18rg tuning, LONG!

[rcodea]

I was thinking today(scary idea!) about developing a compromise as many of you are referring to.

As I said earlier, I have been told that going with the 38mm large venturies is the way to go. But that will effect the atomization and thus efficiency of the carb.

So I thought, why not use the 38mm venturies, and change out the small venturies for something like a 16OD and 10.5ID. that would allow for a HUGE pressure and velocity change, and should allow the fuel to atomize better, thus compensating for the large 38mm venturies.

Any thoughts? I have access to several different combinations, so feel free to recommend just about anything(including that i shut my mouth and listen)

Cheers!

[MWP]

Well if you have access to the parts, give it a go!
You wont get a good tuned combination the first time... trial & error will be required.

[Rodger]

So Ryan,

What do you want to acheive from it all? Unless I have missed it, your are yet to decide.

To compromise I'd go with the #36 outer venturi and leave the inner venturis alone. It is the air speed that plays with the way the fuel is drawn from the jet block and you are correct about the efficency. #38 is really only good as I have said for outright top end improvements.

Yes a smaller inner venturi will speed up the air through it and help pick up the fuel but it is heading towards both ends of the spectrum again, same as the current main jetting you have. Too big of a difference in sizes. Again all to do with air speed and volume.

The engine will lean out at high revs because you cannot get enough fuel from the smaller inner venturi to match the amount of air moving though the main bore.

Keep at it and we will all be more knowledgable.

Regards

Rodger

[rcodea]

First off, thank you guys for your help. I really do appreciate it!

Rodger, i want to achieve a street monster. Something that i can drive on the roads in the city, pick up ladies in, and embarrass the Honda ricers with. Something fun, but not crazy F1 type characteristics.

I think a setup for weekend autocross is about where i want to be, since that is what the car was built for in the first place. No need to push it behond that, but its stupid not to at least use it for what it was made to do.

After reading through this thread about 5 times trying to pick up everything and learn as much as possible, I decided that you all are probably right!

So, I went down to the car today. I replaced the 34mm outer venturies, so those are back in the carbs (I had them out to try and make a trade, but that isn’t happening).

I then got out the old 150 main jets that came in the carbs when I bought it. I put those in place of the 135 mains, and took it for a spin. And guess what happened…

The car DEFANETLY pulls better now! I still would say that it is not perfect, but it is MUCH better!

So, my plan as of right now is to get the new venturies first. It turns out that no one seems to have the 38mm, but they do have 37mm and 39mm, so I will be getting 37mm ones at the end of the month. I will also NOT be playing with the small venturies, as Rodger said.

I am also going to try and get an assortment of jets, and start playing from there. If I am lucky, I will be able to find a tuner here in town that has lots of experience with carbs, but maybe not Mikuni carbs specifically. I figure, if I can supply the jets, he should have everything that he would need to get it perfect.

So, here are the jets that I am going to try and get a hold of:

air - 200(have), 210, 220, 230, 240
main - 135(have),150(have), 160,170,180,190
idle - 55, 57.5, 60(have)
pump - 40(have), 45, 50, 55

I figure that should give a tuner everything he needs. What I don’t use, I will send back and get my money back.

Any thoughts?

A few other things…

Cam timing. As I said, I have been told that the intake should be 100 degrees and the exhaust should be 98 degrees. Those are measured from the lobe center, not opening or closing of the valve. Anyone have any other suggestions?

For my distributor, it was stated earlier that I should have 36 degrees total timing. Since I don’t have vacuum advance, I only have mechanical. So, at what rpm should I achieve this 36 degrees? My timing light is a pretty great one, so I can use it to set my total timing at a particular rpm and leave it. Any thoughts? Anyone else have an opinion on total advance?

Thanks again mates for your help. As you can tell, I am paying attention, and I think I am learning a lot, and have already gained a lot of trust in you all.

CHEERS MATES!

[jonra23]

As a couple others have said get the vacumn advance working, will have no effect when you have you boot into it but at part throttle cruise will give extra advance which will give more efficient combustion.

Cams selection should realy only effect idle vacumn, not part throttle cruise.

regards
Jon

[rcodea]

As a couple others have said get the vacumn advance working, will have no effect when you have you boot into it but at part throttle cruise will give extra advance which will give more efficient combustion.

Cams selection should realy only effect idle vacumn, not part throttle cruise.

regards
Jon

But wouldn’t that also then cause me to get detonation at part throttle, having more advance with a leaner mixture?

