4g32 Dual Mikuni Jetting Advice

[Natedog]

I just got my car up and going again, however the twin carbs I'm running were on an Astron manifold when I got them. Can anyone tell me what would be a good place to start on Jetting sizes? I am running an aftermarket cam and a header as well. Cam info is listed below. Thanks!

Thanks

[75wagon]

Pic from 4G32 solex/ mukini twin carbs close up shots and info......

I'm running the stock jetting on my twins with the No.5 cam (the factory twin carb jetting is the same as that for a 1300). I'm running headers and a larger bore exhaust (I'd have to measure it to give you an actual size).
The easy alternative for upsizing jetting is change all 3 secondary jets to the 1600 jets. But I'm not having any problems at all with fuel supply.

Dave...

[davetrees]

What Dave said ..... put the stock 4G32GS jetting in it for a start.

To fine tune, either take it to someone who can put an AFR sniffer up the exhaust to see if it's lean or rich (and where) while driving, or spend a few $$ on the dyno.

[astronturbo77]

Dave there is only one.main jet and one secondary jet per.carb.

Ive.got standard 1600 jets in.both carbs on my twins, runs well

[75wagon]

Each carb has 3 matching jets for the secondary circuit.
The chart above proves it.

I know I am right.

[astronturbo77]

Your getting confused with enrichment jets and air bleed jets for primary and secondary circuits. The main jet and secondry jets in the bottom of the float bowels meters the amount of fuel taken from the bowel to the venturi.

[davetrees]

Dave,
I think you & Astronturbo are just getting your terminologies mismatched ... Solex/Mikuni use slightly different terms than (for example) Weber carbs.

According to the exploded diagram in the workshop manual, each carb has :
- 2 Main (fuel) jets
- 2 Pilot (idle) jets
- 1 Enrichment (starter) jet

There's no part called a "secondary jet" in a Mikuni/Solex carb.

I think you'll find that the "Main Air Jet" (air corrector, in Weber-speak) and "Pilot Air Jet" referred to in the chart are drilled in the carb body and are not changeable ? (Similar story in Mikuni sidedraughts ... you can change the Main Air Jets, but the Pilot Air Jets are fixed, unlike a Weber sidedraught).

[Natedog]

Awesome, thanks guys. I knew the info was here. I was just having a hard time finding it! As for tuning, my brother works at a motorcycle shop, he's bringing home the "sync tubes" they use on bikes with multiple carbs, as well as a box of Jets, a laser thermometer (to test the temps of the extractors), and I have a wideband 02 that I'm going to wire up. I've got 1 week to get it right before I make the Hot Rod Power Tour (figure you guys know that Magazine on that side?)

Thanks again!

[astronturbo77]

What davetrees said is what i was trying to explain but in better terminology
Im a mechanic, what ive been taught is that all carbs generally have one primary fuel jet and one secondary fuel jet. Now between weber terminology and factory mitsubishi manuals the terms get quite confusing..

This is why i like old dgv webers, simple jets, simple air bleed jets, imulsion tubes etc, was just this week i put a old dgv weber on a ke30, after a few hours of adapting a few cables etc and changing jets/air bleeds its tuned and running sweet!

The term 'enrichment' jet to me means accelerator pump squirter.

Im fairly sure your right regarding the air bleed corrector jets are in the side of the carbs, cant remember if they are a un-screwable type or not.

[davetrees]

Awesome, thanks guys. I knew the info was here. I was just having a hard time finding it! As for tuning, my brother works at a motorcycle shop, he's bringing home the "sync tubes" they use on bikes with multiple carbs, as well as a box of Jets, a laser thermometer (to test the temps of the extractors), and I have a wideband 02 that I'm going to wire up. I've got 1 week to get it right before I make the Hot Rod Power Tour (figure you guys know that Magazine on that side?)

Thanks again!

Looks like you access to everything you need then !

My rally car has been away this week getting the jetting in the twin 40mm sidedraughts etc sorted - I finally found someone who actually knows their way around Mikuni/Solex carbs. Will be interesting to see what changes he has made (on initial measurements, it was running seriously rich).

He balanced the carbs by ear basically (and also measuring exhaust runner temps with laser thermometer). We then stuck a wideband AFR "sniffer" up the tailpipe and took it for a couple of "power runs" through the gears up a long steepish hill to see what the AFR was doing through the rev range up to WOT, plus some lower-rev driving around the streets to see what the part-throttle AFRs were like (remembering that at part throttle you are mainly running on the idle circuit, not the main).

He has now fine-tuned the jetting this way by trial & error over several runs during the week, and apparently it is now sorted. (He said he could dyno it if I "wanted some numbers", but suggested there isn't much value really .... all the dyno does is simulate driving it on the road anyway, with a lot more stress !)

Looking forward to driving it home on the weekend

One point he did make that I hadn't really thought about is that it's important to get the jetting/AFR correct for the revs & load you actually use - eg. there's no point jetting it to run spot on at say 6500rpm if you never normally rev it that high. Makes sense to me !!

(FWIW, he prefers the Solex sidedraughts to Webers in many ways. He tunes a lot of both, and says that Solexes have a smoother transition between the idle & main circuits)

[Natedog]

The fuel jets in mine are currently a 96.3 and 160, so I'd dare to say I can probably get it pretty close like it is. I don't have any of the air jets available at this time, but I'm going to start buying them up when i see cars in the junk yard...

Hoping I can get it somewhat sorted before my brother comes over, anything is better than nothing.

[davetrees]

Try these guys ..... http://www.wolfcreekracing.com/

You might find that the downdraught & sidedraught jets are interchangeable ? They would probably know, anyway ...

I have had great service from them ... emailed what jets i wanted, paid by PayPal, and had the jets landed in Oz in a week (at half the price I could buy them here)

[Natedog]

We got it sorted out, it was primarily running off #3 and 4. The sync tubes he had wouldn't work due to the low vaccum pull. However we used a laser thermometer and got the temp of the extractors at 400-425 across all 4. I can probably lean it out more, but I'm unsure what the exhaust temp is due to the coating on my header. I'm going to snag some 155 jets for the top side when I get back from the power tour. It cruises great, and has pretty good throttle response.

[Superscan811]

my brother works at a motorcycle shop, he's bringing home the "sync tubes" they use on bikes with multiple carbs,

Why not go the whole hog with a set of motorbike carbs?



They'll go well, and your brother can correctly tune them..

You just need to get someone to make the manifold..


Cheers.

[Natedog]

I'd have to relocate my brake cylinder to do so, and I don't want to cut on the car in that fashion.

[75GC]

Hi. Any ideas where I can buy main jets for my 4G30 carb? I'm looking to take mine back to standard 1300 jets. Main jets are 0.875mm and 1.65mm according to my Gregory's manual. I plan to take the top cover off tonight to confirm what the current main jet sizes are.