TDC is close enough to get you ready for really starting the car.

That transistorized ignition box must be grounded. So if there's any signs of corrosion on it or under it, remove it, clean up a little and screw it back down. Fixing it is pretty simple should it die. If you feel the urge to bypass it, do that after the carbs are back on the car is running again (only break one thing at a time).

Also, there are a couple of versions of that box with three basic wiring patterns. Use the right pattern for the box installed. If you get a spare box, ensure it looks like the one you have in every detail.

PS. Welcome to the site and do please share pictures. The W114 coupe is one of my favorites.
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Old 01-06-2021, 07:46 PM
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Join Date: Dec 2020
Posts: 15
Here are some pics.

Tobacco Brown with tan leather? I think Its leather anyway. Interior is MINT. Headliner is tight. Smells so good in there actually. Window gaskets and felt is in good shape. Floors and trunk are good. Toolkit is there. No spare though. Maybe had some paint work on the drivers side and there is a golfball sized hole in the inner fender behind the drivers wheel. Not through and through. EZ fix. Body mounts and suspension pick up points are good. Engine is clean, and free. Been sitting since 95. Tires are dated 1994! So they gotta go once she is ruunning.

Aluminum- thanks for the ATF idea. I did that today. NSS should be here on Sat. So hopefully it will crank and i can do a compression test.

CTH350- does the transistorized box just provide spark, or is it also responsible for the no crank? There is a bit of corrosion about the ground so i WILL clean that up. thanks

i have the carbs off and cleaning one at a time. Waiting on the carb kit, fuel hoses, and Fuel pump from our sponsor. it will be a few weeks but I hope to have it running in my driveway on a portable gas tank -Roadkill style. im in NE PA so weather is an issue. Garage is not heated.
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Old 01-06-2021, 08:16 PM
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GREAT looking car. Good luck!!!
Does a Caddy stretch? No, but a Mercedes bendz.
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Old 01-08-2021, 05:29 PM
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Join Date: Nov 2020
Location: Hermosa Beach
Posts: 10
Very nice car. Much better than my 250C. Carbs can have air leaks from over tightening the screws and warping form heat. Make sure all the carb surfaces fit well.

Get an air volume meter. I don't know why I spent so much time listening to carbs instead of getting a meter. If nothing stupid has been done to the carbs they should be fine. Don't expect great fuel mileage. If worse come to worse, get a Weber kit ~$1000.
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Old 01-09-2021, 09:00 AM
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Join Date: Jun 2010
Location: Miami Beach, Florida
Posts: 494
That is one gorgeous car!

1976 280C
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Old 01-19-2021, 01:22 PM
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Join Date: Mar 2004
Location: Des Moines, WA
Posts: 800
Nice car!

I got mine (hence my username) with the carbs already setup and it ran perfectly but for the 250/8, a carb balancing tool really helped as did making sure there were absolutely no vacuum leaks and that the linkage did not preload one carb more than the other.

Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120
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Old 01-20-2021, 04:43 PM
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Join Date: Dec 2020
Posts: 15
now I need help with the ignition switch

So Good and not so good.

I put the NOS NSS in and I know that works- but I still dont have POWER to the starter bendix with the key in the start position. Its GOTTA be the ignition key switch right??

So... I started to remove it. Did they build the car around this thing?

****Anyone know how to get the 8 PIN ignition switch out? I see 3 screws. Was able to get to two of them I think the entire assembly has to come out but I am at a loss. SO SO close and super frustrating***

But I did jump the starter so it finally turned over- first time since 1995!! pretty awesome.
Got over 180 PSI in all but two cylinders. Added a tsp of oil and they did jump quite a bit and they all evened out.
So it should RUN. just need power to the starter
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Old 01-20-2021, 05:50 PM
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Join Date: Sep 2001
Posts: 1,556
If you can access the wires, confirm voltage to the switch before pulling it out and replacing it. There should be power to the ignition switch and then when in the the crank mode, power to the NSS and eventually the solenoid when in the crank mode.

Good luck!!!
Does a Caddy stretch? No, but a Mercedes bendz.
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Old 01-20-2021, 06:10 PM
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Join Date: Aug 2019
Location: Pacific Northwest
Posts: 788
Originally Posted by richmike114 View Post
So Good and not so good.

I put the NOS NSS in and I know that works- but I still dont have POWER to the starter bendix with the key in the start position. Its GOTTA be the ignition key switch right??...
not necessarily.

hook up a test lead with clips to a test-light or a multimeter to the NSS plug where the power from the ignition switch should come.

You could have power coming through the switch to the nss but somewhere from the nss to the starter there could be a problem.

Diagnose first fix second
1990 300SE (Daily driver)
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Old 03-05-2021, 04:59 PM
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Join Date: Dec 2020
Posts: 15
Ok christuna- if I hold the key to full start position, one of the four leads going to the NSS should have power?
I think they are the thicker purple wires number 50 and 51 on the NSS.
If I touched them together, then I should get power right to the starter?

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