How then should I set the timing? I mean, if I need 36 degrees of total advance, should I just get the car to say 3000 rpm and set the timing to the recomended 36 degrees, while running the vacuum advance? Doing so would mean though, while running the vacuum advance, that at full throttle, I will be getting less than the 36 degrees, thus making my combustion less efficient. Correct? Is there another technique to setting the timing that I should be using?

I know, stupid questions, but I am trying to learn how this ignition theory works in a practical application.

Thanks for the help! Please keep it coming!

[jonra23]

Unusual to get detonation under light throttle conditions unless very lean, because the intake is "throttled" the amount of air getting into the cylnder is a fair bit less give a lower effective compression ratio, because of this you can run more advance without detonation.

As you said at full throttle the vacumn advance will be doing nothing so you should not have to adjust you base timing much, if at all.

There is a fair amount of power and throttle response to be gained from a properly "mapped" ignition machanical and vacumn advance, it is a balancing act though if too rich you will need to run more advance as combustion is slower, if running a bit on the lean side you can run very little advance without detonation.

Definitley not stupid questions, better to ask for more details behind someones statement rather than take it as the truth.

regards
jon

[rcodea]

Well, its taken me a long time to get anything done on the car. With college and the like, its been ruff.

I was able to get the parts to reconnect the vacuum advance, but have not had the chance to do so. I also need to get my hands on a timing light that has adjustable advance. That will allow me to see exactly how much advance I am getting at full throttle from the vacuum.

I have also purchased a cam timing kit to make sure that I have my cams properly timed. I will be doing that as well.

In driving the car recently, I have been making sure to pay attention to how the car acts. There is definitely a hole in the power just after 2000rpm. It is more prominent in full and very light throttle situations. I have noticed though that if I have just the right amount of throttle, there is no hole, and the motor has a very linear powerband. So my guess is that there is definitely a timing issue, and possible a carb mixture problem as well. Time will tell as I get on a dyno with O2 sensors and such.

I have ordered a set of the 37mm venturis, as well as 210 air jets and 170 main. Having those and the jets I currently own should give me enough combinations to play around with and make notes. Who knows, maybe in that playing I’ll find that perfect combination!

I will keep you all updated as things progress.

Thanks again gents, I really do appreciate the help!

[rcodea]

Well, I got the 37mm venturies in there and decided to get it dynoed to see where things are sitting with it.

Here is the chart as well as a blown up view of the A/F ratio.




Any thoughts? I am running the 200 air, 150 main, 60 idle, and 40 accel, with it in the upper hole. How do I get the tuning to level out? I need it leaner low and richer high. Any thoughts?

I was disappointed in the power though. I was expecting something north of 130hp, but obviously that is not there. The dyno guy suggested a better distributor than the points one I have in it now. He also noted that my timing is not advanced enough with the 18degree base and no vacuum. (It was set that low to avoid the detonation I was getting up high in the rpms). He also suggested increasing the fuel pressure to about 12psi.

Anyone care to speculate on the jets that I might need? I do have the 170 main and 210 air, and can always take the ventuis out as well....

[Rodger]

Try a set of #65 pilot. It may flatten out that hole in the A/F at 35mph.

Go up a couple of sizes in main jet (#170) to richen up the top end.

Nice to see dyno graphs and it will be good to see for all the choices being made in jetting and how it shows up on the graphs. I don't get to do this enough and it may help myself and others sort out a few jetting issues.

Keep up the experimenting.

Regards

Rodger

[rcodea]

I don’t know how many of these graphs I will be able to produce. At $60USD per set of 3 runs, it aint too cheep. If I only had a way of measuring and logging the numbers on the road....

It is interesting the differences in tuning suggestions that I get. I have been told that doing a 170 main and 210 air would probably be perfect for my car.

But then, given this chart, I was told that I needed to actually drop to the 145 main and a 180 air. He said that the lean upper rpm was caused by the air jet being too large, and that it is a little rich down low.

But the next time I go in there. I will be doing the "self tune" option, which will cost me some more money on top of the base, but it will allow me to change jets right there on the dyno and get immediate results. I will have at least 2 combinations to through into the carbs, and we will see how it affects everything.

I guess I will see with time. But first I have some things that I need to fix on the car before I do anything else.

It will probably be another month before I can get any more graphs(mostly cause I am already broke for the month!)

