First Things First, I'm talking about a '78 Triumph Spitfire here, not any Volvo. Hope y'all don't mind, but the issue is, I believe, applicable across makes and there are more than a couple knowledgable and helpful folks here.

The backstory: A good friend is at his wits end trying to get his '78 Spitfire to run correctly. He must be desperate because he lent the car to me to try and diagnose. Me, a person who has never tuned a carburetor, much less two SU carburetors (just found out I can't spell the word either).

The car: '78 Triumph Spitfire, 1500cc, converted by PO to dual HS4 SU carbs, 4-2-1 header. Electric fuel pump, new Pertronix branded distributor and coil, new cap, rotor and wires. Carbs were supposedly recently rebuilt by PO, current owner sent them to Joe Curto recently.

The symptoms: As it came to me, starts and runs pretty good when cold. Hot, it has a lumpy idle, misses as it takes up throttle. Stumbles and struggles under load until 3k rpm then it pulls like I think it should, good power.

What I've done: Starting with the assumption that the carbs are just fine, I tried to find fault with the ignition system. Cap and rotor show no obvious signs of faults, plugs gapped. Per Pertronix, I checked voltage in the red wire, ground and voltage tests at the coil, all sat. Owner had the timing way advanced at idle, I removed the vacuum advance tube, backed it up to 10 BTDC and confirmed that it advances at higher RPM.
What else can I check? Pertronix claims that misfiring and breaking up under load is nearly always because of low voltage at that red wire. I moved it from the + side of the coil to the same switched power circuit as the fuel pump.

From there I moved to the induction system. Spraying carb cleaner all around didn't change idle except for the onboard ends of the throttle shafts. Are they to be sealed, or is it normal for the clearance in the bushes to allow fluid past?

I've assumed that the HS4s are working good internally. This based on the fact that they've been in Joe Curto's hands recently, they don't dump fuel out the overflow, and the damper pistons rise and fall freely and evenly. Another thing, there's no FPR just a little plastic gauge between pump and carbs that steadily indicates ~2.75psi.

I followed the guide at https://sucarb.co.uk/technical-hs-type-carburetter-tuning-general to adjust each carb.Adjusting throttle plate, backing off fast idle adjustment, setting jet height, resetting throttle and choke interconnections, setting idle speed and synchronizing (thanks Ron Kwas for your insights @sw-em.com) All went well until the last iterative step of adjusting jet height. Eventually I lost track of how many flats each one had been turned and which way. As it sits, the mixture is too rich (raising the piston lifting pins raises the idle and stays there); When I started the adjustment it was weak, idle dropped when lifting the piston.

I should go readjust the jets, but I get the feeling it won't make any difference, nothing so far has.

So I put it you, dear friends, what am I missing here and where should I go next?

Thanks so much for taking the time,
Will
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XC60 / Odyssey

Is there a ground cable from the block to the frame
?

A quick look reveals a good, newish looking 4awg cable between the block and negative side of the battery. That's the only thing attached to the negative terminal of the battery.

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XC60 / Odyssey

It's important to also have a ground strap between the battery and body--or from the motor to the body - originally Volvos always had a strap from the bellhousing to the body. - Dave

Yes, I was surprised to not see one. I added just that, a strap from the bellhousing to the body. I know to temper my expectations for that to be the magic bullet to fix all this car's woes, but I've been surprised before.

Jet height is reset to two turns below the bridge; I'm ready to fire it up, adjust the mixture and check the timing again per Mr. Mullet's advice...but I might be too nice a neighbor. It's the best day of the year so far and all my neighbors are outside; don't think they'd enjoy the raspy note of this car quite the same as I would. We'll walk the dog instead.

Thanks all for humoring me and my non-Volvo content.

-Will
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XC60 / Odyssey

When you have the timing light connected do you see any misfires as it warms up? My 1800 did the same thing (missing when warm) and I traced it to a failing coil.

What would a misfire look like under the flashes of the timing light?
I'm afraid I'm only knowledgable enough to be dangerous as up until now, K-jet and LH 1.0 is as far back as I've gone technologically.

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XC60 / Odyssey

A properly working ignition system will show as a nice rhythmic series of flashes - approximately 6 flashes per second if the pickup is connected to any one of the four plug wires. What you're looking for are any breaks in that nice rhythmic flashing, like the strobe light won't flash for a split second. Check all 4 cylinders separately as the problem, if it is an ignition issue, might only be affecting one or two cylinders.

You can also connect the pickup to the coil-to-distributor hot lead and you will then be watching for a break in an approximately 24 flashes per second.

Good call Chris! Hot engine at idle, I definitely see breaks in the flashes on cylinders 3 and 4, and in the lead from the too.

All components are new from Pertronix via Summit Racing, so I'll suspect the installation for now, and make sure they purchased the correct coil for the application. Methinks I need to track down the ballast wire to make sure it's not connected somewhere down the line.


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XC60 / Odyssey

Good call Chris! Hot engine at idle, I definitely see breaks in the flashes on cylinders 3 and 4, and in the lead from the too.

All components are new from Pertronix via Summit Racing, so I'll suspect the installation for now, and make sure they purchased the correct coil for the application. Methinks I need to track down the ballast wire to make sure it's not connected somewhere down the line.


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XC60 / Odyssey