Hobbit tuning - 4 stroking at low throttle

I've been trying to get my hobbit tuned and have been running into some issues getting the lower throttle range dialed in.

It's a 78 PA50i with a DR kit, stock head, proma with polini baffle, PHBG 19 on an MLM intake, boysen dual stage reeds on a PA50ii reed block, upgraded clutch springs, and a camino 6 roller variator with a fast arrow ramp plate and weight set.

The carb has a W7 needle in the second notch from top, a 30 slide, 88 main jet, 40 idle jet, still have the disc in the bowl (consensus seems to be to chuck it), not sure about the atomizer.

It loves running in the mid throttle which is great and it gets up to 30-35mph pretty quickly. I haven't pushed it to the main jet for more than a few seconds at a time as the kit's not broken in yet.

At lower throttle positions I can't get it to not 4-stroke, even with a 40 idle jet and the mixture screw all the way in it just sounds ragged. I tried running a little bit like this to see what the plug looked like and counterintuitively it came out white, so not sure how I could be 4-stroking and running lean? (At mid range the plug looks good)

It's worse if I get into the mid range and gain some speed before coming back down to lower throttle.

Being in Portland people have a tendency to drive slow and I need to be able to ride at low throttle (20mph) without feeling like my engine is going to explode.

The tuning spreadsheet hasn't helped very much but after reading through this thread it sounds like I'm not too far off on the main jet, idle jet, and needle but could the slide be the issue, maybe swap out for a 40 or 50 or is it something else?

Re: Hobbit tuning - 4 stroking at low throttle

Neil Burgess /

I'm not 100% sure on what size jets you should need, but 0-1/4 throttle would be idle jet- at 1/4 throttle you're just getting into the needle so maybe drop the needle down one clip and see if it helps. I run the 21 phbg, 40 slide, 50 idle jet, W7 needle 2nd clip from the top. 40 idle seems small to me but I haven't tuned a 19. I'm thinking 45 idle(then you should be able to adjust the screw). The slide may be an issue too- heard of other guys running 40 and 50 slides in phbg's but not a 30. That sounds like a 4 stroke setup. Better check that atomizer- make sure it's an AU.

Re: Hobbit tuning - 4 stroking at low throttle

Thanks. I'll pull the atomizer to double check it and maybe order new slides.

When you say drop the needle down one clip do you mean putting the clip in the second from bottom notch or the top notch (it's in the second from top now)? I've tried it in the second from bottom and it was 4-stroking across almost the entire throttle at that position.

Re: Hobbit tuning - 4 stroking at low throttle

The atomizer is a 262 AU.

Re: Hobbit tuning - 4 stroking at low throttle

I put in a 40 slide, made the needle one notch richer and went to a 92 main and 45 idle. I still have the idle mixture screw turned in almost all the way but am getting pretty good progression across the throttle with little to no 4-stroking.

The problem now is that when the bike is idling at a stop the temperature climbs, a lot, got up over 400 before I killed it. It doesn't matter where I have the idle screw or idle mixture screw set, it just keeps getting hotter. It also runs pretty hot in the low throttle ranges, in the 380's or higher. Seems like that shouldn't be normal, does anyone have any thoughts on why that might be happening?

It's ok temp-wise when running in the needle as long as I start from cooled down, if it's already heated up it'll stay on the hot side.

Re: Hobbit tuning - 4 stroking at low throttle

What is a good operating temp range for a kitted hobbit anyways?

Re: Hobbit tuning - 4 stroking at low throttle

375-450 max. They run hot.

Re: Hobbit tuning - 4 stroking at low throttle

Cool, good to know. Think I'm alright at speed then.

Should I even be concerned about the temp rising at idle/low speed or does that have less to do with possibly being lean and more to do with the bike being at a standstill and not getting any air to cool it down?

Even if I have the idle mixture screw out to the point that it four strokes heavily at low throttle it's still heating up.

Re: Hobbit tuning - 4 stroking at low throttle

Is this a stock ignition with points? Did ya slam on a timing light and see where it sits vs the marks on the case.? Hobbits run super hot often and it's really limiting. You can try if stock ignition making the points gap a bit smaller to retard your timing and see if that cools things a bit at idle. The fins on your stock head plus the stock hemispherical head etc etc are not working in your favor. This problem can take to a variety of complex "fixes" here is one!

And when you go totally insane check out picture number two

But pic number three is at least somewhat sane. It's a stocko shocko which isn't anywhere as sweet as a dr with a somewhat squished modded head. I might have in a bit a dr a moped factory dr head head available to sell. But everybody is looking for this item and I might have to price it similar to a b1 bomber. Plus it's not that amazing at dropping temps but it helps.

Good luck

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