M10 Running Rough at Idle

[CarSnob]

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.

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

I had to look, I never realized the UniSyn was its own tool, I just thought that was sort of a catch-all term for these flow meters. The one I've been using looks just like that picture you posted, so at least I'm good there. I'll have to try syncing them at 2000 RPM to see if that does anything for me, I've been doing it at 1200 RPM.

I've tried pulling the throttle lever (on the carburetor) as closed as I possibly can, no dice. The springs seem to be doing a good enough job holding everything closed. Last week I took the carbs off the car to check the position of the butterflies, and, after some adjustment, they all seemed fine. There's still a tiny bit of light visible around the plate, but I don't think I can get rid of that completely. The needles were replaced as part of the rebuild, but I have removed them and they all seem to be in good shape with no marring or bending or anything. The idle jets are all the same, but I don't remember off the top of my head what number they are. I tried zooming in, but my phone camera doesn't have the resolution for that, I'll have to look when I go home. I'm pretty sure there aren't any vacuum leaks. I didn't have any carb cleaner handy but I held a propane torch (not lit, just the gas flow) around the carbs and didn't notice any idle up or anything. I'll try again though and focus on the throttle rod bushings.

The float level is the one thing I haven't played with yet, so I think I'll have to give that a look when I pull the car out again. Do you know what the level is supposed to be?

Not sure what the timing is at idle. I set it using the directions I got from @starcruiser in the post I linked to in the OP, 25° BTDC @ 2000RPM. Unless I graduate the crank pulley it's going to be hard to tell where I am. Then there's the problem of getting the engine to idle down to 1000RPM...

Thanks for your help so far. Hopefully I'll get this thing sorted. It really doesn't seem all that complicated, but it sure is finicky.

 

[CarSnob]

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.....................

I don't have my brake booster hooked up right now, so I have the inlet plugged. I thought I might have had a leak there too, but I checked everything and it seemed tight. Shame, I would have liked an easy fix.

 

[CarSnob]

We have the car running but the idle is still a bit unstable and at around 800 rpm the engine shakes a lot. The idle adjustment screws on two carbs are just loose maybe a 1/4 turn out the screws on the other two carbs are out about 2 turns. The plugs are all showing signs of running rich with black sooty deposits . If we completely remove the idle adjustment screws the engine runs smooth. We can't seem to lean out the engine introducing a vacuum leak makes it run better. I am wondering if I have late ignition timing and it's the problem . The car is put away until the weekend if anyone has some input let me know . All the jets are factory specs. Thanks for any reply

 

[zinz]

Bummer...

Did you ever determine the fuel level? Sure sounds like that could be your problem if you are over-rich with jets that match factory specs. Are you getting any fuel at all out of the vent tubes of the carbs? (vent tubes are the brass tubes on top of the carbs that point toward the passenger side of the engine. If you have any fuel coming out of those vents, it would be an indicator of too much fuel pressure, or high fuel level. I see you are using the factory fuel pump. If you rebuilt it and replaced the coil spring... it is possible the pump is delivering too high a pressure. I experienced this with a rebuild kit on my CS... the new spring was too strong and pushed fuel past my needle valve. This condition would be readily apparent, though.

Fuel Level Witchery 101:

There is a mark on the carb body between the barrels...a simple line in the casting.

The fuel level can be determined using the toothpick method. You can determine fuel level by removing the top plate off the carb, and removing a jet to look down into the bowl. (Google pic)

Now, with a bright flashlight shining on top of the fuel, slowly insert something like a long toothpick, or bamboo skewer and watch for the surface of the fuel to be disturbed by the skewer (this method is far more accurate than simply dunking the skewer and seeing the wet line). Pinch the skewer with your thumbnail at the top edge of the carb body and remove the skewer. Now you can take that measurement and work out a way to determine where your fuel level is, in relation to the casting mark on the carb body. I've found using a metal straightedge laid across the top of that open jet reservoir gives me something to gauge from when measuring down to the casting mark.

Adjusting the fuel level is a simple process, loosen the brass jamb nut of the fuel inlet and turn the inlet in or out to raise/lower the float level.... it doesn't take much, as I remember; 1/2 turns at time, then 1/4 turns when I get I'm fine tuning tip-in after the car is running well.

Timing/Ignition

Adding more advance can help a side draft M10 a lot; especially at idle and low rpms. 25 degrees at 2000rpm may not be enough. Blue manual says 28-32 degrees at 2000rpm for a BMW 2002Ti (the 2000CS has the same engine). If you don't have an advancing timing light, simply turn the dizzy counter clockwise a teeny bit and see how it reacts.

What spark plugs are you using? Not Platinum plugs, I hope... or resistorized plugs? NGK BP5ES, or BP6ES is all I use in an M10. (the 5 is the hotter plug of the two). Platinum plugs won't work at all...

Hope this helps... Sidedrafts can be frustrating to the uninitiated, but the payout is worth it

Keep plugging away, you'll get it.

Ed Z

 

[CarSnob]

I think you have given me a lot of info there is no fuel comming out of the vents . I have two fuel pumps and when they were on the engine each of them run the same . My timing is not what you recommended and I am using platinum plugs. So this weekend I will readjust the timing and install new plugs to start with . Thanks for your help I welcome any more help from anyone who has any ideas. Funny I thought the 6 plug would be hooter than the 5 plug? Thanks again

 

[zinz]

Eureka moment! Platinum plugs... changing those out will definitely help you. Guaranteed.

Get some NGK BP6ES... make sure there is no “R” in the model #. That would be a resistor plug. Gapped to 0.026, or so....perhaps wider as you sort things out later.

This might not solve the discrepancy you have between the mixture screw adjustments of the two carbs, but now it will firing a healthy spark and you may find the engine wakes up. Always solve ignition first

Add some ignition advance and I bet you can have it purring in no time.

Ed

 

[Stevehose]

Ed, great call on the platinum plugs. Those are no bueno for vintage bimmers

 

[zinz]

Thanks, Steve. Ray tells me he got his “project car” and found platinum plugs in it and it ran terribly...as expected. Changed them out and instantly it ran better.

I’m curious how these Solexes respond with a sorted ignition.

 

[Stevehose]

I remember back in 1984 I put the new bosch platinum plugs in my first coupe. The 4 electodes looked like a winner. Ran like crap.

 

[CarSnob]

Hello, I have not been able to get back to my car this week , I did not get a chance to install the new plugs and reset the timing. Thanks to all who have asked the status of the car.