Engine Tuning Any Gurus??

[Jimpoole]

My carb was never 'set up'. Might be time to find out how!

I am looking forward to your pics.

[rob83ke70]

kylie's 4k with a 280 odd degree camp will idle at 600 revs all day if tuned right :dance: but it has huge carburettors...

 

I'd look at your cut off solenoid, make sure the fuse (labelled "engine") isn't blown. I'd definitely run 12 degrees of ignition advance as well if you have a different cam.

 

My suggestion here though, keeping in mind its been a long time since I played with standard carburettors is that you physically cannot get enough air through the carburettor to keep the engine running when the butterfly is closed far enough to run on the idle jet. solution: fit a larger carburettor. I can't reccommend side draught carburettors enough either as the manifolds seem to be the biggest improvement.

 

but as I said, its been a long time since I played with standard corolla carburettors.

 

Robert.

[Felix]

you physically cannot get enough air through the carburettor to keep the engine running when the butterfly is closed far enough to run on the idle jet.

 

That is a good point that I overlooked. With a bigger cam the throttle plates need to be opened further to enable more airflow due to less vacuum from the increased cam overlap. The problem is (as Rob pointed out) that the Idle drillings in the carb bore are ineffective as the throttle plate is opened too far.

 

I've run into this before when I ran a 32/32 DGV, though it was easy to fix as the model DGV I was running had a throttle plate adjustment screw on the secondary valve, which enabled me to open the secondary and close up the primary butterfly to get the idle circuit running properly.

 

A simple fix I've seen is to drill small holes in the primary throttle plate. How big or how many would need experimentation. Another way would be to open the secondary butterfly slightly at idle, so the primary could be closed up. I'd have to have a look at a stock carb to see if this is a possibility. Maybe this arve if I get time.

[rob83ke70]

long time since I played with these things, and I've moved into the world of SU carburettors and thus no longer have all those pesky jets and circuits and twin butterflys to worry about, but all that being said, I've just had a moment of realisation there as to exactly what that adjustment on the secondary butterfly on a dgv is...

 

Yes, I can be a bit slow!! :dance:

 

if you drill a hole in the butterfly, make sure its a very small one to start and you will have to adjust the mixture richer to get it right.

 

Robert.

[Felix]

I just had a look at a couple of stock carbs.

 

Unfortunately there is no way to crack open the secondary throats. You would really need to drill holes in the butterflys to be able to get the full effect from the idle circuit in them.

 

Attached is a few old pics of my Digital A/F ratio meter and O2 sensor.

[Felix]

springersrolla I'd be looking at this 32/32 DGV. They go really well on a worked 4k or 5k. Way better than a 32/36.

[Felix]

springersrolla I'd be looking at this 32/32 DGV. They go really well on a worked 4k or 5k. Way better than a 32/36.

[springersrolla]

Felix, cool man cheers for the info and the tip on that sale, I'm actually looking at picking up a set of twin 40mm dellortos for super cheap so how do u think they would perform on a 3k or is it not worth it?? i also have a set of twin hitachi SUs 1.5" but got told there not that flash, i did unhook the fuel cut solenoid and nothing at all happened so i guess its off to a tuning shop, and i'll get an 02 sensor along the way thanks heaps for your info and responses guys

[coln72]

did you try to hook it directly to the battery with a couple of wires? Nothing changing after being disconnected means that it is not working and from memory they need power to the solenoid for the engine to idle properly.

[rob83ke70]

i'd be keen on the side draught carburettors. If you drive it on the street the hitachi su clones would probably go really well when set up correctly. if you race it, the dellortos would be the go.

 

pull the solenoid out of the standard carburettor and put a shallow bolt and a washer in to plug the hole.

 

Robert.

[philbey]

Any sidedrafts, dellortos or su's will give good gains. Rebuild them properly, jet them properly and get them dyno tuned.

[springersrolla]

good to hear are u aware of the appropriate sizing of chokes, idle jets, fuel jets etc... I'm looking at a set of DHLA 40's of a 2000cc alfa giuletta and i was reading in the How to build a tuff k motor saying to run 28mm chokes?? is anyone running a 3/4/5k with twin dellortos or can recomend a good tuning book or site?

[ke70rolla]

have you checked the cam timing sounds like it could be off if you can't get the thing idling under a grand 750rpm

should be enough vacuum leaks could be another problem.

[Felix]

This will get you into the ballpark with tuning dellortos

 

Personally I think dellortos/webers are best saved for the race track.

 

the purpose of the main venturi is to increase the vacuum acting on the main jet in order to draw in and effectively atomise the fuel mixture. The smaller the main venturi, the more effective this action is, but a smaller venturi will inhibit flow. A large venturi may give more power right at the top end of the power band, but will give this at the expense of lower RPM tractability. Only a circuit racer will benefit from this sort of compromise, on a road car, driveability is much more important. 95 percent of the time, a road engine is nowhere near its peak power, but is near its peak torque for 75 percent of the time. It is much more important therefore to select the main venturi for best driveability

 

SU's work much better over a wider power band with their variable venturis. I've got a set of 1 3/4" SU's. They go hard, but:

 

My carb of choice for a street K series motor is the 28/36 DCD Weber. The venturis are changeable to suit various engine capacities and states of tune. Super simple to work on, as the main and idle jets are accessible from the outside. You don't even need to remove the aircleaner. :dance: Set and forget.

[rob83ke70]

wouldn't want to go bigger than 1.5" su carburettors on a 4k. maybe bigger on a 5k?

 

Robert.