Hey guys,

Looking for some info/opinion on timing. I know there are plenty of threads with a lot of info, but I wanted to clear up some questions.

A good resource I found is the RB timing chart.. lays things out pretty nice.

http://www.racingbeat.com/manuals/timingchart.pdf

First, I am running a Streetport 12A (RB Dellorto sidedraft, RB Streetport Exhaust) with a stock 82 Distributor (10deg mech advance?). I do not run vacuum advance.

Currently, I have my timing set to stock, L 0BTC and T 20ATC. Just for initial setup, break in etc.

The RB PDF is saying to advance my timing to L 8BTC and T 12ATC.

So, this should run +8 deg more advanced compared to the stock setup. If the Factory specs are correct, I should end up with 18deg advance beyond 4k RPM or so. Is this a good setting for a Streetport?

This seems a bit lower than some other numbers I have seen recommended. Also wondering why the 84/84 Streetport in RB's PDF has very different numbers compared to the earlier 12A? The Mazda docs say mechanical advance for 84/85 is 12.5deg.

Thanks!

EDIT- Did some more searching.. it is making more sense now. The numbers that the Tech Specs and Distributor Curves show must be doubled to translate to the engine advance and engine RPM.

So the stock Mazda distributors run about 20-25deg of total mech advance.

So the RB numbers do make sense... for a 1982 Streetport they suggest +8deg compared to factory. This would be 28deg. That is actually a bit high?!

 

I have a stock port with a hogged out nikki and did run a Dell previously. In both cases my timing is set about 26 deg at 4500 rpm. I have vacuum advanced hooked up. Not sure what it looks like at idle but its not near the stock settings. Btw, vacuum advance really makes it more drivable at low rpms, regardless what folks say. Even on a Dell.

 

Ok that is good info!

What/where did you use as the vacuum source for the distributor pots? My RB Dell manifold only has the single brake booster reference. I have some small NPT fittings to drill and tap another location.

Is it possible to get into trouble with vacuum advance by having too much timing advance at higher load and RPM's?

 

I think I teed off the vacuum port for the brakes when I had my Dell. The only issue with using manifold vacuum is you will be at full advance at idle. Makes for a slightly lumpy idle but fixes itself as soon as you start driving, then it behaves just like ported vacuum. Ported vacuum shows no vacuum, at idle and then you see it off idle. Won't hurt anything unless you are boosted (turbo or SC).

The dizzy is at full advance once you hit 4000 rpm, so advance mechanism, both vacuum and mechanical only have an effect below 4000 rpm.

 

Sorry to re-ignite an old post, but I have (more or less) the same question. I have a new street port running Nikki with primaries machined to 24mm. Looking at the RB timing recommendations they say 8BTDC at idle. My distributor has 24 deg mechanical advance which means that the full advance would be 32 BTDC (unless my math is off). That seems pretty advanced! Anyone have experience with that setting? Previously I ran the stock port, stock Nikki (with some modified jetting) at 24-25 BTDC at full advance. Didn't have any problems with that. This is a race car only.

Want to get max performance, but don't want to break anything either. Heading to the dyno next week to get the new carb dialed in, as well as compare performance to the old unmodified carb.

Cheers,

Carl

 

One thing you can do while on the dyno, after the carb is jetted correctly, is change timing. I did that and even got up to 32* Leading timing for max hp but dialed it back to 28* 'for safety'. There was a very small increase in hp between the two.

 

Try various timings and let us know if there were any big changes in power.

 

OK. Went to the dyno today to check performance of new street port 12A and modded carb. In direct reference to this thread, there was no increase in performance between 25 BTDC and 27 BTDC (actually 27 was a meaningless 1/2 HP lower), so I did not test any additional settings. I have attached though the dyno results from 8 years ago that led me to the 25 BTDC in the first place. In that case, there was an improvement going from about 22.5 to 24 to 25 BTDC. The engine at that time was a freshly rebuilt 12A with a stock Nikki (with 130 primaries and 170 secondaries). I will give a few more details in the thread related to carb modding, but the modded carb with hogged out primaries resulted in a 13HP increase over the "stock" Nikki (both tests with the new street ported 12A) . I was happy with that, whether there is more performance available or not.

Carl

 

Care to let us know what 130 and 170 are? Is it jetting... and um, care to spill the beans on your carbs' tunes (air bleeds) and venturi sizes? It'd be really interesting for those of us who are just starting to learn about the Nikki.

I'm seeing 24mm primaries mentioned in your carb so far. Any other details would be great. Thanks.

 

Actually I had another thread related to the carb mods and I just posted more detailed stuff there. I went simple since it is a pure race car. No need for any kind of performance at less than 4k rpm. All I did was machine the primary venturis from 19mm to 24mm and removed the alignment arms on the booster venturis. No changes to air bleeds, etc. Still have vacuum secondaries that I might change later (they actually weren't working during the dyno test due to gasket installation screw-up so I was the actuator). The car seemed to rev very smoothly on the dyno, but haven't been able to drive it yet other than slowly around the block. The attached dyno sheet does show some weird A/F nos. at "low rpm". It idles smoothly at 12.3 A/F though, so not sure what the high nos. in the curve are about.

I know you are not just starting to learn about Nikkis ;-). Hopefully someone finds the info useful though. .

Carl