hi all
i got the twin carbies on the ol' grey motor, and i was after a few pointers to get them running at their best.
they are both 1 1/32 bore downdraught strommies, both with a kit through them, on a cain manifold. they both have brand new size 44 main jets in them.
after hours of agonising troubleshooting, i discovered you did indeed need to fill the fuel tank with 'petrol' to ensure the fuel pump will perform its duty. so, after doing that, it fired right up, idled a bit high, but nothing i cant live with for a little while. other than that, they idled quite nicely.
so then on the test drive, they behaved strangely. after the initial burst of fuel from the accelerator pumps, they die off really quick, and have no power. to get off the line, you need to jump up and down on the throttle to squirt enough fuel down the carbs to limp off the line. there is a big hesitation when you plant it while cruising.
another strange symptom is when you put your hand over one carb, the idle will drop considerably, and you pull your hand off and it comes good. if you do the same on the other carb, the idle drops considerably, but when you pull your hand off, its covered in fuel!
any ideas? i reaaaally dont want to just take it to the mechanic, because then i didnt learn anything. i will as an absolute last resort though.
trouble with the twins
.
Oh shit I've posted twice 
jesus, thats a good point, i didnt even think of it to be honest. ill check the butterflies out asap.
with the uni-syn, ive heard that you can simply stick a piece of hose in the carb and listen to it. when both carbs make the same 'note', theyre in sync.
that being said, wouldnt adjusting the mixture screws (synchronising the carbs) only affect the idle? it idles fine. it was my understanding that the mixture screws only affected performance at idle. please correct me if im wrong!
with the uni-syn, ive heard that you can simply stick a piece of hose in the carb and listen to it. when both carbs make the same 'note', theyre in sync.
that being said, wouldnt adjusting the mixture screws (synchronising the carbs) only affect the idle? it idles fine. it was my understanding that the mixture screws only affected performance at idle. please correct me if im wrong!
The mixture screws do mainly work at idle, yes , the mixture/idle passage in the carb does play a part in a small way at higher engine speeds but its mainly the intermediate and main passages that come into play at the mid to high end engine speeds.
By synchronising the carbs I mean to get both throttle butterflys opening at the same time and check that they both reach the full throttle position at the same time, if that makes any sense
.
And yes, the uni-syn carb tool DR mentioned is a good one to get hold of to assist the carbs tune.
By synchronising the carbs I mean to get both throttle butterflys opening at the same time and check that they both reach the full throttle position at the same time, if that makes any sense
.And yes, the uni-syn carb tool DR mentioned is a good one to get hold of to assist the carbs tune.
I can't think what to write here so this will do.
cheers for the tips!
i had a play around today, and from what i can tell, the rear carb is sucking more air than the front one. would this have anything to do with the way the vacuum advance is connected? I just have the front carb connected to the dizzy advance line. there is a hole in the manifold that is the same size as this, which i have blocked off. is it possible this was meant for the dizzy advance, and both carbs vacuum outlet should be blocked up?
i thiiiiink the throttle butterflies are pretty close to being in sync, its hard to see i had a play around with the mixture screws, and its actually much better than it was. perhaps i just need to get to it and give the old girl a thorough tune up, instead of the quick adjustments ive been doing.
i had a play around today, and from what i can tell, the rear carb is sucking more air than the front one. would this have anything to do with the way the vacuum advance is connected? I just have the front carb connected to the dizzy advance line. there is a hole in the manifold that is the same size as this, which i have blocked off. is it possible this was meant for the dizzy advance, and both carbs vacuum outlet should be blocked up?
i thiiiiink the throttle butterflies are pretty close to being in sync, its hard to see
i had a play around with the mixture screws, and its actually much better than it was. perhaps i just need to get to it and give the old girl a thorough tune up, instead of the quick adjustments ive been doing.
Gday Muddy
Vacuum hose should be connected to the manifold, not one carb.
To find a base line to work from as far as tuning is concerned,
Set the mixtures on each carb as per the manual.
Disconnect the lingages that connect both carbs together.
use a tube, or better still, a stethoscope with the end pulled off
to listen to the air rushing into each carb.
Make sure when you hold the tube just into the carbs,
that you do the same for both (ie; the depth and the way you hold it)
Adjust the idle screws until they both sound the same.
Keep adjusting the idle screws until the correct idle revs are achieved,
and that the carbs still sound the same.
Once achieved, reconnect the lingages checking that there is no play in them, and that the operate evenly.
All the best
Cheers
Jeff
Vacuum hose should be connected to the manifold, not one carb.
To find a base line to work from as far as tuning is concerned,
Set the mixtures on each carb as per the manual.
Disconnect the lingages that connect both carbs together.
use a tube, or better still, a stethoscope with the end pulled off
to listen to the air rushing into each carb.
Make sure when you hold the tube just into the carbs,
that you do the same for both (ie; the depth and the way you hold it)
Adjust the idle screws until they both sound the same.
Keep adjusting the idle screws until the correct idle revs are achieved,
and that the carbs still sound the same.
Once achieved, reconnect the lingages checking that there is no play in them, and that the operate evenly.
All the best
Cheers
Jeff
weeeell, im still stuffing around with the twin carb setup. been a bit distant from the old girl for a while, uni and stuff. but thats over now, so back at it.
ok so, the carbs are synchronised. ive had a vacuum gauge on there, looks to be no air leaks. mixture screws set.
here is the problem - it appears to be starving for fuel. it will idle, and rev fine. it revs because of the fuel supplied by the accelerator pump, if you hold it at high revs, it will die completely. the main jets are not blocked.
so what the HELL is going on???
ok so, the carbs are synchronised. ive had a vacuum gauge on there, looks to be no air leaks. mixture screws set.
here is the problem - it appears to be starving for fuel. it will idle, and rev fine. it revs because of the fuel supplied by the accelerator pump, if you hold it at high revs, it will die completely. the main jets are not blocked.
so what the HELL is going on???
Re: trouble with the twins
Gday Muddy
Its starving for fuel somewhere by the sounds of it.
Blocked fuel filter maybe?
From your initial post
Cheers
Jeff
Its starving for fuel somewhere by the sounds of it.
Blocked fuel filter maybe?
From your initial post
Does this still happen when you place your hand over the carbs, or do they now behave the same?muddy wrote: another strange symptom is when you put your hand over one carb, the idle will drop considerably,
and you pull your hand off and it comes good. if you do the same on the other carb, the idle drops considerably,
but when you pull your hand off, its covered in fuel!
Cheers
Jeff