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Old 10-17-2020, 11:34 PM   #1
everyhumanvovo
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Default 1 minute starting below 40 F. - SU carbs sync or choke?

Have a B20 with dual SU carbs in the 122. This past summer it took longer to start. Tonight, temp outside down to 45F, had manual choke wide open for about a minute over 5 or 6 tries and afraid this is bad for starter. Carbs were syncronized by good vintage Volvo mechanic a few years ago in Oregon, but now in colder storage for winter.

Thinking may have to tune SU carbs and adjust choke. Runs fine and restarts fine when warm. Have not synced carbs myself, but watched a few videos, and have a syncrometer from ipd. Funny, videos watched just showed both carbs with number match on this meter, but did not point out what number on device they should match at.

Can push choke all the way back in after 5 or 10 minutes and warmed up, and then will start right back up and run fine. Wider viscosity dashpot oil for starting in colder temps?

Any insights on whether this is more of a sync issue, choke adjustment, dashpot oil is much appreciated!

Last edited by everyhumanvovo; 10-26-2020 at 08:00 PM..
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Old 10-18-2020, 01:20 AM   #2
hiperfauto
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It sounds like a choke issue to me. Pull the choke lever and verify that the jet tubes are dropping down at least ¼". You may need to loosen the trunions and reclock the choke levers if they aren't going down far enough.
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Old 10-18-2020, 06:53 AM   #3
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What model SU carbs?
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Old 10-18-2020, 10:51 AM   #4
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Originally Posted by everyhumanvovo View Post
Thinking may have to tune SU carbs and adjust choke. Runs fine and restarts fine when warm. Have not synced carbs myself, but watched a few videos, and have a syncrometer from ipd. Funny, videos watched just showed both carbs with number match on this meter, but did not point out what number on device they should match at.
The exact number isn't super important. It's more important to ensure that both have the same flow, since that means they're synced.
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Old 10-18-2020, 01:25 PM   #5
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Check to make sure you aren't losing power to the coil while cranking. I ran into this on a 122.
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Old 10-26-2020, 07:33 PM   #6
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Default Part of issue fixed!

Turns out (glitch) had not replaced spark plugs and cap since I bought 9 years back and about 15 or 20 thousand miles. Gapped some new NGK BP7's at about .030. Checked before ordering and went with these over NGK BP6's being replaced as I had some engine ping at reasonable, but lower, rpm's in higher gears under acceleration. And, always use real gasoline at 91 or 93, some Lucas octane, and lately add lead sub. Distributor cap had turquoise areas on some of the copper contacts and one looked a little rough, so replaced that and the rotor (although rotor looked fine to my eye).

Spark plug electrodes and ground bars were all uniform, dry brown, but darker oily brown going up threads. Guess these can be cleaned and filed for more life if needed. Wondering about the Bosch plug wires - they looked fine and were not cracked and also replaced same time as plugs and cap change - 20k and 10 years back. Only real mod is the Pertronix ignition replacing points. Do have a Bosch blue coil, but while B20 is in this car, it is a 64 model with armored cable and coil in firewall which one mechanic seemed to say was more than a simple replacement. Will bigger better plug wires make much difference or necessary with current arrangement?

Plugged in block heater for an hour, pulled choke out all the way (new cables were replaced several years ago) and started up in about 5 seconds with only two pumps of pedal - much better on a "cold" start" than last time or this summer.

Able to push choke all the way in after about twenty seconds and it remained idling - much better. But, then as before, when starting was less an issue, and cap and plugs were new or newer, if I slow down for a stop especially after some spirited driving, idle dies. But, if I pull choke about a 1/3 or 1/2 way out, before coming to a stop, can maintain idle. This I expect is a carb adjustment. Idles at about 1,000 rpm after warm with choke off, pushed in. Is it a little lean as indicated by needing choke to keep running when slowing to stop?

Plugs and/or cap made a - Wow, it was faster than I remember - not off the start, as is the case, but when rolling, nice power for passing and beating highway lights.. 65 to 85 jiminy split

On carbs and sync, So if the the number on sync gauge for dual carbs does not matter as have noticed is not mentioned in videos I've found, and if one carb is showing 400 on gauge and one is showing 600, do you sync at 500? Or adjust one down to 400 or other up to 600? There has to be a correct measurement to sync carbs at or am missing something.

Last edited by everyhumanvovo; 10-26-2020 at 08:28 PM..
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Old 10-26-2020, 08:10 PM   #7
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Originally Posted by hiperfauto View Post
It sounds like a choke issue to me. Pull the choke lever and verify that the jet tubes are dropping down at least ¼". You may need to loosen the trunions and reclock the choke levers if they aren't going down far enough.
Thanks. Think I was looking tonight at the screws to loosen for choke cable adjustment for each carb. Not sure about re-clocking, but will research. Did lube some of the exposed cables and linkages couple years ago.

Last edited by everyhumanvovo; 10-26-2020 at 08:30 PM..
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Old 10-27-2020, 08:19 AM   #8
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Pumping the pedal doesn't do anything on an SU carb - there's no accel pump in it.

The choke mechanism bumps up the idle speed slightly - in addition to lowering the jet to enrichen the mixture it also nudges the throttle valve slightly open. So a slow idle on a warming but not warm engine perhaps just means that you need to slowly close the choke as the engine warms, and if the idle is too slow when fully warm, then adjust that up a little.

There's a proper procedure for setting the mixture at idle on a fully warmed engine - using the piston lift pins (around the bottom edge of that large dome structure on top) - if you're interested I could type out a bunch of words, but it's the sort of thing you don't want to just blindly mess with. And the proper method might not work well if your carbs are worn and leaking air around the throttle shafts.

And last but not least, there's a possibility that the carb float bowls are going dry after the car sits for a bit, and perhaps it's taking lots of engine revolutions for the mechanical fuel pump to move fuel into them?

I did notice my PV's motor got much easier to start in the cold when I put a really high (and possible over-) powered ignition system on it. But that's with some DCOE's which act differently. But a weak ignition system can be more sensitive to things like wet, cold plugs.
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