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 DubCty
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| posted on August 26th, 2012 at 08:41 PM |
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IDF wizards needed
G’day guys (and girls)
I have recently fitted twin 36IDF carbys to my 1850cc and I’m going through the tuning/jetting stage trying to get the most from them. I have a
wideband air/fuel gauge fitted to the car to help out with the jetting side if things but im having problems with inconsistent running. I can the
engine to operating temperature and set the idle speed and the car runs great for a few drives, but then all of a sudden the idle speed jumps up from
800 to around 1200rpm and the air fuel ratio leans right out with an impressive backfire problem when coasting, with a bit more driving ( about 10
minutes) everything settles back down and the engine idles and runs as it should.
I have checked all the jets are clear, the timing is as it should be and I can’t find any air leaks in the system.
The engine is an 1850ccTP with electronic ignition, mild cam, Italian 36 IDF’s run 32mm ventures with 150mains at the moment
Any help would be really appreciated.
Cheers AL
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 Bizarre
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| posted on August 26th, 2012 at 08:57 PM |
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My 2c - treat with caution 
The 150's are too rich for the 32's.
Should be 135's - but that isnt your problem
What emulsions are you running??
They usually come with F57's and I could never get them to work
Your symptoms seem to be classic air leak.
Do you have a crack in the manifolds??
Seen exactly your problems on a mates car with kads, but the crack opened and closed with temperature.
When it "speeds up" shut it down and pull the plugs.
At least then you will know what side/pot has the lean issue
Futue te ipsum!!!
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Gracey
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| posted on August 26th, 2012 at 08:58 PM |
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Sounds like a vacuum leak to me. These can be hard to find.
Try spraying aerostart or something similar around the mainfolds, particularly were the manifold bolts to the head and listen for an increase in
engine speed.
The intake gaskets sometimes leak between ports so it may be worthwhile pulling the manifolds off to check.
Could also be a few other things like manifolds warping when hot, does it do it after it's been idling for a while, allowing the manifolds to
heatsoak?
What sort of manifolds are you using?
Could also be the linkages, what sort of linkage are you using?
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DubCty
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| posted on August 26th, 2012 at 09:12 PM |
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Hey guys thanks for the quick reply.
F11 emulsion tubes
I’m running brand new CB performance manifolds with new gaskets and Loctite sealer and A CB Hexbar kit. I had a suspect vacuum leak noise so I
removed and re sealed that side Il’l keep looking and spray some start ya barstard around.
The change over seams to be while driving I can leave home with the car running well and return running like a pig only to sort it’s self out again
with more driving???
Cheers AL
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matberry
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| posted on August 26th, 2012 at 09:20 PM |
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Sounds odd symtoms, but I agree with air leaks or linkage, also possibly timing not returning advance to zero. why at different temps I dont know but
easy to check if no leaks show up.
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Klaus
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| posted on August 26th, 2012 at 10:35 PM |
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on a mild 1600 tp I ended up using 65 idle jets f11 emulsion tubes and 140 mains , hasnt been dynoed but runs pretty decent
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vwo60
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| posted on August 27th, 2012 at 06:15 AM |
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If the gasket surfaces are in perfect condition i do not use a gasket between the manifold and the head, just metal to metal, loctite 518 master
gasket, no gasket to compress over time and start leaking.
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DubCty
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| posted on September 5th, 2012 at 06:56 PM |
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Hey guys,
I’ve finally got around to removing my IDF’s ready to reseal them to the manifolds, while I had them out I decided to have a look at the float
levels when I removed the tops I noticed that the air bleeds on only one the carbies were closed up (soldered and redrilled much smaller around 1.2
by the look). On the other carby one of the air bleeds was completely gunked up with rubbish while the other one was clean and the standard size.
First Carby
Second Carby
My question is what do these holes do? Where do they lead so I can clean them out properly? Could this be why I’m having trouble balancing the se
up? Should I close up the bleeds on both carbies so they and consistent.
The carbs were taken off a type3 1916cc where they ran well and bolted straight on to my 1850.
Any help is really appreciated
Cheers AL
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Bizarre
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| posted on September 5th, 2012 at 07:21 PM |
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Pretty sure they are the air for the idle circuit??
These can be sealed if the modified idle jet holder is used?? (re CB jet doctor)
have a read through here
Lots of info for rebuilding your 36's
http://carburetorclinic.com/36idf.html
Futue te ipsum!!!
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matberry
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| posted on September 5th, 2012 at 07:21 PM |
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They are the idle circuit air jet, they need to be clear (obviously) and the mix air into the idle circuit during the progression stage so the gallery
will lead to the idle jet. Most lean out when hot like you describe would be the head-manifold not sealing
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DubCty
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| posted on September 5th, 2012 at 07:32 PM |
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Thanks guys, I will be resealing manifold to head as well although looking at the gaskets they were sealing well. Do you have a link to modified idle
jet holder info?
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Bizarre
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| posted on September 5th, 2012 at 07:38 PM |
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http://www.cbperformance.com/catalog.asp?ProductID=1507
Futue te ipsum!!!
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DubCty
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| posted on September 5th, 2012 at 07:51 PM |
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Thanks again Bizarre that makes alot of sense.
Cheers AL
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matberry
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| posted on September 5th, 2012 at 08:17 PM |
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You will still need to resize the air jets as the jet doctor won't correct the size
Matt Berry Motorsports...air cooled advice, repairs and mods Ph 0408 704 662
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DubCty
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| posted on September 5th, 2012 at 08:42 PM |
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Has any on here compared standard sized air bleeds to the 1.2mm closed in ones? is there a lot off difference? Also has any one used the jet doctor
here? Any problems?
