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Old 04-20-2021, 10:58 PM   #1
blakemcelroy2000
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Default Volvo Amazon always fouling spark plugs

Hey all. Im having a hell of a time tuning my Amazon, and I have no idea why. B20F with dual SU HS6s. Fully rebuilt engine, fully rebuilt carbs. New plugs, wires, distributor cap, rotor, coil, pertronix pointless ignition. Seems like no matter how I tune the engine, it keeps fouling the spark plugs. Even if the carbs are leaned out to the point where the engine backfires like crazy upon acceleration, the plugs still become sooty within 5 minutes of driving. Car idles pretty rough too. Plugs are Bosch, and gapped to .035". Distributor has no vacuum advance, and has the centrifugal mechanism. Valve lash issue? Wrong temp plugs? Seems like the only way to make the engine run is to advance the hell out of it (the #1 wire output on the distributor cap is closest to the engine block) Any advice helps.
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Old 04-20-2021, 11:05 PM   #2
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Take off crank pulley and make sure it didn’t slip or was installed the wrong way

Then adjust valves
Check adjust timing
Adjust carbs and re check timing
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Old 04-20-2021, 11:19 PM   #3
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Take off crank pulley and make sure it didnít slip or was installed the wrong way

Then adjust valves
Check adjust timing
Adjust carbs and re check timing
Crank pulley? Do you mean the crank gear? The pulley shouldn't affect the timing should it?
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Old 04-20-2021, 11:27 PM   #4
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Crank pulley? Do you mean the crank gear? The pulley shouldn't affect the timing should it?
yes crank pulley
thats why I said Take off crank pulley and make sure it didn’t slip or was installed the wrong way


then check timing gears after all the rest
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Old 04-20-2021, 11:59 PM   #5
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What plugs are you running? I tried a few in my B20F to see how they did after I accidentally ordered NGK BP7HS plugs instead of the often recommended BP6HS. They were a bit too cold and were rather sooty unless I drove the car hard. I popped some hotter plugs into it and that issue went away. Also which coil? I recall in the literature that came with my pertronix unit that they recommended avoiding a certain type of wires, I can't remember which.

Can you get photos of the plugs? What is your tuning procedure for the SUs? I struggled with rough running until I got enough practice tuning them (much to my housemate's annoyance).
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Old 04-21-2021, 12:06 AM   #6
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yes crank pulley
thats why I said Take off crank pulley and make sure it didnít slip or was installed the wrong way


then check timing gears after all the rest
Gotcha. Noobie question, but the dot on crank gear and pulley gear should align at TDC, correct? I had a pretty reputable builder in my area rebuild the engine and installed the cam and crank, so I doubt he screwed that up, but everyone can make mistakes I guess.
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Old 04-21-2021, 12:07 AM   #7
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I would also tighten up your plug gap. The spec for a B20F is 0.028" to 0.032".
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Old 04-21-2021, 12:09 AM   #8
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I would also tighten up your plug gap. The spec for a B20F is 0.028" to 0.032".
Ill try .030". Thanks!
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Old 04-21-2021, 07:12 AM   #9
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Make sure the distributor isn’t installed 180* off. You can check by moving your plug wires around on the distributor. Move 1 to 3, 2 to 4, etc.
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Old 04-21-2021, 09:16 AM   #10
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I know you said the carbs were rebuilt, but if there are air leaks along the throttle shafts, it will lean the mixture out at (or very near) idle. And that's where you set the mixture, so if you set it properly with some extra air leaking in, it will be rich when the throttle is open more. Or you might have the wrong needles for the motor? You could also try an alternative method of setting the carb mixture - clean the plugs well, drive it around without letting it idle much at all, shut it off quickly, take the plugs out and do a visual read. Sooty? Lean the carbs down some. Even if this screws with the mixture at idle.

I'd still set the valve lash - it's something that needs to be done every 5K miles (more or less). .016 - .018. Tighter is quieter, but you're more likely to burn a valve. And unless you have hardened exhaust valve seats, those might be slowly tightening over time as well. Run through them, adjust them, and eyeball the valve lift on each one.

Sad story - but 2 times over the decades when my PV's B20 has developed an uneven idle, it wasn't carb adjustment issues, it was low compression from a cracked piston. And a couple of times when it felt a bit 'off' under power - it was a flat cam lobe. So set the valves, check the valve lift, and do a compression test before casting too much suspicion on the carbs.
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Old 04-21-2021, 12:38 PM   #11
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What are you using for a fuel pump? Max pressure for SU's is 3Ĺ psi. If the pressure is higher it could easily force fuel past the float valves.

Also, if the floats are set too high fuel will dribble into the carb.
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Old 04-21-2021, 02:04 PM   #12
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What are you using for a fuel pump? Max pressure for SU's is 3Ĺ psi. If the pressure is higher it could easily force fuel past the float valves.

