M10 Running Rough at Idle

CarSnob

CarSnob

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Hi all. I'm working on getting the M10 in my 2000CS running smoothly, but I've never really played with side-draft carbs before so I'm having a tough time. I'm running the stock Solex 40PHH carbs that came on the car. No matter what I do I can't seem to get all the cylinders running at the same time. So far, here's what I've checked and done:

1) Compression check with 180 psi across all cylinders
2) Changed out points for a Pertronix electronic module, upgraded to a flamethrower coil, and timed using the procedure from this thread (https://e9coupe.com/forum/threads/m-10-timing.29750/)
3) Adjusted valves using procedure from owners manual
4) Carbs were rebuilt a couple years back from this company (https://www.partsklassik.com/c-22-carburetor.aspx). After the rebuild they never had fuel in them and sat inside for a couple years, but they don't seem to be leaking and I checked that the jets and needle valves were clean before starting.
5) Checked for vacuum leaks while the engine was running with a propane torch

Despite all this, I have a consistently low vacuum through the throat of the carburetor on cylinder 4, and it isnt't running currently. I fooled with the settings for the idle air mixture and I have had it running, but then cylinder 3 was dead. The low vacuum concerns me because I don't think it's dependent on the mixture, or whether the cylinder is running, but I don't know whats causing it because I have good compression and everything else seems right.

If anyone has any other thoughts or advice it'd be much appreciated. Thanks in advance.
 
Here's a video I just took of the engine running. My dad was out there fooling with the carbs and now cylinder 1 isn't running either. It runs pretty smooth for a engine that isn't running on 2 cylinders though. I apologize in advance for the vertical video.
 
The mixture needs enriched or your throttle plates are open too much. Same effect. Weak mixture.

Enough is reaching the center cylinders and not enough on the two outside.
 
I do not think its a spark issue , I removed all the plugs spun the engine and they all had a strong spark. I really think the carbs are the issue I just can not get them dialed in
 
Hello CarSnob,
My initial thought in watching your video and trying to trace the ignition wires, I think possibly you have #1 and # 4 reversed on the dist. cap?

Also do you have a UniSyn or some other carb. tool to check and balance the air intake on each throat?

Here are some pages from the ol' Blue Book to help synchronize and adjust the carbs.

Good Luck, Chris Starcruiser
 

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I wish the wires were switched but they are in the correct spot yes I have a UniSyn but I can't seem to get all the carbs working at the same time . Thanks to all for your input
 
at idle can you see spark from the dead cylinder plug wire when grounded to the block? do the cylinders catch on off idle as you rev up? have someone hold the throttle at >1500 rpm or do same with a linkage screw if you have one then pull the plugs to see if they are firing at that rpm. I have found that on dead cylinders at idle the carb throttle plate and or idle mixture screw is too closed preventing enough fuel from firing the cylinder but will fire once off idle.

EDIT: I just looked at the video, are these carbs standard side drafts with one barrel per cylinder or are they like the ones in the video where he says the car idles on on barrel of each carb?
 
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Yes at idle I have removed the plugs one at a time each plug has good spark. The above link is for PHH 44 carbs my carbs are PHH 40 they are a bit diffrent I am not sure if they tune the same . Thanks again for all the good ideas
 
Pull plugs on the dead cylinders. Are they wet with fuel?

What jet #s do you have for idle, main, air corrector? How many turns out are each of the idle mixture screws?

Also, I’ve found that fuel level is very important for most side drafts. Any idea where yours is set?

Ed
 
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So I managed to get all 4 cylinders running and balanced with the Unisyn, but now I have two new problems. First, the engine won't really idle down. I have the idle adjustment screw all the way out but it refuses to drop below about 1100 RPM. I'm assuming I'm getting excess air in from somewhere, but since it seems to be consistent across all 4 barrels I'm a little thrown off. My second issue is that for the car to run well, I have to run in the rear-most idle mixture screws on each carb (cylinders 2 and 4) until they're almost completely seated. They're probably only about 1/4 to 1/2 turn out, whereas the screws for 1 and 3 are about 1 1/2 turns out.

I think the throttle butterflies might not be completely sealing in the bore and letting some air leak past, but that doesn't explain my issue with the mixture screws. A few people have mentioned that the float level could be off, would that cause these symptoms? I assumed that since the two barrels of each carb share a float bowl if one is running the other should be too.

Again, any input is appreciated. Thanks in advance.
 
What kind of carb to manifold gaskets are you using?

Can you get it to run well with running cylinders 1 and 3 almost closed and the others out or is it confined to those cylinders.
 
Weber tuning tips only go so far with Solex PHHs, but basic balancing crosses over. This is a nice write up from the MB forums for the 44PHH...there are differences mind you, but good pictures and tips. http://www.190slgroup.com/tech/solex adj procedure.pdf


I have personally never used a Unisyn synchronizing tool, but I have never heard many good things about them... I have good success using the squirrel cage style for PHHs. For carbs with vacuum ports, a manometer is the only way to go (this 40PHH does not have vacuum ports, though). Try synchronizing at about 2000 rpm and see if your results vary after you back the idle back down.

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Couple things on high idle... is the throttle return spring pulling both carb's throttle all the way shut? Can you close the throttles levers by hand and lower the rpm? I occasionally have my linkage get lazy and one carb is slow to return to fully closed... causing a high idle. Any slop in the linkage can cause this issue. You can use a small hand mirror and a bright flashlight to look into the carb's throat to inspect if the throttle butterflies are seated equally.

Remove the idle mix screws completely and check that the needle-ends are smooth. It is possible to damage the needles by forcefully seating them. If the needles are marred, it could affect idle speeds.

Are you sure that each carb is jetted equally (same # on all the idle jets, especially?) Your discrepancy between one carb and the other's mixture screws leads me to think the idle jets may be different. The #s are the top of the idle jet at the screw slot (my old eyes won't let me read the #s without removing them, perhaps you can use the zoom feature on your smart phone camera)

Are you positive there are no vacuum leaks? With the engine at idle, spray carb cleaner directly at the throttle rod "bushings"... these are the worst culprits for air leaks on PHHs. If you find the engine bogs after spraying the throttle "bushings", I've had good success in sealing minor leaks with simple axle grease applied to the bushing area with a Q-tip, or long, skinny screwdriver. Vacuum will pull the grease into the void and help seal off the leak.

The fuel level is adjusted at the fuel inlet. The inlet has a brass jamb-nut that you loosen and then screw in/out the top "nut" to adjust the needle seat against the float lever within the carb. Check to see that each carb has the same amount of gap between the inlet and the jamb-nut (it's only a couple mms)

You set the timing... what is the timing at idle? Timing will certainly affect idle speed. Haynes manual for a BMW 2002Ti (same engine) shows 18-22 degrees at 1000rpm.

Hope this helps,

Ed
 
could be a leaking brake servo! on a lot of M10 manifolds the brake vacume is from No.4 inlet area and a leaking servo will allow excess air into No4 cylinder causing a weak mixture on that cylinder.....................
 
Stevehose said:
What kind of carb to manifold gaskets are you using?

Can you get it to run well with running cylinders 1 and 3 almost closed and the others out or is it confined to those cylinders.
Click to expand...

Not sure what brand the gaskets are, if that's what you mean. They seem to just be normal paper or fiber gaskets, one on either side of the fiber spacer that goes between the intake runner from the head and the carburetor itself. If I close off the mixture for 1 and 3 the engine starts to stumble, but I think I see where you're going with that. I know there's a specific order for balancing the air flow through the carbs, is there also an order for setting the idle mixture screws? I've just been adjusting them 1-4.
 
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