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matberry
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| posted on September 6th, 2012 at 08:14 AM |
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No problems fitting Jet Doctor, easy. I'd be heading towards the smaller air bleed.
Matt Berry Motorsports...air cooled advice, repairs and mods Ph 0408 704 662
OFF-ROAD,CIRCUIT,DRAG,STREET,ENDURANCE
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DubCty
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| posted on September 10th, 2012 at 10:13 AM |
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Hey Guys
After a busy weekend I have rebuilt both carbies, installed the jet doctors (piece of Piss) and re sealed my carbie to manifold and manifold to head.
The set up is running much better now and behaving as it should, after installing the jet doctor the idle is running very rich about 10.4 air/fuel. I
can't for the life of me remember what the idle jet size was!!!! what jet sizes would be recommend for this set up both idle and mains as a starting
point? I know there are so many variables but I just want to know what others have tried before?
Cheers AL
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cnfabo
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| posted on September 10th, 2012 at 11:28 AM |
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go down one size size and go from there....
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Bizarre
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| posted on September 10th, 2012 at 06:36 PM |
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I agree with the comments made here for jetting
http://www.aircooled.net/vw-carburetors-jetting-102/
1.25 x body size for idles (36 x 1.25 = 45)
4.2 x vent size for mains (32 x 4.2 = 135)
Futue te ipsum!!!
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DubCty
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| posted on September 10th, 2012 at 08:10 PM |
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Thanks Bizzarre That is a great read and has helped me alot :-)
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DubCty
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| posted on September 23rd, 2012 at 03:34 PM |
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Hey guys,
After a few runs with the new carb set up and playing around with the jetting I am having trouble with a dead spot from around 1950 to 2150RMP where
the car leans right out and loosing power, below is the set up as it is now.
1850 running mild cam and twin 36 IDF and electronic ignition.
Choke size 34mm
Main jets 140
Air jets 200
Idle jets 45
Timing 11 degrees
Plug gap 0.5mm
Mixture screws are around 1.5 - 2 turns out
Idle is a little rich but ok the car takes off well and has good low down torque, Top end pulls well but midrange isn’t too good if I pump the
throttle a bit when it leans out the extra fuel pulls it through.
I did try running 40 idle jets but these were too lean I then slipped some 47.5’s in and these were a little too rich fowling the plugs after a
few 10 minute drives
Any helps appreciated
Cheers AL
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matberry
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| posted on September 23rd, 2012 at 04:57 PM |
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Have you reduced the size of the idle airs and what emulsions, my guess still the F57's
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vassy66T1
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| posted on September 23rd, 2012 at 05:11 PM |
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I am surprised at the size of the idles (small) and feel the jump from small idle to reasonably generous mains is going to leave a gap. My 2c
Are the idles affected by the jet doctor set up? i.e. does the jet doctor make a smaller idle richer?
What are your AF's on cruise (very light throttle)?
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DubCty
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| posted on September 23rd, 2012 at 06:01 PM |
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Yes I closed the air bleeds to 1.3mm and A/F readings on idle sits about 11.3-12 (it was originally 10.0) emulsion tubes are still F11. Cruising
around 2000rpm it is very lean 16.0 but if I cruise below or above 2000rmp im sitting on about 13.5 which is what I’m aiming for.
From idle to 1950 the A/F ration sits around 12.5-13.5
I’m worried that if I put bigger idle jets back in ill just keep fowling plugs.
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matberry
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| posted on September 23rd, 2012 at 08:10 PM |
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When the bigger idles were in, you reset the idle mixtures?
Good you've changed the idle air and emulsions. For me, I'd be starting with 55 idles but I also shoot for richer than 13.5, I'm normally happy at
11.5-13. Plugs shouldn't be a problem even at 10.0. Fuel keeps your engine cool which isn't such an issue in Tasmania.
I also concentrate on throttle position more than rpm. The idle jet only governs the progression ports and idle. The progression ports are only at
very light throttle application, like 3% or so. As rpm come up the mains start to work quicker than you think.
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matberry
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| posted on September 23rd, 2012 at 08:19 PM |
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The other thing to note is sync is SUPER important, both air speed and linkage. 99% of trouble with flat spots and stumbles starts there
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DubCty
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| posted on September 23rd, 2012 at 08:36 PM |
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Ok I might step up the idle jets to 50 and re sync just to be sure, after all the idle jet changes over the last weeks I have been very tempted to cut
and adjust my air filter bases to I can change the idle jets without removing the air cleaner bases has anyone done this before? I cant see a problem
as the air filter bottoms don’t seal with the tops of the jet holders anyway.
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matberry
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| posted on September 23rd, 2012 at 08:40 PM |
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Definately setup airfilter bases to suit your needs. Removing them to change jets is a pita
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Bizarre
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| posted on September 23rd, 2012 at 08:41 PM |
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| Quote: | Originally
posted by DubCty
I have been very tempted to cut and adjust my air filter bases to I can change the idle jets without removing the air cleaner bases has anyone done
this before? I cant see a problem as the air filter bottoms don’t seal with the tops of the jet holders anyway.
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When I had IDF36s I was running a Redline hex bar / filter set up
I cut a "wedge" out of the top so as I could access the idles without taking the whole filter set up off.
this worked fine.
I dont know how it all works if you are running the aluminium CB set up
Futue te ipsum!!!
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DubCty
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| posted on September 23rd, 2012 at 08:47 PM |
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| Quote: | Originally
posted by matberry
Definately setup airfilter bases to suit your needs. Removing them to change jets is a pita
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tell be about it lol but I was quite proud when I could change both sides removing the linkage and airfillter bottoms and drive away in 15
minutes.(the locknuts and long studs slow me down a little)
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