SU Carb Swap

[motomallett]

Hi Guys hope everybody is well. Im looking for some expert advice so here goes. I have fitted an SU hs2 aud 13 carb to the 602cc engine in my PAC threewheeler. I changed the standard needle to a gg weaker needle and changed the spring to a stronger yellow spring and of course the inlet manifold. I was pleasantly surprised when i started the engine it fired up and ran ok the only adjustment i had to make was the tickover as it was to fast.It was great through the rev range no flat spots. Took the car for run of about 10 miles it went really well and put it back in the garage.A few days later went to start the PAC the engine was completely erratic with no throttle it started, but just kept revving up to 4000 revs and dying back to a lumpy fast tickover.I have stripped the carb cleaned it, reset it, tried different oils in the dashpot rebedded all the gaskets in case of air leaks to no avail. Dont want to give up on it as it seemed to go so well with the SU. Cant say i have been pulling my hair out because i have not got any. Would appreciate any suggestions from you clever guys cheers Roger

[howardjs]

Hi, I was looking forward to reading all the expert responses to your post but as no one has offered any here is my twopenn'orth.
As the engine was performing well initially it looks more like one of those automotive coincidences.
You have already checked that the needle hasn't dropped and the piston is moving freely in the suction chamber (not sticking due to a misaligned needle) and the throttle cable is moving freely and the choke / jet tube is not sticking and it is going back to the correct height after the choke has been operated? I'm using Dellortos and the choke setting from cold is critical. It needs a lot of choke to get it started, but too much or too little and it won't start but get it right and it is off first turn every time. But after it has started the choke must be closed immediately or the engine will run VERY lumpy and erratically then cut out.
If all above is OK then how about the timing? Are you still using points? if so are the advance bob weights operating properly?
If you are on an electronic ignition then all the above may be a load of cow poo but you never know... Best of luck
Howard

[billp]

Hi Roger, small point - dash pot oil needs to be quite thin in any SU. I used to use ATF and that always worked fine.

[motomallett]

Hi guys many thanks to those that tried to help,but now i have resolved the problem. Not convinced it was an air leak i stripped the carb again and found the butterfly was not always completely closing and the float needle was allowing too much fuel in the chamber.I adjusted the float and removed the butterfly and spindle cleaned and replaced it.Refitted the carb on to the manifold started the engine and it runs and revs perfectly. The only thing i can think of why it happened is the fuel was very low and perhaps it picked up some debris in the tank although i did not find any bits lodged in the carb. Of course it could have been SODS LAW for changing the engine format. Drive Safe cheers Roger

[billp]

Nice one Roger. Years ago I ran a Gilbern 1800(MGB motor) and that had twin SU carbs. I had an erratic tickover and strange running problems and no matter what I tried I couldn't cure things. My problem - after I'd replaced the float needles and seats - was all down to the float chamber breather on one of the carbs being blocked. Mind you the needles and seats did show signs of wear, so I wasn't too unhappy. If the float chamber breather is blocked then the air inside will compress and the pressure will hold the needle open. Easy enough to check.
Would you believe, I still have the old SU needles and seats in my carb spares drawer - must have a tidy up one day.
Cheers
Bill

[Vyv Cox]

Can I ask whether the change to an SU made an appreciable difference to performance compared with the standard Solex?

[motomallett]

Hi in my opinion the SU is a much simpler and more efficient carb than the Solex.My engine runs much smoother through the rev range no flat spots.The fuel consumption has improved and the spark plugs are more brown than black now. Overall a worthwhile conversion but we all have our own ideas about what we want to achieve from our engines. regards Roger