Also, if the floats are set too high fuel will dribble into the carb.
Ah, you may have figured it out. Im using an electric fuel pump and have my regulator set to 4.5-5 psi. Also, pretty sure I have the non adjustable float types. New float needles too.
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Old 04-21-2021, 02:08 PM   #13
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Ah, you may have figured it out. Im using an electric fuel pump and have my regulator set to 4.5-5 psi. Also, pretty sure I have the non adjustable float types. New float needles too.
They should still be adjustable but you adjust them with washers under the needle valves.
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Old 04-21-2021, 02:08 PM   #14
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Originally Posted by JohnMc View Post
I know you said the carbs were rebuilt, but if there are air leaks along the throttle shafts, it will lean the mixture out at (or very near) idle. And that's where you set the mixture, so if you set it properly with some extra air leaking in, it will be rich when the throttle is open more. Or you might have the wrong needles for the motor? You could also try an alternative method of setting the carb mixture - clean the plugs well, drive it around without letting it idle much at all, shut it off quickly, take the plugs out and do a visual read. Sooty? Lean the carbs down some. Even if this screws with the mixture at idle.

I'd still set the valve lash - it's something that needs to be done every 5K miles (more or less). .016 - .018. Tighter is quieter, but you're more likely to burn a valve. And unless you have hardened exhaust valve seats, those might be slowly tightening over time as well. Run through them, adjust them, and eyeball the valve lift on each one.

Sad story - but 2 times over the decades when my PV's B20 has developed an uneven idle, it wasn't carb adjustment issues, it was low compression from a cracked piston. And a couple of times when it felt a bit 'off' under power - it was a flat cam lobe. So set the valves, check the valve lift, and do a compression test before casting too much suspicion on the carbs.
The carbs were just rebuilt about 3 weeks ago with new throttle shaft bushings and seals, so for my own sanity, Im eliminating that problem from the equation. The engine is basically brand new, as in, new pistons, rods, cam, lifters, bearings, etc. Hardened valve seats, new valves, new valve springs, retainers, etc. Only things that aren't brand new are crank and block of course. So Im highly doubting (and hoping) that none of those serious problems you listed are happening.
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Old 04-21-2021, 02:10 PM   #15
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They should still be adjustable but you adjust them with washers under the needle valves.
Ahh, by placing washers under the brass "nut" type housing that the needles ride in? Ill be sure to measure them up with some feeler gauges. Any rough number of what is correct?
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Old 04-21-2021, 02:13 PM   #16
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Make sure the distributor isnít installed 180* off. You can check by moving your plug wires around on the distributor. Move 1 to 3, 2 to 4, etc.
Im pretty sure I have the wires plugged in correctly. Rotor spins counter clockwise and firing order is 1-3-4-2?
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Old 04-21-2021, 03:58 PM   #17
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Ahh, by placing washers under the brass "nut" type housing that the needles ride in? Ill be sure to measure them up with some feeler gauges. Any rough number of what is correct?
The spec volvo used seems to vary a bit. I've got a PDF of the B20A/B greenbook open and it says 0.189" for the floats on an HS6.



When I was tuning my car it also had a high fuel pressure issue. The mechanical pump was putting out 5psi. I had fuel spilling out of the jet on the rear carb and it would spray out of the vent hole on the front.
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Old 04-21-2021, 04:14 PM   #18
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Even if your floats are set correctly the SUs probably aren’t going to like that extra fuel pressure. Do you have a mechanical pump you can throw on there and see if that helps? I had a similar issue with a pair of “rebuilt” carbs I bought off eBay. They were beautiful but the rebuilder didn’t even bother to set the floats and one of them was super high. The car ran out great but was always really rich at idle and smoked badly on decel.
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Old 04-21-2021, 04:17 PM   #19
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I've always just set fuel levels by bending the tab on the float?

Sounds like you have a regulator, just set it down to 2.5, 3 psi and see if it behaves better.
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Old 04-22-2021, 11:53 AM   #20
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I've always just set fuel levels by bending the tab on the float?

Sounds like you have a regulator, just set it down to 2.5, 3 psi and see if it behaves better.
There are a few styles on the market. There is the original ones with the clear-ish plastic float attached to a metal tab, there are the newer black ones with a metal tab, and there are completely plastic ones that have no metal tab to bend, hence the washers.
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Old 04-24-2021, 02:05 PM   #21
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There are lots of things on a rebuilt engine to check before becoming sick. Fouled plugs within a few miles - you are going to see something obvious. Runs very rough? Maybe it is 180 out of time, but rough running has to be fixed first. Only a couple reasons for sooty fouled plugs - they better be the correct plug, correct heat range and correct gap first (.035 is too wide).
-Mixture is way too rich. Rich enough to cause rough running and smoky exhaust.
Did the carb rebuilders really know what to do? Gas dripping from the carbs?
-Plugs are way too cold
-Spark intensity too wimpy OR electronic ignition box too weak to drive a high capacity coil
-Didn't adjust the valves - oops!
-Check with timing light: mag pickup wires reversed?
-Bad gas. Has it been sitting there for months while you worked on the engine?
-Have a beer after each test to think things over


Keep us posted.
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Old 04-24-2021, 02:24 PM   #22
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We won't use the plastic floats that don't have a metal strap to adjust the float level as there isn't enough thread on the float valve base to adjust properly.

I believe there is a different float valve assembly that has a shallower internal seat that raises the height and then you might be able to shim the base to make the non-adjustable float work.


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There are a few styles on the market. There is the original ones with the clear-ish plastic float attached to a metal tab, there are the newer black ones with a metal tab, and there are completely plastic ones that have no metal tab to bend, hence the washers.
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