Carb Troubleshooting 101 Needed, please

E-Type
1

I have been bothered with a regular sputter at idle and have noticed evidence of rich mixture so I am digging into tuning the carbs. Note: I have converted my '68 to triple SU’s. Also, I am a real noob with carbs. Removing the center carb housing and piston revealed this:

20200604_165820-b
20200604_165820-b3024×4032 2.99 MB

It may be hard to see but the jet/jet bearing has dropped a good 1/8" and is loose in the carb body. Based on this schematic:
SU_HD8_CARB_SCHEMATIC
SU_HD8_CARB_SCHEMATIC652×663 69.5 KB

I don’t understand how this can be unless #21, the “jet bearing lock screw” has fallen? Has that ever happened? Any guidance on how to move forward is greatly appreciated.

Craig

2

It is possible that the spring that presses the jet up, (number 6 in diagram on the right), is broken. Usually rusted away!

1/8th is about 0.125 inches deep. The jet is usually about 0.65 down (plus or minus) when the carb is tuned so that carb is running very rich.

If it were me, I would take the carb out and unscrew the four screws (19 in picture) that hold the jet in and see what it happening.

Dennis 69 OTS with triple SUs

3

I forgot to say, have you checked that the choke is not operating on that jet and keeping it open?
Dennis 69 OTS

4

Thanks for the quick reply Dennis. Yes, I guess it’s got to come out. The picture is hard to make out but both the jet (#1) and the jet bearing (#20) have dropped. Probably closer to 0.200" than 0.125"! Am I seeing the schematic correctly? Does the jet bearing have a collar that the jet bearing lock screw (#21) bears against to keep it from dropping? If so, has anyone ever had the lock screw back out?

5

It may be that the but that holds the sleeve that the jet rides up and down in, has undone

6

You will need to take the carburetor apart and follow the jet centering procedure in the manual. That should fix it unless something is stripped out. Probably just loose.

7

Thanks guys. I will pull it off and see!

8

Sooo… the jet bearing lock screw backed out. The threads appear to be OK so I’ll follow the centering procedure, re-install and hit the road!

I won’t bore you all with the back story but suffice it to say that I didn’t properly follow the setup of the carb after getting it back from having shaft bushes replaced and that most likely allowed the screw to slowly back down.

Another lesson learnt! Thanks.

Craig

9

If the nut hasn’t loosened, it’s also possible the carbs were rebuilt with bad parts. Mine were rebuilt by me 20 years ago and were very hard to tune properly. It wasn’t until some years later I found the reason why. A lot of badly machined brass parts had crept into the supply chain and were being sold even by specialists. The dimensions were measurably off and I installed them without knowing better.