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Recently I installed Jet Doctors from CB Performance. I never really had a need for them, but I had a set on the shelf. Down the rabbit hole......

This spring, I had installed a set of Zero Bypass accelerator pump valves provided by Kelly Frazer, as I sent him a set of old valves due to a "brass thingy" incident in Maryland LOL. That's another story.

Zero bypass accelerator pump valves are NOT for my car. It came with 50s, which means 50 percent of the fuel is bypassed back to the float bowl. The zero bypass valves( I'll just call them Zeros) were either too much fuel for too long or none as I adjusted them off. They were an all or none situation, never just right. The 50s are recommended by CB for our installations and appear to be perfect. Much nicer accel response and adjustment is possible to get the right "shot".

So in the process of doing this, I removed the carbs. Took them apart and cleaned them in my HF ultrasonic cleaner with Simple Green 50-50 with water. They came out looking new, and after a quick rinse with clean water and a blow-through with air, I reassembled. The HF Ultrasonic is $80, and works very nicely, especially when you turn on the solution heater. The carbs don't totally submerge, but I flip them over and do each side through a few cycles.

Anyway, checked float height and it was dead-on 11mm(or 10mm +1mm for gasket) and 25mm on the drop. This seems to work well for my engine.

I made sure all air bypasses closed, idle speed screws one turn in from not touching,  mixture screws 1.5 turns out, and carb bases drilled and cleaned of shavings for the jet doctors. The carbs remain unmodified for this, so if you want to go back you can remove them and plug the carb base hole with a new base gasket and/or a blob of silicone.

Got it all together and warmed it up and now we have a problem. Idle is perfect, low range is perfect, and above 3000 or 3200 is perfect. However, 2200-3000 is basically un-drivable. Revving in the garage is fine, but under any load at all, crap. Hmmmmm?

So I pull out my 57.5 idles and put the 60 in and retune. No better no worse. Put in F7 emulson tubes which are richer than the F11s and no better no worse. 


2016 Vintage Spyder 2165 type1 EFI/Dry Sumped

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Now I'm really scatching my head! So I get out my trusty Innovate MTX-L(wideband A/F gauge), but it goes into error mode and has random LEDs on or off. Basically, junk. DO NOT BUY ONE! A quick search shows lots of failures. I tried reflowing all the solder joints but no joy. Ordered a new APSX gauge with a harness and new O2 sensor on Wednesday. It arrived on Friday with cheap shipping about $5. Way to go US Postal!

My new car has a new exhaust, so no O2 bung. Get out the drill and welder and get to it. After a conversation with Stan I ended up putting the bung after the final center merge at the tailpipe. I was considering putting two so I could alternatively monitor the 2 cylinders on the same head, but he convinced me otherwise. Thanks, Stan!

So now I go for a drive without making any changes at all(60 idles and F7 emulsion). It is PIG rich all over the bottom(under 9 to 10.5), settling down to 12.5(perfect) under full load above about 3500. But especially bad in the transition from 2 to 3K. 

So I start really thinking, because even with the richness, it is still running better with less hesitation than the other day. The other day it was 85F and low humidity and sunny. Today it is cool and cloudy so the air is denser, which translated into leaner(denser air=more air/less fuel). So now I'm wondering exactly how incredibly rich it was the other day.

Then I get an idea. The CB Jet Doctors put the idle air entrance right near the lip of my velocity stacks, which are 1 1/4" high short ones. So I remove my Turbo Hats and quiet 2" cone air filters and swap the velocity stacks for the original 2 1/4" tall stacks. throw the original air filters on and it is driveable again! Pig rich, but driveable. My cam has a very high overlap and there is major intake reversion/pulsation going on above the carbs. You can actually see the fog of fuel hovering above the carbs at higher rpm. Making the velocity stack a good inch above the Jet Doctors isolates the tube from the reversion: EUREKA!

Put the 57.5 idles and F11 Emulsions and now it runs like it used to in the old car: QUICK and SMOOTH! I still think I can lean it a little more with smaller 55 idles and possibly larger air correctors(leans top end of transtion and upper end). Tweaked the pump adjustment a little and darn, she runs SWEET! The A/F gauge reads 13.8 to 14.7 at idle fully warmed up. 12.5 under full load but dipping into the 11s and high 10s in the low range, which is why I think I can use smaller idle jets.

So does this prove that a wideband A/F gauge is a very good(indispensable?) tool expense?

To be continued

Last edited by DannyP

A couple pics for you. There is a good 6" extension that adds onto the exhaust that is not is the picture, so there is about a foot of tailpipe past the sensor.

In the second pic you can see the Jet Doctors and the different heights of the velocity stacks. 

Moral of the story: No good deed goes without adding extra troubleshooting time and expense!!



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Last edited by DannyP

I'm sorry I couldn't have given this more attention when we were texting, Danny-- I was tied up on a crazy service call (1.5 hp fan broke loose from the base and sent a 26" fan blade spinning into a coil-- got it buttoned up at 3:00 AM), but you're a smart and resourceful guy and I knew you'd get it.

