Flailing about with SUs after a motor swap. I have a few questions.

So I attempted to tune my SU carburetors today after a motor swap to a B20F. It didn't really work out so well.

Some background info on the motor. The timing is set at 10 degrees BTDC on a stock '73 B20F and the valve clearances are set according to the green book. The injector holes have freeze plugs and I searched for vacuum leaks with carb cleaner. The SUs are set up with SM needles.

I attempted to synchronize the carburetors by sound or using the trick with two pieces of wire bent at right angles to measure how high the dashpots are sitting. I thought I had it right but my idle was relatively lumpy. Also when reving the motor it blew out one damper and sprayed fuel out of the vent on the carburetor. This made me think that something else is amiss. I can guess the threads on the plastic damper cover are messed up but would that cause fuel to spray out of the float bowl vent?

As of now I have decided to leave it until a uni-syn tool shows up in the mail.

Also the engine runs hotter now and took a while to get air out of the system. Has anyone else encountered higher temperatures running a B20F with an Amazon stamped four blade fan and radiator?

The fuel inlet needle valve can stick and let the carb bowl overflow. You'll want to check or set the float height. Another cause is fuel sprays out of the float bowl vents because your fuel pressure is too high. The float can fail and sink that could overflow the bowl. I had to use a fuel pressure regulator to stop the overflow. Most of the aftermarket fuel pumps have too high a pressure. The only one that works without needing a regulator is the OEM Pierburg pump. Fuel pressure is 1.5-2.5psi for SUs.

Yes, wait for the unisyn. Also Take some time to read up on how to set the mixture. You'll need to get the small jet wrench so you can turn the jet to adjust the mixture. You will count the flats on the hex shape of the jet to know how much you have adjusted the jet. You test the mixture by slightly lifting the damper at idle. How the engine reacts tells you whether too rich or too lean. Another tool to consider is the see through spark plug so you can tune the mixture by viewing the colour of the combustion.

In addition to the above (stuck jets and fuel pressure too high) tiny TINY bits of grit can keep the jets stuck open as well, sometimes they've fallen out and aren't seen if you disassemble. But they really need clean fuel (filter between the pump and carbs is a good idea) and something under 3 or 4 psi of the fuel will jsut push the vale open and flow in regardless. If fuel comes out the vent, it's because the float bowl is overfilling for some reason, that has to be fixed before you set the mixture.

Also - there's the fuel height setting. Hold the fuel bowl lid upside down and measure the gap to the lid. I forget the spec - maybe 7mm? Bend the tab on the float arm that presses the needle valve shut to adjust.

Then - locate the lifting pins on the carbs. Somewhere around the base of the 'dome' on the air cleaner side will be a little metal pin sticking down. Push it up and it will lift the piston slightly, like 1/4 of an inch. That's how you tune the carbs.

1) Make sure the ignition system is in fine shape, no issues with plugs, wires, coil, etc.
2) Warm the engine fully, let everything heatsoak nicely (carbs too) until it's all at operating temp
3) Ensure the choke mechanism turns all the way off and the jets return to their normal position, and the idle is not being set by the choke as well (it affects both)
4) Equalize the airflow as best you can. A unisyn makes it super easy to do. Otherwise... just fiddle with the idle adjusters until a quick half turn in and out makes roughly the same difference in idle speed. Unfortunately, the mixture also affects this, so if you haven't set this precisely with a Unisyn like tool, you'll need to loop through this step multiple times.
5) Working with one carb at a time, lift the piston with the lifting pin (air cleaners off). And observe what the motor does in response.
--a) If the idle speed drops - the mixture is too lean. Unscrew the jet adjuster nut to slightly lower the jet and try again
--b) If the idle speed increases, and then returns more or less to normal - congrats! Mixture is good!
--c) If the idle speed increases and stays higher - the mixture is too rich. Screw the jet adjuster nut in to slightly raise the jet and try again
6) After setting the mixture properly, you may need to revisit step 4 again if you didn't use a Unisyn - just to make sure the carbs are flowing similar amounts of air at idle. Then do step 5 again as well.
7) After both carbs are balanced and have proper mixtures, set the idle speed to what you want.

