Throttle issues

[puchmaxikid]

Hi, just fitted a 1" 3/4 su carb to my 1300 marina engine. my problem is that i dont think the throttle is backing itself down quickly, it seems to continue to rev for a brief sencond and the drop down slowly. i belive that the return sping is strong enough and not fouling with anything i have checked the mixture and it seems good, the one thing that i have done is file the nedle slightly at the fatter end to increase the mixture, the only thing i can think is that were i have filled it i may have over done it so it goes inwards at that point witch allows more fuel in at that point.
anyone got any idea?

Loz

[alainmoran]

have you tried using thinner oil in the damper ... I find that 3 in 1 works a treat

[puchmaxikid]

have you tried using thinner oil in the damper ... I find that 3 in 1 works a treat

Thanks but thats exactly what i have in it.

Loz

[alainmoran]

ick, ok .. and it comes down with a clunk when you push the piston up?

[bmcecosse]

Some of the HIF carbs have a little 'damper' attached to the side - which is designed (I can't think why) to only let the throttle close slowly. Your needle filing ( ) won't be the cause of this! If there is a strange little device on the side - try taking it off!

[puchmaxikid]

ick, ok .. and it comes down with a clunk when you push the piston up?

yep, really nice and smooth with no drag and back down with a positive clunk.

Loz

[puchmaxikid]

Some of the HIF carbs have a little 'damper' attached to the side - which is designed (I can't think why) to only let the throttle close slowly. Your needle filing ( ) won't be the cause of this! If there is a strange little device on the side - try taking it off!

Thankyou for the reply, the only thing that i can see thats on the side is what i would think is a little fuel prima witch just lifts the damper up slightly? The only other thing is on the buterfly there is a like spring with what i would call a little valve, what does this do? oh and the only othere thing that i can see is the shaft that the buterfly attaches to there is slight play in the shaft itself.

Loz

[bmcecosse]

That 'spring and little valve' sounds like the damper I mentioned. Try without.

[puchmaxikid]

That 'spring and little valve' sounds like the damper I mentioned. Try without.

do you no how i would remove it? and the standerd 1.5" su has this on the butterfly.

Loz

[autolycus]

The "spring and little valve" sounds more like an over-run valve - intended to be pulled open under conditions of very high manifold depression, e.g. on rapid throttle closing.

Standard advice on, for instance late MGBs which have HIF carbs is either to remove the butterfly and replace with a solid one, or remove the valve and solder up the hole. A blob of solder on the flap and spring will disable it, but leave an obstruction to the air flow.

Throttle dampers (as distinct from dashpot dampers) were, iirc, fitted to some cars, particularly transverse-engined ones, to reduce lurching when drivers were particularly clod-hoppered. BMC 1100s were so bad for this that aftermarket hydraulic dampers used to be fitted between cylinder head and slam panel.

Kevin

[bmcecosse]

Ahh - sorry - if you mean TINY spring and valve inside the carb on the throttle plate - then that is there to help fudge past the emissions rules. If you can be bothered - take it out and solder the hole.

[puchmaxikid]

Do you belive this could solve my problem?

Loz

[linearaudio]

Definitely could be! It is known for acting erratically and mucking up the low running! I disabled mine on the bench by cropping through the spring and pin with side cutters, then hitting the offending bits with a hammer on a block ( not strictly neccessary, but it makes you feel better while the soldering iron is heating up!) Use a decent size iron (50w did it, just) and some flux if you have any, and glance over the hole in the butterfly with solder. I admit to being a heathen, and using an old iron, heating the tip with a blowlamp to increase its temperature transfer capabilities! ( make sure you have NO petrol around if you go this route!).

Other possibilities could be air leaks, typically from a holed auto advance unit on the distributor, or the right angled joiner from that to the carb being split.
If it is an HIF carb then there have been instances of the "O" ring seal around the choke shaft failing, which could mess the mixture around, or, conceivably, a loose/broken auto advance spring in the distributor could also play up the tickover stability.

[puchmaxikid]

Thankyou for all your replys

Okay so had a play this afternoon. i took out the buterfly and soldered up the whole, was quite pleased with that. i then went to check if have any air leaks, didn't come across anything bad. So then put back on the carb etc. Started it up and it all seemed okay, it did seem to do it still, just not quite as bad though. it didn't seem to do it under hard throttle, but under light throttle it seems to back down slowly and maybe not back of all the way.
Is this just me being fussy?

Thanks again Loz

[bmcecosse]

Well done with the soldering! Does the piston rise and fall freely ? can't see what else it can be - provided there is no 'damper' on the side of the carb.

[puchmaxikid]

Thanks for that. yep couldnt move anymore freely.

The only thing i did notice was that at the top of the needle, (where it goes into the damper) there is a slight bend in it witch makes it sit at a lean, but the needle wobles so would that matter?

Loz