Because I'm a coward and make everything harder than it has to be, I tapped my existing idle air intakes and got some size tiny sockethead plugs when I did my 48 Dells, rather than deal with the lead balls which make it so you can't easily go back. About that time, I talked with Dave from Art Thraen's old shop (now Blackline Racing), who really didn't like the Jet Doctors. I chickened out and took them back off, but I still think they are a great idea. I'm not a fan of the spray-bar setup that replaces the aux venturi (which strikes me as a solution looking for a problem), but the Jet Doctors are a great idea, IMHO.

I'm glad (for selfish reasons) you got it to work. I'm not opposed to tapping my 45s at some point, and putting them in there. The taller velocity stacks can't hurt anything, especially if you've got fuel standoff. I've been considering the turbo-hats for safety reasons, but I think a nice fiberglass breather box setup like BAS makes would be better-- but it won't fit my engine and 550 euro (plus freight) kind've puts a kink in that plan.

So for now, I'm running an open filter like everybody else and hoping for the best.

Once again, Stan, thanks for your input. I don't expect you to drop what you're doing when you're making a living, my friend.

Weber Jet Doctors don't really require a commitment like Dells. That's new information to me.

I knew the problem was the only thing I had changed. I'm going to measure the height difference of the turbo hats versus the regular open air cleaners. That's why I got shorter velocity stacks in the first place, to give enough room above. If the CB turbo hats were the same height, then no problem.

I'd really like to keep them because of the intake noise reduction(very important in a Spyder with gill vents a la 550a) and I believe it adds a bit of backfire/carb fire resistance as there is nothing but metal around the carb throats.

I can't see spending that coin either. I'm actually thinking of making some glass molds of my hats but taller. We'll see. It isn't a whole lot in materials and vacuum bagging some hats would be fairly easy.

Yes. As I stated above, my engine has tons of valve overlap so also tons of reversion of the mixture. I strongly believe the air moving back and forth right near the idle air intake caused the pig-rich problem at transition. Possibly the air bouncing off the aluminum wall of the Cb turbo hat also had an effect.

I know this because the problem is gone with the simple removal of the turbo hats and installing the taller velocity stacks.

Interesting observation Danny ! I applaud you on your methodology to correct the problem as well My guess was that you affected that area where atmosphere plays a part in the air/fuel mixture ratios. Installing taller stacks put that area back to close to what it was originally. I ran into this problem many years ago on some Weber DCNL 40s. I ended up making my own V-stacks to fit them. There were no "Jet Doctors" back then. The reason we installed "Jet Doctors" then was to move the atmospheric vent hole from being right over air correction jets. The way it was in stock configuration, any sloshing around of the fuel in the fuel bowl just dumped over into the main jet assemblies and stumbling would occur. Off roading caused this to happen a lot.

Anyway, we would go in and epoxy the hole shut between the fuel bowl and the Jet assemblies. Then drill a hole the air horn area over the fuel bowl and adjacent to the inside of the air cleaner. Then epoxy a small brass tube in the air horn hole to connect to the area inside the air cleaner.   This solved the blubbering but carelessness in how you did this simple fix could really create other problems. 

One of these is what Danny cleverly discovered and corrected. Like Danny, I ended up making and adding a set of V-stacks. DCNLs didn't come with them like a DCNF did.

The second potential problem has to do with where you vent the fuel bowl. It could be vented to outside atmospheric pressure or to the inside of the air cleaner. Both will work fine until the air cleaner becomes restrictive due to dirt.  Now another pressure imbalance  can occur, creating new and mysterious fuel issues.

Here's a little tip about those Jet Doctors. If you have them. If you suspect that one of your idle jets is clogged up. Buy a small can of compressed air (used to blow off keyboards, switches etc) and keep in your "on-board" tool box. Take off your air cleaners and use the compressed air to back-blow down thru the Jet Doctors. The Idle jet passageway is right underneath and you can blow the debris back down into the fuel bowl. Try this while idling and you can trouble shoot exactly which idle jet was plugged up. Later you're going to have to take off the top to find the debris any way but at least you'll know which one...........Bruce

Alternatively, to check the jets, you could just put your finger over the jet doctor and temporarily block the air. If the rpms don't dip down, you've found your problem jet. If they dip down, that's not the plugged jet. I think that's smarter than blowing air into the jet doctors.

Something I've discovered in my travels: If and when you clean/rebuild your carbs, it seems not to matter even if you are hospital-clean in your labors. VERY soon after your rebuild you are bound to get one or two, maybe three jet clogs. This has bothered me, but I finally figured it out. 

The problem is you have disturbed things and opened the carbs up. There is bound to be a microscopic fleck of garbage or dust that ends up getting in your carburetors. So if you are running fine, try not to even take the air cleaners off. Just taking them off will most probably introduce dirt into the system.

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