[RT104GT]

Thank you for reminding me. As far as i know, there is noone in my local area that has extensive experience with the mikuni carbs, nor do they have the jets that are used in them.I will be continuing to search around, but it is not looking too hopeful as of right now.

thanks!

FYI I have 1973 2000 GT Corona. I remember when I bought it and went on my honey moon.
We droev acorss Victoria Australia and up intothe Australian Alps. The higher we went the worse the performance went down.

That car was good for 7000 rpm in low gears and went like a rocket until we climbed 4500 feet up to the mountains it ran like a dog.

I mean the performance was down 25 -30%.

Solution: No idea, we went back down.

The jetting was 145 mains and 750 air.

The engine lost most of its pulling power and I geuss the engine just did not breath.

As to how you get that back dont know.

In any aircraft as you climb you LEAN.

But eventually you get to FULL THROTTLE HEIGHT (IE THROTTEL TO THE FIREWALL and NO MOR EPOWER) and all that leaning does then is slow you down.

The EGT at low level is high and you have to throtle back, at height above 6000 feet it does not matter and you nevere xceeed the EGT redline no matter waht you do..

How they fix planes is to Go Fuel injection...and Turbo charge it..

2 cents worth.

[Turdinator]

Very interesting read guys. The vacume advance was disconected on my car by the previous owner, supposedly because the vacume signal created by the larger cams and larger carbis is too lumpy. I might have to look at connecting it up again and seeing what happens.

[pilot]

You can try to connect several cylinders to get a vacuum signal (2 or all 4). That´s what I did in my setup and "lumpiness" improved a lot (changing from 1 to 2 opposites).

[Steve M]

My turn now

1. Listen to Rodger for jetting instructions.

2.
Here's what we've found with playing around with ignition and cam timing and mikunis on my brother's 18RG.

Set up:
18RG (88270 head, ports smoothed)
9.8:1 CR (0.05mm OS)
40mm Mikunis (32mm venturis) 200air, 140main, 60slow, 40pump
short inlet manifold and 4-2-1 extractors (header) with 2" ex. and straight through muffler
280degree mystery cams timed based on performance, (no specs) over shimmed, actual duration is now 298deg.
18RC dizzy (22R elec. conversion) with stock vacuum advance

Ignition

Static timing 5 degrees btdc (STD for the high comp 18RGs)
Any different and lost a lot of power, particularly at the top end. It was a slug.
Vacuum advance connected to manifold vacuum, this fixed up the idle a lot and no more surging when cruising.
That all adds up to 20degrees advance at idle, 32degrees total advance at 6000RPM (no vac. adv at WOT)
No pinging even in 44^C weather.

Cams

We based our cam timing on a 110 degree Cam Lobe Separation (each side from lobe centre).
I would think that anything less than a 105degree CLS would be too much overlap, not to mention valve getting very close to pistons.
What are the TRD specs for phase or CLS (same thing)?
We found that any other set up was slow. We initially had the timing at 110 intake and 106 exhaust and it was slow. The change to 110 exhaust made a big difference.

Carbies

Make sure they're balanced and all that stuff, if they're out of balance it will run poorly.
I'm sure you've done that already though.

Currently using 200a/145m/60s jet set up and that has been great. (stock for 88211 manifold)
We also tried a 140air /132.5main /63.8-65slow set up with 31mm venturis.(stock for 88251 manifold)
There was bugger all difference between the two set ups with stock cams.
The top end was slightly better with the top setting.

Performance

Nothing below 2000RPM
noise changes to individual carby throat pulsing at 2000RPM and torque comes in.
gets a increasingly stronger upto 3300RPM
noise goes to very loud pulsing, power kicks in really hard
Power keeps pulling harder until about 6800RPM.
I would expect about 15hp advantage over same engine with stock timings. That's how much better it feels.

Summary

If you're having trouble in multiple areas, just do this.

Take the settings as close to STD as possible for the stock engine that most closely resembles your's. i.e the early 88210 or 88230 18RG. Then make one change at a time from there, starting with ignition, then cams and carbies after ignition.
Settings:
ignition: 5 degrees + vacuum advance
cams: 108-112deg CLS per side (lobe centre)
Carbies: Listen to Rodger
acc. pump discharge time: 0.8-1.1 seconds

I know this was long, but now I will sleep better knowing that I didn't forget anything.

Oh, is your head ported? With big cams like those, I'd be wanting a well ported head.
That would also restrict top end power.