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HS2 carb troubleshooting

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Dutch 1960 Avatar
Dutch 1960 Mark Holland
San Diego, CA, USA   USA
The breather in the rear fuel bowl. I am now intimately familiar with it. A small plate, captured by the fuel feed tube, is creased and keeps the fuel from easily splashing out the hole. No blockage, the vent is fine, I even propped it open a bit more on reinstallation.

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Dutch 1960 Avatar
Dutch 1960 Mark Holland
San Diego, CA, USA   USA
No change on the engine behavior after that project, as mentioned above. Lets pull the plugs. Label everything. Uh oh. Which one of these is not like the others?


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Dutch 1960 Avatar
Dutch 1960 Mark Holland
San Diego, CA, USA   USA
The plugs get progressively lighter and less oily as you go from #1 on the left to #4 on the right.

This is not looking good...

Lets do the wet/dry compression check. The engine is not all the way warmed up, because it is bucking and running so poorly.

As you can see from the photo--

#1 168 dry 220 wet
#2 168 dry 200 wet
#3 171 dry 203 wet
#4 182 dry 212 wet

The 220 wet on #1 is a bit of an anomaly, but we look OK. Whew!

This is a cheap Autozone compression checker. I don't know its accuracy, but the relative numbers seem basically OK.

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Dutch 1960 Avatar
Dutch 1960 Mark Holland
San Diego, CA, USA   USA
So I am kind of back to square one. The rear carb seems to have an awful lot of fuel swimming around in it. The rear plugs, especially #4, look as they might be getting washed clean with raw fuel. Or at least a non-rich burn if there is one, which would suggest the other direction, lean... Checking each plug with the timing light, I get a consistent spark on each one, #1 through #4. The backfires through the carbs, always the rear one first quickly followed by the front, are raw fuel, no spark. No "pow" backfires, kind of a wet, fuel spitting "pfhuut" backfire that gets the wiper motor and fender liner all wet with fuel when the air cleaners are off.

Still no way to get a "hissing" vacuum out of the rear carb. The front one is easy to get hissy.

Is it possible the rear lobes on the cam are scored, and the valves are not opening all the way, so no vacuum and a bunch of fuel swimming around in the carb, as it is adjusted rich to compensate for no engine action on #3 and #4? If anyone has seen that before, it would be helpful to know. I did not measure the height of the open valves, I just put them to maximum open, one by one, to count to nine to measure the clearances of the others. Any guidance on how to check for that damaged/severely worn cam lobe condition, if that is a possible problem?

The rear fuel bowl is not overflowing, as fuel would be coming out the breather hole onto the face of the fuel bowl lid. So the fuel pressure does not appear to be overpowering the needle and seat.

Should I still be considering some sort of semi-massive vacuum leak in the rear carb, since I can't get a hiss out of it?

Whatever the problem is, it now seems to be getting worse.

refisk Avatar
refisk Rick Fisk
Frankenmuth, MI, USA   USA
Doesn't make a huge amount of difference if the vent is open, but someone has swapped your rear carb float cover for an older version. The rear carb should have a brass tube vent just like the front carb. I wonder if the whole carb was swapped for an older carb?

Plug 4 looks to be the only one with a correct mixture.

Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
Just on the off chance, recheck your float height. I had odd symptoms with my carbs recently where it would run worse and worse over time and was impossible to adjust. My oil pressure dropped and I smelled fuel in the oil.

My floats seemed to be adjusted by the book but were jumping gas into the cylinders. I set them a fair bit higher and changed the oil... problem solved...

Dutch 1960 Avatar
Dutch 1960 Mark Holland
San Diego, CA, USA   USA
In reply to # 3695402 by Poundingsand Just on the off chance, recheck your float height. I had odd symptoms with my carbs recently where it would run worse and worse over time and was impossible to adjust. My oil pressure dropped and I smelled fuel in the oil.

My floats seemed to be adjusted by the book but were jumping gas into the cylinders. I set them a fair bit higher and changed the oil... problem solved...

I was looking at the floats this weekend. I had replaced them, and they are a white plastic material, and non-adjustable. I am more familiar with the kind that have a metal tab that can be adjusted. I think I should find a set of those.

By "higher", I assume you mean you run a lower fuel level in the bowls by "raising" the metal tab relative to the float, closing the needle sooner as the fuel level rises.

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Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
I also have the plastic floats. To adjust them you need to shim under the needle valve. I used nylon washers from the hardware store (about .5 mm thick).

Worth looking into!

Dutch 1960 Avatar
Dutch 1960 Mark Holland
San Diego, CA, USA   USA
Clever. Thanks for the tip.

