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posted on March 23rd, 2016 at 10:08 PM
Throttle response
Hi All,
I am just chasing some help from those in the know or with experience.
I have the following engine specs
1641
stock cam, lifters, push rods and rockers
stock german crank unbalanced
Cb performance heads, 40 intakes and 35 exhaust
larger oil pump
alternator
aluminium stock size pulley
stock flywheel
heavier clutch, 1 past stock
extractors & single muffler
single 40 idf webber on a pre-heat manifold, 28 venturi, 120 mains, 65 idles (from memory)
I have self tuned it with a wide band and am currently running round 12:1 on idle after warm up and 14:1 at 100km/h round 2800 rpm (taller 4th
gear)
Runs great but i get poor throttle response under 2000 rpm. I have got smaller venturi made, but would have to get a smaller main jet made to bring
the fuel down as its too rich...
I want to know if i swap to the twin 36 idfs that i have will i get the low end throttle response and keep the top end power? The 36 idfs are for a
1776 that i have planned so could give them a work out.
Cheers,
Matt
modnrod
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posted on March 23rd, 2016 at 10:28 PM
Off-idle and very low speed response is governed mostly by idle jetting and dizzy advance.
Perhaps try 13:1 or 13.5:1 at idle, and/or more off-idle advance with a lighter spring in the dizzy (a vac adv unit like the late 1600s use would be
ideal for this).
Everything is a compromise, but for this small cc engine with a stock cam, any inlet manifold that is bigger than 32mm I.D. will lose airspeed and
make a "snap" response difficult. Perhaps a trial with "inserts" as a band-aid to test it out?
Try 15:1+ at 100kph as a science experiment once you have your SVDA tuned in.
MattY-64
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posted on March 23rd, 2016 at 10:43 PM
Thanks modnrod.
I forgot to add the electricals.
Runs a stock coil and mechanical 009 set to approx 30 - 32 degree advance at max ( from memory has been a while).
I think i have the progression correct, the problem with leaning out the mains and idle is it stumbles and farts too much when cold. Most of my trips
arent long enough to warrant 30 minute warm up.
What if i changed my rocker geometry?
How far can i advance the 009?
modnrod
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posted on March 23rd, 2016 at 11:00 PM
65 idles sounds richer than what I'm used to, I would have expected 55s.
To use 55s you need good manifold heat AND a good advance off-idle. 009s it is possible, but getting the advance at idle to under 10*, and then
getting the advance curve to quickly spin to 18* at 2000rpm needs your small/light spring to be made even lighter again, by quite a bit (this is
trying to simulate the extra 6-8* advance at low throttle and under 2000rpm an SVDA will give you).
check one end of your manifold heat goes to your collector, or it won't work very well, it should be warm and working well within a couple of
minutes. The other thing you could do is get heated air to the air filter, but that is difficult with an open filter.
Twin 36s will work very well, and they are a much easier solution than tuning a centremount sometimes, so before you spend 1/2 your weekends tuning
and tweaking consider starting off with the easier package to begin with maybe, since you already have them?
Centremounts, whether Solex, IDF, Holley, whatever, need LOTS of heat, LOTS of airspeed, and an SVDA, then they are brilliant once tuned.
MattY-64
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posted on March 23rd, 2016 at 11:22 PM
Yeah i have the 36 idfs, just need to finish their rebuild.
The pre-heat manifold is a good one definately passes exhaust gases.
I might be on 60 idle i will have a check tomorrow.
It isnt my primary vehicle and i like to tinker so will try some different distributers.
I was thinking a 010 would be a good distributer choice? Would just have to track one down. I have a couple of old distributers here, where to
connect the vac line to on a webber? I don't think i have a lighter spring in the 009. Where to get these?
When i had the 1600 barrels an pistons install the engine made around 50hp at the wheels so its not too bad at the top end.
With the 1776 and 36 idfs is it possible to get a good responsive curve or are 36s to small for top end? Looking at 7000rpm redline. Balanced and
blueprinted.
modnrod
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posted on March 23rd, 2016 at 11:44 PM
With your parts on hand, I would run the 36s on the 1641 for sure, and I would throw on a set of 1.4s to get your lift to match your valve sizes
better, with 5000rpm ceiling from your stock cam just some .060" shims under stock springs will do if you haven't got HD singles on your CB heads.
It will match the bottom end of a good centremount but start to work better over 3200/3500, as much as 10HP better. Under that there really isn't
much in it (it just sounds gruntier! LOL!).
If you want to tinker then find the light spring (dizzy's have heavy and light springs, light springs have a thinner wire) out of an old Holden
173/186/202 etc, from memory that's the one I settled on. The advance curve is much more important than what your wideband is telling you, the dizzy
will make it jump best down low.
i dunno where to point the vacuum tube at for your IDF, I don't have that much time on them, sorry.
1776 spinning to 7000 (peak power around 6500-ish?) will need 44s or 48s to really buzz.
Send a PM to Daimo.
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posted on March 24th, 2016 at 12:05 AM
The heads have single HD springs.
If i go to 1.4s do i have to shorten the push rods?
modnrod
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posted on March 24th, 2016 at 11:10 AM
Sometimes you can get away with just shims under the posts, but it depends on how fussy you want to be.
If you set the rocker pattern up from scratch so it's really nice everywhere, then it's likely that pushrods will be a different length, but then
again, that's the case with some stock engines too.
The rockers are only worth a few HP, so it's up to you, not necessary really.
