Science of picking the right jet size - theory

[EricB]

OK, Work with me here guys...

My other 32yr old Japanese car has two dual throat Dell'Orto DHLB 35 carbs.

Yes I know... tiny carbs - hahahah

At least one car that was fitted with these same carbs from the factory was the 70s Lancia Fulvia 1.3 and 1.3s which were used as part of Lancia's rallying effort prior to the arrival of the Stratos.

Well my Japanese engine is a bored&stroked 1200cc now with approx 1300cc displacement, but fitted with forged motorcycle pistons which bump compression up to roundabout 10.5:1 & it's got a factory racing cam with much more duration than stock...

So with that said does it make sense for me to select my initial jet sizes based on what the Lancias ran with because their displacement is the same as mine?

Another question: once I've measured combustion chamber volume in CCs, & I know my bore size, what's the formula for calculating my exact compression ratio?

[dogma420]

sorry man don't know anything about lancias, but if you ask a Z question, I'd totally help out!

Sincerely,

[EricB]

The point is does it make sense to use another car's default jet sizes based on the fact it's got the same displacement and used the same carbs... does that reasoning hold?

-e

[240ZX]

The point is does it make sense to use another car's default jet sizes based on the fact it's got the same displacement and used the same carbs... does that reasoning hold?

-e

EricB, I think your point is very logical and should be a starting point however, since your engine is less than stock and the other engine you mentioned is stock I would tend to start out a bit on the fat (rich) side for jet selection so you don't risk overcooking new parts with a too lean mixture. Of course a too rich has its drawback as well. Have you decused this issue with anyone who has extensive experience tuning carbs, to get you in the proverbial ballpark? (I'm no expert, but was blessed with at least an ounce of sense!) Let me know what transpires!

[EricB]

I've been asking around for local Dell'Orto carb experts and have only tracked down a couple motorcycle guys who can't help me on a DHLB and a bunch of dead-ends where so and so just died unfortunately...

I'll find someone...

[240ZX]

Eric, have you thought about trying Z Theropy? I know they're SU guys, but carbs is carbs and jetting should follow the same logic. Might be worth a try.

[EricB]

Good point - I'll give 'em a call.

-e