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

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

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?

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

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.

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

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.

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

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 

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

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?

http://www.cbperformance.com/catalog.asp?ProductID=1507 

Thanks again Bizarre that makes alot of sense.

Cheers AL

You will still need to resize the air jets as the jet doctor won't correct the size

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?

No problems fitting Jet Doctor, easy. I'd be heading towards the smaller air bleed.

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

go down one size size and go from there....

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)

Thanks Bizzarre That is a great read and has helped me alot :-)

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

Have you reduced the size of the idle airs and what emulsions, my guess still the F57's

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)?

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.

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.

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

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.

Definately setup airfilter bases to suit your needs. Removing them to change jets is a pita