Solex P40 ii idle problem

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joseph rossi
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Solex P40 ii idle problem

#1 Post by joseph rossi » Sun Sep 08, 2019 7:39 pm

After 6 to 8 days of frustration, I can't get my Solex's to idle properly. This is a new 912 engine, 1720 kit, 8.7 compression ratio. Carbs have been re-machined and rebuilt by a reputable source. The highest idle I can attain and have the engine return to idle after reving is 750 RPM. At that, the idle adjustment screws are barely turned in. If I try to raise the idle any more and then rev the engine, it will stick in the 1500 RPM range and not return to idle. I've checked for air leaks by pulling the carbs and replacing the gaskets. Base of carbs is good. Throttle plate closes completely. I've also tried spraying carb cleaner and using propane to check for leaks. Couldn't find any! I though it might be the distributor (rebuilt), so I sent it to a another reputable vendor who made a few tweeks. This didn't help. Jets are nothing out of the ordinary. 60 idle jets with 110 air correction. Maybe a little rich, but still reasonable. Set up on the carbs is perfect (float levels, balancing etc.). I've tried numerous combinations of the idle adjustment / idle mixtures screws. Static timing is 3 degrees BTDC. Nothing works. The idle is so weak, when you give it gas, it almost dies. Other than the idle, the car actually runs very well. Any suggestions.

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Vic Skirmants
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Re: Solex P40 ii idle problem

#2 Post by Vic Skirmants » Sun Sep 08, 2019 7:59 pm

Put your question in the regular forum. This section is for questions about the Forum and website,

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John Brooks
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Re: Solex P40 ii idle problem

#3 Post by John Brooks » Sun Sep 08, 2019 8:17 pm

look at the distributor advance and return, I believe the distributor is advancing as you rev the engine then sticking around 25 deg. look at the pulley mark with a timing light, then rev it and when it sticks at 1500 see when it mark lands.

My 912 was doing this, after it got warm, the idle would creep up to 1500 after 15-20 minutes of driving. Rebuilt the distributor and presto its fine now. I think it was corrosion/lack of lubrication on the advance weight pivots. Most solex and zenith problems are electrical.
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Phil Planck
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Re: Solex P40 ii idle problem

#4 Post by Phil Planck » Sun Sep 08, 2019 9:11 pm

Welcome to the forum Joseph. Can you please identify what distributor you have?
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joseph rossi
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Re: Solex P40 ii idle problem

#5 Post by joseph rossi » Mon Sep 09, 2019 4:26 am

It’s an 031.

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Phil Planck
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Re: Solex P40 ii idle problem

#6 Post by Phil Planck » Mon Sep 09, 2019 9:11 am

Ok. Time to start ruling things out. Split shaft or solid shaft carbs?
Disconnect links to carb and increase idle to 800 rpm on each carb individually. If good, try 850 rpm.
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Re: Solex P40 ii idle problem

#7 Post by joseph rossi » Mon Sep 09, 2019 11:40 am

They are split shafts. I’ll check the linkage again. I’m pretty sure they’re OK. When the idle has moved up, I’ve pushed on the throttle arms to make sure they were closed.

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Re: Solex P40 ii idle problem

#8 Post by Doug McDonnell » Mon Sep 09, 2019 12:24 pm

Try a set of 57.5 idle jets viewtopic.php?f=1&t=31257 With your compression ratio you have a pretty mild setup.
1965 356C There is never enough time to do it right, but always enough time to do it over.

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Steve Hatfield
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Re: Solex P40 ii idle problem

#9 Post by Steve Hatfield » Mon Sep 09, 2019 3:32 pm

Air leaks at the manifolds
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Tim Berardelli
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Re: Solex P40 ii idle problem

#10 Post by Tim Berardelli » Mon Sep 09, 2019 6:50 pm

How are you measuring air flow at all 4 carb throats? And at what RPM's?

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Re: Solex P40 ii idle problem

#11 Post by joseph rossi » Mon Sep 09, 2019 7:17 pm

I checked air flow in all 4 carb throats at 750 RPM and 2000 RPM. I use a synchrometer.

I checked for air leaks again today using propane. Found nothing. Tried lowering the float level slightly below factory recommendation. No effect. Tried 55 idle jets vs the 60 and 57.5 I have already tried. No effect. Tried disconnecting the throttle linkage and creeping up on a higher RPM. The minute I reved the engine up after the adjustment, it still stuck at a high RPM, only higher than before.

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Re: Solex P40 ii idle problem

#12 Post by Bruce Smith » Mon Sep 09, 2019 8:13 pm

Sounds like your distributor's mechanical advance might be sticking. Take off the cap. Grab the rotor shaft and turn it manually. It should rotate a few degrees fairly easily with only resistance from the springs, and then spring back. Put a few drops of oil on the wicking felt in the center of the shaft hub under the rotor and work it several times. This is a periodic maintenance item.
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Re: Solex P40 ii idle problem

#13 Post by joseph rossi » Mon Sep 09, 2019 8:22 pm

Thanks Bruce. I’ve already done that plus the distributor was just checked and adjusted by John Jenkins. Also tried anther distributor, an 022, that had just been restored, but that didn’t solve the idle either.

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Re: Solex P40 ii idle problem

#14 Post by Bruce Smith » Mon Sep 09, 2019 8:41 pm

Being a 912 engine, depending on the year, the carb bases and manifold spacers would have had vacuum ports. None are necessary. Are these closed off?
Bruce

'60 356B (Ivory cab)
'63 356B (Ruby Red coupe)
'67 912 (Rally car)
'68 912 (Golden Green)
'70 911T (Bahia Red)
'14 911-50th (GTS)
'18 Beck 550 Spyder (356C motor)
'62 Austin Mini (850 stock)

joseph rossi
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Re: Solex P40 ii idle problem

#15 Post by joseph rossi » Mon Sep 09, 2019 9:13 pm

I’m using fiber block insulators so the only vacuum take off is on the carb body. These are blocked of with small screws.

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