Single HS4 Conversion Troubleshooting

[dklawson]

I've been posting to a related thread in the Spridget forum about carburetor conversions for the 1500 engine. The topic bears repeating here to get some additional thoughts on my situation.

To summarize briefly, I have installed a late model, single HS4 SU in place of the ZS carb on my project '72 Spit (1300). The carb has a 0.090" jet and the needle is an "ADD" which is close to what the Triumph Toledo used when its 1300 engine was outfitted with a single HS4. So... I thought the car would run better than it does.

I took the car on an "around the block" test drive today and it ran terribly. The idle is smooth and steady at 800 RPM and the car will move OK in 1st and 2nd gear. However, give it some gas and try to accelerate in 3rd and the car falls down and backfires out the carb (I think) which implies to me that it's going lean, way lean. The dizzy is a (new, less than 2k miles) late model Lucas 45D4 with new points and condenser and the timing is set for about 8 BTDC Static. ColorTune showed the mixture was rich at idle. Since the idle is rich and steady, I don't suspect a vacuum leak but I will look for one. I also plan to double check the float level.

Are there other areas you guys would look? Are there obvious things I'm missing? Did Triumph install a fuel filter somewhere that I don't know about that could be plugged? All feedback and suggestions are welcome.

 

[tdskip]

Hi Doug - I think you'll have to trace the fuel lines to make sure that there aren't any PO fuel lines but there should be any stock ones hidden anywhere.

I'm sure you check but is the piston working correctly and there is oil in the dashpots?

You mentioned the float and double checking that sounds like a good idea.

Can you rev her up while watching the colortune through the full RPM range?

 

[aeronca65t]

OK, I'm assuming you've truned the main jet nut all the way "up" and then unscrewed it down 2 full turns (or "12 flats").

That's the general setting for the main jet.

I did this about 10 years ago so I'm a little foggy, but I sort of recall that there is a vent that allows manifold vacuum to act on the damper.....you don't have that somehow blocked off with a gasket, do you?

If all this is OK, and you're getting lean misfire, try unscrewing it down more flats (richer). Don't be afraid to turn it down a few extra full turns.

You can also see if it improves when the choke is dropped slightly.

If it's no better, then I'd look at fuel pressure, and/or float setting.

If it seems better, then at least you're on the right track. I've seen folks at the track file down a damper needle while it's turning in an electric drill.....sort of a trail-and-error process. If you have and extra damper needle, might be worth a shot.

 

[startech47]

The vent holes are on the inlet face that the air cleaner fastens to. Make sure these are open to atmosphere or the filtered air inside the air cleaner.

 

[dklawson]

I machined the filter mounting plates myself and I'm fairly certain I included holes to line up with all the holes on the front face of the carb, but I will check to make sure that I did and that they are not covered/closed by a gasket.

The piston is very free to move. Being a late HS4, it moves on linear ball bearings instead of metal to metal contact between tubes so it is very free. There is dashpot oil.

I did the initial 2 turns down from flush when I first installed the jet. What concerns me is that it is now down at least two more full turns (for a total of more than 4).

The ColorTune was yellow to yellow/blue at idle but once you approach 2k RPM it goes to blue. I did not hold it for an extended time at 2k+ though to see what happened to the mix.

After making my initial post I did check one other thing but it was too late to re-test the car on the road. The PO had fit resistor plugs and resistor wires. I know the old ZS carb was not right and I hadn't touched the plugs or attempted to adjust the ZS mixture so the plugs are as they were wit the ZS installed. Plugs #1 & #4 were pitch black (completely covered with carbon). I'm hoping that part of the lack of power and misfires were from these two plugs when the car was under load. I'll re-test with fresh plugs as soon as possible.

The NASS guys pointed me towards similar issues that Paul Tegler had with carbs a while back. In his case, it turned out that his carbs would not work well with the K&N filters he was using. When he went back to stock filters, his problems went away. That's not my case as I'm still using a stock paper element. However, Paul's troubleshooting experience pointed to a couple of other areas for me to check.

Please keep the suggestions coming. I will check them out when I have a chance to attemp this in daylight.

 

[tdskip]

The NASS guys pointed me towards similar issues that Paul Tegler had with carbs a while back. In his case, it turned out that his carbs would not work well with the K&N filters he was using. When he went back to stock filters, his problems went away. That's not my case as I'm still using a stock paper element. However, Paul's troubleshooting experience pointed to a couple of other areas for me to check.

Are all the vent holes on the carb open? Some aftermarket air filters and mounting plates block vents, and nothing good comes from that.

 

[dklawson]

The folks at NASS also pointed towards the vent holes. The filter that's on the HS4 is a "stock" Triumph paper element sandwiched between two aluminum plates I machined. I'm pretty certain that I duplicated the complete hole pattern as on the face of the HS4. Regardless, I will check that when I work on the car next time.