Off idle lean bog- new carb

MilesnMiles · Post by **MilesnMiles** » Thu Nov 05, 15 11:00 am

I have a brand new 650 VS Speed Demon with Idle eze.

The car has an off idle lean bog which the AF guage shows up very clearly.

I only have to progressively open the throttle from idle and it goes lean before enrichment occurs as I add more throttle. its bad enough to cause real hesitation in low speed traffic.

When I got the carb I baselined it and ensured that the transition slot was square as recommended. I then wound the curb idle back to non existent and tuned the idle with the 4 corner adjustment and used the Idle Eze facility to add a little air and get a good tickover.

I've tried 28,31,35 squirters with no change.

Blue has mentioned changing the power valve but before I do that, does the PV actually come into play at such progressive throttle i.e.just off idle.
I understand that if I mash the throttle the PV is definitely involved, but not sure about where it comes into play in the overall fuel transition process.

Blue · Post by **Blue** » Thu Nov 05, 15 12:18 pm

The power valve opens as soon as the engine vacuum drops to it's opening point, so that could be just off idle as well as just above your cruise vacuum. My Dart had a stumble at both points during"normal" driving, upping the power valve was the only thing that would fix it.

It works like this, the pump shot is there to cover the lean spot on the transition from the idle to the main circuit when the throttle is opened.
The power valve enriches the main circuit by approx 10 jet sizes when vacuum demands it.
The pump shot is quite a small enrichment, the power valve considerably bigger.

MilesnMiles · Post by **MilesnMiles** » Thu Nov 05, 15 3:47 pm

Thanks Blue that makes sense. Next time I run it I'll hook up my vac gauge and see what happened vac wise and adjust accordingly.

Adrian Worman · Post by **Adrian Worman** » Thu Nov 05, 15 4:07 pm

Yeah do that Miles, you may well be surprised at the amount of vac your engine can produce at low RPM's & little load.

My 850 Speed Demon has IdlEze but it tuned fine without it.
Try winding the adjuster right down & install a 9.5/10.5 whatever PV as Blue suggests........... he's always right

My carb liked a slightly rich idle, forget 14.7 and don't be afraid to wind it up a bit.

Mick70RR · Post by **Mick70RR** » Thu Nov 05, 15 4:38 pm

Sounds very much like a hesitation I had with the Proform 950. If it does it just off idle in traffic it's nothing to do with the squirter or the power valve, it sounds like the idle fuel jets (IFR's) are too small. I think they are screw in jets on Demon carbs like on the Proform. You could try fitting smaller idle air bleeds first to see if it improves things, (as long as they are also screw in type). Or you could restrict the bleeds with a piece of wire. If it does improve things it's best to fit bigger IFR's.

MilesnMiles · Post by **MilesnMiles** » Thu Nov 05, 15 5:35 pm

Hi Mick, I don't think the bleeds are screw type on this carb, I think that's only on the Mighy Demon ( mines a Speed Demon), but your description is accurate. As I just push the throttle off idle the AF number go lean right off the guage before correcting as I push further on throttle.

Ade, I have the af at 13.5 for idle as I know these carbs like to be quite rich at idle compared to some. I may well retune with the idle Eze screwed in. The reason I used it is because I had the curb idle completely closed to allow the transition slots to be correct.

Powermyster · Post by **Powermyster** » Fri Nov 06, 15 8:24 am

Coincidently I have the opposite issue at present AF goes lean when snapping throttle open. Fine when progressively open, I got pump cams couple weeks ago but haven't had time to tinker.

Hook up vacuum and see if blue is on the right track, I'd say he onto a winner there.