Hey guys,

 

I'm having this problem with my engine of late, I'm running twin sidedraft c40 solex's on 4k, andill be driving along not flatfoot or anything then suddenly my throttle will go WIDE OPEN, without any input from me. its not the cable or the linkages. Any ideas as to what the issue could be? ( fresh rebuild in carbs too)

 

-Kim

Do you have return springs?

Do you have return springs?

 

..and how many? I run three, one returns the cable and one on each SU carb throttle. They don't have to be strong, just enough to do the job. Clutch/brake pedal return springs are nice

ye I'm running a big single return spring. I'm sure its not the spring not returning

Its the POWAAH....

 

If the pedal goes soft at that time then something is holding the throttles open, but if it doesn't then its quite likely something else entirely.

 

I'm sure you have opened and closed the throttle carefully while watching the carb linkages... There are no sticky points anywhere from idle to wide open and back?? Sometimes wear in the axle/housing will jam a little going one way and not the other. Are the two throttles directly connected to the center quadrant with the cable, or can the cable relax back without the throttles closing?? (hence my three springs)

 

How about sticking a webcam under there with a torch and driving with a laptop on the passenger's seat recording it??

 

If they have been rebuilt it might be worth disconnecting the linkages and checking that each throttle plate closes completely and easily. If the plate is not centered in the axle it can hold the throttle open, but usually that causes a high idle & engine racing.

ye I'm running a big single return spring. I'm sure its not the spring not returning

 

That's a grouse photo.

It's happened before on a warm startup so I figure it has to be internal it's not the linkages it's only ever sporadically and When I turn it off pump the accell a couple times turn it on its fine.. Has to be internal right?

And that's the only photo of my setup I had on my phone my car isn't near me haha

 

Next time it happens just kill the ignition and brake to a stop without touching the throttle, then take a look at it all to see if one carb is still open and not closed to idle.

 

For the motor to race it needs more air and enough fuel, so if it is sporadically leaking just air through the brake booster line or similar it will lean out. The only way to get fuel and air together is past the throttle plate really.

ye rightio next time it happens ill have a geez down the barrells and see whats going on! thanks man!

 

if it is sticking open how would i go about fixing?

Find out what is holding it open- Could be something external in a linkage, or a mis-aligned/loose throttle plate.. I dunno, find it first and then tackle it.

Is it at all possible, that under large amounts of vacuum, it is enough to pull the throttle open?

 

It sounds rediculous, but how on earth does a throttle with a huge spring open by itself?

 

I like that photo! one day ill have cool carbies.

I was thinking the possibility of the vacuum being to strong, because it will only ever happen after I've given it some whilst driving but on warm start I'm stumped on that one.

It shouldn't, the throttle axle goes through the middle of the throttle plates so there is no tendency for the air flow (or the inlet vacuum) to suck them open or closed.

 

Apart from the weight of the throttle linkages I can't think of anything that would open a throttle by itself. It will be interesting to see what causes it in the end.

It shouldn't, the throttle axle goes through the middle of the throttle plates so there is no tendency for the air flow (or the inlet vacuum) to suck them open or closed.

 

Apart from the weight of the throttle linkages I can't think of anything that would open a throttle by itself. It will be interesting to see what causes it in the end.

 

Well it happened yesterday with the bonnet open in front of me and several other stationary in neautral and all the links and throttle components returned as they should've.. Stupidly I forgot to check inside the carby throats to see if butterfly's where open. But it has to be internal just has to. The carbs are over jetted at the moment could that have something to do with it?!?