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Dry dashpot and backfiring.

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tweety

tweety

Jedi Trainee
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'79 midget with z-s carb.... California car

The car was up on blocks for a couple of weeks while I fitted a gear reduction starter.... That works great.

The car was backfiring (at the carb)and sputtering after shifting from second to thrid and third to fourth.

I discovered there was no oil in the dashpot so I filled it up. The backfiring and sputtering is MUCH better now, but it will still do it (esp in 3rd) right around 2K rpm. Overall the car's very sluggish.

I'm thinking that the backfiring either damaged the carb or totally messed up the settings.

Any words of wisdom?????
 
It's doubtful that anything has been damaged or knocked out of adjustment by the backfiring. However, if the rubber diaphragm is old though, I would check to make sure that it is intact. The ZS carbs have a reputation for loosing their dashpot oil.

I found the same backfiring and stumbling behavior when I had 10W30 oil in the damper. I changed to 20W50 and things were much improved. If the oil you put in was thin, you may benefit from trying a heavier oil before looking at any major adjustments or carb tuning. I was surprised to see on one of John Twist's (University Motors) YouTube videos that he encouraged his customers to run 90W oil in the carb damper. That's much thicker than the 20W oil our cars carbs' probably left the factory with.
 
dklawson said:
It's doubtful that anything has been damaged or knocked out of adjustment by the backfiring. However, if the rubber diaphragm is old though, I would check to make sure that it is intact. The ZS carbs have a reputation for loosing their dashpot oil.

I found the same backfiring and stumbling behavior when I had 10W30 oil in the damper. I changed to 20W50 and things were much improved. If the oil you put in was thin, you may benefit from trying a heavier oil before looking at any major adjustments or carb tuning. I was surprised to see on one of John Twist's (University Motors) YouTube videos that he encouraged his customers to run 90W oil in the carb damper. That's much thicker than the 20W oil our cars carbs' probably left the factory with.
Click to expand...


I agree with nearly all the above, but note that 90W oil is probably gear oil, which has a different viscocity scale than engine oil, and is closer to 30w.
 
I used the SU/Zenith Dashpot oil sold by Moss and VB

https://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=31273&SortOrder=1

I'll check the diaphragm and replace the oil with something heavier.

I would really love to pitch the ZS but then I would not pass smog. It's tested every two years which really isn't the end world if a change out is needed..... But my test is due in a couple of months.

What I really want to do is replace the engine all together. Maybe drop in a 1275 and tranny. I would have to be sure that I can get it certified as a pre '76 engine by the state..... No more smog checks then!

Right now I just want it to go.
 
Tweety: Move out here to So Dak. Our governor and legislature love "gross pollution". Trying to get one of the biggest oil refinerys ever built in this country with virtually no pollution control. Also a huge coal powerplant.
KA.
 
I had problems with my HIF44 carb loosing it's oil, (I cut the tube down to get it inside my Bugeye hood).
My solution, find some rubber tube that fits tightly the tube, cut a piece 1/8" long, slit it along it's length & slide it around the damper rod, then wedge it inside the dashpot tube. It's a little awkward to get in, but I don't loose any oil.
 
You may be set to lean, that will cause a backfire. Had the same problem w/ mine and a timing & carb adjustment fixed the problems. I use 30W in mine w/o any problems and it gets cold in PA. Had to buy the "special" tool to adjust the carbs. IMHO, I'll take SU's anyday!!!!
 
took a look tonight. The diaphragm was fine. Filled it with 80 90 gear oil. No more backfiring or sputtering.

It's still a little on the sluggish side, but I think that's some fine tuning as the idle speed seems low.

Thanks for the advise, I would never be able to own a LBC without your help
 
Pull the plugs and take a look at the color, that will tell you quite a bit about the carb setting & running condition. Start the engine & run at a high RPM or take it for a spin. Don't check after a long idle or they won't reflect the running condition.
 
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