I am a bit afraid that I might still looking at multiple problems here, which makes solutions harder to find. There seems to be lots of fuel everywhere, and as Rick pointed out, I am basically running rich, so shimming both needles a bit may be the best thing to try now. Then go back to the SU "factory" starting settings, and see what happens.

I am getting to the point where I hope I can just wander into the proper set-up. This car has never been quite right. The John Twist piston drop test showed that the two carbs were waaaay off from each other. Perhaps the rear carb was swapped for an older or some sort of bitsa set-up to create the mismatch? By the time I recently found and fixed that problem, something else or another must have gotten out of whack. The car ran lean with a lot of stumbling and white spark plug coloration when I first started a few years back.

At least the elements away from carburetion seem to all be in good order (fingers crossed).

It's also frustrating because the SUs are so cleverly simple and logical in their operation, and yet...

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Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
It sounds a lot like what I went through. No dialing them in properly. While great around town (although rich) the car was super lean on the freeway and the plugs were always white. I was also blowing a lot of oil through the seals.

I bought a full rebuild kit and added some parts it didn't have like the piston springs. It made a world of difference. The rebuild was actually quite fun.

Since the rebuild the car will run on the freeway, I can actually get the carbs balanced and the underside is dry....BTW it was after the rebuild that I had the float issue...just sorted that out.

Also, the piston springs play a bigger role than you would think...worth replacing as its easy...

Dutch 1960 Avatar
Dutch 1960 Mark Holland
San Diego, CA, USA   USA
Did the piston springs (Moss and VB must love me). Went with standard rate, not heavy.

I have just been experimenting with the oil in the pistons. Was using 20/50, but the pistons seemed to have a lot of resistance to lifting. Now using Marvel Mystery Oil, which is very light, and the pistons seem to move with less resistance. Whether less resistance is a good thing or a bad thing, under the circumstances, appears to be an open question. I just made the change, I would guess the lighter oil might lean things out a bit.

Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
My understanding is that the oil weight only affects the rate of lift during throttle input. The piston height at cruise is unaffected. So not too much to go wrong there.

But perhaps someone with more experience than I can confirm...

hoggie Avatar
hoggie Mike Hogan
Oklahoma City, OK, USA   USA
Just a thought, have you watched the piston heights of both carbs while idling and as you add throttle? They should look and react the same to throttle changes. They will be almost completely down at idle and rise in unison to any throttle changes. If not, then look for the butterfly opening to be the same and sticking pistons. The piston movement is strictly a function of air flow and vacuum retarded by the dashpot oil and valve. Both should behave the same despite any fuel mixture or float level concerns.

Dutch 1960 Avatar
Dutch 1960 Mark Holland
San Diego, CA, USA   USA
Yup. They are acting in unison. It is so cool that you can actually watch these carburetors work in real time, and also pull the bells and watch how your jet adjustments affect the innards of the carb.

I have been thinking about the fuel bowl levels quite a bit. In a “traditional” American style carb, you have three fuel bowl levels. Too high-flooding. Too low-starving. In between-good, but where you are on the “in between” is mostly irrelevant to performance. I need to wrap my head around the idea that fuel bowl levels are absolutely critical in SUs, because the spectrum of settings affects the rest of the carb’s operation. There is no “getting it roughly right, not too high and flooding, not too low and starving”. The settings within the “acceptable” range are critical to getting things right.

A side project is to try to come up with some sort of external fuel bowl height adjustment, relative to the throttle body, so that the fine tuning can be done without opening the fuel bowl lids every time.

hoggie Avatar
hoggie Mike Hogan
Oklahoma City, OK, USA   USA
That's good. In my desperation to resolve my SU carb problems, I think I have read almost every technical instruction I could find on how to adjust the SU float levels and fuel mixture. All indicated that the float level is not critical as long as you are close to the 1/8 in float to lid gap. But there are other issues related to the type of float, nylon or brass, and the type of needle and seat, solid brass, Viton tipped, spring tipped, ball type, etc that muddies up the float level issue. I did find a document that indicated the proper fuel level in the jet (where it really counts) should be between 1/8" to 1/4" below the top of the jet. Another issue is the PSI of fuel pump. I think the SU pump spec was 3.7 PSI, so you probably want to keep the fuel pressure somewhere between 3 to 5 psi. Nice thing about an old contact type SU pump is that you could hear every cycle of the pump so if you turned the ignition on, you would hear several cycles then no more as the pressure built up and held. If the pump kept cycling, then you either had a leak or the pump needed service.

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