Of course, if there are air leaks around the throttle shafts - this will screw up the 'proper' method for setting mixtures - because the vac is high at idle, and even small leaks can cause the mixture to be leaner than it should be. Setting the mixture to account for this air leak at idle will make it too rich elsewhere in the throttle range - where 'real' airflow increase, vac drops and the 'leaked' air shrinks to a miniscule proportion. So if you have leaky throttle shafts, you might need to set it by driving it around on some clean plugs, and then looking at them. Won't idle as well, but you'll have a better mixture setting while driving.

I have the float height set correctly per the greenbook for a B20B, which has a section on HS6 carbs.

I had a spare dashpot damper so I replaced that. It doesn't fly off now.

The tuning procedure I have figured out except for the synchronizing.

Any recommendations on a fuel pressure regulator if it comes to that? I'll probably go after the fuel squirting beyond checking the valve once the carbs are tuned-ish.

You really can't set the tune with fuel squirting out one of the carbs. The level is 'way too high' and extra fuel is going into the motor through the jet.

I've been using one that looks like this on my PV for a long time:
https://www.ebay.com/itm/Universal-...003654?hash=item523942e546:g:jhYAAOSwzApfSKv4

It's feeding DCOE's now, but they need even lower pressure (2, 2.5 psi).

And if you do have the mixture set right, you can balance by going back and forth between the idle adjusters on the carbs. Screw the adjuster in a half turn, listen to the change in idle speed, go back a half turn. Then go to the other carb and do the same. Is one carb affecting the idle more than the other? Lower that one a bit, and/or raise the other to keep the overall idle the same. Then go back and forth again.

The SU manifolds have balance tubes, so they'll actually idle pretty well on only one carb, so it can be a bit subtle. But still each carb should have the same affect on idle speed when you screw the adjuster in 1/2 turn, wait a few seconds, then back to where it was.

JohnMc said:

You really can't set the tune with fuel squirting out one of the carbs. The level is 'way too high' and extra fuel is going into the motor through the jet.

I've been using one that looks like this on my PV for a long time:
https://www.ebay.com/itm/Universal-...003654?hash=item523942e546:g:jhYAAOSwzApfSKv4

It's feeding DCOE's now, but they need even lower pressure (2, 2.5 psi).

Click to expand...

Ok, I was somewhat skeptical of the generic purolator clone regulator. I guess I will pick up one of those.

I used the CR fuel pressure regulator that looks the same. One they sell at most of the major chains. Great post on setting up the mixture on the carbs John!

Once you get comfortable setting up the carbs. You become a member of the Skinners Union.

I picked up a fuel pressure regulator today. Also realized I accidentally bought hotter plugs that what I was originally using with the B18. Switched those back to BP6HS from BP7HS just to minimize the number of variables in play.

I didn't like how it didn't have a mounting bracket and it felt a bit heavy to be supported only by the fuel line. so I put together a prototype using some sheet steel and tin snips. I am thinking that if I put a right angle into the next version that it will sit up against the block nicely. I ordered a small pressure gauge so I used whatever T fitting I had lying around to mock it up until the gauge arrives. I will replace it with another when I can so the fuel hose doesn't have to make that 90 degree bend near the fan.

We use DX needles in HS6 carbs on B20 engines. We will only use high quality Holley brand fuel pressure regulators.

I have seen those cheap pieces of crap fuel pressure regulators you are using break apart and cause a massive fuel leak. Fortunately the customer's car didn't catch on fire so we won't use them.

I've had mine on the car for 15-ish years. But it's also on the firewall and doesn't get shaken around as much.

But you never know what you're getting these days. Knockoff? A knockoff of a knockoff?

JohnMc said:

You really can't set the tune with fuel squirting out one of the carbs.

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yep and keep a fire extinguisher close by

planetman said:

We use DX needles in HS6 carbs on B20 engines. We will only use high quality Holley brand fuel pressure regulators.

I have seen those cheap pieces of crap fuel pressure regulators you are using break apart and cause a massive fuel leak. Fortunately the customer's car didn't catch on fire so we won't use them.

Click to expand...

Whelp. I will be looking for an alternative option then in the near future. Would holley part number 12-804 (the 1-4 psi unit) not burst into flame?

