Revs not dropping...

topcatcustom · Post by **topcatcustom** » Fri Jun 11, 2010 8:56 pm

Hi guys, this problem just materialised tonight- it revs fine but they will not drop below approx 2500rpm when you put your foot on the clutch unless you give the throttle a blip and they nicely fall straight back to idle.

Cant think of anything I have done! It does idle nicely so dont think it's air leak related, all linkages inc accelerator pump are nice and free, fuel pressure is good at 6psi. Its a bloody pain not having any engine braking effect as the revs just want to hover! I can see the linkage and cable out the top of the bonnet so I can see it is all free, and there is some slack on the cable so it always springs back to the stop.

mgbv8 · Post by **mgbv8** » Fri Jun 11, 2010 9:10 pm

Sounds like the flaps are not closing fully Tom. Try some extra springs on the return. Excess cable in a hoop can be heavy enough to hold part throttle. Only has to be 1-2mm of travel to keeps revs quite high off load eh ?

kiwicar · Post by **kiwicar** » Sat Jun 12, 2010 6:36 am

Is the carb mechanical or vac secondarys? just if they are vac they could be holding open.
best regards
Mike

topcatcustom · Post by **topcatcustom** » Sat Jun 12, 2010 10:45 am

That was/is a thought of mine too Mike, vacuum. Strange thing is it is only when warm, and gets worse/holds higher revs the longer it's been driven. I have thought about taking the weights out and building them up a bit to increase the weight, what do you think or is it unrelated?

CastleMGBV8 · Post by **CastleMGBV8** » Sat Jun 12, 2010 11:13 am

Tom just a guess, but if the revs will drop back to normal with a quick flick of the throttle pedal it suggests that there is not enough strength in the spring to counter the vacuum through the carb produced by the blower or as the engine heats up more friction is being produced in the linkage.

First thing to try would be to increase the return spring pressure, for a test you could add a strong rubber band to the return spring and see if this cures the problem, if it does then buy one of those nice double springs which are available which also add a good degree of safety, this has been on my to do list for some time, hate to think of the effect of a throttle spring failure especiall with a blower sucking the carb open!

Kevin

kiwicar · Post by **kiwicar** » Sat Jun 12, 2010 12:13 pm

Hi Tom
as Kevin says defiantly try bigger return springs on the secondarys. If that doesn't work, how is your crank case evac plumbed? if it is plumbed to below the carb but above the blower (which I presume it is) restrict the crank case inlet side ( say to a 1/4" hole maximum) and set the carb to give a leaner mixture at idle as you may have enough flow through the crank case that with the blower mixing everything up and a rich ish mixture at low flow through the carb the engine can burn this fine at closed throttle when hot (remember the charger can pull allot more air through a small hole than the engine alone.
Best regards
Mike

topcatcustom · Post by **topcatcustom** » Sat Jun 12, 2010 3:57 pm

Guys, I shortened the big spring on the throttle linkage so it is slightly stronger now, snaps shut very easily, and Mike I am runnin a -10AN hose from each rocker cover to catch tank and vacuum pump, which easily pulls any gas out of the crank case. All the carb ports are blocked off.

Ian Anderson · Post by **Ian Anderson** » Sat Jun 12, 2010 4:07 pm

May sound stupind but

Too many springs on one side of the butterfly shaft can cause it to drag closed.

Better is to, if possible, put one spring on each side of the shaft (both causing the shaft to close) - it tends to help centralise the shaft in the "bearings / bushes"

Also put a return spring on the pedal - no need to make it drag everything back with the accelerator cable.

And to show I practice what I preach I have 4 springs on my system
1-pedal
1 coil around butterfly shaft
2 working on shaft

Ian

kiwicar · Post by **kiwicar** » Sat Jun 12, 2010 6:41 pm

"Mike I am runnin a -10AN hose from each rocker cover to catch tank and vacuum pump, which easily pulls any gas out of the crank case. All the carb ports are blocked off."

Again the young-ens are well ahead of me, I'll get my bowl of soup and my slippers and sit in front of 'telly and and complain about how young the police look these days and that there are no good programmes any more like there were in my day . . . . .

.
Stick a broot scoop on top of it with three more throttle butterflies in the front, then you will know the damn throttle is shut and it can't argue with that!

http://cgi.ebay.com/ebaymotors/Street-S ... ccessories
Best regards
Mike
PS only joking but they look cool!