SU carbs - holding revs

satancom · Post by **satancom** » Wed Mar 11, 2009 7:11 pm

I have just fitted a pair of rebuilt SU HIF carbs to my Rover 3.5 V8. They replaced a pair of Su's that I used as a known good pair to get running, but the seals were all leaking slightly so were only temporary.

The new carbs were cleaned up, all seals replaced and everything checked and all is well. I set them up according to the factory settings and believe I have the idle/mixture set correctly.

I took it out for a test drive and it ran well, however when releasing the throttle the revs take an age to come down, so much so that it jerks into the next gear as its still reving quit high. If i rev the engine up on the drive it'l rev and then slowly come down even though the throttles snapped shut.

Therefore it is not a problem with the throttle linkage. The only two things I can think of is that I have over filled the dash pots (wasn't really paying attention doh) or the the little poppet valves on the butterfly have some effect?

The only difference between the replacement carbs is that the old carbs I took up had the poppet valve soldered shut. Would this fix my problem?

Other than holding the revs they seem to run perfectly fine with better response form the engine.

harvey · Post by **harvey** » Wed Mar 11, 2009 7:27 pm

satancom wrote: or the the little poppet valves on the butterfly have some effect?

The only difference between the replacement carbs is that the old carbs I took up had the poppet valve soldered shut. Would this fix my problem?

Can do, as the springs get weak and allow air to pass through them while the butterflies are shut. That's why the ones on your old carbs were soldered shut, and why I personally would have soldered the ones on the new (to the car) ones as well. A pain for you as the carbs have to come off again to do it.

satancom · Post by **satancom** » Wed Mar 11, 2009 7:33 pm

Is there anything else to check before I go down that road.. They are such a pain to remove

ian.stewart · Post by **ian.stewart** » Wed Mar 11, 2009 7:35 pm

Vacuum leak will make the revs hang up

kiwicar · Post by **kiwicar** » Wed Mar 11, 2009 7:36 pm

odds on it is the poppet valves, the springs weaken rapidly (I had them go soft on a 2 year old midget years ago), couldnt get the idle set properly, throttle response was terrible and the acceleration enrichment on the dashpots to work correctly and the pistons didn't rise evenly soldered them shut and it sorted it out. The otherthing it could be is a sticking piston that is showing too much vacuum to the poppet valves and efectivly giving the effect of partly open throttle. Either way solder them shut, if it doesn't fix it directly you can atleast look at the pistons and then fix them.
Mike

satancom · Post by **satancom** » Wed Mar 11, 2009 7:42 pm

Well I will have a quick look over the vacuum pipes although I think everything is in order.

It does sound like the carbs will have to come off again though. Would it be bad to swap the throttle butterflys between the sets of carbs as I already have a pair that have been soldered. or are they matched to the carb's?

Many thanks for your input guys

RoverP6B · Post by **RoverP6B** » Wed Mar 11, 2009 11:29 pm

You can swap the butterflies between the two sets of carbs if you wish to.

Also ensure that the pistons are completely free to move.

Ron.

satancom · Post by **satancom** » Thu Mar 12, 2009 8:20 am

RoverP6B wrote:You can swap the butterflies between the two sets of carbs if you wish to.

Also ensure that the pistons are completely free to move.

Ron.

Pistons are free to move. Its fine when the engines cold, only seems to be a problem once it warms up. I am oging to do some phoning about later and see if I can get some plain throttle discs for a good price, if not I will just swap the throttle discs over and hopefully this will cure my problem.

Cheers

satancom · Post by **satancom** » Thu Mar 12, 2009 8:58 am

Ordered a pair of plain discs at £6.70 each, complete with new screws

Happy days!

satancom · Post by **satancom** » Fri Mar 13, 2009 7:44 pm

Well I changed the throttle discs over for blank ones from berling along with all new gaskets on the manifold and such.

It still seems to be holding revs, is it possible that I overfilled the dashpots as they are sluggish to come down, they are free enough but maybe over damped? Would this have an effect?

Could the mixture be too rich? Its gotto be the carbs as thats all I have changed, and Im fairly sure all the seals are good now as I replaced them all. WHere to look next?

While driving it drives a lot better and accelerates better than the old carbs! Which is good

RoverP6B · Post by **RoverP6B** » Fri Mar 13, 2009 9:08 pm

Overfilling the dashpots will not hold the revs high.

If you have the idle mixture far too rich, this will not be the cause either.

Did you reconnect the vacuum advance line from the carburettor to the vacuum advance module on the distributor?

Is everything free to move, no stiff movement in any of the linkages?

Ron.

satancom · Post by **satancom** » Fri Mar 13, 2009 9:35 pm

RoverP6B wrote:Overfilling the dashpots will not hold the revs high.

If you have the idle mixture far too rich, this will not be the cause either.

Ron.

Ok that clears that up then, good.

The vacuum advance has been reconnected, but there is no guarantee it was working properly before as the old carbs were pretty worn. So I may have a play about there, disconnect it and see if it makes any difference (just briefly)

Linkages snap back when operated by hand but I will get her nice and hot and try again as it seems more of a problem when its warm! I can't see anything wrong and can only assume its the carbs, allthough the ignition advance is worth checking!

Thank you

RoverP6B · Post by **RoverP6B** » Sat Mar 14, 2009 2:18 am

If you wish to test the vacuum advance, an easy way is to place a tube over the little pipe on the module and with the distributor cap off, suck hard and see if you can see the base plate move. If not, then the vacuum advance module has failed,..the rubber diaphragm will have perished.

This in itself will not be the cause of the problem that your engine has,...the revs not coming down.

As you indicate that this problem was not happening prior to your overhaul of the carbs, and as you have not adjusted anything else, it would seem most likely that something that you have done is wrong.

It might be advisable to remove the carbs and dismantle them again, that is remove the dashpots, take out the pistons, remove the float chamber covers, check that the floats are set to 40 thou.

Also make sure that when the butterflies are closed, that they seal completely around their circumference.

Did you replace the springs with the same grade, ie yellow?

Ron.

topcatcustom · Post by **topcatcustom** » Sat Mar 14, 2009 11:16 am

What oil did you fill the dash pots with?

harvey · Post by **harvey** » Sat Mar 14, 2009 7:56 pm

Did you centralise the throttle butterflies when you fitted them?

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SU carbs - holding revs

SU carbs - holding revs

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