Edit: Before posting this I put together version 2.0 of my mounting setup.

Yes, the Holley 12-804 regulator is the 1 we use. It ain't perfect, but we do trust it more.

The main problem is the size of the ports which are 3/8 female NPT. I haven't been able to find fittings with 3/16" hose barbs and have 3/8 male NPT ends so reducing bushings are needed and the cost of the hardware adds up. 3/16" fuel hose is what is supposed to used for the outlet side of the fuel pump all the way to the carbs and in between the carbs.

A fuel pressure regulator wouldn't be needed if someone would make new mechanical fuel pumps with properly made and calibrated fuel pump check valves like those used in any of the original fuel pumps Volvos used to sell. I'm pretty sure Volvo sells one of the same pieces of crap currently being sold by others.

And if putting out too much fuel pressure ain't bad enough, the new pieces of crap fuel pumps have additional problems like the inlet and outlet hose barbs leaking and falling out and the machine screws backing out that hold the 2 halves together which when loose causes a fuel leak.

planetman said:

Yes, the Holley 12-804 regulator is the 1 we use. It ain't perfect, but we do trust it more.

The main problem is the size of the ports which are 3/8 female NPT. I haven't been able to find fittings with 3/16" hose barbs and have 3/8 male NPT ends so reducing bushings are needed and the cost of the hardware adds up. 3/16" fuel hose is what is supposed to used for the outlet side of the fuel pump all the way to the carbs and in between the carbs.

A fuel pressure regulator wouldn't be needed if someone would make new mechanical fuel pumps with properly made and calibrated fuel pump check valves like those used in any of the original fuel pumps Volvos used to sell. I'm pretty sure Volvo sells one of the same pieces of crap currently being sold by others.

And if putting out too much fuel pressure ain't bad enough, the new pieces of crap fuel pumps have additional problems like the inlet and outlet hose barbs leaking and falling out and the machine screws backing out that hold the 2 halves together which when loose causes a fuel leak.

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My has been relatively painless but just puts out too much pressure. It luckily doesn't leak. The screws were a bit loose when I moved it over from the B18. I put some blue loctite on them so they shouldn't back out on the road.

I'll probably just go get some reducers and reuse the 1/8 NPT fittings I have in the cheap FPR.

I'll also try tuning it with the SM needles and see how it does for now. I have been playing with the carbs and this FPR (it does work) just to get a feel for adjusting the mixture.

Third time's a charm. It could use some elbow fittings though. I dug through my bin of pipe stuff and found two 3/8 to 1/8 npt adapters. I'll make a better bracket for it or mount it to the inner fender when I know it works.

Here is a good source for parts including a pierburg pump. https://www.cvi-automotive.se/en/articles/20733/2b-fuel-pump-b18b20

I got the gauge set up and set the fuel pressure to 2psi based on the 1.5-2.5psi recommendation. I got them tuned as well as I think I could. The rear carb is very subtle in any rpm change while tuning. The front is more obvious. The unisyn makes the job a lot easier. The idle is a little bit lumpy but I have chalked that up to the D cam's 280 degree duration.

I decided to take it for a drive up hwy 1 a bit and then pulled the plugs to check. Here is what I've got. Any feedback would be welcome.


Cylinder 1


Cylinder 2


Cylinder 3


Cylinder 4


I am getting the impression that the rear carb is running a bit leaner. Maybe richen that up a flat?

Yep, try one flat and see how it is. Then go further if needed. Use John Mc suggestion on checking the mixture with the reaction to the 1/4" lift of the damper at idle. The front plugs look great.

dl242gt said:

Yep, try one flat and see how it is. Then go further if needed. Use John Mc suggestion on checking the mixture with the reaction to the 1/4" lift of the damper at idle. The front plugs look great.

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I went back and re-tuned. The front needed no adjustment. The rear I richened a flat or so until I thought it matched the behavior of the front. I was gauging rpm changes when lifting the piston by sound. The rpm differences were hard to hear on the rear carburetor. Perhaps in the future I should just go get a tach/dwell meter and round out my tuning equipment.

Also how lumpy should the idle be when running a D cam?

The D cam is the cam that was in the fuel injected 1800. So, a little lope at idle is normal. It shouldn't be really chunky.