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Old 03-01-2021, 07:38 PM   #1
spock345
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Default Hard warm start SU HS6

I've got the SU carburetors mostly dialed in on my B20F (now with DX needles as recommended by planetman). Now I have an issue with hot starts. It used to tick over rather quickly when I was running the SM needles but it will hesitate to get started unless at full throttle and cranking for longer. I've dialed them in and verified the mixture setting by looking at the plugs, all of them are a gray-ish tan. I'm running a heat shield.

From what I have read one of the big issues with these is heat and modern ethanol laden fuel's tendency to vaporize more easily. Is there anything off or should I just live with flooring it and cranking for a few seconds? Also is it normal for the inner two cylinders (2 and 3) to seem to run a bit richer than the outer cylinders (1 and 4)?
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Last edited by spock345; 03-01-2021 at 07:44 PM..
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Old 03-01-2021, 07:43 PM   #2
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How is the plug gap? Maybe reduce it a bit like a .02" and see if it starts easier. Also if the timing is too advanced it can be hard to crank. But that doesn't seem like your issue. It might be that the engine could use a bit more spark energy. Are you using a pertronix or is it points?
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Old 03-01-2021, 07:52 PM   #3
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Pertronix with a bosch red coil (1.8 ohm ballast resistor so I don't fry the pertronix). I'll check the plug gap.
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Old 03-01-2021, 10:55 PM   #4
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Originally Posted by spock345 View Post
(1.8 ohm ballast resistor).
Sidebar - The function of a ballast resistor on a point-based ignition is to keep the points from being "fried." When key is in start position, a full 12 volts is sent to coil directly, and when key is switched to run position, then ballast resistor is used to drop down voltage going to coil.

Some OEMs used a ballast resistor, while others used a resistor-wire that went directly to coil, when key was in run position. I have no idea which way Volvo did it then.

RE: inner two cylinders (2 and 3) to seem to run a bit richer than the outer cylinders (1 and 4)?

Carburetor based inline engines (I-4 & I-6) ran richer on inner cylinders. Physics....when the manifold is on the same plane


RE: hot starts

Common with E10 fuel...same problem with a V8 Chevrolet engine. I believe your intake and exhaust manifolds are adjacent to each other, so this adds more heat to intake manifold, making this problem worse.

Also, is your local's fuel vapor pressure change in "winter months?"

Reid vapor pressure - The matter of vapor pressure is important relating to the function and operation of gasoline-powered, especially carbureted, vehicles and is also important for many other reasons. High levels of vaporization are desirable for winter starting and operation and lower levels are desirable in avoiding vapor lock during summer heat.

Most likely, you can't buy regular/premium without alcohol I suspect.
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Old 03-01-2021, 11:04 PM   #5
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RE: inner two cylinders (2 and 3) to seem to run a bit richer than the outer cylinders (1 and 4)?

Outer cylinders need to get adequate fuel/air mixture...that's why inner cylinders ran a pinch richer.
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Old 03-02-2021, 10:00 AM   #6
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None of the cars I've had with SUs ever hot-started as well as they start when they're cold. Floor it, crank a few seconds, and then give it a rev to clear out. If there's a way to fix that I'd like to know.
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Old 03-02-2021, 10:58 AM   #7
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If there's a way to fix that I'd like to know.
Cold Starts - Farm tractors, semi-trucks, and pickups using diesel have used a high pressure Ether System for a cold start....Diagram Pic Here. If I was setting this up, I'd power it up via starter motor circuit hooked to a momentary switch on dash somewhere. Hence, this button switch on dash would only get power when key is on start position.

I've mentioned this before, but 1970s MG vehicles had a heating element between intake manifold and SU(s), so a person had to wait before attempting to start.
=========================

Hot Starts - Open the hood, and wait 15 minutes. Fuel injected vehicles can tolerate fuels with a higher vapor pressure.

Speculating...I suspect what's happening is SU's fuel bowl is being vaporized, with vapors going to atmosphere via air filters (like on B18), and maybe "flooding" intake manifold with vapors also. On a hot day, shut off engine, open hood, and smell around air filters. B18 engine used a mechanical fuel pump, so if a low pressure fuel pump like this could be inserted where it is "cool," this pump could be run for say 5 seconds before starting (I assume fuel can flow thru it when off...I've never owned one...otherwise, this needs to be setup so fuel pump shuts off when engine is off and if a fuel line is cut in an accident; I think 1971 Vega had a oil pressure switch that was used to shut off power to fuel pump when oil pressure dropped; I'd have to review Vega's electrical circuit..don't recall if a relay was used)
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Old 03-02-2021, 11:01 AM   #8
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Note - If intake manifold is being flooded with fuel vapor on a hot start, hold peddle to floor for say several minutes before starting.
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Old 03-02-2021, 12:18 PM   #9
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OP- are you using the phenolic spacers? If not, I'd try that. Rather than flooring it, you could try adjusting the choke cam so that it cracks the throttle before it drops the jet. I think I ended up using a different choke cam, not sure the number, but It would just crack the throttle with 1/8-1/4" of cable pull.
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Old 03-02-2021, 12:36 PM   #10
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Turns out the plug gap was a bit too wide. I narrowed it and the problem went away. Now it starts right up when I floor it.

