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General Category >> Rubber Side Down! >> Stock carb, some rejet, am I thinking right?
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Message started by voldigicam on 07/28/09 at 08:04:09

Title: Stock carb, some rejet, am I thinking right?
Post by voldigicam on 07/28/09 at 08:04:09

52.5 w/holes, 1 3/4 turns, 150.

With stock exhaust, fine coming up from idle, response seems to flatten a bit at around 2800 to 3300 rpm, then goes nicely above 4000 rpm. Not fabulous, but I suspect the stock exhaust (soon to go away) isn't helping out.

I am interpreting this as having a good idle mix, a very slightly lean pilot, and a good main. Coming off idle I get the power and as the pilot takes more and more of the load, the mix leans just a little too much, then with more air the main starts to pull in and gives me a decent mix.

Is this correct thinking?

If so, I may ask my dad if he's ever going to use his watchmaker's reamers. They're very very precision. I can squeek the pilot opening a tiny (and I mean tiny) bit wider with the reamer. Think normal machinist's reamer on a very tiny and very long taper.

I'm pretty sure this will work. I see people looking for that 53.5 jet. No trouble to make one. We used to do this type of thing all the time in cars. I am one of the few with a real SK carb wrench in my toolbox!

Makes me wonder about altitude. I used to have to stop and adjust my Fiat at 3000 ft or it would become unhappy.

Just making sure my thoughts are right. Maybe I could make 53.5 equivalent jets for folks. All it takes is having the two adjacent sizes available, running a reamer into each, and then reaming smaller sizes to 1/2 way in between. The jeweler's reamers cut very clean!

Title: Re: Stock carb, some rejet, am I thinking right?
Post by srinath on 07/28/09 at 08:27:04

2800-3300 rpm - how are you measuring that. Also jetting is related to throttle position not to rpm. So 2800-3300 under full load, say 4/5th gear would be needle.
Pilots control 0-1/8th throttle, so its the take off range more than anything else.

Then, flat power = rich more than lean. Lean is if its surging in that rpm or if it shows a lean misfire or if it sounds dry ...

In your case first I'd check the float, lower it if its high, and if not I think its rich as it starts on the needle. Lower the needle a shade ...

However if you really think its lean, raise the float level and lift the needle.

Pilot is 0-1/8th throttle ... so I doubt it needs to be drilled out.

Cool.
Srinath.

Title: Re: Stock carb, some rejet, am I thinking right?
Post by Moofed on 07/28/09 at 09:49:39

Sounds to me like you need to lose some of the white spacer on the needle.

Title: Re: Stock carb, some rejet, am I thinking right?
Post by voldigicam on 07/28/09 at 11:01:20

"2800-3300 rpm - how are you measuring that."

tach.

"Also jetting is related to throttle position not to rpm. So 2800-3300 under full load, say 4/5th gear would be needle."

I was looking at cruising on level ground, so not too much throttle really.

"Pilots control 0-1/8th throttle, so its the take off range more than anything else."

OK, so this is likely the start of the main.

"Then, flat power = rich more than lean. Lean is if its surging in that rpm or if it shows a lean misfire or if it sounds dry ..."

In your case first I'd check the float, lower it if its high, and if not I think its rich as it starts on the needle. Lower the needle a shade ..."

Float is good - in the middle of the acceptable range. I can easily lower the needle a hair. I have about 1/2 of white spacer thickness, can drop it to about 1/3, see what happens. If it gets worse, then I know I'm going the wrong way.

Good thoughts. Certainly am rusty in this thinking.

Thanks!