I used a spacer when I ran a Weber DGV on the car to mitigate vapor lock symptoms. I was once stuck in a parking lot cranking for five minutes with that carburetor. I eventually had to bump start it because my battery was pretty weak at the time. A spacer and ditching the manifold heater hoses fixed that issue. I may try doing the same here.

I just have the choke adjusted as described in the service manuals. I'll play with it and see if that can reduce the need to floor it. The idea reminds me of the hand throttles some older cars had in addition to a gas pedal as a "cruise control".

Thanks for the explanation about why 2 and 3 were running richer. As far as the ballast resistor, a Pertronix unit is designed to use a coil with the same impedance as points (about 3 ohms at minimum). I got the Bosch red coil (1.5-ish ohms IIRC) because it and a Bosch 1.8 ohm resistor (from a Mercedes 230) was cheaper than a blue coil (the normal replacement). I can also go to a 123 electronic distributor and ditch the resistor at a later date. I had the weird integrated ignition switch and coil with the armored cable but the coil had weakened to where the primary resistance was nearing 20 ohms and put out a wimpy yellow spark.

It isn't that warm here right now and I want to mitigate the problem as much as I can before we get into summer. I will have to do some research to see if the local gas stations switch between winter and summer mixes or if I can get premium without ethanol. 87 and 89 octane definitely have ethanol anywhere you go.

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Old 03-02-2021, 01:53 PM   #11
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The Vapor Rub: Summer versus Winter Gasoline Explained - Every September 15, as we resist switching to long sleeves and heated seats, fuel stations in all 48 continental states must begin dispensing winter-grade gasoline under strict federal deadlines. But topping off in Chicago isn’t the same tankful as in Boise. Enter the confusingly zoned world of winter and summer gasoline.

Vapor Lock - Don't know if a fire resistant tube insulation existed that would wrap around fuel line...insulation that would block infrared also. Heating the fuel line increases fuel pressure, and/or, vapor bubbles, in a closed system...otherwise, fuel vapor is passed on to atmosphere.
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Old 03-02-2021, 04:57 PM   #12
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Rich idle setting?
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Old 03-03-2021, 01:19 AM   #13
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There’s such a thing as a carb heat shield. Prolly only available used, or NOS. This ones gone, but what it looks like. Also wrap or ceramic coat the exhaust manifold?

https://www.ebay.com/itm/carburettor...&ul_noapp=true
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Old 03-03-2021, 01:20 AM   #14
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Are you using a steel 1 piece exhaust/intake manifold or the aluminum 2 piece manifold?

Both my '71 and '72 140's were notorious hard starters when hot, running HIF6's.
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Old 03-03-2021, 09:22 AM   #15
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Quote:
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carb heat shield...wrap or ceramic coat the exhaust manifold
Good ideas....yes, exhaust manifolds can be coated...but in colder climates, intake needs the heat. "Carburetor icing is caused by the temperature drop in the carburetor, as an effect of fuel vaporization, and the temperature drop associated with the pressure drop in the venturi. If the temperature drops below freezing, water vapor will freeze onto the throttle valve, and other internal surfaces of the carburetor."

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Old 03-03-2021, 09:29 AM   #16
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From Wikipedia

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Old 03-03-2021, 11:56 AM   #17
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I'm running a heat shield. Early cast aluminum intake manifold and dual port exhaust manifold from a fuel injected 140.

I live in central California about a mile from the ocean so I am not so worried about icing. There is a shop nearby that does ceramic coating that I was going to send my seat frames to for powder coating. I might just send them the exhaust manifold too.

I did fiddle with the carburetors yesterday after getting it nice and hot. I found the jet on the rear carburetor wasn't centered quite right. It seemed centered when cold but when warm things expanded enough to not let the piston drop all the way. The piston wouldn't go bellow what its normal position at idle was when the car was off.
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Old 03-08-2021, 12:46 PM   #18
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Quote:
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It seemed centered when cold but when warm things expanded enough to not let the piston drop all the way. The piston wouldn't go bellow what its normal position at idle was when the car was off.
That'll do all sorts of weird stuff.
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